BILATERALLY EVOKED MONOSYNAPTIC EPSPS, NMDA RECEPTORS AND POTENTIATION IN RAT SYMPATHETIC PREGANGLIONIC NEURONS IN-VITRO

1. Whole-cell patch clamp and intracellular recordings were obtained f rom 190 sympathetic preganglionic neurones (SPNs) in spinal cord slice s of neonatal rats. Fifty-two of these SPNs were identified histologic ally as innervating the superior cervical ganglion (SCG) by the presen ce of Lucifer Yellow introduced from the patch pipette and the appeara nce of retrograde labelling following the injection of rhodamine-dextr an-lysine into the SCG. 2. Electrical stimulation of the ipsilateral ( n = 71) or contralateral (n = 32) lateral funiculi (iLF and cLF, respe ctively), contralateral intermediolateral nucleus (cIML, n = 41) or ip silateral dorsal horn (DH, n = 34) evoked EPSPs or EPSCs that showed a constant latency and rise time, graded response to increased stimulus intensity, and no failures, suggesting a monosynaptic origin. 3. In a ll neurones tested (n = 60), fast rising and decaying components of EP SPs or EPSCs evoked from the iLF, cLF, cIML and DH in response to low- frequency stimulation (0.03-0.1 Hz) were sensitive to non-NMDA recepto r antagonists. 4. In approximately 50 % of neurones tested (n = 29 of 60), EPSPs and EPSCs evoked from the iLF, cLF, cIML and DH during low- frequency stimulation were reduced by NMDA receptor antagonists. In th e remaining neurones, an NMDA receptor antagonist-sensitive EPSP or EP SC was revealed only in magnesium-free bathing medium, or following hi gh-frequency stimulation. 5. EPSPs evoked by stimulation of the iLF ex hibited a sustained potentiation of the peak amplitude (25.3 +/- 11.4% ) in six of fourteen SPNs tested following a brief high-frequency stim ulus (10-20 Hz, 0.1-2 s). 6. These results indicate that SPNs, includi ng SPNs innervating the SCG, receive monosynaptic connections from bot h sides of the spinal cord. The neurotransmitter mediating transmissio n in some of the pathways activated by stimulation of iLF, cLF, cIML a nd DH is glutamate acting via both NMDA and non-NMDA receptors. Synapt ic plasticity is a feature of glutamatergic transmission in some SPNs where EPSPs are potentiated following a brief high-frequency stimulus. Our data also suggest a differential expression of NMDA receptors by these neurones.