Diurnal variations in rat posterior pituitary catecholamine levels.

Diurnal variations, ranging over a twofold difference in concentrations of dopamine, norepinephrine and epinephrine, occur in the rat posterior pituitary gland during a 24-hour period. Highest catecholamine concentrations were seen during daylight hours and low levels reached a nadir 5 h after the onset of darkness. Dopamine concentrations rapidly returned to high levels at the onset of the daylight period, whilst the increase in norepinephrine and epinephrine concentrations was delayed. These diurnal variations in catecholamine content may reflect their modulation of neural lobe peptide secretion.