Distribution of dopaminergic fibers and neurons in visual and auditory cortices of the harbor porpoise and pilot whale.

The distribution of putative dopaminergic fibers in two sensory cortical areas in the brain of the harbor porpoise (Phocoena phocoena) and pilot whale (Globicephala melaena) was analyzed at the light and electron microscopic levels using tyrosine hydroxylase (TH) immunohistochemistry. The quantitative analysis of the distribution of labeled fibers demonstrates that the primary visual cortex located in the lateral gyrus and entolateral sulcus contains a denser dopaminergic innervation than the auditory cortex within the posterior portion of the presylvian gyrus. In both areas, TH-immunoreactive fibers are densest in layer I, while layers IIIab and VI have intermediate densities and layers II and IIIc-V have the lowest fiber counts. Layer I is characterized by the presence of very thick TH-immunoreactive fiber populations, in addition to the thin and varicose fiber plexus observed throughout the cortical layers. Electron microscopic analyses demonstrated that some of these thick fibers represent the dendrites of TH-immunoreactive neurons located in the deep portion of layer I. The patterns observed in the present study suggest that the dopaminergic projections to the neocortex in whales have a different organization than in terrestrial mammals, particularly rodents and primates. These differences may reflect the fact that during evolution, the cetacean neocortex has retained many of the cytoarchitectonic features that are usually observed only in proisocortical regions in progressive terrestrial mammals.