Mismatch of Synaptic Patterns between Neurons Produced in Regeneration and during Development of the Vertebrate Retina.

Stereotypic patterns of synaptic connections between neurons underlie the ability of the CNS to perform complex but circuit-specific information processing. Tremendous progress has been made toward advancing our understanding of how circuits are assembled during development, but whether the precision of this process can be recaptured after regeneration of neurons in the damaged CNS remains unclear. Here, we harnessed the endogenous regenerative capacity of the zebrafish retina to reconstruct the circuitry of neurons produced after damage. We tracked the input connectivity of identified bipolar cell (BC) types across stages of retinal development and after BC regeneration. We found that BCs of each type generate a unique and stereotypic wiring pattern with cone photoreceptors by gaining synapses with specific photoreceptor types over time. After selective ablation, the targeted BC types are rapidly reproduced and largely re-establish their characteristic morphological features. The regenerated population connects with appropriate photoreceptor types and establishes the original number of synaptic contacts. However, BC types that normally bias their connectivity in favor of red cones fail to precisely recapture this synaptic partner preference upon regeneration. Furthermore, regenerated BCs succeed in forming synaptic specializations at their axon terminals, but in excess of the usual number. Altogether, we find that regenerated BCs reinstate some, but not all, major features of their stereotypic wiring, suggesting that circuit patterns may be unable to regenerate with the same fidelity as in development.