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 bradycardic 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 selective NMDA receptor antagonist. Baroreflex activation was performed before and after bilateral microinjection of AP-5 into the intermediate commissural NTS (0.5 mm lateral to the midline). Microinjection of the vehicle (saline, 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 nl of AP-5 produced a significant reduction in the gain of the baroreflex 2 min after microinjection [-1.43+/-0.22 vs. -0. 43+/-0.03 bpm/mmHg, (n=6)], with a return to control levels 10 min after the microinjections. The dose of 10 nmol/50 nl was selective for NMDA receptors considering that the cardiovascular responses to microinjection of AMPA (0.05 pmol/50 nl), a non-NMDA receptor agonist, were not affected by this dose of AP-5 and the responses to microinjection 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 the NTS, indicating that the neurotransmission of the parasympathetic component of the baroreflex is mediated by NMDA receptors in the NTS.