Facilitation of baroreflex-induced bradycardia by stimulation of specific hypothalamic sites in the rat

Hypothalamic stimulation generally inhibits baroreflex-induced bradycardia. However, we have noted discrete areas of the rat hypothalamus which facilitate reflex bradycardia. The effects of hypothalamic stimulation on baroreflex-induced changes in heart rate were investigated in urethane-anesthetized rats (1.2 g/kg, i.p.; n = 6) instrumented with femoral arterial and venous catheters. Bipolar electrodes (250 μm diameter) were implanted stereotaxically in the hypothalamus. Baroreflex-induced bradycardia was elicited by phenylephrine (PE) injection (8-20 μg/kg). Responses to stimulation (STIM) (50-150 μA, 80 Hz, 0.5 ms), PE, and Stim + PE were studied for 1 min. In the ventral medial and anterior hypothalamus. STIM caused transient increases in blood pressure and no changes in heart rate. Peak blood pressure was lower during STIM + PE than during PE (144 ± 5 vs 164 ± 3 mm Hg; P < 0.05). However, STIM + PE resulted in a lower heart rate compared to PE (194 ± 22 vs 270 ± 17 bpm; P < 0.05). At 1 min, the heart rate in STIM + PE rats remained lower than in PE rats (205 ± 37 vs 319 ± 16 bpm; P < 0.05). Atropine administration indicated that the facilitation was primarily parasympathetic in nature. These results identify specific hypothalamic regions which facilitate baroreflex-induced bradycardia by parasympathetic mechanisms.