The diencephalon of the channel catfish, Ictalurus punctatus. II. Retinal, tectal, cerebellar and telencephalic connections.

In a companion paper, the nuclear organization of the diencephalon of the channel catfish, Ictalurus punctatus, was described and compared to that of other teleosts. The present paper describes the connections of the diencephalon with the retina, optic tectum, corpus of the cerebellum and telencephalon. The principal tracer employed is the indocarbocyanine dye DiI which diffuses along neuronal membranes in fixed tissues. Almost all of the nuclei that were recognized as distinct in the companion study are found to also exhibit distinct sets of connections. Most of these connections have not been described previously in catfishes or other teleosts. When combined with connectional data from the existing literature, the results of the present study allow one to recognize a great number of distinct pathways through the diencephalon of channel catfish, including several visual, auditory, gustatory, electrosensory and mechanosensory pathways to the telencephalon. Almost all of the species differences in diencephalic organization noted in the companion study can be accounted for by changes in one of the major sensory pathways. In contrast, the multimodal and integrative areas of the diencephalon appear to be relatively conservative. A comparison between the diencephalon of teleosts and that of other vertebrates suggests that the dorsal thalamus, the ventral thalamus and the posterior tuberculum are homologous, at least in part, to the dorsal thalamus, the zona incerta and the subthalamic nucleus of mammals, respectively. All three areas project to the telencephalon in both mammals and teleosts. In most vertebrates, however, the dorsal thalamus provides the dominant input to the telencephalon, whereas in teleosts the main telencephalic input derives instead from the posterior tuberculum.