Abstracts

RATIONALE: Patients with temporal lobe epilepsy display neuron loss in the hilus of the dentate gyrus. Some missing cells are interneurons. Whether dentate granule cells are less inhibited is an important but unresolved issue.
METHODS: Rats experienced pilocarpine-induced status epilepticus when they were 51[plusminus]3 (mean[plusminus]sem, n=25) d old. They were video-monitored for seizure activity 40 h/wk. Their first observed spontaneous seizure occurred 26[plusminus]3 d after status epilepticus, and they were used in an experiment 28[plusminus]4 d later. Control animals included age-matched naive controls (n=15) and pilocarpine-treated rats that did not have status epilepticus and did not develop epilepsy (n=17). We found no differences in the results from the 2 types of controls. Horizontal slices were prepared from the temporal hippocampus. Immediately after slicing, the first and last slice were fixed for Nissl-staining and somatostatin- and parvalbumin-immunocytochemistry. Intracellular recordings were obtained in an interface recording chamber (30-32 C) with sharp electrodes. The outer molecular layer was stimulated at an intensity that evoked a maximal amplitude IPSP at 150 ms latency. To visualize biocytin-labeled neurons, slices were fixed, sectioned, and processed using the ABC method.
RESULTS: In epileptic rats Nissl-staining revealed a loss of hilar neurons. The density of parvalbumin- and somatostatin-immunoreactive interneurons was 36-37% of controls (P[lt]0.0001). The density of somatostatin-positive neurons in 3-7 d post-status epilepticus rats (35[plusminus]6 cells/mm², n=15) was similar to that of chronically epileptic rats (33[plusminus]4 cells/mm², n=25). Stimulation of the outer molecular layer revealed hyperexcitability of granule cells in epileptic rats. The maximum number of action potentials discharged per stimulus was higher in epileptic rats (1.4[plusminus]0.1, n= 84 cells) compared to controls (0.7[plusminus]0.1, n=74 cells, P[lt]0.0001). At stimulation intensities below spike threshold, prolonged depolarizations were evident ni epileptic rats. None of the granule cells from control or epileptic rats responded to current step injection with a burst of action potentials, and resting membrane potential and input resistance were similar in both groups. Basal dendrites were evident in 31% of 108 labeled granule cells in epileptic rats and in only 5% of 58 cells in controls (P[lt]0.005). At least one axon collateral from a biocytin-labeled granule cell projected into the molecular layer in 91% of 34 slices from epileptic rats and in only 19% of 27 slices from controls (P[lt]0.005). In normal ACSF the average synaptic conductances at 20 and 150 ms latencies in epileptic rats were lower than those of controls (74 and 77% of controls, respectively), but the differences were not significant. The reversal potential at 20 ms was more depolarized in epileptic rats (P[lt]0.001). Monosynaptic IPSPs recorded in CNQX/D-APV in control and epileptic rats had similar reversal potentials, but in epileptic rats the mean conductances of fast and slow IPSPs were reduced to 23% and 32% of control values, respectively (P[lt]0.03).
CONCLUSIONS: Granule cells in epileptic rats receive less inhibitory synaptic input than controls. However, that change alone appears to be insufficient to cause epilepsy, because interneuron loss is present 3-7 d after status epilepticus when rats do not appear to experience spontaneous seizures.
[Supported by: NINDS]