Distribution of parvalbumin immunoreactivity in the visual cortex of Old World monkeys and humans.

The macaque visual system has been frequently used as a model for understanding functional aspects of human vision. There are, however, few studies directly comparing biochemically defined neuronal populations in the visual cortex of the two species. In this study we compared the distribution and morphological features of the parvalbumin-immunoreactive neuronal subpopulation within humans and Old World monkeys (Macaca fascicularis and Macaca mulatta) by using monoclonal antibodies against the Ca2(+)-binding protein parvalbumin (PV), a neuronal marker in the vertebrate cerebral cortex. Characteristic laminar density and distribution of PV is observed, matching that seen with cytochrome C-oxidase and gamma-aminobutyric acid (GABA) immunoreactivity. Thus, parvalbumin is prominent in the layers receiving afferents from the dorsal lateral geniculate nucleus. Terminal fields are rich in layer IVA and IVC and moderate in the blob-region of layer II-III of the monkey cortex. In the human visual cortex only layer IVC displays rich terminal fields. Parvalbumin is present in neurons within all layers of the cortex except layer I. Parvalbumin-immunoreactive (PV-ir) axons occur in different lamellae of the white matter containing axons belonging to association or projection neurons. The estimation of PV-ir neurons, determined for 50 microns-wide columns through the thickness of area 17, shows that the percentage of the total neuron number in area 17 of humans containing PV is 6.8 +/- 2.0%, and in the macaque monkey, 11.5 +/- 2.9%. The perikaryal area of PV-ir neurons varies according to the layer and is comparable in humans (109.3 +/- 40.8 microns2) and monkeys (94.3 +/- 29.5 microns2). However, the relative number of large PV-ir neurons is higher in humans. The immunoreactive product fills the thinnest cell processes and the shape of PV-ir neurons can be easily traced with the aid of a camera lucida. The shape of the neurons is similar in the two species studied, and they probably belong to non-spiny stellate, double-bouquet, chandelier, and basket cell classes. This study shows that parvalbumin acts as a marker for a subpopulation of interneurons in area 17, but it is also present in the geniculocortical as well as in corticocortical pathways. Moreover, the Old World monkey and human visual cortices have a similar, but not identical, distribution of this important calcium-binding protein.