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 antagonist picrotoxin or bicuculline on the antinociception produced by i.t. serotonin (5-HT), γ-aminobutyric acid (GABA), muscimol - the GABA(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) exhibited a partial and later-emerged blockade on the antinociception produced by 5-HT (120 nmol) or GABA (1.5 nmol); 2) both i.t. picrotoxin and i.t. bicuculline, with the same dosages, completely blocked the antinociception produced by muscimol (1.0 nmol), but showed no effects on that produced by baclofen (0.3 nmol). The results suggest that GABA may mediate the 5-HT-induced antinociception at the spinal level, with the GABA(B)-receptors exhibiting the effect at the early-stage and the GABA(A)-receptors at the later stage of the 5-HT-induced antinociception.