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 hypothalami c dopaminergic neurons. The presence of melanotrophs in primary cultur es of foetal hypothalamic neurons induces an increase of the number of dopaminergic neurons (while the total neuronal population remains unc hanged) and induces a stimulation of their neuritic outgrowth, These e ffects are mediated by diffusible factors since they are reproduced by application of conditioned medium issued from co-cultures with interm ediate lobe cells from newborn rats, Moreover, by immunoneutralization of alpha-melanocyte-stimulating hormone (alpha MSH) in the co-culture or conditioned medium, or by application of the peptide itself, we de monstrate that: the neuritotrophic effect on dopaminergic neurons is m ediated by alpha MSH, the main secretory product of melanotrophs, wher eas the inductive effect on the number of dopaminergic neurons is attr ibutable to another diffusible neurotrophic factor(s) present in foeta l, but not adult, adenohypophysis. Similar effects are observed on cul tures of newborn hypothalamic neurons, However, at this stage of neuro nal development, alpha MSH also increases the number of dopaminergic n eurons, which could be due tcr a change of neuronal receptivity. We sh ow that the neuritotrophic influence of alpha MSH is restricted to the dopaminergic neurons connected to the melanotrophs, and that in addit ion, these neurons systematically co-express the tyrosine hydroxylase and glutamate decarboxylase as the neurons innervating the melanotroph s in situ. These findings indicate that the differentiation of dopamin ergic hypothalamic neurons is influenced by the target cells, melanotr ophs, and that this trophic influence implicates alpha MSH.