Types of Serotonin Receptors

The various serotonin receptor types can be categorized based on which organs and tissues express them. The effects of serotonin across different organ systems are also related to the receptor type expressed at that location.

Nervous System

• Serotonin receptors are located in a variety of sites throughout the brain and central nervous system. For instance, the area of the brain known as the medulla, which is responsible for regulating the vomiting reflex, express serotonin type 3 receptors. These same serotonin receptors are expressed in chemically sensitive nerve endings present in the blood vessels supplying your heart. Additionally, the neurons controlling the activity of the gastrointestinal tract, also known as the enteric nervous system, produce several serotonin receptor subtypes that play important roles in digestion. Throughout the enteric nervous system, serotonin type 1P and 4 are expressed and play important roles in regulating intestinal motility and function.

Respiratory System

• Serotonin appears to have a small stimulatory effect on muscle cells lining the respiratory tract, also known as bronchial smooth muscle. By acting on bronchial tracts, episodes of severe airway constriction have been observed in patients with serotonin-secreting tumors. According to the textbook "Basic and Clinical Pharmacology" by Bertram Katzung, these effects likely occur via stimulation of serotonin type 2A receptors.

Cardiovascular System

• Serotonin has powerful effects on blood vessels throughout the body through its effect on vascular smooth muscle, a specialized type of muscle that controls blood vessel diameter. Through a process known as vasoconstriction, stimulation of serotonin type 2 receptors will decrease blood flow to most tissues throughout the body, especially vessels supplying the kidneys and lungs. Conversely, the blood vessels supplying cardiac and skeletal muscle will increase their diameter in response to serotonin administration, a process known as vasodilation. Serotonin exerts these opposing effects in different vascular tissues by using separate mechanisms. For instance, rather than stimulating receptors directly on smooth muscle, cardiac blood vessels are lined with vascular endothelial cells, a specialized type of tissue that directly controls the diameter of these vital blood vessels. Serotonin will stimulate these endothelial cells to produce a substance known as nitrous oxide, which acts to dilate coronary blood vessels by relaxing the surrounding smooth muscle. The existence of this intermediary cell type allows the same serotonin molecule to have variable effects throughout the vasculature.

Gastrointestinal Tract

• Serotonin has several effects on the function of the gastrointestinal system. Stimulation of serotonin type 2 receptors in the gut will increase constriction of smooth muscle tissue, thus increasing the tone of the digestive tract. Activation of serotonin type 4 receptors throughout the enteric nervous system will increase motility, or the rate of material movement through the gastrointestinal tract. In fact, in cases of serotonin overproduction, severe diarrhea may result from excessive stimulation of these type 4 receptors.

Skeletal Muscle

• Serotonin type 2 receptors are expressed on the surface of skeletal muscle, although their purpose is not well understood. Serotonin syndrome, a condition characterized by excessive muscle contraction, may result from a dangerous combination of anti-depressant drugs, which results in the release of high serotonin levels in the brain.

Blood Clotting

• Stimulation of serotonin type 2 receptors located on blood platelets will lead to cell aggregation and clot formation. Platelets normally produce and store serotonin, which will produce this clotting effect when released under normal physiologic circumstances.