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Notes: Melatonin in the systemic circulation of rats fluctuates with age, and the causes for such changes were investigated. Male rats (aged 7 days, 16 days, 18 days, 20 days, 30 days, 48 days, 60 days, and 〉 17 months) were adapted under a lighting regime of 12L: 12D for at least 7 days. Pineals and blood samples from the trunk or confluens sinuum were collected in the dark period. Melatonin in tissues was extracted, identified, and determined by gas chromatography-mass spectrometry (GC-MS) and/or radioimmunoassay. Tissue melatonin levels obtained by radioimmunoassay correlated closely with those quantified by GC-MS. Thus, the melatonin radioimmunoassay used is a reliable assay method for melatonin in the plasma and pineal of the rat.Plasma melatonin in the confluens sinuum of rats exhibited episodic release superimposed on a basal release pattern. It was suggested that there are two pools of melatonin in the pineal gland, a readily releasable pool and a bound pool. The mean plasma levels of melatonin in the confluens sinuum of rats increased with age with the highest level recorded at 60 days old and declined to a lower level at 〉 17 months old. The above age-related changes, being similar to the alterations in pineal melatonin levels with growth and aging, suggest that, under our experimental conditions, levels of pineal melatonin increase or decrease with its secretory rate.In developing rats, the age-related increase in the rate of secretion of pineal melatonin as reflected by increases in melatonin levels in the confluens sinuum or pineal melatonin content before adulthood is different from the changes in melatonin levels in the systemic circulation which showed an early developmental rise, followed by an active period and then a prepubertal decline. However, when the body weight was taken into consideration, changes in the levels of pineal melatonin content per 100 gm body weight or the calculated blood melatonin levels (plasma melatonin in the confluens sinuum/body:head ratio) correlated well with the fluctuation of serum melatonin in the systemic circulation. Thus, the developmental changes in the concentrations of melatonin in the general circulation are the result of 1) changes in the rate of pineal melatonin secretion and 2) increase in the dilution factor because of increase in body size. In old rats, levels of plasma melatonin in the confluens sinuum and pineal melatonin content decreased indicating a decline in the rate of pineal melatonin secretion. These results suggest that the reduced level of systemic melatonin in aged rats is the result of a decline in the rate of pineal melatonin secretion plus an increase in the body weight.

Notes: The release patterns of pineal melatonin were studied by continuously monitoring melatonin levels in the confluens sinuum plasma in sighted and bilaterally enucleated rats in the light and dark periods. Plasma melatonin was determined by radioimmunoassay, and the data were analyzed by a computerized algorithm developed in our laboratory. Pulsations of melatonin levels were found in the confluens sinuum plasma in all the animals studied, suggesting episodic secretion of pineal melatonin in rats. Because the minimum melatonin levels in the confluens sinuum were over three times the melatonin levels in the general circulation, it is postulated that 1) there is an episodic release pattern of pineal melatonin superimposed on a basal release pattern and 2) there are two pools of melatonin in the pineal gland, a readily releasible pool responsible for the basal release and a bound pool responsible for the pulsatile release. In the sighted rats, there was no diurnal difference in mean melatonin concentration, mean pulse amplitude, mean pulse rate, mean minimum melatonin level, and mean maximum melatonin level in the confluens sinuum. In the bilaterally enucleated rats, with the exception of the mean pulse amplitude, diurnal rhythms were demonstrated in all the other parameters studied with, higher values in the dark period. This experimental model should be employed in future investigations on the regulation of secretory patterns of pineal melatonin. Results of these studies may provide important insight into the regulation of pulsatile release of neuroendocrine secretions in general.