EXPERIENCE AS A FACTOR IN DEVELOPMENT OF TOLERANCE TO ANALGESIC EFFECT OF MORPHINE

The effect of experience in the test procedure on the development of tolerance to morphine was studied by dividing rats into 2 major groups: the tested group and the non-tested group (those which although receiving drug injections were not tested until the final day of the experiment). The hot plate procedure was used to measure analgesia. Morphine (5 mg/kg s.c.) or saline (2 ml/kg s.c.) was given once daily, once evey 3 days, or once a week for varying periods of time. It was observed that (1) control reaction times decreased in the tested group and (2) the effect of chronically administered morphine was less in the tested group than in the non-tested group. It was concluded that experience in the test procedure can play a role in the development of tolerance to morphine. Thus, this possibility, and the consequences thereof, must be considered in studies dealing with the development of tolerance to morphine. © 1969.