Direct projections from Ammon's horn to the septum in the cat.

Direct projections from Ammon's horn to the septum were studied in the cat by the anterograde tracing method after injecting WGA-HRP (wheat germ agglutinin-horseradish peroxidase conjugate) into Ammon's horn. The results were further confirmed by the retrograde WGA-HRP method after injecting WGA-HRP into the septum. Pyramidal neurons in fields CA1, CA2 and CA3 were observed to send their axons ipsilaterally to the lateral septal nucleus; the septal parts of the hippocampus sent projection fibers to the dorsomedial portions of the lateral septal nucleus via the medial aspects of the subcallosal fornix, while the hippocampal regions successively more proximal to the temporal pole sent projection fibers to progressively more ventrolateral portions of the lateral septal nucleus via more lateral aspects of the subcallosal fornix. It was also found that the septal parts of fields CA1, CA2 and CA3 sent projection fibers bilaterally to the dorsomedial aspects of the lateral septal nucleus. Field CA4 appeared to send projection fibers only sparsely, if at all, to the medial septal nucleus. The rudimentary parts of the hippocampal formation, taenia tecta and indusium griseum, were found to have reciprocal ipsilateral connections with the dorsal portions of the lateral septal nucleus.