Impaired drinking response in histamine H3 receptor knockout mice following dehydration or angiotensin-II challenge.

Histamine H3 receptors (H3Rs) are presynaptic receptors that negatively regulate the release of histamine. The present study examined the physiological role of H3Rs in drinking behavior. In water-replete rats, intracerebroventricular (i.c.v.) administration of R-alpha-methylhistamine (RalphaMeHA), an H3R agonist, elicited drinking behavior. In contrast, i.c.v. administration of thioperamide, an H3R inverse agonist, significantly attenuated the drinking behavior elicited by either overnight dehydration or i.c.v. administration of angiotensin-II (AT-II). Inhibition of histamine release with alpha-fluoromethylhistidine, an inhibitor of histidine decarboxylase, did not elicit drinking behavior. Moreover, the inhibitory effects of thioperamide on drinking behavior in water-depleted rats were not mimicked by i.c.v. administration of histamine. These results suggest that the predominant effects of H3Rs on drinking behavior are not mediated by the modulation of histamine release. In H3R-deficient (H3RKO) mice, drinking behavior induced by overnight dehydration or i.c.v. administration of AT-II was significantly impaired compared to wild type mice. Collectively, these observations suggest that brain H3Rs play a pivotal role in drinking behavior in response to dehydration and AT-II, and these effects may be largely independent of the modulation of histaminergic tone.