Microdialysis Studies on the Action of Tolcapone on Pharmacologically‐Elevated Extracellular Dopamine Levels in Conscious Rats

Abstract

Abstract:To elucidate the importance of catechol-0-methyltransferase, we performed striatal microdialysis studies in conscious rats given tolcapone, an inhibitor of catechol-0-methyltransferase, together with four compounds each of which elevates the extracellular dopamine content through a different mechanism. Tolcapone itself did not alter dopamine levels in the striatal microdialysis fluid but increased DOPAC and decreased homovanillic acid levels. However, tolcapone pretreatment (30 mg/kg) multiplied the already high dopamine levels after levodopa, and less so the moderately elevated dopamine levels after GBR-I 2909 (at 20 mg/kg) alone, but the minor (insignificant) dopamine-elevating effects of haloperidol and (+)-U232 were not altered. In all cases, a tolcapone pretreatment decreased homovanillic acid levels and elevated DOPAC levels. In further combination studies, GBR-12909 did not alter significantly the effects of levodopdcarbidopa on dopamine, DOPAC and homovanillic acid levels. In these rats, tolcapone enhanced the effect of GBR-12909 on extracellular dopamine but not on DOPAC. In conclusion, when levodopa and carbidopa are given together, COMT inhibition becomes extremely meaningful, and dopamine levels are multiplied by tolcapone. Otherwise, tolcapone is able to further elevate extracellular dopamine levels only when dopamine turnover is normal or low but not when it is high. Overall, the role of COMT in the elimination of synaptic dopamine remains minor compared to the dominance of the reuptake process. Catechol-0-methyltransferase (COMT) is one of the main enzymes metabolizing catecholamines (Kopin 1985).COMT is considered to be the first enzyme involved in the metabolism of dopamine when released to the synaptic cleft, and therefore the analysis of 3-methoxytyramine, the COMT-derived metabolite of dopamine has been considered as a good indicator of dopamine release (Kehr 1976(Kehr & 1981 Wood & Altar 1988).Recent advances in our understanding of the intracellular distribution of COMT have cast some doubt on this hypothesis. It has been shown that COMT is exclusively an intracellular enzyme. Soluble COMT is cytoplasmic and partially a nuclear enzyme while membrane-bound COMT exists in the rough endoplasmic reticulum but not in the cell membrane Lundstrom et al. 1995). In the membrane vesicles, COMT is oriented hanging towards the cytoplasm (Lundstrom et al. 1995). Old (Kaakkola et al. 1987) and new evidence further suggests that COMT is not located in the presynaptic dopamine nerves, instead it is found in glial cells and perhaps to some extent in the postsynaptic neurones. Hence it seems obvious that a specialized uptake system (called uptakez to differentiate it from uptakel) (Trendelenburg 1990) is needed to conduct the substrate to the enzyme.

Effect of Dopamine Uptake Inhibition on Brain Catecholamine Levels and Locomotion in Catechol-O-methyltransferase-Disrupted Mice

“…Taken together, GBR 12909 induced only minor changes in dopamine metabolism and there were no indications of potentiation by COMT deficiency on these effects. These results fit quite well with previous findings in rats, where GBR 12909 alone had no effect on DOPAC and HVA striatal extracellular and brain tissue concentrations (Westerink et al, 1987;Irifune et al, 1995;Budygin et al, 1999;Huotari et al, 1999). However, when tolcapone, a selective and potent COMT inhibitor (Borgulya et al, 1989), was given together with 20 mg/kg GBR 12909, striatal extracellular DOPAC levels were slightly increased (Huotari et al, 1999), whereas with 10 mg/kg GBR 12909 DOPAC levels remained unaltered (Budygin et al, 1999).…”

“…In previous studies with rats, the selective COMT inhibitor tolcapone was able, to some extent, to potentiate GBR 12909-induced elevation of striatal extracellular dopamine concentration (Budygin et al, 1999;Huotari et al, 1999). In the present study, we have investigated the effects of the dopamine uptake inhibition on locomotor activity, the levels of catecholamines and their metabolites in three different brain regions, and dopamine concentration in striatal extracellular fluid of COMT-deficient mice of both sexes.…”

d -Amphetamine responses in catechol-O-methyltransferase (COMT) disrupted mice

“…Our present neurochemical results in COMT-deficient mice with a lower dose (2.5 mg/kg) of d-amphetamine are in line with previous findings that COMT inhibition did not further potentiate d-amphetamine-induced changes in neurochemical or behavioral parameters Tuomainen et al 1996). This contrasts with the results of similar experiments with low doses of GBR 12909, a pure DAT inhibitor (Andersen 1989), where the effects of the drug on dopamine levels were augmented by both a potent COMT inhibitor tolcapone (Budygin et al 1999;Huotari et al 1999) and the lack of COMT (Huotari et al 2002b).…”

Intravenous levodopa administration in humans based on a two-compartment kinetic model

“…In normal subjects levodopa also has reproducible, regionally specific effects on cerebral metabolism and blood flow (Chen et al, 1996;Hershey et al, 1998Hershey et al, , 2000Hershey et al, , 2003Kobari et al, 1995;Porrino et al, 1987;Trugman, 1995) that mimic the effects of direct dopamine agonists (Black et al, , 2002a. In animal models, levodopainduced increases in striatal extracellular dopamine levels have been measured directly (Brannan et al, 1998;Huotari et al, 1999;Orosz and Bennett, 1992; but compare Rau et al, 2002), along with acute changes in striatal neuronal firing (Chang and Webster, 1997) and other dopamine-receptor-mediated postsynaptic effects (Kimura et al, 1995).…”