Are Brain TRPs Viable Targets for Curing Neurodegenerative Disorders and Improving Mental Health?

Chapter 23


Are Brain TRPs Viable Targets for Curing Neurodegenerative Disorders and Improving Mental Health?



Bernd Nilius1; Arpad Szallasi2,*    1 Katholieke Universiteit of Leuven, Department of Cellular and Molecular Medicine, Laboratory of Ion Channel Research and TRP Research Platform Leuven (TRPLe), Campus Gasthuisberg, Leuven, Belgium
2 Department of Pathology, Monmouth Medical Center, Long Branch, NJ, USA
* Corresponding author: ASzallasi@barnabashealth.org




The Role of TRP Channels in Building the Brain and Regulating its Functions: Introducing the Players


Canonical TRP Channels


Every member of the TRPC subfamily is expressed in the brain (Figure 23.1), some (like TRPC1) ubiquitously, whereas others (e.g., TRPC4) in a highly restricted pattern [1]. Further details are out of the scope of this chapter (interested readers are referred to [24]). In general, TRPCs (a) regulate neural stem cell proliferation; (b) promote neuronal cell survival in response to neurotrophins such as BDNF; (c) are involved in chemoattraction of neurites, growth cone guidance and regulation of neurite outgrowth spine formation; and (d) are increasingly accepted as major players in synaptic plasticity and synaptogenesis (Figure 23.2; see 4 for a review]. These functions are especially prominent in the developing brain.


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Figure 23.1 Schematic representation of TRP channels distribution in the mouse and human brain. Reprinted with permission from Morelli, et al. CNS Neurol Disorder Drug Targets, 2013;12:274–93.

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Figure 23.2 (a) TRPCs promote stem cell proliferation via basic fibroblast growth factor (bFGF). (b). Some TRPCs promote neuronal cell survival in response to brain derived neurotrophic factor (BDNF). (c) Neurotrophins (BDNF) induce chemoattraction, TRPCs are required for proper growth cone turning responses to microscopic gradients, such as netrin-1. (d) Some TRPCs regulate neurite outgrowth and dendrite patterning. (e) TRPCs are required for synaptogenesis and induce spine formation. Reprinted with permission from Tai et al. [5].

TRPC1


TRPC1 in the Developing Brain

In the CNS, the expression pattern of TRPCs changes during embryogenesis, suggesting that coordinated alterations in TRPC gene expression are involved in brain development. One of the main functions of TRPC1 in the developing brain is thought to be the guidance of axonal growth by directing growth cone turning (see for a review [58]). Excitatory synapses in the vertebrate nervous system are characterized by an electron-dense structure, called the postsynaptic density (PSD). Homers are PSD proteins that (in concert with Shank and GRIP) play a pivotal role in building a complex submembrane platform for the anchorage of receptors and scaffolding proteins. By coupling extracellular receptors (such as TRPC1 and metabotropic glutamate receptors, mGluRs) to intracellular receptors (e.g., inositol triphosphate [IP3R] and ryanodine receptors) and signaling pathways, Homers are ideally placed to regulate Ca2 + dynamics within the neural growth cone. Spatiotemporal patterns of Ca2 + are believed to underpin growth cone motility [911]. Indeed, calcium imaging of motile growth cones revealed that Homer-1 was required for the guidance-cue-induced rise in cytosolic Ca2 +. Homer-1 knockdown reversed growth cone turning from attraction to repulsion in response to brain-derived neurotrophic factor (BDNF) and netrin-1 [12]. Of note, in the adult rat brain BDNF promotes neurogenesis and dendritic spine reorganization in the hippocampus [13,14]. BDNF is also an important regulator of striatal neuron survival, differentiation, and plasticity [15].


In different brain nuclei, distinct factors are thought to guide axon growth. Named after their role in building bone and cartilage, bone morphogenic proteins (BMPs) constitute a family of essential morphogenic agents that regulate tissue architecture throughout the body, including the CNS. BMP signaling involves the transcription factor, SMAD (a mammalian homolog of the C. elegans Small Body Size and the Drosophila Mothers Against Decapentaplegic). Disruption of BMP signaling prevents development of the neural plate. TRPC1 has been implicated in BMP signaling. BMP7 gradient causes a bi-directional turning response in the nerve growth cone. This effect is due to activation of the LIM kinase (so-called for the three transcription factors, Lin-1, Isl-1, and Mec-3, it activates) and the phosphatase, Slingshot. Both enzymes regulate actin dynamics via the actin-depolymerizing factor. This interaction requires the presence of TRPC1; apparently, TRPC1 initiates the Ca2 + signal that activates Slingshot via calcineurin, leading to growth cone repulsion [16].


In the corpus callosum, a Wnt5a (Wingless-type MMTV integration site family, member 5a) gradient guides axons after their midline crossing via RYK (Related to Tyrosine Kinase Receptor) proteins, and propels them into the spinal cord. In dissociated cortical neuron cultures, Wnt5a simultaneously promotes axon outgrowth and repulsion by Ca2 + signaling. The signaling pathway involves both Ca2 + release through IP3Rs and Ca2 + entry through TRPCs. Knock down of the RYK receptor reduced postcrossing (but not precrossing) axon growth by 50% and, importantly, led to misrouting [17]. In addition to RYK proteins, the repulsive growth-cone turning requires the protein, Frizzled (Fz) [18].


It should be mentioned here that (at least in expression systems) TRPC1 and TRPC5 exert opposite effects on neurite growth. In PC12 cells, topical expression of TRPC1 promotes, whereas TRPC5 suppresses, neurite outgrowth. Suppression of TRPC1-induced neurite outgrowth by TRPC5 was due to a marked reduction in the surface expression of TRPC1. These findings suggest that TRPC1 acts as a scaffold at the cell surface to assemble a signaling complex to stimulate neurite outgrowth [19]. Furthermore, in PC12 cells nerve growth factor (NGF) markedly up-regulates TRPC1 and, conversely, down-regulates TRPC5 expression while promoting neurite outgrowth. Knockdown by shRNA of TRPC1 decreases, whereas shRNA-mediated knockdown of TRPC5 increases NGF-induced neurite extension. Interestingly, overexpression of TRPC6 (which, similar to TRPC1, is up-regulated by NGF) counteracts TRPC1 effects: it slows down neuritogenesis. Confusingly, hyperforin, a specific TRPC6 activator, decreased TRPC6 expression in NGF-differentiated PC12 cells [20]. TRPC effects on axonal guidance likely depend on PIP3 [21].


In the developing brain, TRPC1 is also involved in the proliferation of oligodendrocytes. In oligodendrocyte progenitor cells, Golli proteins (alternative spliced products of the myelin basic protein gene) regulate migration by modulating intracellular Ca2 + levels. Experiments performed with acute brain slice preparations obtained from Golli knockout and Golli-overexpressing mice point to TRPC1 as an important downstream target for Golli [22].


The Role of TRPC1 in Determining the Excitation State of Hippocampal Pyramidal Neurons

In the hippocampus, the excitability of pyramidal neurons is regulated by an intricate interplay between coincident inputs and the intrinsic electrical activity of the cells. Following firing activity, afterdepolarizations determine the membrane potential of the neuron. There is accumulating evidence that TRPC1 is involved in the generation of small-amplitude (1 mV), long-lasting (for seconds) afterdepolarizations. Such depolarizations are thought to contribute to neural information processing during behavioral tasks [23]. In CA1 pyramidal neurons, mGluR activation leads to intracellular Ca2 + waves. Stimulation of mGluRs triggers a biphasic electrical response composed of a transient hyperpolarization mediated by SK (small conductance Ca2 +-dependent) and KCNQ potassium channels, followed by a sustained depolarization attributed to TRPC1 activation [24]. Of note, TRPC1 has also been implicated in hippocampal neurogenesis. In mice, knockdown of Trpc1 markedly reduced adult neural progenitor cell proliferation. (As discussed later, this observation links TRPC1 to cognitive deficits [25]).


Role of TRPC1 in Synaptic Plasticity and Long-term Depression (LTD)

The evidence linking TRPC1 to synaptic plasticity and LTD is preliminary and speculative. In the brain, presynaptic firing rates are converted to postsynaptic membrane depolarizations by ion channels, especially by ionotropic NMDA and non-NMDA (e.g., AMPA) glutamate receptor. If the firing rate is low (below 20 Hz), ionotropic glutamate receptors may not produce sufficient summation, and other receptors may take over. It was speculated that under these conditions TRPC channels activated by mGluRs could be more effective owing to their slower kinetics [26]. Indeed, downstream of mGluR1 there are only two major sources of Ca2 +: TRPC channels and IP3Rs. In rats, the mGluR1-evoked slow currents are mediated by TRPC channels whose inhibition blocks cerebellar LTD [27].


TRPC1 (along with TRPC4) is also expressed in the mouse olfactory bulb where it is believed to contribute to the central synaptic processing at the reciprocal mitral and tufted cell-granule cell microcircuits. Suprathreshold activation of these synapses causes long-lasting depolarization (LLD) in the granule cells, linked to a global dendritic postsynaptic calcium signal. The LLD is absent in granule cells deficient for the N-methyl-d-aspartate (NMDA) receptor subunit NR1. Interestingly, the LLD is also absent in granule cells from mice deficient for both TRPC1/C4 (double knockouts). The deletion of either TRPC1 or TRPC4 results in only a partial reduction of the LLD [28].


TRPC1 in Thermoregulation and Food Intake

Histamine influences body temperature by interacting at H1 and H3 receptors expressed in distinct subpopulations of thermoregulatory neurons in the median preoptic nucleus. In these cells, single-cell RT-PCR revealed the expression of TRPC1 (and also TRPC5 and TRPC7) channels. Histamine activates a cationic inward current to increase the intracellular Ca2 + concentration. This current has two components, a transient current as well as a sustained one. The sustained component was blocked by intracellular application of a TRPC1-blocking antibody, identifying TRPC1 as a downstream target for histamine receptors [29].


Pro-opiomelanocortin (POMC) neurons of hypothalamic arcuate nucleus regulate food intake, energy homeostasis, and glucose metabolism. A subpopulation of POMC neurons (distinct from those activated by leptins) respond to meta-chlorophenylpiperazine (mCPP), a psychoactive 5-HT receptor agonist that suppresses food intake in rats. It was hypothesized that TRPC channels are a pivotal downstream target for 5-HT receptor activation. By contrast, in ventral premammillary nucleus neurons leptin-induced depolarization was dependent on TRPC channels [30].


