ALTERATION IN NEURONAL NETWORK DURING EPILEPTOGENESIS: FROM INHIBITORY TO EXCITATORY SHIFT IN HIPPOCAMPAL NETWORK

Epileptogenesis is a process in which a normal network changes by abnormal wiring into seizure-generating circuits. However, it has been difficult to study epileptogenesis due to the lack of reliable biomarkers that reflects synaptic changes. Synaptic vesicle glycoprotein 2A (SV2A) is a presynaptic protein and target of antiepileptic drugs. It has emerged as a promising PET biomarker of synapses. This study evaluated SV2A's temporal, spatial, and cell-type-specific regulation in epileptic rat brain. Status epilepticus was induced in adult male rats by intrahippocampal kainic acid injection, with sham-operated controls. Brains (n=4, for all groups) were collected during acute (1 day), latent (10 days), and chronic (90 days) phases. SV2A binding in the hippocampus was assessed by [³H]UCB-J autoradiography. We then conducted single-cell spatial analysis of SV2A expression by multiplex RNA in-situ hybridization (RNAscope) in hilus using confocal microscopy and QuPath image analysis. SV2A-expressing cells were counted based on their coexpression with Vesicular Gaba Transporter (VGAT), Vesicular Glutamate transporter (VGLUT), Somatostatin (SST), Vasointestinal Polypeptide (VIP), and Parvalbumin (Pvalb). [³H]UCB-J binding in hippocampus was shown to be reduced in the latent phase and elevated in the chronic phase. RNAscope revealed increased SV2A accompanied by VGLUT expression. We confirmed a reduction of SV2A mRNA expression from the latent phase onward, but only in the GABAergic interneurons. SV2A loss occurred specifically in Pvalb- and SST-expressing subtypes, and not VIP expressing neurons. These findings demonstrate SV2A is regulated in a cell-type-specific manner and identify pathological synaptic remodeling under epileptogenesis involving enhanced excitatory synaptogenesis in the hippocampus.