The aim of this project is to understand how behaviours are controlled by neuronal circuits in zebrafish. Genetic methods such as the Gal4-UAS system will be used to express the light-gated cation channel channelrhodopsin-2 and the light-gated chloride pump halorhodopsin in defined neuronal populations. These optogenetic tools permit the stimulation (channelrhodopsin) or inhibition (halorhodopsin) of genetically targeted neurons by light in transparent zebrafish in vivo. Effects of these manipulations on behavior will be examined in forward and reverse behavioral screens. In forward experiments, swimming behaviors evoked by uniform or patterned stimulation of neurons will be analyzed in an unbiased manner. In reverse experiments, the effect of silencing defined populations of neurons will be evaluated in the context of an olfactory discrimination learning paradigm. The identification of defined neuronal populations involved in different behaviours will then serve as the starting point for the anatomical and functional analysis of the underlying circuits. The connectivity of the identified neuronal populations will be examined by anatomical methods and transsynaptic viral tracers. The function of neuronal circuits will be analyzed using 2-photon calcium imaging and electrophysiological methods. This integrated approach is expected to provide novel insights into the principles of information processing in the brain and establish a direct relationship between neuronal circuit function and behaviour.