TRPC3


TRPC3 and Motor Coordination

Cerebellar Purkinje cells play a pivotal role in motor coordination. In these cells, TRPC3 is highly expressed [31]. Cerebellar motor coordination is dependent on mGluR1 that partners with glutamate receptor δ2 (GluRδ2) and protein kinase C-γ (PKCγ). There is good evidence that TRPC3 is involved in the formation of mGluR1-dependent slow excitatory postsynaptic potentials (EPSCs). Mutations in GluRδ2 change the subcellular distribution of mGluR1 and TRPC3 to increase their surface expression. Loss of GluRδ2 disrupts mGluR1-dependent synaptic transmission at parallel fiber-Purkinje cells synapses; this will result in deficits in motor coordination [32]. TRPC3 also provides a negative feedback to cytosolic Ca2 + regulation via its C-terminal CIRB (calmodulin and I3PR) domain. Alternative splicing of the TRPC3 mRNA transcript results in a truncated TRPC3 protein referred to as TRPC3c that lacks approximately half of the CIRB domain. TRPC3c expression is brain region specific, with high prevalence reported in the cerebellum and brainstem. When expressed in HEK293 cells, the TRPC3c channel exhibits a high opening rate. Thus, TRPC3c appears to have enhanced efficacy as a neuronal Ca2 + signaling effector [33].


Of note, functional TRPC3 channels are required for BDNF to increase dendritic spine density in CA1 pyramidal neurons. TRPC3 blockade (by siRNA down-regulation or antagonist treatment) attenuated dendritic spine formation in CA1 hippocampal neurons after BDNF application [34].


Cerebellar LTD is induced by pairing the synaptic inputs provided by the climbing fibers with those of the parallel fibers. Cerebellar LTD is impaired by TRPC3-blocking antibodies, implying a pivotal role for TRPC3 in the induction of LTD [35]. In expression systems, TRPC3 activity is inhibited following phosphorylation by PKC. However, in native Purkinje cells the activation of TRPC3-dependent currents is not inhibited by PKC, implying that native TRPC3-dependent currents may differ significantly in their regulation from those studied in expression systems [36].


As discussed later, Trpc3 knockout mice exhibit impaired walking behavior. Also, disruption of mGluR signaling through TRPC3 is one of the major molecular defects in staggerer (sg/sg) mice, a model of human spinocerebellar ataxia [37]. Combined, these observations confirm the pivotal role of TRPC3 in motor coordination and establish TRPC3 as an important postsynaptic channel that mediates mGluR-dependent synaptic transmission in cerebellar Purkinje cells [38]. Of note, TRPC3 is also highly expressed in cerebellar granule neurons, where it is thought to exert a protective function against serum-deprivation cell death [39,40].


GABA projection neurons in the substantia nigra pars reticulata provide critical contribution to movement control by regulating the activity of basal ganglia. These neurons exhibit a sustained, spontaneous high-frequency spike firing that is believed to involve tonically active TRPC3 channels [41]. Moreover, large aspiny cholinergic interneurons provide the sole source for acetylcholine (Ach) in the striatum. These interneurons are important for keeping the balance between dopamine and GABA. ACh is released in response to corticostriatal glutaminergic afferents, which act mainly via mGluRs. The current activated by mGluR resembles TRPC3 (and also TRCP7). Indeed, in heterologous coexpression experiments TRPC3 is activated via mGluR1 and mGluR5 [42].


TRPC3 as a Neuronal Energy Generator

The brain uses more energy than any other human organ. Appropriate mitochondrial transport and distribution are essential for neurons because of the high energy and Ca2 + buffering requirements at synapses. In hippocampal neurons, BDNF halts mitochondrial transport via activation of TRPC3 and TRPC6; this results in the accumulation of mitochondria at the presynaptic sites. The Ca2 + sensor Miro1 plays an important role in this process. Mutant Miro1 (lacking the ability to bind Ca2 +) prevents BDNF-induced presynaptic mitochondrial accumulation and synaptic transmission [43]. Of note, chronic exposure to elevated levels of manganese (Mn2 +) causes neuronal injury. Astrocytes selectively accumulate Mn2 + which, in turn, inhibits mitochondrial respiration via TRPC3 [44].


TRPC4 and TRPC5


Role of TRPC4 and C5 in Neurotransmission and Memory Formation

Cholecystokinin (CCK) is one of the most abundant neuropeptides in the brain where it interacts with two G-protein coupled receptors (GPCRs), CCK-1 and CCK-2. Activation of both CCK receptors influences neurotransmission. For example, CCK-2 receptor activation attenuates dopamine release. The CCK effects are suppressed by the nonselectiveTRPC channel blockers, 2-APB and flufenamic acid; conversely, they are potentiated by lanthanides (Gd3 + and La3 +). Importantly, the CCK-induced enhancement of neuronal excitability was significantly inhibited by intracellular application of a TRPC5-blocking antibody [45].


Persistent neuronal activity lasting seconds to minutes is thought to contribute to the transient storage of memory traces in the entorhinal cortex. In many cortical and subcortical structures, nonsynaptic plateau potentials induced by ACh account for the persistent firing. For example, in layer V of the rat medial entorhinal cortex, carbachol (a cholinomimetic drug that binds to muscarinic receptors) evokes persistent firing via phospholipase C (PLC) activation; this response was suppressed by the TRPC channel blocker, SKF-96365, but not the TRPV channel blocker, ruthenium red (RR). The diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol (OAG), a nonselective activator of TRPC3, C6, and C7 channels, did not alter the firing evoked by carbachol, implicating the involvement of TRPC1, C4, and/or C5 protein. However, the persistent firing was inhibited by intracellular application of the peptide EQVTTRL that disrupts interactions between the C-terminal domain of TRPC4 or C5 subunits and associated PDZ proteins [46]. Furthermore, a TRPC5 dominant negative construct inhibited, whereas the overexpression of wild-type TRPC5 (or TRPC6) enhanced the amplitude of the muscarinic receptor-induced inward after current. These results indicate that TRPC channels (most likely TRPC5 and C6) mediate the muscarinic receptor-induced slow afterdepolarization seen in pyramidal cells of the cerebral cortex and suggest a possible role for TRPC channels in mnemonic processes [47].


TRPC4 and C5 in Arousal, Mood, and Reproductive Functions

The thalamic paraventricular nucleus contains abundant receptors for thyrotropin-releasing hormone (TRH), a neuropeptide known to modulate arousal and mood. In whole cell patch clamp recordings obtained in rat brain slice preparations, TRH induced two concurrent conductances. One conductance featured a K+-dependent current that was suppressed by the GIRK (G protein-coupled inwardly rectifying K+ channel) antagonists, tertiapin Q, and SCH 23390. The second conductance was attenuated by the nonselective TRPC channel blockers, 2-APB, flufenamic acid, and ML204. Based on these findings, it was speculated that TRH excites paraventricular nucleus neurons by a concurrent action on GIRK and a TRPC channel possibly involving TRPC4 and C5 subunits [48].


Hypothalamic kisspeptin-positive neurons are critical for reproductive functions by initiating the synthesis of gonadotropin-releasing hormone (GnRH) at puberty. In slice preparation from female guinea pig brain, arcuate kisspeptin neurons exhibit a negative resting membrane potential, with the majority (~ 80%) showing a pacemaker current. Leptins increase burst firing in kisspeptin-positive neurons, which was potentiated by lanthanum, a TRPC4/C5 channel activator. By contrast, the leptin-activated current was abrogated by the TRPC channel blocker, 2-APB. It was speculated that excitation by leptin of kisspeptin neurons may be involved in the regulation of GnRH-expressing neurons during different nutritional states [49].


Coordinated gene expression changes across the CNS are required to evoke maternal behavior. After giving birth, gene expression profiling revealed changes in a number of genes including Trpc4 in the lateral septum of female mice, a brain region implicated in maternal care [50]. In lateral septum neurons, TRPC4 is coexpressed with group I mGluRs. The group I mGluR agonist (S)-3,5-dihydrophenylglycine (DHPG) causes an immediate increase in firing rate, followed by a pause of firing; these DHPG actions can be correlated to below-threshold-depolarization (BTD) and above-threshold-plateau-depolarization (ATPD), respectively. In neurons obtained from Trpc4−/− mice the early phase of BTD and the entire ATPD are completely absent. These results suggest that TRPC4 integrates mGluR stimulation with intracellular Ca2 + signals to affect the excitability of lateral septum neurons [51].


TRPC5 in Gait and Motor Coordination

Trpc5 knockout mice harbor long, highly branched dendrites on their granule neurons with impaired dendritic claw differentiation in the cerebellar cortex. These animals also show deficits in gait and motor coordination. In the centrosome of cerebellar granule neuron, TRPC5 forms a complex with CaMKIIβ and thereby regulates the CaMKIIβ-dependent phosphorylation of the ubiquitin ligase, Cdc20-APC. Centrosomal CaMKIIβ signaling regulates dendrite morphogenesis. The role of TRPC5 is to couple Ca2 + signaling to the ubiquitin ligase pathway at the centrosome [52]. Of note, a similar signaling pathway regulates dendrite development in the hippocampus in response to neurotrophin-3 (NT-3) [53].


TRPC5 and Brain Development

As discussed earlier, TRPC5 counteracts TRPC1 in regulating neurite outgrowth. TRPC5 also appears to be involved in growth cone collapse that occurs when the growth cone detects a repellant factor. Semaphorin 3A is a secreted guidance cue that keeps the growth cone away from inappropriate targets. Semaphorin 3A-mediated growth cone collapse is reduced in hippocampal neurons obtained from Trpc5 null mice. This effect is reproduced by inhibition of the Ca2 +-sensitive protease, calpain, in wild-type, but not Trpc5−/−, neurons. Calpain-1 and calpain-2 cleave and activate TRPC5. Mutation of a critical threonine at position 857 in the TRPC5 protein inhibits calpain-2 cleavage. These findings identify TRPC5 as a downstream target for semaphorin signaling to cause changes in neuronal growth cone morphology and nervous system development [54]. In addition, Ca2 +– influx via TRPC5 can activate CAMKIIβ, which, in turn, phosphorylates the ubiquitin-ligase complex Cdc20-APC and regulates dendrite patterning. TRPC5 knockdown causes ataxia due to defective dendrite patterning in the cerebellum [52].


TRPC6


TRPC6 in Learning and Exploratory Behavior

In the mouse brain, the peak of TRPC6 expression is between day 7 and 28 after birth [55]. TRPC6 has a postsynaptic location at excitatory synapses. Overexpression of TRPC6 increases the number of spines in hippocampal neurons. Indeed, these transgenic mice show improved spatial learning and memory in the Morris water maze [56]. The Trpc6−/− mice showed no significant differences in anxiety (as determined by the marble burying test), but demonstrated reduced exploration in the square open field and the elevated stair maze, suggesting an important role for TRPC6 in exploratory behavior [57].


