Histochemical distribution of zinc in the brain of the rainbow trout, Oncorhynchos myciss. I. The telencephalon.

The present paper which describes the distribution of zinc in the telencephalon of the rainbow trout, Oncorhynchos myciss, is the first report on the distribution of a heavy metal in the fish brain. Zinc was demonstrated histochemically by silver enhancement using the Neo-Timm method. The staining was mainly confined to the neuropil, but both moderately and intensely stained nerve cell bodies were of common occurrence. Stained fibers were never observed. The staining revealed a specific distribution pattern which could easily be correlated with the telencephalic nuclei defined on the basis of cytoarchitectural features. However, the telencephalon stained much more weakly than the rest of the brain, in striking contrast to the situation in the reptilian, mammalian, and avian brain. In these classes, high staining intensities are observed almost exclusively in the telencephalon. The staining was essentially restricted to the nuclei of the ventral telencephalic area. In the dorsal telencephalic area, only the medial and central zones and medial part of the posterior zone showed comparable staining intensities. The Neo-Timm staining pattern lends support to the view that the pallio-subpallial boundary is between the medial and dorsal zones of the dorsal telencephalic area. The distribution of zinc has been compared with the terminal field of afferent projections, known from experimental mapping, and also with the distribution of substance P and vasoactive intestinal peptide. Finally, the possible functional implications of zinc in synaptic vesicles are considered.