Early investigators noted that when histamine is delivered to the skin, subjects experience the sensation of itch that starts 20–30 s after application and then dissipates by about 10 min (Eppinger 1913; Sollmann and Pilcher 1917; Bickford 1937). The itch is in addition to the triple response of local vasodilation, local edema, and flare described by Lewis and co-workers and summarized in his book The Blood Vessels of the Human Skin (Lewis 1927). Indeed, any kind of injury that produces the triple response will also produce itch due to the release of histamine (Lewis 1942). Early clinical studies showed that the first clinical antihistamine, Antergan, could reduce the itch, wheal, and flare caused by intradermal injection of histamine (Decourt 1942; Parrot 1942), and it was recognized early on that a comparison of the antipruritic activity of antihistamines could be made by measuring the changes in the threshold of histamine concentrations required to induce itch (Cormia and Kuykendall 1954).

Histamine has emerged as the best-characterized human pruritogen; but what are the mechanisms that lead to the sensation of itch? A neuronal basis for histamine-induced itch sensation was suggested by the fact that certain direct stimuli such as scratching or painful stimuli such as a pin prick, heat, or cold can inhibit the itch response (Bickford 1937; Graham et al. 1951). Indeed, this is true even if such stimuli are given distal to the site of itch (Graham et al. 1951; Yosipovitch et al. 2005, 2007). In addition, it appears that the application of local anesthetics can inhibit the itch sensation (Shelley and Melton 1950). These observations suggest that histamine-induced itch is mediated via neuronal activation. This was confirmed by the identification of histamine sensitive C-fibers that transmit the itch response to the spinal cord (Schmelz et al. 1997). These C-fibers can also respond to other nociceptive compounds such as capsaicin and bradykinin in addition to histamine, but do not respond to mechanical stimuli (Schmelz et al. 2003). The possibility that other polymodal C-fibers play a role is not excluded, however (Handwerker et al. 1991; Schmelz et al. 1997; Johanek et al. 2008). The transmission of histamine-induced pruritic signals appears to require TRPV1, since scratching was reduced in TRPV1-deficient mice or those treated with an inhibitor (Shim et al. 2007). Transmission of pruritus to the brain also appears to be mediated via specific spinothalamic tract neurons (Andrew and Craig 2001). Interestingly, these neurons are polymodal and can respond to mechanical stimuli (Schmelz 2001; Simone et al. 2004; Davidson et al. 2007), which may explain why scratching can reduce the histamine-induced activity of these neurons and may provide a mechanism for the inhibition of itch by other stimuli (Davidson et al. 2009). In the brain, peripheral itch stimuli evoke a complex pattern of regional neuronal activation that contains elements of sensation, emotion, and motivation (Hsieh et al. 1994; Darsow et al. 2000; Drzezga et al. 2001; Mochizuki et al. 2003, 2007; Herde et al. 2007; Schneider et al. 2008; Papoiu et al. 2012). However, there is some evidence that different pruritus stimuli result in different patterns of neuronal pathway activation and that the processing of chronic pruritic signals may differ from acute pruritus (Leknes et al. 2007; Schneider et al. 2008; Papoiu et al. 2012). Data have shown that in mice, many itch signals, including those of histamine, are transmitted via MrgprA3-expressing neurons (Han et al. 2013). However, itch-transmitting neurons do not appear to be interchangeable. Roberson et al. (2013) used a method to silence neurons after they have been activated to show that specific afferent fibers mediate itch generated by different pruritogens. In both cases, it appears that these neurons do not process pain sensations (Han et al. 2013; Roberson et al. 2013).

The H₁R is expressed on many cell types and mediates several important physiological functions that have been exploited from a therapeutic perspective. For example, as above, expression on both peripheral and central neurons mediates pruritic responses. The H₁R is also expressed on neurons in the central nervous system with a role in sleep/wake cycles. Blockade of the H₁R by compounds that cross the blood–brain barrier results in sedation, which is undesirable in the treatment of allergy but useful in the treatment of insomnia. On smooth muscle cells and endothelial cells, activation of the H₁R drives vascular permeability responses in the skin and other organs.

The currently marketed H₁R antihistamines are divided into two categories. First-generation H₁R antihistamines are characterized by poor selectivity for the H₁R as well as their ability to easily cross the blood–brain barrier. Second-generation antihistamines are more selective for the H₁R, thus reducing the side-effect burden, and are mildly sedating or nonsedating, since they do not penetrate into the brain as readily. Despite these differences, it appears that antipruritic efficacy is similar between first- and second-generation H₁R antihistamines. However, second-generation antihistamines are preferred, due to safety advantages and dosing convenience. In addition, second-generation H₁R antihistamines have been more rigorously studied in the clinic since they were introduced after many of the clinical trial efficacy and safety requirements went into effect.

