Persistent loss of hippocampal neurogenesis and increased cell death following adolescent but not adult chronic ethanol exposure

摘要

Although adolescence is a common age to initiate alcohol consumption, long-lasting consequences of exposure to alcohol at this time of considerable brain maturation are largely unknown. In studies utilizing rodents, behavioral evidence is beginning to emerge suggesting that the hippocampus may be persistently affected by repeated ethanol exposure during adolescence, but not by comparable alcohol exposure in adulthood. The purpose of this series of experiments was to explore a potential mechanism of hippocampal dysfunction in adults exposed to ethanol during adolescence. Given that disruption in adult neurogenesis has been reported to impair performance on tasks thought to be hippocampally-related, we used immunohistochemistry to assess levels of doublecortin (DCX), an endogenous marker of immature neurons, in the dentate gyrus (DG) of the hippocampus 3–4 weeks after adolescent (P28–48) or adult (P70–90) intermittent ethanol exposure to 4 g/kg ethanol administered intragastrically. We also investigated another neurogenic niche, the subventricular zone (SVZ), to determine if effects of ethanol exposure were region-specific. Levels of cell proliferation and cell death were also examined in the DG via assessing Ki67 and cleaved caspase-3 immunoreactivity, respectively. Significantly less DCX was observed in the DG of adolescent (but not adult) ethanol exposed animals ~4 weeks post-exposure when these animals were compared to control age-mates. Effects of adolescent ethanol on DCX immunoreactivity were specific to the hippocampus, with no significant exposure effects emerging in the SVZ. In both DG and SVZ there was a significant age-related decline in neurogenesis as indexed by DCX. The persistent effect of adolescent ethanol exposure on reduced DCX in the DG appears to be related to significant increases in cell death, with significantly more cleaved caspase-3 positive immunoreactivity observed in the adolescent ethanol group compared to controls, but no alterations in cell proliferation when indexed by Ki67. These results suggest that a history of adolescent ethanol exposure results in lowered levels of differentiating neurons, likely due at least in part to increased cell death of immature neurons. These effects were evident in adulthood, weeks following termination of the chronic exposure, and may contribute to previously reported behavioral deficits on hippocampal-related tasks after the chronic exposure.