Serotonin system development and adult function are regulated by GDNF

Cognitive functions, neuropsychiatric disorders and behaviors from feeding to mood relate to the serotonin (5-hydroxytryptamine; 5-HT) system. We report that expression of glial cell line-derived neurotrophic factor (GDNF), a known potent stimulator of the brain dopamine system, correlates with serotonergic markers in humans, and increased GDNF defines a subset of psychiatric patients with a characteristic 5-HT-related gene expression pattern. A similar ∼1.5- to 2-fold upregulation of endogenous GDNF expression in mice increases brain 5-HT levels and function, both developmentally and during adulthood, and modulates response to fluoxetine. Notably, increasing GDNF more than approximately 2-fold does not increase 5-HT further and instead produces an inverted U-shaped curve of 5-HT levels, suggesting why the GDNF/5-HT correlation has remained controversial. Collectively, our data indicate that GDNF levels fine-tune 5-HT system development and adult function while excess GDNF exclusively associates with neuropsychiatric illness, making it an important target for future research.