Heterogeneity of calretinin expression in the avian cerebellar cortex of pigeons and relationship with zebrin II

The cerebellar cortex has a fundamental parasagittal organization that is reflected in the physiological responses of Purkinje cells, projections of Purkinje cells, afferent inputs from climbing and mossy fibres and the expression of several molecular markers. The most thoroughly studied of these molecular markers is zebrin II (ZII; a.k.a. aldolase C). ZII is differentially expressed in Purkinje cells, resulting in a pattern of sagittal stripes of high expression (ZII+ve) interdigitated with stripes of little or no expression (ZII−ve). The calcium binding protein calretinin (CR) is expressed heavily in mossy fibres terminating throughout the cerebellar cortex, but whether CR is heterogeneously expressed in parasagittal stripes, like ZII, is unknown. In this study, we examined CR expression in the cerebellum of pigeons and compared it to that of ZII. CR was expressed heavily in the granule layer in mossy fibres and their terminal rosettes. Moreover, CR is expressed heterogeneously in the granule layer such that there are sagittal stripes of heavy CR labelling (CR+ve) alternating with stripes of weaker labelling (CR−ve). The CR heterogeneity is most notable in folium IXcd and folia II-IV in the anterior lobe. In the anterior lobe, the central CR+ve stripe spanning the midline is aligned with the central ZII+ve stripe, whereas the other CR+ve stripes are aligned with the ZII−ve stripes. In IXcd, the CR+ve stripes are aligned with the ZII+ve stripes. This combination of aligned and unaligned CR+ve stripes, relative to ZII+ve stripes, differs from that of parvalbumin and other neurochemical markers, but the functional consequences of this is unclear. With respect to the posterior lobe, we suggest that the CR+ve mossy fibres to IXcd originate in two retinal recipient nuclei that are involved in the processing of optic flow.