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

Rationale. We have earlier found that 1) COMT inhibitors did not enhance amphetamine-induced dopamine efflux into striatal extracellular, that 2) they did not increase dopamine levels in striatal tissue and that 3) they did not potentiate amphetamine-induced turning behavior of hemiparkinsonian rats. Further, when COMT knockout mice were challenged with L-dopa or a dopamine transporter (DAT) inhibitor, an accumulation of dopamine occurred and the neurochemical and locomotor effects of L-dopa and GBR 12909 were modified accordingly. Objective. Since DAT inhibitors and amphetamine apparently have different mechanisms of action, we were interested to see how COMT knockout mice would react to d-amphetamine treatment. Methods. We measured the effects of d-amphetamine on locomotor activity and on the levels of catecholamines and their metabolites in striatal microdialysis fluid and in striatal, hypothalamic and cortical brain regions of COMT gene disrupted mice. Striatal dopamine receptor binding was also determined. Results. After d-amphetamine administration, the DOPAC content in homozygous mice was 3-fold in the striatum, 17- to 18-fold in the cortex and 7- to 8-fold in the hypothalamus higher than in wild-type control mice, and there were no indications of genotypexsex interactions. However, the lack of COMT did not potentiate d-amphetamine-induced dopamine levels in brain tissue or in striatal extracellular fluid. d-Amphetamine-induced (10 mg/kg) hyperlocomotion was less suppressed in male COMT knockout mice than in their wild-type counterparts. Striatal dopamine D-1 and D-2 receptor levels in male mice were not altered by COMT gene disruption. Conclusions. Changes in COMT activity modulates dopamine metabolism but the behavioral effects of d-amphetamine in male mice only to a small extent, and this action does not seem to depend on the actual extracellular dopamine concentration. Nor is it mediated through compensatory changes in dopamine D-1 and D-2 receptor levels. In dopaminergic neurons, the contribution of intracellular COMT remains secondary in conditions when dopamine is released by d-amphetamine.