Morphine analgesic tolerance: its situation specificity supports a Pavlovian conditioning model.

Rats were made tolerant to morphine in either of two environments and then assessed for morphine-induced alteration of pain sensitivity in both environments. Analgesic tolerance was displayed when rats were tested in that environment in which they previously received morphine, but not in the alternative environment. The results indicate than an association between environmental cues and the systemic effects of morphine is crucial to tolerance development.