Ionotropic glutamate receptor subunit expression in the rat hippocampus: Lack of an effect of a long-term ethanol exposure paradigm

Background: Studies have shown that acute ethanol exposure inhibits ionotro pic glutamate receptor function and that long-term ethanol exposure results in maladaptive increases in the expression of some of these receptors in n eurons. It has been postulated that these changes, when unopposed by ethano l, contribute, in part, to the hyperexcitability associated with ethanol wi thdrawal. In this study, we compared the effect of long-term ethanol exposu re on the hippocampal expression levels of subunits belonging to the three families of ionotropic glutamate receptors. Methods: Adult male Sprague-Dawley rats were fed an ethanol-containing diet for 16 days. This diet contained 0% ethanol on days 1 and 2, 3% on days 3 and 4, 5% on days 5 to 7, and 6.7% on days 8 to 16. Control rats received a n equivalent amount of an isocaloric diet without ethanol. Rats were killed on day 16 at the peak of ethanol consumption. Hippocampal homogenates were prepared by sonication and analyzed by Western immunoblotting techniques. On a separate group of rats, we measured withdrawal scores and audiogenic s eizures on day 17. Results: Ethanol-exposed rats had significantly higher withdrawal scores, a nd a significantly higher percentage of them developed audiogenic seizures; this indicates that the 16-day ethanol diet induces ethanol dependence. Un expectedly, we found that expression of NR1 (including the expression of NR 1 subunits containing the N1, C1, and C2 inserts), NR2A, NR2B, NR2C, GluR1, GluR2/3, GluR5, GluR6/7, and KA2 subunits was not altered in hippocampal h omogenates from ethanol-exposed rats. Conclusions: These results indicate that maladaptive changes in the hippoca mpal expression levels of ionotropic glutamate receptor subunits do not alw ays occur in ethanol-dependent rats. Consequently, other mechanisms must me diate the hyperexcitability state associated with ethanol withdrawal in the se animals.