NMDA RECEPTORS IN NTS ARE INVOLVED IN BRADYCARDIAC BUT NOT IN PRESSOR-RESPONSE OF CHEMOREFLEX

Activation of carotid chemoreceptors with intravenous potassium cyanid e (KCN) produces increases in arterial pressure, bradycardia, and tach ypnea. In the present study, we activated carotid chemoreceptors with KCN and the neurotransmission of the chemoreceptor reflex into the com missural nucleus tractus solitarii (NTS) was blocked with phosphonoval eric acid (AP-5), an N-methyl-D-aspartate (NMDA)-selective antagonist. The aim of this study was to evaluate the involvement of NMDA recepto rs in the cardiovascular and respiratory responses produced by chemore ceptor activation in unanesthetized rats. The presser response to KCN was not changed after microinjection of three different doses of AP-5 into the NTS, whereas the bradycardic response was reduced in a dose-d ependent manner. The increase in respiratory frequency in response to carotid chemoreceptor activation was also not affected by AP-5 microin jected into the NTS. The data indicate that the activation of the card iovagal component of the chemoreflex in the commissural NTS is mediate d by NMDA receptors, whereas presser and ventilatory responses are not .