Regulation of RGS proteins by chronic morphine in rat locus coeruleus

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Abstract
The present study explored a possible role for RGS (regulators of G protein signalling) proteins in the long term actions of morphine in the locus coeruleus (LC), a brainstem region implicated in opiate physical dependence and withdrawal. Morphine influences LC neurons through activation of µ‐opioid receptors, which, being Gi/o‐linked, would be expected to be modulated by RGS proteins. We focused on several RGS subtypes that are known to be expressed in this brain region. Levels of mRNAs encoding RGS2, ‐3, ‐4, ‐5, ‐7, ‐8 and ‐11 are unchanged following chronic morphine, but RGS2 and −4 mRNA levels are increased 2–3‐fold 6 h following precipitation of opiate withdrawal. The increases in RGS2 and −4 mRNA peak after 6 h of withdrawal and return to control levels by 24 h. Immunoblot analysis of RGS4 revealed a striking divergence between mRNA and protein responses in LC: protein levels are elevated twofold following chronic morphine and decrease to control values by 6 h of withdrawal. In contrast, levels of RGS7 and −11 proteins, the only other subtypes for which antibodies are available, were not altered by these treatments. Intracellular application of wild‐type RGS4, but not a GTPase accelerating‐deficient mutant of RGS4, into LC neurons diminished electrophysiological responses to morphine. The observed subtype‐ and time‐specific regulation of RGS4 protein and mRNA, and the diminished morphine‐induced currents in the presence of elevated RGS4 protein levels, indicate that morphine induction of RGS4 could contribute to aspects of opiate tolerance and dependence displayed by LC neurons.