Reduced inhibition of hippocampal adult-born granule cells by parvalbumin interneurons after TBI

Traumatic brain injury (TBI) is one of the leading causes of acquired temporal lobe epilepsy. TBI drives hippocampal circuit rearrangements that may contribute to increased seizure risk, such as altered inhibitory circuit function and aberrant post-traumatic neurogenesis. In the hippocampal dentate gyrus, adult-born dentate granule cells (DGCs) acquire inhibitory synaptic inputs from parvalbumin-expressing (PV) interneurons early in their maturation. These inputs are important for circuit integration and feedforward inhibition of these neurons. To test whether DGCs born after TBI have functionally altered PV-mediated innervation, we used genetically modified mice, retroviral vectors, and optogenetics to study adult-born and mature DGCs after TBI. Although DGCs born after TBI acquired inhibitory synaptic inputs during their maturation, PV-mediated inhibition of adult-born DGCs was persistently reduced following TBI. This was not observed in mature granule cells and was not due to TBI-induced changes in PV cell density. This deficit in PV-mediated functional innervation was associated with a transient reduction in release probability at these synapses, which normalized as DGCs matured despite ongoing reduction of functional PV input. Surprisingly, although spontaneous inhibitory postsynaptic currents were reduced for mature granule cells after TBI, these were unchanged in adult-born DGCs. Taken together, these data demonstrate distinct differences in the de novo development and maintenance of PV+ synapses in the dentate gyrus after TBI. The addition of neurons with reduced PV+ interneuron-mediated feed-forward inhibition to the dentate gyrus could contribute to hippocampal hyperexcitability after severe brain injury.