Ultrastructural evidence for co-localization of corticotropin-releasing factor receptor and μ-opioid receptor in the rat nucleus locus coeruleus

Abstract

Previous studies have shown that corticotropin-releasing factor (CRF), an integral mediator of the stress response, and opioids regulate the activity of the locus-coeruleus-norepinephrine (LC-NE) system during stress in a reciprocal manner. Furthermore, repeated opiate exposure sensitizes noradrenergic neurons to CRF. Previous studies have shown that μORs are prominently distributed within somatodendritic processes of catecholaminergic neurons in the LC and axon terminals containing opioid peptides and CRF converge within the LC. To further examine cellular sites for interactions between CRF receptor type 1 (CRFr) and μOR, immunofluorescence and electron microscopic analysis of the rat LC was conducted. Triple immunofluorescence showed prominent co-localization of the CRFr and μOR in noradrenergic somata in the LC. Ultrastructural analysis confirmed dual localization of CRFr and μOR in common dendritic processes in the LC. Semiquantitative analysis showed that of the dendrites exhibiting CRFr immunolabeling, 57% expressed μOR immunoreactivity. These data provide ultrastructural evidence that CRFr and μOR are colocalized in LC neurons, a cellular substrate that may underlie opiate-induced sensitization of brain noradrenergic neurons to CRF. Keywordscorticotropin-releasing factor receptor 1; μ-opioid receptor; immunocytochemistry; electron microscopy Corticotropin-releasing factor (CRF), the hypothalamic neurohormone that initiates release of adrenocorticotropin in response to stress [20] also serves as a neurotransmitter that activates neurons of the noradrenergic nucleus locus coeruleus (LC). CRF administration increases the spontaneous discharge rate of LC neurons [21]. Microinfusion of CRF receptor antagonists into the LC abrogates increases in LC discharge activity elicited by both intracerebroventricularly (icv) administered CRF [6] and certain stimuli [24]. Likewise, hypotensive stress-induced activation of LC neurons is blocked by microinfusion of CRF directly into the LC [24]. Taken together, these data suggest that CRF release in the LC increases activity of noradrenergic neurons. In addition, CRF potently activates forebrain electroencephalographic (EEG) activity [7]. Consistent with physiological findings, Corresponding Author: Beverly A. S. Reyes, Ph.D., Department of Neurosurgery, Farber Institute for Neurosciences, Thomas Jefferson University, 900 Walnut Street, Suite 400, Philadelphia, PA 19107, Voice: (215) FAX: (215) 955-7921, e-mail: bsr103@jefferson.edu Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. The LC is densely innervated by process...

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