DIFFERENCES IN THE BIMODAL EFFECTS OF MORPHINE IN THE VAS-DEFERENS OF 2 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 sensitive 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.