Abstracts

RATIONALE: GAG causes seizure activity when injected into the entorhinal cortex of anesthetized adult rats. After an acute period of repetitive seizures and a latent period, electrographic seizures reappear spontaneously. Initially, neuronal damage is limited to the entorhinal cortex, but subsequently spreads to the hippocampus. This study examined the acute effects of GAG using combined slices of the hippocampus and entorhinal cortex, and the ability of NMDA receptor antagonists to modulate its effects.
METHODS: Horizontal slices (400 [mu]m thick) containing both the hippocampus and attached entorhinal cortex were made from adult male Sprague-Dawley rats using a Vibroslice. Conventional methods were used to record extracellularly and intracellularly from entorhinal and hippocampal neurons using sharp microelectrodes. A monopolar 75 [mu]M-wide Teflon-coated stainless steel wire was used to stimulate the trisynaptic pathways or white matter. GAG (100 [mu]M - 1 mM) was applied focally using a picospritzer or by bath-application.
RESULTS: GAG enhanced evoked potentials after focal application or bath-application of 100 [mu]M (n= 5 slices). After perfusion with 500 or 750 [mu]M (n=14), the effect was greater, and in the dentate gyrus or CA1, 2 population spikes could be evoked in response to the second of two identical stimuli (40 ms interstimulus interval). GAG (1 mM) caused spontaneous epileptiform discharges in both the entorhinal cortex and area CA3/CA1 (n=19). Spontaneous repetitive discharges did not fully develop until 1-2 hours after perfusion began. Intracellular recordings showed no effect of GAG on membrane properties of entorhinal pyramidal cells (n=5 cells). Evoked IPSPs were robust in superficial entorhinal neurons after GAG (n=3), as was dentate gyrus paired-pulse inhibition (n=5 slices). Pretreatment with the NMDA receptor antagonist DL-APV (50 [mu]M, 30 min), followed by 1 mM GAG and APV, blocked entorhinal spontaneous epileptiform discharges (15/15 slices), but was less effective in CA3 (4/15 slices); in both areas, epileptiform events could still be evoked by stimulation. After 1 mM GAG had induced spontaneous discharges, treatment with kynurenic acid (10 [mu]M; KYNA) blocked the events in the entorhinal cortex (5/7 slices) but not in CA3 of the same slices (0/8 slices), although CA3 afterdischarges were reduced in duration.
CONCLUSIONS: GAG appears to have a widespread ability to enhance excitatory transmission in many afferent pathways of the entorhinal cortex and hippocampus, which is likely to contribute to its ability to induce electrographic seizures in vivo. The differential effects of APV and KYNA in the entorhinal cortex vs. area CA3 indicate a possible dissociation of mechanisms underlying spontaneous epileptiform activity in these two regions.
Support: This work was supported by NS 16102.
Disclosure: Grant - The author (Scharfman) is a co-investigator on an NIH grant that covers the subject material. This grant is [dsquote]Kynurenines, Glia and Epilepsy[dsquote] (NS 16102) and is written in the Abstract at the bottom.