Zebrin II Expression in the Cerebellum of a Paleognathous Bird, the Chilean Tinamou (Nothoprocta perdicaria).

Zebrin II (ZII) is a glycolytic enzyme expressed in cerebellar Purkinje cells. In both mammals and birds, ZII is expressed heterogeneously, such that there are sagittal stripes of Purkinje cells with a high ZII expression (ZII+) alternating with stripes of Purkinje cells with little or no expression (ZII-). To date, ZII expression studies are limited to neognathous birds: pigeons (Columbiformes), chickens (Galliformes), and hummingbirds (Trochilidae). These previous studies divided the avian cerebellum into 5 transverse regions based on the pattern of ZII expression. In the lingular region (lobule I) all Purkinje cells are ZII+. In the anterior region (lobules II-V) there are 4 pairs of ZII+/- stripes. In the central region (lobules VI-VIII) all Purkinje cells are ZII+. In the posterior region (lobules VIII-IX) there are 5-7 pairs of ZII+/- stripes. Finally, in the nodular region (lobule X) all Purkinje cells are ZII+. As the pattern of ZII stripes is quite similar in these disparate species, it appears that it is highly conserved. However, it has yet to be studied in paleognathous birds, which split from the neognaths over 100 million years ago. To better understand the evolution of cerebellar compartmentation in birds, we examined ZII immunoreactivity in a paleognath, the Chilean tinamou (Nothoprocta perdicaria). In the tinamou, Purkinje cells expressed ZII heterogeneously such that there were sagittal ZII+ and ZII- stripes of Purkinje cells, and this pattern of expression was largely similar to that observed in neognathous birds. For example, all Purkinje cells in the lingular (lobule I) and nodular (lobule X) regions were ZII+, and there were 4 pairs of ZII+/- stripes in the anterior region (lobules II-V). In contrast to neognaths, however, ZII was expressed in lobules VI-VII as a series of sagittal stripes in the tinamou. Also unlike in neognaths, stripes were absent in lobule IXab, and all Purkinje cells expressed ZII in the tinamou. The differences in ZII expression between the tinamou and neognaths could reflect behavior, but the general similarity of the expression patterns across all bird species suggests that ZII stripes evolved early in the avian phylogenetic tree.

Laminar segregation of GABAergic neurons in the avian nucleus isthmi pars magnocellularis: a retrograde tracer and comparative study.

The isthmic complex is part of a visual midbrain circuit thought to be involved in stimulus selection and spatial attention. In birds, this circuit is composed of the nuclei isthmi pars magnocellularis (Imc), pars parvocellularis (Ipc), and pars semilunaris (SLu), all of them reciprocally connected to the ipsilateral optic tectum (TeO). The Imc conveys heterotopic inhibition to the TeO, Ipc, and SLu via widespread γ-aminobutyric acid (GABA)ergic axons that allow global competitive interactions among simultaneous sensory inputs. Anatomical studies in the chick have described a cytoarchitectonically uniform Imc nucleus containing two intermingled cell types: one projecting to the Ipc and SLu and the other to the TeO. Here we report that in passerine species, the Imc is segregated into an internal division displaying larger, sparsely distributed cells, and an external division displaying smaller, more densely packed cells. In vivo and in vitro injections of neural tracers in the TeO and the Ipc of the zebra finch demonstrated that neurons from the external and internal subdivisions project to the Ipc and the TeO, respectively, indicating that each Imc subdivision contains one of the two cell types hodologically defined in the chick. In an extensive survey across avian orders, we found that, in addition to passerines, only species of Piciformes and Rallidae exhibited a segregated Imc, whereas all other groups exhibited a uniform Imc. These results offer a comparative basis to investigate the functional role played by each Imc neural type in the competitive interactions mediated by this nucleus.