CHANGES IN EXCITATORY NEUROTRANSMISSION IN THE CA1 REGION AND DENTATE GYRUS IN A CHRONIC MODEL OF TEMPORAL LOBE EPILEPSY. Lothman, Eric W., David A. Rempe, and Patrick S. Mangan. Department of Neurology and Neuroscience Program, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908, U.S.A.
APStracts 2:0118N, 1995.
SUMMARY AND CONCLUSIONS
1) This report compares changes of excitatory neurotransmission within the CA1 region and the dentate gyrus (DG) in a model of chronic temporal lobe epilepsy (TLE). Extracellular and intracellular recordings were obtained from in vitro hippocampal-parahippocampal slices one month or more following a period of self-sustaining limbic status epilepticus (SSLSE) induced by continuous hippocampal stimulation (CHS). Pyramidal cells in CA1 were activated by electrodes in the stratrum lacunosum/moleculare or stratum radiatum. Granule cells in the DG were similarly activated by electrodes positioned in the perforant path. 2) Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked in CA1 pyramidal cells in post-SSLSE tissue were always longer than those evoked in control tissue, irrespective of whether hyper-responsiveness was present or not. EPSPs elicited by stimulus subthreshold for AP in post- SSLSE and in control slices and matched in amplitude had a statistically greater duration in the post-SSLSE slices. Durations of monosynaptic EPSPs elicited by stimuli subthreshold for AP in DG granule cells in post-SSLSE slices were not longer than EPSPs of equal amplitude elicited in control slices. 3) Higher intensity stimuli produced EPSPs with associated AP and, in certain cases in the post-SSLSE tissue, hyper-responsive events with multiple ( ¯> 3) AP. Durations of depolarizing profiles with stimuli producing AP were overall longer in both CA1 pyramidal cells and DG granule cells and correlated with the degree of hyper-responsiveness. 4) Neither the amplitudes nor durations of monosynaptic EPSPs evoked in CA1 pyramidal cells in slices from control animals were affected by the addition of D(-)-2-amino-5- phosphonovaleric acid (APV), a blocker of the N-methyl-D-aspartate (NMDA) receptor, to the artificial cerebrospinal fluid (ACSF) bathing the slices. In contrast to the situation in control tissue, in post-SSLSE tissue APV shortened EPSPs evoked in CA1 pyramidal cells while not changing their amplitudes. After APV, IPSPs remained greatly diminished or absent in CA1 pyramidal cells. APV did not statistically decrease amplitudes of monosynaptic EPSPs evoked in DG granule cells in either control slices or post-SSLSE slices. APV decreased EPSP durations in both types of slices, more so in the post-SSLSE tissue. 5) In control slices, APV did not change the amplitudes or durations of depolarizing profiles of responses evoked by stimuli producing AP in CA1. Similarly APV did not change the amplitudes of such responses in the DG. However, APV did reduce the durations of such responses in the DG in control slices. APV decreased the amplitude and durations of depolarizing responses in post-SSLSE tissue in CA1 and the durations of such responses in the DG. In both CA1 and the DG, the net effect of APV in post-SSLSE tissue depended on the amount of hyper-responsiveness present, reducing the number of evoked action potentials, usually to a single spike. 6) The issue of interactions of loss of GABAergic inhibition versus enhancement of glutamatergic excitation in promoting hyper-responsiveness in the CA1 region was further explored by using a stimulus protocol that directly activated inhibitory interneurons to elicit monosynaptic inhibitory postsynaptic potentials (IPSPs) (see previous report, Mangan et al. , submitted). In control tissue in ACSF that did not contain glutamate antagonists, such stimuli produced EPSPs with IPSPs at lower stimulus intensity and EPSPs with superimposed action potentials followed by robust biphasic IPSPs with early (GABA A receptor-mediated) and late (GABA B receptor-mediated) components. Infusion of ACSF containing APV along with 6-cyano-7-nitroquinoxaline-2,3- dione (CNQX) blocked EPSPs and AP but did not alter the IPSPs. 7) Enactment of the protocol to elicit monosynaptic IPSPs in CA1 pyramidal cells in post- SSLSE slices in ACSF without glutamate blockers produced responses with, relative to controls, broadened EPSPs, absent IPSPs, and, in many cases, hyper-responsiveness with multiple action potentials. Addition of APV blocked the hyper-responsiveness but did not restore IPSPs. In the presence of APV and CNQX, the same stimuli produced IPSPs with the early component comparable to that in control tissue (either with or without APV/CNQX) but a markedly diminished second (GABA B receptor-mediated) component. 8) These findings indicate that in chronically epileptic tissue there is an enhancement of ionotropic glutamatergic excitation. One form of this enhancement is in the augmentation of NMDA receptor-mediated excitation which correlates closely with hyper-responsiveness in the post-SSLSE model, as it does in tissue taken from humans with chronic epilepsy. This enhancement seems to be sufficient to impart epileptic hyper-responsiveness since an excess number of discharges occur in DG granule cells in the post-SSLSE model in the face of preserved GABAergic inhibition. The converse of this is that a diminution of GABAergic inhibition, while not necessary for hyper-responsiveness does contribute to it, as demonstrated by greater hyper-responsiveness in CA1 wherein GABAergic inhibition is diminished than in the DG where it is not. 9) The other form of enhanced excitation in the post-SSLSE model involves non-NMDA ionotropic glutamate receptors. Two lines of evidence support the argument for such enhancement. One is the residual hyper-responsiveness detected in severely hyper-responsive tissue after APV. The second is that even though potent monosynaptic IPSPs are activated with near-site stimuli in APV-containing ACSF in post-SSLSE tissue, they are over-ridden by EPSPs which are blocked by CNQX. This indicates that an augmentation of excitation is capable of overwhelming inhibition and setting the stage for epileptiform discharges. 

Received 8 August 1994; accepted in final form 17 March 1995.
APS Manuscript Number J494-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  1 May 1995.