Histamine H1- and H2-receptors in the central thermoregulatory pathways of the rat.

The effects of pyrilamine and burimamide (histamine H1- and H2-receptor blockers, respectively) on the hypothermic response to histidine loading have been studied in rats. The antagonists were injected into one of three sites in the brain: the preoptic/anterior hypothalamic nuclei, the lateral ventricle, or the third ventricle. Only injections of burimamide into the third ventricle blocked the hypothermic response to systemic injection of histidine. Behavioral studies revealed that, after histidine, the animals increased their time of exposure to a heat lamp, which suggests that efferent heat loss pathways had been activated. Previous studies have shown that H1-receptor antagonists will block the hypothermic effect of histamine injected into the rostral hypothalamus. These data suggest that there are, at least, two sets of histaminergic receptors on the neuronal pathway through the hypothalamus: H1-receptors in the rostral hypothalamus and H2-receptors on neurons lying close to the wall of the third ventricle. Activation of this thermoregulatory pathway leads to increased heat loss and a fall in body temperature.