Evidence for the involvement of opioidergic systems in medprotine-induced analgesia in the mouse

Intramuscular administration of medprotine, a protein fraction of human placenta (30-300 kDa), produced dose-dependent and long lasting analgesia as evaluated by various analgesic tests in mice. Activity was found in a dose range of 0.05-0.6 mg/kg in the phenylbenzoquinone and acetic acid writhing tests and 1.5-5 mg/kg in the hot-plate test. In this latter model, medprotine enhanced morphine-induced analgesia. Pretreatment with naloxone antagonized the effect of medprotine in all assays. The antinociceptive response of medprotine was not modified after a ten day pretreatment and no overt withdrawal symptom could be observed after either interruption of chronic treatment or administration of a precipitating dose of naloxone. It is concluded that the analgesic activity of medprotine may be mediated through either the opiate receptors or activation of endogenous opioidergic systems.