Endocrine changes associated with luteal regression in the ewe; the secretion of ovarian oestradiol, progesterone and androstenedione and uterine prostaglandin F2alpha throughout the oestrous cycle.

The concentrations of oestradiol, androstenedione, progesterone and prostaglandin F2alpha (PGF2alpha) were measured in utero-ovarian autotransplants. The secretion of oestradiol was closely correlated with that of androstenedione (r = 0-67, P less than 0-001) indicating a common origin from the Graafian follicle. The concentration of these two steroids fluctuated at random throughout the luteal phase with the maximum secretion occurring about 2 days before the onset of oestrus. Functional regression of the corpus luteum, as indicated by a fall in the secretion of progesterone, began on day 12 or day 13, i.e. about 4 days before the onset of oestrus. In five of the six cycles the first significant rise in the secretion of PGF2alpha occurred on days 12-14 at the time of decline of progesterone secretion, although the release of PGF2alpha was maximal on the day before the onset of oestrus. There was very little release of PGF2alpha from the uterus before day 12. The temporal relationship of these events suggests that the uterus will only release PGF2alpha after it has been primed for 7-10 days with progesterone. The initiation of luteal regression is independent of secretion of oestradiol by the pre-ovulatory follicle which may, however, stimulate the further release of PGF2alpha responsible for irreversible structural luteolysis on the day of pro-oestrus.

Selection among males for the genetic improvement of female fertility.

The quantitative physiological study of genetic variation in reproductive performance has shown that differences in the litter size and incidence of lambing in sheep are associated with variation in the release of LH. This variation is detectable in young animals of both sexes, and the injection of LHRH may aid its recognition. In the male it is associated with variation in mating behavior and testis growth. The ovarian activity of both sheep and mice has been shown to change following selection for testis growth. The use of measures of testis growth as criteria for selection to improve the reproductive performance of females is discussed.

Reproduction in young sheep: some genetic and environmental sources of variation.

"The extent of genetic and environmental variation" in the development of reproduction in sheep is illustrated by examples with particular reference to variation among breeds and to the effects of photoperiod. The interactions between genetic and environmental effects are introduced; these may be so great that genetic groups may reverse their ranking for rate of development in different environments. The "physiology of puberty" is then discussed. The difficulty of separating puberty from seasonal variation is stressed, and a possible contrast is drawn between the physiological characteristics of genetic variation and those of environmental variation in reproductive development. Finally the physiological factors associated with sterility in young females are discussed; most studies, however, have been conducted during the time of year when adult females would also be expected to be sterile, so that conclusions are difficult and a "missing link" cannot be identified.

Transgenic livestock.

Single genes can now be added routinely to the genome of mice by molecular manipulation as simple Mendelian dominants; this complements the normal process of reproduction to give 'transgenic' animals. Success in ruminants is limited to a few examples in sheep and although gene expression has yet to be documented, there is every reason to expect that it will be achieved. The application of this technology to livestock improvement depends on the identification of circumstances in which the phenotype is limited by the deficiency of a single protein. While there is little evidence to indicate that single dominant genes are in general likely to have favourable effects, it is argued that there are likely to be exceptions. These include particular combinations of promoter and structural gene sequences to alter feedback control, for example through a change in tissue specificity, and the alteration of definitive proteins such as those of milk. A mouse model has been established to study the molecular manipulation of sheep milk proteins. The sheep beta lactoglobulin gene has been incorporated and the sheep whey protein is secreted by the mammary gland of transgenic mice. For the future, means to delete or reduce the expression of existing genes are likely to be important, as are more effective means of incorporation such as retroviral based methods and the incorporation of multigene constructs. The resources required to test transgenic livestock will, however, be greater than those required to create them.

Plasma luteinizing hormone levels and testis diameters of ram lambs of different breeds.

Plasma LH levels were examined at 1 1/2-hr intervals in ram lambs of the Finnish Landrace, Border Leicester and Cheviot breeds on three occasions at 3-week intervals by radioimmunoassay. On each occasion, the mean plasma LH concentration was found to be significantly higher in the Finnish Landrace than the other two breeds, although big variations occurred between individual samples in the same animal. This form of hormonal pulsatile release was most marked in the Finnish Landrace lambs. Testis growth was also measured and was found to be correlated, independently of body weight, with LH concentration on the final sampling occasion.

