Gamma-aminobutyric acid (GABA) is regarded as the most important inhibitory neurotransmitter in the central nervous system, including the retina. However, the roles of GABA-immunolabeled retinal ganglion cells (RGCs) have not been explored. Here, we report the expression of GABAergic RGCs that project to many brain areas in mice, including the superior colliculus. Selective ablation of the superior colliculus-projecting GABAergic RGCs, leaving other GABAergic RGCs intact, reduces the looming stimulus-induced defensive response without affecting image-forming functions; it also significantly enhances glucose metabolism in the superior colliculus, as determined by [18F]-fluorodeoxyglucose PET (FDG PET). Our findings demonstrate that superior colliculus-projecting GABAergic RGCs control the visually active defensive response by regulating superior colliculus neurons.

40The looming stimulus-evoked flight response is an experimental paradigm for studying 41 innate defensive behaviors. However, how the visual looming stimulus is transmitted from the 42 retina to the brain remains poorly understood. Here, we report that superior colliculus (SC)-43 projected GABAergic RGCs (spgRGCs) transmit the looming signal from the retina to the 44 brain to mediate the looming-evoked flight behavior by releasing GABA. In the mouse retina, 45 GABAergic RGCs are capable of projecting to many brain areas, including the SC. spgRGCs 46 are mono-synaptically connected to the parvalbumin-positive SC neurons known to be 47 required for the looming-evoked flight response. Optogenetic activation of spgRGCs triggers 48 GABAergic responses in SC neurons. The ablation or silence of spgRGCs compromises 49 looming-evoked flight response but not image-forming functions. Therefore, this study shows 50 that spgRGCs control the looming-evoked flight response by regulating SC neurons via 51 GABA, providing novel insight into the regulation of innate defensive behaviors. 52 5 130 responses in SC neurons, channel Rhodopsin 2 and EYFP fusion protein (ChR2-EYFP) were 131 expressed in the AAV-infected GABAergic RGCs. When spgRGCs were activated by 470 nm 132 laser light, post-synaptic SC responses were recorded (Fig. 2A and 2B). Brief pulses of light 133 can induce sustained inward currents in the ChR2-EYFP-expressing RGCs, indicating that 134ChR2-EYFP can be functional in GABAergic RGCs (Fig. 2C). In the SC, light-mediated 135 ChR2 activation in the ChR2-EYFP-expressing RGC axonal fibers induces inhibitory 136 postsynaptic currents (IPSCs) when Vholding was set to 0 mV, which is close to the reversal