CHANGES IN EXCITATORY NEUROTRANSMISSION IN THE CA1 REGION AND DENTATE GYRUS IN A CHRONIC MODEL OF TEMPORAL-LOBE EPILEPSY

1. In this report we compare 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 greater than or equal to 1 mo after a period of self-sustaining limbic status epilepticus (SSLSE) induced by continuous hippocampal stimulation. Pyramidal cells in CA1 were activated by electrodes in the stratum lacunosum/moleculare or stratum radiatum. Granule cells in 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 hyperresponsiveness was present or not. EPSPs elicited by stimulus subthreshold for action potentials (APs) 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 APs 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 APs and, in certain cases in the post-SSLSE tissue, hyperresponsive events with multiple (greater than or equal to 3) APs. Durations of depolarizing profiles with stimuli producing APs were overall longer in both CA1 pyramidal cells and DG granule cells and correlated with the degree of hyperresponsiveness. 4. Neither the amplitudes nor the 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, inhibitory postsynaptic potentials (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 APs in CA1. Similarly, APV did not change the amplitudes of such responses in DG. However, APV did reduce the durations of such responses in DG in control slices. APV decreased the amplitudes and durations of depolarizing responses in post; SSLSE tissue in CA1 and the durations of such responses in DG. In both CA1 and DG, the net effect of APV in post-SSLSE tissue depended on the amount of hyperresponsiveness present, reducing the number of evoked APs, usually to a single spike. 6. The issue of interactions of loss of GABAergic inhibition versus enhancement of glutamatergic excitation in promoting hyperresponsiveness in the CA1 region was further explored by using a stimulus protocol that directly activated inhibitory interneurons to elicit monosynaptic IPSPs. 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 APs followed by robust biphasic IPSPs with early [gamma-amino-butyric acid-A (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 APs 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, hyperresponsiveness with multiple APs. Addition of APV blocked the hyperresponsiveness but did not restore IPSPs. In the presence of APV and CNQX, the same stimuli produced IPSPs with the early component comparable with 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 the augmentation of NMDA-receptor-mediated excitation, which correlates closely with hyperresponsiveness 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 hyperresponsiveness because an excess number of discharges occurs 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, although not necessary for hyperresponsiveness, does contribute to it, as demonstrated by greater hyperresponsiveness in CA1, where GABAergic inhibition is diminished, than in 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 hyperresponsiveness detected in severely hyperresponsive 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 overridden by EPSPs that are blocked by CNQX. This indicates that an augmentation of excitation is capable of overwhelming inhibition and setting the stage for epileptiform discharges.