Central projections of the nervus terminalis and the nervus praeopticus in the lungfish brain revealed by nitric oxide synthase.

Lungfishes possess two cranial nerves that are associated with the olfactory system: the nervus terminalis enters the telencephalon with the olfactory nerve, and the nervus praeopticus enters the diencephalon at the level of the optic nerve. We investigated the central projections of the nervus terminalis and the nervus praeopticus in the Australian lungfish (Neoceratodus forsteri) and in the African lungfish (Protopterus dolloi) by NADPH-diaphorase histochemistry (nitric oxide synthase; NOS) and compared them with the projections of the nervus terminalis of the frog (Xenopus laevis). In Neoceratodus, NOS-positive fascicles of the nervus terminalis divide and project with a ventral component through the septum and with a dorsal component through the pallium; fibers of both trajectories extend caudally beyond the anterior commissure and join the lateral forebrain bundle. In the nervus praeopticus, about 300 fibers contain NOS; they innervate the preoptic nucleus and continue their course through the diencephalon; many fibers cross in the commissure of the posterior tuberculum. In Protopterus, ganglion cells of the nervus terminalis and of the nervus praeopticus contain NOS. NOS-positive fibers of the nervus terminalis project through the septal region but not through the pallium. Several major fascicles cross in the rostral part of the anterior commissure, where they are joined by a small number of NOS-containing fibers of the nervus praeopticus. Both nerves innervate the preoptic nucleus. The number and pathways of the fascicles of the nervus terminalis are not always symmetric between the two sides. The nervus terminalis fascicles remain in a ventral position, whereas the nervus praeopticus gives rise to the more dorsal fascicles. Many fibers of the two nerves extend throughout the diencephalon and cross in the commissure of the posterior tuberculum. These findings demonstrate many similarities but also significant differences between the contributions of the nervus terminalis and the nervus praeopticus to forebrain projections in the two lungfishes. They support the view that the nervus praeopticus is part of a nervus terminalis system comparable to that in frogs and other nonmammalian vertebrates.