Collateral projections from the rat hippocampal formation to the lateral and medial prefrontal cortex.

Projections from the hippocampal formation to the medial prefrontal cortex are well known. In this report we used two retrogradely transported tracers to show that a small but significant subpopulation of pyramidal neurons in area CA1 and subiculum of the hippocampal formation projects to the lateral prefrontal cortex. About half of these neurons also possess collateral projections to the medial prefrontal cortex. The neurons projecting only to the lateral PFC are found in the intermediate hippocampal formation and in the most ventral part of the temporal subdivision. On the other hand, most of the neurons projecting to the medial prefrontal cortex only are present in the temporal and ventral intermediate hippocampal formation, and their number decreases in the dorsal intermediate subdivision. The distribution of neurons having collateral projections is comparable to that of neurons projecting to the medial prefrontal cortex only. In view of proposed functional differences between the septal one-third and the temporal two-third of the hippocampal formation, it is of interest that the neurons projecting to the prefrontal cortex are only present in the temporal two-thirds.