ULTRASTRUCTURE OF BOMBESIN-LIKE IMMUNOREACTIVE NERVE-TERMINALS IN THENUCLEUS OF THE SOLITARY TRACT AND THE DORSAL MOTOR NUCLEUS

Bombesin (gashin-releasing peptide 14-27) inhibits gastric function an d feeding when microinjected into the nucleus of the solitary tract (N TS)/dorsal motor nucleus of the vagus (DMV) complex. We performed a pr eembedding immunoelectron microscopic study in rats to describe the bo mbesin containing nerve terminals and to characterize their postsynapt ic structures. 228 bombesin-LI nerve terminals which made synaptic con tacts in the NTS/DMV complex were studied. Labeling was heaviest over dense core vesicles and lighter over small clear vesicles. The dense c ore vesicles were typically located along the plasmalemma away from th e synaptic face, a finding that is typical of neuropeptide containing nerve terminals. The postsynaptic structures were most often medium si zed dendrites (56%) and small sized dendrites (27%), with similar perc entages in the NTS and DMV. In the DMV, synapses on cell bodies (8%) w ere more frequent than in tile NTS (1%). In the NTS, synapses on dendr itic spines (10%) were more frequent than in the DMV (4%). Only a sing le axe-axonal contact was identified. These findings add to the increa sing body of evidence that bombesin is a neurotransmitter/neuromodulat or in the NTS/DMV complex. Bombesin rarely makes presynaptic (axo-axon al) contacts that might inhibit the release of excitatory neurotransmi tters but rather makes postsynaptic contacts potentially effecting vag al motoneurons.