Temporal and environmental cues in conditioned hypothermia and hyperthermia associated with morphine.

The effectiveness of temporal and environmental cues in eliciting conditioned hypothermia and hyperthermia was studied in male Wistar rats using as an unconditioned stimulus an IP injection of 20 mg/kg of morphine sulfate. The relevance of temporal stimuli was minimized in Experiment 1 by administering morphine at irregular times on alternate days. For one group (Cond) morphine injections were preceded and followed by periods in distinctive environments. Group Pseudo animals, though exposed to the environments, received morphine on the intervening days in the home cage; group Saline received only saline. All animals receiving morphine showed a non-specific hypothermia when not under the direct influence of morphine. A "conditioned hyperthermia" was evident in group Cond animals in the distinctive environments. In Experiment 2, in which animals remained in their home cages at all times, the relevance of temporal cues was emphasized by administering morphine at exactly 24 h intervals. These animals became hypothermic only around the time of the expected injection. Animals in another group that received morphine at irregular times showed the non-specific hypothermia seen previously. There was no evidence for a conditioned hyperthermia in this second experiment.