Perirhinal cortex hyperexcitability in pilocarpine-treated rats

The perirhinal cortex (PC), which is heavily connected with several epileptogenic regions of the limbic system such as the entorhinal cortex and amygdala, is involved in the generation and spread of seizures. However, the functional alterations occurring within an epileptic PC network are unknown. Here, we analyzed this issue by employing in vitro electrophysiology and immunohistochemistry in brain tissue obtained from pilocarpine-treated epileptic rats and age-matched, non-epileptic controls (NECs). Neurons recorded intracellularly from the PC deep layers in the two experimental groups had similar intrinsic and firing properties, and generated spontaneous depolarizing and hyperpolarizing postsynaptic potentials with comparable duration and amplitude. However, spontaneous and stimulus-induced epileptiform discharges were seen with field potential recordings in over one fifth of pilocarpine-treated slices but never in NEC tissue. These network events were reduced in duration by antagonizing NMDA receptors and abolished by NMDA+non-NMDA glutamatergic receptor antagonists. Pharmacologically isolated IPSPs had reversal potentials for the early GABAA receptor-mediated component that were significantly more depolarized in pilocarpine-treated cells. Experiments with a potassium-chloride co-transporter 2 antibody identified in pilocarpine-treated PC a significant immunostaining decrease that could not be explained by neuronal loss. However, interneurons expressing parvalbumin and neuropeptide Y were found to be decreased throughout the PC, whereas cholecystokinin-positive cells were diminished in superficial layers. These findings demonstrate synaptic hyperexcitability that is contributed by attenuated inhibition in the PC of pilocarpine-treated epileptic rats, and underscore the role of PC networks in temporal lobe epilepsy.