Loss of striatal DA innervation increases striatal trophic activity directed at DA neurons in culture

Male rats received intraventricular infusions of the dopamine (DA) neurotoxin 6-hydroxydopamine (6-OHDA; 0, 75, 150, and 250 micrograms) in order to determine if DA neuron loss was associated with an increase in striatal trophic activity. After 4 weeks, the animals were sacrificed and perfused with normal saline, and the brains were removed, immediately frozen, and processed. Intraventricular infusions of 6-OHDA were associated with a dose-dependent reduction in striatal DA content and tyrosine hydroxylase-immunoreactive (THir) cell counts in the substantia nigra while striatal DA activity ([HVA]/[DA]) was increased. Extracts of the striatum from these animals increased the survival of E15 primary, dissociated rostral mesencphalic cultures growing at low cell density. This growth effect was positively correlated with the dose of 6-OHDA infused. THir cell counts present in high-cell-density mesencephalic cultures following 72 h of extract incubation were similarly correlated to 6-OHDA dose but inversely correlated with striatal DA content and THir cell counts in the substantia nigra. Trophic activity in the cerebellar extracts from these animals was significantly lower than that present in striatal extracts and was not influenced by 6-OHDA lesions. These data suggest that loss of DA innervation in the striatum is associated with an increase in striatal trophic activity directed at DA neurons. A compensatory response to the loss of DA neurons involving increased striatal trophic activity may result in increased DA terminal sprouting of remaining viable DA neurons that, in turn would serve to help reinstate normal DA tone.