P. C. Y. Yip

Bio: P. C. Y. Yip is an academic researcher from University of Hong Kong. The author has contributed to research in topics: Melatonin & Pineal gland. The author has an hindex of 1, co-authored 1 publications receiving 22 citations.

Secretory patterns of pineal melatonin in the rat.

Abstract: 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.

The effect of food deprivation on brain and gastrointestinal tissue levels of tryptophan, serotonin, 5-hydroxyindoleacetic acid, and melatonin.

Abstract: In order to investigate the effect of food deprivation on the levels of indoles in the brain and the gastrointestinal tissues, we have determined tissue levels of tryptophan (TRP), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), and melatonin in the brain and the gastrointestinal tract (GIT) of mice on ad libitum diet as well as in mice deprived of food for 24 and 48 hr. The reduction of food intake 1) had no effect on TRP levels in the brain, but increased TRP concentrations in the stomach and the gut, especially in the colon; 2) decreased 5-HT levels in the brain, but increased values in the stomach and the intestines; 3) decreased 5-HIAA levels in the brain, but increased them in the stomach and the intestines; 4) did not change 5-HT conversion to 5-HIAA in the brain, stomach, and the jejunum, but increased the conversion in the ileum and colon and; 5) increased melatonin levels in all tissues investigated, particularly in the stomach and the brain. The changes of indole levels induced by food deprivation were compared to their known function in the brain and the individual segments of the GIT. A possible serotonin-melatonin antagonism in the brain and GIT function is considered.

Fluctuation of blood melatonin concentrations with age: result of changes in pineal melatonin secretion, body growth, and aging

Abstract: 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 greater than 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 greater than 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.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuroendocrine effects of light.

Abstract: The light/dark cycle to which animals, and possibly humans, are exposed has a major impact on their physiology. The mechanisms whereby specific tissues respond to the light/dark cycle involve the pineal hormone melatonin. The pineal gland, an end organ of the visual system in mammals, produces the hormone melatonin only at night, at which time it is released into the blood. The duration of elevated nightly melatonin provides every tissue with information about the time of day and time of year (in animals that are kept under naturally changing photoperiods). Besides its release in a circadian mode, melatonin is also discharged in a pulsatile manner; the physiological significance, if any, of pulsatile melatonin release remains unknown. The exposure of animals including man to light at night rapidly depresses pineal melatonin synthesis and, therefore, blood melatonin levels drop precipitously. The brightness of light at night required to depress melatonin production is highly species specific. In general, the pineal gland of nocturnally active mammals, which possess rod-dominated retinas, is more sensitive to inhibition by light than is the pineal gland of diurnally active animals (with cone-dominated retinas). Because of the ability of the light/dark cycle to determine melatonin production, the photoperiod is capable of influencing the function of a variety of endocrine and non-endocrine organs. Indeed, melatonin is a ubiquitously acting pineal hormone with its effects on the neuroendocrine system having been most thoroughly investigated. Thus, in nonhuman photoperiodic mammals melatonin regulates seasonal reproduction; in humans also, the indole has been implicated in the control of reproductive physiology.

Effect of continuous melatonin infusions on steady-state plasma melatonin levels in rats under near physiological conditions

Abstract: It was the aim of this study to measure the actual amount of melatonin required for elevating the circulating hormone from low daytime levels to the 10-fold higher nocturnal steady-state concentrations in rats. For this purpose, escalating doses of melatonin were continuously infused into the right jugular vein and blood samples were repeatedly drawn from the left jugular vein for a period of 2 hr in freely moving catheterized rats. In order to achieve an about 10-fold elevation of the plasma melatonin concentration, 500 ng melatonin/hr had to be infused, i.e., about 300 times the normal nocturnal melatonin content of the pineal. Infusions of up to 61 ng melatonin/hr (equivalent to the melatonin content of 40 pineals at darkness) failed to cause a significant rise of the low daytime steady-state concentrations in the blood. If the dose of 500 ng melatonin/h was infused at night, a less-pronounced rise of the blood levels was observed, as compared to that caused by the infusion of the same dose during daytime. No differences were found in the rate of metabolism between daytime and nighttime. The results of this study indicate 1) that the low basal concentrations of melatonin in the blood are not affected by an increased melatonin supply up to a certain critical threshold, 2) that the rat pineal gland would have to release all its melatonin content almost every 10 sec in order to sustain the elevated steady-state level of melatonin in the circulation during the dark period, and 3) that significant day/night differences exist in the disposition of circulating melatonin if administered in near physiological amounts and under near physiological conditions.