A direct projection from the entorhinal cortex to the mammillary nuclei in the rat

A direct projection from the entorhinal cortex to the mammillary nuclei was found in the rat, by using retrograde and anterograde transport of wheat germ agglutinin-horseradish peroxidase. The projection originates from cells in layer III of the medial entorhinal area and terminates almost ipsilaterally within the ventrolateral region of the pars lateralis of the medial mammillary nucleus. The terminal fields consist of two distinct spots; one is located ventral to the fornix fibers entering the mammillary nuclei and the other situated more caudoventrally in the most ventrolateral region of the pars lateralis. The results suggest that cells in the medial entorhinal area are directly connected with cells in the two restricted regions of the medial mammillary nucleus.