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Restoration of Dendritic Complexity, Functional Connectivity, and Diversity of Regenerated Retinal Bipolar Neurons in Adult Zebrafish

Adult zebrafish (Danio rerio) are capable of regenerating retinal neurons that have been lost due to mechanical, chemical, or light damage. In the case of chemical damage, there is evidence that visually mediated behaviors are restored after regeneration, consistent with recovery of retinal function. However, the extent to which regenerated retinal neurons attain appropriate morphologies and circuitry after such tissue-disrupting lesions has not been investigated. Adult zebrafish of both sexes were subjected to intravitreal injections of ouabain, which destroys the inner retina. After retinal regeneration, cell-selective markers, confocal microscopy, morphometrics, and electrophysiology were used to examine dendritic and axonal morphologies, connectivities, and the diversities of each, as well as retinal function, for a subpopulation of regenerated bipolar neurons (BPs). Although regenerated BPs were reduced in numbers, BP dendritic spreads, dendritic tree morphologies, and cone–bipolar connectivity patterns were restored in regenerated retinas, suggesting that regenerated BPs recover accurate input pathways from surviving cone photoreceptors. Morphological measurements of bipolar axons found that numbers and types of stratifications were also restored; however, the thickness of the inner plexiform layer and one measure of axon branching were slightly reduced after regeneration, suggesting some minor differences in the recovery of output pathways to downstream partners. Furthermore, ERG traces from regenerated retinas displayed waveforms matching those of controls, but with reduced b-wave amplitudes. These results support the hypothesis that regenerated neurons of the adult zebrafish retina are capable of restoring complex morphologies and circuitry, suggesting that complex visual functions may also be restored. SIGNIFICANCE STATEMENT Adult zebrafish generate new retinal neurons after a tissue-disrupting lesion. Existing research does not address whether regenerated neurons of adults successfully reconnect with surrounding neurons and establish complex morphologies and functions. We report that, after a chemical lesion that ablates inner retinal neurons, regenerated retinal bipolar neurons (BPs), although reduced in numbers, reconnected to undamaged cone photoreceptors with correct wiring patterns. Regenerated BPs had complex morphologies similar to those within undamaged retina and a physiological measure of photoreceptor–BP connectivity, the ERG, was restored to a normal waveform. This new understanding of neural connectivity, morphology, and physiology suggests that complex functional processing is possible within regenerated adult retina and offers a system for the future study of synaptogenesis during adult retinal regeneration.