Calbindin immunoreactivity in the auricular lobe and interauricular granular band of the cerebellum in bullfrogs.

Calcium binding protein (CaBP) immunoreactivity in the cerebellum of bullfrogs was examined, concentrating on cells associated with the auricular lobe. While anti-calretinin and anti-parvalbumin also immunoreacted with the same cell populations, anti-calbindin exhibited the most robust and typical pattern of immunostaining. Calbindin immunoreactivity was observed in various populations of cells in the auricular lobe and interauricular granular band of the cerebellum, in the cerebellar peduncle, and in a bundle of interauricular commissural fibers which course through the dorsal, marginal, part of the molecular layer. Cells in the granular layer of the ventral part (i.e., corpus cerebelli) of the cerebellar plate were not CaBP-immunoreactive, nor were any fibers in the molecular layer of this cerebellar region. We believe that axons of CaBP-immunoreactive granule-like cells of the auricular lobes contribute to the formation of the interauricular fiber bundle, which corresponds to the lateral commissure of urodele amphibians. The pattern of calbindin immunoreactivity in the auricular lobes and marginal part of the cerebellar plate provides additional evidence that this cerebellar compartment, which is already present in tadpoles, has a distinct origin, biochemical characterization and connectivity and is separate from the compartment that forms the corpus cerebelli of frogs during metamorphosis.