Abstracts

Rationale: Inhibitory neurons of the dentate gyrus are crucial in maintaining the functional “gate” that controls the spread of activity in the trisynaptic circuit. Specifically, parvalbumin-positive Axo-axonic cells (PV-AACs) regulate granule cell firing by inhibiting the axon initial segment and are distinct from PV-basket cells (PV-BCs), which mediate somatic feed-forward and feedback inhibition. In the human epileptic tissue, axonal cartridges of hippocampal dentate axo-axonic cells have been shown to undergo reorganization. Moreover, cortical axo-axonic cells have been shown to depolarize postsynaptic neurons. However, the intrinsic and synaptic characteristics of dentate PV-AACs and their regulation of granule cell activity and how this is altered after experimental status epilepticus (SE) is not known. This study examined the functional alterations in PV-AACs early after experimental SE. Methods: Adult mice (6-10 weeks) of both sexes expressing ChR2-EYFP in Parvalbumin neurons were treated with pilocarpine to induce SE and or injected with saline for controls. Whole-cell patch clamp recordings were obtained from EYFP positive cells in the inner molecular layer of hippocampal slices prepared from mice one week after SE (post-SE) and age-matched saline-injected and naiive controls. Cells were filled with biocytin during recordings and processed to reveal morphology. PV-AACs were distinguished from PV-BCs by the presence of vertical axonal cartridges and intrinsic electrophysiological parameters. Polygon400 DMD was used to optically stimulate somatic ROIs (3-5 ms pulses of blue light) and activate individual PV-ChR2 expressing neurons. Results: One week after SE, PV-AACs showed a significant decrease in firing rate in response to somatic current injection compared to controls (Two way RM ANOVA F (1, 16) = 5.162, p = 0.037). However, input resistance and action potential threshold in PV-AACs were not altered after early SE. The post-SE alterations in intrinsic physiology are unique to PV-AACs and were not observed PV-BCs (Yu et al., 2015). Similar to findings in PV-BCs, the frequency (Control: Median=14.5; IQR=6.8 - 30.2, n=7; Post-SE: Median=6.1; IQR=2.4 - 11.4, n=8, p<0.05 by Mann-Whitney U test) and amplitude of spontaneous inhibitory synaptic currents in PV-AACs were decreased early after SE. Similarly, the frequency of spontaneous excitatory synaptic currents in PV-AACs decreased after SE. In paired patch-clamp and optically evoked recordings of AAC evoked unitary inhibitory postsynaptic current (uIPSC) in GCs. The average peak amplitude of AAC evoked uIPSC in GCs was significantly decreased (in pA; Control: 128.47±21.9, n=20; post-SE: 40.42±7.1 pA, n=9 by unpaired T-test p<0.05) without a change in probability of release. Conclusions: Early after SE, dentate PV-AACs undergo a robust decrease in intrinsic excitability and also receive fewer excitatory and inhibitory synaptic inputs, suggesting that recruitment of PV-AACs during network activity may be impaired. Additionally, the significant decrease in uIPSC amplitude at AAC→GC synapses indicates that, even when recruited, the ability of PV-AACs to inhibit GC axon-initial segment is reduced following SE. Such a functional disconnection of dentate AACs after SE could compromise the dentate inhibitory gate early after status epilepticus and contribute to epileptogenesis. Funding: No funding