mu-Opioid receptors are present in vagal afferents and their dendritic targets in the medial nucleus tractus solitarius

Ligands of the mu-opiate receptor (MOR) are known to influence many functio ns that involve vagal afferent input to the nucleus tractus solitarius (NTS ), including cardiopulmonary responses, gastrointestinal activity, and cort ical arousal. The current study sought to determine whether a cellular subs trate exists for direct modulation of vagal afferents and/or their neuronal targets in the NTS by ligands of the MOR. Anterograde tracing of vagal aff erents arising from the nodose ganglion was achieved with biotinylated dext ran amine (BDA), and the MOR was detected by using antipeptide MOR antiseru m. The medial subdivision of the intermediate NTS was examined by electron microscopy for the presence of peroxidase-labeled, BDA-containing vagal aff erents and immunogold MOR labeling. MOR was present in both presynaptic axo n terminals and at postsynaptic sites, primarily dendrites. In dendrites, M OR immunogold particles usually were located along extrasynaptic portions o f the plasma membrane. Of 173 observed BDA-labeled vagal afferent axon term inals, 33% contained immunogold labeling for MOR within the axon terminal. Many of these BDA-labeled terminals formed asymmetric, excitatory-type syna pses with dendrites, some of which contained MOR immunogold labeling. MORs were present in 19% of the dendrites contacted by BDA-labeled terminals but were present rarely in both the vagal afferent and its dendritic target. T ogether, these results suggest that MOR ligands modulate either the presyna ptic release from or the postsynaptic responses to largely separate populat ions of vagal afferents in the intermediate NTS. These results provide a ce llular substrate for direct actions of MOR ligands on primary visceral affe rents and their second-order neuronal targets in NTS. (C) 2000 Wiley-Liss, Inc.