Cytotoxic effects of exposure to the human immunodeficiency virus type 1 protein Tat in the hippocampus are enhanced by prior ethanol treatment

Background: Long-term ethanol exposure leads to increases in the expression and/or sensitivity of NMDA-type glutamate receptors, effects that may contribute to the development of cytotoxicity in the brain. The human immunodeficiency virus 1 (HIV-1) transcription factor Tat is one of many viral proteins that may contribute to the development of HIV-associated dementia (HAD) by indirectly or directly promoting excess function of NMDA receptors. Thus, these studies examined the hypothesis that long-term ethanol pre-exposure would sensitize the hippocampus to Tat-induced cytotoxicity in an NMDA receptor-dependent manner. Methods: Organotypic slice cultures of rat hippocampus were exposed to a recombinant 86-amino acid form of Tat (Tat1-86) or a Tat deletion mutant devoid of amino acids 31 to 61 (TatΔ31-61; 0.1-100 nM) for 24 hr alone or during withdrawal from 10 days of ethanol exposure (50 mM in culture medium). Additional cultures were exposed to NMDA (5 μM) or the NMDA receptor channel blocker MK-801 (1 μM) during these treatments. Cellular injury in the CA1, CA3, and dentate gyrus regions of slice cultures was assessed by microscopy of propidium iodide fluorescence. Results: Twenty-four hours of withdrawal from ethanol exposure did not produce overt cellular injury in any region of slice cultures. However, NMDA-induced toxicity was markedly increased in ethanol-pre-exposed cultures, an effect prevented by MK-801 (1 μM) coexposure. Treatment of cultures with Tat1-86 alone (≥0.1 nM) produced modest toxicity in each region of hippocampal cultures that was also blocked by MK-801 coexposure. In contrast, exposure to TatΔ31-61 did not alter propidium iodide fluorescence. Exposure of cultures to Tat1-86 (≥0.1 nM) during ethanol withdrawal resulted in a marked potentiation of Tat's toxic effects in each region of slice cultures, particularly the CA1 region. This potentiation of Tat neurotoxicity was significantly attenuated by coexposure of cultures to MK-801 (1 μM). Conclusions: These results indicate that long-term ethanol exposure sensitizes the hippocampus to the cytotoxic effects of Tat in an NMDA receptor-dependent manner. This may suggest that HIV-1-positive individuals who are alcohol dependent possess a heightened risk for the development of HAD. Furthermore, the NMDA receptor, particularly allosteric modulatory sites such as polyamine-sensitive sites, may be a therapeutic target to be investigated in the treatment of HAD.