An Effect of Hypothalamic Stimulation on Cardiac Output in the Rabbit

Abstract

Electrical stimulation of a discrete region of the rabbit brainstem, in the lateral hypothalamic area near the mammillothalamic tract, caused an increase in arterial blood pressure accompanied by bradycardia in anaesthetized animals. Stimulation of the brainstem of the anaesthetized rabbit, in a small region of the lateral hypothalamic area dorsal to the lateral mammillary nucleus and near to the mammillothalamic tract 1-2mm from the midline, can evoke peripheral vasoconstriction with rises in arterial blood pressure and bradycardia. During strong stimulation the heart rate may fall to 20-30 % of the resting rate. These cardiovascular responses may be accompanied by signs of arousal, pupillodilatation, exophthalmos and a transient apnoea or shallow tachypnoea superimposed on a tonic inspiratory effort [Evans, 1976[Evans, , 1980Evans and Pepler, 1974].Although the location of this responsive point in the rabbit hypothalamus bears some resemblence to the caudal extension of the so-called 'defence area' of the cat [Abrahams, Hilton and Zbrozyna, 1960; Hess and Briigger, 1943] and although several of the effects mentioned above have also been obtained by stimulation of the 'defence area' in the cat, the chronotropic responses of the heart are qualitatively opposite in these two species. Stimulation of the 'defence area' in the anaesthetized cat evokes tachycardia [Eliasson, Folkow, Lindgren and Uvnas, 1951] during which the baroreflex cardioinhibition that would have accompanied the rise in blood pressure is inhibited by influences from the hypothalamus