Retention of embryonic features by an adult neuronal system capable of plasticity: polysialylated neural cell adhesion molecule in the hypothalamo-neurohypophysial system.

The neural cell adhesion molecule, N-CAM, changes at the cell surface during development, from a highly sialylated form [polysialic acid (PSA)-linked N-CAM, PSA-N-CAM] to several isoforms containing less sialic acid. N-CAM and its polysialic acid may serve to regulate cell apposition, thus affecting a variety of cell interactions. In the nervous system, PSA-N-CAM has until now been localized in developing tissues where it is thought to participate in the structuring of neuronal groups and tissue pattern formation. It has been proposed, however, that PSA-N-CAM may also be expressed in the adult, where it may take part in plasticity and cell reshaping. In the present study, the use of immunoblot and immunocytochemical procedures with a monoclonal antibody that specifically recognizes PSA-N-CAM revealed that the adult rat hypothalamo-neurohypophysial system, which undergoes important neuronal-glial and synaptic rearrangements in response to physiological stimuli, contains high levels of PSA-N-CAM immunoreactivity. The use of a polyclonal serum reacting with all N-CAM isoforms indicated that PSA-N-CAM is expressed together with "adult" forms of N-CAM. Light and electron microscopy demonstrated the presence of PSA-N-CAM immunoreactivity in the supraoptic and paraventricular nuclei of the hypothalamus and in the neurohypophysis; the immunoreactivity was seen in dendrites, axons, and terminals and in associated astrocytes but not in neuronal somata. We propose that the continued expression of PSA-N-CAM confers to magnocellular neurons and their astrocytes the ability to reversibly change their morphology in adulthood. In addition, our observations suggest that evidence for polysialylation may serve to identify other neuronal systems capable of morphological plasticity in the adult central nervous system.