[The effect of triamterene on the ERG of Long-Evans rats]

Dopamine is an important mediator of the neuronal adaptation mechanism of the retina. Tetrahydrobiopterin (BH4) is cofactor of the rate-limiting enzymatic tyrosine-hydroxylase step in the biosynthetic pathway of dopamine. This enzymatic step causes an oxidation of BH4 to dihydrobiopterin (BH2), which is reduced again to BH4 by action of dihydropterin-reductase (DHPR) and dihydrofolate-reductase (DHFR). Triamterene, a pterin derivate, is known to induce DHFR after initial inhibition. Concomitant elevation of retinal BH4 will result in a higher dopamine concentration. ERG changes of Long-Evans rats mediated by this triamterene effect on dopamine concentration were examined after the rats received triamterene with tap water in two experimental series for 5 and 11 days. After 5 days of triamterene administration a significant decrease of scotopic a- and b-wave ERG amplitudes were registered. After 11 days of triamterene application the decrease could be shown to be even more substantial. This effect was completely reversible: 10 weeks after cessation of triamterene supply ERG amplitudes increased to previous levels. The observed transient ERG changes support the concept of biopterin as an mediator of neuronal adaptation mechanisms of the retina.