Somatostatin-like immunoreactivity in the pigeon visual system: developmental expression and effects of retina removal.

The distribution of somatostatin (SS)-containing neurons was investigated by immunocytochemical methods in the central visual system of adult, developing, and retina-ablated pigeons. In normal adult brains, SS-positive cells and processes were present in the optic tectum, the nucleus of the basal optic root, the visual Wulst, and the ectostriatum. During development, progressive increase or decrease in the numerical density and the total number of SS-containing neurons occurred as determined by quantitative analysis. Changes in SS immunoreactivity also occurred as a consequence of unilateral and bilateral retina removal immediately after hatching, i.e. before retinofugal connections have been established. In spite of the segregation of visual inputs due to the almost completely crossed retinal projections, unilateral and bilateral deafferentation differentially affected SS-containing visual regions. In addition, different effects were observed on the relative packing density of labeled cells as compared to their total number. A possible role of retinal axons in regulating the distribution of SS immunoreactivity was suggested by its altered expression induced by retinal deafferentation. In addition, parallels with the distribution of SS immunoreactivity in the pigeon's visual system were used to suggest possible equivalence between cell populations in the avian and the mammalian brains.