Serotonin is a hormone that helps the body control satiety, the feeling of fullness after eating. The anti-obesity drug, fenfluramine, which acts by increasing serotonin levls and affecting the appetite, was becoming quite popular until it was found to do serious damage to the heart and was withdrawn from the market.

The metabolic biosynthetic pathway from tryptophan to serotonin is shown here. Serotonin plays multiple roles in the nervous system, including neurotransmission. It is a precursor to melatonin, which is involved in regulation of sleepiness and wakefulness. In the intestine, serotonin regulates intestinal peristalsis. Serotonin is a potent vasoconstrictor, which helps regulate blood pressure.

The metabolic biosynthetic pathway from tryptophan to serotonin is shown here.

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