| Epilepsy is a common brain disorder characterized by abnormal synchronous discharges of neurons in the brain.Temporal lobe epilepsy(TLE)is one of the most common types of focal epilepsy and a major form of refractory epilepsy.Its pathological features include changes in various subregions of the hippocampus and abnormal local network activity.Previous studies have indicated that the hippocampal CA1,CA3,and dentate gyrus(DG)are closely related to the onset and progression of epilepsy.However,recent research suggests that the CA2 subregion of the hippocampus,with distinct cellular composition and function,might play a unique role in seizure propagation.To date,the exact role of the CA2 subregion in TLE remains unclear,primarily due to research focusing on ex vivo brain slices,which cannot fully replicate the complex in vivo physiological environment.This study employs techniques such as immunohistochemistry,electrical lesioning,chemogenetic intervention,optogenetic intervention,behavioral analysis,and biomedical signal processing to systematically investigate the role of the CA2 subregion in a TLE animal model.The main findings are as follows:1.In a pilocarpine-induced TLE mouse model,the expression levels of c-fos in the hippocampus were observed before and after seizures to explore the activation state of neurons in various hippocampal subregions during acute seizures.The study found that during TLE seizures,the expression levels of c-fos protein significantly increased in the CA1,CA3,and DG regions of the hippocampus but remained low and unchanged in the CA2 subregion.Moreover,the spike discharge levels of CA2 neurons increased before seizures and decreased during major seizures.The spike discharge levels of narrow-wave neurons(inhibitory interneurons)were similar to the overall neuronal discharge levels and patterns;the coupling relationship between narrow-wave neuron spikes and local field potentials(LFPs)showed enhanced coupling before and during major seizures.This suggests that the CA2 subregion maintains a low level of stable activity under physiological conditions,exhibits "inert" characteristics to external disturbances,but shows enhanced narrow-wave neuron activity before seizures,potentially forming the basis of its "inert" neural activity.2.To further determine the impact of the CA2 subregion on seizures,we employed an electrical lesion method to disrupt the CA2 subregion.The results showed that lesioning the CA2 subregion significantly prolonged the latency to seizures,reduced seizure frequency,and decreased the frequency of epileptiform discharges.This indicates that although the CA2 subregion may exhibit relatively "inert" characteristics under physiological conditions,it still promotes seizures to some extent.After lesioning the CA2,excitatory input from CA2 to CA1 decreased,manifesting as reduced severity of epileptic behavior and electrographic seizures.3.Given the rich diversity and functional characteristics of CA2 neurons,it is hypothesized that the excitation-inhibition balance in CA2 may regulate epileptic activity.Therefore,this study used chemogenetic and optogenetic interventions to respectively inhibit excitatory pyramidal neurons and activate inhibitory interneurons in the CA2 subregion to investigate the effects of these interventions on seizures.The results showed:(1)Chemogenetic inhibition of CA2 pyramidal neurons prolonged seizure latency,reduced seizure frequency and epileptiform discharges,inhibited wide-wave neuron spike discharge rates,and decoupled narrow-wave neuron spikes from LFPs.(2)Optogenetic activation of CA2 interneurons prolonged seizure latency and reduced epileptiform discharges.This suggests that the excitation-inhibition balance of CA2 neurons is crucial for maintaining their neural activity’s homeostasis,and regulating the excitatory or inhibitory activity of CA2 neurons can alleviate seizures,with changes in narrow-wave neuron activity being closely related to this regulation.4.The study investigated information flow among DG,CA2,and CA1 subregions in hippocampal circuits based on electrophysiological data under different epileptic and intervention states.The results showed:(1)Before seizures,information interaction between DG and CA1,CA2 increased,but decreased between CA2 and CA1;during major seizures,information interaction among all three subregions increased.(2)After chemogenetic inhibition of CA2 pyramidal neurons,information interaction between DG and CA2 increased before and during seizures,with no significant change between CA2 and CA1,and decreased interaction between DG and CA1.(3)Optogenetic activation of CA2 interneurons enhanced information interaction among all three subregions before seizures;during major seizures and seizure transitions,interaction between DG and CA2,DG and CA1 decreased.These results suggest that before seizures,CA2 filters DG input and reduces output to CA1,exhibiting an "inert" response to external stimuli;during major seizures,CA2 promotes epileptic activity.Intervening in the excitation-inhibition balance of CA2 affects the transmission of epileptic activity in the DG-CA2-CA1 microcircuit,alleviating seizures.In conclusion,this study systematically analyzed the changes in the hippocampal CA2 subregion in a pilocarpine-induced TLE mouse model,revealing that the CA2 subregion exhibits strong neural "inertia" during epilepsy,showing low sensitivity to epileptogenic treatment.Intervening in the excitation-inhibition balance of the CA2 subregion alleviates seizures,potentially related to the activity of CA2 interneurons.Due to the complex neural projections in hippocampal circuits,the information integration patterns of hippocampal microcircuits vary at different stages of seizures,with CA2 activity regulation significantly impacting this circuit.These findings provide new perspectives on the function of CA2 and its role in epilepsy propagation and offer potential targets for epilepsy intervention.However,the effects of excitatory or inhibitory changes in neurons on local microcircuit activity need further clarification. |
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