NR2B-dependent plasticity of adult-born granule cells is necessary for context discrimination

Abstract

Adult-generated granule cells (GCs) in the dentate gyrus (DG) exhibit a period of heightened plasticity 4-6 weeks postmitosis. However, the functional contribution of this critical window of plasticity to hippocampal neurogenesis and behavior remains unknown. Here, we show that deletion of NR2B-containing NMDA receptors from adult-born GCs impairs a neurogenesis-dependent form of LTP in the DG and reduces dendritic complexity of adult-born GCs, but does not impact their survival. Mice in which the NR2B-containing NMDA receptor was deleted from adult-born GCs did not differ from controls in baseline anxiety-like behavior or discrimination of very different contexts, but were impaired in discrimination of highly similar contexts. These results indicate that NR2B-dependent plasticity of adult-born GCs is necessary for fine contextual discrimination and is consistent with their proposed role in pattern separation.

Figure 1.

Deletion of NR2B in adult-born GCs does not affect neurogenesis in the DG. A, B, Three-mouse design using an inducible reporter, EYFP, to measure GC survival and TMX-induced recombination (A) with representative images of YFP and DCX expression 6 weeks after TMX injection (B). Scale bar, 100 μm. C, Total number of adult-born GCs did not differ between NR2B-deficient and WT littermate controls 6 weeks after TMX injection, as measured by total numbers of EYFP cells. D, Genetic and experimental timeline for neurogenesis measurement. E, No effect on cell survival as measured by BrdU number at specified times postinjection. F, Representative images for BrdU (green, NeuN; red, BrdU; white, GFAP) and DCX experiments. Scale bar: top, 30 μm; bottom, 100 μm. G, No effect of treatment on total Ki67-immunoreactive cells. H, No significant difference in phenotype of BrdU cells 8 weeks after induction. I, J, No significant difference in total number of DCX cells or DCX+ cells with tertiary dendrites. K, L, Sholl analysis revealed a decrease in dendritic complexity in adult-born GCs lacking NR2B. K, Representative images and tracings. Scale bar, 20 μm. Data are mean ± SEM.

Figure 2.

Impaired ACSF-LTP in the DG of iNR2B^Nes mice. A, Experimental timeline for electrophysiology experiments. B, C, No differences in input–output relationship (B) or paired-pulse depression (C; paired-pulse ratio (PPR), 50 ms interstimulus interval) of MPP inputs to the DG after deletion of NR2B. D, Significantly impaired ACSF-LTP in slices from iNR2B^Nes mice compared with controls. Inset, Representative average traces before and after HFS. Scale bars: verticle, 0.5 mV; horizontal, 5 ms. E, No difference in magnitude of LTP in slices in the presence of 10 μm bicuculline. Data are mean ± SEM.

Figure 3.

iNR2B^Nes mice in depression/anxiety-related behavioral assays. A–F, Open-field test under low and high lux conditions with total distance traveled (A, D), percentage center distance (B, E), and percentage center time (C, F). G, Forced-swim test. H, Elevated plus maze. I, Novelty suppressed feeding task. J, K, iNR2B^Nes mice did not differ in weight or latency to feed in their home cage. iNR2B^Nes mice did not differ from controls in FST, EPM, or NSF. Data are mean ± SEM.

Figure 4.

Impaired object exploration and contextual fear discrimination in iNR2B^Nes mice. A, Experimental design for novelty exploration. B, C, Total approaches to the objects during training trials and testing reveal iNR2B^Nes mice explored the novel objects significantly less than control mice. D, E, iNR2B^Nes mice did not differ from controls in a one-shock contextual fear conditioning protocol, and did not generalize freezing to a novel context. F, G, iNR2B^Nes mice did not differ from controls in discriminating between two different contexts (experimental design in F). H, I, iNR2B^Nes mice were slower to discriminate two similar contexts than VEH-treated controls (experimental design in H). J, VEH-treated controls could discriminate by the fourth day of training, while iNR2B^Nes could not discriminate until the eighth day of training. *p < 0.05. Data are mean ± SEM.