RESPONSES OF THE SUPERFICIAL ENTORHINAL CORTEX IN VITRO IN SLICES FROM NAIVE AND CHRONICALLY EPILEPTIC RATS. Bear, Jonathan, Nathan B. Fountain, and Eric W. Lothman. Department of Neurology and Neuroscience Program, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908, U.S.A..
APStracts 3:0156N, 1996.
SUMMARY AND CONCLUSIONS
1. The main purposes of this study are to characterize the intracellular and extracellular responses of cells in superficial layers of entorhinal cortex (EC) in chronically epileptic animals, determine whether their altered physiology is dependent on being connected to hippocampus, and investigate whether there is evidence of augmented excitation and inhibitory interneuron disconnection. 2. Functional connectivity was maintained between the hippocampal area and the EC in vitro in a combined rat hippocampal- parahippocampal slice preparation by slicing with a vibratome at a 30 degrees angle to the base of the brain. Three groups of animals were studied: naive animals; animals that had experienced a previous episode of (nonconvulsive) self sustaining limbic system status epilepticus (SSLSE) induced by electrical stimulation resulting in a chronically epileptic state; and an electrode control group. In chronically epileptic rats and the electrode control group, studies were done on tissue contralateral to the side of electrode implantation. 3. Extracellular and intracellular recordings were made from the superficial layers of EC. Neurons in the superficial layers of the EC were activated by stimulation of the deep layers within the EC or the angular bundle adjacent to the EC, which contains axons from EC neurons. Responses could be elicited by antidromic and synaptic mechanisms by stimulation at either site. In addition, a monosynaptic protocol was used which involved direct activation of interneurons with a stimulating electrode placed near the recording electrode in the presence of the ionotropic glutamate blockers D(-)- 2-amino-5-phosphonovaleric acid (APV) and 6,7-dinitroquinoxaline-2-3-dione (DNQX). 4. Responses were collected over a range of stimulus intensities, from very low to high intensities, to construct Input/Output function (I/O) curves. Amplitudes and durations were measured at the lowest stimulus intensity that elicited a maximum response. 5. Extracellular field potential responses from electrode controls did not differ from naives qualitatively with respect to morphology of field potential responses or quantitatively with respect to response duration and amplitude. Field potential responses in tissue from post-SSLSE rats differed markedly in morphology from naive and electrode controls, being more complex, significantly longer in duration and decreased in amplitude. These epileptiform responses were shortened markedly by blockade of NMDA receptors with APV, but this manipulation did not convert responses to a normal morphology. These responses were abolished by blockade of non-NMDA mediated ionotropic glutamate receptors with DNQX. 6. During intracellular recordings of neurons in slices from both control and epileptic animals, neurons were quiescent under resting conditions, in the absence of electrical stimulation. 7. Intracellular responses in electrode controls were identical to naive, and together were considered "controls." In control tissue, evoked intracellular responses were similar to those previously described and most commonly consisted of an excitatory postsynaptic potential (EPSP) which was partially blocked by the NMDA-receptor antagonist APV, followed by hyperpolarizing potentials which were identified electrophysiologically and pharmacologically as GABA A and GABA B receptor-mediated inhibitory postsynaptic potentials (IPSPs). EPSPs were completely blocked by DNQX. 8. In chronically epileptic tissue, evoked intracellular responses differed markedly from responses in control animals, exhibiting all-or-none prolonged paroxysmal depolarizing events with multiple superimposed action potentials in response to a single shock. These depolarizing events were reduced in duration and amplitude, but not abolished, in APV. IPSPs were not seen or markedly reduced at all stimulus intensities. These intracellular responses never resembled control responses. Intracellular responses correlated precisely in morphology and duration with extracellular field potentials. 9. In chronically epileptic tissue with the monosynaptic protocol used to directly activate inhibitory interneurons, hyperpolarizing potentials were evoked which were electrophysiologically and pharmacologically identified as GABA A and GABA B receptor-mediated IPSPs and did not differ from monosynaptic IPSPs in control animals. 10. The findings are consistent with the hypothesis that superficial layer EC cells are hyperexcitable, as in CA1, in this model and that this is due to augmented excitation and impaired inhibition, possibly because GABAergic inhibitory interneurons are dormant (disconnected from excitatory input). 11. These results constitute the first study of the EC in an animal model of chronic temporal lobe epilepsy and provide evidence of electrophysiologic changes outside of the hippocampus. 

Received 1 June 1995; accepted in final form 27 June 1996.
APS Manuscript Number J357-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 25 July 1996