Modulation of NCAM Polvsialvlation is Associated with Morphofunctional Modifkations in the Hypothalamo‐neurohypophysial System During Lactation

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Abstract
Post‐transcriptional modification of the neural cell adhesion molecule (NCAM) by polysialic acid significantly decreases NCAM adhesiveness and more generally modifies cell‐cell interactions. Polysialic acid‐NCAM (PSA‐NCAM) is mainly expressed in the developing nervous system. In the adult, its expression is restricted to regions that retain morphological plasticity, such as the hypothalamo‐neurohypophysial system during lactation in rats. Since cell‐cell interactions and synaptic contacts in the hypothalamo‐neurohypophysial system are greatly increased during lactation, we examined whether PSA‐NCAM expression is modified during this period. lmmunohistochemistry and immunoblotting showed that, compared with virgin rats, PSA‐NCAM dramatically decreased during lactation in both the supraoptic nuclei and the neurohypophysis, and returned to its initial level only after weaning. This decrease was progressive and became significant only at the end of the first week of lactation. By contrast, modifications in the level of NCAM protein or changes in the splicing pattern of NCAM mRNAs could not be detected. The decline in polysialic acid on the NCAM molecule could strengthen membrane appositions, thereby stabilizing the newly established synapses and neurohaemal contacts in the hypothalamo‐neurohypophysial system that accompany the increased neuronal activity that occurs during lactation. We also studied the regulation of the phosphorylated microtubule‐associated protein‐1B (MAP1B‐P), whose distribution pattern largely overlaps with that of PSA‐NCAM in the adult brain. Expression of MAP1B‐P was greatly increased during lactation in the hypothalamic axons projecting into the neurohypophysis. Thus, the expression patterns of both PSA‐NCAM and MAP1B‐P may reflect the permanent structural plasticity characterizing the hypothalamo‐neurohypophysial system in the adult.