Regulatory influence of histamine receptor activation and inhibition on the synthesis and level of hypothalamic histamine.
The blockade of histamine receptors by repeated i.p. administration of 10-20 mg/kg of chloropyramine and tripelennamine, the potent H1-receptor antagonists, or by the i.c.v. administration of 2 mg/kg of metiamide and cimetidine, the highly selective H2-receptor antagonists, led to significant enhancement in the hypothalamic HD2(L-histidine carboxylase; EC 4.1.1.22.) activity and the histamine content; whereas the activation of the histamine H1-receptor by 4 mg/kg i.e.v. doses of 2-pyridylethylamine, the specific histamine H1 agonist, resulted in significant diminution in both the synthesis and level of this amine. These compounds either do not influence the hypothalamic HD in vitro or cause opposite effects in relatively high concentrations. After repeated administration of either agonists or antagonists, no significant alteration have been observed in the hypothalamic HNMT (histamine-N-methyl-transferase; EC 2.1.1.8.) activity. There were, however, two exceptions: 2 mg/kg i.c.v. doses of 2-methylhistamine and 4-methylhistamine produced remarkable inhibitions in the hypothalamic HNMT activity. These effects do not seem to correspond to the agonistic character of the compounds, but mask the indirect actions and create difficulties in the discovery of regulatory events. The regulatory influence which suppresses or stimulates the basal activity of HD under the activation or the inhibition of the functional state of histamine receptor, is assumed to be mediated through the cyclic AMP system.