Ultrastructural identification of entorhinal cortical synapses in CA1 stratum 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 cortical (EC) axons in the hippocampal CA1 stratum lacunosum-moleculare of the adult rat. Phaseolus vulgaris leucoagglutinin was ionotophoresed at various loci throughout the mediolateral and dorsoventral extent of the EC to label EC-CA1 synapses. Virtually all labeled synapses were asymmetric and axospinous. EC axons did not preferentially synapse with any particular type of dendritic spine; rather, EC axons formed synapses with the range of dendritic spine morphologies observed in CA1 s. lacunosum-moleculare. Spines with either perforated or nonperforated postsynaptic densities were contacted by EC axons. Occasionally both a labeled and an unlabeled axon synapsed on a single dendritic spine head. The data are discussed in relation to the morphology of other afferent systems synapsing in s. lacunosum-moleculare and to the physiology of the EC-CA1 system.