Calcium-binding protein distribution in the retina of strepsirhine and haplorhine primates

Calcium-binding proteins are involved in numerous functional roles in the retina and are widely distributed in almost all retinal neurons. The present study aimed to characterize the distribution of the calcium-binding proteins calbindin, calretinin, parvalbumin and recoverin in relation to retinal cell types in a strepsirhine primate (mouse lemur, Microcebus) in comparison with primate species of the three main haplorhine lineages (marmoset, macaque and human), as well as a rodent (gerbil, Taterillus). The main findings show that whereas the recoverin antibody labels both rod and cone photoreceptors in all species, calbindin consistently labels cones, but not rods, in the haplorhine primates marmoset, macaque and human, but none of the photoreceptors in the mouse lemur. Marmoset and macaque also show a distinct label of cone outer segments with calretinin. Depending on the species, bipolar cells express calbindin and/or recoverin, while amacrine, horizontal and ganglion cells are labeled to varying degrees with calbindin, calretinin and parvalbumin. Haplorhine and strepsirhine primates clearly differ in the expression of calcium-binding protein expression in horizontal cells. In all haplorhine species, horizontal cells are densely labeled with parvalbumin whereas in mouse lemur horizontal cells express calbindin but not parvalbumin. Several characteristics of the calcium-binding immunostaining in the retina of the mouse lemur are similar to those observed in the rodent, and distinguish this species from the diurnal haphorhine primates. These differences may be related to adaptations of retinal structure and function to the nocturnal niche, since nocturnal strepsirhine and haphorhine (Tarsius and Aotus) primates share some features of calcium-binding expression.