Hiroki Hamura1, Motoyuki Yoshida1, Kenji Shimizu1,
Takeo Matsukura1, Hajime Suzuki1,
Minoru Narita2 and Tsutomu Suzuki2
1Research & Development Division, Grelan Pharmaceutical Co., Ltd., 4-3, Sakaecho, 3-chome, Hamura-city, Tokyo 205-0002, Japan
2Department of Toxicology, School of Pharmacy, Hoshi University, 4-41, Ebara, 2-chome, Shinagawa-ku, Tokyo 142-8501, Japan
Abstract: 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°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°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.
Keywords: Pentazocine, Morphine, Antinociceptive effect, Tail-immersion test, Scald-pain test