Role of histamine H(3) receptors in the control of gastrointestinal motility. An overview.

Over the last few years, the biochemical and functional characterization of H3 receptors has been a matter for extensive investigation, culminating in the cloning of the human, guinea pig and rat receptor protein from brain tissues. This discovery contributed to determine the distribution of receptors in the body and to define the molecular mechanisms which follow activation. The major breakthrough in the histamine H3 receptor field came with the synthesis of selective and potent agonists and antagonists, which unravelled the function of this receptor subtype in the different tissues. As expected from the ubiquitous location of histamine in the body, histamine H3 receptors have also been identified in virtually every tissue, although they are quantitatively less abundant than H1 and H2 receptors. Concerning the gastrointestinal tract, this new receptor subtype seems to have multiple cellular locations, which include neurons, enteric ganglia, paracrine and immune cells and, in some tissues, also smooth muscle cells. Therefore it might be regarded as a general regulatory system of different digestive functions, including motility. The effects mediated by histamine H3-receptors mainly reflect the presynaptic inhibition of the release of either excitatory or inhibitory neurotransmitters from the myenteric plexus. The molecular mechanism of presynaptic inhibition seems to involve a restriction of calcium entry into the nerve endings, but other mechanisms (reduction of cAMP), possibly associated to different H3 receptor subtypes, may be involved. Despite the widespread distribution and the well defined inhibitory effects evoked in the majority of in vitro models of intestinal motility, no clear cut evidence of its involvement in the control of peristalsis could be provided. In vivo models of gastrointestinal transit, indeed, did not reveal a defined effect of histamine H3 receptor ligands, even though the possibility of a central inhibition was pointed out in several studies. Therefore, it is not clear at the present what is the physiological meaning of the histamine H3 receptor in the control of gastrointestinal motility and whether it could represent a potential target for novel therapeutic interventions in deranged motility, taking into account that human gastrointestinal tissues are apparently devoid of this receptor.