Effect of apomorphine on the antinociceptive activity of morphine

Apomorphine pretreatment potentiated the analgesic effect of morphine in a dose-dependent manner both in rats and in mice measured by five different tests (writhing, hot plate, inflamed foot, tail-pinch and tail-flick procedures). Furthermore, apomorphine augmented the antinociceptive activity of morphine in tolerant animals as well. In morphine dependent mice the nalorphine precipitated jumping--a withdrawal symptom--was found inhibited by apomorphine treatment. The results are discussed in the light of the numerous but contradictory data available in the literature.