The looming stimulus-evoked flight response to approaching predators is a defensive behavior in most animals. However, how looming stimuli are detected in the retina and transmitted to the brain remains unclear. Here, we report that a group of GABAergic retinal ganglion cells (RGCs) projecting to the superior colliculus (SC) transmit looming signals from the retina to the brain, mediating the looming-evoked flight behavior by releasing GABA. GAD2-Cre and vGAT-Cre transgenic mice were used in combination with Cre-activated anterograde or retrograde tracer viruses to map the inputs to specific GABAergic RGC circuits. Optogenetic technology was used to assess the function of SC-projecting GABAergic RGCs (scpgRGCs) in the SC. FDIO-DTA (Flp-dependent Double-Floxed Inverted Open reading frame-Diphtheria toxin) combined with the FLP (Florfenicol, Lincomycin & Prednisolone) approach was used to ablate or silence scpgRGCs. In the mouse retina, GABAergic RGCs project to different brain areas, including the SC. ScpgRGCs are monosynaptically connected to parvalbumin-positive SC neurons known to be required for the looming-evoked flight response. Optogenetic activation of scpgRGCs triggers GABA-mediated inhibition in SC neurons. Ablation or silencing of scpgRGCs compromises looming-evoked flight responses without affecting image-forming functions. Our study reveals that scpgRGCs control the looming-evoked flight response by regulating SC neurons via GABA, providing novel insight into the regulation of innate defensive behaviors.
Animals ; Superior Colliculi/physiology* ; Retinal Ganglion Cells/physiology* ; GABAergic Neurons/physiology* ; Mice, Transgenic ; Mice ; Optogenetics ; Visual Pathways/physiology* ; Mice, Inbred C57BL ; Photic Stimulation/methods* ; gamma-Aminobutyric Acid/metabolism* ; Male

Objective:To observe the central projection of GABAergic retinal ganglion cells (RGCs) and the distribution and morphology of GABAergic RGCs projected to the superior colliculus (SC) in retina.Methods:Six vesicular GABA transporter ( vGAT)- cre transgenic mice (experimental group I) and 6 wide-type C57BL/6 mice (control group I) were given injection of adeno-associated virus rAAV-EF1α-DIO-EGFP-EGFP into the bilateral vitreous chamber; 6 vGAT-cre transgenic mice (experimental group II) and 6 wide type C57BL/6 mice (control group II) accepted injection of adeno-associated virus rAAV-EF1α-DIO-EGFP-EGFP into the bilateral SC using stereotactic technique. Forty-two d after virus injection, 6 mice in the experimental group I, 6 mice in the control group I, 3 mice in the experimental group II, and 3 mice in the control group II were chosen; their retina, optic nerves and brain tissues were cut into slices for double-labelling immunofluorescent staining to detect the green fluorescent protein (GFP) and GABA expressions. Three mice in the experimental group II and 3 mice in the control group II were chosen to prepare stretched preparations of retina; they were performed double-labelling immunofluorescent staining to detect the GFP and BRN3A expressions, count the number of positive cells, and measure the cell body size. Results:In the experimental group I, the green fluorescence signal expressing GFP and red fluorescence signal expressing GABA were co-standardized in the inner plexiform layer, inner nuclear layer and ganglion cell layer of the retina, the green fluorescent signal expressing GFP could be observed in the optic nerves in a linear arrangement along the axon fibers, and green fluorescent signal expressing GFP was observed in the dorsal lateral geniculate nucleus and the superficial SC; while there was no green fluorescent signal in the retina of control group I. In the experimental group II, the green fluorescence signal expressing GFP and red fluorescence signal expressing GABA were co-standardized; the GABAergic RGCs projected to the SC were mainly small/medium-sized cells (diameter 12-16 μm), which was about (37.70±5.33)% of the total cell number and was about (7.27±0.81)% of the ganglion cells (with positive BRN3A).Conclusion:GABAergic RGCs projected to the dorsal lateral geniculate nucleus and SC are specified, and they are small/medium-sized cells and uniformly distributed in the retina.