Effect of training on the analgesic action of morphine

Abstract

Groups of mice injected with saline were tested at regular intervals using the hot plate or the tail pinch procedures for measuring analgesia. In the tail pinch test, a decreased analgesic response to an injection of morphine was observed in mice subjected to repeated stimuli as compared to mice receiving a single stimulus. No statistical differences were observed between groups of mice receiving either single or repeated stimuli when analgesia was assessed by the hot plate method. When stimuli were repeated under pentobarbital or a muscle relaxant at dose levels that abolished response to the tail pinch, the effect of a subsequent dose of morphine was comparable to that obtained in mice receiving only a single stimulus.