Circadian organization in quail retina:: Differential regulation of melatonin synthesis and iodopsin gene expression in vitro

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.