A STUDY OF SYMPATHETIC PREGANGLIONIC NEURONAL-ACTIVITY IN A NEONATAL RAT BRAIN-STEM-SPINAL CORD PREPARATION

Extracellular recordings were made from 46 sympathetic preganglionic n eurones (SPNs) in a neonatal rat brainstem-spinal cord preparation. Ne urones were identified as SPNs as they were: (i) activated at constant latencies (2-10 ms) following stimulation of the ventral root, which indicated antidromic activation and (ii) recorded at sites located eit her in the intermediolateral cell column or the intercalated nucleus o f the thoracic spinal cord. Over one-third of the neurones (n = 17) re corded displayed ongoing activity with firing frequencies of 0.3-5 Hz. Of the neurones analyzed only one showed a very obvious phasic firing pattern. Dorsal root stimulation evoked firing in 16 of 26 SPNs recor ded from the same spinal segment (6 of 10 with ongoing activity). The types of responses observed varied between neurones. The excitation of all neurones was characterised by a response occurring at a latency o f 6-50 ms. In addition, SPNs in 'spinalised' preparations (n = 2) resp onded with latencies of 10-40 ms, similar to those observed in the int act preparation. The latencies of responses in SPNs were longer and mo re variable than those observed in ventral horn motor neurones. This i ndicates that a spinal polysynaptic pathway was involved in mediating these responses. In 7 SPNs dorsal root stimulation also elicited longe r latency responses which were observed up to 1000 ms after stimulatio n. These responses may involve activation of bulbospinal and/or propri ospinal pathways. These results show that the neonatal rat brainstem-s pinal cord preparation is viable for studying SPNs and that dorsal roo t-SPN reflexes are intact.