Decoded: How we perceive happiness or pain

Using a combination of advanced genetic and optical techniques, researchers have established the effect of serotonin on sensitivity to pain.

Serotonin is a small molecule known to be implicated in a wide range of brain functions - from the control of sleep and appetite to the regulation of complex emotional behaviours.

This neurotransmitter is also popularly thought to contribute to feelings of well-being and happiness, as some anti-depression medications work through increasing serotonin in the brain.

"Most of the cells that produce serotonin are located in a defined cell group called the Dorsal Raphe Nucleus (DRN).

"This cell group is small and located deep in the brain, which makes targeting it difficult," said Zachary Mainen, director at Champalimaud Neuroscience Programme, a private biomedical research in Lisbon, Portugal.

In addition, "other cells that produce and release different molecules are also present in the DRN, which means that general stimulation of the area may result in the release of other molecules besides serotonin", he said.

Using genetic techniques, researchers expressed a light-sensitive protein specifically in the serotonin-producing cells of mice, so that when the researchers shed light on these cells, the cells released serotonin.

"The effect of the serotonin was clear. Mice that we stimulated to release serotonin showed a significant decrease in sensitivity to pain when compared with mice in the control group," added Guillaume Dugue, a former postdoctoral researcher in Mainen's lab.

The results provide a new level of evidence on the importance of serotonin in gating the influence of sensory inputs to behavioural outputs, Mainen concluded.

The study appeared in the scientific journal PLoS One.