Laminar distribution of cytochrome oxidase staining in cetacean isocortex

The distribution of cytochrome oxidase activity was studied in the cerebral cortex of two species of cetaceans, the harbour porpoise Phocoena phocoena, and the bottlenose dolphin Tursiops truncatus. Two main patterns of distribution of cytochrome oxidase were detected. The first, characteristic of the visual and auditory cortices of the lateral and suprasylvian gyri, is typified by a peak density in layer III, contrasting with low levels in layers II, V and VI. The second is found in wide areas of the limbic lobe, the insular cortex, the temporal operculum and the occipital cortex. In these regions, distribution of cytochrome oxidase is more uniform, with little difference between layers III, V and VI. A transitional pattern is found in the most dorsal parts of the limbic lobe, the parietal operculum, the ectosylvian gyrus and in orbitofrontal cortex. As areas of high cytochrome oxidase activity have been described in various land mammals to correspond to zones of major excitatory input and, in particular, to characterise the cortical layers that receive thalamocortical afferents, we propose that the thalamocortical input to cetacean sensory cortex, in which a typical layer IV is absent, may be mainly to layer III. This view is supported by the high density of neurons positive for the inhibitory transmitter gamma-aminobutyric acid that is also found in layer III of cetacean cortex, another typical feature of thalamocortical recipient zones.