Effects of histamine agonists and antagonists (H1 and H2) on ganglionic transmission and on accumulation of cyclic nucleotides (cAMP and cGMP) in rat superior cervical ganglion in vitro.

Histamine and 4-methylhistamine inhibited ganglionic transmission in the rat superior cervical ganglion in vitro via H2-histaminergic receptors. Under blockade of H2-receptors, 4-methylhistamine sometimes showed slight facilitation of ganglionic transmission, when repetitive stimuli were applied. The H1-receptor agonist, 2-pyridylethylamine, was ineffective. Histamine and 4-methylhistamine increased both cyclic AMP (cAMP) and cyclic GMP (cGMP) levels in concentrations depressing ganglionic transmission, whereas 2-pyridylethylamine increased only cAMP concentrations in the isolated ganglia. Histamine-induced accumulation of cyclic nucleotides was only partially prevented by either histamine--H1- and H2-receptor antagonists, but abolished by their combination. It is concluded that changes in intraganglionic cyclic nucleotides induced by histaminergic receptor agonists did not apparently correlate with their effect on ganglionic transmission in vitro.