Effect of photoperiod on LH, FSH, prolactin and melatonin patterns in ovariectomized prepubertal heifers

Rhythmic Release Pattern of Pineal Melatonin in Rodents

“…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. 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 detectable basal lev els of melatonin in the plasma of jugular vein or confluens si nuum support the contention of the presence of a readily re leasable pool in the pineal gland responsible for the basal re lease of melatonin once it is synthesized within the pineal gland. However, it has been shown in conscious heifers that the nighttime melatonin level in the jugular blood did approach zero level [Critser et al, 1987], while in conscious ewes the day time melatonin level was undetectable in both the cerebral sinus [Cozzi et al, 1988] and the jugular vein [English et al, 1987;Cozzi et al, 1988]. Reasons for the presence or absence of a basal melatonin level in blood await elucidation.…”

“…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.…”

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

“…Our results further indicate that in both the light and dark phases within the 24-hour period the level of melatonin in the confluens sinuum never reached an undetectable level. Similar findings were observed in the jugular blood of conscious ewes in the nighttime (with 10-min sam pling intervals) [22], In conscious heifers, however, 15-min sam pling intervals revealed that on several occasions the nighttime melatonin level in the jugular blood approached zero level [23], It should also be noted that the daytime melatonin levels in some conscious ewes may become so low that they are unde tectable in both the cerebral sinus [12] and jugular vein [12,22]. These low levels of blood melatonin argue against the presence of basal release of pineal melatonin (i.e.…”

Plasma Profiles of Growth Hormone, Prolactin and Insulin-like Growth Factor-I during Gestation, Lactation and the Neonatal Period in Goats.

“…In these published observations, however, blood samples were usually collected for only a few weeks around parturition, and plasma profiles of GH and PRL have never been examined throughout gestation. Moreover, the blood samples were collected only about once a day, although GH and PRL are known to be secreted in transient pulses [3,[10][11][12][13][14]. PRL is required for the initiation of lactation [15,16]; therefore, it is of interest to determine how the pulsatile secretory patterns of both GH and PRL change throughout gestation and during the lactation period in animals.…”