Newly generated neurons at 2 months post-status epilepticus are functionally integrated into neuronal circuitry in mouse hippocampus

Abstract

Emerging evidence has linked chronic temporal lobe epilepsy to dramatically reduced neurogenesis in the dentate gyrus. However, the profile of different components of neurogenesis in the chronically epileptic hippocampus is still unclear, especially the incorporation of newly generated cells. To address the issue, newly generated cells in the sub-granular zone of the dentate gyrus were labeled by the proliferation marker bromodeoxyuridine (BrdU) or retroviral vector expressing green fluorescent protein 2months after pilocarpine-induced status epilepticus. The newly generated neurons that extended axons to CA3 area or integrated into memory circuits were visualized by cholera toxin B subunit retrograde tracing, and detecting activation of BrdU+ cells following a recall of spatial memory test at the chronic stage of TLE. We found that the microenvironment was still able to sustain significant neuronal differentiation of newly generated cells at 2months post-status epilepticus time-point, and newly added neurons into granular cell layer were still able to integrate into neuronal circuitry, both anatomically and functionally. Quantified analyses of BrdU+ or Ki-67+ cells demonstrated that there was a reduced proliferation of progenitor cells and diminished survival of newly generated cells in the epileptic hippocampus. Both decreased levels of neurotrophic factors in the surrounding milieu and cell loss in the CA3 area might contribute the decreased production of new cells and their survival following chronic epilepsy. These results suggest that decreased neurogenesis in the chronically epileptic hippocampus 2months post status epilepticus is not associated with altered integration of newly generated neurons, and that developing strategies to augment hippocampal neurogenesis in chronic epilepsy might be protective.