<P> The dopamine system plays a central role in several significant medical conditions, including Parkinson's disease, attention deficit hyperactivity disorder, schizophrenia, and addiction . Aside from dopamine itself, there are many other important drugs that act on dopamine systems in various parts of the brain or body . Some are used for medical or recreational purposes, but neurochemists have also developed a variety of research drugs, some of which bind with high affinity to specific types of dopamine receptors and either agonize or antagonize their effects, and many that affect other aspects of dopamine physiology, including dopamine transporter inhibitors, VMAT inhibitors, and enzyme inhibitors . </P> <P> A number of studies have reported an age - related decline in dopamine synthesis and dopamine receptor density (i.e., the number of receptors) in the brain . This decline has been shown to occur in the striatum and extrastriatal regions . Decreases in the D, D, and D receptors are well documented . The reduction of dopamine with aging is thought to be responsible for many neurological symptoms that increase in frequency with age, such as decreased arm swing and increased rigidity . Changes in dopamine levels may also cause age - related changes in cognitive flexibility . </P> <P> Other neurotransmitters, such as serotonin and glutamate also show a decline in output with aging . </P> <P> Parkinson's disease is an age - related disorder characterized by movement disorders such as stiffness of the body, slowing of movement, and trembling of limbs when they are not in use . In advanced stages it progresses to dementia and eventually death . The main symptoms are caused by the loss of dopamine - secreting cells in the substantia nigra . These dopamine cells are especially vulnerable to damage, and a variety of insults, including encephalitis (as depicted in the book and movie "Awakenings"), repeated sports - related concussions, and some forms of chemical poisoning such as MPTP, can lead to substantial cell loss, producing a parkinsonian syndrome that is similar in its main features to Parkinson's disease . Most cases of Parkinson's disease, however, are idiopathic, meaning that the cause of cell death cannot be identified . </P>

Which of the following describes the function of dopamine in the brain