Effect of nicotine on the pharmacokinetics of levodopa.

OBJECTIVES: Some patients with Parkinson disease improved their symptoms on treatment with nicotine patch or gum. Nicotine has also been studied for its antidyskinetic effect on levodopa-induced dyskinesia. We determined the effects of nicotine on levodopa pharmacokinetics and gastric emptying in healthy subjects and on levodopa transport in Caco-2 monolayers in vitro. METHODS: Healthy subjects received transdermal nicotine patch application followed by oral levodopa/benserazide, 100/25 mg, in a fasting state and with enteral nutrition. Levodopa pharmacokinetics was determined, and gastric emptying was evaluated by carbon 13 ((13)C)-labeled acetic acid breath testing. In vitro studies using intestinal Caco-2 cell monolayers evaluated whether the intestinal transport of levodopa was affected by nicotine and its metabolite, cotinine. RESULT: Nicotine did not increase mean plasma concentration significantly during fasting or with enteral nutrition, although the extent of levodopa absorption was reduced by 34% to 60% in some individuals and the mean plasma concentration of levodopa was statistically decreased by nicotine in subjects who received enteral nutrition. However, gastric parameters were not significantly affected by nicotine. Nicotine and cotinine at 0.1 µmol/L significantly reduced levodopa uptake by Caco-2 cells (P < 0.01). CONCLUSIONS: We found that nicotine reduced plasma levodopa concentration in some healthy subjects but with no alteration of gastric emptying rate. In vitro, nicotine inhibited levodopa transport by Caco-2 cell monolayers in an α-methyl amino isobutyric acid-independent, 2-amino-norbornanecarboxylic acid-dependent manner. These results suggest that nicotine may inhibit the transport of levodopa by the system L-amino acid transporter.

Pramipexole upregulates dopamine receptor D2 and D3 expression in rat striatum.

Randomized clinical trials have shown that pramipexole has an antidepressant effect in patients with Parkinson's disease. We investigated the comparative efficacy of pramipexole toward dopamine receptor D(2) and D(3) expression in rat brain. Groups of rats were treated subacutely with pramipexole (1 mg/kg), imipramine (10 mg/kg), or bromocriptine (5 mg/kg), with appropriate controls. Using real-time RT-PCR and immunoblotting, dopamine receptor D(2) and D(3) expression was up-regulated in the striatum following pramipexole treatment, while imipramine and bromocriptine had no significant effects. These findings support that pramipexole exerts additional therapeutic benefits such as decreasing depression by increasing dopamine receptor D(3) expression in the striatum.