Effects of ethanol and ethanol withdrawal on nociception in rats

The effect of acute and chronic administration of ethanol and ethanol withd rawal on a radiant heat tail-flick assay of nociception was examined in rat s. Acute administration of ethanol (2.0 g/kg, ip) produced peak antinocicep tion (68% of maximum) by 30 min, and effects were gone by 120 min. Cumulati ve doses of ethanol (0.5-2.0 g/kg, ip) produced dose-dependent increases in latencies to 49% of maximum. During chronic administration, a liquid diet containing ethanol (6.5%) was given for 10 days. Tail-flick latencies were measured on day 0 (baseline), day 2, 4, 6, 8, and 10 of chronic ethanol and at 3, 6, 12, and 36 hr after removal of ethanol. To test for behavioral to lerance, both between- and within-group designs were used. In both between- and within-group experiments, the antinociceptive effects of chronic ethan ol peaked by day 4 of exposure to the liquid diet, and tolerance developed by day in. When the liquid diet was removed, hyperalgesia was detected at 6 and 12 hr after withdrawal, and was gone by 36 hr after withdrawal. When c umulative doses of ethanol (0.5 2.0 g/kg) were administered starting 12 hr after withdrawal, ethanol (0.5 g/kg) fully reversed the hyperalgesia induce d by ethanol withdrawal, even though this dose was without antinociceptive effect in the absence of withdrawal. Higher doses of ethanol during ethanol withdrawal did not increase tail-flick latencies over baseline. In summary : (1) ethanol produces antinociception when administered acutely or chronic ally; (2) tolerance to the antinociceptive effects develops during chronic administration; (3) ethanol withdrawal induced hyperalgesia, which was reve rsed by ethanol; and (4) repeated testing did not produce behavioral tolera nce.