Expression of calcium-binding proteins in pathways from the nucleus of the basal optic root to the cerebellum in pigeons.

Calcium-binding protein expression has proven useful in delineating neural pathways. For example, in birds, calbindin is strongly expressed in the tectofugal pathway, whereas parvalbumin (PV) is strongly expressed in the thalamofugal pathway. Whether neurons within other visual regions also differentially express calcium-binding proteins, however, has not been extensively studied. The nucleus of the basal optic root (nBOR) is a retinal-recipient nucleus that is critical for the generation of the optokinetic response. The nBOR projects to the cerebellum both directly and indirectly via the inferior olive (IO). The cerebellar and IO projections originate from different neurons within the nBOR, but whether they can also be differentiated based on calcium-binding protein expression is unknown. In this study, we combined retrograde neuronal tracing from the cerebellum and IO with fluorescent immunohistochemistry for PV and calretinin (CR) in the nBOR of pigeons. We found that about half (52.3%) of the cerebellar-projecting neurons were CR+ve, and about one-third (33.6%) were PV+ve. Most (90%) of these PV+ve cells were also labeled for CR. In contrast, very few of the IO-projecting neurons expressed CR or PV (<or=2%). Thus, the direct nBOR-cerebellar and indirect nBOR-olivocerebellar pathways to the cerebellum can be distinguished based on the differential expression of CR and PV.