DIFFERENT RADIANT-HEAT INTENSITIES DIFFERENTIATE INTRACEREBROVENTRICULAR MORPHINE-INDUCED FROM BETA-ENDORPHIN-INDUCED INHIBITION OF THE TAIL-FLICK RESPONSE IN THE MOUSE

The antinociceptive effects of morphine and beta-endorphin given intracerebroventricularly (i.c.v.) or intrathecally (i.t.) were evaluated by inhibition of the tail-flick and hot-plate paw-licking responses evoked by three different intensities of heat stimulation (low, intermediate and high) in male ICR mice. Morphine given i.c.v. was more potent in inhibiting the tail-flick response evoked by low than high intensity of heat stimulus. Beta-endorphin given i.c.v., on the other hand, was equally potent in inhibiting the tail-flick response evoked by different intensities of heat. This differential effect of morphine and beta-endorphin was not shown when morphine and beta-endorphin were given i.t. Morphine and beta-endorphin given i.c.v. or i.t. were equally potent in inhibiting the paw-licking response at different intensities of hot-plate temperature. Our results suggest that morphine given i.c.v. may inhibit high intensity heat-evoked tail-flick response spinally and inhibits low intensity heat-evoked tail-flick response supraspinally. Our results also provide additional evidence for different neural mechanisms of antinociceptive action of morphine and beta-endorphin.