Odor conditioning with morphine: Conditioned preference, aversion, and analgesia

The dose-dependent effects of morphine on odor conditioning in rats were examined in two experiments. Subjects received one odor paired with either a low dose of morphine (0.5 mg/kg; Experiment 1A) or a high dose of morphine (10.0 mg/kg; Experiment 1B), alternating with another odor paired with saline. In Experiment 1A, subjects spent significantly more time over the morphine-paired odor than over the saline-paired odor in a spatial-odor preference test (odor preference). In Experiment 1B, subjects spent significantly less time over the morphine-paired odor than over the saline-paired odor (odor aversion). In Experiment 2, pairing an odor with a high dose of morphine (10.0 mg/kg) produced a conditioned analgesic response, measured by paw-lick latency. These studies demonstrate that an odor may serve as an effective stimulus to which the motivational and analgesic effects of morphine can be conditioned.