Presence of mu-opioid receptors in targets of efferent projections from the central nucleus of the amygdala to the nucleus of the solitary tract

Opioids acting at mu-opioid receptors (MORs) within the nucleus of the soli tary tract (NTS) potently modulate autonomic functions that are also known to be influenced by inputs from the central nucleus of the amygdala (CEA). In addition, many of the physiological effects of MOR agonists have been at tributed to interactions with neurons that contain gamma-aminobutyric acid (GABA), one of the neurotransmitters present in CEA-derived terminals and t heir targets in the medial NTS. Together, these observations suggest that M ORs are present at pre- or postsynaptic sites within the CEA to NTS circuit ry. To test this hypothesis, we combined anterograde transport of biotinyla ted dextran amine (BDA) with immunogold-silver localization of an antipepti de antiserum against the MOR in the NTS of adult rats. In animals receiving bilateral CEA injections of BDA, anterogradely labeled axons were seen thr oughout the rostrocaudal NTS. Electron microscopy of the medial NTS at rost ral and intermediate levels showed anterograde BDA-labeling in many small u nmyelinated axons and axon terminals, none of which contained detectable MO R. The BDA-labeled axon terminals formed mainly symmetric, inhibitory-type synapses with somata and dendrites. Over half of the somatic and approximat ely 10% of the dendritic targets showed nonsynaptic plasmalemmal immunogold labeling for MOR. The BDA-labeled axon terminals were also frequently appo sed by other small axons that contained MORs. These results suggest that wi thin the medial NTS, MOR agonists modulate the postsynaptic inhibition prod uced by CEA afferents and also play a role in the presynaptic release of ot her neurotransmitters. (C) 1999 Wiley-Liss, Inc.