Route of melatonin secretion in sheep.

To determine the proportion of the total melatonin output which is secreted directly into the third ventricle of sheep, samples of blood from the jugular vein and of cerebrospinal fluid (CSF) from the cisterna magna were obtained during 128-min intervals on three consecutive nights from four ewes. Blood was withdrawn continuously at a rate of 3 ml/min and pooled into 1-min fractions. Simultaneously with the collection of blood, 0.5-ml samples of CSF were obtained from the cisterna magna every 4 min. Melatonin concentrations were quantified in all samples by radioimmunoassay. During the sampling interval of the first night, lights were left on for 32 min, turned off for 64 min, and then turned back on for an additional 32 min. These samples were utilized to obtain the concentration profiles of melatonin in jugular blood and cisternal CSF during an endogenous surge of melatonin secretion. Plasma concentrations of melatonin remained between 10 and 100 pg/ml in two Corriedale crossbred ewes and between 200 an...