Diurnal rhythms in hypothalamic/pituitary AVT synthesis and secretion in rainbow trout: Evidence for a circadian regulation

► Trout plasma and pituitary AVT levels display diurnal ryhthms under normal LD cycle. ► Peak values were found in the last time of the light phase. ► Those rhythms persist for two consecutive days in constant darkness. ► Hypothalamic proVT mRNa was also rhythmically expressed in both LD and DD situations. ► Endogenous AVT synthesis rhythm might have a role in fish adaptation to cyclic changes.