Monoamine Theories of Depression
Catecholamine Hypothesis. This theory postulates diminished activity of catecholamines in the central nervous system (CNS) (e.g., NE) (
10,
11). Conversely, mania involves a relative increase in their activity.
Norepinephrine. The ascending NE pathway in the CNS begins with projections from the locus coeruleus, an anatomical site encompassing neurons (containing 85% to 90% of central NE stores) that project to the following:
The effect of antidepressants on this system may subserve their efficacy. The TCAs and MAOIs increase the activity of this transmitter by two different mechanisms. TCAs block the reuptake pump that recovers NE from the synaptic cleft shortly after its release from the presynaptic neuron. Thus, reuptake inhibition is occurring during both the acute and the chronic phases of therapy. Interestingly, antidepressant response usually occurs during the chronic phase. MAOIs interfere with enzymatic deamination. In either case, the outcome is increased synaptic NE concentrations.
The earliest investigations of this hypothesis measured the major metabolites of NE (e.g., 3-methoxy-4-hydroxyphenylacetic acid [MHPG]) in cerebrospinal fluid (CSF), plasma, and urine. The purpose was to elucidate the biological mechanisms subserving mood disorders, to develop potential markers, to facilitate diagnosis, and to aid in the prediction of treatment response. Although initially promising, this line of inquiry was impeded by various methodological obstacles (e.g., the relative contribution of peripheral vs. central sources) and conflicting results. For example, most studies found that CSF MHPG concentrations in depressed patients were identical to normal control subjects. Because it is in equilibrium with plasma MHPG, however, the failure to find low CSF or plasma MHPG does not negate the NE hypothesis. This is in part because CSF MHPG may not be an accurate reflection of NE activity in the CNS.
The Depression-Type (D-type) score was developed as a predictive tool by Mooney et al.
(56) and exemplifies one attempt to pursue this line of investigation. Janicak et al.
(57) summarized this issue while reporting negative results on the predictive value of urinary MHPG in unipolar depressed patients treated with standard antidepressants.
In animals,
subchronic treatment (i.e., several days or weeks) with TCAs, MAOIs, and electroconvulsive shock (ECS) coincides more closely with the time to maximal clinical response (
58,
59). This time frame also correlates with the most consistent adaptive change (i.e., a reduced sensitivity of postsynaptic receptors), leading to diminished adenylate-cyclase activity. In the original hypothesis, depression was postulated to be secondary to decreased NE levels, decreased NE release, or subsensitive NE receptors. A
downregulation (or reverse catecholamine hypothesis) subsequently proposed a decreased number of postsynaptic β
2-adrenergic receptors in peripheral tissues (e.g., leukocytes) after chronic antidepressant treatment
(59). Thus, depression may be the result of a hyperadrenergic rather than a hypoadrenergic state (i.e., increased levels or release, or supersensitive receptors are the critical characteristics). This hypothesis is further supported by the neuropharmacological effects of chronic antidepressant treatment, which decrease the following:
-
Brain tyrosine hydroxylase and NE
-
Postsynaptic β-adrenergic receptor sensitivity and density
-
The basal firing rate of NE neurons in the locus coeruleus
Within this context, the original theory may still be valid because a defect in presynaptic neurotransmission should result in a compensatory upregulation of postsynaptic receptors. Thus, normalization of presynaptic activity should downregulate (or “normalize”) postsynaptic receptor function.
In addition, β-adrenergic receptors are increased in the brain of suicide victims, as are the number of α
2-adrenergic receptor binding sites in the brains of suicide completers and in the platelets of depressed patients
(60). The implication is that the pathological increased activity of these autoreceptors may reduce NE output secondary to a short loop, negative feedback mechanism. Further, Crews and Smith
(61) found the α
2-adrenergic receptors adapted (i.e., downregulated) after 3 weeks of treatment with desipramine, ultimately enhancing NE transmission.
