Fig 2. The action spectrum using the photoperiodically induced beta subunit of thyroid-stimulating hormone.
(A) Schedule for light exposure and sampling. Eye-patched birds lacking pineal organ and maintained under short-day conditions (6 h:18 h light/dark cycle: 6L18D) were given a long-day stimulus by extending the 6-h light period by 10 h with light of seven different wavelengths. (B) The effects of four intensities of light at each wavelength on the expressions of photoperiodically induced mRNA encoding the beta subunit of thyroid-stimulating hormone (TSHB) in the pars tuberalis of the pituitary gland were evaluated by in situ hybridization. (C) Each expression level of TSHB was plotted to examine the dependence of photoperiodic responses on irradiance with monochromatic lights of seven different wavelengths. Each light intensity (photons m2 s-1) was adjusted to the light intensity at the deep-brain region level using the percentage of T/Tmax. Each point represents the mean ± SEM (n = 4). (D) The action spectrum for photoperiodic TSHB induction. The half-saturation constant (EC50) derived from sigmoidal fits of the light intensity–response curves were plotted against wavelength. The action spectrum was then fitted with a curve for retinal1-based photopigment using the least-squares method. Peak sensitivity was approximately 479.2 nm with a low value for the goodness of fit (adjusted R2 = -0.938).