Dopamine Agonist: Effects, Uses & Risks

A dopamine agonist or dopamine antagonist is a drug that can stimulate dopamine receptors. Dopamine agonists are used to treat Parkinson’s disease, restless legs syndrome, and as emetics, among other conditions.

What is a dopamine agonist?

Dopamine agonists are used to treat Parkinson’s disease, restless legs syndrome, or as emetics, among other conditions. Dopamine agonists, like the neurotransmitter dopamine, can bind to dopamine receptors (D receptors). Depending on receptor selectivity, agonists are divided into D1/5 and D2/3/4 agonists. Due to receptor binding, the dopamine agonists elicit a dopamine-like effect. The selective D1/5 agonists such as SKF 81297 or dihydrexin play no role in the treatment of disease. Agonists that bind to the D2 receptors play a role in the therapy of various medical conditions. Known D2 receptors are the drugs ropinirole, rotigotine, piribedil or pramipexole. When dopamine agonists are used, side effects such as nausea, vomiting, low blood pressure, hallucinations, or confusion may occur.

Pharmacologic effects

Dopamine agonists act like dopamine by stimulating dopamine receptors. Dopamine is a neurotransmitter that belongs to the catecholamine group. It is produced in the human body from the amino acids tyrosine and phenylalanine. The neurotransmitter belongs to the group of sympathomimetics. These substances enhance the action of the sympathetic nervous system. In a low concentration, dopamine increases blood flow to the abdominal and renal vessels. One dopaminergic processing pathway is the mesostriatal system, which originates in the substantia nigra in the midbrain. Here, dopamine performs important tasks in the control of movements. Disturbances in this system are one of the causes of the hypokinetic movement disorders in Parkinson’s disease. In the mesolimbic system, a lack of dopamine leads to listlessness. Dopamine antagonists can compensate for a dopamine deficiency and help patients become more active and enjoy life. If the mesocortical system is underactive, psychoses of the schizophrenic type can develop. Dopamine agonists also become effective in this system by binding to the appropriate receptors. Dopamine agonists also exert their effect in the tuberoinfundibular system. They inhibit the release of the hormone prolactin at the neurons that run from the arcuate nucleus to the anterior lobe of the pituitary gland. Prolactin is a hormone responsible for milk secretion (lactation) during lactation.

Medical use and application

One of the main indications for the use of dopamine agonists is Parkinson’s disease. Degenerative changes in the extrapyramidal motor system (EPMS) result in dopamine deficiency. This upsets the neurotransmitter balance. This manifests itself in neurological disorders and, in particular, motor function disorders. Typical symptoms of Parkinson’s disease are immobility, slowed voluntary motor activity, increased basic tension of the skeletal muscles and tremor. To slow the progression of the disease and alleviate clinical symptoms, patients are usually administered dopamine in the form of the precursor L-dopa. However, the efficacy of this precursor alone is usually not sufficient, so the D2 receptors are additionally simulated with the help of agonists. Another indication for dopamine agonists is restless legs syndrome (RLS). This disorder of the extrapyramidal motor system belongs to the group of hyperkineses. The disorder is manifested by twitching, sensory disturbances, and pain in the legs. Similar to Parkinson’s disease, restless legs syndrome is treated with a combination of L-dopa and dopamine agonists. But the dopamine agonists not only play a role in motor function, they also have an inhibitory effect on prolactin secretion. This is why they are also used for weaning. They are also used in the treatment of hormonal complaints associated with increased production of prolactin. Thus, prolactinomas are treated with dopamine agonists such as cabergoline or bromocriptine. Prolactinomas are hormone-producing tumors of the anterior pituitary gland. The resulting prolactinemia leads to hypoestrogenemia in women with absent ovulation and menstrual bleeding.In half of the patients, breast milk spontaneously leaks from the mammary glands (galactorrhea). Some dopamine agonists are used to treat erectile dysfunction. The term erectile dysfunction refers to the lack of erection of the penis during arousal.

Risks and side effects

Typical side effects of dopamine agonists include nausea, vomiting, and headache. There may be a drop in blood pressure and fatigue. Some patients experience somnolence or sleep disturbances. Restlessness is also frequently observed. Other side effects of dopamine agonists include incontinence, edema, hair loss, or dizziness. In rare cases, patients develop hallucinations or even psychosis after taking dopamine agonists. In some patients, inhibition of prolactin production is desirable. However, during lactation, the effect of dopamine agonists on milk flow must be considered. Otherwise, milk flow may inadvertently cease. Dopamine agonists must not be administered in cases of organ fibrosis. There is then a risk of connective tissue transformation (fibrosis) of the heart valves. Pleural effusion and high blood pressure are also contraindications. The use of dopamine agonists in the presence of hepatic insufficiency is also counterproductive. The drugs should not usually be combined with neuroleptics.