As discussed earlier, intradermal injection of histamine into the skin of humans produces a sensation of itch as well as a wheal and flare response. This is also true with intradermal injection of an H₁R agonist, 2-methylhistamine, whose effects can be blocked by an H₁R antagonist (Davies and Greaves 1980). Much of histamine activity in the skin is mediated via activation of the H₁R, since antihistamines that target this receptor can inhibit these reactions (Hagermark 1973; Davies et al. 1979; Hagermark et al. 1979; Davies and Greaves 1980; Levander et al. 1985, 1991; Coulie et al. 1991; Lahti and Haapaniemi 1993; Weisshaar et al. 1997; Clough et al. 2001; Furue et al. 2001; Denham et al. 2003; Morita et al. 2005; Kupczyk et al. 2007; Tanizaki et al. 2012). For example, two highly selective H₁R antagonists, cetirizine and its active enantiomer levocetirizine, were able to almost completely reduce histamine-induced itch in humans, as well as significantly inhibit the wheal and flare response (Coulie et al. 1991; Levander et al. 1991; Lahti and Haapaniemi 1993; Clough et al. 2001; Denham et al. 2003). This effect could also be mimicked by another selective H₁R antagonist, fexofenadine (Tanizaki et al. 2012).

When injected into the skin, other agents such as compound 48/80, platelet-activating factor, prostaglandin E2, substance P, and other neuropeptides can cause itch as well as a wheal response that can be blocked by H₁R antagonists (Hagermark et al. 1978; Fjellner and Haegermark 1981, 1985; Rukwied et al. 2000; Hosogi et al. 2006). These agents appear to be producing their effects by causing the release of histamine at the site of injection, although it is also possible that they directly or indirectly stimulate c-afferent fibers and generate an upstream histamine-mediated neuronal response that is H₁R mediated. Some pruritogens appear to work via a different mechanism, though, since the itch generated by intradermal injection of substances such as serotonin or cowhage is not inhibited significantly by H₁R antihistamines (Weisshaar et al. 1997; Hosogi et al. 2006; Johanek et al. 2007).

The role of the H₁R in histamine-induced itch in the skin provides a clear rationale for the use of antihistamines that target this receptor for dermal pruritus. The best example of success in this area is with urticaria. Urticaria is characterized by highly pruritic skin wheals, which can be either acute or chronic, persisting for 6 weeks or longer. For many patients, the triggers are unknown, with the exception of physical urticarias that are induced by exposure to heat, cold, sun, pressure, vibration, water, exercise, or contact (Greaves 1995; Zuberbier and Maurer 2007). The similarity in symptoms between urticaria and histamine injection into the skin is suggestive for a role of histamine. This is supported by the observations of increased histamine levels and mast cell numbers in the affected skin of urticaria patients (Kaplan et al. 1978; Natbony et al. 1983). One of the first antihistamines, diphenhydramine, was shown to be effective in treating urticaria, with one early study showing that 25 of 35 patients with urticaria experienced immediate and complete relief of symptoms (Curtis and Owens 1945; O’Leary and Farber 1945). Several H₁R antihistamines are approved for the treatment of urticaria, including fexofenadine. In double-blind, placebo-controlled studies, fexofenadine was shown to reduce pruritus by about 1 point on a 0–4 scale and to improve the wheal score as well (Finn et al. 1999; Kaplan et al. 2005). In addition to fexofenadine, clinical studies (Table 3) have shown efficacy with bilastine (Zuberbier and Maurer 2010), loratadine (Monroe 1992; Dubertret et al. 1999), cetirizine (Breneman 1996), desloratadine (Monroe et al. 2003; Ortonne et al. 2007), levocetirizine (Nettis et al. 2006), mizolastine (Brostoff et al. 1996), and rupatadine (Gimenez-Arnau et al. 2007). These clinical studies have resulted in the recommendation of second-generation H₁R antihistamines as the standard of care for patients with urticaria, according to several clinical guidelines and expert opinions (Kaplan 2002; Powell et al. 2007; Khan 2008; Zuberbier et al. 2009; Ortonne 2011). Of note, the clinical data are mainly concerned with chronic idiopathic urticaria and, with the exception of several studies of cold urticaria (Bonadonna et al. 2003; Juhlin 2004; Magerl et al. 2007; Siebenhaar et al. 2009; Krause et al. 2013), the benefits of treatment for other physical urticarias (e.g., heat and solar) have not been well characterized.

