Possible physiological control of morphine analgesia in mice

Several laboratories have reported that dynorphin and vasopressin may be present in the same neurons and released together in a fixed ratio. In this study, we report that vasopressin and dynorphin can regulate morphine-induced analgesia in the mouse tail-flick test. Its action is opposite that observed for dynorphin because, when administered simultaneously with morphine, vasopressin in a dose-related manner potentiated morphine-induced analgesia while dynorphin antagonized it. When vasopressin and dynorphin is administered together with morphine, morphine ED₅₀ returned to control level.