The Effect of Preemptive Analgesia with Intrathecal Morphine after Thermal Injury in the Rat

Jun, Jong Hun; Shin, Hong Ki

Abstract

BACKGROUND
The pre-emptive analgesic effect of intrathecal administered morphine, on the thermal hyperalgesia state induced by a mild burn injury in rats was examined to study the influence of the timing of the administration of morphine and the effects of its evaluation. METHODS: The thermal injury was induced by applying the left hind paw to a thermal surface (52.5 degrees C) for 45 seconds. The paw withdrawal latency (PWL) of the hind paw was determined using an underglass thermal stimulus. In the pretreated group, first morphine (3, 10 and 30 microgram) was injected through intrathecal catheters into the rat, and 15 minutes later, the mild burn injury was induced on the right hind paw. The procedure for the post treatment group was performed contrary to that in the pretreated group. First, the mild burn injury was induced, and 15 minutes later, the morphine (3, 10 and 30 microgram was injected. The PWLs were measured in the injured hind paws every 30 minutes, for about three hours. RESULTS: The PWL of the 3 and 10 microgram pretreated groups were significantly increased compared with those in post treatment groups, at 120 minutes, but there was no difference between the two groups treated with 30 microgram of morphine, at 120 minutes, and with all doses after 60 minutes. CONCLUSIONS: Pretreatment with intrathecal morphine produced a pre-emptive analgesic effect, at 120 minutes, in the thermal hyperalgesia evoked a mild burn injury. These results suggested that the thermal injury in the hyperalgesia induced by a mild burn injury plays an important role in the pre-emptive analgesic effect.