The effects of morphine (M) on pain threshold and on the Ach outflow from the cerebral cortex of unanaesthetized guinea-pigs were investigated. pCPA pretreatment slightly attenuated M analgesia and significantly reduced Ach release inhibition caused by the drug. α-MT or reserpine pretreatment had little effect on M analgesia, but potentiated and prolonged its anti-release effect. Conversely, amphetamine showed, per se, a clear-cut analgesic effect and acted synergically with M as regards analgesia. On the other hand, the amine fully prevented the inhibition of Ach outflow. These results are discussed in the light of the hypothesis that 5-HT neurones positively co-operate to the entire pattern of M action, whereas DA neurons potentiate analgesia but antagonize M influence on cortical Ach release. © 1979 The Italian Pharmacological Society.
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