Dopamine neurons from the midbrain of fetal rats and/or newborn dogs will be grown in explant cultures. The growth, development and survival of the dopamine neurons will be monitored by measuring tyrosine hydroxylase activity, dopamine content and 3H-dopamine uptake, as well as evaluating the catecholamine histofluorescence and ultrastructural morphology of the dopamine neurons. We will examine whether the in vitro growth, maturation and survival of the dopamine neurons is influenced by the intracellular levels of neurotransmitter or by the presence in the same culture of brain areas that in vivo provide afferent input or are targets of the dopamine neurons. We will also grow the dopamine neurons in serum free medium and study the effects of hormones, such as glucocorticoids and thyroxine, on their growth and development. Finally we will study the regenerative capacity of the dopamine neurons at different stages of the in vitro development and examine whether regeneration is affected by the presence of hormones in the medium or by increased firing through depolarization of the neuronal membrane. The results of this study will provide information on the mechanisms that control and/or modulate growth and development of dopamine containing neurons. This information could be helpful in our efforts to understand parkinsonism, a disease involving a selective destruction of the midbrain dopaminergic neurons.