TRPC6 as a Target for Hyperforin

Hyperforin, the principal bioactive ingredient in the medicinal plant Hypericum perforatum (St. John’s wort), is well known for its antidepressant action. The mechanism of action of hyperforin is, however, poorly understood. Hyperforin elevates [Ca2 +]I in cortical neurons [58]. In mouse brain cortex, chronic hyperforin treatment (daily injection of hyperforin at a dose of 4 mg/kg for 4 weeks) increases the expression of both TrkB (a receptor for BDNF) and TRPC6 [59]. It is not clear how hyperforin drives TRPC6 expression but it is believed to involve CREB (cAMP response element binding protein) phosphorylation [60]. In addition, hyperforin inhibits the reuptake of serotonin through the activation of TRPC6 channels. This is important because selective serotonin reuptake inhibitors are clinically useful antidepressant drugs. Hyperforin by interacting at TRPC6 also modulates dendritic spine morphology in CA1 and CA3 hippocampal pyramidal neurons [61]. Last, hyperforin was shown to modify intracellular Zn2 + levels and to act as an NMDA receptor antagonist; there is some evidence that these actions may also be mediated by TRPC6 [62]. The physiological function of TRPC6 in cortical neurons remains unclear. Indeed, a variety of compounds that are known to block TRPC channels (including 2-ABP, flufenamic acid, lanthanum, SKF-96365, and Pyr-3) had little, if any, impact on cholinergic afterdepolarization potentials, nor were these potentials affected by genetic deletion of Trpc1, Trpc5, or Trpc6 (single knockouts), or both Trpc5 and Trpc6 together (double knockouts) [63].


TRPC7


Substance P (SP) plays an important role in modulating rhythmic activities driven by central pattern generators including respiration. In mouse brainstem slices containing the pre-Bötzinger complex, SP activates TRPC3 and C7 to enhance the respiratory rhythm regularity [64]. It was postulated that TRPC channel dysfunction may be responsible for the irregular respiratory rhythms in some central neuronal diseases [65].


Vanilloid TRP Channels


Of the vanilloid TRP subfamily, the presence of TRPV1 to V4 has been reported in the brain (Figure 23.1). The pharmacology of these channels have been detailed elsewhere [2].


TRPV1


The literature on brain TRPV1 is highly controversial, ranging from widespread expression throughout the whole neuraxis (reviewed in [66,67]) to minimal expression in a few, discreet nuclei [68]. Clearly, a careful re-evaluation of the relevant findings is in order. Until it is done, the most one can do (especially if the author is biased by contributing data to this field) is a balanced review of the literature (pro and contra), allowing readers make up their own minds.


Three lines of evidence support the existence of functional TRPV1 in the brain. First, capsaicin evokes responses in brain slices, neurons in culture, and in whole animal models. Second, both TRPV1 mRNA (by RT-PCR or ISH) and protein (by immunostaining or [3]RTX autoradiography) can be detected (albeit at lower levels than in sensory ganglia) in the brains of wild-type, but not Trpv1 null, mice. And third, Trpv1 (−/−) animals show distinct behavioral changes. Unfortunately, none of this proves the existence of brain TRPV1 unequivocally. Capsaicin (especially at high concentrations) is not selective for TRPV1; anti-TRP antibodies are known to detect proteins other than their expected target; and knockouts may have compensatory changes in the expression of other receptors. With these sobering thoughts in mind, let’s review some recent findings to suggest that brain TRPV1 not only exists but also serves crucial functions.


TRPV1 in Fear and Anxiety

In mice, functional magnetic resonance imaging (fMRI) of brain activity revealed that intragastric infusion of capsaicin at doses at which it evokes nociceptive behavior activates several brain regions linked to fear and anxiety, including the amygdala and the periaqueductal gray matter. These brain nuclei were not activated in the Trpv1 knockout animals. Interestingly, capsiate (a nonirritant capsaicin congener) activated the same brain regions with the sole exception of the periaqueductal gray matter [69].


The endogenous compounds (so-called endovanilloids) that activate brain TRPV1 are subject to extensive research. Anandamide (an endocannabinoid that also activates TRPV1 at high concentrations) exerts a biphasic effect when injected into the dorsolateral periaqueductal gray matter in rats submitted to threatening situations. Lower doses of anandamide induce anxiolytic-like effects, presumably by activating cannabinoid CB1 receptors. This beneficial effect is, however, no longer observed at higher doses, possibly due to the simultaneous activation of TRPV1. Indeed, blockade of TRPV1 by capsazepine potentiates the anxiolytic-like effects of anandamide [70]. The marble-burying behavior is a useful murine model of obsessive-compulsive disorder. Anandamide (1-10 μg/mouse icv) inhibits marble-burying behavior, indicating an anticompulsive activity. Conversely, at higher doses (20 to 40 μg/mouse) anandamide increased the marble-burying activity, and this effect was mimicked by capsaicin (100 μg/mouse) [71].


N-Arachidonoyl-serotonin (AA-5-HT) is a dual blocker of the endocannabinoid-inactivating enzyme, fatty acid amide hydrolase (FAAH), and the TRPV1 channel. Injection of AA-5-HT into the basolateral amygdala exerts a strong anxiolytic action. Indeed, in the elevated maze test, the AA-5-HT-treated animals spend significantly more time in the open arms than the controls. So, which target mediates the anxiolytic action of AA-5-HT, CB1 or TRPV1? Neither capsazepine, nor URB597 (a selective FAAH inhibitor) has a noticeable anxiolytic action on their own. Coadministration of capsazepine and URB597, however, mimicked the anxiolytic effect of AA-5-HT. Taken together, these findings imply that a simultaneous blockade of FAAH activity and TRPV1 activation are required for anxiolytic activity [72].


The periaqueductal gray and the amygdala (part of the limbic system) play complimentary roles in the innate and learned fear responses. A subset of lateral amygdala neurons express TRPV1: these cells are believed to store and recall established fear memory [73]. The amygdala is also involved in the stress modulation of learning and memory formation. In the mouse amygdala, the “endovanilloid” N-oleoyldopamine (OLDA) suppresses LTP in the lateral nucleus of wild-type, but not TRPV1-deficient mice. The specific TRPV1 receptor antagonist AMG 9810 also prevented the OLDA effect on LTP. At the behavioral level, OLDA enhanced LTP in mice subjected to the forced swim test [74].


Contextual fear is evoked by reexposing an experimental animal to an environment that has been previously paired with an aversive or unpleasant stimulus. A marked increase in neuronal activity is associated with contextual fear conditioning, especially in limbic structures involved with defense reactions such as the ventral portion of medial prefrontal cortex. Capsazepine microinjected into the medial prefrontal cortex reduces the freezing behavior and cardiovascular responses in the high aversive protocol. Conversely, capsaicin potentiates fear-associated responses [75]. By contrast, capsazepine microinjected in the ventral portion of the medial prefrontal increased exploration of open arms in the elevated plus maze test, suggesting an anxiolytic-like effect [76]. Of note, the anxiolytic-like response evoked by capsazepine is similar to that of diazepam. Desensitization to capsaicin attenuates the anxiolytic effect of diazepam, whereas coadministration of capsazepine and diazepam at subeffective doses exhibit anxiolytic-like effect. These findings imply that the anxiolytic effect of diazepam, at least in part, involves TRPV1 [77], which appears to be tonically active.


Of note, activation of TRPV1 in the ventrolateral periaqueductal gray also exerts antinociceptive effects (which are out of the scope of this chapter). Briefly, microinjection of capsaicin into the ventrolateral periaqueductal gray reduced noxious heat sensation in the rat (measured in the hot plate test); this effect was blocked by CB1-R and mGluR antagonists. These observations imply that capsaicin activates TRPV1 to release glutamate, which, in turn, activates postsynaptic mGlu5-R. The end result of this cascade activation of a descending pain inhibitory pathway [78].


TRPV1 and Synaptic Plasticity

In amygdala neurons, 2-arachidonoylglycerol (2-AG) and anandamide mediate different forms of synaptic plasticity: 2-AG (acting on presynaptic CB1 receptors) triggers a retrograde short-term depression, whereas LTD is mediated by anandamide acting on postsynaptic TRPV1 receptors. This function sharing between CB1 and TRPV1 receptors is in contrast to the striatum where 2-AG (acting on CB1 receptors) mediates both forms of plasticity [79]. In the striatum, ACh controls both excitatory and inhibitory synaptic transmission by activating muscarinic M1 receptors. Capsaicin prevents the effects of M1 receptor activation on inhibitory postsynaptic potentials (IPSPs). Elevation of the anandamide tone by the FAAH inhibitor URB597 mimicked the effects of capsaicin, indicating that endogenous anandamide may act as the endovanilloid. In support of this model, URB597 effects were absent in mice lacking TRPV1 channels [80,81].


In the hippocampus and cortex of the mouse, TRPV1 expression was detected by a combination of real-time PCR and Western blot. The TRPV1 mRNA and protein expression levels showed dynamic changes during brain development, with a progressive increase between 4 and 8 weeks after birth [82]. Within the hippocampus, TRPV1 was predominantly expressed in the dentate molecular layer, an area linked to long-term synaptic plasticity. By high-resolution electron microscopy, TRPV1 immunoparticles were found to be highly concentrated in the postsynaptic dendritic spines; by contrast, TRPV1 was poorly expressed at the excitatory hilar mossy cell synapses. Importantly, this pattern of TRPV1 expression was completely absent in the Trpv1 null mice [83]. It was speculated that TRPV1 modulates synaptic plasticity (both LTP and LTD) in hippocampal CA1 pyramidal cells, possibly by changing the activity of CA1-inhibitory GABAergic interneurons. Indeed, the GABA antagonist picrotoxin eliminated the enhancement of LTP in CA1 neurons in response to TRPV1 agonists [84]. The calcineurin antagonists, cyclosporine and FK-506, also blocked TRPV1-dependent LTD [85].


In humans, a number of single-nucleotide polymorphisms (SNPs) have been described in the TRPV1 gene, some of which significantly alter the activity of the channel. In cell expression systems, the “G” allele of rs222747 was found to enhance the activity of the channel, whereas rs222749 had no functional effect. In a cohort of 77 healthy individuals, study subjects homozygous for the G allele in rs222747 exhibited larger short-interval intracortical facilitation (a measure of glutamate transmission) explored through paired-pulse transcranial magnetic stimulation of the primary motor cortex [86].


On a technical note, in brain slices that contain the basolateral complex of amygdala, capsaicin changes the magnitude of LTP in a manner that is determined by the anesthetic agent (ether or isoflurane) used before euthanasia. After ether anesthesia, capsaicin had a suppressive effect on LTP, which was completely blocked by the nitric oxide synthase (NOS) inhibitor L-NAME and was absent in neuronal NOS as well as in TRPV1-deficient mice. However, after isoflurane anesthesia capsaicin caused the opposite effect on LTD: a TRPV1-mediated increase in the magnitude of the response [87]. This may account for some of the discrepant findings in the literature.


