Limited influence of Pavlovian conditioning on tolerance to the anorexic effect of morphine

Rats that developed tolerance to the anorexic effect of morphine under conditions supportive of associational tolerance were tested for Pavlovian conditioned tolerance. Initially sensitive and nonsensitive rats received chronic morphine injections of 10 and 20mg/kg respectively in the presence of an explicitly paired compound cue until tolerant to the morphine induced suppression of milk intake. In a test for compensatory responding, only nonsensitive rats increased their intake in response to a placebo injection of saline in the presence of the morphine cue. In tests of associative tolerance, morphine injections in the presence of an explicitly unpaired saline cue, or in the absence of either cue, did not produce a loss of tolerance. After the temporal pattern of morphine administration was extinguished, only nonsensitive rats showed both a loss of tolerance in the presence of the saline cue, and tolerance retention in the presence of the morphine cue.