PHOTIC REGULATION OF MELATONIN IN HUMANS - OCULAR AND NEURAL SIGNAL-TRANSDUCTION

Light is a potent stimulus for regulating the pineal gland's productio n of melatonin and the broader circadian system in humans. It initiall y was thought that only very bright photic stimuli (greater than or eq ual to 2500 lux) could suppress nocturnal melatonin secretion and indu ce other circadian responses. It is now known that markedly lower illu minances (less than or equal to 200 lux) can acutely suppress melatoni n or entrain and phase shift melatonin rhythms when exposure condition s are optimized. The elements for physical/biological stimulus process ing that regulate photic influences on melatonin secretion include the physics of the light source, gaze behavior relative to the light sour ce, and the transduction of light energy through the pupil and ocular media. Elements for sensory/neural signal processing become involved a s photons are absorbed by retinal photopigments and neural signals are generated in the retinohypothalamic tract. Aspects of this physiology include the ability of the circadian system to integrate photic stimu li spatially and temporally as well as the wavelength sensitivity of t he operative photoreceptors. Acute, light-induced suppression of melat onin is proving to be a powerful tool for clarifying how these element s of ocular and neural physiology influence the interaction between li ght and the secretion of melatonin from the human pineal gland.