Synaptic and intrinsic properties of neurons of origin of the perforant path in layer II of the rat entorhinal cortex in vitro

Abstract

Layer II of the entorhinal cortex (EC) provides the first step in the hippocampal trisynaptic loop via the perforant path projection to the dentate gyrus. While a great deal is known about this projection and the properties of the dentate granule cells, much less information is available concerning the properties of and synaptic inputs to the cells of origin of the pathway in layer II. The present experiments have employed a slice preparation of the rat EC to study the intrinsic membrane properties and synaptic organization of layer II neurons. Two types of neurons could be identified electrophysiologically. The majority were designated type I and displayed a pronounced time‐dependent inward rectification in the hyperpolarizing direction. Type II displayed little evidence of this characteristic. However, morphological examination suggested that both types were spiny stellate neurons projecting via the perforant path. Synaptic responses of both types displayed evidence of excitatory inputs mediated by both N‐methyl‐D‐aspartate (NMDA) and non‐NMDA glutamate receptors. In general, however, at low frequencies the responses were dominated by inhibitory inputs mediated by both GABA_A and GABA_B receptors. At higher frequencies the bias was shifted much more toward excitation. The contribution of synaptic and intrinsic properties of layer II neurons to the processing capabilities of the EC is discussed.