PROJECTIONS FROM THE PRESUBICULUM AND THE PARASUBICULUM TO MORPHOLOGICALLY CHARACTERIZED ENTORHINAL-HIPPOCAMPAL PROJECTION NEURONS IN THE RAT

The relations between the inputs from the presubiculum and the parasub iculum and the cells in the entorhinal cortex that give rise to the pe rforant pathway have been studied in the rat at the light microscopica l level. Projections from the presubiculum and the parasubiculum were labeled anterogradely, and, in the same animal, cells in the entorhina l cortex that project to the hippocampal formation were labeled by ret rograde tracing and subsequent intracellular filling with Lucifer Yell ow. The distribution and the number of appositions between the afferen t fibers and hippocampal-projection neurons in the various layers of t he entorhinal cortex were analyzed. The results show that layers I-IV of the entorhinal cortex contain neurons that give rise to projections to the hippocampal formation. The morphology of these projection neur ons is highly variable and afferents from the presubiculum and the par asubiculum do not show a preference for any specific morphological cel l type. Both inputs preferentially innervate the dendrites of their ta rget cells. However, presubicular and parasubicular projections differ with respect to the layer of entorhinal cortex they project to. The n umber of appositions of presubicular afferents with cells that have th eir cell bodies in layer III of the entorhinal cortex is 2-3 times hig her than with cells in layer II, In contrast, afferents from the paras ubiculum form at least 2-3 times as many synapses on the dendrites of cells located in layer II than on neurons that have their cell bodies in layer III. Cells in layers I and IV of the entorhinal cortex receiv e weak inputs from the presubiculum and parasubiculum. Not only is the presubiculum different from the parasubiculum with respect to the dis tribution of projections to the entorhinal cortex, they also differ in their afferent and efferent connections. In turn, cells in layer II o f the entorhinal cortex differ in their electrophysiological character istics from those in layer III. Moreover, layer II neurons give rise t o the projections to the dentate gyrus and field CA3/CA2 of the hippoc ampus proper, and cells in layer III project to field CA1 and the subi culum. Therefore, we propose that the interactions of the entorhinal-h ippocampal network with the presubiculum are different from those with the parasubiculum.