Evidence for a direct projection from the postrhinal cortex to the subiculum in the rat

Behavioral data indicate that three of the areas which form the parahippoca mpal region in the rat, i.e., the entorhinal, perirhinal, and postrhinal co rtices, have different, although related functions that also differ from th ose of the hippocampal formation. These functional differences might be rel ated to differences in connectivity, on the one hand with parts of the asso ciation cortex, and on the other with the hippocampal formation. In a previ ous study, we showed the existence of both a direct and an indirect project ion from the perirhinal cortex to areas CA1 and subiculum of the hippocampu s. Here we present the result of a second study, demonstrating a similarly organized projection from the postrhinal cortex to the subiculum, comprisin g both a direct and an indirect route. Electrical stimulation of the postrhinal cortex in vivo evoked field potent ials throughout the subiculum and the dentate gyrus. Current source density analysis in both the subiculum and dentate gyrus revealed the presence of sink-source pairs, indicative of a synaptic termination. Based on compariso n with the sink-source pairs found after stimulation of the medial entorhin al cortex, we conclude that the connection between the postrhinal cortex an d the dentate gyrus most likely is formed by a polysynaptic pathway mediate d via the medial entorhinal cortex, while the pathway from the postrhinal c ortex to the subiculum is likely monosynaptic. In order to substantiate the se findings, we carried out several tracer experiments. Anterograde tracer injections in the postrhinal cortex resulted in labeled fibers in limited p arts of the subiculum, but no anatomical evidence for a projection of the p ostrhinal cortex to the dentate gyrus was found. Additional retrograde trac er injections in the subiculum also showed evidence for a direct postrhinal -to-subiculum projection with a strong topological organization. Based on t hese combined anatomical and electrophysiological data, we conclude that th e postrhinal cortex indeed can reach the subiculum via both a direct and an indirect pathway. Hippocampus 2001;11:105-117. (C) 2001 Wiley-Liss, Inc.