There has always been considerable interest in elucidating the roles of particular neurons and groups of neurons in behaviour, but it has been difficult to carry out such investigations on unrestrained animals. The recent use of biological 'labels' in particular neural populations has provided greater flexibility in this field, and research publised in Cell now reports the use of photostimulation to investigate the neural basis of movement in flies.

Lima and Miesenböck expressed optically gated ion channels in certain Drosophila melanogaster neurons and used pulses of unfocused laser light to allow them to link particular behaviours to specific populations of neurons. Furthermore, by using a mutation that blocks phototransduction, they were able to ensure that their results reflected the stimulation of target neurons and not simply activation of the visual system.

Escape movements — including jumping, wing beating and flight — were seen in a high percentage of flies in response to photostimulation of giant fibre neurons. However, flight and wing beating endured for considerably longer time periods than the stimulus, which, the authors suggest, indicates that giant fibre neurons simply act to set the thoracic oscillations that control wing movement in motion.

Dopaminergic neurons are also involved in movement control, and Lima and Miesenböck used this model to show that an increase in dopaminergic signalling leads to an increase in locomotor activity and variations in walking patterns. However, the speed at which the flies moved did not change, and the authors ascribe the increase in locomotion to shorter and less frequent pauses during walking.

The authors conclude that the expression of phototriggers in neurons not only opens the door for further research, but might also provide a means of treating diseases and injuries of the CNS in the future.