Development of light response and GABAergic excitation-to-inhibition switch in zebrafish retinal ganglion cells

AbstractThe zebrafish retina has been an important model for studying morphological development of neural circuits in vivo. However, little is known about its functional development. To investigate the functional development of zebrafish retina, we developed in vivo patch-clamp whole-cell recording technique in intact zebrafish larvae. We first examined the developmental profile of light-evoked responses (LERs) in retinal ganglion cells (RGCs) from 2 to 9 days post-fertilization (dpf). Unstable LERs were first observed at 2.5 dpf. By 4 dpf, RGCs exhibited reliable light responses. As the GABAergic system is critical for retinal development, we then performed in vivo gramicidin perforated-patch whole-cell recording to characterize the developmental change of GABAergic action in RGCs. The reversal potential of GABA-induced currents (EGABA) in RGCs gradually shifted from depolarized to hyperpolarized levels during 2 - 4 dpf and the excitation-to-inhibition (E-I) switch of GABAergic action occurred at around 2.5 dpf when RGCs became light-sensitive. Meanwhile, GABAergic transmission upstream to RGCs also has become inhibitory by 2.5 dpf. Furthermore, down-regulation of K+/Cl- co-transporter (KCC2) by morpholino oligonucleotide-based knockdown approach, which shifted RGC EGABA towards a more depolarized level and thus delayed the E-I switch by one day, postponed the appearance of RGC LERs by one day. In addition, RGCs exhibited correlated giant inward currents (GICs) during 2.5 - 3.5 dpf. The period of GIC existence was shifted to 3 - 4.5 dpf by KCC2 knockdown. Taken together, the GABAergic E-I switch occurs coincidently with the emergence of light responses and GICs in zebrafish RGCs, and may contribute to the functional development of retinal circuits.