Determinants of the analgesic action of opiates.

Nociceptive reactivity (tail-withdrawal reflex at 60 degrees C) during morphine administration was investigated in rats of the same population but of different batches in animals during restraint stress after their housing under different environmental conditions. The variability of the analgesic effects of opiates strongly depended on the functional state of animals at the moment of morphine administration, on the aversion to the living conditions prior to morphine and on some individual peculiarities of the organism with regard to the adaptive changes in opiate receptors. The direct action of naloxone, administered periodically, on opiate receptors induced an enhancement of the analgesic effects of morphine, which might result from the compensatory supersensitivity of opiate receptors after their blocking.