Retinal GABA(A) receptors participate in the regulation of circadian responses to light

A role for retinal gamma-aminobutyric add Type A (GABAA) receptors in the r egulation of circadian responses to light was examined. Intraocular injecti ons of the GABAA antagonist, bicuculline, were performed during the early ( Circadian Time [CT] 13.5) and late subjective night (CT 20), followed by a Light pulse. Bicuculline significantly deceased the magnitude of phase dela ys induced by Light to 65%, whereas it had no effect on phase advances. To explore the nature of the inhibition elicited by bicuculline, an intensity- response curve was performed. Intraocular injections of bicuculline inhibit ed phase delays only when induced by high-saturating light illuminances (20 and 100 lux). No effect was observed at light intensities less than or equ al to 5 lux. These results suggest that retinal GABA, receptors modulate th e responsivity of the circadian system to light.