EXPRESSION OF GALANIN AND NITRIC-OXIDE SYNTHASE IN SUBPOPULATIONS SEROTONIN NEURONS OF THE RAT DORSAL RAPHE NUCLEUS

The dorsal raphe nucleus (DR) of the rat was studied with triple label ing immunofluorescence histochemistry to evaluate the quantitative rel ationships between neurons expressing 5-hydroxytryptamine (5-HT), the neuropeptide galanin (GAL) and the nitric oxide (NO) synthesizing enzy me NO synthase (NOS). In addition retrograde tracing studies were perf ormed. It could be established that a high percentage (between 40 and 60%) of the 5-HT neuron profiles in the ventromedial and dorsomedial s ubgroups of the DR contained both GAL and NOS after colchicine treatme nt. This triple coexistence was lower in the dorso-lateral subgroup an d much lower in the lateral subgroup (down to 5%). All GAL neuron prof iles contained 5-HT, and they constituted up to 80% of all labeled pro files in the rostral ventromedial and caudal dorsomedial subgroups, wi th the lowest percentage in the lateral subgroup (45%). The percentage of 5-HT-alone neuron profiles in these four subgroups varied between 15-40%. The proportion of 5-HT/NOS neurons was low (a few percent) at all levels, and this was the case also for NOS-alone neuron profiles e xcept in the lateral subgroup, where 10-20% were of this type. It coul d be established that some 5-HT/GAL/NOS neurons project to the striatu m. These retrogradely labeled cells were mainly found in the mid-line subgroups. In the striatum a moderately dense 5-HT fiber network and n umerous NOS-positive cell bodies and fibers could be observed. However , only a few, weakly fluorescent GAL fibers were found and in a small number of cases it could be shown or was likely that 5-HT and GAL coex isted. No evidence for coexistence between 5-HT and NOS was obtained. The present findings strongly suggest that a large proportion of the D R 5-HT neurons can synthesize and release two additional messenger mol ecules, GAL and NO. Furthermore, even if the 5-HT/GAL/NOS neurons proj ect to the striatum, the amounts of GAL and NOS transported to the ter minal ramifications in this area are very low. This is in agreement wi th a very low GAL synthesis in the DR under normal circumstances, whic h is also indicated by the fact that colchicine treatment is needed to visualize GAL-like immunoreactivity in DR cell bodies. It is possible that NO, and perhaps GAL, in 5-HT neurons exert their main actions at the somatic and dendritic level in the DR. (C) 1997 Elsevier Science B.V.