TRPV1 in Appetite Control and Goal-Directed Behavior

There is anecdotal evidence that capsaicin suppresses appetite, leading to weight loss. The phenotype of the Trpv1 null mouse is, however, confusing: it is lean when it is young, but it is overweight when it grows old. Gastrin-releasing peptide (GRP) is a bombesin-like peptide with a widespread distribution in the mammalian CNS, where it has been implicated in food intake. The paraventricular thalamic nucleus (that participates in arousal, motivational drives, and stress responses) has a dense network of GRP-positive fibers. GRP is thought to interact at postsynaptic bombesin type-2 (BB2) receptors that connect downstream to TRPV1. Indeed, the TRPV1 antagonists, capsazepine and SB-366791, ameliorate the GRP-induced membrane depolarization and rhythmic burst or tonic firing [88].


TRPV1 is also expressed in the nucleus accumbens, implying a role in goal-directed behavior and reward-dependent learning. Medium-size spiny neurons participate in two independent parallel circuits that likely subserve distinct behavioral functions: (1) direct pathway medium spiny neurons express D1 dopamine receptors and target midbrain dopamine centers, whereas (2) indirect pathway medium spiny neurons express D2 receptors and project to the ventral pallidum. In the indirect pathway, synaptic activation of group-I mGluRs leads to generation of endocannabinoids, which, in turn, activate postsynaptic TRPV1 channels [89].


TRPV2 and TRPV3


Compared to TRPV1, the literature on TRPV2 and TRPV3 is very limited (using TRPV1 and brain as key words, a recent Medline search has found 509 papers, whereas the combination of TRPV2 or TRPV3 with brain yielded only 30 and 24 hits, respectively).


Cloned from rat brain as a “capsaicin receptor homolog with a high threshold for noxious heat,” TRPV2 is highly expressed in the brain, both in neurons and astrocytes. The functional role of brain TRPV2 is, however, puzzling because the Trpv2 knockout mouse has no relevant phenotype. Of note (cancer is discussed in a different chapter), TRPV2 was reported to negatively control glioma progression and increase survival.


In spinal motor neurons, TRPV2 expression was first detected at embryonic day 10 when TRPV2 was localized in axon shafts and growth cones, implying a role in axon outgrowth regulation. It was suggested that TRPV2 in developing neurons is directly activated in a membrane stretch-dependent manner [90].


TRPV3 expression was reported in the rat brain [91], where it may be a target for incensole acetate (discussed later). Furthermore, in the hippocampus TRPV3 has been implicated in LTD at excitatory synapses on interneurons [92].


TRPV4


TRPV4 expression in the brain (including the cerebral cortex, hippocampus, thalamus, basal nuclei, cerebellum, and spinal cord) is increasing as the rats grow older. (As discussed later, it was speculated that TRPV4 may be involved in the pathogenesis of age-related neurodegenerative diseases [93].) TRPV4 is expressed both in neurons and astrocytes, where it may be activated by changes in osmolality.


TRPV4 in Neurons

In sensory and spinal motor neurons, both overexpression and chemical activation of TRPV4 promote the formation of neurites. Conversely, its knockdown and/or pharmacologic inhibition exerts the opposite effect. Furthermore, NGF up-regulates TRPV4 expression [94]. These findings imply a role for TRPV4 in brain development and neuroregeneration.


In mature neurons, TRPV4 may function as an osmosensor. It is well documented that abrupt changes in the osmotic pressure of the CSF can alter the excitability of the brain. For example, exposure of hippocampal neurons to hypotonic solutions increases the field EPSPs; this response is prevented by the TRPV4 antagonist, HC-067047 [95]. Moreover, TRPV4 may be involved in the generation of miniature EPSPs in the paraventricular nucleus of the hypothalamus [96]. It is not completely clear how TRPV4 regulates synaptic transmission. According to a recent model, TRPV4 promotes presynaptic glutamate release and thereby increases postsynaptic AMPA receptor function [97].


TRPV4 in Astrocytes

Astrocytes are specialized glial cells in the brain with essential functions in maintaining cerebral homeostasis. Astrocytes give rise to a highly branched network of processes that form an “endfeet” around the cerebral blood vessels. These “endfeet” form the blood-brain barrier along with the endothelium and the pericytes. TRPV4 in astrocytes evokes Ca2 + oscillations that may lead to the release of gliotransmitters [98]. TRPV4-positive astrocytes may also play active roles in the regulation of synaptic transmission [99].


Melastatin TRPM Channels


TRPM2


TRPM2 is Involved in Brain Development

TRPM2 is highly expressed in the embryonic brain. Knockdown of Trpm2 markedly increased, whereas its overexpression, conversely, inhibited axonal growth. Furthermore, TRPM2 was a target for CSF rich in lysophosphatidic acid to induce neuronal retraction. Importantly, neurons isolated from the brain of Trpm2-deficient mice have significantly longer neurites with a greater number of spines than those obtained from the brain of wild-type mice. Combined, these observations imply an important role for TRPM2 in brain development [100].


Is TRPM2 a Redox Sensor in Mature Neurons?

In the rat substantia nigra, TRPM2 is expressed both in GABAergic and dopaminergic neurons. The pars reticulata of the substantia nigra contains GABAergic neurons that project to target brain nuclei. These neurons exhibit spontaneous regular firing, but also exhibit burst firing in the presence of excitatory glutamatergic input (parenthetically, an increase in burst firing is a seen in Parkinson disease). Both the spontaneous firing rate and the burst activity of the cells are modulated by the reactive oxygen species (ROS) acting via TRPM2 channels [101]. The pars compacta of the substantia nigra possesses dopaminergic neurons that are strongly inhibited by the nonselective TRPM2 channel blockers clotrimazole and flufenamic acid. In these cells, challenge with H2O2 initiates a rise in [Ca2 +]i which is partially blocked by clotrimazole, implying an involvement of TRPM2 [102].


TRPM2 channels are coactivated by intracellular ADP-ribose (a substance that is produced under conditions of oxidative stress) and Ca2 +. Glutathione, a thiol redoxant, inhibits TRPM2 channel activity. In cultured hippocampal pyramidal neurons, l-buthionine-sulfoximine (a blocker of γ-glutamylcysteine synthetase, a key enzyme in glutathione biosynthesis) augments TRPM2-mediated currents [103]. In CA1 hippocampal pyramidal neurons, knockdown of Trpm2 by shRNA reduced the amplitude of the ADP-ribose-dependent current [104]. Moreover, in hippocampal slices obtained from Trpm2 null mice, a selective impairment of NMDA-R-dependent LTD was observed [105]. Combined, these findings create a strong case for TRPM2 being a neuronal redox sensor. The implications of this hypothesis for neurodegenerative disorders will be discussed later.


TRPM3


TRPM3 is thought of as a target for neuroactive steroids. Pregnenolone sulfate is an excitatory neurosteroid that acts as a negative allosteric modulator of GABA-A and a weak positive allosteric modulator of NMDA receptors. In the embryonic cerebellar cortex (where TRPM3 is abundantly expressed), pregnenolone sulfate is crucial for the normal development of Purkinje cells. In neonatal Purkinje cells, pregnenolone sulfate potentiates spontaneous glutamate release. This effect is mimicked by TRPM3 agonists and is blocked by the TRPM3 antagonist, mefenamic acid [106]. In mature neurons, as expected for a positive allosteric NMDA receptor modulator, pregnenolone sulfate increases the frequency of AMPA receptor-mediated miniature EPSPs, and this effect is blocked by the nonselective TRP channel antagonist, La3 + [107]. Of note, pregnenolone sulfate also increases glutamatergic-simulated EPSPs in acutely isolated dentate gyrus hilar neurons of the hippocampus. This increase was completely abolished by nonselective TRP channel blockers. It is unclear which TRP channel mediates this action but TRPM3 seems to be a good candidate [108].


TRPM4 and M5


Consistent with its role as a molecular pacemaker [109], TRPM4 is thought to regulate the activity of respiratory neurons in the pre-Bötzinger complex [110]. The pre-Bötzinger complex is located in the lower brainstem, where it generates the respiratory motor output. In functionally intact acute brainstem slices, brief hypoxia causes a biphasic response: a transient decrease in bursting activity followed by augmentation. These changes seem to be mediated by rhythmic Ca2 + transients in a TRPM4-dependent fashion [111].


The physiological role of TRPM4 in dopaminergic neurons is less clear. In the substancia nigra, dopaminergic neurons that respond to high-frequency glutamatergic inputs are believed to relay reward-associated information. These cells exhibit transient bursts of spikes that are absent after pretreatment with flufenamic acid and/or 9-phenanthrol, suggesting the involvement of TRPM2 and TRPM4 in the burst activity [112].


Recently, an intriguing role for TRPM4 and M5 has been suggested in mediating social interactions in rodents. Pheromones are secreted (or excreted) chemical agents that impact behavior including mate selection, aggression, and defense against predators. The main and accessory olfactory systems detect and process pheromonal stimuli. In mice, mitral cells isolated from the accessory (but not the main) olfactory bulb show a sustained firing activity (lasting up to several minutes), which is at least partially mediated by TRPM4 [113]. Mitral cells in the main and accessory olfactory bulbs directly project to the medial amygdala. Neurons that connect the main olfactory bulb to the amygdala respond to volatile urine exposure by the opposite sex. These olfactory sensory neurons express TRPM5 [114].


Of note, TRPM4 and M5 are coexpressed in cerebellar Purkinje cells. Depolarization-induced slow currents are attenuated (but not abolished) in Purkinje cells derived from Trpm4 null, Trpm5 null, and double knockout mice, as well as in wild-type mice with Trpm4 shRNA knockdown. These findings imply a role for TRPM4 and M5 in the generation of cerebellar depolarization-induced slow currents [115].


TRPM7 and TRPM8


TRPM6 and M7 are closely related channels. Indeed, TRPM6 cannot efficiently form functional channels by itself and needs to assemble with TRPM7. TRPM6 has a highly restricted tissue expression pattern (mostly in intestines and kidney) that does not involve the brain; therefore, this channel is out of the scope of this chapter. TRPM7 is, however, expressed in neurons where it is believed to function as a cellular Mg2 + transporter [116].


TRPM8 is expressed in cultured hippocampal neurons, but little is known about its in vivo expression pattern in the brain. Under voltage-clamp conditions, TRPM8 activation seems to evoke an inward current that does not alter synaptic transmission [117]. Menthol (150 to 750 μM), a TRPM8 agonist, prolongs inhibitory postsynaptic currents in the hippocampus, but this effect is most likely mediated by a positive allosteric modulation of recombinant GABA-A receptors. In keeping with this hypothesis, menthol actions were unaffected by TRPM8 antagonists [118].


