Prolactin response in Border-Leicester x merino ewes to administration of melatonin, melatonin analogues, a melatonin metabolite and 6-methoxybenzoxazolinone.

The effect of structural modifications of the melatonin molecule on plasma half-life of the analogues and basal prolactin secretion was studied in Border-Leicester x Merino ewes. Halogenation at position 6 and/or unsaturation of the 2,3-double bond of the melatonin molecule slightly lengthened the half-life of the analogues. Melatonin, 6-chloromelatonin, 2,3-dihydromelatonin and 6-chloro-2,3-dihydromelatonin decreased plasma prolactin to 31, 45, 54 and 48% of control levels respectively when administered daily (100 micrograms at 1600 h) for 21 days. The brain metabolite of melatonin, N-acetyl-N'-formyl-5-methoxykynurenamine, and the putative natural melatonin analogue, 6-methoxybenzoxazolinone, failed to affect prolactin levels when administered in a similar manner. These results indicate that certain structural modifications to the melatonin molecule can be tolerated biologically; however, the modifications reported here still did not prevent rapid clearance from the circulation.

Pinealectomy delays puberty in ewe lambs.

Fifteen pinealectomized and 15 unoperated ewes were exposed to constant light for 3 weeks before and 10 weeks after lambing. Fourteen pinealectomized and 15 unoperated ewes were allowed to lamb outdoors. Five ewe lambs born in constant light to the 2 groups of dams were pinealectomized at 10 weeks of age. Ewes and lambs were then returned to the field. Puberty (determined by weekly progesterone analysis) was significantly delayed (P less than 0.05) in the pinealectomized ewe lambs. Median pubertal age in pineal-intact ewe lambs was 37 weeks compared to 49 weeks in pinealectomized lambs. Constant light during the first 10 weeks of life had no effect upon puberty onset nor did the pineal status of the dam. Control lambs entered seasonal anoestrus at the time pinealectomized ewe lambs were entering puberty. Pinealectomized lambs entered anoestrus at the same time as control lambs were beginning their second breeding season. These results confirm a key role of pineal-mediated hormonal signals in the control of puberty in the sheep.

Effects of pinealectomy, oestradiol and melatonin on plasma prolactin and LH secretion in ovariectomized sheep.

The study examines whether the pineal gland mediates in the seasonal rhythm in the response of the hypothalamo-pituitary axis to oestrogen in the sheep. Five groups of ewes, each comprised of five sham-pinealectomized ewes and five pinealectomized ewes, were maintained under field conditions over a 2-year period. Group I ewes were ovariectomized and treated with two oestradiol capsules in the first year and one oestradiol capsule in the second year; group 2 ewes were ovariectomized and treated with two oestradiol capsules and three melatonin sachets in the first year and one oestradiol capsule and three melatonin sachets in the second year; group 3 ewes were ovariectomized and treated with three melatonin sachets in the first year and one oestradiol capsule in the second year; group 4 ewes were ovariectomized and treated with empty implants; group 5 had intact ovaries. Blood samples were taken at weekly intervals for determination of plasma prolactin, LH and FSH levels. Cyclic ovarian activity, determined by rams fitted with a marking harness, occurred in both sham-pinealectomized and pinealectomized ewes in group 5 during December to May in both years. A circannual rhythm in plasma prolactin levels was apparent in all ten groups of animals. The levels were highest in spring (September to November) and decreased before the onset of breeding activity in midsummer (December). Ovariectomized ewes with intact pineal glands and treated with oestradiol capsules (group 1) exhibited marked changes in LH and FSH concentrations during the study.(ABSTRACT TRUNCATED AT 250 WORDS)

A definition of the breeding season of Pool Dorset ewes.

The seasonality of oestrus and ovulation of 62 Poll Dorset ewes was examined over a period of 16 months commencing October, 1981. Laparoscopic observations showed that the majority of these ewes were anovular during the South Australian spring and early summer (September to December). The number of ewes with corpora lutea (93.6%) and the mean ovulation rate (1.62) reached a peak in June. Oestrus was stimulated in anovular ewes in late spring by the introduction of vasectomised rams. The mean ovulation rate at the teased ovulation was 1.68, although teased ewes did not exhibit oestrus at this ovulation.

Ultradian and seasonal rhythms in plasma gonadotropins, prolactin, cortisol, and testosterone in pinealectomized rams.

Plasma concentrations of PRL, LH, cortisol, FSH, and testosterone were determined in pinealectomized (Px) and control rams at four times during a year. Basal levels of PRL decreased between spring and winter in controls from 44 +/- 2 ng/ml (mean +/- SE) to 16 +/- 3 ng/ml, while in Px rams, a different biphasic pattern was observed (56 +/- 9 ng/ml in the spring; 12 +/- 3 ng/l in the summer, 45 +/- 8 ng/ml in the autumn, and 25 +/- 5 ng/ml in the winter). There were no other significant seasonal differences or treatment effects in PRL episodes frequency or peak height, nor did pinealectomy alter the ultradian rhythm. The control rams had a significant seasonal difference in LH peak frequency, being higher in summer than at any other time. The Px rams had no seasonal changes in this or any other parameter of LH secretion. There were no significant differences between Px and control rams in any aspect of cortisol secretion and no seasonal trends. No seasonal or treatment effects were observed in plasma testosterone levels. Plasma FSH tended to be highest in controls in autumn, but this peak was not observed in Px rams. These results indicate that the pineal gland of sheep is involved in some aspects of seasonal breeding, possibly synchronizing hormone secretion with environmental changes. (Endocrinology 108: 639, 1981)