Morphological and numerical analysis of synaptic interactions between neurons in deep and superficial layers of the entorhinal cortex of the rat

Neurons providing connections between the deep and superficial layers of the entorhinal cortex (EC) constitute a pivotal link in the network underlying reverberation and gating of neuronal activity in the entorhinal-hippocampal system. To learn more of these deep-to-superficial neurons and their targets, we applied the tracer Neurobiotin pericellularly in layer V of the medial EC of 12 rats. Labeled axons in the superficial layers were studied with light and electron microscopy, and their synaptic organization recorded. Neurobiotin-labeled layer V neurons displayed "Golgi-like" staining. Two major cell types were distinguished among these neurons: (1) pyramidal neurons with apical spiny dendrites traversing all layers and ramifying in layer I, and (2) horizontal neurons with dendrites confined to the deep layers. Labeled axons ramified profusely in layer III, superficially in layer II and deep in layer I. Analysis of labeled axon terminals in layers I-II and III showed that most synapses (95%) were asymmetrical. Of these synapses, 56% occurred with spines (presumably belonging to principal neurons) and 44% with dendritic shafts (presumably interneurons). A small fraction of the synapses (5%) was of the symmetrical type. Such synapses were mainly seen on dendritic shafts. We found in two sections a symmetrical synapse on a spine. These findings suggest that the deep to superficial projection is mainly excitatory in nature, and that these fibers subserve both excitation and feed-forward inhibition. There is an additional, much weaker, inhibitory component in this projection, which may have a disinhibitory effect on the entorhinal network in the superficial layers.