Changing responsiveness of developing midbrain dopaminergic neurons for extracellular growth factors

Numerous purified growth factors as well as yet-unidentified neurotrophic activities within mesencephalic glia support the survival of dopaminergic neurons. To further characterize the functional role of these multiple growth factor influences in dopaminergic cell development, various purified growth factors as well as mesencephalic glial-conditioned medium (CM) were screened for effects on dopaminergic cell survival and glial numbers in serum-free low density cultures of the dissociated embryonic day (E) 15 and E17 rat mesencephalon. In E15 mesencephalic cultures, dopaminergic cell survival increased with brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), basic fibroblast growth factor (bFGF), transforming growth factor alpha (TGFalpha), insulin-like growth factor-1 (IGF-1), platelet-derived growth factor-BB (PDGF-BB), and interleukin-6 (IL-6). bFGF, TGFalpha, PDGF, and IL-6 also stimulated glial proliferation as demonstrated by autoradiographic labeling for 3H-thymidine. Moreover, CM derived from the mesencephalic glial cell line Mes42 completely prevented the death of E15 dopaminergic neurons within the initial days of cultivation. In E17 mesencephalic cultures, survival-promoting effects on dopaminergic neurons were present with BDNF, GDNF, and bFGF. TGFalpha, IGF-1, PDGF-BB, and IL-6 stimulated glial proliferation but did not affect dopaminergic cell survival. Similarly, mesencephalic glial-CM completely failed to support the survival of E17 dopaminergic neurons. These observations demonstrate that during embryonic development, dopaminergic cell survival sequentially depends on distinct sets of growth factors. The concomitant loss of sensitivity of developing dopaminergic neurons for mesencephalic glial-CM as well as TGFalpha, IGF-1, PDGF-BB, and IL-6 further provides evidence that these growth factors indirectly affect early dopaminergic neurons through glial-mediated processes and suggests a crucial role of glia during the initial stages of neuronal development.