Studies on the cardiovascular actions of central histamine H1 and H2 receptors

Histamine administered i.c.v. to conscious freely moving rats results in dose-related pressor responses and bradycardia. The selective H1 agonist pyridylethylamine (PEA) and the H2 agonist impromidine (IMP) were utilized to characterize the central receptor subtypes involved in the central cardiovascular actions of histamine. Blood pressure and heart rate were monitored directly via indwelling carotid catheters, and drugs were administered i.c.v. through permanently implanted cannulas. Central administration of PEA or IMP produced dose-dependent pressor responses and bradycardia. The H1 antagonist chlorpheniramine blocked the pressor response, but not the bradycardia produced by PEA. In contrast, the H2 antagonist BMY-25405 blocked both the increase in blood pressure and the bradycardia induced by IMP. Crossover experiments were carried out to examine the specificity of the antagonists' actions. Chlorpheniramine failed to block the actions of IMP and BMY-25405 likewise was ineffective in blocking the actions of PEA. These results suggest that both central H1 and H2 receptors mediate the central pressor effects of histamine, and that PEA and IMP are selectively acting at their respective receptors in the brain. The heart rate effects appear to be mediated directly by central H2 actions, as BMY-25405 was capable of blocking the heart rate changes. The finding that IMP is less potent in the central nervous system, compared to its known greater potency in the periphery, suggests that peripheral and central H2 receptors may be pharmacologically distinct.