Retinal ganglion cells encode differently in the myopic mouse retina?

Mechanisms of emmetropization and what might go wrong in myopia

Studies in animal models and humans have shown that refractive state is optimized during postnatal development by a closed-loop negative feedback system that uses retinal image defocus as an error signal, a mechanism called emmetropization. The sensor to detect defocus and its sign resides in the retina itself. The retina and/or the retinal pigment epithelium (RPE) presumably releases biochemical messengers to change choroidal thickness and modulate the growth rates of the underlying sclera. A central question arises: if emmetropization operates as a closed-loop system, why does it not stop myopia development? Recent experiments in young human subjects have shown that (1) the emmetropic retina can perfectly distinguish between real positive defocus and simulated defocus, and trigger transient axial eye shortening or elongation, respectively. (2) Strikingly, the myopic retina has reduced ability to inhibit eye growth when positive defocus is imposed. (3) The bi-directional response of the emmetropic retina is elicited with low spatial frequency information below 8 cyc/deg, which makes it unlikely that optical higher-order aberrations play a role. (4) The retinal mechanism for the detection of the sign of defocus involves a comparison of defocus blur in the blue (S-cone) and red end of the spectrum (L + M-cones) but, again, the myopic retina is not responsive, at least not in short-term experiments. This suggests that it cannot fully trigger the inhibitory arm of the emmetropization feedback loop. As a result, with an open feedback loop, myopia development becomes "open-loop".

The response of retinal ganglion cells to optical defocused visual stimuli in mouse retinas

Myopia and astigmatism are two primary types of refractive errors characterized by inaccurate focusing images on the retina. This study aimed to investigate the response characteristics of Retinal Ganglion Cells (RGCs), represented by alpha (α) RGCs, when exposed to focused, simulated spherically defocused images and astigmatically defocused images projected onto mouse retinas. Negative pressure was applied to stretch the soma of RGC in vitro to simulate myopia using a 7-8 μm diameter glass microelectrode, resulting in a 5% increase in the cell's diameter. A custom-made device was utilized to project spherically (equal to ±10 and ± 20 D) and astigmatically (+6.00 D) defocused images onto the retinas. As a control for a deficient intact retinal circuit, αRGCs of connexin 36 knockout (Cx36 KO) mice were used. The response of αRGCs varied significantly in terms of spikes, excitatory postsynaptic currents (EPSCs) and capacitances under stretching conditions to mimic myopia. Significant differences in the amplitudes of EPSCs were observed in the majority of αRGCs when exposed to focused and spherically defocused images in normal and mechanically simulated myopic retinas. However, this difference was not observed in αRGCs of Cx36 KO mice. αRGCs demonstrated significant differences in response between focused and astigmatically defocused images. Once again, αRGCs of Cx36 KO mice did not display differences. αRGCs have the ability to detect focused, spherically, and astigmatically defocused images and exhibit differential responses ex vivo. Gap junction subunit Cx36 may play a crucial role in transmitting visual signals associated with developing and perceiving refractive errors.