Comparison of the ability of opioid analgesics to increase endogenous opioid-like activity in the cerebrospinal fluid of rabbits

We have previously demonstrated that the acute administration of morphine increases the level of endogenous substances, which have antinociceptive activity, in cerebrospinal fluid (CSF). The present study was conducted to determine whether other opioid analgesics exert a similar effect. CSF was withdrawn from the cisterna magna of anesthetized rabbits before and after s.c. injections of meperidine, pentazocine, levorphanol and methadone, and was bioassayed for opioid-like activity in the mouse tail-flick and phenylquinone writhing tests. The opioid-like activity of CSF taken 60 min after meperidine (50 mg/kg) was significantly increased in both bioassays, and the CSF level of meperidine was insufficient to account for this effect. Pentazocine (25-75 mg/kg) also significantly increased opioid-like activity in rabbit CSF, but the effects of methadone (5-10 mg/kg) and levorphanol (20 mg/kg) were less marked. Dextrorphan (20 mg/kg), diazepam (10 mg/kg) and pentobarbital (20 mg/kg) administration did not significantly increase opioid-like activity in CSF. It is concluded that the antinociceptive action of some opioid analgesics in rabbits may be mediated in part by the release of endogenous antinociceptive substances.