Melatonin synthesis in retinal photoreceptors is
stimulated at night by a circadian oscillator and suppressed
acutely by light. To identify photoreceptor mechanisms
involved in the acute suppression of melatonin synthesis,
an action spectrum was measured for dark-adapted Xenopus
laevis eyecups at night. Intensity–response
curves at six wavelengths from 400 to 650 nm were parallel,
suggesting that a single photopigment predominates in melatonin
suppression. Half-saturating intensities at 400, 440, 480,
and 533 nm were not significantly different from one another,
at 1–2 × 108 quanta cm−2
s−1. Significantly higher intensities
of 580- and 650-nm light were required for melatonin suppression.
These results indicate a predominant role for the principal
green-absorbing rods in acute regulation of retinal melatonin
synthesis in response to light, and argue against an important
role for the red-absorbing cones. Higher than expected
sensitivity at short wavelengths suggests that photoreceptors
sensitive to blue and/or violet light may also contribute
to melatonin suppression.