Bradycardia evoked by hypothalamic stimulation in the rabbit: dependence upon the arterial blood pressure

The spinal cord was transected in adult New Zealand White rabbits anaesthetized with urethane plus chloralose. The level of transection was in the mid-cervical region. The animals were than ventilated mechanically and the arterial blood pressure was maintained with an intravenous infusion of noradrenaline solution. Stimulation of the hypothalamus 1-2 mm lateral to the third ventricle and 1.5-3 mm dorsal to the mammillary nuclei, in a region known to evoke pressor responses and bradycardia in normal anaesthetized rabbits, never evoked pressor responses in the spinally transected rabbits. Bradycardia was evoked only when the mean arterial blood pressure was maintained above 44-49 mmHg. At higher pressures stimulation evoked a greater bradycardia and the relationship between bradycardia and pressure was approximately linear over much of the range of pressures tested (up to 116 mmHg). Because the threshold mean arterial blood pressure at which hypothalamic stimulation evoked bradycardia was similar to the threshold pressures reported in the literature for baroreceptor activation in the rabbit and because the curve of bradycardia:pressure was similar to published curves of baroreceptor and baroreflex activity against blood pressure, it is concluded that the bradycardia evoked by hypothalamic stimulation in the rabbit is mediated by a neural pathway in the hypothalamus that can increase the gain of the cardio-inhibitory baroreceptor reflex.