Spatial and temporal growth factor influences on developing midbrain dopaminergic neurons.

Despite the large number of growth factors shown to affect dopaminergic cell survival and differentiation in vitro, presently little is known about the role these growth factors play during normal in vivo development of dopaminergic neurons. To address this issue, glia and neurons of both the mesencephalic home region as well as the striatal and cortical target areas have been screened for effects on dopaminergic cell survival in serum-free dissociated cell cultures of the embryonic day (E) 15 and E17 rat mesencephalon. In E15 mesencephalic cultures, the number of surviving tyrosine hydroxylase-immunoreactive dopaminergic neurons maximally increased 2.6-fold with medium conditioned by glia of the E15-E20 mesencephalon, the E17-E20 striatum, or the E20 cortex. In marked contrast, all glial-conditioned media (CM) failed to affect dopaminergic cell survival in E17 mesencephalic cultures. Similarly, E17 dopaminergic cell survival was not affected by CM derived from striatal or mesencephalic neurons. This absence of survival-promoting effects was not due to a general lack of sensitivity of the late embryonic dopaminergic neurons to growth factors. Basal survival of cultured E17 dopaminergic neurons declined with PD98059 (20 microM), a potent inhibitor of growth factor-activated microtubule-associated protein (MAP) kinase cascade. Moreover, irrespective of the age of the cultured mesencephalic tissue, dopaminergic growth factors with potential autocrine functions such as brain-derived neurotrophic factor (BDNF; 50 ng/ml) and glial cell line-derived neurotrophic factor (GDNF; 10 ng/ml) promoted dopaminergic cell survival 1.5- to 1.9-fold. These findings suggest that dopaminergic cell survival is predominantly affected by, as yet unknown, growth factors derived from mesencephalic, cortical, and striatal glia during early embryonic development, and by autocrine-acting growth factors during late developmental stages.