Selective distribution of μ‐opioid receptors in C1 adrenergic neurons and their afferents

Abstract

Agonists of the mu-opioid receptor (MOR) have profound effects on blood pressure, heart rate, and respiration that may be mediated by C1 adrenergic neurons in the rostral ventrolateral medulla (RVL). C1 neurons are sympathoexcitatory and are involved in both tonic and reflex regulation of sympathetic outflow. This study was designed to determine whether C1 neurons, or their afferents, contain MOR. C1 neurons were identified by using an antibody against the epinephrine synthesizing enzyme phenylethanolamine-N-methyl transferase (PNMT), whereas MOR was localized by using an antipeptide antibody that recognizes the cloned MOR, MOR1. Combined immunoperoxidase and immunogold methods were used to examine the cellular distribution of MOR1 relative to PNMT-containing neurons in the RVL. MOR1 was found in 22% of PNMT-containing dendrites (n = 392), whereas MOR1-containing axons or axon terminals contacted 14% of PNMT-containing dendrites. This distribution was heterogenous with regard to dendritic size: PNMT-labeled dendrites containing MOR1 were usually large (60% were >1.2 microm), whereas PNMT-containing dendrites that received MOR1-labeled afferents were usually small (79% were <1.2 microm). Individual dendrites rarely contained MOR1 at both pre- and postsynaptic sites. Together these results suggest that MOR agonists may directly influence the activity of C1 neurons, as well as the activity of select afferents to these cells. Plasmalemmal membrane labeling for MOR1 was more frequent in smaller PNMT-containing dendrites, suggesting that postsynaptic receptors are more readily available for ligand binding in small dendrites, although the receptor was more frequently detected in larger PNMT dendrites. The selective distribution of MORs to specific pre- and postsynaptic sites suggests the receptor may be selectively trafficked to positions where it may regulate afferent activity that is heterogeneously distributed along the dendritic tree of C1 neurons.

C1 catecholamine neurons form local circuit synaptic connections within the rostroventrolateral medulla of rat

“…Immunopositive boutons exhibited the same ultrastructural features previously documented for PNMT+ terminals in RVLM (Milner et al, 1988a, Milner et al, 1989a, Aicher et al, 2001). Each axonal varicosity contained electron lucent axoplasm and concentrations of 40 nm lucent spherical vesicles (figures 3 & 4).…”

“…Importantly, restricted labeling of C1 neurons in either the rostral or caudal RVLM revealed axons that projected into portions of the C1 column that did not contain infected neurons. The light microscopic data were complemented by ultrastructural localizations demonstrating labeled axonal varicosities that exhibited morphology consistent with that previously shown for adrenergic terminals within RVLM (Milner et al, 1989a, Aicher et al, 2001) and formed asymmetric synaptic contacts with both C1 and non-C1 neurons in all regions of the C1 cell column. The data are also consistent with the findings of Lipski and colleagues in which fills of barosensitive bulbospinal neurons in rostral RVLM revealed local collaterals terminating in RVLM (Lipski et al, 1995).…”

Heterogeneous Receptor Distribution in Autonomic Neurons

“…This study again used an antibody against PNMT to detect adrenergic neurons in the RVL and an antibody directed against the cloned μ‐opioid receptor, MOR1. We found MOR1 in the dendrites of C1 adrenergic neurons in the RVL, but it was preferentially detected in larger dendrites 11. We also found MOR1 in axons and axon terminals contacting adrenergic dendrites.…”

“…We also found MOR1 in axons and axon terminals contacting adrenergic dendrites. Interestingly, the profiles containing presynaptic MOR1 were preferentially apposed to smaller PNMT‐containing dendrites that did not contain postsynaptic MOR1 11. These results indicate that there is a targeting of MOR1 within adrenergic neurons, as well as within specific afferents that contact these neurons (Fig.…”

“…In several recent studies, we have examined the pre‐ and postsynaptic distribution of important receptor proteins in relation to neurons that are likely to mediate baroreceptive sympathoinhibition. All of these studies used dual immunogold and immunoperoxidase labeling to identify the following: (1) a particular cell group of interest and (2) a specific receptor protein 9–12. This tissue was then examined by electron microscopy to determine the subcellular distribution of the receptor in relation to the cells of interest.…”