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, interacti ons between guanine nucleotide-binding regulatory protein subunits and adenylyl cyclase affecting the level of cyclic adenosine 3',5'-monoph osphate accumulation have been demonstrated. In this study we evaluate d such an interaction by measuring paw-withdrawal thresholds to mechan ical stimuli in Sprague-Dawley rats in the presence of two direct-acti ng hyperalgesic agents, prostaglandin E(2) and the adenosine A(2)-agon ist, CGS21680. The effects of two agents expected to liberate inhibito ry guanine nucleotide-binding regulatory protein subunits were also st udied: [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (a mu-opioid recepto r agonist) and N-6-cyclopentyladenosine (an A(1)-adenosine agonist). I njection of [D-Ala(2),N-Me Phe(4),Gly(5)-ol]enke phalin immediately be fore prostaglandin E(2) or CGS21680 significantly attenuated the hyper algesia 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 prostaglandi n E(2) or CGS21680 significantly enhanced the hyperalgesia observed. I njection of the adenosine A(1)-agonist N-6-cyclopentyladenosine immedi ately before and 5 min after prostaglandin E(2) or CGS21680 had a simi lar effect to [o-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin. The decrease in sensitivity to prostaglandin E(2-) and CGS21680-induced hyperalges ia 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 guani ne nucleotide-binding regulatory protein, suggesting the involvement o f an inhibitory guanine nucleotide-binding regulatory protein. Also, t he injection of the alpha subunit of retinal transducin, a scavenger o f beta gamma subunits, reversed the enhancing and inhibitory effects o f [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin, suggesting that beta gam ma subunits, released after activation of inhibitory guanine nucleotid e-binding regulatory protein, are involved in both phenomena. Injectio n 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 pro staglandin E(2) hyperalgesia. The enhancement of hyperalgesia observed by postadministration of [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin m ay be understood in terms of the recently described ''conditional co-s timulation'' of adenylyl cyclase II by the beta gamma subunit of guani ne nucleotide-binding regulatory proteins. The inhibitory effects of p readministered [D-Ala(2),N Me-Phe(4),Gly(5)-ol]enkephalin and N-6-cycl opentyladenosine also appear to depend on beta gamma subunit action, b ut 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-bindin g regulatory proteins on adenylyl cyclase isoenzyme (II or IV) or by p arallel processing of transduction signals in separate pathways.