On balance, these actions could support a decrease rather than an increase in the functional state of brain NE transmission if depression is conceptualized as a state of
supersensitive catecholamine receptors secondary to decreased NE availability. This reasoning is consistent with the original hypothesis of diminished NE functioning, with antidepressants returning receptors to a more normal state of sensitivity. Siever and Davis
(62) further elaborated on this concept by suggesting the possibility of dysregulation in the homeostatic mechanisms of one or more neurotransmitter systems, culminating in an unstable or erratic output.
Dopamine. In part, based on the role DA plays in the reward system circuitry, this neurotransmitter is also postulated to be important in the pathophysiology of depression
(63). In 1975, Randrup et al.
(64) postulated a role for DA in depressive disorders. A reanalysis of the data from several groups indicated a bimodal distribution of CSF homovanillic acid (HVA) levels in depressed patients, with one group comparable to normal control subjects and the other with decreased levels
(65). In addition, Roy et al.
(66) reported on the potential predictive value of lower urinary HVA output in depressed patients who attempted suicide versus those who did not. These reports indicate a decreased turnover in DA in at least a subset of depressed patients.
Consistent with earlier studies, Muscat et al.
(67) reported on chronic exposure to mild unpredictable stress in rats as a model to study the antidepressant-reversible decreases in the consumption of palatable sweets. They found that certain DA agonists (i.e., quinpirole, bromocriptine) administered intermittently had the same positive effects as TCAs. They further postulated that the infrequent, intermittent administration of DA agonists (e.g., psychostimulants) avoided problems with tolerance and abuse while providing a clinically relevant antidepressant strategy. In this context, other DA agonists with potential antidepressant properties include
A subsequent report by Kapur and Mann
(73) reviewed the role of DA in depressive disorders. They discussed several lines of evidence, including
-
The lower CSF HVA levels in some depressed patients
-
An increased incidence of depression in Parkinson disease as well as in patients receiving DA-depleting or antagonistic agents
-
The antidepressant effect of agents that enhance DA transmission
-
The ability of various classes of antidepressants and ECS to enhance DA effects in animal models
This last point is supported by
autoradiographic evidence indicating that chronic treatment with several different antidepressants modulates postsynaptic DA function. This increases the density of D
2 and D
3 receptors, particularly in the nucleus accumbens and striatum (
74,
75,
76 and
77). Lammers et al.
(78) also found that chronic antidepressant treatment with several TCAs, MAOIs, and ECS produced a selective increase in D
3 receptor gene expression in the shell of the nucleus accumbens. Although fluoxetine decreased D
3 messenger RNA (mRNA) when given alone, fluoxetine and imipramine prevented downregulation of D
3 receptors caused by handling stress.
These findings support impairment in the nucleus accumbens associated with the experience of pleasure, a hallmark of depression.
Indolamine Hypothesis. The second neurotransmitter implicated in the monoamine theory is serotonin (5-HT)
(79). This neurotransmitter is contained in a few pathways, with those in the
midbrain raphe nuclei projecting to the limbic-septal area where the hippocampus and amygdala may be of particular importance. Serotonin abnormalities are widely reported in patients with depression.
Chapter 6 notes several of these in the section on “Suicide.” Other abnormalities include
-
Decreased 5-HT uptake in the platelets (V
max) of depressed patients is linked to a decrease in the number of platelet imipramine binding sites
(80).
-
Blunting of the maximal prolactin response to intravenous tryptophan (the precursor of 5-HT) is found in depressed patients. Similar results have also been observed with fenfluramine and m-chlorophenylpiperazine (mCPP)
(81).
-
p-Chlorophenylalanine, which decreases 5-HT synthesis, reverses the clinical efficacy of antidepressants
(82).
-
Depletion of plasma tryptophan precursors may reverse antidepressant-induced remissions
(83).
-
Tryptophan and 5-hydroxy’tryptophan, the precursors of 5-HT, may have antidepressant effects, alone or in combination with other drugs
(84).
-
Antidepressants reduce 5-HT receptor number but not their affinity (85), in a manner analogous to β-adrenergic receptor downregulation.
-
Electroconvulsive therapy (ECT) potentiates prolactin response to thyrotropin-releasing hormone (TRH), which is mediated by serotonin
(86).
-
ECS
enhances 5-HT2 receptor functional activity and binding characteristics in postmortem studies on animals
(87).
