Central metabolic and pituitary-adrenocortical stimulatory action of histamine and clonidine.

This review presents our new findings regarding the centrally-induced effects of histamine and clonidine. We have found for the first time that histamine administered intracerebroventricularly (icv) induces a dose-related increase in serum free fatty acids (FFA) in conscious rats. Both H1 and H2 receptors participate in this stimulation. Histamine interacts with central alpha 1 and beta-adrenoceptors and with cholinergic muscarinic receptors when inducing hyperlipemic response in non-stressed rats. Clonidine given icv induces also hyperlipemia which, as shown by us, is elicited by a central alpha 2-adrenergic mechanism. In hypothermia caused in rats by clonidine not only already known central alpha-adrenergic but also an H2-histaminergic mechanism participates to an equal extent. We have found for the first time that in conscious rats both under normal and stress conditions not only central histamine H1- but also H2-receptors mediate the stimulation of the pituitary-adrenocortical response measured indirectly through corticosterone secretion. In our study evidence has for the first time been obtained that in non-stressed rats brain histaminergic mechanism interacts with alpha 1, alpha 2- and beta-adrenoceptors and with cholinergic muscarinic receptors when stimulating the pituitary-adrenocortical response. By contrast, in stressed animals central histamine H1- and H2-receptors interact with alpha 1- and alpha 2- but not beta-adrenergic and cholinergic muscarinic receptors when increasing the corticosterone response. We have also demonstrated that in contrast to a known inhibitory action on adrenocortical secretion in anesthetized dogs, clonidine given icv increases the corticosterone response in both non-stressed and stressed rats by stimulating alpha-adrenoceptors. In stressed animals this effect of clonidine is also mediated, to an equal extent, by H2-receptor mechanism. Our data strongly suggest that in some central effects clonidine affects both alpha 2 and H2 receptor mechanism. This challenges the view that clonidine is a selective alpha-adrenergic agonist.