Increasing adult hippocampal neurogenesis is sufficient to improve pattern separation

Abstract

Adult hippocampal neurogenesis is a unique form of neural circuit plasticity that results in the generation of new neurons in the dentate gyrus throughout life. Neurons that arise in adults (adult-born neurons) show heightened synaptic plasticity during their maturation and can account for up to ten per cent of the entire granule cell population. Moreover, levels of adult hippocampal neurogenesis are increased by interventions that are associated with beneficial effects on cognition and mood, such as learning, environmental enrichment, exercise and chronic treatment with antidepressants. Together, these properties of adult neurogenesis indicate that this process could be harnessed to improve hippocampal functions. However, despite a substantial number of studies demonstrating that adult-born neurons are necessary for mediating specific cognitive functions, as well as some of the behavioural effects of antidepressants, it is unknown whether an increase in adult hippocampal neurogenesis is sufficient to improve cognition and mood. Here we show that inducible genetic expansion of the population of adult-born neurons through enhancing their survival improves performance in a specific cognitive task in which two similar contexts need to be distinguished. Mice with increased adult hippocampal neurogenesis show normal object recognition, spatial learning, contextual fear conditioning and extinction learning but are more efficient in differentiating between overlapping contextual representations, which is indicative of enhanced pattern separation. Furthermore, stimulation of adult hippocampal neurogenesis, when combined with an intervention such as voluntary exercise, produces a robust increase in exploratory behaviour. However, increasing adult hippocampal neurogenesis alone does not produce a behavioural response like that induced by anxiolytic agents or antidepressants. Together, our findings suggest that strategies that are designed to increase adult hippocampal neurogenesis specifically, by targeting the cell death of adult-born neurons or by other mechanisms, may have therapeutic potential for reversing impairments in pattern separation and dentate gyrus dysfunction such as those seen during normal ageing.

Figure 1

Bax ablation in neural stem cells in the adult brain increases hippocampal neurogenesis and neurogenesis-dependent LTP. a, Schematic illustrating genetic gain of function strategy to increase adult hippocampal neurogenesis. (i) In the adult DG, substantial fraction of adult-born neurons undergo Bax-dependent programmed cell death (shown in light red). (ii) Nestin CreER^T2 mediated ablation of Bax in type I and type II cells results in generation of adult-born neurons lacking Bax thereby preventing their cell death. b, Experimental design. c, Representative Dcx immunostained coronal hippocampal sections of Veh and TAM treated NCff mice. Arrows in insets indicate Dcx neurons with at least tertiary dendrites. Quantification of Dcx population. Total Dcx+ neurons: 6974 ± 600 (NCff+Veh), 12636 ± 1764 (NCff+TAM), *P=0.038. Dcx+ neurons with at least tertiary dendrites: 1800 ± 340 (NCff+Veh), 4090 ± 285 (NCff+TAM), ** P=0.006, n=3 mice per group. d, Representative coronal hippocampal sections of TAM treated NCY and NCffY mice and Veh treated NCffY mice immunostained for YFP and NeuN. Quantification of YFP+ neuronal population. Average number of YFP+ neurons per section, Whole hippocampus: 29.3± 2.1 (NCY+TAM), 87.6± 14.3 (NCffY+TAM), *P=0.015. Septal: 28.76 ± 2.3 (NCY+TAM), 76.3± 15.4 (NCffY+TAM), *P=0.03. Temporal: 30.66 ± 5.7 (NCY+TAM), 103.6± 20 (NCffY+TAM), *P=0.02. n=3 mice per group. e, NCff+TAM mice show enhanced medial perforant path-dentate gyrus LTP compared to NCff+Veh mice. Repeated measures ANOVA (50 minutes) shows significant effect of treatment, F_{(1, 17)}=5, P=0.039. Post-tetanic potentiation was significantly different between the two groups P=0.003. f, Medial perforant path-DG LTP evoked in presence of bicuculline was similar between the groups, repeated measures ANOVA, last 30 minutes: F_{(1, 8)}<1. (e) n=8 slices, 6 mice (Veh), n=11 slices,7 mice (TAM); (f) n=4 slices, 3 mice (Veh), n=6 slices, 3 mice (TAM). Results are mean ± SEM. Scale bar: 100μm.

Figure 2

Increasing adult hippocampal neurogenesis is sufficient to improve discrimination between similar contexts. a, Experimental design to test rapid one trial contextual encoding. b, On day 1, both groups show negligible levels of freezing in context A prior to a single 2 second 0.75mA foot-shock delivery. Controls (NCff+Veh)(n=14) and mice with more adult-born neurons (NCff+TAM)(n=14) showed comparable levels of conditioning to the training context A and negligible levels of freezing in a distinct context C. TAM treatment on its own does not affect contextual encoding as reflected in similar levels of freezing of ff+Veh (n=15) and ff+TAM (n=16) mice in contexts A and C. c, Experimental paradigm to test discrimination between two similar contexts A and B. d, Analysis of discrimination ratios. NCff+TAM mice show significantly higher levels of discrimination between the two contexts than NCff+Veh mice. e, Freezing behaviour of mice with increased adult hippocampal neurogenesis and controls over duration of experiment. Although both groups show comparable and extensive generalization between the two contexts at the beginning of the experiment, NCff+TAM mice (n=11) distinguished between context A and B more rapidly than NCff+Veh mice (n=9). f, NCff+Veh mice are able to discriminate between the two contexts by day 9 of testing. *P<0.05, **P<0.01. Results are mean ± SEM.

Figure 3

Increasing adult hippocampal neurogenesis does not produce anxiolytic or antidepressant-like behavioural effects. a–b, NCff+Veh and NCff+TAM mice showed comparable locomotor activity, rearing events and anxiety-like behaviour in the Open Field test under two different lighting conditions. c, NCff+Veh and NCff+TAM mice show similar anxiety-like behaviour in the Novelty Suppressed Feeding Paradigm. d, Total mobility of NCff+Veh and NCff+TAM mice did not differ significantly in the Forced Swim Test. n=9–14 mice per group. Results are mean ± SEM.

Figure 4

Mice with more adult-born neurons display increased exploratory behaviour and decreased anxiety-like behaviour in the open field test following a voluntary exercise regimen. a, Experimental design. Mice were transferred to cages with running wheels five weeks following Veh/TAM treatment. b, Representative images of Dcx and BrdU (red)/NeuN (green) immunostained coronal hippocampal sections of Veh and TAM treated NCff mice. Quantification of Dcx population. Total Dcx+ neurons: 14527 ± 987 (NCff+Veh), 19893 ± 2022 (NCff+TAM), P=0.05. Number of BrdU+ cells in the GCL: 2119 ± 204 (NCff+Veh), 9324 ± 463 (NCff+TAM), ** P<0.0001. n=4–5 mice per group. c, NCff+TAM mice show significantly greater locomotor activity, reduced anxiety-like behaviour and a significant increase in rearing events in the Open Field test compared to NCff+Veh mice. n=10 (Veh) and n=11 (TAM) mice. *P<0.05, **P<0.01. Results are mean ± SEM. Scale bar 100μm.