Morphine dependence in rats assessed in a shock discrimination task.

The effects of chronic morphine sulfate (5 mg/kg/day for 14 days) on shock discrimination performance of rats was assessed in Experiment 1. Significant tolerance developed to the disruptive effects of morphine on performance. A significant increase in discrimination performance was found 48 h after the last injection in tests conducted without the injection environment cues present. Ten days after the chronic drug regimen, tests conducted 30 min after exposure to the injection-environment cues revealed no differences between animals previously administered morphine and control animals administered saline. In Experiment II, shock discrimination performance was assessed in a separate group of rats after exposure to a single injection of morphine sulfate (30 mg/kg), which eliminated associative processes, e.g., Pavlovian or instrumental conditioning, as factors in the subsequent behavioral tests for hyperalgesia. Significant changes in discrimination performance (primarily enhanced percent correct to the high shock stimulus) indicative of increased pain sensitivity were obtained in tests conducted 1, 2, and 3, but not 9 days after the injection. These experiments indicate that increased pain sensitivity, as opposed to hyperresponsivity operationally measured in traditional analgesia tests (e.g., hot plate, jump-flinch, and tail flick), is a component of morphine withdrawal of a nonassociative origin.