Reduced cholecystokinin-expressing interneuron input contributes to disinhibition of the hippocampal CA2 region in a mouse model of temporal lobe epilepsy

A significant proportion of temporal lobe epilepsy (TLE) patients experience drug-resistant seizures associated with mesial temporal sclerosis, in which there is extensive cell loss in the hippocampal CA1 and CA3 subfields, with a relative sparing of dentate gyrus granule cells and the CA2 pyramidal neurons. A role for CA2 in seizure generation was suggested based on findings of a reduction in synaptic inhibition ([Williamson & Spencer, 1994][1]) and the presence of interictal- like spike activity in resected hippocampal tissue from TLE patients ([Wittner et al., 2009][2]). We recently found that in the pilocarpine-induced status epilepticus mouse model of TLE there was an increase in CA2 intrinsic excitability associated with a loss of CA2 synaptic inhibition. Furthermore, chemogenetic silencing of CA2 significantly reduced seizure frequency, consistent with a role of CA2 in promoting seizure generation and/or propagation ([Whitebirch et al., 2022][3]). In the present study we explored the basis of this inhibitory deficit using immunohistochemical and electrophysiological approaches. We report a widespread decrease in the density of pro- cholecystokinin-immunopositive interneurons and a functional impairment of cholecystokinin- expressing interneuron-mediated inhibition of CA2 pyramidal neurons. We also found a decrease in the density of CA2 parvalbumin-immunopositive interneurons and disruption to the pyramidal neuron-associated perisomatic perineuronal net in the CA2 subfield. These data reveal a set of pathological alterations that may disrupt inhibition of CA2 pyramidal neurons and their downstream targets in epileptic mice. ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-102 [2]: #ref-104 [3]: #ref-101