Excitatory and indirect inhibitory actions of 5-hydroxytryptamine on sympathetic preganglionic neurones in the neonate rat spinal cord in vitro.

The action of 5-hydroxytryptamine (5-HT) on sympathetic preganglionic neurones (SPN) was studied by intracellular recordings in thin slices of neonatal rat spinal cord in vitro. Superfusion of 5-HT (1-270 microM) to SPN caused a concentration dependent slow depolarization or inward current and an increase in synaptic activity consisting of both EPSPs and IPSPs. The slow depolarization was still present after superfusion with TTX. Similar effects were seen during superfusion with 5-carboxamidotryptamine (5-CT) or alpha-methyl-5-hydroxytryptamine (alpha-me-5-HT). A comparison with the potency of 5-HT was made for 5-CT or alpha-me-5-HT on the same neurone by determining the magnitude of the slow depolarization to different concentrations of agonist. This showed that the apparent potency of the agonists was 5-CT > 5-HT > alpha-me-5-HT even in the presence of fluoxetine, a 5-HT uptake inhibitor. The 5-HT-induced slow depolarization was partially blocked by ketanserin but full recovery was not observed. The results suggest that the excitatory action of 5-HT on SPN is mediated via an atypical 5-HT2 receptor or a 5-HT1C-like receptor. The 5-HT-induced IPSPs were reversibly blocked by superfusion with strychnine, suggesting they were mediated by glycine.