Non-hippocampal cortical projections from the entorhinal cortex in the rat and rhesus monkey.

The entorhinal cortices are known to give rise to powerful projections that terminate in the hippocampus and dentate gyrus. Collectively, these link the hippocampal formation to many parts of the cortex and to subcortical structures like the amygdala. Non-hippocampal projections from the entorhinal cortices are understood poorly. Such projections to neighboring temporal areas in the rat and rhesus monkey have been investigated using the autoradiographic and horseradish peroxidase (HRP) tracing procedures. In the rat, HRP-labeled neurons were observed in the intermediate and lateral fields of the entorhinal cortices after injections of temporal cortical areas 20, 35, 36 and 41. They were located predominantly in layers II, III and IV. In the monkey, HRP-labeled neurons were observed in the entorhinal cortices after injections of the rostral superior temporal gyrus (area TA or 22); the temporal polar cortex (area TG or 38); the inferior temporal cortex (area TE or 20); the perirhinal cortex (area 35) and the posterior parahippocampal cortices (areas TF and TH). Unlike the rat, labeled entorhinal neurons in the monkey were located in layer IV. Autoradiographic experiments in the monkey yielded complimentary results. In view of the fact that layer IV of the entorhinal cortex in both the rat and monkey receives a powerful projection from the subicular-CAl fields of the hippocampal formation, the results imply that this layer mediates an indirect non-fornical connection between the hippocampal formation and the temporal cortex.