Striatal target-induced axonal branching of dopaminergic mesencephalic neurons in culture via diffusible factors.

The effects of striatal target cells on the morphological development of dopaminergic neurons were studied in dissociated cultures of embryonic rat mesencephalon. Mesencephalic neurons were cultured for four days in presence of target striatal cells or non target cerebellar ones. The outgrowth of dopaminergic neurons, visualized after tyrosine hydroxylase immunohistochemistry, was examined by quantitative morphometry. In cocultures, the increased complexity of dopaminergic neurites (branching) was the most striking pattern. It was dependent on the presence of target striatal cells as compared to non target ones. Cultures raised in presence or absence of serum lead to suggest the implication of striatal neurons rather than glia. Using MAP2 and phosphorylated neurofilaments immunohistochemistry in combination with tyrosine hydroxylase immunolabelling, it could be shown that the target-induced branching effect concerned only axonal and not dendritic processes. To further define whether diffusible factors from the striatal target would participate in the axonal branching effect, mesencephalic cells were cultured in conditioned medium from striatal neurons. Striatal conditioned medium enhanced dopamine uptake and dopamine neuron branching to the same extent as that observed in striatal cocultures. These findings demonstrate that soluble factors secreted by striatal neurons themselves selectively influence the branching of dopaminergic axons in vitro.