Light/Dark Adaptive Regulation of GABAA Receptor and Chloride Cotransporter Expression and Activity Modulates the ON-Pathway Surround

Abstract

Purpose: : Ganglion cells (GCs) display a center-surround receptive field (RF) following prolonged (>30 min) light adaptation, but lose their surround responses following prolonged dark adaptation (Barlow et al., 1957). Bipolar cell (BC) surround strength also decreases when the intensity of background illumination is reduced (Fahey & Burkhardt, 2001). Under light-adapted conditions, horizontal cells (HCs) contribute to ON-GC surround responses (Mangel, 1991) possibly by releasing GABA (Guo et al., 2010) directly onto ON-cone BC dendrites, which express Cl-permeable GABA_A receptor (GABA_AR) channels and the Cl cotransporter NKCC. We thus tested whether light/dark adaptive regulation of GABA_ARs and NKCC modulates the ON-pathway surround.

Methods: : Pigmented rabbits were bright light (photopic)- or dark (low scotopic)-adapted for 1 h or rabbit retinas were incubated in Ringer for 1 h under light-adapted conditions with or without test drugs. Retinal sections were processed in an identical manner for immunostaining with specific antibodies against the β2/3 subunit-containing, GABA_AR (bd-17) and NKCC (T4). The effects of current injections into HCs on the activity of nearby ON-GCs in rabbit eyecups and dopamine application on neurons in the INL of rabbit retinal slices were studied.

Results: : Intense GABA_AR and NKCC antibody labeling were observed in ON-BC and HC dendrites under light-adapted control conditions, but both immunosignals were minimal in dark-adapted retinas or in light-adapted retinas pre-treated with the dopamine D1 antagonist SCH23390 or the nitric oxide (NO) synthase inhibitor L-NAME. Under light-adapted conditions, picrotoxin, a GABA_A/C antagonist, and bumetanide, a specific NKCC inhibitor, blocked the effect of HC polarizations on ON-GC spike activity. Picrotoxin also eliminated the reduction in the ON-GC center response produced by hyperpolarizing nearby HCs. Preliminary data indicate that dopamine application decreased the size of the RF center and strengthened the RF surround of putative ON-cone BCs.

Conclusions: : These results suggest that prolonged light adaptation increases D1 receptor activation and NO synthesis so that GABA_AR and NKCC expression and activity on the dendrites of ON-cone BCs are enhanced, producing the RF surrounds of ON-cone BCs and ON-GCs in part via direct GABA excitation of ON-cone BC dendrites by HCs. Conversely, the decrease in D1 receptor activation and NO synthesis following prolonged dark adaptation substantially reduce GABA_AR and NKCC expression and activity, weakening the RF surrounds of ON-cone BCs and ON-GCs.

Keywords: receptive fields • inhibitory receptors • ion transporters