Role of Circadian Clock-Induced Enhancement of GABA_A Receptor-Mediated Feedback to Cones at Night

Abstract

Purpose: Because GABA_AR expression and activity increase when intracellular cAMP increases (Luscher et al., 2011) and the retinal circadian clock, by activating the dopamine D₂Rs of cones in the day, increases intracellular cAMP in cones at night compared to the day (Iuvone et al., 2005), we studied 1) whether the GABA_AR expression and activity of cones increase at night compared to the day and 2) the function of this day/night difference.

Methods: Rabbits and goldfish were light-adapted for 1 h in the day or dark-adapted for 1 h in the day or night. Retinal sections were processed in an identical manner with specific antibodies against the β2/3 subunit of GABA_ARs (rabbits: bd-17; fish: 62-3G1). Double labeling with cell type-specific antibodies determined whether GABA_ARs are located on cone pedicles (rabbits: PNA; fish: zpr-1) and ON-bipolar cell (BC) (Goα) and horizontal cell (HC) (calbindin) dendrites. The effects of spiperone, a D₂R antagonist that increases cAMP in cones, and gabazine, a GABA_AR antagonist, on cones in fish retinas were studied in the day and night using whole-cell patch-clamp recording.

Results: In horizontal sections through cone pedicles and in vertical sections, intense GABA_AR antibody labeling was observed on cone terminals following prolonged darkness at night, but not in the day following light or dark adaptation, and not on HC or cone-BC dendrites following prolonged darkness at night. Gabazine increased the input resistance of fish cones at night, but had minimal effect in the day. Mid-mesopic light stimulation for 5 min at night increased cone response size by 50%, an effect that was blocked by spiperone but which occurred in the presence of both spiperone and gabazine.

Conclusions: The results suggest that 1) GABA_ARs are expressed on cone pedicles and functional to a greater extent at night in the dark, compared to the day following light or dark adaptation, and 2) the retinal clock, by elevating cAMP in cones at night, increases GABA_AR expression, so that GABA_AR-mediated feedback to cones is more effective at night than in the day. As cAMP levels in cones decrease at dawn due to increased D₂R activation by the clock and/or by mid-mesopic light, the decrease in GABA_AR expression and activity of cones may enhance the size of cone light responses. Conversely, as cAMP levels increase at dusk, the increase in GABA_AR expression and activity may reduce cone light response size.

Keywords: 560 inhibitory receptors • 648 photoreceptors • 458 circadian rhythms