Effects of glial cell line-derived neurotrophic factor (GDNF) on dopaminergic neurons require concurrent activation of cAMP-dependent signaling pathways

Glial cell line-derived neurotrophic factor (GDNF) is a highly selective neurotrophic factor for midbrain dopaminergic neurons and might thus be of potential use in the therapy of Parkinson's disease. In this study, we present evidence that the survival-promoting action of GDNF on dopaminergic neurons requires the concurrent activation of cAMP-dependent signaling pathways. In serum-free low density cultures of the dissociated embryonic day 15 mesencephalon, dopaminergic neurons undergo constant cell death as evidenced by a 90% reduction in tyrosine hydroxylase-immunoreactive (TH-IR) cell numbers between days 1 and 9 of cultivation. This decline was not affected by GDNF (5 ng/ml) within the initial 3 days of cultivation, but was in part attenuated with prolonged treatment. In contrast, stimulation of 3-day-old mesencephalic cultures with GDNF induced c-fos expression in 73% of all TH-IR neurons, indicative for the early presence of efficient signal-transduction coupling in these neurons. Combined treatment of mesencephalic cultures with dibutyryl cyclic AMP (dbcAMP; 100 mu M) and GDNF accelerated the onset of the survival effects of GDNF on dopaminergic neurons, resulting in a 1.5-fold increase in the number of surviving TH-IR neurons at 3 days in vitro. In addition, activation of cAMP-dependent signal pathways significantly potentiated the survival-promoting effects of GDNF on dopaminergic neurons in older cultures. dbcAMP alone had no effect on dopaminergic cell survival. Taken together, our findings suggest that the action of GDNF on midbrain dopaminergic neurons is modulated by other extracellular signals.