Pentobarbital attenuates the antinociceptive effect of intranigral morphine

This study was conducted to characterize the temporal relationship between intranigral injection of morphine and the onset of antinociception. The principal findings are: 1) morphine produces antinociception on the hot plate test within three minutes after intranigral injection, 2) the tail flick reflex cannot be measured within the first 30 minutes following intranigral morphine due to motor effects, and 3) pentobarbital suppresses the antinociceptive effect of intranigral morphine on the tail flick test. These findings support the conclusion that the antinociceptive effects of intranigral morphine are mediate by the substantia nigra.