Calmodulin and calbindin localization in retina from six vertebrate species

Calmodulin is abundant in the central nervous system, including the retina. However, the localization of calmodulin in the retina has not been described in detail. We therefore decided to investigate calmodulin localization in retinae from six vertebrate species, by using immunohistochemical labeling with four different rabbit polyclonal antibodies against calmodulin. The localization of calbindin-D28k, another calcium-binding protein already well described in retina, was compared. We found that calmodulin distribution is more highly conserved among species, contrasting with calbindin variability. The most striking result emerging is that calmodulin could not be detected in photoreceptors although other layers are intensely calmodulin-immunoreactive, casting doubt about a direct role of calmodulin in phototransduction. Horizontal cells are weakly calmodulin-immunoreactive, bipolar cells are calmodulin-immunoreactive except in turtle retina, numerous amacrine and ganglion cells are labeled in all species, and the fiber layer is always labeled. These data demonstrate that, while the calmodulin distribution in retina is similar among vertebrate species, selective differences in localization can be detected not only among the same cell types in different species but also among different cell types in the same species. The results showing differences in calmodulin immunoreactivity among cell types also provide further evidence that calmodulin expression in eukaryotes is not constitutive, in the sense that not every cell expresses similar levels of calmodulin.