Abstract
A circadian oscillator in the chicken pineal cell regulates the diurnal oscillation of melatonin production. This circadian rhythm synchronizes to the environmental dark-light cycle through the intracellular photon-signal transduction machinery. One approach to the molecular events in the pineal circadian oscillator is to follow this photic input pathway toward the oscillator. To this end we identified a chicken pineal opsin, which binds 11-cis-retinal resulting in formation of a blue-light sensitive pigment with an absorption maximum at 470 nm. This photoreceptive molecule was named pinopsin. Immunohistochemical examination revealed the localization of pinopsin in the luminal membranes of all the follicles and in cilia-like structures and enlarged cilia with concentric lamellar arrays in the parafollicular zone. All the immunoreactive structures seemed to represent degenerated outer segments of photosensitive pinealocytes in the gland. The primary structure of cytoplasmic loops of pinopsin was similar to that of rhodopsin, raising a possibility that transducin (Gtαβγ) or Gt-like G-protein is involved in the pineal light-signal transduction process. We were able to clone a chicken pineal cDNA encoding the α-subunit of rod-type transducin (Gtlα), but the deduced amino acid sequence of the pineal Gtlα had a putative ADP-ribosylation site for pertussis toxin. Since the photic input pathway in the chicken pineal cell is insensitive to the toxin, it seems unlikely that this rod-type transducin mediates the pathway. Chicken pineal transducin may mediate the light-dependent acute inhibition of melatonin production.
