ULTRASTRUCTURAL IDENTIFICATION OF ENTORHINAL CORTICAL SYNAPSES IN CA1STRATUM LACUNOSUM-MOLECULARE OF THE RAT

The goal of the present study was to identify and characterize, at the electron microscopic level, the synapses formed by entorhinal cortica l (EC) axons in the hippocampal CA1 stratum lacunosum-moleculare of th e adult rat. Phaseolus vulgaris leucoagglutinin was ionotophoresed at various loci throughout the mediolateral and dorsoventral extent of th e EC to label EC-CA1 synapses. Virtually all labeled synapses were asy mmetric and axospinous. EC axons did not preferentially synapse with a ny particular type of dendritic spine; rather, EC axons formed synapse s with the range of dendritic spine morphologies observed in CA1 s. la cunosum-moleculare. Spines with either perforated or nonperforated pos tsynaptic densities were contacted by EC axons. Occasionally both a la beled and an unlabeled axon synapsed on a single dendritic spine head. The data are discussed in relation to the morphology of other afferen t systems synapsing in s. lacunosum-moleculare and to the physiology o f the EC-CA1 system. (C) 1994 Wiley-Liss, Inc.