Elimination of extracellular dopamine in the medial prefrontal cortex of conscious mice analysed using selective enzyme and uptake inhibitors.

We have shown in a previous study that in the medial prefrontal cortex (mPFC) of Comt knockout animals, uptake1 followed by oxidation accounts for approximately 50% and uptake2 followed by O-methylation for the remaining 50% of dopamine clearance. However, compensatory mechanisms in genetically modified animals may have affected the result. Therefore, in the present study, we gave a high dose (30 mg/kg) of tolcapone in combination with pargyline and reboxetine to C57BL/6J mice to see whether the earlier findings could be confirmed. The three drugs were also given together. We used intracerebral microdialysis to determine the levels of extracellular dopamine and its metabolites in the mPFC. In addition, we analyzed dopamine, 3,4-dihydroxyphenyl acetic acid (DOPAC) and homovanillic acid (HVA) contents in cortical and striatal synaptosomes to estimate the amount of releasable dopamine and dopamine turnover. In the prefrontal cortex of male C57BL/6J mice, the combination of two drugs (pargyline + tolcapone or reboxetine + tolcapone) generally elevated extracellular dopamine levels more than any single drug. Similar responses, although much weaker, were observed in female mice. Unexpectedly, triple treatment with pargyline, reboxetine and tolcapone did not increase dopamine outflow in the mPFC in either sex, and the treatment actually diminished dopamine outflow in the dorsal striatum. This seems to indicate that such an extensive treatment induces a fast and effective shut-down of dopamine release both in the mPFC and striatum to protect the brain from excess dopaminergic stimulation. The observed decrease in extracellular dopamine levels was not due to the depletion of releasable dopamine because abundant amounts of dopamine were present in synaptosomes. These results imply that the relative proportion of COMT-induced dopamine clearance may be somewhat lower than earlier estimated.

[Effect of Catechol-O-methyltransferase deficiency on reinforcing effects of cocaine (experimental study)].

Catechol-O-methyltransferase (COMT) remains an important regulatory element in prefrontal cortex dopamine homeostasis. The literature data suggest that individual differences in COMT activity (Val158Met polymorphism) might have indirect downstream effects on the reward system. The aim of the present study was to examine whether COMT deletion affects reinforcing effects of cocaine in mice. The study was conducted in male mice with homozygous COMT deletion as well as their C57BL/6J wild-type littermates. Animals were trained to nose-poke to receive response-contingent intravenous infusions of cocaine (0.3 mg/kg per infusion; final schedule of reinforcement - fixed ratio (FR) 3 time out 30 s). Following the initial acquisition phase, cocaine self-administration dose-effect functions (0.03, 0.1, 0.3, 1, and 3 mg/kg per infusion) were determined under FR3 and progressive ratio (PR) schedules of reinforcement. Cocaine dose-dependently maintained responding under FR3 and PR schedule of reinforcement when the unit dose of cocaine was varied across the sessions. The total cocaine intake did not differ in COMT deletion mice and wild-type mice. The results of this study suggest that individual differences in COMT activity do not affect primary reinforcing effects of cocaine in mice.

Importance of membrane-bound catechol-O-methyltransferase in L-DOPA metabolism: a pharmacokinetic study in two types of Comt gene modified mice.

BACKGROUND AND PURPOSE: Catechol-O-methyltransferase (COMT) metabolizes compounds containing catechol structures and has two forms: soluble (S-COMT) and membrane-bound (MB-COMT). Here we report the generation of a mouse line that expresses MB-COMT but not S-COMT. We compared the effects of deleting S-COMT only or both COMT forms on the pharmacokinetics of oral L-DOPA. EXPERIMENTAL APPROACH: L-DOPA (10 mg kg(-1)) and carbidopa (30 mg kg(-1)) were given to mice by gastric tube, and samples were taken at various times. HPLC was used to measure L-DOPA in plasma and tissue samples, and dopamine and its metabolites in brain. Immunohistochemistry and Western blotting were used to characterize the distribution of COMT protein isoforms. KEY RESULTS: Lack of S-COMT did not affect the levels of L-DOPA in plasma or peripheral tissues, whereas in the full COMT-knock-out mice, these levels were increased. The levels of 3-O-methyldopa were significantly decreased in the S-COMT-deficient mice. In the brain, L-DOPA levels were not significantly increased, and dopamine was increased only in females. The total COMT activity in the S-COMT-deficient mice was 22-47% of that in the wild-type mice. In peripheral tissues, female mice had lower COMT activity than the males. CONCLUSIONS AND IMPLICATIONS: In S-COMT-deficient mice, MB-COMT in the liver and the duodenum is able to O-methylate about one-half of exogenous L-DOPA. Sexual dimorphism and activity of the two COMT isoforms seems to be tissue specific and more prominent in peripheral tissues than in the brain.