Perirhinal cortex input to the hippocampus in the rat: evidence for parallel pathways, both direct and indirect. A combined physiological and anatomical study

The possibility of a direct projection from the perirhinal cortex (PER) to areas CA1 and subiculum (SUB) in the hippocampus has been suggested on the basis of tracer studies, but this projection has not unequivocally been sup ported by physiological studies. The demonstration of such a functional pat hway might be important to understand the functioning of the hippocampal me mory system. Here we present physiological and further anatomical evidence for such a connection between PER and the hippocampus. Electrical stimulati on of PER in vivo evoked field potentials (EFPs) at the border area of CA1/ SUB, consisting of a short latency and a longer latency component. Current source density analysis revealed that the sink of the short latency compone nt was situated in the molecular layer of area CA1/SUB, while the longer la tency component had its sink in the outer molecular layer of the dentate gy rus (DG). Anterograde tracer injections in PER showed labelled fibres in th e border area of CA1/SUB, but anatomical evidence for a projection of PER t o DG was not found. When synaptic transmission in the entorhinal cortex was partly blocked, the amplitude of the longer latency component of the recor ded EFPs in the hippocampus was decreased while the short latency component was not affected, which suggests that the indirect pathway originating in PER is mediated through a synaptic relay in the entorhinal dorter. From the present results we conclude that information originating in PER reaches ar ea CA1/SUB by parallel, direct and indirect, routes. The existence of this parallel organization appears to form an essential feature for the proper f unction of the medial temporal lobe memory system.