The Ankyrin TRPA1 Channel


In the rat brain, TRPA1 is expressed in neurons, astrocytes, and cells lining the choroid plexus and the ventricles. In the hippocampus, TRPA1 has been implicated in neuronal cell death. The synthetic cannabinoid WIN 55,212-2 protects hippocampal neurons against ischemic damage. This protective action was suspended by the CB1 receptor antagonist AM251, but not the CB2 receptor antagonist AM630. The TRPA1 blocker HC-030031 enhanced the neuroprotective efficacy of WIN 55,212-2. In contrast, the TRPA1 agonist icilin or allyl isothiocyanate led to a stronger neurodegeneration. These data suggest that WIN 55,212-2 has a biphasic action: at low concentrations it protects neurons by activating CB1 receptors, and at high concentrations it additionally activates TRPA1 that interferes with the CB1 receptor-mediated neuroprotection [119].


Somewhat confusingly, TRPA1 plays a protective role in astrocytes. In these glial cells, H2S2 generates polysulfides that, in turn, induce Ca2 + influx. This effect is prevented by the TRPA1 antagonist HC-030031, as well as by Trpa1 gene silencing via siRNAs [120]. In, addition, astrocytic TRPA1 contributes to basal Ca2 + levels that are required for constitutive release of d-serine, a signaling molecule used to communicate with neurons [121].


TRPA1 is expressed in both the choroid plexus and ventricular lining epithelium [122], as well as in magnocellular neurosecretory cells of the supraoptic nucleus that produce vasopressin [123]. Given its role as a chemosensor, one might speculate that TRPA1 may respond to chemical stimuli to regulate CSF synthesis and vasopressin release.


TRP Channel Dysfunction in Epilepsy


Epilepsy is one of the most common diseases seen in neurology between departments. It is caused by various perturbances that disturb the normal balance excitation and inhibition within the CNS. Current antiepileptic drugs target ion channels, neurotransmitter transporters, and neurotransmitter metabolic enzymes. They could control symptoms in the majority (70-80%) of the patients. A significant subset of patients (20-30%), however, develops intractable epilepsy. Even worse, existing antiepileptic drugs do not cure the disease, only alleviate symptoms, and possess significant dose-limiting adverse effects. Clearly, there is a dire need for novel antiepileptic drugs. Given their postulated role in modulating neuronal excitability, TRP channels are promising drug targets to explore. There is increasing evidence to implicate TRPC channels (in particular, TRPC1/C4, TRPC3, and TRPC5) and TRPV1 in the pathogenesis of epilepsy.


TRPC Channel and Epilepsy


Unlike TRPC1, which is ubiquitously expressed in the brain, TRPC4 expression is highly restrictive, with the highest expression level reported in the lateral septum. A dysfunction of the septo-hippocampal network has been implicated in chronic epilepsy. In cats, injection of kainate (an agonist of a subset of AMPA receptors) evokes seizures and epileptiform EEG activity. The large depolarizing plateau potential that underlies the epileptiform burst firing was completely abolished in the Trpc1/c4 double knockout mice and was also absent in the majority (74%) of lateral septal neurons in Trpc1 null mice.


The muscarinic agonist, pilocarpine, evokes seizures in experimental animals; this is a widely used model of temporal lobe epilepsy. Severe pilocarpine-induced seizures may cause neuronal cell death in the lateral septum; this effect is ameliorated in the Trpc1/c4 double knockout mouse. These results imply an essential role for TRPC1 and TRPC4 (likely as a heteromultimer) in forming the intrinsic membrane conductance that mediates the plateau potential in lateral septal neurons [124,125].


The Trpc5 knockout mouse also exhibits both significantly reduced seizure activity and attenuated seizure-induced neuronal cell death in the hippocampus. Yet, epileptiform bursting activity induced by mGluR agonists (which is normal in the Trpc1/c4 double knockout animals) is unaltered in Trpc5 null mice. By contrast, long-term potentiation is (which is normal in the Trpc1/c4 double knockout animals) is greatly reduced in TRPC5-deficient mice. The distinct phenotype of these knockout animals suggests that TRPC5 and TRPC1/C4 contribute to seizure activity by distinct cellular mechanisms [124]. In support of this model, muscarinic activation by carbachol in hippocampal neurons was shown to initiate a current through TRPC5 by promoting the membrane insertion of the channel. As expected, the muscarinic antagonist, atropine, prevented the increase in TRPC5 surface expression. TRPC5-like currents were also inhibited by the nonselective TRPC antagonists, 2-APB and SKF-96365, as well as the PI3K inhibitor, wortmannin, which blocks TRPC5 translocation from the cytosol to the plasma membrane. In conclusion, the rapid translocation of TRPC5 contributes to the generation of the cholinergic-induced plateau potentials through a Ca2 +/calmodulin and PI3K-dependent pathway, providing new insights into the pathology of epilepsy [126].


In the motor cortex, TRPC3 expression is strong during embryogenesis but is weak to absent in the mature brain. In dysplastic cortex, TRPC3 expression, however, returns to the high levels seen in developing neurons. In pyramidal neurons, combinations of low Ca2 + and Mg2 + increases the amplitude of depolarization, which, in the dysplastic cortex, is sufficient to provoke epileptiform activity. Importantly, this activity was suppressed by the TRPC3 inhibitor, Pyrazole-3 (Pyr3) [127]. Following status epilepticus, TRPC3 expression is elevated (whereas TRPC6 is reduced) in pyramidal neurons and dentate granule cells. This increase in TRPC3 is blocked by Pyr3. Furthermore, hyperforin (a TRPC6 activator) prevents down-regulation of TRPC6 induced by epileptic seizures. Apparently, both increased TRPC3 and decreased TRPC6 are relevant because both Pyr3 and hyperforin (alone or in combination) protected neurons against damage by severe epileptic seizures [128]. Interestingly, status epilepticus induced TRPC3 expression in endothelial cells that did not contain this protein in control animals. The neo-expression of TRPC3 in endothelial cells was accompanied by a loss of SMI-71, a blood-brain barrier marker, and correlated to the development of vasogenic edema and resultant neuronal damage. The vasogenic edema response was attenuated by Pyr3. It was speculated that TRPC3 neo-expression in endothelial cells may contribute to the neuronal damage during and after status epilepticus by disrupting the blood-brain barrier [129].


In summary, TRPC channels could represent shared downstream target for a number signaling pathways that may contribute to seizure and excitotoxicity, such as NMDA receptor-mediated Ca2 + influx, or mGluR activation [130]. Of TRPC channels, TRPC3 is particularly interesting given its preponderance in dysplastic (and low expression in normal) motor cortex and postulated role in disrupting the blood-brain barrier during status epilepticus.


TRPV1 as a Potential Target in Epilepsy and Febrile Seizures


TRPV1 is an interesting, though highly controversial (the ongoing debate whether or not TRPV1 is expressed at all in the brain is discussed elsewhere), antiepileptogenic target. In rats, TRPV1 activation was reported to modulate activity-dependent synaptic efficacy in the hippocampus by facilitating LTP and suppressing LTD. In brain slices obtained from Trpv1 (−/−) mice, LTD was absent, and capsaicin was inefficient [131,132]. It was hypothesized that TRPV1 selectively inhibits excitatory synapses at hippocampal interneurons. 4-aminopyridine triggers epilepsy-like symptoms in the rat. In this model, the first-generation TRPV1 antagonist, capsazepine, suppresses epileptiform activity. By contrast, capsaicin potentiates the seizures [133]. To further strengthen the link between TRPV1 and epilepsy, it was pointed out that NGF (a known driver of TRPV1 expression) triggers epileptogenesis. Furthermore, the levels of the endocannabinoid anandamide (also an endogenous TRPV1 agonist) are increased in epilepsy. It was speculated that TRPV1 activation (possibly by high anandamide concentrations) may trigger apoptotic neuronal death (at least in the rat brain) that leads to chronic epilepsy [134].


The cannabinoid link to epilepsy is a hot topic. Those who favor legalization point out that medical marijuana (or its main active ingredient, tetrahydrocannabinol) can control seizures in experimental animals not responsive to other treatments. Also, there are anecdotal reports that medical marijuana can stop intractable seizures in children with Duvet syndrome, also known as myoclonic epilepsy of infancy. Boosting endogenous anandamide in the brain by blocking FAAH (the enzyme that hydrolyzes anandamide) has anticonvulsant effects, which are mediated by CB1 receptors. The trick is to keep anandamide in the beneficial dose range because high anandamide concentrations can cause paradoxical seizures by activating TRPV1 [135,136]. One strategy to achieve this goal is by dual FAAH and TRPV1 blockade with N-arachidonoyl-serotonin (AA-5-HT). Indeed, seizures induced by pentylenetetrazole in mice were prevented by AA-5-HT [137].


Importantly, there is experimental evidence linking TRPV1 to epilepsy in humans. Both tuberous sclerosis complex and focal cortical dysplasia type IIb cause intractable epilepsy. In brain specimens removed during surgery from patients with tuberous sclerosis or focal cortical dysplasia IIb, TRPV1 was detected in the abnormal cell types, such as dysmorphic neurons, balloon cells, and giant cells. Interestingly, TRPV1 appeared to have both cytoplasmic and nuclear distribution, suggesting a potential nuclear role of TRPV1 [138]. Elevated TRPV1 was also detected in the brains of patients with mesial temporal lobe epilepsy compared to controls (brain surgery unrelated to epilepsy). TRPV1 was mainly present in the cell bodies and dendrites of glutaminergic and GABAergic neurons and GFAP-positive astrocytes [139]. At present, it is unclear if this abnormal TRPV1 expression is a cause or consequence of the disease.


As a heat-activated channel, TRPV1 seems to be an interesting candidate to explore for mediating febrile seizures, the most common seizure type in children under the age of five. In mice, pentylene tetrazol induces clonic seizures that are made worse in febrile animals. In this model, Trpv1 gene deficiency decreased the intensity of experimental febrile seizures [140]. In rats treated with subconvulsive doses of pentylene tetrazol, the TRPV1 receptor agonist OLDA (injected intracerebroventricularly 30 min prior to pentylene tetrazol administration) accelerated the incidence of seizures, whereas the TRPV1 antagonist AMG-9810 ameliorated the seizure activity [141].


TRPM2 and Juvenile Myoclonic Epilepsy


Mutations in the gene that encodes the EF-hand motif-containing protein EFHC1 have been linked to the pathobiology of juvenile myoclonic epilepsy (JME). In hippocampal neurons, TRPM2 and EFHC1 are coexpressed with a physical interaction between the N- and C-terminal cytoplasmic regions of TRPM2 with the EFHC1 protein. In recombinant TRPM2 expression systems, coexpression of EFHC1 was shown to potentiate the H2O2-induced currents with a resultant increase in cell death. Based on these findings it was speculated that TRPM2 mediates the disruptive effects of JME mutations of EFHC1 [142].