Testis growth and plasma LH concentration following hemicastration and its relation with female prolificacy in sheep.

The mean testis diameter of 20- to 25-week-old Blackface, Finn and Merino rams was ranked in the same order as the ovulation rates of females of their breeds. The removal of one testis at 12 or 16 weeks of age resulted in hypertrophy of the remaining testis. The relative increase in testis growth following hemicastration was greatest in the Merino rams (72%), least in the Finns (42%) and intermediate in the Blackfaces (57%), so that it was inversely related to their breed ovulation rates. This hypertrophy was associated with increases in the concentration of plasma LH in all breed types. The results indicate that differences in testis growth rate are associated with differences in gonadotrophic stimulation rather than in intrinsic growth potential, and it is postulated that these may arise from breed differences in sensitivity to negative feed-back from the testes.

Effect of suppression of plasma prolactin on ovulation, plasma gonadotrophins and corpus luteum function in LH-RH-treated anoestrous ewes.

Nineteen Scottish Blackface ewes were given LH-RH (3 X 30 micrograms i.v., 90-min intervals) during anoestrus when prolactin levels were elevated. Plasma levels of prolactin were suppressed with CB 154 (twice daily, i.m.) on Days -5 to 0 (N = 5), 0 to +5 (N = 5) or -5 to +5 (N = 5) around the day of LH-RH treatment (Day 0). Control animals (N = 4) received saline on Days -5 to +5. Nine animals ovulated forming corpora lutea as judged by laparoscopy on Day +7. No difference in FSH or LH levels was found between treatments and ovulations occurred equally in all treatment groups. Progesterone levels were less than ng/ml in all animals up to Day 14. It is concluded that short-term suppression of prolactin does not affect the incidence of ovulation or corpus luteum progesterone production in LH-RH-treated anoestrous ewes.

Oestradiol levels in sheep plasma during the oestrous cycle.

The concentration of oestradiol was measured by radioimmunoassay in ovarian venous plasma collected from ewes of three breeds (Finnish Landrace, Scottish Blackface and Tasmanian Merino) on Day 9 of the oestrous cycle and in jugular venous plasma collected daily around oestrus in two of these breeds. The mean +/- s.e.m. concentration in the ovarian venous plasma of the Merino (44.1 +/- 7.6 pg/ml) was lower than that in Blackface (72.2 +/- 10.2 pg/ml) and Finn ewes (66.8 +/- 10.2 pg/ml). The overall fitted mean concentration in peripheral venous plasma was 1.7 pg oestradiol-17beta/ml, with no difference between the Finn and Blackface ewes, in which the highest preovulatory values were 3.9 +/- 0.5 and 3.4 +/- 0.5 pg/ml respectively.

Dynamic studies of prostaglandin F-2 alpha in the utero-ovarian circulation of the sheep.

The concentration of prostaglandin (PG) F-2alpha in ovarian arterial blood was compared to that in aortic and carotid blood at different stages of the ovine oestrous cycle, and its concentration measured in the ovarian artery following the infusion of radioactive PGF-2alpha to the uterine vein. In all cases the concentration of PGF-2alpha increased during passage through the ovarian artery, and the increase was proportional to the log of the concentration in the uterine vein. No such change was observed in the concentration of progesterone in the ovarian artery or of PGF in the uterine artery. It was concluded that PGF-2alpha can be and is normally transferred from the uterine vein to the ovarian artery. These results are compatible with the hypothesis that PGF-2alpha of uterine origin is the normal luteolytic agent in the sheep.

The effects of nutrition on the reproductive performance of Finn x Dorset ewes. II. Post-partum ovarian activity, conception and the plasma concentration of progesterone and LH.

