Effects of the benzomorphan κ-opiate, MR 2266 and its (+) enantiomer MR 2267, on thermonociceptive reactions in different strains of mice

Abstract

The effects of the benzomorphan κ opiate MR 2266 and its dextro enantiomer MR 2267 were assessed on thermonociception in Swiss Webster, C57BL/6, BALB/c and DBA/2 strains of mice. In the hot plate (60 ± 0.5°C, cut-off time 120 s) and tail immersion tests, MR 2266 (10 mg/kg, s.c., 15 min before) decreased, while MR 2267 (10 mg/kg s.c., 15 min before) increased the reaction latencies. In the hot plate test, the sensitivities for the effects of MR 2266 and MR 2267 on jump latency in different strains of mice were as follows: MR 2266; BALB > Swiss > C57BL > DBA and MR 2267; DBA > BALB = Swiss > C57BL. In the immersion test, for the hyperalgesic response of MR 2266, the rank order of strains was; BALB > c57BL and DBA ≥ Swiss while the rank order for the analgesic effect of MR 2267 was; Swiss > DBA and BALB. The results indicate the presence of tonic κ-receptor-mediated regulation of the spinal and supraspinal thermonociceptive reactions which is stereospecific and strain dependent.