Rat strain differences in the potentiation of morphine-induced analgesia by stress

Restraint stress has been shown to increase the magnitude and duration of morphine-induced analgesia; however, this phenomenon has only been investigated using the Sprague-Dawley rat strain. The purpose of this study was to determine if other rat strains would also exhibit a potentiated analgesic response to morphine compared to their unrestrained controls. Dose-response and time course curves for the analgesic effect of morphine (1.0, 3.0, 5.6, 10 mg/kg) were generated in adult, male Wistar, Lewis, Fischer 344, Long-Evans Hooded, and Sprague-Dawley rats either unrestrained or restrained in Plexiglas cylinders, using the tail flick assay. Morphine produced dose-dependent increases in tail flick latencies, and this effect was potentiated by restraint stress in the Sprague-Dawley, Wistar, Lewis, and Fischer 344 strains, but not in the Long-Evans Hooded rats. Because Sprague-Dawley and Wistar rats displayed the most robust stress effect, the use of either of these rat strains is appropriate in studying the mechanisms of stress-induced potentiation of analgesia. The differences among rat strains demonstrated in this study may serve as a basis for correlation with opioid function.