Differential effects of morphine on two-way avoidance in selectively bred rat strains

Effects of morphine administration were investigated on avoidance behavior in five rat strains. Strain-dependent facilitation or suppression of avoidance acquisition was found in animals tested 1 h after the last administration of morphine. Significant strain differences were found in the incidences of sexual stimulation, diarrhea and wetness, reactivity, and death due to toxicity.