1. Whole‐cell patch clamp recordings were made from twenty‐three sympathetic preganglionic neurones (SPNs) in the upper thoracic segments of a neonatal rat brainstem‐spinal cord preparation to study their synaptic responses to stimulation of the rostral ventrolateral medulla (RVLM) and the receptors involved. 2. SPNs were identified by their antidromic activation following stimulation of a ventral root, their morphology and their location in the spinal cord. 3. Electrical stimulation within 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 responded to RVLM stimulation with a constant‐latency fast EPSP that increased in size as the membrane was hyperpolarized. This EPSP was reduced in amplitude by the non‐NMDA receptor antagonist 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (10‐20 microM). 5. In thirteen SPNs the response to RVLM stimulation was a complex EPSP consisting of a fast EPSP and a slow EPSP that either followed or summed with the fast EPSP. The amplitude 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 microM). 6. Selective activation of neuronal cell bodies in the RVLM by chemical stimulation elicited slow depolarizations and increases in synaptic activity in SPNs. 7. These results provide evidence that an excitatory amino acid is involved in transmitting sympathoexcitatory drive from the RVLM, partly via a monosynaptic pathway. Both non‐NMDA and NMDA receptors play a role in mediating this drive.
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