Study of the intraplantar injection of lidocaine and morphine on pain perception and the influence of morphine dependence and withdrawal on lidocaine-induced analgesia in rats.

BACKGROUND Morphine and lidocaine are known to influence the perception of pain. The present study sought to determine the influence of local administration of morphine on lidocaine-induced analgesia in morphine non-dependent (MND), morphine dependent (MD) and morphine withdrawal (MW) animals. METHODS Adult male Wistar rats were divided into four groups: Control, MND, MD and MW rats. Lidocaine (0.5, 1 and 2%) and morphine (200, 400 and 800 µg) were injected in the plantar surface of the right paw. MD animals received chronic oral morphine (0.1, 0.2, 0.3 and 0.4 mg/ml in their drinking water) for 20 days. Twenty four hours before experiment, the animals in the MW group were deprived of morphine in their drinking water (physical dependence was observed by precipitating an abstinence syndrome with naloxone 2 mg/kg i.p.). Analgesia was assessed using hot-plate apparatus. RESULTS Morphine (400 µg) and lidocaine (2%) produce local analgesia in MND group. In MND rats, non-analgesic doses of each drug (200 µg morphine and 1% lidocaine) were used in combination and produced analgesia. In MD animals, all doses of lidocaine produced analgesia, while in MW animals, it failed to produce analgesia. In this situation, local administration of morphine could eventually influence the analgesic effect of lidocaine. CONCLUSION Opioid withdrawal is one of the most common problems in clinic. This study determined the analgesic effect of lidocaine in MW animals in which lidocaine had no analgesic effect. In this regard, local administration of morphine with combination of lidocaine could probably produce an effective analgesia.