Differentiation of rat hypothalamic dopaminergic neurons is stimulated in vitro by target cells: the melanotrophs.

We have investigated in vitro the influence of pituitary intermediate lobe melanotrophs on the differentiation of their afferent hypothalamic dopaminergic neurons. The presence of melanotrophs in primary cultures of foetal hypothalamic neurons induces an increase of the number of dopaminergic neurons (while the total neuronal population remains unchanged) and induces a stimulation of their neuritic outgrowth. These effects are mediated by diffusible factors since they are reproduced by application of conditioned medium issued from co-cultures with intermediate lobe cells from newborn rats. Moreover, by immunoneutralization of alpha-melanocyte-stimulating hormone (alphaMSH) in the co-culture or conditioned medium, or by application of the peptide itself, we demonstrate that the neuritotrophic effect on dopaminergic neurons is mediated by alphaMSH, the main secretory product of melanotrophs, whereas the inductive effect on the number of dopaminergic neurons is attributable to another diffusible neurotrophic factor(s) present in foetal, but not adult, adenohypophysis. Similar effects are observed on cultures of newborn hypothalamic neurons. However, at this stage of neuronal development, alphaMSH also increases the number of dopaminergic neurons, which could be due to a change of neuronal receptivity. We show that the neuritotrophic influence of alphaMSH is restricted to the dopaminergic neurons connected to the melanotrophs, and that in addition, these neurons systematically co-express the tyrosine hydroxylase and glutamate decarboxylase as the neurons innervating the melanotrophs in situ. These findings indicate that the differentiation of dopaminergic hypothalamic neurons is influenced by the target cells, melanotrophs, and that this trophic influence implicates alphaMSH.