Morphine: some puzzles of well-known substance

The study of morphines influence on vocalization and movement, nociceptive reactivity in rats in free behavior, during trauma of an extremity, intraperitoneal acetic acid administration and two-hour restraint allow us to reveal specific analgesic actions of this substance in doses comparable to that used for pain relief in man. This action consisted in the almost disappearance of trauma-induced hyperalgesia or a significant reduction of visceral stimulation-induced hypalgesia without any significant changes in movement reactivity depression typical to these states. Moreover, analysis or morphines influence on movement reactivity depression in free behaving rats of different groups or parts of one group previously housed in different aversive conditions allow to understand the reasons of variability in the action of opiates and the significance of previous living conditions in determination of their effects. Furthermore, the enhancement of morphines action on movement reactivity 3-4 hours after a single naloxone administration was observed, which indicates the possibility of artificial regulation of opiate effects by direct action on opiate receptors by naloxone. Subchronic naloxone administration (0.5 mg/kg, 3 times per day for 3 days) led to substantial and longterm enhancement of morphines depressive effect on movement reactivity (20 and 105 hours after the last naloxone use) and to decrease in movement reactivity depression typical to restraint stress.