Striatal interneurons expressing calretinin, parvalbumin or NADPH-diaphorase: a comparative study in the rat, monkey and human.

The present study is aimed at evaluating the relative number and comparing the pattern of distribution of interneurons containing calretinin (CR), parvalbumin (PV) or NADPH-d in the striatum of rats, squirrel monkeys and humans. A series of adjacent coronal sections taken at three comparable rostrocaudal levels in the three species were treated to reveal the three neuronal markers and the density of each type of chemospecific interneurons was analyzed with a computerized image analysis system. In primates, the most abundant interneurons were those expressing CR. The ratio of CR+/PV+ neurons was approximately 2-3:1 compared to a ratio of 3-4:1 for CR+/NADPH-d+ neurons. In contrast, the most frequently encountered interneurons in the rat striatum were those expressing PV. In rodents, all three interneurons were more abundant rostrally than caudally, but CR+ neurons displayed a particularly striking rostrocaudal decreasing gradient. In monkeys and humans, the three striatal interneurons were distributed rather uniformly rostrocaudally, but CR+ and PV+ interneurons were significantly more numerous in the caudate nucleus than in the putamen in humans. In monkeys, only PV+ neurons were more abundant in the caudate nucleus than in putamen. Overall, the density of the three striatal interneurons was much higher in monkeys than in rats and humans. These results reveal important species differences in respect to the relative density and pattern of distribution of striatal interneurons. These findings should be taken into account when evaluating the effect of neurodegenerative processes on cell densities in the human striatum or when studying animal models of the such diseases.