SEROTONIN INPUTS TO RABBIT SYMPATHETIC PREGANGLIONIC NEURONS PROJECTING TO THE SUPERIOR CERVICAL-GANGLION OR ADRENAL-MEDULLA

The input from serotonin-containing nerve fibres to rabbit sympathetic preganglionic neurons projecting to either the superior cervical gang lion or the adrenal medulla was investigated by combining retrograde t racing with the B subunit of cholera toxin and immunocytochemistry for serotonin. There were pronounced rostrocaudal variations in the densi ty of serotonin fibres in the rabbit intermediolateral cell column fro m T1 to L4; maximum numbers of fibres were found in T3-6 and L3-4 and minimum numbers in T1 and T10-12. By light microscopy, retrogradely la belled sympathetic preganglionic neurons projecting to the superior ce rvical ganglion or the adrenal medulla received variable densities of close appositions from serotonin-immunoreactive fibres. Some neurons f rom each population received many close appositions, whereas others re ceived moderate numbers or few appositions. Appositions occurred on th e cell bodies, dendrites, and occasionally axons of sympathetic pregan glionic neurons. Rare neurons in both groups of retrogradely labelled cells received no appositions from serotonin-containing nerve fibres. At the ultrastructural level, synapses were found between serotonin-po sitive boutons and sympathetic preganglionic neurons projecting either to the superior cervical ganglion or to the adrenal medulla. These re sults indicate that, through direct synaptic contacts, serotonin-immun oreactive, presumably bulbospinal, nerve fibres affect the activity of the vast majority of sympathetic preganglionic neurons that send axon s either to the superior cervical ganglion or to the adrenal medulla. This serotonin input may be sympathoexcitatory and could mediate incre ases in sympathetic nerve activity and in the release of catecholamine s from the adrenal medulla. (C) 1995 Wiley-Liss, Inc.