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

Abstract

Although adolescence is a common age to initiate alcohol consumption, the long-term 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 (postnatal day, PD28-48) or adult (PD70-90) intermittent ethanol exposure to 4 g/kg ethanol administered intragastrically. We also investigated another neurogenic niche, the subventricular zone (SVZ), to determine if the 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 about 4 weeks after exposure when these animals were compared to control age-mates. The effects of adolescent ethanol on DCX immunoreactivity were specific to the hippocampus, with no significant exposure effects emerging in the SVZ. In both the DG and the 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, probably 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 chronic ethanol exposure in adolescence.

Figure 1

Overview and timeline of manipulations experienced by animals in the current study (n=10/grp). Note that the behavioral data is published elsewhere (see Broadwater & Spear, 2013).

Figure 2

Weight did not differ between adolescent exposure groups during the exposure period or in adulthood. Adult ethanol exposure resulted in weight deficits starting on the 6^th i.g. exposure day (P80), an effect that persisted ~4 weeks after the exposure period [F(11, 198)= 10.92, p<.05]. Asterisks (*) indicate significant difference from H20 controls.

Figure 3

DCX+IR in the dentate gyrus of the hippocampus in P74 and P116 brains after adolescent or adult exposure, respectively. Adolescent (but not adult) ethanol-exposed animals showed reduced DCX+IR compared to water controls [F(1, 33)= 101.30, p< .05]. An age-related decline in DCX+IR was also evident [F(1, 33)= 6.26, p< .05]. Asterisk (*) indicates significant difference from H20 controls.

Figure 4

DCX+IR in the subventricular zone (SVZ) after adolescent or adult exposure, respectively. DCX+IR was not influenced by prior exposure at either age in the SVZ, although an age-related decline in DCX+IR emerged [F (1,34)= 11.38, p<.05].

Figure 5

Ki67+IR in the DG of adolescent exposed adults (P74) did not significantly differ between adolescent exposure groups, suggesting that proliferation of NPC was not influenced by prior adolescent ethanol exposure. See arrows for examplex of ki67+ cells in the pictures on the right panel.

Figure 6

Cleaved caspase-3+IR, an index of cell death, in the DG of adolescent exposed adults (P74) was significantly increased in the ethanol group compared to H20 controls [F(1,18)= 16.63, p< .05] (see *). See arrows for examples of caspase-3+ cells (dark stained cells) in the pictures on the right panel.