Day-night variations in plasma melatonin and arginine vasotocin concentrations in chronically cannulated flounder (Platichthys flesus)

Chronically catheterised, free swimming flounder (Platichthys flesus) have been used in experiments examining the day-night variations in circulating levels of melatonin (Mel) and arginine vasotocin (AVT). Under normal photop eriod (16 h light/8 h dark) serial blood samples taken from individual fish demonstrated a Mel rhythm with daytime levels at 09.00 and 15.00 h (238 +/ - 14 and 179 +/- 12 fmol (.) ml(-1), respectively) lower than those at 23.0 0 h (1920 +/- 128 fmol (.) ml(-1)). Maintenance of fish in 24-h light aboli shed the light/dark Mel rhythm and circulating levels were comparable to th ose measured during the day in fish under normal photoperiod illumination. In fish maintained under 24 h dark, although a daily rhythm was still appar ent, at the time when it would be normally dark, plasma Mel concentration w as reduced and at times when it would be normally light, levels were higher than in fish maintained under normal light/dark illumination. Plasma AVT c oncentrations were higher in fish during the day (4.4 +/- 0.8 fmol (.) ml(- 1)) than those at night (1.5 +/- 0.4 fmol (.) ml(-1)), the opposite to that seen with Met. During acute study infusion of AVT resulted in reduced leve ls of plasma Mel, although this did not achieve statistical significance. I nfusion of Mel did not alter circulating AVT concentration. (C) 2001 Elsevi er Science Inc. All rights reserved.