Compensatory network changes in the dentate gyrus restore long-term potentiation following ablation of neurogenesis in young-adult mice

Abstract

It is now well established that neurogenesis in the rodent subgranular zone of the hippocampal dentate gyrus continues throughout adulthood. Neuroblasts born in the dentate subgranular zone migrate into the granule cell layer, where they differentiate into neurons known as dentate granule cells. Suppression of neurogenesis by irradiation or genetic ablation has been shown to disrupt synaptic plasticity in the dentate gyrus and impair some forms of hippocampus-dependent learning and memory. Using a recently developed transgenic mouse model for suppressing neurogenesis, we sought to determine the long-term impact of ablating neurogenesis on synaptic plasticity in young-adult mice. Consistent with previous reports, we found that ablation of neurogenesis resulted in significant deficits in dentate gyrus long-term potentiation (LTP) when examined at a time proximal to the ablation. However, the observed deficits in LTP were not permanent. LTP in the dentate gyrus was restored within 6 wk and this recovery occurred in the complete absence of neurogenesis. The recovery in LTP was accompanied by prominent changes within the dentate gyrus, including an increase in the survival rate of newborn cells that were proliferating just before the ablation and a reduction in inhibitory input to the granule cells of the dentate gyrus. These findings suggest that prolonged suppression of neurogenesis in young-adult mice results in wide-ranging compensatory changes in the structure and dynamics of the dentate gyrus that function to restore plasticity.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Suppression of neurogenesis disrupts LTP in the dentate gyrus of young adult mice. (A) After 10 min of baseline recording, a tetanus of four bouts of 100-Hz stimulation (500 ms) separated by 30 s was delivered (arrows). Following the tetanus, the average fEPSP slope (as a percentage of baseline) was significantly enhanced in slices from wild-type mice treated for 28 d with GCV (wt/GCV). In contrast, slices from nestin-tk mice treated with GCV (tk+/GCV) exhibited no significant increase in fEPSP slope after the tetanus. (B) LTP in response to a high-frequency tetanus was examined in the presence of artificial cerebrospinal fluid that contained 50 μM picrotoxin. Slices from wild-type mice (wt/GCV) and nestin-tk mice (tk+/GCV) treated with 28 d of GCV both exhibited robust and comparable LTP. All data are mean ± SEM. (C) LTP is restored 42 d after termination of GCV treatment. Wild-type mice and nestin-tk mice were treated with 28 d of GCV (wt/GCV and tk+/GCV, respectively), and hippocampal slices prepared 42 d later. Slices from both groups exhibited robust LTP following the high-frequency tetanus. All data are mean ± SEM. Insets present representative extracellular fEPSP recordings made before (average of first 10 min) and after (average of last 10 min) the tetanus. (Scale bars, 0.25 mV/5 ms.)

Fig. 2.

GCV treatment persistently disrupts neurogenesis. Immunoreactivity for doublecortin was examined in both frozen-brain sections (A–D) and in dentate gyrus slices used for electrophysiological recordings (E–H). Intracerebroventricular infusion of GCV for 28 d dramatically reduces immature neurons in nestin-tk (tk+) mice, shown here by absence of doublecortin immunoreactivity (B and F) vs. controls receiving GCV (A and E). No recovery of doublecortin expression was seen after 28 d of GCV administration plus a 42-d recovery (D and H) vs. similarly treated controls (C and G). (Scale bar, 100 μm.)

Fig. 3.

Ablation of newborn neurons increases survival of cells born before starting the GCV infusion. (A) A cohort of proliferating cells labeled with BrdU 5 to 10 d before GCV infusion was examined after 14 or 28 d of GCV infusion. (B) After 28-d GCV infusion, more BrdU-labeled cells were evident in the SGZ and GCL of tk+ mice than WT controls (arrowheads). (C) Quantification of BrdU labeling revealed no difference after 14 d of GCV treatment, but a significantly greater number of surviving BrdU+ cells in tk+ mice after a 28-d GCV infusion. Data presented as mean ± SEM. *P < 0.02. (Scale bar, 100 μm.)

Fig. 4.

Ablation of neurogenesis in young adult mice decreases inhibitory innervation in the dentate gyrus. (A) Staining for VGAT in the dentate gyrus (Top row at 20× and Middle row at 63×) and fornix (Bottom row). The left two columns are immediately after 28-d GCV treatment, and the right two columns are 70 d after initiating GCV infusion (28 d + 42 d recovery). Immunoreactivity for VGAT (lighter staining) is maximal in the outermost portion of the GCL and in the OML, and is significantly decreased in tk+ mice only after treatment with GCV for 28 d followed by a 42-d recovery (70 d; right column of Top and Middle). The white matter tracts of the fornix (Bottom) show minimal VGAT immunoreactivity. [Scale bars: 100 μm (Top and Bottom), 25 μm (Middle.)] (B) Quantification of the optical densities (normalized to the white matter of the fornix). Although a trend toward lower normalized optical density was evident after 28 d of GCV treatment, VGAT immunoreactivity was significantly decreased after 42 d of recovery from GCV treatment. Data presented as mean ± SEM. *P < 0.05, **P < 0.001.

Fig. 5.

Miniature IPSC frequency is reduced in tk+ mice 42 d after treatment with GCV. (A) Representative whole-cell voltage clamp recordings from wild-type (A1) and tk+ (A2) mice 42 d after termination of GCV treatment. (B) Plot of cumulative fraction of the log10 interevent intervals revealed an increase in time between events as evidenced by a significant rightward shift in the tk+ curve (Kolmogorov-Smirnov, P < 0.0001). (C) Histograms of the number of events observed in recordings from slices prepared from wild-type mice (C1) and tk+ mice (C2). Log10 (interval) bins were set to 0.1 ms. Average mIPSC amplitude (D), area (E), and decay (F) were not significantly different 42 d after treatment with GCV (P > 0.5, Student t test). Data in D to F presented as mean ± SEM. (Scale bars, 10 pA/200 ms.)