(−)-nicotine ameliorates corticosterone's potentiation of N-methyl- d-aspartate receptor-mediated cornu ammonis 1 toxicity

Abstract

Hypercortisolemia, long-term exposure of the brain to high concentrations of stress hormones (i.e. cortisol), may occur in patients suffering from depression, alcoholism, and other disorders. This has been suggested to produce neuropathological effects, in part, via increased function or sensitivity of N-methyl- d-aspartate (NMDA)-type glutamate receptors. Given that cigarette smoking is highly prevalent in some of these patient groups and nicotine has been shown to reduce toxic consequences of NMDA receptor function, it may be suggested that nicotine intake may attenuate the neurotoxic effects of hypercortisolemia. To investigate this possibility, organotypic hippocampal slice cultures derived from rat were pre-treated with corticosterone (0.001–1 μM) alone or in combination with selective glucocorticoid receptor antagonists for 72-h prior to a brief (1-h) NMDA exposure (5 μM). Pre-treatment with corticosterone (0.001–1 μM) alone did not cause hippocampal damage, while NMDA exposure produced significant cellular damage in the cornu ammonis (CA)1 subregion. No significant damage was observed in the dentate gyrus or CA3 regions following NMDA exposure. Pre-treatment of cultures with corticosterone (0.1–1 μM) markedly exacerbated NMDA-induced CA1 and dentate gyrus region damage. This effect in the CA1 region was prevented by co-administration of the glucocorticoid receptor antagonist RU486 (≥1 μM), but not spironolactone (1–10 μM), a mineralocorticoid receptor antagonist. In a second series of studies, both acute and pre-exposure of cultures to (−)-nicotine (1–10 μM) significantly reduced NMDA toxicity in the CA1 region. Co-administration of cultures to (−)-nicotine (1–10 μM) with 100 nM corticosterone prevented corticosterone's exacerbation of subsequent CA1 insult. This protective effect of (−)-nicotine was not altered by co-exposure of cultures to 10 μM dihydro-β-erythroidine but was blocked by co-exposure to 100 nM methyllycaconitine, suggesting the involvement of nicotinic acetylcholine receptors possessing the α7* subunit. The present studies suggest a role for hypercortisolemia in sensitizing the hippocampal NMDA receptor system to pathological activation and indicate that prolonged nicotine exposure attenuates this sensitization. Thus, it is possible that one consequence of heavy smoking in those suffering from hypercortisolemia may be a reduction of neuronal injury and sparing of cellular function.