The organization of feline entopenduncular nucleus projections: anatomical studies

The organization of entopeduncular nucleus (EPN) projections was studied in cats using autoradiographic and horseradish peroxidase (HRP) techniques. In autoradiographic studies, EPN axons were found to terminate in a J-shpaed region in the dorsal and medial part of the ventral anterior nucleus (VA) and the rostral portion of the adjacent ventral lateral nucleus (VL). EPN axons also terminated in the rostral portion of the centrum medianum (CM), the ventrolateral portion of the lateral habenular nucleus (LHB), and the pedunculopontine nucleus (PP). The VA included the largest terminal field although the LHB had the greatest density of terminals. Regardless of the region of EPN into which amino acids were injected, the terminal fields were the same: there was no localization within the EPN of the cells projecting to one region. HRP-containing cells were distributed throughout the EPN following injections into the VA, LHB, or PP, although many more cells were labeled following injections into either VA or LHB than PP. EPN cells containing HRP following injections into either VA or LHB were not morphologically different from those not containing HRP in the same respective animals. Following HRP injections into stria medullaris, only cells in the rostral part of the EPN were labeled, providing evidence that rostrally and caudally located EPN neurons have different paths to LHB. Although there may be a rostrocaudal organization of pathways to LHB, individual regions of the nucleus project to the same areas.