Histaminergic H1 and H2 receptors located within the ventromedial hypothalamus regulate food and water intake in rats

The aim of the present study was to investigate the effect of the pharmacological blockade of histamine H1 and H2 receptors located within the ventromedial hypothalamus (VMH) on overnight food and water intake and on water intake elicited by two physiological stimuli: hyperosmolarity induced by an acute intragastric salt load and water deprivation. During the overnight period, the pharmacological blockade of both H1 and H2 VMH receptors significantly increased food intake and decreased water intake. In hyperosmotic rats, the blockade of H1 VMH receptors reduced water intake, while the blockade of H2 receptors in this same region yielded no significant effect. Additionally, in water-deprived rats, the blockade of both H1 and H2 receptors located within the VMH induced a significant decrease in water intake. The inhibitory effects on drinking behavior observed in this study do not seem to be a consequence of any "illness-inducing" effect provoked by the central administration of the antihistaminergic agents employed here, because an aversion test indicated that the injection of those compounds into the VMH does not induce any "illness-like" effect. In addition, the central administration of either mepyramine or cimetidine to dehydrated and hyperosmotic rats did not produce any reduction in locomotor activity measured in an open-field arena. Injections of the antihistaminergic agents used here into the regions that circumscribe the VMH produced no significant effects on water or food intake, indicating that the actions observed here may be specifically attributed to the set of histaminergic receptors situated within the VMH.