-
Frontolimbic 5-HT
2A receptor binding may identify vulnerability to mood disorders
(88).
-
5-HTT binding potential in the amygdala and midbrain is lower during a major depressive episode (MDE)
(89).
From a therapeutic perspective, the most successful application is the clinical efficacy of agents that impact this system by
Regarding this last point, there is interest in partial 5-HT agonists that stimulate their receptor target in the absence of the natural, full agonist (i.e., endogenous neurotransmitter). For example, in the early 1990s, there was considerable interest in the azapirones (i.e., 5-HT
1A partial agonists) as possible antidepressants and anxiolytics. Eison
(92) argued for a common underlying pathology for anxiety and depression based on early evidence that azapirones (e.g., buspirone, gepirone, ipsapirone, and tandospirone) appeared to have both anxiolytic and antidepressant effects. Eison postulated that this is a result of the ability of the azapirones to modulate central 5-HT function through their partial agonism of 5-HT
1A receptors. In support of their possible antidepressant effects, Eison noted that azapirones downregulate 5-HT
2 receptors, as well as desensitize presynaptic 5-HT
1A autoreceptors, and may also affect postsynaptic 5-HT
1A receptors differently than their presynaptic counterparts (i.e., buspirone is a partial agonist of only postsynaptic receptors). Thus, he postulated that azapirones could normalize central 5-HT neurotransmission either up or down depending on the baseline level of activity (i.e., excess—anxiety; deficit—depression). Agents were then developed to block the 5-HT transporter and also act as 5-HT
1A receptor agonists
(93).
Other Neurotransmitter Hypotheses
Glutamate. Evidence from preclinical studies, postmortem studies, and clinical trials suggests that increases in glutamate and its ionotropic receptors (e.g., N-methyl-D-aspartate [NMDA]; α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid [AMPA]) may play an important role in the pathophysiology of depression
(94). For example, some NMDA
modulators, including 2-aminophosphonoheptanoic acid (a competitive NMDA antagonist), 1-aminocyclopropanecarboxylic acid (ACP, a glycine partial agonist), and dizocilpine (a use-dependent channel blocker),
reduce immobility time in the forced swim test (95). Chronic antidepressant treatment also
reduces high-affinity glycine sites in the cerebral cortex but not in other brain regions and causes more widespread alterations in mRNA levels coding for NMDA receptor subunits
(95). Postmortem studies also find a decrease in glycine binding sites on the NMDA receptor in the frontal cortex of suicide victims
(96). One small clinical trial reported that intravenous
ketamine improved depressive symptoms for up to 72 hours in patients not responsive to standard antidepressants
(97). A subsequent trial also suggests that repeateddose IV ketamine could benefit treatmentresistant depression (TRD) with acceptable tolerability
(98). Other investigations also implicate AMPA glutamate receptors with the rapid onset and longer lasting antidepressant effects of ketamine versus other NMDA antagonists
(99). The activation of ligand-gated NMDA receptors results in an influx of Ca
2+ into the cell, which activates nitric oxidase synthase (NOS) and the formation of nitric oxide. In this context, there is preclinical evidence that NOS
inhibitors demonstrate antidepressant-like activity. Thus, the antidepressant effect of NMDA receptor antagonists may be mediated in part through its effects on NOS (
100,
101).
γ-Aminobutyric Acid. Since glutamate and γ-aminobutyric acid (GABA) systems have reciprocal links throughout the brain, GABA may play a role in the pathophysiology of depression. Injections of bicolline (a GABA-A receptor antagonist) into the hippocampus promoted
learned helplessness in rats, whereas injections of
GABA into the hippocampus reversed or prevented its development
(102). Martin et al.
