Circadian variations of catecholamine levels in brain, heart, and plasma in the eel, Anguilla anguilla L., at three different times of year.

The amounts of catecholamine (dopamine, adrenaline, and noradrenaline) in the brain, heart, and plasma of the eel (Anguilla anguilla L.) during a 24-hr period and at three different times of year were determined by using a radioenzymatic method. Seasonal variations of catecholamine average values were found to be different when considering catecholamine levels in the same tissue (one exception: heart levels in May) or the same amine in different tissues. Circadian rhythms of catecholamine levels were evident only in the brain; the maximum amount generally occurred during the light phase. No correlation could be found between the 24-hr variations in the different tissues. The most important variations were phased with the dark-light cycle but were also dependent on the annual cycle.