Optogenetics allows the manipulation of neural activity in freely moving animals with millisecond precision, but its application in Drosophila has been limited. Here we show that a recently described Red activatable Channelrhodopsin (ReaChR) permits control of complex behavior in freely moving adult flies, at wavelengths that are not thought to interfere with normal visual function. This tool affords the opportunity to control neural activity over a broad dynamic range of stimulation intensities. Using time-resolved activation, we show that the neural control of male courtship song can be separated into probabilistic, persistent and deterministic, command-like components. The former, but not the latter, neurons are subject to functional modulation by social experience, supporting the idea that they constitute a locus of state-dependent influence. This separation is not evident using thermogenetic tools, underscoring the importance of temporally precise control of neuronal activation in the functional dissection of neural circuits in Drosophila.

How brains are hardwired to produce aggressive behavior, and how aggression circuits are related to those that mediate courtship, is not well understood. A large-scale screen for aggression-promoting neurons in Drosophila identified several independent hits that enhanced both inter-male aggression and courtship. Genetic intersections revealed that 8-10 P1 interneurons, previously thought to exclusively control male courtship, were sufficient to promote fighting. Optogenetic experiments indicated that P1 activation could promote aggression at a threshold below that required for wing extension. P1 activation in the absence of wing extension triggered persistent aggression via an internal state that could endure for minutes. High-frequency P1 activation promoted wing extension and suppressed aggression during photostimulation, whereas aggression resumed and wing extension was inhibited following photostimulation offset. Thus, P1 neuron activation promotes a latent, internal state that facilitates aggression and courtship, and controls the overt expression of these social behaviors in a threshold-dependent, inverse manner.DOI: http://dx.doi.org/10.7554/eLife.11346.001

Innate behaviors involve both reflexive motor programs and internal states. In Drosophila, optogenetic activation of male-specific P1 interneurons triggers courtship song, as well as a persistent behavioral state that prolongs courtship and enhances aggressiveness.Here we identify pCd neurons as persistently activated by repeated P1 stimulation. pCd neurons are required for P1-evoked persistent courtship and aggression, as well as for normal social behavior. Activation of pCd neurons alone is inefficacious, but enhances and prolongs courtship or aggression promoted by female cues. Transient female exposure induced persistent increases in male aggressiveness, an effect suppressed by transiently silencing pCd neurons. Transient silencing of pCd also disrupted P1-induced persistent physiological activity, implying a requisite role in persistence. Finally, P1 activation of pCd neurons enhanced their responsiveness to cVA, an aggression-promoting pheromone. Thus, pCd neurons function within a circuit that integrates P1 input, to promote a persistent internal state that enhances multiple social behaviors.