Patterns of Pineal Melatonin Secretion in Rabbits: Diurnal Variation of Basal and Pulsatile Release

Abstract

The level and release pattern of plasma melatonin in the confluens sinuum of 28 sighted and 18 blinded (i.e. acute bilateral orbital enucleation) rabbits anesthetized with pentobarbital were studied. The animals had been adapted to a 12:12 h light-dark regime. Blood samples were collected from the cannulated confluens sinuum and/or the femoral artery at either 2- or 4-min intervals in both the light and dark phases. Plasma melatonin was determined by radioimmunoassay. In all rabbits studied, plasma melatonin in the confluens sinuum exhibited an episodic release pattern, with pulses superimposed on a basal level. At 4-min sampling intervals, the pulsatile release of melatonin in sighted rabbits were 3.8 peaks/h in the light phase; shorter sampling interval (2-min) revealed more frequent pulsatile release of melatonin (5.1 peaks/h). In sighted animals in the light phase, the level of melatonin in the plasma of confluens sinuum was 7–15 times higher than that obtained from the plasma collected at the same time from the peripheral artery where the level of melatonin also exhibited pulsatile pattern. In blinded animals, melatonin levels in terms of mean concentration, mean maximum level and mean minimum (or baseline) level obtained in the dark phase were 12–14 times higher than those obtained in the light phase. These results suggest that the level of melatonin exhibits a diurnal rhythm in the confluens sinuum of rabbits.

Rhythmic Release Pattern of Pineal Melatonin in Rodents

“…With 4-min sampling intervals, the plasma melatonin in the confluens si nuum showed a pulse rate of 3.8 peaks/h in the daytime. The pulse rates of plasma melatonin found in the confluens sinuum and that in the femoral artery were similar [Chan et al, 1989a[Chan et al, , 1990, In rats, the plasma melatonin concentration found in the confluens sinuum of rats was 5-8 times that of the peripheral circulation [Withyachumnamlcul and Knigge, 1980;Pang, 1985], while the rate of episodic release of melatonin revealed at 4-min blood sampling interval was found to be 2.9 peaks/h in the daytime [Pang and Yip, 1988]. Comparable pulse rates of melatonin release was also observed in sheep [Cozzi et al, 1988] and chickens [Pang et al, 1990b] using 4-and 5-min blood sampling intervals, respectively.…”

“…With the aid of a cannula inserted through the sa gittal sinus, a method was recently developed by which blood can be collected directly from the confluens sinuum of anesthe tized rabbits and rats for 1-7 h Pang et al, 1985;Pang and Yip, 1988;Chan et al, 1989aChan et al, , 1990. Such a system enables one to investigate the secretory pattern of pineal melatonin into the blood and the regulating parameters for pi neal melatonin secretion.…”

“…It should also be noted that cerebral blood flow in rats [Goldman and Sapirstein, 1973] was found to be reduced by pentobarbital. Since the surgical procedures and blood collection processes were per formed in rats and rabbits under pentobarbital anesthesia Pang et al, 1985;Pang and Yip, 1988;Chan et al, 1989aChan et al, , 1990, the possible influence of this on pi neal melatonin secretion has to be considered.…”

“…In pentobarbital-anesthetized rabbits and rats, the concen tration of plasma melatonin in the confluens sinuum exhibited an episodic fluctuation, with pulses superimposed on a basal level Pang et al, 1985;Pang and Yp, 1988;Chan et al, 1989aChan et al, , 1990. Similar short-term fluctuation of melatonin levels was also observed in the jugular veins or confluens sinuum of sheep [English et al, 1987;Cozzi et al, Time (min) 1988] and heifers [Critser et al, 1987], In sighted rabbits, the daytime level of plasma melatonin in the confluens sinuum (90.2 pg/ml) was 11-fold of that found in the plasma collected at the same time from the femoral artery (7.9 pg/ml) where the level of melatonin also exhibited pulsatile pattern.…”

“…levels are high at night and low during the day. Superimposed on the diurnal rhythm, the level of plasma melatonin in the pe ripheral circulation of rabbits Chan et al, 1989aChan et al, , 1990, sheep [English et al, 1987;Cozzi et al, 1988], heifers [Critser et al, 1987] and humans [Vaughan et al, 1979;Weinberg et al, 1979;Penny, 1985;Claustrat et al, 1986] was reported to exhibit episodic fluctuations. However, in many of these studies in humans, the mean values of daytime blood mel atonin levels were found to be > 50 pg/ml of plasma or serum.…”