Differential expression of two adhesion molecules of the immunoglobulin superfamily, F3 and polysialylated NCAM, in hypothalamic magnocellular neurones capable of plasticity

The adult hypothalamo-neurohypophysial system undergoes activity-dependent, reversible morphological changes which result in reduced astrocytic covera ge of its neurones and an increase in their synaptic contacts. Our recent o bservations show that neurones and glia of the hypothalamo-neurohypophysial system continue to express 'embryonic' molecular features which may underl ie their capacity to undergo such plasticity. These include expression of c ell surface molecules Like the glycosyl phosphatidyl inositol (GPI)-linked glycoprotein F3, which intervenes in axonal outgrowth, and the polysialylat ed isoform of the neural cell adhesion molecule (PSA-NCAM), which reduces c ell adhesion and promotes dynamic cell interactions. F3 is colocalised with vasopressin and oxytocin hormones in neurosecretory granules and follows a n activity-dependent, regulated pathway for surface expression on neurohypo physial axons. In contrast, PSA-NCAM appears to follow a constitutive pathw ay, independent of the activity of the hypothalamo-neurohypophysial system, for expression on axonal and glial surfaces, in the hypothalamic magnocell ular nuclei and in the neurohypophysis. The role of F3 remains to be determ ined but in view of its presumptive functions during development, we propos e that it promotes remodelling of neurosecretory terminals. On the other ha nd, we provide direct evidence that surface expression of PSA on NCAM is es sential to morphological plasticity since its specific enzymatic degradatio n in vivo inhibited the neuronal-glial and synaptic changes normally induce d by stimulation of secretion from the hypothalamo-neurohypophysial system.