Abstracts

In situ hybridization studies have revealed the presence of mRNA encoding subunits for kainate receptors (KARs) in neurons of the rat medial entorhinal cortex (mEC). The functional presence and roles of these KARs have not yet been examined despite the fact that layer III pyramidal neurons in this region, like neurons in other brain regions that are known to express KARs, rapidly degenerate in patients with temporal lobe epilepsy (TLE) and animal models of this disease. Characterization of these receptors[apos] contribution to the combined AMPA/KAR EPSC may provide further insight into the roles KARs play in mediating the deleterious effects of systemic kainate injection and excitotoxic injury seen in animal models of TLE. Whole cell voltage clamp recordings of locally evoked EPSCs were made from mEC layer II and layer III neurons in combined entorhinal cortex - hippocampal brain slices (400 [mu]m). Non-NMDA receptor-dependent EPSCs were recorded by perfusing slices with oxygenated ACSF containing APV (50 [mu]M), picrotoxin (50 [mu]M), and CGP 54626 (400 nM) to inhibit NMDA, GABA[sub]A[/sub], and GABA[sub]B[/sub] receptors respectively. To reveal the KAR-dependent portion of this current, 100 [mu]M GYKI 52466 (a selective AMPA receptor antagonist) was applied. GYKI-resistant EPSCs were evoked by single or multiple stimuli (5 at 100 Hz). Recording pipettes contained 2.5 mg/mL biocytin, and after recordings were complete, slices were fixed with paraformaldehyde and processed for biocytin to assess neuron location and morphology. Three varieties of neurons located in layers II and III of the mEC were identified by their electroresponsive membrane properties and morphology. Consistent with currents mediated by KARs, all of these neurons had EPSCs evoked by single or multiple stimuli that were resistant to inhibition by GYKI 52466, had relatively slow rise and decay kinetics, and were inhibited by CNQX (10 [mu]M). However, KAR-mediated EPSCs in layer III pyramidal neurons of the mEC contributed significantly more to the combined non-NMDA EPSC than did those from the two varieties of neurons in layer II. These results represent the first demonstration of functional postsynaptic KARs in neurons of the rat mEC. While layer II neurons survive, layer III neurons of the mEC are selectively susceptible to degeneration in human TLE and animal models of TLE such as kainate-induced status epilepticus in rats. The relatively greater functional expression of postsynaptic KARs in layer III neurons of the mEC may contribute to this cell loss. Characterization of postsynaptic receptors in principle neurons of the mEC represents an important first step in understanding the functional changes in these neurons seen in TLE. (Supported by NIH NS44210 (KSW).)