Abstracts

A common feature in patients and models of temporal lobe epilepsy is the loss of layer III neurons in the medial entorhinal cortex. The consequences of layer III neuron loss are unclear, but previous studies show that layer II neurons become hyperexcitable, despite the sparing of GABA-ergic interneurons. Recently we found that in rats 3-7 days following pilocarpine-induced status epilepticus the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) in layer II neurons is significantly reduced. It was not known whether the reduced frequency of sIPSCs persists in epileptic rats, so in the present study we evaluated the anatomy and electrophysiology of entorhinal cortical slices from pilocarpine treated rats that experienced status epilepticus and eventually went on to become epileptic, displaying spontaneous seizures.
Whole-cell voltage clamp recordings of spontaneous (s) and miniature (m) IPSCs from visualized layer II principal cells in the entorhinal cortex were obtained in 400[mu]m-thick horizontal slices from control and epileptic rat brains (P50- P65) at 32[plusmn]1[deg]C. mIPSCs were isolated in TTX (1[mu]M), NBQX (10[mu]M) and D-APV (50[mu]M). Following pilocarpine-induced status epilepticus animals were allowed to recover and were video-monitored (40 Hr/week) for spontaneous seizures. Layer III neuron loss in the medial entorhinal cortex was verified in epileptic rats by NeuN-immunocytochemistry, and the location of the recorded neurons within the cortical laminae was confirmed by biocytin-labeling.
The pattern of layer III cell loss in epileptic rats was similar to that seen 3-7 days after status epilepticus. In layer II neurons in epileptic rats there was a significant decrease in sIPSC frequency (19.4 vs 12.2 Hz, [italic]p[/italic] [lt] 0.0001, t-test; n = 22 cells), but sIPSC amplitude was normal (26.6 vs 25.6 pA). mIPSC frequency, which was normal 3-7 days after status epilepticus, was reduced in epileptic rats (12.3 vs 8.6 Hz, [italic]p[/italic] [lt] 0.05; n = 20), but mIPSC amplitude and charge transfer were normal (22.2 vs 22.1; 172 vs 197 fC). The averaged 10-90% rise times were also similar in epileptic and control rats (1.56 vs 1.62 ms and 1.63 vs 1.63 ms for s- and m-IPSCs, respectively).
The reduction in mIPSC frequency in epileptic animals, but not in rats 3-7days after status epilepticus, suggests that GABA-ergic inhibition of layer II neurons in epileptic rats is further compromised either by a loss of inhibitory synapses and/or a decrease in probability of GABA release. The reduced frequency of sIPSCs that was found in layer II neurons 3-7 days after status epilepticus persists in epileptic animals. These findings suggest that reduced inhibition of layer II neurons precedes the onset of epilepsy and continues into the stage of spontaneous, recurrent seizures. Therefore, reduced inhibition of layer II neurons in the entorhinal cortex might contribute to the epileptogenic process.
[Supported by: [italic]NIH[/italic]]