Measurements of Somatostatin and Neuropeptide Y in the Visual Cortex of Monocularly Deprived Rats

Abstract

The levels of somatostatin and neuropeptide Y were measured with radioimmunoassay bilaterally in visual cortical areas 17, 18, and 18a of rats which had received monocular enucleation at birth. Neuropeptide levels were consistently three- to fourfold higher for neuropeptide Y than for somatostatin. Monocular enucleation did not change somatostatin levels within areas 17 or 18 of either hemisphere but significantly increased somatostatin levels in contralateral area 18a when compared to contralateral areas 17 or 18 3 months after enucleation. The concentrations of neuropeptide Y are significantly greater in areas 17 and 18a than those in area 18, however, neonatal enucleation had no significant effect on neuropeotide Y levels within any visual cortical area of either hemisphere. Visual cortical areas 17, 18, and 18a show differences in the relative concentrations of neoropeptide Y compared to somatostatin. Furthermore, these two peptides respond distinctively to neonatal enucleation. Enuclestion had no effect on the concentration of either peptide in samples of frontal cortex. Immunohistochemical analysis showed that area 17 contains far fewer somatostatin neurons than areas 18 or 18a, in marked contrast to the uniform levels of somatostatin measured in all visual cortical areas by radioimmunoassay. Immunohistochemically identified neuropeptide Y-immunoreactive neurons are evenly distributed between areas 17, 18, and 18a and represent about half of the number of somatostatin-immunoreactive cells. While neuropeptide Y levels are significantly different between these visual cortical areas, the numbers of immunoreactive neurons are similar. Thus, relatively few neuropeptide Y cells are accompanied by neuropeptide Y concentrations that are four- to fivefold higher than those for somatostatin, the more abundant cell type. In view of the high degree of colocalization of neuropeptide Y and somatostatin in cortical nonpyramidal neurons, these observations imply differences in the biosynthetic and degradation/release mechanisms in these neurons.