Hypotensive actions of microinjections of angiotensin II into the dorsal motor nucleus of the vagus.

Angiotensin II (ANG II) was injected via double-barrelled micropipettes into brain stem nuclei of 83 chloralose anaesthetized rats at doses between 8 fmol and 250 pmol. Femtomole injections of ANG II confined to the dorsal motor nucleus of the vagus (dmnX) and the ventral portion of the medial nucleus tractus solitarius (NTS) produced falls in blood pressure (BP) and heart rate (HR) of 28 and 12%, respectively, which were significantly greater than those produced by vehicle injections at these same sites (9 and 6% for BP and HR, respectively). ANG II injections into either the medial NTS or the hypoglossal nucleus were not different from vehicle. Prior administration of methylatropine (intraperitoneally) or bilateral cervical vagotomy significantly attenuated the bradycardia without affecting the fall in blood pressure. These data provide the first evidence for a direct action of low doses of ANG II within discrete nuclei of the dorsal medulla. The findings suggest that activation of the parasympathetic nervous system primarily accounts for the bradycardia. However, inhibition of sympathetic nervous system activity may contribute to the hypotension. Additionally, these data are consistent with a neurotransmitter or neuromodulator role for ANG II in the brain stem of rats.