RGSZ1 and GAIP regulate μ- but not δ-opioid receptors in mouse CNS: Role in tachyphylaxis and acute tolerance

Abstract

In the CNS, the regulators of G-protein signaling (RGS) proteins belonging to the Rz subfamily, RGS19 (Gα interacting protein (GAIP)) and RGS20 (Z1), control the activity of opioid agonists at μ but not at δ receptors. Rz proteins show high selectivity in deactivating Gαz-GTP subunits. After reducing the expression of RGSZ1 with antisense oligodeoxynucleotides (ODN), the supraspinal antinociception produced by morphine, heroin, DAMGO ([D-Ala 2, N-MePhe4,Gly-ol5]-enkephalin), and endomorphin-1 was notably increased. No change was observed in the effect of endomorphin-2. This agrees with the proposed existence of different μ receptors for the endomorphins. The activities of DPDPE ([D-Pen 2,5]-enkephalin) and [D-Ala2] deltorphin II, agonists at δ receptors, were also unchanged. Knockdown of GAIP and of the GAIP interacting protein C-terminus (GIPC) led to changes in agonist effects at μ but not at δ receptors. The impairment of RGSZ1 extended the duration of morphine analgesia by at least 1 h beyond that observed in control animals. CTOP (Cys2, Tyr3, Orn5, Pen7-amide) antagonized morphine analgesia when given during the period in which the effect of morphine was enhanced by RGSZ1 knockdown. Thus, in naive mice, morphine tachyphylaxis originated in the presence of the opioid agonist and during the analgesia time course. The knockdown of RGSZ1 facilitated the development of tolerance to a single dose of morphine and accelerated tolerance to continuous delivery of the opioid. These results indicate that μ but not δ receptors are linked to Rz regulation. The μ receptor-mediated activation of Gz proteins is effective at recruiting the adaptive mechanisms leading to the development of opioid desensitization.