Chemogenetic activation of hippocampal area CA2 promotes acute and chronic seizures in a mouse model of epilepsy

Abstract

Pyramidal cells (PCs) of hippocampal area CA2 exhibit increased excitability in temporal lobe epilepsy (TLE) and in mouse models of TLE. In epileptic mice, selective inhibition of CA2 PCs reduces chronic seizures. Here we asked if activating CA2 PCs increases seizures. Mice expressing Cre recombinase in CA2 PCs (Amigo2-Cre mice) were injected with the convulsant pilocarpine to induce a period of severe seizures (status epilepticus, SE), which leads to chronic seizures after 3-4 weeks (epilepsy). Epileptic mice were injected with a Cre-dependent adeno-associated virus (AAV) to express an excitatory designer receptor exclusively activated by designer drug (eDREADD; hM3Dq) in dorsal CA2 bilaterally and implanted with subdural EEG electrodes. After recovery, mice were recorded continuously using video and EEG for 6 weeks, 3 weeks with drinking water containing the eDREADD activator clozapine-N-oxide (CNO) and 3 weeks without CNO. CA2 activation with CNO caused a significant increase in seizure frequency and duration. Seizures occurred in clusters (many seizures per day over several consecutive days) and mice given water with CNO had a greater maximum number of seizures per day during a cluster compared to water without CNO. CNO had no significant effect in control mice. In naive Amigo2-Cre mice expressing hM3Dq, pre-treatment with CNO before pilocarpine administration shortened the latency to SE and increased EEG power at the start of SE. Taken together with prior findings, the results suggest that CA2 is a control point for regulating seizures in the pilocarpine mouse model of TLE. HIGHLIGHTSO_LIChemogenetic excitation of CA2 increased chronic seizure frequency and duration in epileptic mice. C_LIO_LIWhen seizures occurred in clusters, CA2 excitation increased the peak of the cluster. C_LIO_LIWhen sexes were separated, males showed effects on seizure frequency but both sexes showed effects on seizure duration and clusters. C_LIO_LIChemogenetic excitation of CA2 promoted pilocarpine-induced status epilepticus in normal mice. C_LI GRAPHICAL ABSTRACTLegend: In adult mice, injecting the convulsant pilocarpine induces severe seizures (status epilepticus, SE) and ultimately leads to chronic intermittent seizures (epilepsy). Selective activation of hippocampal area CA2 during the period of chronic seizures led to greater seizure frequency and duration, as well as a greater number of seizures/day at the peak of a seizure cluster. Effects were greatest in males. In naive mice, activation of CA2 prior to pilocarpine administration facilitated SE. Therefore, CA2 influences acute and chronic seizures in this mouse model. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=73 SRC="FIGDIR/small/696896v1_ufig1.gif" ALT="Figure 1"> View larger version (16K): org.highwire.dtl.DTLVardef@174f63dorg.highwire.dtl.DTLVardef@1d2478corg.highwire.dtl.DTLVardef@573c56org.highwire.dtl.DTLVardef@13a72e7_HPS_FORMAT_FIGEXP M_FIG C_FIG