MEDULLARY-EVOKED EPSPS IN NEONATAL RAT SYMPATHETIC PREGANGLIONIC NEURONS IN-VITRO

1. Whole-cell patch clamp recordings were made from twenty-three sympa thetic preganglionic neurones (SPNs) in the upper thoracic segments of a neonatal rat brainstem-spinal cord preparation to study their synap tic responses to stimulation of the rostral ventrolateral medulla (RVL M) and the receptors involved. 2. SPNs were identified by their antidr omic activation following stimulation of a ventral root, their morphol ogy and their location in the spinal cord. 3. Electrical stimulation w ithin the RVLM elicited EPSPs in all SPNs tested (n = 23). These EPSPs consisted of one or more components that had different time courses, voltage relationships and pharmacological sensitivities. 4. All SPNs r esponded to RVLM stimulation with a constant-latency fast EPSP that in creased in size as the membrane was hyperpolarized. This EPSP was redu ced in amplitude by the non-NMDA receptor antagonist 6-cyano-7-nitroqu inoxaline-2,3-dione (10-20 mu M). 5. In thirteen SPNs the response to RVLM stimulation was a complex EPSP consisting of a fast EPSP and a sl ow EPSP that either followed or summed with the fast EPSP. The amplitu de of the slow EPSP was (i) either reduced in size or not affected as the membrane was hyperpolarized, and (ii) reduced by the NMDA receptor antagonist, D,L-2-amino-5-phosphonovaleric acid (50 mu M). 6. Selecti ve activation of neuronal cell bodies in the RVLM by chemical stimulat ion elicited slow depolarizations and increases in synaptic activity i n SPNs. 7. These results provide evidence that an excitatory amino aci d is involved in transmitting sympathoexcitatory drive from the RVLM, partly via a monosynaptic pathway. Both non-NMDA and NMDA receptors pl ay a role in mediating this drive.