L-DOPA (Levodopa) is an amino acid precursor of dopamine with antiparkinsonian properties. Levodopa is a prodrug that is converted to dopamine by DOPA decarboxylase and can cross the blood-brain barrier. When in the brain, levodopa is decarboxylated to dopamine and stimulates the dopaminergic receptors, thereby compensating for the depleted supply of endogenous dopamine seen in Parkinson's disease. To assure that adequate concentrations of levodopa reach the central nervous system, it is administered with carbidopa, a decarboxylase inhibitor that does not cross the blood-brain barrier, thereby diminishing the decarboxylation and inactivation of levodopa in peripheral tissues and increasing the delivery of dopamine to the CNS.

Levodopa produces at 25-200 μM concentrations a dose-dependent reduction of 3H-DA uptake in foetal rat midbrain cultures. Levodopa results in a decrease in the number of viable cells and tyrosine hydroxylase (TH) positive neurones, plus disruption of the overall neuritic network. [1] Levodopa induces dyskinesia in the absence of dopamine by excessive inhibition of neurons of the putamen-globus pallidus (GPe) projection and subsequent disinhibition of the globus pallidus (GPe). Levodopa results in a decrease in cytochrome oxidase messenger RNA expression in the globus pallidus (GPi). [2]

Levodopa elicits the development of a variety of abnormal movements in monkeys with parkinsonism induced by the neurotoxin MPTP. Levodopa administrations result in an ectopic induction of the dopamine D3receptor expression in the CdPu in 6-OHDA-lesioned rats. [3] Levodopa (50 mg/kg) increases anandamide concentrations throughout thebasal ganglia via activation of dopamine D1/D2 receptors in intact rats. Levodopa produces increasingly severe oro-lingual involuntary movements which are attenuated by the cannabinoid agonist R(+)-WIN55,212-2 (1 mg/kg) in lesioned rats. [4] Levodopa administration reverses the up-regulation of D2 dopamine receptors seen in severely lesioned rats provided evidence that Levodopa reaches a biologically active concentration at the basal ganglia. [5]

Briefly, transfected HEK-293 cells, incubated in charcoal-treated Dulbecco's modified Eagle's medium for 24 h, are washed once with Hanks' solution and resuspended in a buffer containing 100 mM NaCl, 1 mM MgCl2, 1 mM EDTA, 1 mM EGTA, 250 mMsucrose, 20 mM Tris-HCl, pH 7.4. Cells are lysed by freezing in liquid nitrogen. Dehydrogenase activity is measured in a final volume of 20 μL containing the appropriate concentration of bile acid, 30 nCi of [3H]cortisol, and unlabeled cortisol to a final concentrations of 50 nM. The reaction is started by mixing cell lysate with the reaction mixture. Alternatively, endoplasmic reticulum microsomes are prepared from transfected HEK-293 cells and incubated with reaction mixture containing various concentrations of cortisol and CDCA. Incubation proceeded for 20 min, and the conversion of cortisol to cortisone is determined by thin layer chromatography (TLC). Because of the inaccuracy of the TLC method at low conversion rates and the end-product inhibition of 11βHSD2 at conversion rates higher than 60-70%, only conversion rates between 10 and 60% are considered for calculation. The inhibitory constant IC50 is evaluated using the curve-fitting program. Results are expressed as means±S.E. and consist of at least four independent measurements.