The mechanism of the vagal bradycardia evoked by diencephalic stimulation in the rabbit.
Blood pressure, in the rostral arterial system, was varied in the range 38-122 mm Hg by impeding the flow through the descending thoracic aorta in rabbits, anaesthetized with urethane and chloralose, in whom the spinal cord had been transected at mid-cervical level. Resting heart rate (H.R.) varied inversely with mean rostral arterial blood pressure (B.P.). The linear regression of H.R. on B.P. had a slope that averaged -0.38 beats X min-1 X mm Hg-1. Electrical stimulation in the lateral hypothalamic area, using 10 s trains of 300 microA X 1 ms constant current cathodal pulses at 60 Hz, caused H.R. to slow, provided that B.P. was above a threshold value and that the baroreceptor afferent pathway was intact. The mean threshold pressure was 49.5 mm Hg. Above threshold, the linear regression of the fall in H.R. during stimulation (delta H.R.) on B.P. had a slope that averaged -1.33 beats X min-1 X mm Hg-1. Linearity was good (r = 0.96). The results suggest that an inflow of baroreceptor impulses is essential for electrical stimulation of the cardioinhibitory region in the lateral hypothalamic area of the rabbit to cause bradycardia. This action is brought about by an increase in the gain of the vagal cardiodecelerator limb of the baroreceptor reflex.
D000818:Animals; D001794:Blood Pressure; D001919:Bradycardia / Q000503:physiopathology*; D001931:Brain Mapping; D001933:Brain Stem / Q000502:physiology; D004027:Diencephalon / Q000502:physiology*; D004558:Electric Stimulation; D005071:Evoked Potentials; D005260:Female; D006339:Heart Rate; D011311:Pressoreceptors / Q000502:physiology; D011817:Rabbits; D014630:Vagus Nerve / Q000503:physiopathology*