Cerebellar Ataxia as a “TRP Channelopathy”


As we saw earlier, TRPC3 is abundantly expressed in Purkinje cells, where it mediates slow mGluR-mediated synaptic responses. Heterozygous moonwalker mice represent a model of cerebellar ataxia. These mice carry a dominant gain-of-function mutation (T635A) in the Trpc3 gene [143]. This mutation leads to dysmorphism and/or loss of Purkinje cells, which have been suggested to cause the ataxia. Interestingly, the ataxic phenotype is present from a very early age (before weaning) although Purkinje cell loss does not appear until much later (several months of age). The intrinsic excitability of the mutant Purkinje cells is, however, altered as early as 3 weeks after birth. To explain these seemingly contradictory findings, it was pointed out that (in addition to Purkinje cells) type-II unipolar brush cells in the cerebellum also express functional TRPC3 channels. These cells are ablated in moonwalker mice by 1 month of age, much earlier than the loss of Purkinje cells. Combined, these findings suggest that the ataxic phenotype of the moonwalker mouse reflects both the altered excitability of Purkinje cells (secondary to the gain-of-function TRPC3 mutant) and the TRPC3-mediated loss of type II unipolar brush cells [144].


If the overactivity of TRPC3 leads to ataxia, the loss of channel activity should not impair motor coordination. Apparently, this is not the case because animals carrying gain-of-function (moonwalker) and loss-of-function (Trpc3 null) mutations have a similar phenotype. For a detailed discussion on how opposing aspects of TRPC3 channel activation can lead to the same phenotype, see [145].


In cerebellar Purkinje cells, PKCγ is involved in the “pruning” of the climbing fiber synapses. Spinocerebellar ataxia type-14 (SCA14) is an autosomal dominant neurodegenerative disorder, caused by mutations in PKCγ. Interestingly, 18 of the 22 mutations described in SCA14 patients are concentrated in the C1 domain of the enzyme, which is responsible for the membrane binding of DAG that drives the translocation and regulation PKCγ. Wild-type, but not C1 domain mutant, PKCγ inhibits Ca2 + influx in response to muscarinic receptor stimulation. The C1 domain mutants are constitutively active, and they are unable to phosphorylate the TRPC3 protein, resulting in sustained Ca2 + entry. This alteration in Ca2 + homeostasis in Purkinje cells may contribute to neurodegeneration see in SCA14 patients [146]. The mutant PKCγ colocalizes with wild-type PKCγ, and the mutant PKCγ acts in a dominant-negative manner [147].


Hereditary cerebellar ataxia is rare (less than 1 in 100,000), but sporadic ataxia is somewhat more common (10 per 100,000). If a functionally overactive TRPC3 channel is responsible for SCA14 (a hereditary cerebellar ataxia), the TRPC3 gene could be a promising candidate for screening ataxic patients with unknown genetic etiology. Somewhat disappointingly, in a cohort of 98 patients with sporadic cerebellar ataxia a genetic screen for TRPC3 mutations did not reveal any potentially causative variants [148].


TRP Channels in Neuroprotection and Their Dysfunction in Neurodegenerative Disorders


Parkinson Disease


Parkinson disease (PD) is a devastating, relentlessly progressive neurodegenerative disorder characterized by motor impairment (such as bradykinesia and resting tremor) and dementia. Autopsy brain of PD patients show Lewy body formation (of note, Lewy bodies are not specific for PD because they can also be seen in Pick disease and other forms of dementia) and loss of dopaminergic neurons in the basal ganglia. The loss of dopaminergic neurons results in a chemical imbalance that affects the whole basal ganglia-thalamus-cortex circuit. The cause of PD is unknown but a number of environmental factors (e.g., pesticide exposure) have been associated with increased risk.


In animal models, neurotoxins (e.g., rotenone, a pesticide) that kill dopaminergic neurons in a Ca2 + overload-dependent manner mimic some aspects of the human disease. The rise in [Ca2 +]i evoked by rotenone was attenuated by the TRPM2 blocker, N-(p-amylcinnamoyl) anthranilic acid [149]. In SH-SY5Y cells, salsolinol (a toxic condensation product of dopamine and acetaldehyde) and 1-methyl-4-phenylpyridinium (MPP, a neurotoxin that causes parkinsonism in primates) induce apoptosis, which is reversed by TRPC1 activation. Exposure of SH-SY5Y cells to these toxins blocks thapsigargin-mediated Ca2 + influx and decreases TRPC1 in the plasma membrane by translocation to the cytoplasm. It was speculated that neurotoxins may alter Ca2 + homeostasis (in a way that involves TRPC1 possibly via PLC) and induce mitochondrial-mediated caspase-dependent cytotoxicity, an important characteristic of PD [150].


Unexpectedly, capsaicin microinjected into the substantia nigra provides partial protection against neuronal cell death induced by MPP by suppressing the production of microglia-derived ROS. These capsaicin effects are reversed by capsazepine. Bases on these findings, it was speculated that TRPV1 agonists may have a therapeutic value in PD [151]. (How one can safely deliver a TRPV1 agonist into the CNS is a completely different problem, not addressed in this article!)


GABAergic interneurons that express TRPC3 control the activity of striatal projection neurons. Parkinsonian movement disorders are often associated with abnormalities in the firing intensity and/or pattern of these cells. TRPC3 channels expressed by GABAergic interneurons are tonically active and mediate an inward, Na+-dependent current, leading to a substantial depolarization. Conversely, inhibition of TRPC3 channels induces hyperpolarization and thereby decreases the firing frequency [152].


The mainstay of pharmacotherapy in PD is dopamine replacement. Unfortunately, the long-term use of levodopa (l-DOPA) as a side effect causes abnormal involuntary movements, called L-DOPA-induced dyskinesia. In a hemiparkinsonian model of PD (mice with 6-hydroxydopamine-induced striatal lesions), chronic L-DOPA treatment leads to the development of intense axial, forelimb, and orolingual dyskinetic symptoms, resembling the human adverse effect. Treatment with oleoylethanolamide (OEA) ameliorated these symptoms without altering the therapeutic motor effects of L-DOPA. The antidyskinetic action of OEA was most likely mediated by TRPV1 because it was absent in mice desensitized to capsaicin [153].


Excessive glutamate can cause neuronal dysfunction and degeneration. Glutamate excitotoxicity has been implicated in acute brain insults (such as ischemic and traumatic brain injury that are discussed later) and also in chronic neurodegenerative disorders including PD. 2-APB (a “universal” TRP channel blocker) reduces glutamate-induced cell death in hippocampal organotypic slice cultures presumably by inhibiting TRPC1 channels. In support of this hypothesis, knockdown by iRNA of Trpc1 in slice cultures prevents glutamate-induced cell death [154]. Apparently, in these cells TRPC1 is an important downstream target for mGluRs [155].


Lou Gehrig’s Disease and Amyotrophic Lateral Sclerosis-Parkinson Dementia Complex


Amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig’s disease) is a progressive neurodegenerative disorder that predominantly affects the spinal motor neurons, leading to muscle weakness and eventual paralysis. The etiology of ALS is unknown but a subset of cases (~ 10%) appears to be inherited. In 1993, mutations in the gene that encodes the SOD1 protein (superoxide dismutase-1, alternatively referred to as superoxide dismutase [Cu2 +-Zn2 +]), have been linked to familial ALS. Sod1 (G93A) mutant transgenic mice represent an animal model of ALS. In these mice, increased expression of TRPV4 was noted in the cerebral cortex, hippocampal formation, thalamus, cerebellum, and spinal cord. Both in the cerebral cortex and the hippocampus, TRPV4 was especially increased in the pyramidal cells. It was speculated that TRPV4 may be involved in the pathogenesis of ALS, but the functional implications of increased TRPV4 remain unclear [156].


Amyotrophic lateral sclerosis-Parkinson dementia complex (ALS-PDC) is a mysterious neurodegenerative disorder that combines symptoms of ALS, Parkinsonism, and Alzheimer disease. ALS-PDC is prevalent in the Guam population but is a rarity elsewhere [157]. The etiology of ALS-PDC remains unknown, but the extremely limited geographic distribution of the disease strongly suggests an environmental factor specific to Guam. It was speculated that β-N-methyl-amino alanine (BMAA) may be the toxic agent that damages the neurons to cause ALS-PDC. BMAA is a mixed mGluR agonist present in the cycad plant, a traditional food source in Guam, which causes reversible membrane depolarization and initiates a rise in [Ca2 +]i in dopaminergic neurons. The inward current was mainly mediated by mGluR1 via TRPC channels. Indeed, the TRP channel blockers, SKF 96365 and ruthenium red (RR), reduced the BMAA-induced current. Prolonged exposure to BMAA leads to Ca2 + overload, cell shrinkage, massive cytochrome-c release into the cytosol, and ROS production; ultimately, these changes kill the treated neurons [158].


There is increasing evidence that L-BMAA per se is not sufficient to cause ALS-PDC; another environmental factor is needed. Recent studies have focused on drinking water, which has a unique mineral composition in these Pacific islands; it is extremely low in Ca2 + and Mg2 +, but rich in metals like Mn2 +, Al3 +, and Fe3 +. Indeed, rats fed diets that mimic the mineral composition of drinking water in Guam showed significant loss of nigral dopaminergic neurons. A TRPM7 missense mutation (T1482I) was also identified in a subset of ALS-PDC patients [159]. Recent clinical studies, however, have questioned the relevance of the TRPM7 T1482I variant in ALS-PDC. Parametric linkage analyzes of the TRPM7 locus in a large extended family with ALS-PDC patients did not reveal any evidence supporting the linkage to the TRPM7 locus. Resequencing of the entire coding region of TRPM7 did not reveal any pathogenic mutations in an affected individual in this family. The allele frequencies of the T1482I in affected individuals in this family or in those from other families are not significantly different from those in regional controls in Japan [160].


Alzheimer Disease


Alzheimer disease (AD) is the most common form of dementia. Although some cases appear to be familial (and have an early onset), the most important risk factor is advanced age. Indeed, after age 85 the risk for developing AD approaches 50%. Hyperphosphorylated tau aggregated into neurofibrillary tangles is a hallmark lesion of AD. In animal models, cold water exposure causes reversible tau hyperphosphorylation, associated with cognitive deficits. In rats, intragastric capsaicin (10 mg/kg) was reported to mitigate the cognitive decline induced by cold water exposure. Furthermore, capsaicin attenuated the cold water stress-induced spatial memory impairment and prevented tau hyperphosphorylation [161]. These findings are puzzling because capsaicin absorbed from the GI tract undergoes extensive hepatic metabolism and is unlikely to reach pharmacologically meaningful concentrations in the CNS.


