Effects of intracerebroventricular injection of clonidine on metabolic, respiratory, vasomotor and temperature responses in the rabbit.

1. The thermoregulatory outputs (including metabolic, respiratory and vasomotor activities) produced by an injection of clonidine or 5-hydroxytryptamine (5-HT) into the third cerebral ventricle of conscious rabbits were assessed at three different ambient temperatures (Ta) of 2, 22 and 32 degrees C. 2. When injected into the third cerebral ventricle, both clonidine and 5-HT produced a dose-dependent hypothermia in rabbits at both 2 and 22 degrees C Ta. The hypothermia was due to a decrease in metabolic heat production (M) at 2 degrees C Ta, while at 22 degrees C Ta the hypothermia was due to cutaneous vasodilatation. There were no changes in respiratory evaporative heat loss. 3. Furthermore, the clonidine-induced hypothermia was greatly reduced by pretreatment of the animals with either 5,6-dihydroxytryptamine (impairment of central 5-HT pathways) or yohimbine (alpha-adrenergic blocking agent), but not by 6-hydroxydopamine (impairment of central catecholamine pathways). 4. The results indicate that clonidine may act on the alpha-adrenergic receptors located on central 5-HT pathways to produce a hypothermic action by promoting a decrease in heat production or an increase in heat loss in the rabbit.