Circadian variations of melatonin binding sites in the goldfish brain

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. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.