Influence of ω-conotoxin on morphine analgesia and withdrawal syndrome in rats

The effect of ω-conotoxin on opiate analgesia and withdrawal syndrome was investigated in rats. ω-Conotoxin given i.c.v. and i.p. caused weak analgesia in the tail-flick test. When the toxin (20 ng/rat) was given i.c.v. immediately before morphine (1.5 μg/rat i.c.v.) the resultant analgesic effect was additive. In contrast, the analgesia elicited by morphine (3 μg/rat i.c.v.) was greatly reduced after 24-h pretreatment with the toxin (20 ng/rat i.c.v.). The systemic administration of the toxin (10 μg/kg i.p.) did not affect morphine analgesia whether ω-conotoxin was coadministered with morphine (2.5 mg/kg i.p.) or was given 24 h before the opiate (5 mg/kg i.p.). ω-Conotoxin i.c.v. injected in morphine-dependent rats 15 min before naloxone challenge significantly attenuated the abstinence syndrome. On the contrary systemic administration of ω-conotoxin failed to suppress the morphine withdrawal syndrome. The present results suggest that ω-conotoxin affects both acute and chronic effects of morphine. © 1992.