Noxious behaviors from subcutaneous injection of formalin and morphine tolerance effects on the responses of addicted male rats

Formalin as a chemical noxious stimulus evokes a biphasic pain that its second phase as a chronic inflammatory pain is similar to clinical pain. In this study, we used 40 NMRI male rats (250-300 g), and experiments were carried out considering ethical rules. Experiments were performed in normal and morphine addicted rats, the addiction was induced by Leung method. Behaviors of flinching and licking scored as quantifying pain after injection of 50 ?l formalin (2.5%) into plantar region of hind paw. Assessment of morphine tolerance was carried out by intraperitoneal injection of morphine (10 mg/Kg) 10 minute before formalin injection. In the control group (normal rats, n=10) we observed a biphasic model of formalin test, but in the other normal group (n=10) morphine injection depressed noxious behaviors. In the addicted groups (n=10) flinching and licking responses evoked by formalin showed a biphasic model and morphine did not affect their behaviors. Only licking scores in addicted rats have significant difference from normal rats. Addicted rats showed tolerance to morphine analgesia after formalin injection.