Tracing of single fibers of the nervus terminalis in the goldfish brain.

Central projections of the nervus terminalis (n.t.) in the goldfish were investigated using cobalt- and horseradish peroxidase-tracing techniques. Single n.t. fibers were identified after unilateral application of cobalt chloride-lysine to the rostral olfactory bulb. The central course and branching patterns of individual n.t. fibers were studied in serial sections. Eight types of n.t. fibers are differentiated according to pathways and projection patterns. Projection areas of the n.t. include the contralateral olfactory bulb, the ipsilateral periventricular preoptic nucleus, both retinae, the caudal zone of the periventricular hypothalamus bilaterally, and the rostral optic tectum bilaterally. N.t. fibers cross to contralateral targets in the anterior commissure, the optic chiasma, the horizontal commissure, the posterior commissure, and possibly the habenular commissure. We propose criteria that differentiate central n.t. fibers from those of the classical secondary olfactory projections. Branching patterns of eight n.t. fiber types are described. Mesencephalic projections of the n.t. and of secondary olfactory fibers are compared and discussed with regard to prior reports on the olfactory system of teleosts. Further fiber types for which the association with the n.t. could not be established with certainty were traced to the torus longitudinalis, the torus semicircularis, and to the superior reticular nucleus on the ipsilateral side.