Presynaptic H3 autoreceptors modulate histamine synthesis through cAMP pathway.

Histamine H3 receptors modulate histamine synthesis, although little is known about the transduction mechanisms involved. To investigate this issue, we have used a preparation of rat brain cortical miniprisms in which histamine synthesis can be modulated by depolarization and by H3 receptor ligands. When the miniprisms were incubated in presence of forskolin, dibutyryl-cAMP, or 3-isobutyl-1-methylxanthine (IBMX), histamine synthesis was stimulated in 34, 29, and 47%, respectively. These stimulations could be prevented by the selective cAMP protein kinase blocker Rp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPs). Preincubation with the H3 receptor agonist imetit prevented IBMX- (100% blockade) and forskolin- (70% blockade) induced stimulation of histamine synthesis. The H3 inverse agonist thioperamide enhanced histamine synthesis in the presence of 1 mM IBMX or 30 mM potassium (+47 and +45%, respectively). Similarly, the H3 antagonist clobenpropit enhanced histamine synthesis in the presence of 30 mM potassium (+ 59%). The cAMP-dependent protein kinase blockers Rp-cAMPs and PKI14-22 could impair the effects of thioperamide and clobenpropit, respectively. These results indicate that the adenylate cyclase-protein kinase A pathway is involved in the modulation of histamine synthesis by H3 autoreceptors present in histaminergic nerve terminals.