After lambing forty-five ewes were allocated to three groups, two of sixteen and one of thirteen ewes. The lambs of the two groups of sixteen ewes were weaned on Day 1 after lambing and the ewes were fed a diet of 100% (Group H) or 50% (Group R) of maintenance energy requirements. The thirteen ewes in the third group (Group L) suckled twin lambs and were fed freely. During the first 3 weeks after lambing, oestrus was observed for 11/16 (Group H) and 8/16 (Group R) ewes; of the ewes which had shown oestrus in the two groups, ovulation occurred in 5/8 and 5/7 respectively. Only 1/13 Group-L ewes showed oestrus and ovulated during the same period. The mean plasma concentrations of progesterone and LH were unaffected by the treatments and were around 0-4 and 1-5 ng/ml, respectively. Restricted feeding had no effect on oestrus, ovulation or the hormone levels during the oestrus cycle following synchronization. The onset of oestrus and the start of the preovulatory discharge of LH were 3 and 6 hr later, respectively, in the lactating ewes (Group L) than in those in Groups H and R. Ewes in Group L also had a higher ovulation rate, 2-8 +/- 0-2 versus 2-1 +/- 0-2 (P less than 0-05). Restricted feeding reduced the number of ewes lambing; only 1/11 ewes in Group R, considered to have conceived because of the presence of high progesterone levels 17 days after mating, subsequently lambed compared with 6/12 in Group H and 5/9 in Group L.

Genetic and environmental variation in the LH response of ovariectomized sheep to LH-RH.

The influence of breed and season on the sensitivity of the pituitary gland of sheep to LH-RH was assessed. Ovariectomized ewes of 3 breeds (Finnish Landrace, Scottish Blackface and Tasmanian Merino) with differing normal breeding seasons and with differing ovulation rates were injected (i.v.) with 3 doses of LH-RH (1.56, 6.25 or 25.0 micrograms) at 3 different times of the year covering the anoestrous and the breeding seasons of intact ewes; 9 ewes of each breed (3 per sub-class) were examined on the first and third occasions, 6 (2 per sub-class) on the second. The response was measured in terms of the concentration of LH in peripheral plasma 20, 40, 60 and 80 min after injection. Time of year, but not the breed of sheep, affected the magnitude of the response; the data indicated that the duration of LH secretion was greater during the breeding season than during anoestrus. It was concluded that changes in the spontaneous activity of the hypothalamus/hypophysis could contribute to seasonal changes in LH secretion independently of the modifying effects of gonadal steroids. Such variation in unmodulated activity apparently does not contribute to the differences in ovulation rate among the 3 breeds.

Increased testicular growth of Tasmanian Merino ram lambs treated with antisera to oestrogens.

Passive immunization against oestrone and oestradiol increased the rate of testicular growth (P less than 0.01) without a statistically significant change in the concentrations of LH and FSH at the time of measurement. Although the concentration of testosterone was higher in the group with the largest testes, the difference was not statistically significant. Within groups, the concentration of FSH was correlated with the oestrone antibody titre (r = 0.5), suggesting that oestrone may have a particular role to play in the feedback control of FSH release in the ram; in addition, the concentration of testosterone was correlated with that of LH (r = 0.65). The data indicate a role of oestrogens in the control of testicular growth and are compatible with earlier reports that the testes of Merino lambs grow more quickly when gonadal feedback is reduced.

Plasma FSH, LH, the positive feedback of oestrogen, ovulation and luteal function in the ewe given bromocriptine to suppress prolactin during seasonal anoestrus.

The effects of pharmacological reduction of the high plasma prolactin concentration typical of seasonal anoestrus in sheep were assessed with respect to positive feedback of oestrogen on LH release, ovulation, and progesterone secretion. Treatment of 16 Scottish Blackface ewes with 1 mg bromocriptine, i.m. twice daily for 12 days, reduced prolactin concentrations in peripheral plasma from 64 +/- 10 ng/ml before treatment to < 4 ng/ml. This treatment had no effect on the proportion of ewes discharging LH and FSH in response to 12.5 microgram oestradiol benzoate (3/8 before compared with 5/16 during treatment) or the proportion of ewes ovulating in response to oestrogen treatment. Plasma progesterone concentrations remained low even in ovulating ewes. It is concluded that treatment with bromocriptine alone is unlikely to restore oestrous cycles to ewes in seasonal anoestrus.