Differences in the bimodal effects of morphine in the vas deferens of two strains of rats

1. The effect of morphine on the smooth muscle of the isolated vas deferens of Sprague-Dawley and Wistar rats was studied. 2. Morphine produces pre- and post-synaptic effects on the contractile activity of the vasa deferentia. 3. The pre-synaptic action is an increase in the basal tension of the vas deferens. 4. The post-synaptic effect is a bimodal change in the height of the muscular twitch induced by neurogenic transmural stimulation. 5. Sprague-Dawley rats are more sensitivite than Wistar rats to the actions of morphine. 6. The morphine induced responses were antagonized by naloxone better in Wistar than Sprague-Dawley rats. 7. The findings obtained suggest a genetic difference in the synthesis of opioid subtype receptors.