However, not all chronic urticaria patients experience complete symptomatic relief with second-generation H₁R antihistamines. Some of these subjects are given sedating first-generation H₁R antihistamines, especially when the symptoms interfere with sleep at night. In clinical guidelines it has been recommended that these refractory patients may benefit from increasing the dose of the second-generation H₁R antihistamine up to four times the standard dose (Zuberbier 2012). Clinical data showing greater efficacy with increasing doses of levocetirizine, desloratadine, and rupatadine have been used to support this claim (Kameyoshi et al. 2007; Gimenez-Arnau et al. 2009; Siebenhaar et al. 2009; Staevska et al. 2010). One study compared the effects of increasing the dose of either levocetirizine or desloratadine (Staevska et al. 2010). Urticaria patients were randomized in a blinded fashion to 5 mg of either levocetirizine or desloratadine. After 1 week, patients who did not respond were instructed to double the dose and then to double the dose again if they did not respond after an additional week. Increasing the dose of either drug was associated with an increase in treatment success rate, suggesting that the higher doses were beneficial in patients who did not respond to lower doses. However, it should be noted that in this study, the subjects were not blinded to the increase in dose. An early study with fexofenadine showed an advantage of doses of 60–240 mg bid over 20 mg bid, but did not detect differences between the higher doses (Finn et al. 1999). However, an open-label study in Japanese subjects with chronic urticaria showed that 120 mg fexofenadine bid was more effective than 60 mg in reducing itch and severity score (Tanizaki et al. 2013a). This study also included a double-blind, placebo-controlled crossover study assessing the effects on histamine-induced wheal, flare, and itch in healthy volunteers. The higher dose of fexofenadine was more effective at reducing itch at early time points, but at 6 and 12 h after dosing, the results were the same for both doses. Similar effects on histamine-induced itch have been reported for increased doses of levocetirizine (Tanizaki et al. 2013b). Inhibition of the itch intensity and the wheal area was significantly greater with 10 mg levocetirizine compared to 5 mg at both 1 and 24 h. The difference at intermediate time points was not statistically significant. Increasing the dose of bilastine has also been reported to improve its effectiveness with respect to the wheal threshold in cold contact urticaria, but not on the pruritus score (Krause et al. 2013). It should be noted that it is currently unclear as to whether increasing the dose is more effective due to more complete inhibition of the H₁R- or to other non-H₁R-mediated anti-inflammatory properties (Wang et al. 2005; Weller and Maurer 2009).

Mosquito bites also cause pruritic wheals that are similar to those seen in urticaria and are mediated in part by the local release of histamine (Horsmanheimo et al. 1996). As with urticaria, H₁R antihistamines have shown efficacy in clinical trials with ebastine, cetirizine, and levocetirizine decreasing bite pruritus by 60–70 % (Reunala et al. 1993; Karppinen et al. 2000, 2006).

The H₁R also mediates histamine-induced nasal itch. Nasal challenge with histamine or the H₁R agonist, betahistine, leads to itching, congestion, rhinorrhea, and sneezing (Mygind 1982; Miadonna et al. 1987; Shelton and Eiser 1994). Antihistamines that target the H₁R completely block all of the histamine-induced symptoms, with the exception of congestion, indicating that these effects are due to activation of the H₁R (Secher et al. 1982; Kirkegaard et al. 1983; Mygind et al. 1983; Naclerio and Togias 1991; Hilberg 1995; Wood-Baker et al. 1996; Wang et al. 2001; Taylor-Clark et al. 2005). For example, cetirizine was shown to almost completely inhibit histamine-induced itch, secretions, and sneezing, but only partial effects were observed for congestion-related parameters (Hilberg 1995).

The efficacy of H₁R antihistamines in blocking histamine-induced nasal symptoms provides a mechanistic rationale for their effects in allergic rhinitis. Allergic rhinitis is a very common allergic disease with symptoms that include nasal and pharyngeal pruritus, as well as sneezing, rhinorrhea, coughing, and congestion: effects that are very similar to those observed with administration of histamine. H₁R antihistamines have been one of the mainstays for the treatment of allergic rhinitis ever since diphenhydramine, one of the first antihistamines, showed effects in patients with hay fever (Koelsche et al. 1945). Since then, several topical and oral H₁R antihistamines have been approved for use in allergic rhinitis based on controlled clinical studies (for a review see Benninger et al. 2010). Many H₁R antihistamines have shown efficacy in reducing nasal pruritus as well as other symptoms in well-conducted, placebo-controlled studies in allergic rhinitis. These include bilastine (Kuna et al. 2009), cetirizine (Howarth et al. 1999; Murray et al. 2002; Noonan et al. 2003), desloratadine (Berger et al. 2002; Kim et al. 2006), fexofenadine (Bernstein et al. 1997; Wilson et al. 2002), levocetirizine (Potter 2003), and loratadine (Oei 1988; Storms et al. 1989) and the intranasal H₁R antihistamines, including azelastine (LaForce et al. 2004; Lumry et al. 2007) and olopatadine (Meltzer et al. 2005; Ratner et al. 2005). For example, 5 mg of desloratadine reduced nasal itching by about 0.7 points on a four-point scale in patients with seasonal allergic rhinitis compared to placebo, which only had a reduction of 0.4 points (Berger et al. 2002). A similar magnitude of effect was also seen for all of the other symptoms measured (Berger et al. 2002). There is also some limited evidence that the combination of an H₁R antihistamine with intranasal corticosteroids has an additive benefit on nasal itch (Simpson 1994; Anolik 2008; Ratner et al. 2008).

