The relationship between serotonin, dopamine beta hydroxylase and GABA immunoreactive inputs and spinal preganglionic neurones projecting to the major pelvic ganglion of Wistar rats.

Preganglionic neurones in the lumbosacral spinal cord give rise to nerves providing the parasympathetic and sympathetic innervation of pelvic organs. These neurones are modulated by neurotransmitters released both from descending supra-spinal pathways and spinal interneurones. Though serotonin has been identified as exerting a significant influence on these neurones, few studies have investigated the circuitry through which it achieves this particularly in relation to sympathetic preganglionic neurones. Using a combination of neuronal tracing and multiple immunolabeling procedures, the current study has shown that pelvic preganglionic neurones receive a sparse, and probably non-synaptic, axosomatic/proximal dendritic input from serotonin-immunoreactive terminals. This was in marked contrast to dopamine beta hydroxylase-immunoreactive terminals, which made multiple contacts. However, the demonstration of both serotonin, and dopamine beta hydroxylase immunoreactive terminals on both parasympathetic and sympathetic preganglionic neurones provides evidence for direct modulation of these cells by both serotonin and norepinephrine. Serotonin-containing terminals displaying conventional synaptic morphology were often seen to contact unlabeled somata and dendritic processes in regions surrounding the labeled preganglionic cells. It is possible that these unlabeled structures represent interneurones that might allow the serotonin containing axons to exert an indirect influence on pelvic preganglionic neurones. Since many spinal interneurones employ GABA as a primary fast acting neurotransmitter we examined the relationship between terminals that were immunoreactive for serotonin or GABA and labeled pelvic preganglionic neurones. These studies were unable to demonstrate any direct connections between serotonin and GABA terminals within the intermediolateral or sacral parasympathetic nuclei. Colocalization of serotonin and GABA was very rare but terminals immunoreactive for each were occasionally seen to contact the same unlabeled processes in close proximity. These results suggest that in the rat, the serotonin modulation of pelvic preganglionic neurones may primarily involve indirect connections via local interneurones.