DIFFERENT GABA-RECEPTOR TYPES ARE INVOLVED IN THE 5-HT-INDUCED ANTINOCICEPTION AT THE SPINAL LEVEL - A BEHAVIORAL-STUDY

The effects of intrathecally (i.t.) administered GABA(A)-receptor anta gonist picrotoxin or bicuculiine on the antinociception produced by i. t. serotonin (5-HT), gamma-aminobutyric acid (GABA), muscimol - the GA BA(A) agonist or baclofen - the GABA(B) agonist were investigated and compared using the tail-flick assay in rats. The results showed that 1 ) both i.t. picrotoxin (1.5 nmol) and i.t. bicuculline (0.5 nmol) exhi bited a partial and later-emerged blockade on the antinociception prod uced by 5-HT (120 nmol) or GABA (1.5 nmol); 2) both i.t. picrotoxin an d i.t. bicuculline, with the same dosages, completely blocked the anti nociception produced by muscimol (1.0 nmol), but showed no effects on that produced by baclofen (0.3 nmol). The results suggest that GABA ma y mediate the 5-HT-induced antinociception at the spinal level, with t he GABA(B)-receptors exhibiting the effect at the early-stage and the GABA(A)-receptors at the later stage of the 5-HT-induced antinocicepti on. (C) 1998 Elsevier Science.