Comparison of visual sensitivity for suppression of pineal melatonin and circadian phase-shifting in the golden hamster

Visual sensitivity for light suppression of pineal melatonin was measured in golden hamsters using 300 s stimuli of monochromatic light (503 nm) in constant darkness. Increasing stimulus irradiance caused a monotonic decrease in pineal-melatonin content. Irradiance greater than 3.5 × 10¹⁰ photons cm^-2·s^-1 caused significant reductions of melatonin in the hamster pineal. Saturation of the response occurred above 10¹¹ photons cm^-2·s^-1 and melatonin levels were suppressed to approximately 7% of levels measured in unstimulated animals. Using a 4-parameter Naka-Rushton function to fit the data, the half-saturation constant for suppression of pineal melatonin was 1.3 × 10¹⁰ photons cm^-2· s^-1. The sensitivity of the photic-entrainment pathway for circadian rhythms has also been measured in the hamster using identical stimulus parameters. Light suppression of pineal melatonin was 25 times (1.4 log units) more sensitive to irradiance than the phase-shifting response measured for the circadian rhythm of running-wheel activity (comparing the half-saturation constants for the two responses). Both of these responses, however, are much less sensitive to light than other visual responses measured behaviorally in the golden hamster.