(103) found that GABA-B receptors are downregulated in the frontal cortex of animals with learned helplessness and are normalized by successful treatment with chronic TCA administration. A recent study in depressed patients using proton magnetic resonance spectroscopy (MRS) indicated that reduced GABA in the occipital cortex may predict treatment resistance
(104). Other human data include
Histamine. Histamine (H) interacts with both monoamines and acetylcholine (ACh). Specifically, the
H3 receptors can act as heteroreceptors to modulate the release of monoamines and ACh in the brain
(112). Stress decreases H
3 receptors in the rat cortex, while chronic amitriptyline treatment reverses this effect
(113). Finally, thioperamide (an H
3 antagonist) and iodophenpropit (a very selective H
3 antagonist) demonstrate antidepressant-like activity in the forced swim test in rats
(79).
Others. Data support the potential role of the muscarinic
cholinergic antagonist, scopolamine, which demonstrated rapid and robust antidepressant effects in one trial
(114).
At one time,
opioids were first-line antidepressants, but they are no longer clinically acceptable, in part because of the severity of their adverse effects. Clinical data, as well as basic science observations, suggest a role for opioids in the pathophysiology of depression. For example, SSRIs and TCAs increase the concentration of enkephalins in the brainstem and µ-opioid receptors in the rat forebrain
(115). Opioids can inhibit the adrenocorticotropic hormone and cortisol overactivity often associated with depressed patients. Finally, cholecystokinin (CCK) is a neuropeptide that colocalizes with opioids in a number of brain regions. In one report, CCK-B (but not CCK-A) receptors enhanced the antidepressant-like activity of opioids
(116).
Interactional Theories of Depression.
A single neurotransmitter theory does not sufficiently explain all known evidence. As a result, models that include two or more systems can consider their modulatory interactions.
Permissive Hypothesis. The “permissive” hypothesis proposes that decreased function in central serotonin transmission sets the stage for either a depressive or a manic phase
(117). Although not sufficient to produce a mood disturbance, when superimposed on aberrations in NE function, serotonin changes may determine the phase of an affective episode (i.e., decreased 5-HT and decreased NE subserves depression; decreased 5-HT and increased NE subserves mania). Data from animal studies to support this theory include
Adrenergic-Cholinergic Balance Hypothesis. A second interactional theory postulates an imbalance between the cholinergic and the noradrenergic systems
(119). The central cholinergic system consists of projections primarily from the
nucleus basalis. A relative increase in this system’s activity in comparison with central NE activity may play a role in producing depression. Conversely, a decrease relative to central NE activity may play a role in producing mania. Clinically, agents with cholinomimetic effects (e.g., precursors, cholinergic agonists, cholinesterase inhibitors) have shown some benefit in mania (see
Chapter 10). Cholinergic abnormalities are also thought to underlie some of the abnormal sleep patterns (e.g., decreased rapid eye movement [REM] latency; increased
REM density) found in depression. Consistent with this theory is evidence that ECT
-
Decreases brain ACh levels
-
Increases activity of choline acetyltransferase, the enzyme most prominently involved in ACh breakdown
-
Causes release of CSF ACh
-
Produces cholinergically mediated electroencephalographic (EEG) slowing following a series of treatments
The numerous links between smoking and clinical depression (or schizophrenia) also suggest a role for the cholinergic system in the pathophysiology of depression. Thus, patients with depression have a higher incidence of smoking compared with the general population
(120). Depression can also emerge when someone stops smoking
(121) and can be reversed by either resuming smoking or with antidepressant treatment
(122). Nicotine increases the concentration of all three biogenic amine neurotransmitters (i.e., DA, NE, and 5-HT) (
123,
124) and can reverse learned helplessness in rats
(125).
Bidimensional Model Hypothesis. Proponents of this hypothesis identify three types of abnormal neurochemistry:
As others before them, Emrich and Wolf
(126), propose that a single neurochemical imbalance is not sufficient to explain many of the inconsistencies and contradictions in studies with various mood stabilizers. Instead, they speculate that several neurotransmitter imbalances relating to different brain areas should be anticipated.
Assuming differing mechanisms of action for mood stabilizers such as lithium, valproate, and carbamazepine (CBZ), a bidimensional model of mood regulation postulates two “gating zones” (one for depression and one for mania). These zones are subserved by different neurochemical abnormalities, leading to a situation in which both could be impacted by certain agents (i.e., mood stabilizers) or, alternatively, could individually be affected by unidirectional compounds (e.g., TCAs).
