Abstract: It is thought that applications and use of tetrahydrobiopterin (BH4), which is used as a pharmaceutical, will expand due to its superior efficacy. However, in addition to BH4 being extremely expensive, since it is unstable with respect to oxidation, applications other than pharmaceuticals are considered to be difficult. Further, it is difficult for BH4 to permeate through a blood-brain barrier, so that BH4 concentrations in the brain tend not to increase under the present conditions. Biopterins are oxidized form of BH4, stable to oxidation, easy to handle, and can be produced inexpensively. The inventors found that administration of biopterin can be expected to demonstrate effects equal to or greater than those of administration of BH4.