Catechol-O-methyltransferase inhibition protects against 3,4-dihydroxyphenylalanine (DOPA) toxicity in primary mesencephalic cultures: new insights into levodopa toxicity

Inhibition of catechol-O-methyltransferase (COMT) has protective effects on levodopa (L-DOPA), but not D-DOPA toxicity towards dopamine (DA) neurons in rat primary mesencephalic cultures [Mol. Pharmacol. 57 (2000) 589]. Here, we extend our recent studies to elucidate the mechanisms of these protective effects. Thus, we investigated the effects of all main L-DOPA/DA metabolites on survival of tyrosine hydroxylase immunoreactive (THir) neurons in primary rat mesencephalic cultures. 3-O-Methyldopa, homovanillic acid, dihydroxyphenyl acetate and 3-methoxytyramine had no effects at concentrations up to 300 micro M after 24h, whereas DA was more toxic than L-DOPA with toxicity at concentrations of >or=1 micro M. The coenzyme of COMT, S-adenosyl-L-methionine (SAM), and its demethylated product S-adenosylhomocystein caused no relevant alteration of THir neuron survival or L-DOPA toxicity. In contrast, inhibition of SAM synthesis by selenomethionine showed time- and dose-dependent increase of THir neuron survival, but did not affect L-DOPA toxicity. L-DOPA-induced lipid peroxidation in mesencephalic cultures was not modified by the COMT inhibitor Ro 41-0960 (1 micro M). Increased contamination of the cultures with glial cells attenuated L- and D-DOPA toxicity, but caused significant enhancement of protection by COMT inhibitors against L-DOPA toxicity only. Investigations of L-DOPA uptake in rat striatal cultures using HPLC revealed a significant reduction of extracellular L-DOPA concentrations by Ro 41-0960. Our data confirm that L-DOPA toxicity towards DA neurons is mediated by an autooxidative process, which is attenuated by glial cells. In addition, we demonstrate a second mechanism of L-DOPA toxicity in vitro mediated by a COMT- and glia-dependent pathway, which is blocked by COMT inhibitors, most likely due to enhanced glial uptake of L-DOPA.