SYNAPTIC AND INTRINSIC-PROPERTIES OF NEURONS OF ORIGIN OF THE PERFORANT PATH IN LAYER-II OF THE RAT ENTORHINAL CORTEX IN-VITRO

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 t he properties of the dentate granule cells, much less information is a vailable concerning the properties of and synaptic inputs to the cells of origin of the pathway in layer II. The present experiments have em ployed a slice preparation of the rat EC to study the intrinsic membra ne properties and synaptic organization of layer II neurons. Two types of neurons could be identified electrophysiologically. The majority w ere designated type I and displayed a pronounced time-dependent inward rectification in the hyperpolarizing direction. Type II displayed lit tle evidence of this characteristic. However, morphological examinatio n suggested that both types were spiny stellate neurons projecting via the perforant path. Synaptic responses of both types displayed eviden ce of excitatory inputs mediated by both N-methyl-D-aspartate (NMDA) a nd non-NMDA glutamate receptors. In general, however, at low frequenci es the responses were dominated by inhibitory inputs mediated by both GABA(A) and GABA(B) receptors. At higher frequencies the bias was shif ted much more toward excitation. The contribution of synaptic and intr insic properties of layer II neurons to the processing capabilities of the EC is discussed. (C) 1994 Wiley-Liss, Inc.