Responses of the superficial entorhinal cortex in vitro in slices from naive and chronically epileptic rats

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)