A. Cepedabenito et St. Tiffany, TEST-SPECIFIC MANIFESTATIONS OF ASSOCIATIVE TOLERANCE TO THE ANALGESIC EFFECTS OF MORPHINE IN THE RAT, Psychobiology, 24(4), 1996, pp. 327-332
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.