The gain of the baroreflex bradycardia is reduced by microinjection of NMDA receptor antagonists into the nucleus tractus solitarii of awake rats

The baroreflex activation with phenylephrine infusion produces a bradycardi c response. in the present study, the role of NMDA receptors in the nucleus tractus solitarii (NTS) in the processing of the parasympathetic component of the baroreflex was evaluated using acid phosphonivaleric (AP-5), a sele ctive NMDA receptor antagonist. Baroreflex activation was performed before and after bilateral microinjection of AP-5 into the intermediate commissura l NTS (0.5 mm lateral to the midline). Microinjection of the vehicle (salin e, 0.9%) or a dose of 2 nmol/50 nl of AP-5 into the NTS produced no effect on the gain of the baroreflex while a dose of 10 nmol/50 ni of AP-5 produce d a significant reduction in the gain of the baroreflex 2 min after microin jection [-1.43+/-0.22 vs. -0.43+/-0.03 bpm/mmHg, (n=6)], with a return to c ontrol levels 10 min after the microinjections. The dose of 10 nmol/50 nl w as selective for NMDA receptors considering that the cardiovascular respons es to microinjection of AMPA (0.05 pmol/50 nl), a non-Nh-IDA receptor agoni st, were not affected by this dose of AP-5 and the responses to microinject ion of NMDA (2 nmol/50 nl) were blocked. The data show that the bradycardic response to baroreflex activation was blocked by AP-5 microinjected into t he NTS, indicating that the neurotransmission of the parasympathetic compon ent of the baroreflex is mediated by NMDA receptors in the NTS. (C) 2000 El sevier Science B.V. All rights reserved.