Stimulation of α₂-adrenergic receptors in nucleus tractus solitarius is required for the baroreceptor reflex

Bilateral injection into the nucleus tractus solitarius (NTS) of the α₂-adrenergic receptor antagonist yohimbine produced a dose-related (10-500 pmol) increase in arterial pressure, with a maximal response of approximately 60 mm Hg. Idazoxan, also an α₂-adrenergic receptor antagonist, produced a similar response although idazoxan was less potent than yohimbine. The pressor response elicited by these drugs was attenuated by stimulation of adrenergic receptors in the NTS by local administration of either clonidine or tyramine. Doses of yohimbine (200 pmol) or idazoxan (5 nmol) that maximally increased arterial pressure also completely inhibited the depressor and bradycardic responses to electrical stimulation of the aortic depressor nerve. These results indicate that tonic stimulation of α₂-adrenergic receptors in the NTS is required for baroreceptor reflex function.