Projections from the parahippocampal region to the prefrontal cortex in the rat: evidence of multiple pathways.

The purpose of the present study was to investigate, by means of anterograde tracing methods, the detailed organization of the parahippocampal-prefrontal projections in the rat brain. Efferents from the perirhinal cortex were found to terminate principally in both the ventromedial (prelimbic and infralimbic cortices) and lateral (agranular insular cortex) regions of the prefrontal cortex. Terminal fields were observed mainly in the superficial layers of the prefrontal cortex. Projections arising from the dorsolateral entorhinal cortex, which borders the perirhinal cortex along its ventral extent, were similarly directed to the ventromedial and lateral prefrontal cortices but also encompassed other frontal areas (dorsomedial and orbital prefrontal regions). Terminal fields of entorhinal projections were also found in the superficial layers of the prefrontal cortex. A third pathway, taking its source in the post-rhinal cortex, presented striking topographical differences with the two other output systems. Hence, post-rhinal efferences terminated only in the ventrolateral orbital area. The results indicate that two main routes originate from the parahippocampal region to reach the prefrontal cortex. One pathway involves the rostral and lateral portions of the parahippocampal region (perirhinal and dorsolateral entorhinal cortices), and the other relies on its most caudal region, the post-rhinal cortex. The presence of such different multiple parahippocampal-prefrontal pathways may have functional relevance for learning and memory processes.