Abstracts

Both the hippocampus and extrahippocampal areas have been implicated in temporal lobe epilepsy (TLE). To clarify the relative contribution of the hippocampus and entorhinal cortex (EC), we compared them in slices of pilocarpine-treated rats with spontaneous seizures. We also examined relative pharmacological sensitivity of kynurenic acid (KYNA), because of its potency in the EC to inhibit epileptiform activity induced by decreasing [Mg²⁺][sub]o[/sub] (Scharfman et al., Neurosci. Lett., 274:111-114,1999). We also tested the selective [alpha]7 nicotinic cholinergic receptor ([alpha]7nAChR) antagonist methyllycaconitine (MLA), because KYNA acts at [alpha]7nAChRs (Hilmas et al., J. Neurosci., 21:7463-7473, 2001).
Adult male Sprague-Dawley rats were treated with atropine methylbromide (1 mg/kg, s.c.), and 30 min later with pilocarpine (380 mg/kg, i.p.). After 1 hr of status, diazepam was injected (5 mg/kg, i.p.). Controls received all drugs except pilocarpine, which was replaced by an equal volume of saline. All rats had recurrent spontaneous seizures. At least 3 weeks later, 400 [mu]m-thick horizontal slices were made with a Vibroslice, placed in an interface recording chamber, and maintained at 32[deg]C. Extracellular and intracellular recordings were made in all subfields using standard techniques.
Spontaneous epileptiform activity was recorded in the EC in all slices that were viable, (defined as a [gt]3 mV antidromic spike recorded in layer VI after a white matter stimulus; n=16). In 10 of these slices, no spontaneous field potentials were recorded in area CA1, CA3, or the dentate gyrus cell layers, and intracellular recordings from pryamidal cells and granule cells confirmed the extracellular findings. In other slices, which were from older rats (1-3 vs. [gt]5months after pilocarpine), area CA3 and the EC both demonstrated spontaneous epileptiform discharges. Paroxysmal depolarization shifts were recorded in many cell types within the EC (n=10). In dorsal EC, surviving layer III neurons demonstrated epileptiform events (n=5). KYNA blocked spontaneous EC discharges reversibly (0.5 [mu]M, n=2; 1 [mu]M, n=1) without affecting spontaneous bursts in CA3. MLA blocked epileptiform discharges in the EC (50 nM, n=4).
The results suggest that robust epileptiform activity occurs in the EC of epileptic rats, even when there is an absence of this activity in the hippocampus. These data strengthen the argument that extrahippocampal areas contribute to TLE, and may be more important than the hippocampus. Furthermore, KYNA inhibits these events even at very low doses, similar to the levels of endogenous KYNA in epileptic tissue. This suggests that endogenous KYNA could have an anticonvulsant role. The efficacy of MLA indicates that [alpha]7nAChRs may be important modulators of epileptiform activity, and supports previous suggestions that low concentrations of KYNA may act either at NMDA or [alpha]7nAChRs.
[Supported by: NIH grant 16102.]