The epithalamus of the developing and adult frog: calretinin expression and habenular asymmetry in Rana esculenta.

Expression of the calcium binding protein (CaBP) calretinin (CR) was studied with immunohistochemistry in the pineal complex and habenular nuclei (HN) of the developing and adult frog Rana esculenta. The frog pineal complex is a medial structure formed by two interconnected components, the frontal organ and the pineal organ or epiphysis; the habenular nuclei are bilateral and are asymmetric due to subdivision of the left dorsal nucleus into medial and lateral components. In the pineal complex, calretinin immunostaining of cells and fibers was consistently observed in developing and adult frogs. In the habenulae, calretinin immunoreactivity exhibited instead marked variations during development, and was expressed only in cells of the medial subnucleus of the left dorsal habenula. In particular, calretinin was detected at larval stages, peaked during metamorphosis, was markedly downregulated at the end of metamorphosis, and was evident again in adulthood. This sequence of calretinin expression was confirmed by quantitative analysis of immunoreactive cells in the left habenula. In tadpoles, calretinin-positive cells exhibited a dorsoventral gradient of density, while in adulthood, they were distributed throughout the dorsoventral extent of the medial subnucleus. The study demonstrates a peculiar developmental pattern, with transient downregulation, of asymmetric calretinin expression in the frog epithalamus. The findings indicate that calcium and calcium buffering systems may play critical roles in neurogenetic and neuronal migration processes implicated in the formation of the asymmetric habenular portion in amphibians. In addition, the reappearance of calretinin expression in the adult frog supports a distinct functional role of the asymmetric habenular component in amphibians.