Opposing regulation of the locus coeruleus by corticotropin-releasing factor and opioids. Potential for reciprocal interactions between stress and opioid sensitivity.
Substantial clinical and preclinical findings support an association between stress and opiate abuse. To understand the mechanisms underlying this association, it is important to identify substrates of the stress response and endogenous opioid systems that interact and specific points at which stress circuits and endogenous opioid systems intersect. This review focuses on corticotropin-releasing factor (CRF), a critical substrate of the stress response, and its potential for interactions with endogenous opioid systems within the pontine nucleus, locus coeruleus (LC), a brain region that has been implicated as a target in response to stress and opiates. Evidence is reviewed supporting the hypothesis that CRF and endogenous opioids interact to co-regulate the LC. Thus, CRF- and enkephalin-immunoreactive fibers innervating LC dendritic fields overlap, and some axon terminals in this region co-localize CRF and enkephalin. CRF and opioids have opposing effects on LC neuronal discharge and on intracellular signaling mechanisms within LC neurons. Finally, a history of stress or opiate use induces plasticity in CRF-LC or opiate-LC interactions, respectively. Disruptions in the CRF/opioid balance as a result of this plasticity are proposed to result in hyperactivity or hyperresponsiveness of the LC-norepinephrine (NE) system. Co-regulation of the LC-NE system by CRF and opioids may be important in acute adaptation to stress. Potential clinical consequences of an imbalance in this regulation as a result of prior stress include increased risk of opiate self administration and decreased sensitivity to opiates used in clinical settings. Conversely, chronic exposure to opiates may predispose individuals to stress-related psychiatric disorders.