Parkinson’s disease destroys the brain cells that make the nerve signaling chemical dopamine, causing muscle tremors, muscle stiffness and limited mobility. Drugs to treat Parkinson’s disease attempt to replace or supplement the body’s supply of dopamine. There are three main classes of drugs: dopamine precursors; dopamine inhibitors, which slow the breakdown of dopamine; and dopamine agonists, which mimic the effects of dopamine. Doctors often prescribe dopamine agonists together with the preferred dopamine precursor Levodopa to maximize their effectiveness.
Bromocriptine
Like all dopamine agonists, bromocriptine binds to dopamine receptors, activating them in the same way the body’s natural dopamine would. According to the Merck Manual, 10 to 40 milligrams (mg) once per day is a typical dosing regimen for bromocriptine. However, doctors rarely prescribe bromocriptine for Parkinson’s disease because this drug can cause serious side effects, including hardening and scarring of the lungs and valves of the heart.
Like the other dopamine agonists, bromocriptine can cause side effects such as upset stomach, overwhelming drowsiness, low blood pressure on standing up from sitting or lying down, and loss of muscle control with spasmodic or repetitive movements (known as dyskinesia). Bromocriptine and other dopamine agonists can cause mental symptoms ranging from confusion and hallucinations to delirium and psychosis.
Ropinirole
Ropinirole, another dopamine agonist, can be taken by itself in the early stages of Parkinson’s disease, or doctors might prescribe it together with Levodopa for patients in more advanced stages of the disease. Experts believe that it works by binding dopamine receptors in an area of the brain called the caudate putamen. A typical dosing regimen is 3 to 4 mg, three times per day.
Like other agonists, ropinirole can cause patients to have trouble controlling impulsive behavior. The Merck Manual states that some reports document a link between ropinirole and compulsive gambling, hypersexuality and compulsive eating.
Pramipexole
Pramipexole agonists preferentially bind the D2 subtype of dopamine receptors, but also bind the D3 and D4 subtypes, so that this drug primarily works in the striatum and substantia nigra of the brain. A typical dosing regimen consists of 0.5 to 1 mg, three times per day. According to the Merck Manual, pramipexole can cause hallucinations, especially in older patients.
Other Forms
Bromocriptine, ropinarole and pramipexole are taken orally in a tablet form. By contrast apomorphine, another dopamine agonist, is injected. Doctors use apomorphine only for acute treatment during times when Levodopa fails to ameliorate symptoms, called “off” periods. Apomorphine works to relieve symptoms very quickly—within 5 to 10 minutes—but lasts only 60 to 90 minutes, according to the Merck Manual. Patients can have injections up to five times per day if necessary.
The dopamine agonist rotigotine comes as a skin patch. Like the oral dopamine agonists, doctors usually prescribe it together with Levodopa.
