New submitter baalcat quotes a report from Neuroscience News: A new study in mice shows that increasing serotonin, one of the major mediators of brain communication, affects motivation -- but only in certain circumstances. Furthermore, the study revealed that the short and long term effects of increased serotonin levels are opposed -- a completely unforeseen property of this neurotransmitter's functional system. A surprising behavioral effect, discovered in mice by neuroscientists at the Champalimaud Centre for the Unknown (CCU), in Lisbon, Portugal, strongly suggests that serotonin is involved in a biological mechanism which affects the animals' motivation. The study has now been published in the online open access journal eLife. Serotonin, one of the chemical "messengers," or neurotransmitters, in the brain, is used by neurons to communicate with each other. It plays an important role in the regulation of sleep, movement and other behaviors which are essential for animal survival. But for motivation in particular, it was unclear whether serotonin was involved. Using optogenetics, the team stimulated the release of serotonin from neurons in the raphe nuclei. They first induced "peaks" of serotonin by stimulating these neurons with pulses of light, lasting three seconds every ten seconds, over three five-minute time periods. The mice, placed in a box, were left free to explore their environment. In these conditions, their most frequent spontaneous behaviors are walking around, rearing, grooming, digging holes or keeping relatively still, but nevertheless alert. The only difference the scientists saw was that stimulation caused the mice to reduce their locomotive speed by about 50%. In general, this stimulation of serotonin-producing neurons did not affect other behaviors. The effect of these serotonin "peaks" on locomotion was almost instantaneous (speed reduction manifested one second after stimulation) and transient, with things going back to normal after five seconds. But during this short period of time, "the animals acted as if they weren't motivated," says Zach Mainen, who led the study.