Organization of major telencephalic pathways in an elasmobranch, the thornback ray Platyrhinoidis triseriata

Abstract

The forebrain of elasmobranchs is well developed, and in some species the relative brain/body weight is comparable to that in mammals. However, little is known about the organization of major telencephalic pathways. We injected biotinylated dextran amines into the olfactory bulb, lateral pallium, dorsomedial pallium, and the forebrain bundles of the thornback ray, Platyrhinoidis triseriata. Secondary olfactory fibers from the bulb innervate the lateral pallium, the ventral division of the rostral telencephalon and area superficialis basalis. Retrogradely labeled cells were seen exclusively in the lateral periventricular area. The projections of the lateral pallium appeared basically similar to those of the olfactory bulb, but labeling was much denser in the superficial part of area basalis. Some fibers were also seen to innervate the posterior tuberal nucleus. Injections into the dorsomedial pallium revealed a major input from area basalis. Only a few cells were retrogradely labeled in the dorsal thalamus and posterior lateral thalamic nucleus. Major efferents of the dorsomedial pallium appear to reach the contralateral inferior lobe of the hypothalamus and the lateral mesencephalic nucleus. Tracer injections into the forebrain bundles retrogradely labeled many cells in the diencephalon and the mesencephalon and also revealed terminal fields in area superficialis basalis. In addition, a large number of cells were labeled in the dorsomedial pallium. Descending telencephalic fibers innervate heavily the inferior lobes and the lateral mesencephalic nucleus. Our results show that higher order olfactory pathway courses from the lateral pallium through area basalis to the dorsomedial pallium and that ascending non-olfactory input is integrated in area superficialis basalis and the dorsal pallium along with olfactory information, rather than being processed in separate, non-olfactory centers.