Distribution of μ-opioid receptors in rat visceral premotor neurons

Agonists of the g-opioid receptor (MOR) can modulate the activity of visceral premotor neurons, including cardiac premotor neurons. Neurons in brainstem regions containing these premotor neurons also contain dense concentrations of the MORI. This study examined the distribution of MORI within two populations of visceral premotor neurons: one located in the dorsal motor nucleus of the vagus and the other in the nucleus ambiguus. Visceral premotor neurons contained the retrograde tracer Fluoro-Gold following injections of the tracer into the pericardiac region of the thoracic cavity. MORI was localized using immunogold detection of an anti-peptide antibody. Visceral premotor neurons in both regions contained MORI at somatic and dendritic sites, although smaller dendrites were less likely to contain the receptor than larger dendrites, suggesting there may be selective trafficking of MORI within these neurons. MORI labeling in nucleus ambiguus neurons was more likely to be localized to plasma membrane sites, suggesting that ambiguus neurons may be more responsive to opioid ligands than neurons in the dorsal motor nucleus of the vagus. In addition, many of the dendrites of visceral premotor neurons were in direct apposition to other dendrites. MORI was often detected at these dendro-dendritic appositions that may be gap junctions. Together these findings indicate that the activity of individual visceral premotor neurons, as well as the coupling between neurons, may be regulated by ligands of the MOR. (C) 2002 IBRO. Published by Elsevier Science Ltd. All rights reserved.