antiparkinsonian drugs

Drugs used to treat Parkinson's disease or parkinsonism. These conditions result from a disease process affecting the basal ganglia, the part of the brain responsible for controlling voluntary movement at a subconscious level, and are associated with a deficiency of the neurotransmitter dopamine. This gives rise to an imbalance between the actions of dopamine and another neurotransmitter, acetylcholine, and results in disorders of movement, notably tremor, rigidity, and slow movements. A distinction is sometimes made between Parkinson's disease, a degenerative condition associated with ageing, and parkinsonism, parkinsonian symptoms due to other causes; for example, the long-term use of antipsychotic drugs (see extrapyramidal reactions).

A variety of drugs are available for treating the condition. Levodopa, which is converted to dopamine in the body, is used as replacement therapy. Selegiline and rasagiline prevent the breakdown of dopamine in the body; entacapone and tolcapone prevent the breakdown of levodopa. Dopamine agonists (see bromocriptine; cabergoline; pergolide; pramipexole; ropinirole; rotigotine) increase the action of dopamine by stimulating dopamine receptors in the brain. Antimuscarinic drugs block the activity of acetylcholine in the brain and thus act to correct the imbalance between dopamine and acetylcholine. They are less effective than levodopa, although they often supplement its action. Patients with mild symptoms may be treated initially with antimuscarinic drugs (alone or with selegiline), levodopa being added at a later stage as symptoms progress. They are also useful in reversing drug-induced extrapyramidal reactions. The common antimuscarinic drugs used in treating parkinsonism are trihexyphenidyl hydrochloride, orphenadrine, and procyclidine. See also amantadine hydrochloride.

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