Ethanol Increases Mechanical Pain Sensitivity in Rats via Activation of GABAA Receptors in Medial Prefrontal Cortex
BACKGROUND Acute and chronic ethanol produces antinociception, and ethanol withdrawal induces hyperalgesia. METHODS A radiant heat tail-flick assay was used to assess the effects of benzodiazepine ligands on ethanol-induced changes in nociception in rats. Acute activity of cumulative doses of ethanol (0.5-2.0 g/kg) and diazepam (0.1-10 mg/kg), a benzodiazepine agonist, was tested alone and after pretreatment with flumazenil (1.0-10 mg/kg), a benzodiazepine antagonist. Chronic effects of ethanol were tested in three groups of rats that received a liquid diet for 10 days. One group received ethanol alone; one group received ethanol and twice-daily injections of flumazenil (10 mg/kg); and one received a dextrin control diet. Acute withdrawal was tested at 12 hr after removal of the liquid diet. Effects of cumulative doses of diazepam (1.0-10 mg/kg) were tested during withdrawal (12 hr) in the ethanol-alone group. RESULTS Acute doses of ethanol produced a small but significant degree of antinociception, which was fully suppressed by flumazenil. Acute doses of diazepam did not produce antinociception. Chronic exposure to ethanol produced antinociception on days 2 through 8. Tolerance developed by day 10, and hyperalgesia was seen 12 hr after removal of ethanol. Administration of diazepam or ethanol during withdrawal reversed the hyperalgesia induced by ethanol withdrawal. However, flumazenil (10-50 mg/kg) failed to reverse the antihyperalgesic effect of either diazepam or ethanol. No antinociception was seen in either the ethanol/flumazenil or dextrin control groups. CONCLUSIONS These results suggest that the antinociceptive effects of both acute and chronic ethanol are at least partially mediated by GABA receptors, and that diazepam's antihyperalgesic effects may not be mediated by the GABA acid receptor.