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

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.

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).

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.