Antinociceptive effect of the combination of pentazocine with morphine in the tail-immersion and scald-pain tests in rats

We investigated the antinociceptive effect of pentazocine hydrochloride (pentazocine) in combination with morphine hydrochloride (morphine) using two antinociceptive tests; i.e., the tail-immersion and scald-pain tests, in rats. In the tail-immersion test, the rat's tail was immersed in warm water at 47 degrees C, and the latency to a nociceptive response was measured. In the scald-pain test, the right hind foot was scalded by immersion into hot water at 57 degrees C. Two hours later, additional thermal stimulus was applied to the same foot, and the latency to a nociceptive response was measured. Subcutaneous treatment with either pentazocine (6, 12, 24 mg/kg) or morphine (1.5, 3, 6 mg/kg) alone dose-dependently showed antinociceptive effects in both tests. The ED50 values (95% confidence limit) of pentazocine and morphine were 13.0 (5.4-31.5) and 2.4 (1.6-3.7) mg/kg in the tail-immersion test and 11.0 (4.5-26.6) and 3.8 (1.8-7.2) mg/kg in the scald-pain test, respectively. Simultaneous treatment with pentazocine at the similar dose augmented the morphine (1.5 mg/kg)-induced antinociception, but did not diminish the morphine (6 mg/kg)-induced antinociception in both tests. These results suggest that the simultaneous administration of pentazocine at the antinociceptive dose and morphine exerts additional antinociceptive activity against thermal and scald-induced inflammatory pain.