What is the pharmacological action of carbidopa?  

Answer: Carbidopa is a blood-brain barrier impermeable dopa decarboxylase inhibitor.

carbidopa levodopa

L-DOPA is the most effective therapy for the treatment of the motor symptoms of Parkinson's disease. L-DOPA relieves the symptoms of Parkinson's disease by restoring dopaminergic tone.

The L-DOPA drug that is given to patients is a combination of levodopa and carbidopa. Levodopa is the biochemical precursor to dopamine. It is converted to dopamine by the enzyme dopa decarboxylase. Levodopa readily crosses into the blood-brain barrier.

Carbidopa, on the other hand, is an inhibitor of the dopa decarboxylase enzyme. Carbidopa cannot permeate through the blood-brain barrier. It acts in the periphery to prevent the premature synthesis of dopamine.

This has two major consequences. First, it prevents dopamine from acting at nonspecific sites throughout the periphery, where the effects of elevated dopamine may be detrimental. Second, it ensures that as much dopamine will enter brain as possible. Since very little will be synthesized peripherally, the majority of the dose will go into the brain.