Mu-opioid agonist enhancement of prostaglandin-induced hyperalgesia in the rat: A G-protein βγ subunit-mediated effect?

Guanine nucleotide-binding regulatory protein stimulation of adenylyl cyclase has been shown to be an important second messenger system for many processes, including mechanical hyperalgesia. Recently, interactions between guanine nucleotide-binding regulatory protein subunits and adenylyl cyclase affecting the level of cyclic adenosine 3′,5′-monophosphate accumulation have been demonstrated. In this study we evaluated such an interaction by measuring paw-withdrawal thresholds to mechanical stimuli in Sprague-Dawley rats in the presence of two direct-acting hyperalgesic agents, prostaglandin E2 and the adenosine A2-agonist, CGS21680. The effects of two agents expected to liberate inhibitory guanine nucleotide-binding regulatory protein subunits were also studied: [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (a μ-opioid receptor agonist) andN6-cyclopentyladenosine (an A1-adenosine agonist). Injection of [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin immediately before prostaglandin E2 or CGS21680 significantly attenuated the hyperalgesia subsequently induced by these agents, i.e the sensitivity to these hyperalgesia agents was decreased. On the other hand, injection of [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin 5 min after prostaglandin E2 or CGS21680 significantly enhanced the hyperalgesia observed. Injection of the adenosine A1-agonistN6-cyclopentyladenosine immediately before and 5 min after prostaglandin E2 or CGS21680 had a similar effect to [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin. The decrease in sensitivity to prostaglandin E2- and CGS21680-induced hyperalgesia by pre-administration of [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin orN6-cyclopentyladenosine and the enhancement by postadministration were all reversed by pertussis toxin, an inhibitor of inhibitory guanine nucleotide-binding regulatory protein, suggesting the involvement of an inhibitory guanine nucleotide-binding regulatory protein. Also, the injection of the ∝ subunit of retinal transducin, a scavenger of βγ subunits, reversed the enhancing and inhibitory effects of [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin, suggesting that βγ subunits, released after activation of inhibitory guanine nucleotide-binding regulatory protein, are involved in both phenomena. Injection of 2′,5′-dideoxyadenosine or SQ22536, inhibitors of adenylyl cyclase, before prostaglandin E2, significantly attenuated prostaglandin E2 hyperalgesia, suggesting the involvement of adenylyl cyclase in prostaglandin E2 hyperalgesia. The enhancement of hyperalgesia observed by postadministration of [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin may be understood in terms of the recently described “conditional co-stimulation” of adenylyl cyclase II by the βγ subunit of guanine nucleotide-binding regulatory proteins. The inhibitory effects of preadministered [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin andN6-cyclopentyladenosine also appear to depend on βγ subunit action, but the mechanism(s) involved is unclear. It is suggested that [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin andN6-cyclopentyladenosine enhance prostaglandin E2-induced peripheral mechanical hyperalgesia via the action of βγ subunits of inhibitory guanine nucleotide-binding regulatory proteins on adenylyl cyclase isoenzyme (II or IV) or by parallel processing of transduction signals in separate pathways.