Mu-opioid receptor is present in dendritic targets of Endomorphin-2 axon terminals in the nuclei of the solitary tract

Endomorphins represent a group of endogenous opioid peptides with high affinity for the mu-opioid receptor. In the brainstem, Endomorphin-2 is found in trigeminal dorsal horn and the nuclei of the solitary tract, suggesting its presence in both nociceptive and visceral primary afferents. If Endomorphin-2 were an endogenous ligand for the mu-opioid receptor, we would expect to find the receptor at cellular sites in close association with the peptide. We used dual-labeling immunocytochemistry combined with electron microscopy to examine interactions between Endomorphin-2-immunoreactive and mu-opioid receptor-immunoreactive profiles within the nuclei of the solitary tract in the rat. Endomorphin-2-immunoreactivity was found primarily in unmyelinated axons and axon terminals in nuclei of the solitary tract and the majority of these terminals contained dense core vesicles. Endomorphin-2-immunoreactive axon terminals often formed asymmetric synapses with dendritic spines lacking mu-opioid receptor-immunoreactivity, but mu-opioid receptor-immunoreactivity was found in many of the larger dendritic targets of Endomorphin-2-immunoreactive terminals. Thus, mu-opioid receptor-immunoreactivity was found in the postsynaptic targets of Endomorphin-2-immunoreactive axon terminals, consistent with the hypothesis that Endomorphin-2 is an endogenous ligand for this receptor within the nuclei of the solitary tract. A small number of Endomorphin-2-immunoreactive somata, dendrites, and axon terminals also contained mu-opioid receptor-immunoreactivity. Cells that contain both the opioid peptide and its receptor may be a substrate for potential autoregulation of nuclei of the solitary tract neurons by opioid ligands. Finally, using tract tracing and confocal microscopy, we found Endomorphin-2-immunoreactivity in a subset of vagal afferents. Together these findings support the hypothesis that Endomorphin-2 is a ligand for the mu-opioid receptor within nuclei of the solitary tract and that the peptide is at least partially derived from primary visceral afferents.