No abstract

The N-methyl-D-aspartate (NMDA) receptor is a type of glutamate receptor, which is involved in neuronal function, plasticity and development in the mammalian brain. However, how the NMDA receptors contribute to adult neurogenesis and development of the dentate gyrus is unclear. In this study, we investigate this question by examining a region-specific knockout mouse line that lacks the NR1 gene, which encodes the essential subunit of the NMDA receptors, in granule cells of the dentate gyrus (DG-NR1 KO mice). We found that the survival of newly-generated granule cells, cell proliferation and the size of the granule cell layer are significantly reduced in the dorsal dentate gyrus of adult DG-NR1 KO mice. Our results also show a significant reduction in the number of immature neurons and in the volume of the granule cell layer, starting from 3 weeks of postnatal age. DG-NR1KO mice also showed impairment in the expression of an immediate early gene, Arc, and behavior during the novelty-suppressed feeding and open field test. These results suggest that the NMDA receptors in granule cells have a role in adult neurogenesis in the adult brain and contributes to the normal development of the dentate gyrus. Significance statementA type of glutamate receptors, the NMDA receptors, are known to be generally important in brain development. However, a previous study, which genetically ablated the NMDA receptors in the developing dentate gyrus, did not detect malformation of the dentate gyrus. Here, we performed more detailed analyses of the knockout mice and showed that the extent of neurogenesis and the size of the neuronal layer are reduced in the postnatal and adult dentate gyrus. We found that the knockout mice exhibit behavioural abnormality during adulthood. These results indicate that the NMDA receptor is essential for normal development of the dentate gyrus and that a malfunction of the NMDA receptors during the postnatal period could lead to life-long disturbance in brain functions.

Neurons of the central nervous system do not regenerate spontaneously after injury. As such, biofunctional tissue scaffolds have been explored to provide a growthpromoting environment to enhance neural regeneration. In this regard, aligned electrospun fibers have proven invaluable for regeneration by offering guidance for axons to cross the injury site. However, a high fiber density could potentially limit axonal ingrowth into the scaffold. Here, we explore which fiber density provides the optimal environment for neurons to regenerate. By changing fiber electrospinning time, we generated scaffolds with different fiber densities and implanted these in a rat model of spinal cord injury (SCI). We found that neurons were able to grow efficiently into scaffolds with high fiber density, even if the gaps between fiber bundles were very small (<1 μm). Scaffolds with high fiber density showed good host-implant integration. Cell infiltration was not affected by fiber density. Efficient blood vessel ingrowth likely requires larger gaps between fibers or faster degrading fibers. We conclude that scaffolds with high fiber densities, and thus a large number of small gaps in between fiber bundles, provide the preferred environment for nerve regeneration after SCI.

Efforts to understand the role of the dentate gyrus have been hampered by the limitation of extracellular recordings to identify cell types. By combining extracellular recording with optogenetics, it has become possible to identify and characterize the activity of specific cell types. Here, we assessed the feasibility of this technique to identify granule cells of the dentate gyrus in freely behaving mice. We were able to detect units that responded to light stimulation with short latency and high reliability. Although the short latency suggested that units were directly activated by light stimulation, we could not exclude the possibility that they were indirectly activated postsynaptic neurons. We also detected voltage fluctuations after the light stimulus, which were presumably caused by photovoltaic artefacts and synchronous activation of multiple cells. We believe that optogenetic identification of granule cells could be a valuable technique provided that the uncertainty about the response latency is resolved.