Sepiapterin Enhances Brain Tetrahydrobiopterin BH4-Dependent Serotonin Synthesis with Regional Specificity

Sepiapterin (SP) serves as a rate-limiting precursor of tetrahydrobiopterin (BH₄), an essential cofactor for tryptophan hydroxylase (TPH) and tyrosine hydroxylase, which are rate-limiting enzymes in the synthesis of brain monoamines. This study investigated the enhancement of brain serotonin metabolism through peripheral SP administration using comprehensive, in vitro and in vivo methodologies. In serotonin-producing RBL2H3 cells, SP exhibited a 21-fold greater cellular uptake efficiency compared to 6R-BH₄, leading to enhanced intracellular BH₄ levels and TPH activation. Kinetic analysis indicated cooperative TPH behavior (Hill coefficient = 2.1) with an apparent K _M of 18.1 µM, which closely aligned with the endogenous BH₄ levels. In C57BL/6J mice, systemic SP administration demonstrated a notable biphasic dose–response pattern with a distinct threshold at approximately 20 mg/kg, delineating two mechanistically distinct regimes. Below this threshold, SP does not reach the brain; however, peripherally-generated BH₄ enters the brain parenchyma but remains predominantly extracellular, stimulating region-specific serotonin release without enhancing synthesis, manifesting as decreased 5-HIAA in brainstem serotonergic nuclei while increasing it in projection areas. Above the threshold, SP directly penetrates the blood–brain barrier and enters brain cells, elevating intracellular BH₄ levels and enhancing TPH activity, thereby producing uniform 5-HIAA increases across all brain regions. These findings establish SP as an effective strategy for enhancing brain serotonin synthesis through targeted intracellular BH₄ elevation, addressing the fundamental limitations of 6R-BH₄ supplementation that have persisted since the pioneering work of Niederwieser in the 1980s. This mechanistic breakthrough suggests a substantial therapeutic potential for various monoaminergic disorders.