Muscarinic excitation of parvalbumin‐positive interneurons contributes to the severity of pilocarpine‐induced seizures

SummaryObjectiveA common rodent model in epilepsy research employs the muscarinic acetylcholine receptor (mAChR) agonist pilocarpine, yet the mechanisms underlying the induction of pilocarpine‐induced seizures (PISs) remain unclear. Global M1mAChR(M1R) knockout mice are resistant to PISs, implying that M1R activation disrupts excitation/inhibition balance. Parvalbumin‐positive (PV) inhibitory neurons express M1Rs, participate in cholinergically induced oscillations, and can enter a state of depolarization block (DB) during epileptiform activity. Here, we test the hypothesis that pilocarpine activation of M1Rs expressed onPVcells contributes toPISs.MethodsCA1PVcells inPV‐CREmice were visualized with a floxedYFPor hM3Dq‐mCherry adeno‐associated virus, or by crossingPV‐CREmice with the RosaYFPreporter line. To eliminate M1Rs from PV cells, we generated PV‐M1knockout (KO) mice by crossing PV‐CRE and floxed M1mice. Action potential (AP) frequency was monitored during application of pilocarpine (200 μm). In behavioral experiments, locomotion and seizure symptoms were recorded in wild‐type (WT) or PV‐M1KO mice during PISs.ResultsPilocarpine significantly increased AP frequency in CA1 PV cells into the gamma range. In the continued presence of pilocarpine, a subset (5/7) of PV cells progressed to DB, which was mimicked by hM3Dq activation of Gq‐receptor signaling. Pilocarpine‐induced depolarization, AP firing at gamma frequency, and progression to DB were prevented in CA1 PV cells of PV‐M1KO mice. Finally, compared to WT mice, PV‐M1KO mice were associated with reduced severity of PISs.SignificancePilocarpine can directly depolarizePV+ cells via M1R activation, but a subset of these cells progress toDB. Our electrophysiologic and behavioral results suggest that this mechanism is active duringPISs, contributing to a collapse ofPV‐mediated γ‐aminobutyric acid (GABA)ergic inhibition, dysregulation of excitation/inhibition balance, and increased susceptibility toPISs.