Morphology and immunohistochemistry of efferent neurons of the goldfish corpus cerebelli

In teleosts, cerebellar efferent neurons, known as eurydendroid cells, are dispersed within the cerebellar cortex rather than coalescing into deep cerebellar nuclei. To clarify their morphology, eurydendroid cells were labeled retrogradely by biotinylated dextran amine injection into the base of the corpus cerebelli. Labeling allowed the cells to be classified into three types—fusiform, polygonal, and monopolar—depending on their somal shapes and numbers of primary dendrites. The fusiform and polygonal type cells were distributed not only in the Purkinje cell layer but also in the molecular and granule cell layers. The monopolar type cells were distributed predominantly in the Purkinje cell layer of the ventrocaudal portion of the corpus cerebelli. These results suggest that there are some functional differences between these eurydendroid cell types. The eurydendroid cells were double‐labeled by retrograde labeling and immunohistochemistry using specific antibodies against GABA, aspartate, and zebrin II. No GABA‐like immunoreactivity was detected in the retrogradely labeled eurydendroid cells. About half of retrogradely labeled cells were immunoreactive to the anti‐aspartate antibody, suggesting that some eurydendroid cells utilize aspartate as a neurotransmitter. Zebrin II reacts with cerebellar Purkinje cells but left all retrogradely labeled neurons nonreactive, although some of these were surrounded by immunopositive fibers. This relationship between the eurydendroid and Purkinje cells is similar to that between the deep cerebellar nuclei and Purkinje cells in mammals. J. Comp. Neurol. 487:300–311, 2005. © 2005 Wiley‐Liss, Inc.