The neurotransmitters dopamine and serotonin act as chemical messengers that relay nerve signals through the brain. Released from one nerve ending, these chemicals pass through a small space called a synapse and bind to specialized receptors on neighboring nerve cells, causing them to fire an electrical impulse. Dopamine and serotonin control different functions, use different receptors, work in different parts of the brain, are synthesized in different pathways and are implicated in different disorders.
Role
The Merck Manual claims that there are at least 100 chemicals that act as neurotransmitters and considers dopamine and serotonin, along with only 16 others, to be of "major importance." Dopamine regulates muscle movement, motivation and the sensation of pleasure. Serotonin primarily affects mood, impulsiveness and social behaviors.
Receptors
Dopamine interacts with five different receptors in nerve cell membranes named D1 through D5. Serotonin, whose chemical name is 5-hydroxytryptamine or 5-HT, interacts with 15 different receptors, including 5-HT1, 5-HT2 and 5-HT3 and their sub-types, according to the Merck Manual.
Brain Areas
Dopamine acts primarily in the dopaminergic neurons of the midbrain, hypothalamus and substantia nigra. Serotonergic neurons use serotonin to propagate nerve signals primarily in an area called the raphe nucleus, and in the neurons along the center line of the brain stem and the pons, the region connecting the brain stem to the cerebellum.
Production
The Merck Manual describes how the serotonergic nerve cells make serotonin by hydroxylation of the amino acid tryptophan, followed by decarboxylation. The synthesis of dopamine in dopaminergic nerve cells follows the same chemical steps but starts with the amino acid tyrosine. The synthesis steps require specific enzymes: tyrosine hydroxylase for dopamine and tryptophan hydroxylase for serotonin. An enzyme called monoamine oxidase or MAO degrades both dopamine and serotonin back to inactive forms once they have played their roles in neurotransmission.
Disease
Problems with maintaining the correct levels of dopamine and serotonin can contribute to various diseases. People with Parkinson's disease do not have enough dopamine and the most effective treatment, according to the Mayo Clinic, is the drug levodopa to mimic the actions of natural dopamine. Problems with regulating the levels of serotonin can contribute to depression and other anxiety disorders, and the Merck Manual outlines how various classes of drugs that modulate serotonin levels can treat those conditions. People with schizophrenia often have an imbalance of both serotonin and dopamine, explains the Mayo Clinic. According to the Merck Manual, drugs that stimulate D4 dopamine receptors help control the symptoms of schizophrenia.
