Circadian variations of melatonin binding sites in the goldfish brain

Abstract

Daily and circadian variations in melatonin binding sites in the brain of goldfish ( Carassius auratus) were examined by radioreceptor assay. Under light–dark cycles of 12:12 h, the density ( B max) of melatonin binding sites in the brain exhibited daily variations with a peak and a trough seen around the light offset and 2 h before light onset, respectively. The affinity ( K d) exhibited no variation. The rhythm in the B max persisted even under constant darkness with higher levels during the late subjective-day. These results indicate that the density of melatonin binding sites in the goldfish brain is regulated by the circadian clock.