The GBeta5 subunit that associates with the R7 subfamily of RGS proteins regulates mu-opioid effects

Abstract

The G β5 protein, which is similar in sequence to other G-protein beta subunits, mainly associates with the G-protein γ-like (GGL) domains of the R7 subfamily of regulators of G-protein signalling (RGS) proteins. This paper reports the presence of the G β5 protein and its mRNA in all areas of mouse CNS, and also its involvement in the cellular signals initiated at mu- and delta-opioid receptors. The expression of G β5 and RGS9-2 proteins (member of the R7 subfamily of RGS) was reduced by blocking their mRNAs with antisense oligodeoxynucleotides (ODN). Knock-down of these proteins enhanced the potency and duration of antinociception promoted by morphine and [D-Ala 2, N-MePhe 4,Gly-ol 5]-enkephalin (DAMGO), agonists at mu opioid receptors. However, the activity of the selective agonist at delta opioid receptors, [D-Pen 2,5]-encephalin (DPDPE), appeared to be reduced. A single intracerebroventricular (icv) ED 80 analgesic dose of morphine gave rise to acute tolerance in control mice, but did not promote tolerance in G β5 or RGS9-2 knock-down animals. In a model of sustained morphine treatment, the impairment of G β5 proteins facilitated the development of tolerance. This treatment did not alter the incidence of jumping behaviour precipitated by naloxone 3 days after commencing with chronic morphine. These results show differences in the signalling regulation of G-proteins when activated by mu or delta opioid agonists. For mu opioid receptors, acute tolerance, but probably not long-term tolerance, appears to depend on the function of G β5 subunits and associated RGS proteins.