Antinociceptive effects of baclofen and muscimol upon intraventricular administration.

The effects of intraventricularly administered baclofen and muscimol were investigated on tail-flick responding and on vocalization and motor responses to nociceptive pinch. Baclofen (1 microgram) and muscimol (0.1 microgram) strongly reduced responding to pinch, particularly vocalization, without altering tail-flick responding. When given systemically, however, baclofen markedly attenuated tail-flick as well as pinch responding. Muscimol produced only weak antinociception by systemic administration, suggesting that it may have poor access to brain. At antinociceptive doses, i. vent. muscimol produced less apparent muscle relaxation than did baclofen. These results suggest that baclofen's antinociceptive action may be mediated in part by a supraspinal, GABAergic substrate, in addition to a spinal component which may not directly involve GABA.