CHANGES IN INHIBITORY NEUROTRANSMISSION IN THE CA1 REGION AND DENTATE GYRUS IN A CHRONIC MODEL OF TEMPORAL LOBE EPILEPSY. Mangan, Patrick S., David A. Rempe, and Eric W. Lothman. Department of Neurology and Neuroscience Program, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908, U.S.A.
APStracts 2:0117N, 1995.
SUMMARY AND CONCLUSIONS
1) This report compares changes in inhibitory 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 in combined hippocampal-parahippocampal slices one month or more following a period of self-sustaining limbic status epilepticus (SSLSE) induced by continuous hippocampal stimulation (CHS). 2) Polysynaptic IPSPs were induced by positioning electrodes to activate specific afferent pathways and evoking responses in the absence of glutamate receptor antagonists (D(-)-2-amino-5- phosphonovaleric acid [APV] and 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX]). Polysynaptic IPSPs were evoked in CA1 pyramidal cells from electrodes positioned in stratum radiatum and in stratum lacunosum/moleculare. Polysynaptic IPSPs were evoked in DG granule cells from electrodes positioned over the perforant path located in the subiculum. Monosynaptic IPSPs were induced by positioning electrodes within 200 [mu]m of the intracellular recording electrode (near-site stimulation) and stimulating in the presence of APV and CNQX to block ionotropic glutamate receptors. Monosynaptic IPSPs were evoked in CA1 pyramidal cells with electrodes positioned in the stratum lacunosum/moleculare and stratum pyramidale. Monosynaptic IPSPs were evoked in DG granule cells with electrodes positioned in the stratum moleculare. 3) Population spike (PS) amplitudes were employed to assure that a full range of stimulus strengths, from subthreshold for action potentials to an intensity giving maximal amplitude PS, was used to elicit polysynaptic IPSPs in CA1 pyramidal cells in both post-SSLSE and control slices. In control tissue, polysynaptic IPSPs were biphasic, comprised of early and late events. In post- SSLSE tissue polysynaptic IPSPs were markedly diminished. The diminution of polysynaptic IPSPs was detected at all levels of stimulus intensity. Both early IPSPs (mediated by GABA A receptors) and late IPSPs (mediated by GABA B receptors) were diminished. Polysynaptic IPSPs were diminished with both stratum radiatum and with stratum lacunosum/moleculare stimulation. 4) Reversal potentials for either polysynaptic early or polysynaptic late IPSPs evoked in CA1 pyramidal cells by stratum radiatum stimulation were not different in slices from post-SSLSE animals as compared with control animals. Likewise, reversal potentials for either polysynaptic early or polysynaptic late IPSPs evoked by stratum lacunosum/moleculare stimulation did not differ in the two groups. These findings, combined with the observations in the previous report (Rempe et al. , submitted) that resting membrane potentials (RMP) in CA1 pyramidal cells were not different between the post-SSLSE and control groups, excluded changes in driving force as an explanation for the diminished amplitude of IPSPs in CA1 pyramidal cells in the post-SSLSE model. 5) Temporal aspects of polysynaptic IPSPs were determined for maximal amplitude responses evoked in CA1 pyramidal cells in the two study groups. Latencies to peaks of polysynaptic early IPSPs and latencies to peaks of polysynaptic late IPSPs did not differ between the control and post-SSLSE tissue, for either stratum radiatum or stratum lacunosum/moleculare stimulation. Times of half-decay from peaks of IPSPs in post-SSLSE tissue were significantly shorter than those in control tissue. This can be attributed to a preferential deterioration of late, GABA B receptor-mediated IPSPs. 6) Only a late, GABA B receptor-mediated component was detected in polysynaptic IPSPs in DG granule cells, even with depolarization of the membrane potential well away from the chloride reversal potential, a procedure that enhanced early, GABA A receptor-mediated IPSPs in CA1 pyramidal cells. In contrast to the situation in CA1, no reduction in the amplitude of polysynaptic IPSPs was found in DG granule cells. In fact, polysynaptic IPSPs elicited in post-SSLSE cells were larger than those seen in control cells. However, this difference did not reach statistical significance. Times to peak and times for half-decay for IPSPs in control and in post-SSLSE tissue were not different. 7) Reversal potentials for polysynaptic IPSPs in DG granule cells obtained in post-SSLSE tissue were not different from those obtained in control tissue, both being at the expected potassium reversal potential. 8) Monosynaptic IPSPs evoked in CA1 pyramidal cells in control tissue during blockade of ionotropic glutamatergic receptors displayed early (GABA A -mediated) and late (GABA B -receptor mediated) IPSPs, both with stratum pyramidale and with stratum lacunosum/moleculare stimulation. The morphologies of monosynaptic IPSPs evoked in CA1 in post-SSLSE tissue were more variable than those in control tissue. Typically monosynaptic late IPSPs were absent although they were detected in some cases but were of small amplitude; this was the case for both stimulus sites. On the other hand, monosynaptic early IPSPs in post-SSLSE tissue did not differ appreciably from those in control tissue with respect to amplitude, latency to peak, or reversal potential; this was the case for both stimulus sites. Concomitant with the loss of the monosynaptic late IPSP, times for half-decay from the IPSP peaks were significantly shorter in the post- SSLSE tissue than in control tissue. 9) Monosynaptic IPSPs evoked in DG granule cells had both early and late components. The early component had a reversal potential near the expected chloride potential and was blocked by antagonists effective at GABA A receptors. The late component had a reversal potential near the expected potassium potential and was blocked by antagonists effective at GABA B receptors. 10) We conclude that area CA1 and the dentate gyrus respond fundamentally differently in the post-SSLSE chronic model of TLE with respect to GABAergic inhibition. In area CA1, polysynaptic IPSPs are reduced or eliminated, whereas they are retained in the DG. Data presented in this report indicate that the defect in GABAergic neurotransmission in CA1 has two mechanisms. One deficit is a functional "disconnection" of inhibitory interneurons from excitatory drive which may result from either a defect in excitatory terminals impinging on the interneurons or a reduction in the ability of interneurons themselves to respond to normal activity of excitatory terminals. This disconnection appears to extend to at least 2 types of GABAergic cells in CA1, basket cells and interneurons in stratum lacunosum/moleculare. The second deficit is a preferential diminution in the potency of late (GABA B receptor mediated) IPSPs, which appears to reside at the level of CA1 pyramidal cells. The slight increase in amplitude of IPSPs in DG granule cells in post-SSLSE suggests that sprouting of GABAergic interneurons or of the excitatory connections onto them may occur in chronic TLE. 

Received 8 August 1994; accepted in final form 17 March 1995.
APS Manuscript Number J493-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  1 May 1995.