NMDA RECEPTOR ANTAGONIST BLOCKS THE BRADYCARDIAC BUT NOT THE PRESSOR-RESPONSE TO L-GLUTAMATE MICROINJECTED INTO THE NUCLEUS-TRACTUS-SOLITARIUS (NTS) OF UNANESTHETIZED RATS

In the present study we evaluated the role of NMDA receptors on the pr esser and bradycardic responses to L-glutamate (L-Glu) microinjected i nto the nucleus tractus solitarius (NTS) of unanesthetized rats. L-Glu (1 nmol/100 nl) was microinjected into the NTS before and 10 min afte r microinjection of phosponovaleric acid (AP-5), a selective NMDA rece ptor antagonist, into the NTS of three different groups of rats (0.5, 2.0 and 10.0 nmol/100 nl). Microinjection of AP 5 into the NTS produce d a dose-dependent reduction in the bradycardic response to L-Glu. How ever, no significant change in the presser response to L-Glu was obser ved. These results indicate that the activation of the cardiovagal com ponent (bradycardia) by L-Glu involves NMDA receptors and suggest that the activation of the sympatho-excitatory component (presser response ) by L-Glu in the commissural NTS is mediated by non-NMDA receptors. ( C) 1997 Elsevier Science B.V.