Spatial learning depends on both the addition and removal of new hippocampal neurons

Abstract

The role of adult hippocampal neurogenesis in spatial learning remains a matter of debate. Here, we show that spatial learning modifies neurogenesis by inducing a cascade of events that resembles the selective stabilization process characterizing development. Learning promotes survival of relatively mature neurons, apoptosis of more immature cells, and finally, proliferation of neural precursors. These are three interrelated events mediating learning. Thus, blocking apoptosis impairs memory and inhibits learning-induced cell survival and cell proliferation. In conclusion, during learning, similar to the selective stabilization process, neuronal networks are sculpted by a tightly regulated selection and suppression of different populations of newly born neurons.

Figure 1. Spatial Learning in a Water Maze Increases Cell Death and Decreases Cell Genesis in the Dentate Gyrus

(A) Latency to find the escape platform. The syringes represent BrdU injections. (B–D) Apoptotic cell death measured by the number of fractin-IR cells (B), active-caspase-3-IR cells (C), and both pyknotic and karyorrhexic dying cells (Apoptotic cells) (D). (E) Cell genesis measured by the number of BrdU-IR cells. C, Control group; D, day; L, Learning group; Y, Yoked group. A single asterisk (*) indicates p ≤ 0.05, double asterisks (**) indicate p ≤ 0.01, and triple asterisks (***) indicate p ≤ 0.001 compared to Control. A single circle (°) indicates p ≤ 0.05, double circles (°°) indicate p ≤ 0.01 compared to Yoked. Error bars represent the standard error of the mean (s.e.m.).

Figure 2. Schematic Representation of the Main Experiment

Syringes represent XdU BrdU (gray), IdU (green), and CldU (blue) injections. Black arrows represent the time of sacrifice. Red squares represent intracerebroventricular infusions (zVAD or Vehicle).

Figure 3. Representative Examples of Apoptotic, Newborn, and Proliferative Cells

(A) Fractin-IR pyknotic cell. (B) Fractin-IR karyorrhexic cell. (C) Active-caspase-3-IR cell. (D) BrdU-IR cells. (E) Ki67-IR cells. (F) HH3-IR cell in anaphase. (G) BrdU-labeled cells (red) are stained with Dcx (green), a typical marker for immature newborn neurons. (H) Healthy BrdU-DAB-IR cell. (I) Pyknotic BrdU-DAB-IR cell. (J and K) Small BrdU-IR cell (red) exhibiting nuclear shrinkage using Fluoro Nissl Green counterstaining. Bar scales: (A–C), (F), and (H–K) indicate 5 μm; (D), (E), and (G) indicate 10 μm.

Figure 4. The Stabilization of Spatial Performances Increases Cell Death and Cell Proliferation in the Dentate Gyrus

(A, E, I, and M) Latency to find the escape platform. The syringes represent BrdU injections. D, day. (B, F, J, and N) Apoptosis measured by the number of fractin-IR cells. (C, G, K, and O) Apoptosis measured by the number of both pyknotic and karyorrhexic dying cells. (D, H, L, and P) Cell proliferation measured by the number of Ki67-IR cells. A single asterisk (*) indicates p ≤0.05, double asterisks (**) indicate p ≤ 0.01, and triple asterisks (***) indicate p ≤ 0.001 compared to Control groups. Error bars represent the s.e.m.

Figure 5. Spatial Learning Promotes the Death of Newborn Neurons Generated a Few Days before Training

(A) Latency to find the escape platform. Learning groups were injected with BrdU either 4 or 3 d before initiation of training as indicated by the syringes. D, day. (B) Newly born cells measured by the number of BrdU-IR cells. (C and D) Apoptotic cells measured by the number of fractin-IR cells (C) and both pyknotic and karyorrhexic dying cells (D). (E) Number of BrdU-IR cells exhibiting characteristics of dying cells. (F) Percentage of BrdU-IR cells expressing the immature neuronal marker Dcx. (G) Extrapolated number of newborn neurons. A single asterisk (*) indicates p ≤ 0.05, double asterisks (**) indicate p ≤ 0.01, and triple asterisks (***) indicate p ≤ 0.001 compared to Control groups. Error bars represent the s.e.m.

Figure 6. zVAD Infusion Blocks Learning-Induced Cell Death and Impedes Spatial Memory

(A) Latency to reach the hidden platform in animals infused with zVAD (•) or vehicle (○). The dashed line represents the mean escape latency across the first four training days. (B) Memory of platform location during a probe test (seventh day of testing) measured by the time spent in the target quadrant (open bar). A single asterisk (*) indicates p ≤ 0.05. (C) Spatial histograms of the animals' locations and examples of swim paths to reach the platform location during the probe test (see Protocol S1 for more information). (D and E) Effects of zVAD (filled bars) and vehicle (open bars) treatments on cell death measured by the number of fractin-IR cells (D) and both pyknotic and karyorrhexic dying cells (E). (F and G) Effects of zVAD (filled bars) and vehicle (open bars) treatments on cell proliferation measured by the number of Ki67-IR cells (F) and HH3-IR cells (G). A single asterisk (*) indicates p ≤ 0.05, double asterisks (**) indicate p ≤ 0.01, and triple asterisks (***) indicate p ≤ 0.001, zVAD group compared to Veh group; a single circle (°) indicates p ≤ 0.05, and triple circles (°°°) indicate p ≤ 0.001, Learning group (L) compared to Control group (C). Error bars represent the s.e.m

Figure 7. Learning Increases the Number of New Neurons, and This Effect Is Blocked by zVAD Infusion

(A) Latency to reach the hidden platform in animals infused with zVAD (•) or vehicle (○). The dashed line represents the mean escape latency across the first four training days. The syringes represent IdU and CldU injections. D, day. (B) Effects of zVAD (filled bars) and vehicle (open bars) treatments on the survival of IdU-IR cells generated 7 d before exposure to the task. (C) Effects of zVAD (filled bars) and vehicle (open bars) treatments on the number of IdU-IR cells exhibiting characteristics of pyknotic cells. (D) Effects of zVAD (filled bars) and vehicle (open bars) treatments on the survival of CldU-IR cells generated 3 d before exposure to the task. (E) Effects of zVAD (filled bars) and vehicle (open bars) treatments on the number of CldU-IR cells exhibiting characteristics of pyknotic cells. A single asterisk (*) indicates p ≤ 0.05, double asterisks (**) indicate p ≤ 0.01, and triple asterisks (***) indicate p ≤ 0.001, zVAD group compared to Veh group; a single circle (°) indicates p ≤ 0.05, double circles (°°) indicate p ≤ 0.01, and triple circles (°°°) indicate p ≤ 0.001, Learning group (L) compared to Control group (C) .

Figure 8. Spatial Learning Depends upon a Selective Stabilization Process Where the Production of New Neurons Is Followed by an Active and Selective Removal of Others

The early phase of learning in the water maze, characterized by a fast improvement in performance, increases the survival of newborn neurons that were produced 1 wk before exposure to the task. Once the task has begun to be mastered, during the late phase of learning, learning induces apoptosis of newborn neurons that are a few days younger than the ones for which survival has been increased. This wave of cell death is followed by an increase in cell proliferation. Learning-induced apoptosis plays a pivotal role in this intermingled chain of events since it is necessary for the survival of the older, newly born neurons, but also for the increase in cell proliferation occurring during the late phase of learning.