The H₁R also mediates ocular pruritus. H₁R antihistamines completely block the itch induced with ocular administration of histamine in addition to having a partial effect on redness (Abelson et al. 1980; Kirkegaard et al. 1982; Abelson and Smith 1988), supporting their use in allergic conjunctivitis. This ocular allergy is triggered by exposure of the conjunctiva to environmental allergens that initiate an inflammatory cascade. One of the primary symptoms is pruritus, but tearing, lid and conjunctival edema and erythema, and photophobia also occur. In clinical studies, the symptoms of allergic conjunctivitis are induced by the application of allergen in a controlled setting. Antihistamines that target the H₁R such as alcaftadine (Greiner et al. 2011; Torkildsen and Shedden 2011), azelastine (Ciprandi et al. 1997; Horak et al. 1998), bepotastine (Abelson et al. 2009; Macejko et al. 2010), emedastine (Abelson and Kaplan 2002; D’Arienzo et al. 2002; Borazan et al. 2009), epinastine (Lanier et al. 2004; Whitcup et al. 2004), ketotifen (Crampton 2002; Greiner and Minno 2003), levocabastine (Zuber and Pecoud 1988; Abelson et al. 1995), and olopatadine (Abelson 1998; Spangler et al. 2001; Mah et al. 2007) demonstrated superiority over placebo in such studies. For example, both alcaftadine and olopatadine were able to inhibit ocular itching after conjunctival allergen challenge with a mean difference compared to placebo of about 1.7–1.9 points on a 0–4 pruritus scale (Greiner et al. 2011). In a natural allergen exposure setting, an ophthalmic solution of bepotastine gave a mean improvement of 30–40 % greater than placebo (Carr et al. 2013). These drugs are able to reduce many of the symptoms of allergic conjunctivitis including pruritus and are typically used as first-line therapy for the disease. Topical ocular H₁R antihistamines appear to have superior efficacy to oral antihistamines (Abelson and Welch 2000; Spangler et al. 2003), although the combination of ocular and oral H₁R antihistamines is superior to either treatment alone (Abelson and Lanier 1999; Crampton 2003). Finally, head-to-head studies have shown that topical H₁R antihistamines are more effective than mast cell stabilizers, such as cromolyn or nedocromil (Orfeo et al. 2002; Greiner and Minno 2003), or topical NSAIDS such as ketorolac (Discepola et al. 1999; Yaylali et al. 2003).

Despite the successes in treating itch associated with urticaria, allergic rhinitis, and allergic conjunctivitis, not all pruritic diseases are controlled by H₁R antihistamines. One such disease is atopic dermatitis, which is an inflammatory skin disease where pruritus is one of the most common and characteristic symptoms (Williams 2005). Despite evidence of elevated histamine levels and increased numbers of mast cells in pruritic skin of atopic dermatitis patients (Johnson et al. 1960; Juhlin 1967; Ikoma 2009), there is little clinical evidence that H₁R antihistamines have any effect. Several clinical studies have been conducted, and reviews of these studies suggest that any effect seen is due to sedation by blocking the central H₁R or off-target anti-inflammatory or skin barrier effects (Klein and Clark 1999; Akdis et al. 2006; Amano et al. 2007; Tamura et al. 2008; Saeki et al. 2009; Buddenkotte et al. 2010).

Other pruritic diseases where antihistamines that target the H₁R are ineffective are psoriasis, cutaneous T-cell lymphomas (CTCL), and chronic cholestatic liver disease. Psoriasis symptoms include inflamed, dry and scaling skin lesions that are pruritic in many patients (Yosipovitch et al. 2000). Histamine has been associated with the disease; it has been reported that histamine is increased in psoriatic skin and in addition there are increases in the number of mast cells and in particular degranulated mast cells (Brody 1984; Krogstad et al. 1997). However, no controlled clinical trials have been conducted, and it is not thought that H₁R antihistamines are effective in treating the pruritus of psoriasis. Clinical studies have been conducted testing H₁R antihistamines in the itch associated with CTCL, but it was found that they were not effective (Meyer et al. 2010; Ahern et al. 2012). Pruritus is also a frequent symptom in patients with either primary biliary cirrhosis or primary sclerosing cholangitis (Wiesner et al. 1985; Mela et al. 2003), but histamine is not thought to play a major role, although first-generation H₁R antihistamines are prescribed for their sedative effects (Herndon 1975; Bergasa and Jones 1991). Similarly, histamine has been discounted as cause for pruritus in patients with end-stage renal disease. With the exception of a few small studies where doxepin, ketotifen, and terfenadine showed some benefit, trials of oral H₁R antihistamines have not in general demonstrated efficacy in relieving uremic pruritus (Russo et al. 1986; Francos et al. 1991; Pour-Reza-Gholi et al. 2007). Despite the lack of clinical evidence, first-generation H₁R antihistamines are occasionally used for their sedative properties.

