The involvement of a rhodopsin-like photopigment in the photoperiodic response of the Japanese quail

Abstract

A technique has been developed for the investigation of the photopigment involved in the photoperiodic control of reproduction in Japanese quail,Coturnix coturnix. When these photoreceptors were exposed to white or monochromatic light a clear relationship was found between light intensity and the extent of photo-induced luteinizing hormone (LH) secretion. A spectroradiometric investigation of the passage of light through the skull and brain enabled us to illuminate the hypothalamic region with equal numbers of photons at a range of wavelengths. Action spectra were then conducted and showed a photopigment with a peak sensitivity at wavelengths near 500 nm. An excellent match was obtained when the standard absorption spectrum for a rhodopsin was fitted to the action spectrum, suggesting a rhodopsin maximally sensitive at 492 nm. The absolute sensitivity of the photoreceptors was calculated at a range of wavelengths: with light at 500 nm, 2.85×10−12 μE·cm−2·s−1 triggered the photoperiodic response. This level of sensitivity is matched only by the rhodopsin visual pigments.