Abstract
BackgroundIt is well established that the absolute sensitivity of the suprachiasmatic nucleus to photic stimulation received through the retino-hypothalamic tract changes throughout the 24-hour day. It is also believed that a combination of classical photoreceptors (rods and cones) and melanopsin-containing retinal ganglion cells participate in circadian phototransduction, with a spectral sensitivity peaking between 440 and 500 nm. It is still unknown, however, whether the spectral sensitivity of the circadian system also changes throughout the solar day. Reported here is a new study that was designed to determine whether the spectral sensitivity of the circadian retinal phototransduction mechanism, measured through melatonin suppression and iris constriction, varies at night.MethodsHuman adult males were exposed to a high-pressure mercury lamp [450 lux (170 μW/cm2) at the cornea] and an array of blue light emitting diodes [18 lux (29 μW/cm2) at the cornea] during two nighttime experimental sessions. Both melatonin suppression and iris constriction were measured during and after a one-hour light exposure just after midnight and just before dawn.ResultsAn increase in the percentage of melatonin suppression and an increase in pupil constriction for the mercury source relative to the blue light source at night were found, suggesting a temporal change in the contribution of photoreceptor mechanisms leading to melatonin suppression and, possibly, iris constriction by light in humans.ConclusionThe preliminary data presented here suggest a change in the spectral sensitivity of circadian phototransduction mechanisms at two different times of the night. These findings are hypothesized to be the result of a change in the sensitivity of the melanopsin-expressing retinal ganglion cells to light during the night.
Highlights
It is well established that the absolute sensitivity of the suprachiasmatic nucleus (SCN) to photic stimulation received through the retino-hypothalamic tract (RHT) changes along the 24-hour day [1,2,3,4]
The results suggested that the spectral sensitivity of melatonin suppression may change during the night, because the relative contribution of the candidate photopigments to best fit the suppression data seemed to systematically change during the night
Post-hoc statistical tests were conducted to determine whether there was a significant change in melatonin suppression from session 1 to session 2 for each lighting condition, light emitting diodes (LEDs) and high-pressure mercury (Hg) (Figure 4)
Summary
It is well established that the absolute sensitivity of the suprachiasmatic nucleus to photic stimulation received through the retino-hypothalamic tract changes throughout the 24-hour day. It is believed that a combination of classical photoreceptors (rods and cones) and melanopsin-containing retinal ganglion cells participate in circadian phototransduction, with a spectral sensitivity peaking between 440 and 500 nm. Changes in photoperiod affected the two SCN peak activity periods differently, demonstrating that the phases of the two peaks are not locked but are independently linked to the environmental cycle of dusk and dawn. They showed that the two peaks responded differently to a pulse of glutamate (the neurotransmitter that conveys light information from the eye to the SCN). Unknown is whether there is temporal variation in the sensitivity of the circadian phototransduction mechanism itself throughout the 24-hour cycle
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