After the discovery of the first antihistamines, it was recognized that they could not block all of the physiological effects of histamine. It was proposed that the receptor blocked by the known antihistamines be called the H₁ receptor and that another histamine receptor mediated the histamine role in functions such as gastric acid secretion and the positive chronotropic response in isolated atria (Ashford et al. 1949; Trendelenburg and Hobbs 1960; Ash and Schild 1966). The receptor that modulates gastric acid secretion was defined as the H₂ receptor (H₂R) after specific ligands were found that block this effect of histamine (Black et al. 1972; Wyllie et al. 1972). While the H₂R is expressed in many tissues and cell types, its most clinically relevant pharmacological activity is mediated via its expression on parietal cells in the gastric mucosa. Parietal cells are the main acid-secreting cells in the gastrointestinal system. Histamine release from enterochromaffin-like cells enhances this secretion by activating H₂R on the parietal cells. Therefore, antagonists of the H₂R will decrease gastric acid secretion. This has led to the clinical use of such antagonists for lowering gastric acid levels in patients with gastric or duodenal ulcers or with gastroesophageal reflux disease. However, since the development of proton pump inhibitors, H₂R antagonists are no longer considered the frontline therapy. The most commonly used H₂R antagonists are cimetidine, ranitidine, and famotidine.

There has been some work suggesting that the H₂R is involved in dermal responses to histamine. In one study, neither the H₂R antagonist cimetidine nor the H₁R antagonist chlorpheniramine was able to shift the threshold for itch induction in response to increasing concentrations of histamine, although chlorpheniramine exhibited a trend toward increasing this threshold when compared to placebo (Davies et al. 1979). These results may have been complicated by a high placebo response. Nevertheless, the combination of cimetidine and chlorpheniramine did significantly suppress histamine-induced itch (Davies et al. 1979). A second paper indicated that both the H₂R antagonist metiamide and cimetidine given intradermally could inhibit histamine-induced itch, but the inhibition was modest and not dose dependent, leading the authors to conclude that the effects were not due to the H₂R (Hagermark et al. 1979). Another study showed that the H₂R antagonist ranitidine given orally was able to inhibit itch when subjects were simultaneously given skin-prick testing of histamine, codeine, and grass pollens (Kupczyk et al. 2007). The levels of inhibition were statistically significant compared to placebo and similar in magnitude to the H₁R antagonist loratadine, but since all skin tests were given at the same time, it was impossible to tell if there was any specificity to the response. The effects of H₂R antagonists on histamine-induced wheal and flare responses are modest at best (Marks and Greaves 1977; Hagermark et al. 1979; Meyrick-Thomas et al. 1985a, b; Kupczyk et al. 2007). Finally, preclinical data do not show any effect of H₂R antagonists on histamine-induced itch in mice (Bell et al. 2004; Dunford et al. 2007). Therefore, the current data do not support a major role for the H₂R in mediating histamine effects in the skin. This also appears to be true in the nasal mucosa where H₂R antagonists do not reduce symptoms induced by histamine, although there is some indication of an effect in combination with H₁R antagonists (Secher et al. 1982; Havas et al. 1986; Wood-Baker et al. 1996).

Despite this, H₂R antagonists are sometimes prescribed to treat itch in refractory chronic urticaria and psoriasis. Currently, there is little clinical trial evidence supporting this use. Ranitidine failed to show benefit in a large placebo-controlled study in psoriasis patients (Zonneveld et al. 1997). In addition, a Cochrane review concluded that the data on the use of H₂R antagonists did not support their use in urticaria (Fedorowicz et al. 2012). However, one study in subjects with acute allergic conditions suggested that a combination of ranitidine and diphenhydramine could reduce wheal formation in subjects with urticaria (Lin et al. 2000). Therefore, currently there is no conclusive clinical evidence that H₂R antagonists are effective against itch in either of these two conditions.

