The Developmental and Circadian Variation of Melatonin Receptors in the Chicken Spinal Cord

The developmental variation of 2-[125I]iodomelatonin binding sites in membrane preparations of the chicken spinal cords was investigated. In 19-day-old embryos, the maximum number of binding sites (Bmax) was the highest during the development. With the increase in age posthatch, the Bmax decreased gradually and significantly from 1-day-old to 4-month-old chickens. However, there were no significant changes in equilibrium dissociation constant (Kd) among all age-groups studied. To study the circadian variation, the 2-[125I]iodomelatonin binding sites in the spinal cords of 3-week-old chickens were determined at 4-hour intervals throughout a 24-hour period under a 12-hour light/12-hour dark cycle. There was a diurnal rhythm in both Bmax and Kd of [125I]iodomelatonin binding sites in which Bmax decreased and Kd increased in the dark period. The characteristics of developmental and circadian variation of melatonin receptors in the chicken spinal cord suggest that melatonin may exert a direct action on spinal cord functions and that it plays a more important role in the young animal.