In the rat, TRPV4 is highly expressed in hippocampal astrocytes where it is thought to play a major role in oxidative stress-induced cell damage. In aged monkey, circulating amyloid- β40 peptide crosses the blood-brain barrier and is deposited in cerebrovascular endothelial cells, causing amyloid angiopathy. In rat hippocampal slices, synthetic amyloid-β40 peptide initiates cell death after cultures were preconditioned with sublethal concentrations of buthionine sulfoximine (1.5 μM), a compound that enhances endogenous ROS production. The damage is predominantly in the granule cell layer of the dentate gyrus, with additional cell loss in pyramidal neurons. The dying neurons evoke reactive gliosis with altered (accentuated) TRPV4 expression. In this model, neuronal damage is attenuated by the universal TRP blocker, RR [162]. Of note, the PARP (poly(ADP-ribose)polymerase) inhibitor SB-750139 also attenuates cell death initiated by amyloid β-peptide in rat striatum neurons [163]. PARP (in concert with Ca2 +) activates endogenous TRPM2 channels, a neuronal “redox sensor.” Thus, it is entirely possible that TRPV4 up-regulation is coincidental and the real target for ROS-mediated cell death is TRPM2. Last, hyperforin (a TRPC6 activator) blocks the formation of amyloid-β aggregates in vitro and decreases astrogliosis and microglia activation in vivo. Importantly, hyperforin improves spatial memory formation in animal models of AD [164]. Indirectly, these findings implicate TRPV4, TRPM2, and TRPC6 in the pathogenesis of AD. If these observations hold true in humans, combined pharmacological blockade of TRPM2 and TRPV4 and stimulation of TRPC6 may be beneficial in AD patients.


Autoimmune Encephalomyelitis and Multiple Sclerosis


Cholera toxin-B binds to the branched pentasaccharide moiety of the ganglioside, GM1. This protein-ganglioside interaction is instrumental in initiating the signal transduction pathway that is responsible for diarrhea. In murine experimental autoimmune encephalomyelitis (EAE), administration of the GM1 cross-linking unit of cholera toxin (the so-called B-toxin) is beneficial. Cholera toxin-B is a binding partner for the endogenous lectin, galectin-1. In these animals, both cholera toxin-B and galectin-1 caused symptomatic improvement. Conversely, mice lacking GM1 demonstrate enhanced susceptibility to EAE. Polyclonal activation of murine regulatory T (Treg) cells up-regulates galectin-1. Furthermore, activation of CD4 + and CD8 + effector T (Teff) cells elevates GM1. Of importance to the topic of this chapter, activation of Teff cells also up-regulates TRPC5, which, in turn, mediates the Ca2 + influx on GM1 cross-linking by either galectin-1 or cholera toxin-B. Knockdown by shRNA of Trpc5 in Teff cells blocks the contact-dependent proliferation inhibition by Treg cells. These observations suggest a role for TRPC5 in the negative control (suppression) of autoimmunity [165].


Multiple sclerosis (MS) is a chronic, relentlessly progressive demyelinating disorder. Although the pathogenesis of MS is unknown, increased glutamate production is thought to play a role in the inflammation-driven neurodegenerative process. In support of this hypothesis, CSF collected from MS patients potentiates glutamate-mediated neuronal swelling through a mechanism that seems to involve both IL-1β signaling and increased AMPA-R stimulation. Indeed, IL-1β is significantly higher in the CSF of patients with active MS. It was hypothesized that TRPV1 is an essential mediator for the synaptic action of IL-1β on central glutamatergic synapses [166]. In keeping with this hypothesis, in a mouse model of EAE Trpv1 null animals show reduced infiltration of the CNS by autoreactive T-cells [167]. In MS patients, a missense SNP in the TRPV1 gene was correlated to the risk of progressive disease [167]. Taken together, these findings identify TRPV1 as a potential novel therapeutic target in MS patients.


TRP Channels in Psychiatric Disorders and Mental Retardation


Anxiety Disorders, Panic Attacks, and Depression


Anxiety disorders are the most common form of mental illness in the United States, affecting one out of six Americans age 18 and older. In mice, several TRP channels (including TRPC4, TRPC5, and TRPV1) are expressed in brain areas implicated in the control of fear and anxiety. In behavioral experiments, constitutive ablation of Trpc4 or Trpc5 was associated with diminished innate fear and anxiety levels. Selective knockdown of Trpc4 in the lateral amygdala via lentiviral-mediated gene delivery of RNAi mimicked the behavioral phenotype of the Trpc4 knockout mouse. It was speculated that TRPC4 is a crucial downstream target for two Gαq11 protein-coupled signaling pathways, activated via Group-I mGluRs and CCK2 receptors, respectively [168]. These observations imply that TRPC4 and/or TRPC5 blockers may constitute a new class of anxiolytic drugs.


Several lines of experimental evidence implicate TRPV1 in anxiety and panic responses. In behavioral studies, TRPV1 activation by capsaicin (microinjected into the periaqueductal gray) initiates anxiety-like behavior, including less frequent entry into the open arm of the maze test, as well as reduced number of stretched-attend postures and head dippings. TRPV1 blockade by capsazepine did not change the behavior of the animals in the elevated maze test. However, when given before capsaicin, capsazepine completely blocked the anxiogenic-like effect. These findings imply that (1) TRPV1 activation in the periaqueductal gray causes anxiety-like behavior in the mouse, and (2) these TRPV1 receptors are not tonically active [169]. Other studies, however, are more consistent with a tonically active TRPV1. For example, capsazepine inhibits the escape response in the elevated T-maze test [170], and Trpv1 null animals exhibit diminished innate fear response and anxiety-like behavior [171]. In addition, it was shown that TRPV1 might be a functional tool to prevent the risks associated with the long-term use of benzodiazepines [77].


So, what is the endogenous substance that evokes anxiety by activating TRPV1? An interesting candidate molecule is anandamide. In the T-maze assay, microinjection of the selective FAAH inhibitor, URB 597, into the hippocampus produces anxiolytic-like effects, presumably by elevating endogenous anandamide levels. Further increase in anandamide levels, however, exerts the opposite effect. The TRPV1 antagonist AMG 9810 does not interfere with the anxiolytic effect of URB 597 but blocks the anxiogenic action. These findings suggest that the beneficial effect of anadamide (or URB 597) is mediated by CB1-Rs, whereas TRPV1 activation contributes to increased anxiety [172]. These findings add TRPV1 antagonists to the list of potential new anxiolytic agents.


Unfortunately, the TRPV1 literature is confusing with some studies suggesting that it is TRPV1 activation that improves mood and reduces anxiety. In mice, nicotine induces depression-like behavioral alterations, similar to those seen in other murine models of depression such as the repeated immobilization stress. In both the nicotine and immobilization stress models, capsaicin and olvanil administered intraperitoneally exhibited significant antidepressant-like activity. By contrast, anandamide and N-arachidonyldopamine (NADA) lack antidepressant-like effects. In accord, the antidepressant-like effect of capsaicin and olvanil was reversed by capsazepine, but not the CB1R antagonist, AM 251 [173]. These observations imply that TRPV1 activation may improve mood, especially in “cold turkeys” suffering from nicotine withdrawal symptoms.


Incensole acetate, the main active ingredient in Boswellia resin, activates TRPV3 in expression systems. In vivo, incensole acetate causes anxiolytic-like and antidepressive-like behavioral effects in wild-type, but not Trpv3 knockout, mice, with concomitant changes in c-Fos activation in the brain [174]. These findings imply a therapeutic potential for TRPV1 agonist in anxiety disorders and depression.


Schizophrenia


Schizophrenia is a disabling mental disorder that affects ~ 1% of the population worldwide. It runs in families (identical twins have a ~ 50% chance of developing the disease), suggesting a genetic predisposition for neurochemical malfunction in the brain. Disrupted in schizophrenia-1 (DISC1) is a protein implicated in schizophrenia (also in bipolar disorder, major depressive disorder, and autism). The function of DISC1 is to modulate cAMP signaling by increased cAMP catabolism. DISC1 disruption by shRNA knockdown increases intracellular Ca2 + waves in response to mGluR activation. Furthermore, it decreases TRPC-mediated sustained depolarization. It was hypothesized that diminished DISC1 function disrupts the normal pattern of prefrontal cortex activity through the loss of cAMP regulation of mGluR-mediated intracellular Ca2 + waves; this eventually leads to perturbations in TRPC channel activity [175]. As of today, it is unclear which member of the TRPC subfamily is involved in this response.


A link between TRPV1 and schizophrenia was postulated, but the experimental evidence is at best preliminary. In rats, capsaicin desensitization leads to behavioral changes (e.g., learning impairments in the novel object recognition test) that are somewhat reminiscent of those seen in patients with schizophrenia [176]. Furthermore, spontaneously hypertensive (SH) rats show schizophrenia-like deficits in social interactions that are ameliorated by atypical antipsychotics. Capsaicin (2.5 mg/kg) increases social interaction of in both SH and normotensive rats and decreases locomotion in SH rats [177]. These findings might be interpreted to imply that loss of TRPV1 function could contribute to the pathogenesis of schizophrenia, whereas TRPV1 activation may be beneficial in these patients.


Rett Syndrome


Rett syndrome is a neurodevelopmental disorder that affects intellectual ability. Children with Rett syndrome exhibit autism-like behavior combined with impaired motor functions. In the hippocampus, BDNF activates TRPC3 to increase neuronal dendritic spine density. Indeed, knockdown of Trpc3 prevents the increase in spine density caused by BDNF application. It was hypothesized that dysfunction in the BDNF-TRPC3 interaction may contribute to the pathomechanism of Rett syndrome by causing abnormal dendritic spine density [178]. This model is supported by findings in mice that lack the methyl-CpG-binding protein-2 (Mecp2), a model of Rett syndrome. In symptomatic Mecp2 mutant mice, membrane currents and dendritic Ca2 + signals evoked by recombinant BDNF are impaired. In the hippocampus of Mecp2 mutants, both TRPC3 and TRPC6 are decreased. BDNF mRNA and protein levels are also lower in Mecp2 mutant hippocampus and dentate gyrus granule cells. Chromatin immunoprecipitation suggest that Trpc3 is a target of Mecp2 transcriptional regulation. According to these results, correction of the impaired BDNF-TRPC6 signaling is a potential therapeutic strategy in Rett syndrome [179].


Bipolar Disorder


Two indirect findings implicate TRPM2 in the pathogenesis of bipolar disorder. First, B-lymphoblast cell lines established from bipolar disorder patients when exposed to rotenone (a broad-spectrum, neurotoxic pesticide and a known activator of TRPM2) exhibit reduced cell viability compared to healthy controls [180]. Second, in case-control studies, a number of SNPs in the TRPM2 gene were reported to increase the risk for developing bipolar disorder. Interestingly, the C-T-A haplotype of SNPs rs1618355, rs933151, and rs749909 was significantly associated with early age at onset of the disease [181].


