Wyart et al expressed the light-gated ion channel LiGluR as well as the UAS promoter in some zebrafish and crossed these with zebrafish containing the GAL4 transcription factor in various classes of ventral spinal neurons. When the progeny was photo-stimulated in the caudal spinal cord, 94% displayed tail oscillations. The effects were localized down to Kolmer-Agduhr cells and since its action potentials were blocked by the GABA-A antagonist bicuculline, the researchers were able to identify the mechanism as GABAergic, of note because GABA is typically inhibitory. This is a good example of how a given behavior can be elicited in a non-intuitive way by a specific subset of neurons in a particular region of the nervous system.
Wyart C, et al. Optogenetic dissection of a behavioural module in the vertebrate spinal cord. Nature 461:407-410. doi:10.1038/nature08323.