Input from the presubiculum to dendrites of layer-V neurons of the medial entorhinal cortex of the rat.

The entorhinal cortex (EC) and the hippocampus are reciprocally connected. Neurons in the superficial layers of EC project to the hippocampus, whereas deep entorhinal layers receive return connections. In the deep layers of EC, pyramidal neurons in layer V possess apical dendrites that ascend towards the cortical surface through layers IIII and II. These dendrites ramify in layer I. By way of their apical dendrites, such layer-V pyramidal cells may be exposed to input destined for the superficial entorhinal neurons. A specific and dense fiber projection that typically ends in superficial entorhinal layers of the medial EC originates in the presubiculum. To investigate whether apical dendrites of deep entorhinal pyramidal neurons indeed receive input from this projection, we injected the anterograde tracer PHA-L in the presubiculum or we lesioned the presubiculum, and we applied in the same experiments the tracer Neurobiotin trade mark pericellularly in layer V of the medial EC of 17 rats. PHA-L labeled presubiculum axons in the superficial layers apposing apical segments of Neurobiotin labeled layer-V cell dendrites were studied with a confocal fluorescence laserscanning microscope. Axons and dendrites were 3D reconstructed from series of confocal images. In cases in which the presubiculum had been lesioned, material was investigated in the electron microscope. At the confocal fluorescence microscope level we found numerous close contacts, i.e. appositions of boutons on labeled presubiculum fibers with identified dendrites of layer-V neurons. In the electron microscope we observed synapses between degenerating axon terminals and spines on dendrites belonging to layer-V neurons. Hence we conclude that layer-V neurons receive synaptic contacts from presubiculum neurons. These findings indicate that entorhinal layer-V neurons have access to information destined for the superficial layers and eventually the hippocampal formation. At the same time, they have access to the hippocampally processed version of that information.