TEST-SPECIFIC MANIFESTATIONS OF ASSOCIATIVE TOLERANCE TO THE ANALGESIC EFFECTS OF MORPHINE IN THE RAT

The acquisition of associative tolerance to the analgesic effects of m orphine as assessed on the hotplate test was investigated in two exper iments. Associative contingencies were manipulated by giving rats eigh t moderately high morphine doses (20 mg/kg) either paired or unpaired with a distinctive context at a 96-h interdose interval. Tolerance was evaluated as shifts in dose-response curves of morphine-experienced r elative to morphine-naive animals tested in the distinctive context. E xperiment 1, in which independent groups of animals were tested with h ot-plate temperatures of 50 degrees C (N = 302) and 54 degrees C (N = 175), produced no evidence of context-specific tolerance to morphine. Experiment 2 (N = 200) evaluated the effect of contextual contingencie s of drug delivery on the development of morphine tolerance in animals tested on either the hot-plate or the tailflick test. As found in Exp eriment 1, animals tested on the hot plate showed no context-specific tolerance, whereas animals tested on the tailflick displayed pronounce d associative-tolerance effects. The data suggest that the hot plate, as opposed to other tests of analgesia, does not provide a sensitive m easure of associative morphine tolerance.