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 micron diameter) were implanted stereotaxically in the hypothalamus. Baroreflex-induced bradycardia was elicited by phenylephrine (PE) injection (8-20 micrograms/kg). Responses to stimulation (STIM) (50-150 microA, 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 less than 0.05). However, STIM + PE resulted in a lower heart rate compared to PE (194 +/- 22 22 vs 270 +/- 17 bpm; P less than 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 less than 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.
D000818:Animals; D007025:Anterior Hypothalamic Nucleus / Q000502:physiology; D001794:Blood Pressure; D001931:Brain Mapping; D006339:Heart Rate; D007031:Hypothalamus / Q000502:physiology*; D008297:Male; D010275:Parasympathetic Nervous System / Q000502:physiology; D011311:Pressoreceptors / Q000502:physiology*; D051381:Rats; D011919:Rats, Inbred Strains; D012018:Reflex / Q000502:physiology*; D014697:Ventromedial Hypothalamic Nucleus / Q000502:physiology