S-cone contribution to the acute melatonin suppression response in humans

Area of Science:

• Ophthalmology
• Chronobiology
• Neuroscience

Background:

• Light exposure significantly impacts human physiology, including circadian rhythms and sleep.
• Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) are key mediators of non-visual light effects.
• The spectral sensitivity underlying melatonin suppression is debated, with potential roles for melanopsin and cone photoreceptors.

Purpose of the Study:

• To investigate the spectral sensitivity of acute melatonin suppression.
• To determine the contribution of S-cones to light-induced melatonin suppression.
• To refine models of non-visual spectral sensitivity.

Main Methods:

• Six healthy males were exposed to 30-minute sessions of 415 nm monochromatic light at varying irradiances.
• Data were aggregated with a previously established action spectrum for melatonin suppression.
• Multi-opsin models were used to analyze the spectral sensitivity data.

Main Results:

• The aggregated data were inconsistent with single-opsin models or existing multi-opsin models like Circadian Stimulus.
• The extended action spectrum was best approximated by a combination of melanopsin (~2 parts) and S-cone (~1 part) signals.
• This 2:1 melanopsin-S-cone model accurately predicted melatonin suppression for shorter light exposures but not longer ones.

Conclusions:

• Human melatonin suppression exhibits an initial sensitivity to short-wavelength light mediated by S-cones.
• The contribution of S-cones to melatonin suppression is transient and decays rapidly under sustained light exposure.
• A combined melanopsin and S-cone model provides a more accurate representation of acute non-visual spectral sensitivity.