Temporal lobe epilepsy (TLE) is among the most common forms of epilepsy in adults. A significant proportion of patients experience drug-resistant seizures associated with hippocampal sclerosis (HS), in which there is extensive cell loss in the hippocampal cornu ammonis 1 (CA1) and cornu ammonis 3 (CA3) subfields. The dentate gyrus (DG) and cornu ammonis 2 (CA2) subfield are more resilient to neurodegeneration, and a prior report found that CA2 neurons in tissue from TLE patients show interictal-like firing and receive aberrant perisomatic excitatory synapses from DG granule cell (GC) mossy fibers (Wittner et al. Brain. 2009;132:3032–3046). Furthermore, findings from a collaborative study in the laboratory of Dr. Helen Scharfman demonstrated that chronic chemogenetic inhibition of CA2 pyramidal neurons (PNs) in vivo significantly reduced the frequency of spontaneous recurring convulsive seizures in epileptic mice. I therefore explored the hypothesis that pathophysiological changes to CA2 PN excitability or synaptic connectivity may be associated with chronic epilepsy by examining CA2 properties in a mouse model of TLE. Pilocarpine-induced status epilepticus in mice leads to a pattern of hippocampal sclerosis-like neurodegeneration and recurring spontaneous seizures, and thus recapitulates key features of TLE. I performed whole-cell electrophysiological recordings from PNs in acute hippocampal slices from pilocarpine (PILO)-treated mice in the chronic phase of epilepsy as well as age-matched controls. In some experiments I used Cre-expressing mouse lines to selectively express a light-activated excitatory channel in CA2 PNs or DG GCs. I also performed immunohistochemistry to examine CA2 interneuron (IN) populations following PILO-induced status epilepticus. I found that in healthy tissue CA2 PNs, like those in CA3, both directly excited other CA2 PNs via a recurrent CA2-CA2 PN circuit and indirectly inhibited other CA2 PNs by recruiting local INs. The CA2 and CA3 subfields also form reciprocal excitatory and feedforward inhibitory circuits. These recurrent and reciprocal circuits constitute an auto-associative network in which INs crucially control CA2/CA3 population excitability. DG GC mossy fibers made direct but relatively weak excitatory synapses onto CA2 PNs. Following PILO-induced status epilepticus, feedforward inhibition is diminished in the DG GC mossy fiber circuit to CA2, in the CA2/CA3 recurrent network, and in the forward-projecting circuit from CA2 PNs to CA1. I found a modest decrease in the density of parvalbumin-immunopositive INs and a profound decrease of cholecystokinin-immunopositive IN density, combined with degradation of the pyramidal neuron-associated perisomatic perineuronal net, which together may contribute to this inhibitory disruption. DG GC mossy fiber excitatory input to CA2 PNs is strengthened, along with CA2 PN excitatory input to CA1 PNs. Finally, in hippocampal slices from PILO-treated mice I found an increase in CA2 PN input resistance and thus elevated intrinsic excitability, leading to a higher firing rate upon direct current injection. The combined effect of these changes may drive the emergence of epileptiform synchronization in the CA2 network and facilitate the propagation of seizure activity from the DG and entorhinal cortex directly to CA1 via the CA2-centered disynaptic (EC LII → CA2 → CA1) and alternate trisynaptic circuits (EC LII → DG → CA2 → CA1).
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