3-O-Methyldopa inhibits astrocyte-mediated dopaminergic neuroprotective effects of l-DOPA

Background: We evaluated the effects of 3-O-methyldopa (3-OMD), a metabolite of l-DOPA which is formed by catechol-O-methyltransferase (COMT), on the uptake, metabolism, and neuroprotective effects of l-DOPA in striatal astrocytes. We examined changes in the numbers of dopaminergic neurons after treatment with l-DOPA and 3-OMD or entacapone, a peripheral COMT inhibitor, using primary cultured mesencephalic neurons and striatal astrocytes. Results: The number of tyrosine hydroxylase-positive dopaminergic neurons was not affected by l-DOPA treatment in mesencephalic neurons alone. However, the increase in viability of dopaminergic neurons in the presence of astrocytes was further enhanced after methyl-l-DOPA treatment (25 μM) in mixed cultured mesencephalic neurons and striatal astrocytes. The neuroprotective effect of 25 μM l-DOPA was almost completely inhibited by simultaneous treatment with 3-OMD (10 or 100 μM), and was enhanced by concomitant treatment with entacapone (0.3 μM). The uptake of l-DOPA into and the release of glutathione from striatal astrocytes after l-DOPA treatment (100 μM) were inhibited by simultaneous exposure to 3-OMD (100 μM). Conclusions: These data suggest that l-DOPA exerts its neuroprotective effect on dopaminergic neurons via astrocytes and that 3-OMD competes with l-DOPA by acting on target molecule(s) (possibly including glutathione) released from astrocytes. Since some amount of entacapone can cross the blood-brain barrier, this reagent may enhance l-DOPA transportation by inhibiting COMT and increase the astrocyte-mediated neuroprotective effects of l-DOPA on dopaminergic neurons.