Daily cycles in plasma melatonin levels under long or short photoperiod in the common carp, Cyprinus carpio.

A radioimmunoassay (RIA) for plasma melatonin (MLT) was simplified for use with the common carp (Cyprinus carpio). Plasma was partially purified with Sep-Pak C18 cartridge before RIA. The inhibition curves for the Sep-Pak C18 fraction from the plasma of carp, goldfish (Carassius auratus), yellow tail (Seriola quinqueradiata), Japanese eel (Anguilla japonica), bora (Mugil cephalus cephalus), kisu (Sillago japonica), ishigarei (Kareius bicoloratus), and shimaisaki (Rhyncopelates oxyrhynchus) were parallel with the MLT standard curve. There was a highly significant correlation between MLT added to carp plasma and that which was recovered (r = 0.997, P less than 0.01). Intraassay coefficients of variation at low, medium, and high levels were 4.7, 5.2, and 6.4, respectively. Interassay coefficients of variation at low, medium, and high levels were 11.8, 8.2, and 24.1%, respectively. The lower limit of detection was 11 pg/tube. Plasma MLT levels were investigated every 2 hr in carp under 16L-8D and 8L-16D at 24 degrees. Under both photoperiods, MLT levels showed marked daily patterns, i.e., the levels were high (220-540 pg/ml) during the dark phase and low (23-104 pg/ml) during the light phase. Tricaine methanesulfonate markedly interfered with the MLT measurements, while ethylcarbamate did not show any significant influence. Sexual difference in plasma MLT levels in carp was not observed. These observations suggest that MLT is an important hormone in photoperiodism and/or a circadian rhythm in fish.