Histamine H3 receptors--general characterization and their function in the cardiovascular system.

The histamine H3 receptor was initially identified as a presynaptic autoreceptor controlling histamine release and synthesis in the brain. It belongs to the superfamily of G protein-coupled receptors. The existence of the H3 receptor which has not yet been cloned was definitely established by the design of highly potent and selective agonists (R-(-)-alpha-methylhistamine, imetit) and antagonists (thioperamide, clobenpropit). These receptors also occur as heteroreceptors both in the central nervous system and on peripheral neurons of the gastrointestinal and bronchial tract, where they regulate the release of a variety of neurotransmitters. In the cardiovascular system, histamine H3 receptors are mainly located presynaptically on the postganglionic sympathetic nerve fibers innervating the blood vessels and the heart. Their activation leads to the inhibition of noradrenaline release and consequently to the reduction of the neurogenic vasopressor and cardiostimulatory responses. The presence of such receptors has been shown both in vitro (human, pig, guinea-pig, rabbit, rat isolated tissues) and in vivo (rat, guinea-pig). The vascular and cardiac presynaptic H3 receptors may be activated by endogenous histamine. The vascular H3 receptors appear to be operative in hypertension and interact with presynaptic alpha 2-adrenoceptors. Postsynaptic vasodilatatory H3 receptors have been detected in several vascular beds as well. H3 receptor ligands affect basal cardiovascular parameters in conscious and anesthetized guinea-pigs but not rats. Presynaptic H3 receptors may play a role in the pathophysiology of headache and cardiac ischemia.