Bradycardic and hypotensive responses to microinjection of L-glutamate into the lateral aspect of the commissural NTS are blocked by an NMDA receptorantagonist

Baroreflex activation by phenylephrine infusion produces a bradycardic resp onse while microinjection of L-glutamate into the most lateral aspect of th e commissural nucleus tractus solitarius (NTS, 0.8 mm lateral to the midlin e) produces bradycardic and hypotensive responses. In the present study we investigated the role of NMDA receptors in the lateral aspect of the commis sural NTS (0.8 mm lateral to the midline) in the bradycardic and hypotensiv e responses to microinjection of L-glutamate as well as in the processing o f the bradycardic response to the baroreflex activation. The hypotensive an d bradycardic responses to L-glutamate microinjection into the NTS were blo cked by methyl-atropine (intravenous, i.v.), indicating that the hypotensiv e response was secondary to the bradycardia. Microinjection of L-glutamate (1 nmol/50 nl) into the NTS was performed before and after microinjection o f increasing doses of phosphonovaleric acid (AP-5, a selective NMDA antagon ist) at the same site. The microinjection of AP-5 [0.5 (n = 9), 2.0 (n = 8) and 10.0 nmol/50 nl (n = 7)] into the;LTS (0.8 mm lateral to the midline) produced a dose-dependent blockade of the bradycardic and hypotensive respo nses to L-glutamate. In a specific group of mts the microinjection of 10 nm ol/50 nl of AP-5 produced a significant reduction in baroreflex sensitivity 2 min after microinjection into the lateral NTS [gain = -1.48 +/- 0.12 vs. -0.5 +/- 0.2 beats/mmHg, (n = 5)], which was reversible. The data show tha t the bradycardic responses produced by microinjection of L-glutamate into the most lateral aspect of the commissural NTS or by activation of the baro reflex were blocked by microinjection of AP-5, indicating that the neurotra nsmission of the parasympathetic component of the baroreflex in the neurons of the lateral aspect of the commissural NTS involves NMDA receptors. (C) 2000 Elsevier Science B.V. All rights reserved.