Use this URL to cite or link to this record in EThOS:
Title: The spectral sensitivity of light-induced melatonin suppression in humans
Author: Thapan, Kavita
ISNI:       0000 0001 3516 6586
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2001
Availability of Full Text:
Access from EThOS:
Access from Institution:
The eye is a complex organ involved in the input pathway of circadian entrainment. Non-image forming, irradiance-dependent responses mediated by the human eye include synchronisation of the circadian axis and suppression of pineal melatonin production. Light exposure at night causes an acute suppression of melatonin levels. In humans the photoreceptors mediating this and other circadian responses to light are not known. Identification of a photopigment requires an investigation of the spectral sensitivity (action spectrum) of the light-dependent response. The aim of this thesis was to construct an action spectrum for light-induced melatonin suppression. Melatonin suppression was quantified in 22 volunteers in 215 light exposure trials using monochromatic light (30 minute pulse administered at circadian time (CT) 16-18) of different wavelengths (max 424, 456, 472, 496, 520 and 548 nm) and irradiances (0.7-65.0 μW/cm2). At each wavelength, suppression of plasma melatonin increased with increasing irradiance. Irradiance response curves (IRCs) were fitted and the generated half-maximal responses (IR50) were corrected for lens filtering and used to construct an action spectrum. The resulting action spectrum showed unique short-wavelength sensitivity very different to the classical scotopic and photopic visual systems. The lack of fit (R2 < 0.1) of the action spectrum with the published rod and cone absorption spectra precluded these photoreceptors from having a major role. Cryptochromes 1 and 2 also had a poor fit to the data. Fitting a series of Dartnall nomograms generated for rhodopsin-based photopigments showed that rhodopsin templates between max 457-462 nm fit the lens corrected data well (R2 > 0.73). Of these, the best fit was to the rhodopsin template with max 459 nm (R2 = 0.74). The findings demonstrate that short wavelength light is the most effective at suppressing melatonin and this sensitivity is clearly not matched to the spectral sensitivities of the known retinal photoreceptors that mediate human vision. As well as providing evidence for a novel photoreceptor system in humans, these data will allow optimisation of the spectral composition of light needed to manipulate the circadian axis.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Eye; Vision; Circadian system; Action spectrum