mu-opioid receptors are present in functionally identified sympathoexcitatory neurons in the rat rostral ventrolateral medulla

Agonists of the mu -opioid receptor (MOR) produce profound hypotension and sympathoinhibition when microinjected into the rostral ventrolateral medull a (RVL). These effects are likely to be mediated by the inhibition of adren ergic and other presympathetic vasomotor neurons located in the RVL. The pr esent ultrastructural studies were designed to determine whether these vaso motor neurons, or their afferents, contain MORs. RVL bulbospinal barosensit ive neurons were recorded in anesthetized rats and filled individually with biotinamide by using a juxtacellular labeling method. Biotinamide was visu alized by using a peroxidase method and MOR was identified by using immunog old localization of an antipeptide antibody that recognizes the cloned MOR, MOR1. The subcellular relationship of MOR1 to RVL neurons with fast- or sl ow-conducting spinal axons was examined by electron microscopy. Fast- and s low-conducting cells were not morphologically distinguishable. Immunogold-l abeling for MOR1 was found in all RVL bulbospinal barosensitive neurons exa mined (9 of 9). MOR1 was present in 52% of the dendrites from both types of cells and in approximately half of these dendrites the MOR1 was at nonsyna ptic plasmalemmal sites. A smaller portion of biotinamide-labeled dendrites (16%) from both types of cells were contacted by MOR1-containing axons or axon terminals. Together, these results suggest that MOR agonists can direc tly influence the activity of all types of RVL sympathoexcitatory neurons a nd that MOR agonists may also influence the activity of afferent inputs to these cells. The heterogenous distribution of MORs within individual RVL ne urons indicates that the receptor is selectively targeted to specific pre- and postsynaptic sites. (C) 2001 Wiley-Liss, Inc.