Abstracts

Rationale:
Precise timing and magnitude of inhibitory inputs are critical for maintaining proper hippocampal circuit function. Even small changes in the level of inhibitory control can have significant effects on the activity and function of the circuit. Brain stimulation is used increasingly in patients with refractory epilepsy through the presumed activation of impaired inhibitory networks. Close-loop optogenetic intervention of seizures via activating parvalbumin (PV) interneurons improves spatial memory in a chronic temporal lobe epilepsy (TLE) mouse model. Our previous studies demonstrate that circuit-based interventions via inhibitory chemogenetic receptors in the DG rescues epilepsy-associated spatial discrimination in a pilocarpine-induced (Pilo-) TLE mouse model. However, the precise mechanisms underlying the effectiveness of brain stimulation and the identity of interneurons controlling dentate gyrus (DG) granule cells hyperactivation in epilepsy remain elusive. The present study examines the pathological alterations of hippocampal GABAergic networks during chronic epilepsy development in a Pilo-TLE mouse model.
Method:
C57BL/6 mice at 7–9 weeks old were injected with pilocarpine to trigger status epilepticus (SE). Chronically epileptic mice were perfused with fixative, and were brains harvested at early stages (2 weeks -3 months) and late stages ( > 6 months) post-SE. Brain sections were subjected to double immunofluorescence, followed by confocal scans with identical thickness and imaging parameters for control and Pilo-TLE groups.  Image J  and sholl analysis were used to quantify the intensity and dendritic branching of PV+-interneurons.
Results:
In the dorsal hippocampus of early epileptic mice, there is a marked reduction of PV levels, especially in the DG, as well as a partial loss of hilar somatostatin (SST+) interneurons, and a dramatic increase of axonal terminals in the molecular layers compared with controls. Surprisingly, in late epileptic mice, while the number of PV+ interneurons in the DG is not altered, the PV levels and dendritic arborization of PV+ interneurons are increased substantially compared with controls. Conversely, the SST levels in reorganized SST+ networks returned to control levels. Furthermore, we found that a group of small interneurons called neurogliaform family cells (NFGC) are activated specifically all across the hippocampus in early and late epileptic mice. Endogenous c-Fos immunoreactivities were increased gradually in NFGC interneurons, while decreased in DG granule cells and CA1 pyramidal neurons. These activated interneurons showed prominent GABAAa2 expression in the soma.
Conclusion:
Our findings demonstrate chronic reorganization and enhancement of hippocampal PV- and NFGC- GABAergic networks in the Pilo-TLE mouse model, suggesting early seizure activity-induced disruption of GABAergic networks may cause chronic remodeling and enhancement of hippocampal GABAergic networks, contributing to abnormal neuronal synchronization and cognitive deficits in epilepsy patients. Our findings not only reveal novel pathogenic mechanisms at the circuit levels, but also provide rationales for timing- and circuit-specific precise brain stimulation in epilepsy patients.
Funding:
:NIH NS082046 (DC) and CHOP Pediatric Department Fund (HL)