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) and the presence of interictal- like spike activity in resected hippocampal tissue from TLE patients (Wittner et al., 2009). 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). 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.