A direct projection from Ammon's horn to prefrontal cortex in the rat.

A topographically organized input to layers I and V of the infralimbic area from the temporal half of field CA1 of Ammon's horn has been demonstrated in the rat with autoradiographic and fluorescent retrograde tracer methods. Double-labeling studies with the fluorescent dyes True blue and bisbenzimide indicate that most of the pyramidal cells in field CA1 that project to the infralimbic area also send an axon collateral to the entorhinal area. This is the first demonstration of a projection from Ammon's horn to a cortical region outside of the hippocampal formation.