Serotonin is an important neurotransmitter in the function of the central nervous system and the gastrointestinal tract in both non-vertebrates and vertebrates.

Non-vertebrates

Serotonin is involved in the physiological and behavioral processes of non-vertebrates as well as vertebrates:

In insects, serotonin regulates heart beat and secretion as well as growth, memory, learning and the circadian rhythm. In parasites and worms, serotonin has been shown to regulate movement. Serotonin is also produced by some infective pathogens to stimulate gut movement and diarrhea in the host.

Serotonin modulates eating behaviour by influencing how animals respond according to available food resources. In the case of the roundworm, for example, if the worm is starved when food is encountered, serotonin is released to slow the animal down so that the time spent in the presence of food is increased. The muscles involved in feeding are also stimulated by the release of the serotonin.

Vertebrates

Some of the functions of serotonin in vertebrate animals include:

Serotonin is a mood regulator in vertebrate animals, including humans. Activation of one form of serotonin receptor in the brain called the 5-HT1A receptor has been shown to prevent aggression. In addition, mutations in the gene that code for this receptor have been associated with an increased risk of suicide.

Within the intestines, serotonin regulates intestinal movement.

In the brain, serotonin is involved in regulating several important functions including sleep, appetite, feeding, and body weight. Abnormal serotonin levels are also associated with problems such as suicidal tendency, obsessive compulsive disorder, alcoholism and anxiety.

Serotonin is also stored by blood platelets which release the chemical when they bind to a clot, in order to promote blood clotting.