6R-L-erythro-5,6,7,8-Tetrahydrobiopterin and Dopamine Release

Summary

We previously reported that intracerebroventricular administration of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (6R-BJL), a cofactor for aromatic amino acid hydroxylases and nitric oxide synthase, induces an increase in DOPA levels (as an index of DA biosynthesis) and in levels of DA metabolites (as an index of DA release) in the rat brain: the increase in DOPA levels was larger than that of DA metabolites, indicating that 6R-BH₄enhances DA release. In the present study, we examined the effects of 6R-BH₄on DA release in vivo from the striatum of male Wistar rats and its mechanism of action using brain microdialysis. When various concentrations of 6R-BJL, 6S-BH₄ (a diastereoisomer of 6R-BH₄ ) or 6-methyltetrahydropterin (6-MPH₄) were added to the perfusion fluid of microdialysis probes, DA levels collected in dialysates increased in a concentration-dependent manner: the 6R-BJL-induced increase was far greater than the increase induced by 6S-BH₄ or 6-MPH₄. Biopterin had little effect on the DA levels. After i.p. injection of 250 mg/kg of α-methyl-p-tyrosine (α-MT; an inhibitor of tyrosine hydroxylase), most of the 6RBH₄- induced increase persisted but the increase induced by 6S-BJL or 6-MPH4 was abolished. The 6R-BH₄ -induced increase in DA levels in dialysates was abolished after addition of tetrodotoxin (50 μM) to the perfusion fluid, but it persisted after i.p. injection of 100 mg/kg of nomifensine which completely inhibited DA reuptake. The increase in DA levels also persisted after administration of nitric oxide synthase inhibitors. The 6R-BJL-induced increase in DA levels was inhibited by co-administration of 6S-BJL or 6-MPH₄. Administration of sepiapterin (an immediate precursor of 6R-BJL) increased tissue levels of reduced biopterin (mainly consisting of 6R-BH₄) but not its extracellular levels monitored by brain microdialysis, indicating that it selectively increases the intracellular 6R-BJL levels. In contrast, administration of 6R-BH₄increased both tissue levels of reduced biopterin and its extracellular levels, indicating that it increases both the intra- and extra-cellular 6R-BH₄leveles. DA levels in dialysates was elevated following administration of sepiaterin (an immediate precursor of 6R-BH₄), but the elevation was abolished after pretreatment with α-MT. Furthermore, 6R-BH₄ increased firing rates of neurons in the dorsal motor nucleus of vagus monitored by a slice patch method. These results suggest that 6R-BH₄ has a DA releasing action independent of its cofactor for tyrosine hydroxylase and nitric oxide synthase, which is mediated by a specific recognition site for 6R-BH₄ on DA neurons or related cells.