Melanopsin expression in the chick retina and pineal gland

The chick retina and pineal gland exhibit circadian oscillations in biochemical and physiological processes in vivo and in vitro and entrain to 24 h LD cycles. However, the phototransductive mechanisms responsible for entrainment are largely unknown. Experiments utilizing mice lacking functional rods and cones reveal normal circadian responses to light. These studies also implicate a subset of photoresponsive cells in the ganglion cell layer (GCL) which express melanopsin (opn4). The current study investigates opn4 mRNA distribution and regulation in the chick. Opn4 mRNA is widespread in the central nervous system and peripheral tissues, with highest levels of transcript abundance in the pineal gland and retina. Opn4 mRNA is also regulated on a circadian basis in the pineal gland, reaching peak values of accumulation in the late subjective night. In the retina, opn4 is not robustly regulated at the transcriptional level, exhibiting a slight increase in mRNA abundance during subjective night. In situ hybridization (ISH) revealed opn4 mRNA in areas associated with phototransduction and the visual system, including the pineal gland and optic tectum of the brain, along with GCL, inner nuclear layer (INL), and photoreceptor layer (PL) of the retina. Opn4 is also present in cerebellum. Our results present an opsin-like gene located in two circadian oscillators associated with circadian phototransduction and melatonin biosynthesis that may play a role in entrainment of these tissues' clocks.