<P> Serotonin has been implicated in cognition, mood, anxiety and psychosis, but strong clarity has not been achieved . </P> <P> The gut is surrounded by enterochromaffin cells, which release serotonin in response to food in the lumen . This makes the gut contract around the food . Platelets in the veins draining the gut collect excess serotonin . </P> <P> If irritants are present in the food, the enterochromaffin cells release more serotonin to make the gut move faster, i.e., to cause diarrhea, so the gut is emptied of the noxious substance . If serotonin is released in the blood faster than the platelets can absorb it, the level of free serotonin in the blood is increased . This activates 5 - HT3 receptors in the chemoreceptor trigger zone that stimulate vomiting . The enterochromaffin cells not only react to bad food but are also very sensitive to irradiation and cancer chemotherapy . Drugs that block 5HT3 are very effective in controlling the nausea and vomiting produced by cancer treatment, and are considered the gold standard for this purpose . </P> <P> In mice and humans, alterations in serotonin levels and signalling have been shown to regulate bone mass . Mice that lack brain serotonin have osteopenia, while mice that lack gut serotonin have high bone density . In humans, increased blood serotonin levels have been shown to be significant negative predictor of low bone density . Serotonin can also be synthesized, albeit at very low levels, in the bone cells . It mediates its actions on bone cells using three different receptors . Through 5 - HT receptors, it negatively regulates bone mass, while it does so positively through 5 - HT receptors and 5 - HT receptors . There is very delicate balance between physiological role of gut serotonin and its pathology . Increase in the extracellular content of serotonin results in a complex relay of signals in the osteoblasts culminating in FoxO1 / Creb and ATF4 dependent transcriptional events . These studies have opened a new area of research in bone metabolism that can be potentially harnessed to treat bone mass disorders . </P>

Where is serotonin released in the nervous system