Homeostatic Plasticity Shapes the Retinal Response to Photoreceptor Degeneration.

Homeostatic plasticity stabilizes input and activity levels during neural development, but whether it can restore connectivity and preserve circuit function during neurodegeneration is unknown. Photoreceptor degeneration is the most common cause of blindness in the industrialized world. Visual deficits are dominated by cone loss, which progresses slowly, leaving a window during which rewiring of second-order neurons (i.e., bipolar cells) could preserve function. Here we establish a transgenic model to induce cone degeneration with precise control and analyze bipolar cell responses and their effects on vision through anatomical reconstructions, in vivo electrophysiology, and behavioral assays. In young retinas, we find that three bipolar cell types precisely restore input synapse numbers when 50% of cones degenerate but one does not. Of the three bipolar cell types that rewire, two contact new cones within stable dendritic territories, whereas one expands its dendrite arbors to reach new partners. In mature retinas, only one of four bipolar cell types rewires homeostatically. This steep decline in homeostatic plasticity is accompanied by reduced light responses of bipolar cells and deficits in visual behaviors. By contrast, light responses and behavioral performance are preserved when cones degenerate in young mice. Our results reveal unexpected cell type specificity and a steep maturational decline of homeostatic plasticity. The effect of homeostatic plasticity on functional outcomes identify it as a promising therapeutic target for retinal and other neurodegenerative diseases.