Olfactory bulb projections to the parahippocampal area of the rat.

Recent evidence suggests that the main olfactory bulb projects caudally beyond the prepiriform cortex and the cortical amygdaloid nuclei to the region of the piriform lobe called the parahippocampal area. Included within this area is the entorhinal cortex, which is composed of six major subdivisions. Since questions remain as to which of these subdivisions receives centripetal fibers from the bulb, we reexamined these projections using autoradiography and HRP histochemistry and correlated the sites of termination with the cytoarchitecture of the entorhinal cortex. The results indicate that olfactory bulb axons reach all parts of the parahippocampal area, including the cortex which forms the medial and lateral banks of the amygdaloid sulcus (area TR), and both subdivisions of the laterally located entorhinal cortex (28L' and 28L). Also, label is observed over the more medially located fields of the entorhinal cortex, including the cortex posterior to the cortical amygdaloid nucleus (28M'), as well as the ventrolateral parts of medial entorhinal cortex (28M). In addition, evidence of label occurs over the full extent of the transition zone (28i) which separates areas 28L and 28M. These results suggest that the olfactory bulb has a more extensive projection to the parahippocampal area in the rat than previously thought, and may provide at least some input to all of the parahippocampal areas which project to the hippocampal formation.