Central projections of the nervus terminalis in lampreys, lungfishes, and bichirs

Central projections of the nervus terminalis were investigated in a cyclostome (Lampetra planeri), in a sarcopterygian (Protopterus dolloi), and in an actinopterygian fish (Polypterus palmas), following the injection of horseradish peroxidase into the olfactory epithelium. Despite differences in forebrain morphology (inversion versus eversion of the hemispheres), projections of the terminal nerve are similar in the species investigated. The nervus terminalis courses through the subpallium (septum) and mainly innervates periventricular nuclei in the telencephalon and diencephalon. In lampreys, the majority of labeled fibers terminate in the hypothalamus, while in bony fishes the main projection is to periventricular nuclei of the anterior commissure. The course of the nervus terminalis through the dorsomedial telencephalon in lungfishes supports the interpretation that this part of the brain constitutes the septum, and not a pallial structure. Nervus terminalis projections are compared with those in teleosts and in amphibians. The presumed lack of gonadotropin-releasing hormone in the nervus terminalis of lampreys is discussed.