Effect of apomorphine, alpha-methylparatyrosine, haloperidol and reserpine on DOPA production in clonal cell lines (PC-12 and N1E-115)

Intracellular dopamine oxidation mediates rotenone-induced apoptosis in PC12 cells

AIM

To study the role of dopamine (DA) in rotenone-induced neurotoxicity in PC12 cells.

METHODS

Cell viability was assessed by detecting the leakage of lactate dehydrogenase (LDH) into the medium. Apoptosis rate was measured by flow cytometry. Caspase-3-like activity was measured by fluorescence assay using the probe Ac-DEVD-AMC. The level of intracellular hydrogen peroxide and other peroxides in PC12 cells were quantified by loading cells with 2'-7'-Dichlorodihydrofluorescein diacetate (DCFH-DA) in fluorescence assay. Lactic acid was measured spectrophotometrically. The DA levels in PC12 cells were determined by HPLC-ECD.

RESULTS

A 48-h incubation of PC12 cells with rotenone caused an apoptotic cell death and elevated intracellular reactive oxygen species (ROS) and lactic acid accumulation. Intracellular DA depletion with reserpine significantly attenuated rotenone-induced ROS accumulation and apoptotic cell death. No change was found in rotenone-induced ROS accumulation when cells were co-treated with deprenyl. Brief treatment with reserpine at the end of rotenone treatment had no effect on rotenone-induced neurotoxicity. However, when cells were first incubated with deprenyl, a monoamine oxidase-B inhibitor for 30 min then co-incubated with rotenone plus deprenyl, a brief treatment with reserpine enhanced cell injury.

CONCLUSION

Rotenone-induced apoptosis in PC12 cells was mediated by intracellular dopamine oxidation.

Two PC12 pheochromocytoma lines sealed in hollow fiber-based capsules tonically release L-dopa in vitro

Two PC12 cell-derived lines have been studied following encapsulation into polymer-based hollow fibers with respect to secreted catecholamines and their metabolites. Cellular encapsulation provides a chronic microperfusion environment within which basally secreted PC12 products can be readily measured. Encapsulated PC12 cells grown and held under the conditions specified in this report basally release amounts exceeding their total cellular stores of the dopamine precursor L-DOPA and the electrochemically active dopamine metabolites DOPAC and HVA during 45-min static incubations. Under these same conditions, these cells release less than 0.1% of their total cellular store of dopamine. Depolarizing incubations enhance dopamine secretion eightyfold and enhance secretion of L-DOPA, HVA, and DOPAC about twofold. The relative composition of products basally secreted differs between PC12-derived cell lines, and an inverse relationship exists between basal release of L-DOPA and total cellular store of dopamine. These results further indicate that selected PC12 cell lines are potentially a source of both dopamine and L-DOPA in therapeutic cellular replacement applications.

Apomorphine does not decrease tissue levels of tetrahydrobiopterin in vivo

It was reported that R(-)apomorphine and other catechols are potent inhibitors of dihydropteridine reductase in vitro. It was suggested that decreased levels of tetrahydrobiopterin may represent a mechanism by which R(-)apomorphine inhibits catecholamine synthesis in vivo. This paper demonstrates that tetrahydrobiopterin levels are not affected either in vitro (PC12 cells) or in vivo (rat liver and corpus striatum) by treatment with R(-)apomorphine, whereas DOPA (3,4-dihydroxyphenylalanine) production (PC12 cells, corpus striatum) is reduced. This indicates that R(-)apomorphine does not inhibit DOPA production by reducing 6(R)-L-erythro-tetrahydrobiopterin) levels.

Direct inhibition of tyrosine hydroxylase from PC-12 cells by catechol derivatives

Several drugs with a catechol moiety were studied for their potency to inhibit tyrosine hydroxylase (TH) from PC-12 cells in vitro. When the natural compounds tested were compared, dopamine, norepinephrine and 2(3,4-dihydroxyphenyl)-ethanol (DOPET) were most effective (IC50 between 1.4 and 3.6 microM with 0.5 microM 6(R,S)-L-erythro-5,6,7,8-tetrahydrobiopterin as cofactor). 3,4-Dihydroxyphenylalanine (DOPA; IC50: 35 microM) and 3,4-dihydroxyphenylacetic acid (DOPAC; IC50: 180 microM were less potent inhibitors. Among the synthetic drugs possessing catechol moiety, isoproterenol, (+/-)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (6,7-ADTN) and (+/-)-2-dimethylamino-6,7-dihydroxy-tetrahydronaphthalene (TL-99) had the same inhibitory effects as the natural catecholamines (IC50 between 1.6 and 3.9 microM), whereas the apomorphine derivatives and 2,3,4,5-tetrahydro-1-phenyl-1 H-3-benzazepine-7,8-diol (SKF 38393) were even more potent (IC50: 0.5-0.8 microM). These results demonstrate that natural catechols and certain drugs (e.g. 6,7-ADTN, TL-99, SKF 38393) are more effective direct blockers of tyrosine hydroxylase than generally assumed provided appropriate assay conditions are used. In the case of dopamine and norepinephrine, these findings suggest a reevaluation of their role for feedback control of tyrosine hydroxylase in vivo.