Searching for relief: Drosophila melanogaster navigation in a virtual bitter maze

Animals use pain-relief learning to discern which actions can diminish or abolish noxious stimuli. If relief from pain is provided in a specific location, place learning is the mechanism used to pinpoint that location in space. Little is known about how physiological and non-directly damaging stimuli can alter visual-based searching behaviour in animals. Here we show how the optogenetically-induced activation of bitter-sensing neurons urges Drosophila melanogaster to seek relief from bitter taste stimulation and that this distressful, but ecologically relevant stimulus, innately wired to the threat of intoxication, is sufficient to elicit pain-relief-like behavioural responses. Specifically, freely walking flies inside an open circular arena are trained to seek relief from the unpleasant stimulation by searching for a safe area alternatively positioned in the proximity of a pair of identical, diametrically opposed, visual markers. Moreover, and perhaps more importantly, under this paradigm flies develop visual place learning manifested by their seeking relief in the zone associated with bitter relief during the last trial of training, even when exposed to constant bitter stimulation with no relief provided. An important implication is that this form of learning does not lead to operant conditioning generalization. We further propose that kinematic indexes, such as the spatially-specific reduction of locomotor velocity, may provide immediate evidence of relief-based place learning and spatial memory.