Stress-induced antinociception and GABAergic mechanism.

The antinociceptive effects of GABAergic agents in presence or absence of swim-stress were investigated in mice, using the tail-flick test. Swim-stress, the GABAB agonist baclofen and the GABAA agonist muscimol raised the threshold of withdrawal reactions to nociceptive stimulation. The antinociception, induced by an interaction of stress and GABA agonists, was higher than that of stress or a GABA agonist alone. The GABAB antagonist phaclofen and the GABAA antagonists bicuculline and picrotoxin decreased the antinociceptive action induced by stress plus baclofen. Picrotoxin administration also decreased the response of baclofen. Picrotoxin and biculline, but not phaclofen, reduced the antinociceptive response induced by muscimol in stressed mice. Administration of picrotoxin, bicuculline or phaclofen alone did not decrease the stress-induced antinociception. The antagonists even increased the base line latencies in both normal and stressed mice. It is concluded that stimulation of both GABAA and GABAB receptor sites has an antinociceptive effect, but that the involvement of a GABAergic mechanism in stress-induced antinociception is unlikely.