CENTRAL EFFECTS OF CHOLINERGIC-RECEPTOR BLOCKING DRUGS ON THE CONSCIOUS RABBIT'S THERMOREGULATION AGAINST BODY COOLING

¹ Division of Medical Physiology, Faculty of Medicine, The University of Calgary, Calgary, Alberta, Canada

The production and conservation of body heat in the rabbit may depend on the activity of cholinergic neurones within thermoregulatory regions of the brain stem. Cholinergicreceptor blocking drugs were therefore injected into the cerebral ventricles of conscious rabbits when the latter exhibited cutaneous vasoconstriction and intense shivering to oppose body cooling. Moderate hypothermia without anesthesia was achieved by passing ice-cold water through a metal water-jacketed cuff which was chronically implanted to enclose a section of the abdominal vena cava. The cold water cooled the venous return to the heart, thereby producing a gradual decrease in body temperature. Shivering, oxygen consumption, the rate of body heat removal and ear-skin and rectal temperature were measured. Intraventricular injection of atropine (100-500 µg), mecamylamine or hexamethonium (500 or 1000 µg) during bloodstream cooling augmented the rate of fall in rectal temperature, and reduced the shivering and O₂ consumption, but caused no change in ear-skin temperature. When rabbits were not subjected to bloodstream cooling and were maintained in an ambient temperature of 18.5-23.5°C, intraventricular injection of atropine or hexamethonium caused a spontaneous fall in rectal temperature associated with panting, and vasodilatation in the ears. Mecamylamine was without these effects. The interference with thermoregulation against body cooling may have resulted from blockade of cholinergic synapses which drive heat production effectors such as those for shivering. The activation of heat-loss mechanisms under thermoneutral conditions suggests that central cholinergic neurons control the activity of heat-loss effectors. The results support the central neurotransmitter model of Bligh et al. [J. Physiol. (London) 212: 377-392, 1971] for thermoregulation in the goat, sheep and rabbit.