Like the H₂R, the H₃R was identified based on a pharmacological activity of histamine that could not be attributed to the known receptors. It was shown that histamine inhibited its own release in neurons but that neither H₁R nor H₂R ligands could reproduce this effect. Although the H₃R was defined on a pharmacological basis in 1983 (Arrang et al. 1983), it was not cloned until years later (Lovenberg et al. 1999). The H₃R is a presynaptic autoreceptor that is expressed on histamine synthesizing neurons that project throughout all major areas of the brain (Panula and Nuutinen 2013). The receptor also acts as a heteroreceptor on non-histamine-containing neurons in both the central and peripheral nervous systems where it impacts the release of a variety of neurotransmitters (Panula and Nuutinen 2013). Histamine and the other neurotransmitters modulated by the H₃R play a role in a variety of physiological functions such as sleep/wake cycles, cognition, and attention. This, therefore, has attracted a lot of interest in developing H₃R ligands as treatments for a variety of neurological disorders. Antagonists to the H₃R have been shown to reduce daytime sleepiness in patients with narcolepsy or Parkinson’s disease (Lin et al. 2008; Arnulf 2009; Iannone et al. 2010; Inocente et al. 2012). Positive results have been reported with one H₃R antagonist in a clinical study in patients with attention-deficit hyperactivity disorder (Schwartz 2009); however, others have not shown a benefit (Kuhne et al. 2011; Herring et al. 2012; Weisler et al. 2012). In addition to these neurological disorders, H₃R antagonists have also been studied in nasal allergen challenge models in humans where reductions in nasal symptoms, including itch, were observed (Stokes et al. 2012; Barchuk et al. 2013).

Expression of the H₃R in the nervous system suggests that it could be involved in the transmission of neuronal itch signals. However, systemic administration of a selective H₃R antagonist could not block histamine-induced itch in mice (Dunford et al. 2007). The story appears to be different when the antagonists are given locally. Scratching could be induced in mice by intradermal injection of iodophenpropit, clobenpropit, or thioperamide, H₃R antagonists that also have H₄R activity (Hossen et al. 2003; Sugimoto et al. 2004). The scratching induced by iodophenpropit and clobenpropit still occurred in H₁R-deficient mice and could not be inhibited by H₁R antagonists (Hossen et al. 2003, 2006). As further evidence for the role of the H₃R in this effect, the H₃R/H₄R agonist R-α-methylhistamine was able inhibit the scratching induced by thioperamide (Sugimoto et al. 2004). These previous studies employed compounds with both H₃R and H₄R activity, but the highly selective H₃R antagonist, pitolisant, has shown the same effect. Intradermal injection of pitolisant induced scratching in mice and this effect was completely blocked by the H₃R agonist, immethridine (Rossbach et al. 2011). Of particular interest is the fact that the scratching could also be partially inhibited by either the H₁R antagonist, cetirizine, or the H₄R antagonist, JNJ 7777120, with the combination completely inhibiting scratching. All three receptors were found to be expressed on TRPV1-positive dorsal root ganglion neurons, and in these neurons, pitolisant induced a calcium response similar to that of an H₁R or H₄R agonist (Rossbach et al. 2011). The mechanisms of the effects of the H₃R antagonist are not clear, but it is known that activation of the receptor can inhibit neuronal histamine release as well as the release of other neurotransmitters (Panula and Nuutinen 2013). Therefore, the H₃R may serve as an inhibitory receptor in the peripheral or central nervous system that dampens histamine signals. Antagonism of the receptor could lead to enhanced release of histamine or other neurotransmitters that either directly or indirectly activate itch pathways. Ligands for the receptor are still in clinical development and so there are limited tools to address this in humans. Kavanagh et al. (1998) studied intradermal injections of the H₃R agonist, R-α-methylhistamine, in humans and showed that it caused a weak wheal and flare response; however, no assessments of itch were included. One study has shown an effect of an H₃R antagonist on nasal itch in response to allergen challenge. Here the H₃R antagonist, JNJ 39220675, given orally, significantly reduced the itch component of the total nasal symptom score compared to placebo as was the total symptom score itself (Barchuk et al. 2013). Pseudoephedrine was also included in the study, and while it did have a significant reduction in the total nasal symptom score versus placebo, there was no effect on the itch component of the score. For the H₃R antagonist, the other three components of the total nasal symptom score were also reduced so it is difficult to determine whether the effects on itch are direct or indirect. While the hypothesis that the H₃R may be involved in itch in humans is intriguing, more clinical work is needed.

With the cloning of the H₃R, searches of genomic databases using the H₃R sequence uncovered a novel receptor that bound to histamine (O’Donnell et al. 2006). The discovery of histamine H₄ receptor (H₄R) was almost simultaneously reported by six different groups in 2000–2001 (Nakamura et al. 2000; Oda et al. 2000; Liu et al. 2001a; Morse et al. 2001; Nguyen et al. 2001; Zhu et al. 2001). After the identification of the new receptor, investigators searched the previous literature to find examples of histamine function that were not mediated by the H₁R, H₂R, or H₃R. The first example noted was an increase in calcium induced in eosinophils treated with histamine. Here, the rank order of potency of histamine receptor agonists did not match the pharmacology of the known histamine receptors (H₁R–H₃R), and it was speculated that this represented a novel receptor (Raible et al. 1994). Pharmacological characterization of the H₄R on eosinophils identified it as the receptor that mediated the calcium response (Buckland et al. 2003; Ling et al. 2004).

