MAPPING OF THE DISTRIBUTION OF POLYSIALYLATED NEURAL CELL-ADHESION MOLECULE THROUGHOUT THE CENTRAL-NERVOUS-SYSTEM OF THE ADULT-RAT - AN IMMUNOHISTOCHEMICAL STUDY

In the nervous system, the neural cell adhesion molecule changes at the cell surface during development, from a form highly enriched in polysialic residues to several isoforms containing much less sialic acid, and is thought to participate in the structuring of neuronal groups and in the establishment of neuronal connections. Recent observations have indicated, however, that it may not be restricted to developing tissues since it is still present in certain adult neuronal centres which can undergo morphological reorganization. In this study, therefore, we examined systematically the distribution of polysialylated neural cell adhesion molecule immunoreactivity throughout the central nervous system of adult male and female rats, using fight microscopic immunocytochemistry and immunoblot analysis with an antibody that specifically recognizes the polysialic residues of the molecule. Concomitantly, we compared this immunoreactivity to that due to all isoforms of the neural cell adhesion molecule, detected with a polyclonal serum raised against the NH2-terminal of the protein. Immunoreactivity due to the polysialylated isoform was consistently visualized in several discrete areas of the adult brain and spinal cord. An intercellular punctate immunolabelling characterized the staining in certain hypothalamic and thalamic nuclei, superficial laminae of the dorsal horn of the spinal cord, ventral portion of the dentate gyrus of the hippocampus, lateral geniculate, parabrachial and habenular nuclei, bed nucleus of the stria terminalis, mesencephalic central gray and olfactory bulb. In other areas, such as the piriform cortex, dorsal aspect of the dentate gyrus and fimbria and lamina X of the spinal cord, isolated neuronal-like cells were either completely filled with immunolabel or showed a surface reaction on their cell bodies and processes. Highly immunoreactive isolated glial-like cells were also noted within the ependymal layer of the central canal and lateral ventricles and at times in the peripheral white matter of the spinal cord. In contrast to this discrete localization, staining due to all isoforms of the neural cell adhesion molecule was widespread and diffuse throughout the brain and spinal cord. The expression of the polysialylated isoform in the supraoptic nucleus and hippocampus was confirmed by immunoblot analysis; it occurred together with weakly sialylated isoforms. No obvious differences were detected in the amount or distribution of immunoreactivity due to the polysialylated isoform in relation to the sex or age of the animals (between three and 12 months of age). Our study thus demonstrates that well-defined areas of the central nervous system of the adult rat continue to express the polysialylated isoform of the neural cell adhesion molecule. Since many of these areas are known to undergo structural reorganization under particular physiological and experimental conditions, our observations support the hypothesis that polysialylation may be a crucial factor in allowing certain neurons and glial cells to manifest their capacity for structural plasticity in adulthood.