Dopamine agonists must not be confused with dopamine precursors, such as levodopa and carbidopa, which are central nervous system agents that get converted to dopamine before binding to the dopamine receptors.
The term 'neurotransmitter' refers to a chemical that transmits nerve impulses across a synapse. Dopamine is one such chemical act that is both excitatory and inhibitory in nature, which means that it can determine the probability of the neuron firing or not firing an action potential (message that is transmitted). Thus, the activation of the receptors can promote or suppress the generation of action potential. Out of the 10 billion cells in the cerebral cortex, only one million dopaminergic cells are present in the brain. Though the number of dopaminergic neurotransmitter is less, dopamine plays a vital role in the proper functioning of the human body. It controls the cognitive function (memory, attention, and problem solving), movements, and emotional behavior.
Moreover, it also controls the reward and pleasure centers of the brain. When the link between dopaminergic cells that are present in a part of midbrain called substantia nigra and the corpus striatum (mass of white and gray matter located in front of the thalamus in both cerebral hemispheres in the brain) is adversely affected, it results in a dopamine deficiency. Such a deficiency is bound to give rise to impaired movement and a host of untoward symptoms. Under such circumstances, medicines belonging to a class of drugs called dopamine agonists are prescribed. These drugs mimic the action of dopamine and activate its receptors in the brain, even in the absence of dopamine.
Types of Dopamine Agonists
Dopamine agonists can be classified into ergoline and non-ergoline agonists. Drugs belonging to these subclasses work by targeting dopamine D2 receptors. Drugs belonging to ergoline dopamine agonists include:➠
The drugs belonging to non-ergoline agonists include:➠
Uses of Dopamine Agonists
Deficiency of dopamine is a major reason behind the development of Parkinson's disease, which is why some of the aforementioned drugs are prescribed for the treatment of this medical condition.Parkinson's Disease
Parkinson's disease is characterized by symptoms such as tremors, rigidity, speech changes, difficulty in writing, bent posture, loss of automatic movements, and slowness of movement. Subcutaneous administration of a drug called apomorphine also helps in the treatment of Parkinson's disease. This drug works by targeting both D1 and D2 receptors. Another drug from the non-ergoline class of medications called Neupro (Rotigotine) is available in the form of a patch. It is approved by the FDA for alleviating the early signs of Parkinson's disease and restless legs syndrome.
It must be noted that Bromocriptine and Pergolide are no longer used for the treatment of Parkinson's disease in the United States due to the increased risk of pulmonary fibrosis and valvular heart disease, respectively. Lisuride (low strength lisuride is classified as N02CA, an ergot alkaloid) is not used in the United States, as it was less successful in comparison to other dopamine receptor agonists. Amongst the ergoline derivates, Cabergoline and Cabaser are commonly prescribed for individuals affected by hyperprolactinemia and early stages of young-onset Parkinson's disease, respectively, which occur due to the deficiency of dopamine. Here is a list of common dopamine agonist drugs that have been approved by the FDA:➠
Pramipexole (Mirapex®) is believed to be effective in the early treatment of the motor symptoms of Parkinson's Disease. It helps control the motor fluctuations.➠
Ropinirole (Requip®) also helps control the motor symptoms of Parkinson's Disease.➠
Rotigotine (Neupro® patch) is a transdermal patch that is changed every 24 hours.➠
First used in the treatment of Parkinson's disease in 1950, concerns were raised about Apomorphine (Apokyn®) due to its side effects. In 1990s, it was released in an injectable form. These days, it is only prescribed in severe cases, especially during the 'off periods' as a rescue drug. It is believed that it might delay the onset of the 'on-off' phenomena. In people affected by Parkinson's disease, the 'on' period is the time when the patient feels that their medication is able to control the symptoms. On the other hand, the 'off' period refers to the period when the patient feels that the medication is not providing the desired effects. Thus, the on-off phenomenon describes the sudden onset of the motor symptoms (stiffness, slowness or tremor) or non-motor symptoms (anxiety, nausea, depression).
While dopamine receptor agonists can be taken alone in the early stages of Parkinson's disease, it can be safely used with a dopamine precursor (Levodopa/Carbidopa) later. These can be safely taken, as they only mimic the action of dopamine in its absence, and don't affect the production, transport, or release of real dopamine.Hyperprolactinemia
Thus, the uses of dopamine agonists are not restricted to the treatment of Parkinson's disease, they also act as prolactin-inhibiting factors, which is why it is prescribed for the treatment of elevated levels of prolactin in the blood. It reduces the production of prolactin from the pituitary gland. More often than not, the use of a long-acting dopamine receptor agonist called cabergoline is suggested for the treatment of hyperprolactinemia. It works by activating the D2 receptors that are found in the pituitary gland, which in turn reduce the amount of prolactin secreted by the pituitary gland. Low strength bromocriptine, low strength cabergoline tablets, and high strength lisuride are classified as G02CB (Prolactin inhibitors).
Dopamine agonists are also prescribed for the treatment of conditions such as Attention Deficit/Hyperactivity Disorder (ADHD), restless legs syndrome, pituitary tumors, etc. FDA has approved drugs called Requip (Ropinirole) and Mirapex (Pramipexole) for treating restless legs syndrome.
Side Effects of Dopamine Agonists
One of the major side effects of these drugs is impulse control disorder. In fact, these drugs put the patient at a greater risk of impulse control disorder (Compulsive behavior such as gambling, uncontrolled shopping, binge eating, sexual urges, etc.) in comparison to dopamine precursors such as levodopa. Other adverse effects that are associated with dopamine agonist use include:➠
Excessive daytime sleepiness➠
Extreme weakness or fatigue➠
Skin irritation in case of transdermal delivery➠
Difficulty in performing voluntary muscle movements (in some cases)
Most of the aforementioned side effects can be alleviated by changing the dosage, depending on the patient's response to the drug. More often than not, the side effects occur with large doses, which is why, very small doses are given initially, gradually building up to an effective dose over weeks or months. In order to avoid adverse drug interactions, inform your healthcare provider about all the drugs that you are taking. Comply with the doctor's directions regarding the dosage schedule and diet. Don't change the dose or double it in case of a missed dose. Don't stop taking the drug, as sudden discontinuation of these drugs can cause the symptoms to worsen.
Though dopamine receptor agonists are quite effective in the treatment of hyperprolactinemia and the early stages of Parkinson's disease, they can cause impulse control issues or compulsive behavior. The risk is higher, if the patient is taking a dopamine agonist with levodopa. Thus, medical assistance must be immediately sought, if the patient exhibits such behavior.Disclaimer
: The information provided in this article is solely for educating the reader. It is not intended to be a substitute for the advice of a medical expert.