Characterization of peptidergic efferents from the lateral parabrachial nucleus to identified neurons in the rat dorsal raphe nucleus.

The peptidergic content of the lateral parabrachial nucleus (LPB) efferents to the dorsal raphe nucleus (DRN) was studied by combining visualization of the anterogradely transported tracer Phaseolus vulgaris leucoagglutinin within fibers that were immunocytochemically stained for neurotensin (NT), calcitonin gene-related peptide (CGRP) or galanin (GAL). The identity of DRN target neurons was determined with simultaneous immunocytochemical labelling for serotonin, the major transmitter within the nucleus. Within the DRN, we estimated that about two-thirds of the anterogradely labelled fibers arising from the LPB also showed peptidergic immunoreactivity. NT was the most commonly observed neuropeptide in LPB neuronal efferents directed to the DRN, followed by CGRP and GAL. The peptidergic afferents in the DRN were oriented preferentially in the dorsoventral plane. Peptidergic fibers from the LPB possessed varicosities (diameters not exceeding 3 microns) and were apposed on serotoninergic neuronal somata. Some of the anterogradely labelled peptidergic fibers were not associated with cells showing immunoreactivity for serotonin. The present results suggest that NT-ergic, CGRP-ergic and GAL-ergic neurons within the LPB are in contact with serotoninergic and non-serotoninergic neurons within the DRN. Since the DRN is known to project to the LPB, it is likely that bi-directional interconnections between these nuclei exist. Such linkages may provide anatomical substrates for coordinated autonomic responses.