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We investigated whether entorhinal cortex (EC) layer IV neurons are hyperexcitable in the post-selfsustaining limbic status epilepticus (post-SSLSE) animal model of temporal lobe epilepsy. We studied naive rats (n = 44), epileptic rats that had experienced SSLSE resulting in spontaneous seizures (n = 45), and electrode controls (n = 7). There were no differences between electrode control and naive groups, which were pooled into a single control group. Intracellular and extracellular recordings were made from deep layers of EC, targeting layer IV, which was activated by stimulation of the superficial layers of EC or the angular bundle. There were no differences between epileptic and control neurons in basic cellular characteristics, and all neurons were quiescent under resting conditions. In control tissue, 77% of evoked intracellular responses consisted of a short-duration [8.6 +/- 1.3 (SE) ms] excitatory postsynaptic potential and a single action potential followed by gamma-aminobutyric acid-A (GABAA) and GABAB inhibitory post synaptic potentials (IPSPs). Ten percent of controls did not contain IPSPs. In chronically epileptic tissue, evoked intracellular responses demonstrated prolonged depolarizing potentials (256 +/- 39 ms), multiple action potentials (13 +/- 4), and no IPSPs. Ten percent of epileptic responses were followed by rhythmic "clonic" depolarizations. Epileptic responses exhibited an all-or-none response to progressive increases in stimulus intensity and required less stimulation to elicit action potentials. In both epileptic and control animals, intracellular responses correlated precisely in morphology and duration with extracellular field potentials. Severing the hippocampus from the EC did not alter the responses. Duration of intracellular epileptic responses was reduced 22% by the N-methyl--aspartate (NMDA) antagonist (-)-2-amino-5-phosphonovaleric acid (APV), but they did not return to normal and IPSPs were not restored. Epileptic and control responses were abolished by the non-NMDA antagonist 6, 7-dinitroquinoxaline-2-3-dione (DNQX). A monosynaptic IPSP protocol was used to test connectivity of inhibitory interneurons to primary cells by direct activation of interneurons with a stimulating electrode placed near the recording electrode in the presence of APV and DNQX. Using this protocol, IPSPs similar to control (P > 0.05) were seen in epileptic cells. The findings demonstrate that deep layer EC cells are hyperexcitable or "epileptiform" in this model. Hyperexcitability is not due to interactions with the hippocampus. It is due partially to augmented NMDA-mediated excitation. The lack of IPSPs in epileptic neurons may suggest inhibition is impaired, but we found evidence that inhibitory interneurons are connected to their target cells and are capable of inducing IPSPs.

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-deg 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 animals in 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 that 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 responses. 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 N-methyl-D-aspartate (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) that was blocked partially by the NMDA-receptor antagonist APV, followed by hyperpolarizing potentials, which were identified electrophysiologically and pharmacologically as gamma-aminobuturic acid-A (GABAA)- and GABAB-receptor-mediated inhibitory postsynaptic potentials (IPSPs). EPSPs were blocked completely 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. Intracellur responss correlated precisely in morphology and duration with extracellular field potentials. (ABSTRACT TRUNCATED)