Influence of ethanol and morphine on pain perception evoked by deep tissue injury

Abstract

The aim of this study was to evaluate the effect of ethanol and morphine on nociceptive behavioral responses evoked by the injection of formalin into the temporomandibular joint region of rats (the TMJ formalin test). In experiment 1, animals were given an ethanol solution (6.5%) or tap water to drink for 4 and 10 days, before the procedure for TMJ pain. In the group treated for 4 days, significant analgesia was observed in the TMJ formalin test, whereas the group treated for 10 days did not show this effect, revealing the development of tolerance to ethanol antinociceptive effects. In experiment 2, animals were submitted to chronic regimen of ethanol (6.5% for 10 days) and the control group was given tap water to drink. After this period, morphine (10 mg/kg i.p.) was administrated 30 minutes before the TMJ formalin test. Morphine had the same analgesic effect in both groups, showing that the treatment with ethanol was not able to alter the analgesic potency of morphine. The results showed that ethanol can affect nociceptive behavioral responses related to pain from deep tissues, like the TMJ, and the absence of interaction between ethanol and morphine suggest that ethanol-induced analgesia was mediated by nonopiate mechanisms.