A noncanonical inhibitory circuit dampens behavioral sensitivity to light

Retinal ganglion cells that release GABA Retinal ganglion cells (RGCs) communicate light signals from the retina to the brain and were previously considered to signal exclusively through release of excitatory neurotransmitters. There have been earlier hints of RGCs producing the inhibitory neurotransmitter γ-aminobutyric acid (GABA), but the specific cells have never been identified and their function was entirely unknown. Sonoda et al. found that a subpopulation of intrinsically photosensitive RGCs (ipRGCs) releases GABA (see the Perspective by Ding and Wei). Removal of GABA signaling from ipRGCs led to an increased light sensitivity of the pupillary light reflex and of circadian photoentrainment. GABA release thus moved the dynamic range of these non–image-forming behaviors to bright light intensities. These results explain why these behaviors are so much less sensitive to environmental lighting conditions than conscious visual perception. Science , this issue p. 527 ; see also p. 471