Differential projections to the hippocampus by neurons of the medial septum and vertical limb of the diagonal band.

Neurons of the medial septum and vertical limb of the diagonal band of Broca project topographically to the hippocampus through the fornix and fimbria, the supracallosal stria and via a ventral route through the amygdala. Injection of the fluorescent dyes Fluoro-Gold or Diamidino yellow into the fimbria-fornix retrogradely labeled neurons in the medial septum and in the ventromedial portion of the vertical limb of the diagonal band. Injection of True blue into the vicinity of the supracallosal stria labeled only neurons in the dorsolateral portion of the diagonal band. No double labeled neurons were observed. These results indicate that (1) neurons of the medial septum and the ventromedial diagonal band project to (or towards) the hippocampus via the fornix and fimbria, (2) neurons in the dorsolateral portion of diagonal band project via the supracallosal stria, and (3) neurons of the medial septum and diagonal band do not send collaterals via both routes. The differential projection of the two groups of neurons may explain the differences in degenerative changes in the two nuclei after damage to their axons.