Adult Japanese quail have an endogenous circadian
clock located in their eyes that has been shown to regulate
melatonin biosynthesis. We investigated if a circadian
oscillator is present in cultures of dispersed embryonic
quail retina. Melatonin release in retinal cell culture
is modulated by the light cycle, indicating that there
are functional photoreceptors in culture. However, when
cultures were placed in constant darkness no rhythm of
melatonin was observed, indicating that at this period
of development the circadian oscillator does not influence
melatonin release. To explore further the question of whether
a circadian oscillator is present in embryonic cell culture,
we examined expression of iodopsin, the red visual pigment.
Iodopsin mRNA is expressed in a circadian rhythm with peak
levels occurring late in the afternoon (ZT 9). Analysis
indicates that the clock influence is at the level of gene
transcription. These results suggest that a clock is not
“hooked up” to melatonin release embryonically
or that a different oscillator regulates photopigment expression
versus melatonin release.