Receptors of dopamine can be classified into five subcategories, the D1, D2, D3, D4, and D5 receptors. Each of these types of receptors serves different functions, depending on the area of the body where they are located. Motor activity, memory, and learning are all functions of these receptors. Dysfunctional receptors of dopamine can cause many different diseases and disorders, such as addiction, schizophrenia, and Parkinson's disease.
One of the many functions of dopamine receptors includes controlling motor activity. Forward locomotion is controlled by activation of the D1, D2, and D3 receptors in the ventral striatum region of the brain, where the limbic system is located. The activation of different types of D2 receptors, such as autoreceptors or postsynaptic receptors, has a direct impact on increasing or decreasing forward locomotion. D3 receptors work similarly to the D2 receptors, in that they can either evoke or inhibit motor activity. The D1 receptors do not impact motor activity on their own, but are necessary for maximum motion when paired with the D2 receptors.
Reward and reinforcement also constitute a primary function of dopamine receptors. The most well-known example of this is shown in cases of drug addiction and abuse. When certain psychostimulant drugs, such as cocaine, are ingested, the D1 and D2 receptors are activated, creating the euphoric feeling associated with the drug. Dopamine is released when drugs are taken, and during withdrawal, the amount of dopamine produced in reduced. During withdrawal, the D2 receptors cause the person to seek further cocaine or other stimulant drug reinforcement, while the D1 receptors try to reduce the drive to seek out the drug.
Despite some inconsistencies in the data, there is general agreement that dopamine receptors have a function in learning and memory. D1 and D2 receptors are activated in the hippocampus, the region of the brain responsible for memory, and are shown to strongly improve retention in rats. In monkeys, studies show that D1 and D2 activation in the prefrontal cortex, which is the area of the brain that controls behavior and expression, improves memory. The D5 receptors are believed to be the strength behind the effects of the D1 receptors in the hippocampus. The D3 and D4 receptors remain a bit of a mystery to scientists, but they are believed to support the D2 receptors in the hippocampus and the prefrontal cortex.
Dysfunction of dopamine receptors is responsible for a variety of diseases and disorders. Social phobias, attention-deficit hyperactivity disorder, and schizophrenia are some examples of mental health disorders that can develop from dysfunction or over-stimulation of the dopamine receptors. Parkinson's disease, neuroleptic malignant syndrome, and genetic hypertension can also develop from dysfunctional receptors.