MORPHINES INTEROCEPTIVE STIMULI AS CUES FOR THE DEVELOPMENT OF ASSOCIATIVE MORPHINE-TOLERANCE IN THE RAT

This study examined the extent to which a 4-mg/kg dose of morphine pai red explicitly with a 12-mg/kg dose could gain associative control ove r tolerance development in rats. The analgesic effects of morphine wer e measured on the tail-flick test. Tolerance was defined as the dose s hift to the right of the dose-response curve (DRC) of drug-experience animals relative to saline control animals. DRC shifts to the right we re greater in the explicitly paired than in the explicitly unpaired an imals. The results suggest that the pharmacological effects of morphin e may function as a conditioned stimulus for the development of associ ative morphine tolerance.