Serotonin (5-Hydroxytryptamine, 5-HT) has long been known to be involved in a variety of behavioral, cognitive and emotional processes and is commonly pharmacologically targeted in the treatment of some major psychiatric and neurological diseases. Most of the brain 5-HT innervation arises from a specific neuronal network in the brainstem, the raphe nuclei RN. For a better understanding of the brain 5-HT functions an in depth knowledge of RN network is required with a special emphasis on the activity of 5-HT neurons, since these can be directly linked to 5-HT release. Despite great interest in 5-HT function, relatively little is known about the activity of 5-HT cells in relation to behavior mainly because technical difficulties in identifying the neurochemical identity of the cells recorded. Here we propose an optogenetic approach which will allow us to to identify the neurochemical and anatomical identity of neurons recorded in behaving animals. In combination with specific behavioral paradigms, these experiments could provide insights into novel aspects of 5-HT function. The results obtained could have important pathophysiological implications.