Using stem cells, researchers at the University of Wisconsin-Madison have created a specialised nerve cell that makes serotonin, a signalling chemical with a broad role in the brain.

Serotonin affects emotions, sleep, anxiety, depression, appetite, pulse and breathing. It also plays a role in serious psychiatric conditions like schizophrenia, bipolar disorder and depression.

"Serotonin essentially modulates every aspect of brain function, including movement," said one of the researchers Su-Chun Zhang said.

A small number of neurons localised on one structure at the back of the brain are responsible for making this chemical.

Serotonin exerts its influence because the neurons that make it project to almost every part of the brain, the researchers said.

The study began with two types of stem cells: one derived from embryos, the other from adult cells.

As serotonin neurons form before birth, the researchers had to recreate the chemical environment found in the developing brain in the uterus, Zhang said.

Because the neurons can be generated from induced pluripotent stem cells, which can be produced from a patient's skin cells, "these could be useful for finding treatments for psychiatric disorders like depression, where we often see quite variable responses to drugs," study first author Jianfeng Lu said.

"By identifying individual differences, this could be a step toward personalised medicine," Lu noted.

The findings were reported in the journal Nature Biotechnology.