MU-OPIOID AGONIST ENHANCEMENT OF PROSTAGLANDIN-INDUCED HYPERALGESIA IN THE RAT - A G-PROTEIN BETA-GAMMA 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 E(2) and the adenosine A(2)-agonist, CGS21680. The effects of two agents expected to liberate inhibitory guanine nucleotide-binding regulatory protein subunits were also studied: [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (a mu-opioid receptor agonist) and N-6-cyclopentyladenosine (an A(1)-adenosine agonist). Injection of [D-Ala(2),N-Me Phe(4),Gly(5)-ol]enke phalin immediately before prostaglandin E(2) or CGS21680 significantly attenuated the hyperalgesia subsequently induced by these agents, i.e. the sensitivity to these hyperalgesic agents was decreased. On the other hand, injection of [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin 5 min after prostaglandin E(2) or CGS21680 significantly enhanced the hyperalgesia observed. Injection of the adenosine A(1)-agonist N-6-cyclopentyladenosine immediately before and 5 min after prostaglandin E(2) or CGS21680 had a similar effect to [o-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin. The decrease in sensitivity to prostaglandin E(2-) and CGS21680-induced hyperalgesia by preadministration of [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin or N-6-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 alpha subunit of retinal transducin, a scavenger of beta gamma subunits, reversed the enhancing and inhibitory effects of [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin, suggesting that beta gamma subunits, released after activation of inhibitory guanine nucleotide-binding regulatory protein, are involved in both phenomena. Injection of 2',5'-dideoxyadenosine or S422536, inhibitors of adenylyl cyclase, before prostaglandin E(2), significantly attenuated prostaglandin E(2) hyperalgesia, suggesting the involvement of adenylyl cyclase in prostaglandin E(2) hyperalgesia. The enhancement of hyperalgesia observed by postadministration of [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin may be understood in terms of the recently described ''conditional co-stimulation'' of adenylyl cyclase II by the beta gamma subunit of guanine nucleotide-binding regulatory proteins. The inhibitory effects of preadministered [D-Ala(2),N Me-Phe(4),Gly(5)-ol]enkephalin and N-6-cyclopentyladenosine also appear to depend on beta gamma subunit action, but the mechanism(s) involved is unclear. It is suggested that [D-Ala(2),N-Me-Phe4,Gly(5)-ol]enkephalin and N-6-cyclopentyladeno sine enhance prostaglandin E(2)-induced peripheral mechanical hyperalgesia via the action of beta gamma 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.