Much of the attention as to the physiological function of the H₄R has focused on its role in the immune response primarily driven by the original observations that the receptor was most highly expressed on immune cells. Anti-inflammatory activity of antagonists for the receptor has been noted in preclinical models of colitis, asthma, allergic rhinitis, and atopic dermatitis (Varga et al. 2005; Dunford et al. 2006; Seike and Furuya 2007; Deml et al. 2009; Takahashi et al. 2009; Cowden et al. 2010; Suwa et al. 2011; Ohsawa and Hirasawa 2012; Shiraishi et al. 2013). In addition to inflammatory conditions, the H₄R has also been suggested to mediate nervous system disorders, based on expression in the brain, spinal cord, and dorsal root ganglion although this data is controversial (Strakhova et al. 2009; Galeotti et al. 2013).

Preclinical studies suggest a role of the H₄R in itch. Histamine-induced itch in mice was shown to be reduced in mice that lack the H₄R and in ones that are treated with H₄R antagonists (Dunford et al. 2007; Yamaura et al. 2009; Shin et al. 2012). Further support of a role for the H₄R in mediating pruritus is given by the fact that agonists to the H₄R can induce scratching in mice when injected intradermally, although not when administered to H₄R-deficient mice (Bell et al. 2004; Dunford et al. 2007; Yu et al. 2010). These findings have now been validated in humans. A selective H₄R antagonist, JNJ 39758979, was shown to inhibit histamine-induced pruritus in healthy volunteers (Kollmeier et al. 2014). This was clearly attributable to the H₄R as the H₁R-mediated wheal and flare responses were no affected. These are the first clinical results showing the therapeutic utility of H₄R antagonists.

Antagonists to the H₄R have also been shown to inhibit itch induced by other pruritogens such as substance P or H₃R antagonists (Yamaura et al. 2009; Rossbach et al. 2011). The H₄R also appears to mediate itch driven by allergic responses to haptens. Acute application of fluorescein isothiocyanate to sensitized mice led to immediate scratching behavior followed by dermal inflammation. The H₄R antagonist, JNJ 7777120, was able to inhibit the scratching and the inflammation in this model (Cowden et al. 2010). JNJ 7777120 also blocked itch generated by the acute application of either toluene-2,4-disiocyanate or 2,4-dinitrochlorobenzene, but in this case, there was no effect on the inflammation (Rossbach et al. 2009). A role has also been observed in more chronic models. One study used application of 2,4,6-trinitrochlorobenzene three times a week over 99 days (Suwa et al. 2011). Starting on day 63, JNJ 7777120 was given orally once a day, and both the scratching and the clinical score of skin symptoms were inhibited, whereas the H₁R antagonist fexofenadine had no effect on either parameter (Suwa et al. 2011). Effects on inflammation were also noted in a similar study (Matsushita et al. 2012). When NC/Nga mice where sensitized and then challenged once a week for 10 weeks with picryl chloride, JNJ 7777120 reduced the scratching to a similar extent as the H₁R antagonist olopatadine (Ohsawa and Hirasawa 2012). However, this effect may be specific for hapten allergens; when a protein allergen was used to induce dermatitis in NC/Nga mice, the H₄R antagonists, JNJ 7777120 and JNJ 28307474, failed to inhibit scratching or reduce disease scores (Kamo et al. 2014). Similar results on lesion scores were found in a model where dogs were challenged with Dermatophagoides farinae (house dust mite), but scratching was not assessed (Baeumer et al. 2011).

Similar effects are seen when studies were conducted looking at ocular or nasal itch. Topical JNJ 7777120 was shown to be as effective as the H₁R antagonist, levocabastine, in reducing scratching associated with ocular application of histamine, but it did not impact the symptom score (Nakano et al. 2009). When allergen was used to induce the ocular symptoms, a trend toward inhibition of scratching was seen for both drugs, but did not reach statistical significance. In addition it was found that the H₄R agonist, 4-methylhistamine, could induce scratching and allergic-like symptom when given ocularly and this could be inhibited by JNJ 7777120, but not levocabastine (Nakano et al. 2009). Both JNJ 7777120 and levocabastine inhibited scratching and symptoms induced by the H₁R agonist, HTMT (Nakano et al. 2009). Of potential interest, alcaftadine, a topical antihistamine approved for allergic conjunctivitis, does exhibit some H₄R antagonist activity that may contribute to its efficacy (Namdar and Valdez 2011; Gallois-Bernos and Thurmond 2012). In an allergen-induced allergic rhinitis model in mice, JNJ 7777120 was able to suppress the sneezing and nasal rubbing to a similar extent as ketotifen when given intranasally or orally. Improvement in upper airway function has also been shown in another model of allergic rhinitis with either JNJ 7777120 or H₄R-deficient mice (Shiraishi et al. 2013).

