Central effects of alpha-adrenergic blocking agents on thermoregulation against venous blood-stream cooling in unanesthetized rabbits.

Abstract

The production or conservation of heat in the rabbit may be mediated in part by adrenergic neurones within thermoregulatory regions of the hypothalamus. The effect of intracerebroventricular injections of α-adrenergic receptor blocking drugs was therefore tested in unanesthetized rabbits subjected to body cooling. Cold water was passed 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 blood returning to the heart and produced a gradual reduction in body temperature. Rectal temperature, shivering, heat loss from the ears, and the rate of body heat extraction were measured.Injection of 50 μg phenoxybenzamine or 350 μg phentolamine into a lateral cerebral ventricle during venous-blood cooling increased the rate of fall in body temperature. Shivering was reduced or abolished, but there was no increase in heat loss from the ears which remained vasoconstricted. After the drug injection, the rabbit lost less heat to the cold water flowing through the cooling cuff. When rabbits were not subjected to blood-stream cooling, the same amount of phentolamine or phenoxybenzamine injected into the lateral ventricle caused vasodilatation and increased heat loss from the ears, and a fall in rectal temperature of 0.23–1.66 °C. These effects on shivering and ear heat loss were not observed following intravenous drug injections at the same dosage, or following intraventricular injections of the drug vehicle.It is concluded that in rabbits undergoing blood-stream cooling, the augmented rate of fall of rectal temperature following intraventricular injection of alpha-receptor blocking agents is caused by reduced heat production. This effect may be due to the blockade of adrenergic synapses in the central nervous system pathways mediating shivering. The vasodilatation in the ears and the fall in body temperature produced in rabbits not undergoing venous cooling indicates that central adrenergic synapses may also mediate the control of blood flow to the skin.