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

Abstract

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 μM after 24 h, whereas DA was more toxic than l-DOPA with toxicity at concentrations of ≥1 μM. 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 μM). 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.