POLYSIALYLATED NEURAL CELL-ADHESION IS INVOLVED IN TARGET-INDUCED MORPHOLOGICAL-DIFFERENTIATION OF ARCUATE DOPAMINERGIC-NEURONS

We have previously shown that the morphological and biochemical matura tion of developing rat hypothalamic dopaminergic neurons is accelerate d when they are cocultivated with pituitary intermediate robe cells, o ne of their targets. Only two subsets of hypothalamic dopaminergic neu rons (arcuate, A12, and periventricular, A14, nuclei) may project to t he pars intermedia. In order to determine whether the two populations are equally responsive to coculture conditions, we microdissected the hypothalamus of 17-day-old rat fetuses in two fragments containing cel l bodies from the A12 and from the A14 regions, prepared neuronal cult ures from both portions and incubated them separately with intermediat e robe cells. The presence of intermediate lobe cells increased tyrosi ne hydroxylase levels in both dopaminergic neuron subsets, but morphol ogical differentiation was accelerated in dopaminergic neurons origina ting in the arcuate nucleus only. We then investigated whether physica l contact between developing arcuate neurons and their target cells wa s a prerequisite of the morphological effect by interposing a semiperm eable membrane between cultivated neurons and intermediate lobe cells in transwell culture dishes. The morphological effect was no longer ob served under transwell coculture conditions, pointing to the involveme nt of membrane-bound molecules. Accordingly, the stimulating effect of coculture on arcuate dopaminergic neurons was completely abolished by the removal of polysialic acid on neural cell adhesion molecules by e ndoneuraminidase N treatment. Thus, maturation of A12 and A14 dopamine rgic neurons exhibits differential susceptibility to intermediate robe target cells, and polysialylated-NCAM is required for the contact-dep endent effect.