A COMPARISON OF SPONTANEOUS EPSCS IN LAYER-II AND LAYER-IV-V NEURONS OF THE RAT ENTORHINAL CORTEX IN-VITRO

1. We have compared the characteristics of spontaneous excitatory post synaptic currents (sEPSCs) in neurons of layer IV-V and layer II of th e rat entorhinal cortex (EC) using whole cell voltage-clamp recordings in a slice preparation.2. The frequency of sEPSCs was similar in the two layers, but the events in layer IV-V had a larger mean amplitude, faster rise time, and were faster to decay. The difference in amplitud e could be attributed to the presence of a population of larger events in the layer IV-V neurons that were not present in layer II. 3. Elect rotonic length was greater in layer II neurons, suggesting that the di fference in kinetics of the sEPSCs may be explained partly by electrot onic attenuation. 4. The frequency of sEPSCs in both layers was reduce d by tetrodotoxin (TTX) to a similar extent (15-20%). However, the amp litude distribution was unchanged in layer II, whereas in layer IV-V T TX abolished most of the larger amplitude sEPSCs. 5. 6-cyano-7-nitroqu inoxaline-2,3-dione or 6-nitro-7-sulphamoylbenzo (f)-quinoxaline-2,3-d ione, abolished most of the sEPSCs in neurons of both layers. However, even at negative holding potentials, a population of slower time-cour se sEPSCs remained in the presence of these antagonists. 6. The slow s EPSCs were more frequent in layer IV-V but had similar characteristics in both layers, being increased in amplitude at more positive holding potentials or in Mg2+-free medium, and blocked by 2-amino-5-phosphono valerate (AP5). 7. AP5 alone (i.e., without addition of lpha-amino-3-h ydroxy-5-methyl-4-isoxazolepropionic acid antagonists) reduced the pea k amplitude and decay phase of sEPSCs in layer IV-V neurons but appear ed to have little effect on amplitude and only a weak effect on decay phase in layer II. 8. Thus both layer IV-V and layer II neurons of the EC suffer continuous spontaneous excitation. However, layer IV-V neur ons exhibit larger amplitude sEPSCs, probably mediated by release of m ultiple quanta of neurotransmitter. In addition, although both types o f neurons display spontaneous excitation mediated by N-methyl-D-aspart ate receptors, this component appears more pronounced in the deeper la yers.