IMMUNOLABELING OF MU-OPIOID RECEPTORS IN THE RAT NUCLEUS OF THE SOLITARY TRACT - EXTRASYNAPTIC PLASMALEMMAL LOCALIZATION AND ASSOCIATION WITH LEU(5)-ENKEPHALIN

Activation of the mu opioid receptor (MOR) by morphine within the caud al nucleus df the solitary tract (NTS) is known to mediate both cardio respiratory and gastrointestinal responses. Leu(5)-enkephalin (LE), a potential endogenous ligand for MOR, is also present within neurons in this region. To determine the cellular sites for the visceral effects of MOR ligands, including LE, we used immunogold-silver and immunoper oxidase methods for light and electron microscopic localization of ant isera against MOR (carboxyl terminal domain) and LE in the caudal NTS of rat brain. Light microscopy of coronal sections through the NTS at the level of the area postrema showed MOR-like immunoreactivity (MOR-L I) and LE labeling in punctate processes located within the subpostrem al, dorsomedial and medial subnuclei. Electron microscopy of sections through the medial NTS at this level showed gold-silver particles iden tifying MOR-LI prominently distributed to the cytoplasmic side of the plasma membranes of axons and terminals. MOR labeled terminals formed mostly symmetric (inhibitory-type) synapses but sometimes showed multi ple asymmetric junctions, characteristic of excitatory visceral affere nts. MOR-LI was also present along extrasynaptic plasma membranes of d endrites receiving afferent input from unlabeled and LE-labeled termin als. We conclude that MOR ligands, possibly including LE, can act at e xtrasynaptic MORs on the plasma membranes of axons and dendrites in th e caudal NTS to modulate the presynaptic release and postsynaptic resp onses of neurons. These are likely to include local inhibitory neurons and both gastric and cardiorespiratory afferents known to terminate i n the subnuclei with the most intense MOR-LI. (C) 1996 Wiley-Liss, Inc .