OLFACTORY CUES AND MORPHINE-INDUCED CONDITIONED ANALGESIA IN RATS

In a Pavlovian conditioning procedure, rats were exposed to an odor co nditioned stimulus (CS) and then mere given morphine with its effect s erving as the unconditioned stimulus (US). After four CS-US pairings, the CS was tested alone to assess the presence of an analgesic conditi oned response (CR) using a hot-plate test. In Experiment la, two group s were conditioned by pairing either 10 mg/kg morphine or saline with an odor CS. In Experiment Ib, two groups were given an odor CS paired or unpaired with 10 mg/kg morphine. These results established that an odor cue can support a morphine-induced analgesic CR. Experiment 2 cha racterized the dose-effect curve (0, 3, 10, and 30 mg/kg morphine) usi ng an odor conditioning procedure. The dose-effect curve showed an inv erted U-shaped function, with the 10 mg/kg morphine group having signi ficantly longer paw-lick latencies compared to all other groups. This finding contrasts with the monotonically ascending dose-effect curve f or the analgesic unconditioned response (UR) to morphine. (C) 1998 Els evier Science Inc.