Opposing effects of ethanol and nicotine on hippocampal calbindin-D _28k expression

Long-term ethanol exposure produces multiple neuroadaptations that likely contribute to dysregulation of Ca²⁺ balance and neurotoxicity during ethanol withdrawal. Conversely, nicotine exposure may reduce the neurotoxic consequences of Ca²⁺ dysregulation, putatively through up-regulation of the Ca²⁺-buffering protein calbindin-D_28k. The current studies were designed to examine the extent to which 10-day ethanol exposure and withdrawal altered calbindin-D_28k expression in rat hippocampus. Further, in these studies, we examined the ability of nicotine, through action at α₇*-bearing nicotinic acetylcholine receptors (nAChRs), to antagonize the effects of ethanol exposure on calbindin-D _28k expression. Organotypic cultures of rat hippocampus were exposed to ethanol (50-100 mM) for 10 days. Additional cultures were exposed to 500 nM (-)-nicotine with or without the addition of 50 mM ethanol, 100 nM methyllycaconitine (an α₇*-bearing nAChR antagonist), or both. Prolonged exposure to ethanol (≥50 mM) produced significant reductions of calbindin-D_28k immunolabeling in all regions of the hippocampal formation, even at nontoxic concentrations of ethanol. Calbindin-D _28k expression levels returned to near-control levels after 72 h of withdrawal from 10-day ethanol exposure. Extended (-)-nicotine exposure produced significant elevations in calbindin-D_28k expression levels that were prevented by methyllycaconitine co-exposure. Co-exposure of cultures to (-)-nicotine with ethanol resulted in an attenuation of ethanol-induced reductions in calbindin-D_28k expression levels. These findings support the suggestion that long-term ethanol exposure reduces the neuronal capacity to buffer accumulated Ca²⁺ in a reversible manner, an effect that likely contributes to withdrawal-induced neurotoxicity. Further, long-term exposure to (-)-nicotine enhances calbindin-D_28k expression in an α₇* nAChR-dependent manner and antagonizes the effects of ethanol on calbindin-D_28k expression.