Differential modulation by baclofen on antinociception induced by morphineand beta-endorphin administered intracerebroventricularly in the formalin test

Our previous studies have demonstrated that supraspinal GABAergic receptors are differentially involved in the antinociception induced by morphine and beta-endorphin given intracerebroventricularly (i.c.v.) in the tail-flick and hot-plate tests. These two models employed a phasic, thermal nociceptiv e stimulus, The present study was designed to examine the possible involvem ent of supraspinal GABAergic receptors in opioid-induced antinociception in the formalin test. Morphine (1 mu g) and beta-endorphin (1 mu g) given i.c .v. displayed the almost complete inhibitory effects against the hyperalges ic response in both phases. Muscimol (75-100 ng) and baclofen (5-10 ng) inj ected i.c.v. produced the hypoalgesic response in the both phases. The hypo algesic response induced by muscimol and baclofen observed during the secon d phase was more pronounced than that observed during the second phase. Bac lofen (2.5 ng), at the dose which did not affect the hyperalgesic response, resulted in a significant reversal of the i.c.v. administered beta-endorph in-induced hypoalgesic response observed during the second, but not the fir st, phase, However, the hypoalgesic response induced by i.c.v, administered morphine was not changed by the same dose of muscimol or baclofen injected i.c.v. Our results indicate that, at the supraspinal level, GABA(B) recept ors appear to be involved in the modulation of antinociception induced by s upraspinally administered beta-endorphin, but not morphine, in the formalin test model. (C) 1999 Harcourt Publishers Ltd.