serotonin

serotonin. Serotonin, scientifically known as 5-hydroxytryptamine (5-HT), is a crucial monoamine neurotransmitter and hormone found extensively in the gastrointestinal tract, blood platelets, and the central nervous system of animals. It is synthesized from the amino acid tryptophan through a series of enzymatic reactions. This biogenic amine is implicated in a vast array of physiological processes, ranging from the regulation of mood and sleep to the control of vascular tone and intestinal motility.

Biochemistry and synthesis

The biosynthesis of this neurotransmitter begins with the essential amino acid tryptophan, which is hydroxylated by the enzyme tryptophan hydroxylase (TPH) to form 5-hydroxytryptophan (5-HTP). This reaction is the rate-limiting step in the pathway. The aromatic L-amino acid decarboxylase (AADC) then rapidly decarboxylates 5-HTP to produce the active molecule. Two isoforms of TPH exist: TPH1, primarily expressed in the pineal gland and enterochromaffin cells of the gut, and TPH2, which is found almost exclusively in neurons of the raphe nuclei within the brainstem. Following synthesis, it is packaged into synaptic vesicles by the vesicular monoamine transporter (VMAT) and released into the synaptic cleft upon neuronal depolarization. Its action is terminated primarily through reuptake into the presynaptic neuron via the serotonin transporter (SERT), followed by degradation by the enzyme monoamine oxidase (MAO) to 5-hydroxyindoleacetic acid (5-HIAA).

Functions and physiological roles

This monoamine exerts its diverse effects by binding to and activating a large family of G protein-coupled receptors and ligand-gated ion channels, classified into at least 14 distinct receptor subtypes (5-HT₁ to 5-HT₇). Within the central nervous system, it is a key modulator of behaviors and states including mood, anxiety, appetite, sleep-wake cycle, cognition, and pain perception. Projections from the raphe nuclei innervate virtually all regions of the brain, including the cerebral cortex, hippocampus, amygdala, and hypothalamus. Peripherally, it is a potent vasoconstrictor and plays a critical role in hemostasis by promoting platelet aggregation; over 90% of the body's total content is stored in platelets and enterochromaffin cells of the gastrointestinal tract, where it regulates gut motility and secretion.

Clinical significance

Dysregulation of serotonergic signaling is strongly implicated in numerous psychiatric and neurological disorders. A deficit in central serotonergic activity is a core component of the monoamine hypothesis of major depressive disorder. Abnormalities are also associated with anxiety disorders, obsessive-compulsive disorder (OCD), post-traumatic stress disorder (PTSD), and migraine. The carcinoid syndrome, caused by neuroendocrine tumors (often in the appendix or ileum) that secrete excessive amounts, leads to symptoms like flushing, diarrhea, and cardiac valvular disease. Measurement of its major metabolite, 5-HIAA, in a 24-hour urine collection is a diagnostic test for these tumors.

Pharmacology

A major class of antidepressant drugs, the selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine (Prozac) and sertraline (Zoloft), work by blocking SERT, thereby increasing synaptic concentrations. Other pharmacological agents include serotonin-norepinephrine reuptake inhibitors (SNRIs) like venlafaxine, tricyclic antidepressants (TCAs) such as amitriptyline, and the older monoamine oxidase inhibitors (MAOIs) like phenelzine. Triptans (e.g., sumatriptan), used to treat migraine, are agonists at 5-HT_1B/1D receptors. Conversely, antipsychotic drugs like clozapine and risperidone act as antagonists at several receptor subtypes, particularly 5-HT_2A. The potent psychedelic drugs LSD (lysergic acid diethylamide) and psilocybin (found in Psilocybe mushrooms) are partial agonists primarily at the 5-HT_2A receptor.

History and discovery

The story of this compound began in the mid-20th century. In 1935, the Italian physiologist Vittorio Erspamer isolated a substance from enterochromaffin cells in the gastrointestinal tract of rodents that caused smooth muscle contraction; he named it "enteramine." Independently, in 1948, a team at the Cleveland Clinic led by Maurice M. Rapport, Arda Green, and Irvine Page isolated a vasoconstrictor substance from blood serum and platelets; they determined its structure and named it "serotonin," from "sero-" (blood serum) and "tonin" (affecting vascular tone). In 1952, Rapport and Page successfully synthesized the molecule, confirming its identity as 5-hydroxytryptamine. It was later established that enteramine and serotonin were the same compound. The pivotal discovery of its presence in the brain and role as a neurotransmitter was made by Betty Twarog and Irvine Page in 1953, and further elucidated by the work of Julius Axelrod on neurotransmitter reuptake mechanisms, for which he shared the Nobel Prize in Physiology or Medicine in 1970.

Category:Neurotransmitters Category:Monoamine neurotransmitters Category:Biogenic amines Category:Gastrointestinal hormones