Serotonin(5-HT) is a neurotransmitter and a hormone. It is the brain’s key monoamine neurotransmitter. It is mainly produced in the brain stem in clusters of nuclei called ” raphe nuclei” from there it is projected to the rest of the central nervous system. It is removed from the synaptic cleft via reuptake by a transport protein called “serotonin transporter”(SERT). Serotonin has 7 families of receptors (5-HT1 to 5-HT7) and has 14 receptor subtypes. Whether serotonin’s effects are inhibitory or excitatory depends on the receptor it binds.

Serotonin is involved in a long list of functions. It regulates mood, appetite, aggression, body temperature, sleep and wakefulness, endocrine function and more. In many cases, the exact role of serotonin in these functions is still not completely understood.

Serotonin is linked to depression because increasing serotonin levels decreases the symptoms of depression. Many of the antidepressants increase the serotonin levels for example, the SSRI (selective serotonin reuptake inhibitor). Serotonin is linked to Major Depressive Disorder, phases of depression in bipolar disorder, Dysthymia, Depressive episodes and substance-induced depression (during withdrawal from drugs that raise serotonin like cocaine).

Pineal gland is a small endocrine gland located inside the brain that converts tryptophan into serotonin and then this serotonin into melatonin. Melatonin is best known for its function in regulating circadian rhythm. It regulates the sleep and wake cycle via the suprachiasmatic nucleus present in the hypothalamus.

One of the functions of serotonin is the regulation of aggression and impulsiveness. When serotonin levels in the brain are depleted by diet (decreasing the amount of tryptophan intake) people show more impulsive and aggressive behaviours. The 5-HT1A receptor primarily mediates the anti-impulsive and anti-aggressive function of serotonin. Drugs that act as agonists for 5-HT1A receptors have anti-impulsive and anti-aggressive effects.

Understanding serotonin helps us see the impact that a single neurotransmitter can have.