Second Messenger Dysbalance Hypothesis. Receptors are glycoproteins imbedded in the lipid bilayer of neuronal membranes and can detect minute amounts of specific ligands (e.g., neurotransmitters, hormones). The ligand-receptor interaction sets in motion a transduction system (e.g., an enzyme, an ion channel) that orchestrates various intracellular biochemical events. Further, investigators look beyond the receptor-ligand binding relationship to study intraneuronal events stimulated by this interaction. Two primary areas are the adenylate cyclase (AC) and the phosphoinositol (PI) second messenger systems.
Such investigations led to the postulation that functional disturbances in intraneuronal signal transmission distal to the receptors of classic neurotransmitters (i.e., the first messengers) are pathogenetically important in mood disorders (127). Further, it suggests that these disorders are caused by a dysfunction in the “crosstalk” between major intraneuronal signal amplification systems (e.g., AC and the phospholipase C systems). Thus, depression may result from a diminished functioning in cyclic adenosine monophosphate (cAMP)-mediated effector cell responses, together with an absolute or relative dominance of the inositol triphosphate diacylglycerol-mediated responses. In the context of bipolar disorder, mania is conceptualized as resulting from the reverse circumstances.
Work in this area has yielded results that link the changes in receptor physiology to changes in second messenger systems. For example, blunted β-adrenergic receptor responsivity of noradrenergic receptor-coupled AC occurs after repeated doses of most, but not all, antidepressants
(57). Yet, an increase in AC activity was demonstrated in the hippocampus and cortex following chronic antidepressant treatment and ECS (
128,
129). This suggests that, even though there is a relative decrease in β-adrenergic receptors after chronic as compared with acute antidepressant administration, levels of cAMP remain elevated compared with the no treatment condition. Thus, despite downregulation of β-adrenergic receptors, there is an overall increase in the activity of the cAMP system because of an increase in biogenic amine levels in the synapse as a result of antidepressant action on amine reuptake mechanisms. Thus, current antidepressants, including NE and serotonin reuptake inhibitors, may exert their effects through activation of the cAMP
pathway, which in turn leads to regulation of cAMP-dependent protein kinase and subsequently to activation of the
cAMP response element binding protein (CREB).
The latter is a downstream component of the cAMP cascade system and, in its phosphorylated form (pCREB), induces the expression of neuroprotective factors such as BDNF (
25,
129). BDNF in turn mediates neuronal genesis, survival, and plasticity
(130). BDNF is the most widespread growth factor in the brain and is activated by a number of stimuli in addition to pCREB. BNDF infusions into the adult rat neocortex result in 5-HT nerve terminal growth and regrowth after parachloroamphetamineinduced destruction, especially in the CA3 region of the hippocampus
(131). CREB, the BDNF transmembrane receptor tyrosine kinase B, and BDNF itself are all elevated following chronic antidepressant and ECS exposure (
132,
133,
134 and
135). Stress decreases BDNF mRNA expression in rat brain consistent with stress as a precursor to the onset of depression in humans
(136).
The administration of BDNF into the brain produces antidepressant-like activity in both the forced swim and the learned helplessness animal model. In one study, BDNF was continuously infused into the midbrain by an osmotic minipump over 14 days
(137). In another, BDNF was administered as a single injection into the dentate gyrus or CA3 region of the hippocampus. In both of these animal models, BDNF produced an antidepressant effect lasting over 10 days
(138). Although the postmortem findings from the brains of depressed patients are inconsistent (
139,
140), BDNF levels were low in the serum of depressed patients and increased in the hippocampus and cerebral cortex of postmortem brains of patients treated with antidepressants
(141).
Taken together, the effect of increased neurotrophins could mitigate hippocampal changes associated with exposure to stress. Although theoretical, this model is supported by empirical studies of the pathophysiology of depression in patients (
142,
143) as well as in animal studies (
144,
145). These theories are also consistent with the neuroanatomical findings of cell loss and decreased hippocampal volume, as well as increased ventricular volume consistent with cortical atrophy, reported in the brains of depressed patients (
146,
147).