One common finding in the preclinical models is that there may be a benefit of combining an H₄R antagonist with an H₁R antagonist. The combination of JNJ 7777120 and diphenhydramine completely eliminated histamine-induced itch in mice, and the residual scratching to histamine observed in H₄R-deficient mice could be completely eliminated by an H₁R antagonist (Dunford et al. 2007). The combination of an H₄R antagonist and an H₁R antagonist was also more effective in inhibiting itch induced by acute application of toluene-2,4-disiocyanate and fluorescein isothiocyanate, as well as chronic picryl chloride-induced itch (Rossbach et al. 2009; Cowden et al. 2010; Ohsawa and Hirasawa 2012). The combination was also more effective on the skin lesions in the chronic models (Matsushita et al. 2012; Ohsawa and Hirasawa 2012). In the allergen-induced ocular model, neither JNJ 7777120 nor levocabastine had a statistically significant effect on scratching or symptoms, but the combination of the two drugs was effective (Nakano et al. 2009). These data suggest that for both itch and inflammation, the combination of an H₁R antagonist with an H₄R antagonist may be more effective than either on its own.

The mechanism by which the H₄R drives pruritic responses is still unknown. In mice it was shown that H₄R-mediated scratching did not require mast cells and the scratching could not be restored by reconstituting H₄R-deficient mice with wild-type bone marrow (Dunford et al. 2007). These results show that in mice, the effect is not mediated by hematopoietic cells. The H₄R effects are most likely modulated via specific neuronal pathways as has been shown for the H₁R. In support of this hypothesis, there is some evidence, albeit controversial, that the H₄R is expressed in the spinal cord, dorsal root ganglia, and brain. Expression has been detected by RT-PCR in several regions of the human brain including the amygdala, cerebellum, corpus callosum, cortex frontal cortex, hippocampus, and thalamus (Strakhova et al. 2009). Immunohistochemistry has detected H₄R expression in laminae I–VI of the cerebral cortex with the strongest expression in layer IV (Connelly et al. 2009), although the selectivity of the antibody used has been questioned (Beermann et al. 2012). Expression has also been shown in the hippocampus, granule cell layer of the cerebellum, and the thalamus (Connelly et al. 2009). Of note, in a panel of human tissues including the CNS regions, heart, liver, and spleen, H₄R mRNA expression was highest in the spinal cord (Strakhova et al. 2009). Expression in mouse and rat brains appeared to be similar. In the rat, expression of the H₄R is prominent in dorsal root ganglion neurons of all size classes, with roughly one third of neurons showing H₄R immunoreactivity. H₄R immunoreactivity is also present in the spinal cord, in a pattern consistent with primary afferent terminal staining, in laminae II and IV and more intensely in laminae I–II (Strakhova et al. 2009). In whole cell clamp recordings of layer IV somatosensory cortex cells from mice, activation of the H₄R induced hyperpolarization in the majority of neurons (Connelly et al. 2009). In addition it has been shown that an H₄R antagonist can block the depolarization-induced firing of rat vestibular ganglion neurons (Desmadryl et al. 2012). The receptor also mediates action potential discharge in human submucous neurons (Breunig et al. 2007). In the mouse, skin-specific sensory neurons have been identified that express functional H₄R (Rossbach et al. 2011). Direct stimulation of neurons by compound 48/80 induces scratching in mice that is inhibited by an H₄R antagonist or in H₄R-deficient mice indicating that the H₄R is downstream of sensory fiber activation (Dunford et al. 2007). The scratching induced by compound 48/80 is not due to degranulation of mast cells since it still occurs in mast cell-deficient mice and it is known to activate sensory neurons directly (Eglezos et al. 1992; Dunford et al. 2007). Therefore, the current data suggest that itch stimuli in the skin, whether due to histamine or other pruritogens, lead to the activation of the H₄R on sensory neurons which then transmit the signal to the brain. Whether these pathways are similar to those mediated by the H₁R is currently unknown, but in the work of Rossbach et al. (2011), none of the mouse neurons tested responded to both an H₁R agonist and an H₄R agonist. The possible neuronal activity of the H₄R may also explain the efficacy of H₄R antagonists in both nociceptive and neuropathic pain models (Coruzzi et al. 2007; Altenbach et al. 2008; Cowart et al. 2008; Hsieh et al. 2010).