The “Addictive” TRP Channels


Cocaine Abuse


Two members of the canonical TRP subfamily, TRPC1 and TRPC6, have been linked to cocaine abuse and addiction. Conditional forebrain Trpc5 knockdown mice exhibit an increase in self-administration of cocaine without prior operant training. This observation was interpreted to imply a negative control of TRPC5 over addictive behavior [182]. Mice whose brain expresses the HIV-1 Tat protein show a heightened response to cocaine and appear to be vulnerable to relapse. This observation establishes a connection between HIV infection and drug abuse. Pretreatment of rat hippocampal neuronal progenitor cells with platelet-derived growth factor-BB (PDGF-BB) restores proliferation that had been impaired by HIV-1 Tat. TRPC1 appears to be a downstream target for PDGF-BB. These findings highlight TRPC1 as a novel target that regulates cell proliferation mediated by PDGF-BB with implications for therapeutic intervention in cocaine addiction [183].


Opioid Addiction


In the rat, TRPC6 is expressed in CSF-contacting neurons, with increased levels found during morphine dependence and withdrawal [184]. In mice, repeated morphine administration up-regulates TRPV1 expression in the dorsal striatum. TRPV1 agonists potentiate, whereas TRPV1 antagonists attenuate, morphine reward in the conditioned place preference paradigm. TRPV1 antagonist treatment also suppresses morphine-induced increases in μ-opioid receptor binding. Thus, brain TRPV1 may represent a novel therapeutic target to treat morphine-addictive disorders [185].


TRP Channels in Stroke and Traumatic Brain Injury


Stroke


Intracerebral hemorrhage (hemorrhagic stroke) is a devastating event that stems from the rupture of blood vessels in the brain, with the subsequent accumulation of blood in the parenchyma. There is good evidence that blood-derived factors induce excessive inflammatory responses that, in turn, contribute to the progression of brain injury. When the blood-brain barrier is disrupted, thrombin leaks to the brain parenchyma to cause astrogliosis. Furthermore, thrombin up-regulates TRPC3. In mice, the TRPC3 blocker Pyr3 improves functional outcome and attenuates astrogliosis after hemorrhagic stroke. These findings highlight TRPC3 as a novel therapeutic target for the treatment of hemorrhagic brain injury [186].


Intracerebroventricular injection of hyperforin, a TRPC6 activator, ameliorates the brain damage that occurs after transient focal cerebral ischemia in rats. When applied immediately after middle cerebral artery (MCA) occlusion, hyperforin reduced the infarct volumes and increased neurologic scores at 24 hours after reperfusion. These beneficial effects were accompanied by elevated TRPC6 activity [187]. Resveratrol is another natural product that protects the brain from ischemia/reperfusion injury. In the transient MCA occlusion model, resveratrol protects neurons by inhibiting the proteolysis of TRPC6 by calpain. There are striking similarities between hyperforin and resveratrol actions, suggesting a shared biochemical pathway. For example, both hyperforin and resveratrol elevate TRPC6 and CREB (cAMP-response element binding protein) activities. When the MEK (MAPK/ERK kinase) or CaMK-IV activity was inhibited, the neuroprotective effect of resveratrol was lost. Taken together, these findings suggest that hyperforin and resveratrol protect the brain from ischemic injury through the TRPC6-MEK-CREB and TRPC6-CaMKIV-CREB pathways [188].


Excitotoxicity induced by NMDA receptor-mediated intracellular Ca2 + overload is a major cause of delayed neuronal death after ischemic stroke. As we saw earlier, TRPC6 protects neurons from ischemic brain damage. For example, the infarct volume in Trpc6 transgenic mice is much smaller than that in wild-type littermates after MCA occlusion. The Trpc6 transgenic mice also had better behavior performance and lower mortality than the control animals. These observations imply that increasing TRPC6 activity could be a potential strategy for stroke prevention and therapy [189].


Chronic cerebral hypoperfusion is a risk factor for the development of vascular dementia. In men, vascular dementia accounts for 20-40% of all dementia cases. In mice, bilateral carotid artery occlusion is used to induce global chronic cerebral hypoperfusion. These animals show impairment in locomotion and motor coordination, as well as deficits in learning and memory formation. TRPV1 antagonists are protective against hypoperfusion-induced motor coordination impairment and cognitive decline [190]. Of note, following MCA occlusion the expression of TRPV1 is significantly increased in the hippocampus [191].


Inducing mild hypothermia is a promising therapeutic approach in stroke patients. In rodents, capsaicin evokes a rapidly developing and transient drop in body temperature. Rinvanil is a potent, synthetic capsaicin congener. Intraperitoneal rinvanil administration induces mild hypothermia and protects neurons from the ischemic damage that develops following transient MCA occlusion [192]. This observation highlights the therapeutic potential of TRPV1-mediated hypothermia to minimize brain damage in stroke patients.


Incensole acetate protects against the neurological deficit caused by head trauma. In mice, incensole acetate also attenuates ischemic neuronal damage and reperfusion injury by limiting neuro-inflammation. The protective effects of incensole acetate were absent in Trpv3 deficient mice and were reversed by TRPV3 inhibitors [193].


In adult rat cortical and hippocampal astrocytes, TRPV4 expression is markedly increased 7 days after hypoxia-ischemia insult. The increase in TRPV4 expression coincides with the development of astrogliosis (astrogliosis is an abnormal astrocytic proliferation that occurs in response to neuronal death). In brain slices or cultured hippocampal astrocytes, the TRPV4 agonist 4αPDD elevates Ca2 + and activates a cationic current that is prevented by the universal TRP channel antagonist, RR. Importantly, hypoxic-ischemic injury augments the responses of astrocytes to 4αPDD. These observations imply a role for TRPV4 in the development of astrogliosis that follows ischemic insults [194]. Of note, microglial cells also express TRPM2 with increased levels following ischemic brain injury [195].


In addition to astrocytes, TRPV4 is expressed in hippocampal CA1 pyramidal neurons. These neurons are activated by both 4αPDD and hyposmotic insult; these responses were prevented by the TRPV4 antagonist, HC-067047, and the NMDA-R antagonist, AP-5, indicating that TRPV4 activation potentiates NMDA-R response. When given 60 minutes after MCA occlusion, HC-067047 reduced the size of the cerebral infarct. These findings indicate that activation of TRPV4 increases NMDA-R function, which in turn potentiates glutamate excitotoxicity. If this hypothesis holds true, TRPV4 antagonists may exert potent neuroprotection against cerebral ischemia injury [196].


In stroke models, TRPM2 shows a very unusual behavior. TRPM2 expression is similar in male and female control rats. During reperfusion following in vitro ischemia, TRPM2 channels get activated only in neurons of male animals [197]. In keeping with this finding, shRNA-mediated knockdown of Trpm2 expression protected male, but not female, neurons following in vitro oxygen-glucose deprivation. Importantly, the TRPM2 inhibitor, clotrimazole, reduced infarct volume in male mice, while having no effect in female animals [198]. If these observations hold true in humans, TRPM2 represents a potential target for protection against cerebral ischemia in male stroke patients.


Traumatic Brain Injury


Traumatic brain injury initiates a cascade of complex biochemical changes, including oxidative stress, edema, inflammation, and excitotoxicity. Deferoxamine is a chelator that attenuates Fe2 +-induced toxicity in rats with traumatic brain injury. These animals show a significant increase in brain Fe2 + on day 28, accompanied by a corresponding elevation in TRPC6 levels. Deferoxamine ameliorated the deficits in spatial learning and memory, supporting the notion that deferoxamine may reduce brain injury accentuated by Fe2 + overload by interfering with a biochemical pathway that involves TRPC6 [199]. In an impact-acceleration model of diffuse traumatic brain injury in rats, TRPM2 mRNA and protein levels are elevated in the cerebral cortex [200].


Microvascular failure exacerbates the damage caused by traumatic brain injury via a combination of cerebral edema formation and progressive secondary hemorrhage. In rat models of traumatic brain injury, two transport proteins have been identified in brain endothelial cells as critical mediators of edema formation: the constitutively expressed Na+-K+-Cl cotransporter, NKCC1, and the trauma/ischemia-induced SUR1/TRPM4 channel. Whereas NKCC1 function requires ATP, activation of SUR1/TRPM4 occurs only after ATP is depleted. With critical ATP depletion, sustained opening of SUR1/TRPM4 channels may result in the oncotic death of endothelial cells, leading to capillary fragmentation and progressive secondary hemorrhage. Bumetanide and glibenclamide are antidiabetic drugs that inhibit NKCC1 and the SUR1/TRPM4 channel, respectively. In animal models of traumatic brain injury, bumetanide and glibenclamide protects neurons by preventing injury-associated capillary failure [201204]. Following spinal cord injury, Trpm4 mRNA and protein are up-regulated in capillaries, preceding their fragmentation and formation of petechial hemorrhages. Trpm4 gene suppression (by antisense) or deletion (Trpm4 knockout mice) preserved capillary structural integrity and eliminated secondary hemorrhage. In these TRPM4-deficient animals the lesion volume was reduced and the neurological function was improved compared to controls [205]. In cerebral arteries, TRPM4 might be a regulator of the myogenic constrictor response that occurs in response to increases in intravascular pressure, the so-called Bayliss effect. In rats, suppression of cerebrovascular TRPM4 expression by antisense oligonucleotides reduces myogenic constriction by 70% to 85% [206].


In mice, suppressing the expression of Trpm7 in hippocampal CA1 neurons confers resistance to ischemic cell death [207]. In these animals, hypoxia increases Mg2 + via TRPM7 in the hippocampus, and this contributes to cell death [208]. In men, conversely, depletion of intracellular Mg2 + is known to occur after stroke when it heralds poor neurological outcome. TRPM7 and TRPM6 play important roles in regulating Mg2 + homeostasis; whether or not these channels contribute to neuronal injury remains controversial [209]. To determine whether TRPM7 gene variants may increase (or decrease) the risk of ischemic stroke, a large cohort (14 916) of healthy American men were genotyped for 16 SNPs in the TRPM7 gene. Of these men, 245 subsequently suffered ischemic stroke. All SNPs were in Hardy-Weinberg equilibrium. Overall allele, genotype, and haplotype distributions were similar between cases and controls. Furthermore, marker-by-marker conditional logistic-regression analysis adjusted for potential risk factors showed no evidence for an association between any of the SNPs tested and ischemic stroke [210].


Conclusions


Brain TRP channels represent a challenging but potentially lucrative area of research. There are a number of hurdles for drug development. For example, some TRP channels (e.g., TRPC1) implicated in brain disorders are ubiquitously expressed and are involved in various physiological responses. It remains to be seen if these channels can be modulated without causing unacceptable adverse effects. Also, many TRP channels may exist both as homomeric and heteromeric channels. Thus, it will be necessary to search for compounds that are selective for channels composed of different sets of subunits. We hope that this review will encourage further studies into brain TRPs and lead to the synthesis of compounds that will find their way to the clinics.

Nov 18, 2017 | Posted by in PHARMACY | Comments Off on Are Brain TRPs Viable Targets for Curing Neurodegenerative Disorders and Improving Mental Health?
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