Heterogeneity of parvalbumin expression in the avian cerebellar cortex and comparisons with zebrin II

The cerebellar cortex has a fundamental parasagittal organization that is reflected in the physiological responses of Purkinje cells, afferent and efferent connections, 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 interdigitated with stripes of little or no expression. In this study, we examined the expression of the calcium binding protein parvalbumin (PV) in the cerebellum of several avian species (pigeons, hummingbirds, zebra finches) and compared it to the expression of ZII. We found that PV immunoreactivity was distributed across the cerebellar cortex such that there were sagittal stripes of PV immunopositive (PV+) Purkinje cells alternating with PV immunonegative (PV-) Purkinje cells. Although most Purkinje cells in the anterior lobe were PV+, there were several thin (i.e. only a few Purkinje cells wide) PV- stripes spanning the folia. In the posterior lobe, PV+ and PV- stripes were also apparent, but the PV- stripes were much wider than in the anterior lobe. In sections processed for both ZII and PV, the expression was generally complementary: PV+ stripes were ZII-, and vice-versa. This complementary expression was most apparent in folia II-IV and VIII-IXcd. The complementary expression was not, however, absolute; some Purkinje cells co-expressed PV and ZII whereas others lacked both. These novel findings relate to the complex neurochemical organization of the cerebellum, and are likely important to issues regarding cerebellar plasticity.