Abstract
A role for retinal [.gamma]-aminobutyric acid Type A (GABAA) receptors in the regulation of circadian responses to light was examined. Intraocular injections 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 decreased the magnitude of phase delays 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 inhibited phase delays only when induced by high-saturating light illuminances (20 and 100 lux). No effect was observed at light intensities [.lessequal] 5 lux. These results suggest that retinal GABAAreceptors modulate the responsivity of the circadian system to light.