Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor

Abstract

The photopigment in the human eye that transduces light for circadian and neuroendocrine regulation, is unknown. The aim of this study was to establish an action spectrum for light-induced melatonin suppression that could help elucidate the ocular photoreceptor system for regulating the human pineal gland. Subjects (37 females, 35 males, mean age of 24.5 +/- 0.3 years) were healthy and had normal color vision. Full-field, monochromatic light exposures took place between 2:00 and 3:30 A.M. while subjects' pupils were dilated. Blood samples collected before and after light exposures were quantified for melatonin. Each subject was tested with at least seven different irradiances of one wavelength with a minimum of 1 week between each nighttime exposure. Nighttime melatonin suppression tests (n = 627) were completed with wavelengths from 420 to 600 nm. The data were fit to eight univariant, sigmoidal fluence-response curves (R(2) = 0.81-0.95). The action spectrum constructed from these data fit an opsin template (R(2) = 0.91), which identifies 446-477 nm as the most potent wavelength region providing circadian input for regulating melatonin secretion. The results suggest that, in humans, a single photopigment may be primarily responsible for melatonin suppression, and its peak absorbance appears to be distinct from that of rod and cone cell photopigments for vision. The data also suggest that this new photopigment is retinaldehyde based. These findings suggest that there is a novel opsin photopigment in the human eye that mediates circadian photoreception.

Fig. 1.

This diagram illustrates the experimental electronic, optic, and ganzfeld dome exposure array. This apparatus provides a uniform, patternless stimulus that encompasses the subject's entire visual field. For clarity, the subject's head is shown slightly withdrawn from the opening of the ganzfeld dome. During all light exposures, the subjects' bony orbits are completely enclosed in the dome walls, providing complete exposure of their visual fields.

Fig. 2.

In the top two graphs, thebars represent group mean + SEM values of plasma melatonin relative to 460 nm monochromatic light exposure at different irradiances in one group of eight healthy subjects. Thetop shows plasma melatonin values before and after light exposure. There were no significant differences (F= 0.70; p = 0.69) across preexposure mean melatonin values. Light irradiances at or above 3.1 μW/cm²elicited significant melatonin suppression. The middleillustrates the subjects' plasma melatonin percent control-adjusted change scores. Progressively higher irradiance exposures at 460 nm produce progressively greater plasma melatonin percent control-adjusted change scores (p < 0.0001). Thebottom demonstrates the best-fit fluence–response curve for 460 nm exposures and percent control-adjusted melatonin suppression (R² = 0.97). Each data point represents one group mean ± SEM.

Fig. 3.

This figure illustrates the fitted univariant fluence–response curves for monochromatic light exposures and percent control-adjusted melatonin suppression for eight wavelengths of visible light. Each fluence–response curve is derived from eight healthy volunteers who participated in a complete, within-subjects experimental design. In each graph, the data points represent group means ± SEM. Each curve has a high coefficient of correlation (0.95–0.81).

Fig. 4.

In this graph, the bars represent group mean ± SEM plasma melatonin values before and after exposure to 31.8 μW/cm² monochromatic light at 420 nm in eight healthy subjects. This light irradiance induced a significant melatonin suppression (p < 0.003).

Fig. 5.

This graph demonstrates the action spectrum for percent control-adjusted melatonin suppression in 72 healthy human subjects. The filled circles represent the half-saturation constants of eight wavelengths from 440 to 600 nm that were normalized to the maximum response and plotted as log relative sensitivity. The open circle represents the estimated half-saturation constant derived from the 420 nm data. The solid curve portrays the best-fit template for vitamin A₁ retinaldehyde photopigments, which predicts a maximal spectral absorbance (λ_max) of 464 nm (Partridge and De Grip, 1991). There is a high coefficient of correlation for fitting this opsin template to the melatonin suppression data (R² = 0.91).

Fig. 6.

This figure illustrates a comparison of the melatonin suppression and visual action spectra. The maximal spectral response and long wavelength limb of the melatonin suppression template is plotted along with the maximal spectral response and long wavelength limbs of the human rods and cones that support vision (Stockman and Sharpe, 1999). The shaded area around the 464 nm template represents ±SD from the data presented above.