NON-NMDA RECEPTORS TRANSMIT CARDIOPULMONARY C FIBER INPUT IN NUCLEUS-TRACTUS-SOLITARII IN RATS

1. We sought first to determine whether neurones in caudomedial aspect s of commissural nucleus tractus solitarii (NTS) received input from c ardio pulmonary C fibre endings supplied by the pulmonary versus syste mic circulation. We then examined the role of N-methyl-D-aspartate (NM DA) and non-NMDA receptors in transmitting cardiopulmonary C fibre inp ut to such NTS neurones. 2. Extracellular NTS unit activity, phrenic n erve activity and arterial blood pressure were recorded in urethane-an aesthetized rats. Unit responses to right atrial and left ventricular phenylbiguanide injections were compared in rats with arterial barorec eptors, carotid chemoreceptors and subdiaphragmatic vagal inputs elimi nated. Right atrial phenylbiguanide injections produced greater peak r esponses (27 +/- 11 impulses s(-1)) than did left ventricular injectio ns(11 +/- 3 impulses s(-1)) (n = 9). 3. The non-NMDA receptor agonist quisqualic acid (QUIS) and NMDA were ionophoresed onto NTS neurones th at were synaptically activated by right atrial phenylbiguanide injecti on. Responses were compared before and during ionophoresis of the non- NMDA receptor antagonist -dihydroxy-6-nitro-7-sulphamoylbenzo(f)quinox aline (NBQX), the NMDA receptor antagonist DL-2-amino-5-phosphovaleric acid (AP5), or the broad spectrum antagonist kynurenic acid (KYN). 4. NBQX, which blocked QUIS- but spared NMDA-evoked responses, significa ntly attenuated synaptic activation by 65% (n = 9). AP5, which blocked NMDA- but spared QUIS-evoked responses, did not significantly diminis h synaptic activation (11%; n = 7). KYN, which blocked QUIS- and NMDA- evoked responses, decreased synaptic activation by 70% (n = 9). 5. The results suggest that input from cardiopulmonary C fibre endings, prim arily supplied by the pulmonary circulation, is transmitted to this co mmissural NTS region largely via non-NMDA receptors.