What is the mechanism of action of carbidopa in carbidopa-levodopa treatment?

Carbidopa plays a crucial role in the treatment of Parkinson's disease by inhibiting the enzyme DOPA decarboxylase. This enzyme is responsible for the conversion of levodopa (the precursor to dopamine) to dopamine in the peripheral tissues before it can cross the blood-brain barrier. By inhibiting DOPA decarboxylase, carbidopa ensures that more levodopa reaches the brain, where it can be converted into dopamine, thus enhancing its therapeutic effect. Additionally, this inhibition helps to reduce the peripheral side effects associated with levodopa treatment, such as nausea and vomiting, by limiting the production of dopamine outside the central nervous system.

Other mechanisms of action listed do not pertain to carbidopa specifically. For instance, agonism of postsynaptic dopamine receptors refers to direct stimulation of dopamine receptors in the brain, which is not the function of carbidopa. Similarly, catechol-O-methyltransferase (COMT) inhibition relates to the action of different medications used in conjunction with levodopa to extend its effects. Lastly, acetylcholine receptor antagonism would imply a mechanism aimed at the cholinergic system, which does not directly relate to the action of carbidopa in the context