Inhibition of inflammatory pain by naloxone and its N methyl quaternary analogue.

Naloxone hydrochloride (Nx) and methylnaloxone methylsulfate (MeNx) antagonized the inflammatory pain, but not oedema, produced by intraplantar carrageenan in rats. The liminar effective dose (3 microgram/kg) was the same for the two drugs after intraplantar or subcutaneous injection. The s.c. dose effects curves were, as a whole, similar for the two drugs. The effect was long lasting and stereospecific. The low "analgesic" s.c. doses of Nx were able to antagonize morphine analgesia, those of MeNx were not. The interpretation is difficult; a peripheral action or better the local production of morphinomimetic metabolite(s) might account for most facts.