The commentary by Seifert et al. (Seifert et al., 2011) provides a very important discussion on the pharmacology of the histamine H4 receptor (H4R) and summarizes recent work indicating that our understanding of the receptor is incomplete. Most importantly it has been shown that the receptor, as for many GPCRs, can exhibit functional selectivity (Kenakin, 2011; Rosethorne and Charlton, 2011). This is a very important point with regard to applying pharmacological tools to understand receptor function. Indeed it underscores a very important principle that is largely ignored – data from a single ligand should never be used to make overall conclusions about the function of the receptor it targets. The reason for this is that despite our best efforts it is almost impossible to completely understand the pharmacology of any ligand and how it may differ in different species, cell types or conditions. Because of these factors, researchers must use a combination of tools including agonist/antagonist pairs, ligands of from different chemical classes, and/or knock-out animals before making conclusions about the function of receptor. The use of a single ligand only gives firm conclusions about the action of that ligand under the given experimental conditions.

However, Seifert et al. overstate the case concerning the H4R tool compound, JNJ 7777120. JNJ 7777120 was described as a potent and selective antagonist of the H4R receptor and has been widely adopted as the standard antagonist in the field. However as Seifert et al. summarize, recent data in artificial transfected systems has uncovered other pharmacology of the ligand at the receptor besides antagonism including functional selectivity and even agonism. This led Seifert et al. to suggest that many of the activities attributed to H4R antagonism by JNJ 7777120 may in fact be something else. However, while these other pharmacological effects may exist, most of the current data support the fact that JNJ 7777120 is an antagonist in vivo and in primary cells. Perhaps one of the best examples of this is in the role of the receptor in mediating pruritic responses in mice. Dunford et al. (Dunford et al., 2007) reported that histamine- induced scratching in mice could be blocked by JNJ 77777120 and did not occur in H4R-deficient mice. Furthermore, other H4R agonists also induce scratching that can be blocked by JNJ 7777120, but cannot induce scratching in H4R-deficient mice (Dunford et al., 2007; Yu et al., 2010). Other antagonists also block this response (Cowart et al., 2008; Koenig et al., 2010). The data then clearly support the role on JNJ 77777120 as an antagonist for this effect. The compound reverses the effect of agonists and mimics the findings in animal lacking the H4R. However, this cannot be generalized. For example JNJ 7777120 also has been shown to block substance P induced itch (Yamaura et al., 2009). While is it appropriate to speculate that H4R activation is involved in substance P induced itch based on data in other models, it would take studies in H4R-deficeint mice or with other structurally distinct antagonist to firmly conclude this.

Similar conclusions arise when looking at the effects on JNJ 7777120 in models of asthma and dermatitis, where the results are consistent between JNJ 77777120, other structurally distinct ligands and H4R- deficient mice (Cowden et al., 2010; Dunford et al., 2006). Despite of this Seifert et al. use the mouse asthma data as an example of how it is difficult to interpret the pharmacology of the H4R. Once again the data do not completely support this. JNJ 7777120 shows anti-inflammatory activities in mouse models of asthma that are mimicked by H4R-deficeint mice (Dunford et al., 2006). Furthermore, other described H4R antagonists show the same effects (Afton et al., 2010; Dunford et al., 2006). The only confusing data is that of Morgan et al. (Morgan et al., 2007) that shows that an inhaled H4R agonist, 4-methylhistamine, is anti-inflammatory. This more likely reflects differences in systemic versus local delivery or the fact that 4-methylhistamine is also an H2R agonist as pointed out by Seifert et al. rather than supporting the conclusion that H4R agonism is anti-inflammatory.

Some of the in vivo data also yield insight as to the role of functional selectivity at the H4R. In the models of pruritus, asthma and dermatitis the phenotype is the same between JNJ 7777120-treated and H4R- deficient mice (Cowden et al., 2010; Dunford et al., 2006; Dunford et al., 2007). This suggests that in if the induction of beta-arrestin shown by Rosethorne and Charlton occurs in mice, it has little impact on the activity of the compound in these models. If JNJ 7777120-induced arrestin pathways are anti-inflammatory then they would not exist in the receptor deficient animals. Once again the most likely explanation is that the compound functions as an antagonist of the receptor. This does not mean, however, that the possibility of functional selectivity should be dismissed and it may suggest that not all antagonists are created equal if they block only a subset of the receptor pathways.

In conclusion, although in some reporter systems JNJ 7777120 can act as an agonist or exhibit functional selectivity, the bulk of the current data in vivo and in primary cell lines indicate that it functions as an antagonist and that the anti-inflammatory, anti-nociceptive and anti- pruritic activities exhibited by JNJ 77777120 are clearly due to antagonism of the H4R. This does not mean that the one should ignore the findings of Seifert et al. and Rosethorne and Charlton, indeed the opposite is true, however these findings need to be confirmed in primary cell systems. As with any receptor system findings with a single ligand, including JNJ 7777120, should be validated with receptor-deficient animals or with a structurally distinct compounds before conclusions about receptor function can be made. Therefore, while I agree with the statement by Seifert et al. that “…great caution must be exerted when interpreting effects of JNJ7777120 as H4R antagonism�, the current data clearly do not support their conclusion that there is no “standard� H4R antagonist available.

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