Developmental switch of GABAergic signaling in starburst amacrine cells driven by chloride transporter dynamics

During early postnatal development, γ-aminobutyric acid (GABA) signaling undergoes a functional switch from excitation to inhibition, driven by age-dependent shifts in intracellular chloride concentrations ([Cl⁻]_i). In the retina, starburst amacrine cells (SACs) are pivotal for establishing direction-selective circuitry. However, the molecular mechanisms and the precise timing of the reversal potential of GABA-induced currents (E_GABA) shift in mouse SACs remain to be fully elucidated. We investigated the maturation of GABA responsiveness and chloride homeostasis in mouse SACs during the neonatal period. Ca²⁺ imaging in dissociated retinal neurons revealed that the application of GABA markedly increased intracellular Ca²⁺ concentrations in SACs on postnatal day (P) 0-P2, whereas these excitatory responses were largely absent by P7-P9. Gramicidin-perforated patch recordings showed that E_GABA underwent a significant hyperpolarizing shift with development, accompanied by a marked decrease in [Cl⁻]_i. Immunohistochemical analyses demonstrated a developmental decline in Na⁺-K⁺-2Cl⁻ co-transporter (NKCC1) expression and the concomitant up-regulation of K⁺-Cl⁻ co-transporter (KCC2) in SACs. Consistent with these expression patterns, the pharmacological inhibition of NKCC1 or KCC2 selectively changed E_GABA in a stage-dependent manner. Collectively, these results demonstrate a developmentally regulated GABAergic switch in SACs orchestrated by coordinated changes in chloride transporter expression, providing insights into the physiological maturation of the retinal circuitry underlying direction selectivity.