Hiroki Hamura1, Motoyuki Yoshida1, Kenji Shimizu1,
Takeo Matsukura1, Hajime Suzuki1,
Minoru Narita2 and Tsutomu Suzuki2
1Researchﾊ& Development Division, Grelan Pharmaceutical Co.,
Ltd., 4-3, Sakaecho, 3-chome, Hamura-city, Tokyo 205-0002, Japan
2Departmentﾊ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ﾊmg/kg) or morphine
(1.5, 3, 6ﾊ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ﾊmg/kg)-induced antinociception, but did not
diminish the morphine (6ﾊ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