Oestrous Cycle In Sheep Research Articles

Endocrine actions of central neuropeptide Y in the ewe: activation of the hypothalamo-pituitary-adrenal axis by exogenous neuropeptide Y and role of endogenous neuropeptide Y in the secretion of luteinizing hormone during the oestrous cycle.

Neurons immunoreactive for neuropeptide Y (NPY) are abundant in the hypophysiotrophic areas of the brain. In particular, there is considerable anatomical evidence for the influence of this neuropeptide on the reproductive and hypothalamo-pituitary-adrenal axes. We therefore investigated whether central administration of NPY can alter the activities of the reproductive and hypothalamopituitary-adrenal axes in the ewe, and whether ovarian steroids are involved in the modulation of these events. We also attempted to investigate whether endogenous NPY is important in the control of luteinizing hormone-releasing hormone/luteinizing hormone (LHRH/LH) secretion in the sheep oestrous cycle. Central injection of NPY (0.15 and 1.5 nmol in 50 microliters saline), delivered by gravity flow into the third cerebral ventricle, had no effect on LH levels in ovariectomized (OVX) ewes (n = 6) or OVX ewes implanted with oestradiol (OVX/E2) (n = 7), nor was LH secretion altered by central NPY (1.5 nmol) in intact cycling animals in either the follicular or the luteal phase (n = 5). However, central administration of 1.5 nmol NPY to intact ewes during both the follicular (P < 0.05) and the luteal phase (P < 0.01), and in OVX/E2 ewes (P < 0.05) caused a large and significant increase in plasma cortisol levels. High titre antibodies were raised to NPY in sheep and the effects of peripheral and central (intracerebroventricular) administration of anti-NPY antibodies on the timing and/or characteristics of the E2-induced LH surge in anoestrous ewes and of the preovulatory surge of LH in cycling ewes were determined. Intravenous administration of anti-NPY antibodies (n = 6) had no effect on the oestradiol benzoate-induced LH surge, compared with the control injection of non-immune plasma (n = 6). Likewise, passive systemic immunization against NPY (n = 10) was without effect on the characteristics of the preovulatory LH surge, compared with the control group (n = 10). However, central (intracerebroventricular) administration of anti-NPY antibodies (n = 4) delayed or abolished the preovulatory LH surge when compared with non-immune plasma treatment in the same animals. In summary, tonic LHRH/LH secretion is unaffected by centrally administered NPY at the doses used in this study. However, the same doses of NPY activate the hypothalamo-pituitary-adrenal axis, thus lending clear support to the hypothesis that NPY is involved in the multifactorial regulation of adrenocorticotrophin and cortisol secretion in this species, probably by stimulating corticotrophin-releasing hormone and/or arginine vasopressin secretion within the hypothalamus.(ABSTRACT TRUNCATED AT 400 WORDS)

Hormone production in vivo and in vitro from follicles at different stages of the oestrous cycle in the sheep

This experiment was undertaken in order to investigate the production of inhibin, oestradiol and androstenedione by ovarian follicles at different stages of the oestrous cycle in sheep. Twenty-four Scottish Blackface ewes were allocated to four groups of six ewes, i.e. those operated on during the luteal phase (day 10), and those operated on during the follicular phase 24-30, 36 and 60 h after the induction of luteal regression by an injection of 125 micrograms cloprostenol on day 10 of the luteal phase. Samples of jugular and ovarian venous blood were collected under anaesthesia and ovaries were then removed and all follicles larger than 3 mm diameter dissected out and incubated in medium for 2 h. After injection of cloprostenol, luteal regression occurred as indicated by a fall in the secretion rate of progesterone. The ovarian secretion rate of inhibin was similar at all stages of the follicular phase and during the luteal phase while, in contrast, the secretion rate of oestradiol was significantly (P less than 0.05) elevated in the group 24 h after injection of cloprostenol. There was good correlation between the in-vivo ovarian secretion rate and production rate during incubation in vitro for both inhibin (r = 0.57) and oestradiol (r = 0.60). When follicle diameter was compared with in-vitro hormone production there was good correlation for inhibin (r = 0.72) with larger follicles producing more inhibin, while the value for oestradiol was somewhat lower (r = 0.57) owing to the presence of large atretic follicles with low oestradiol production. Androstenedione production showed a lower correlation with follicle diameter (r = 0.39). When the four time periods were compared separately, there were significantly (P less than 0.05) more follicles with high in-vitro oestradiol production (greater than 90 fmol/min) in the group at 36 h than in the other three groups, while inhibin release in relation to follicle size was similar in the four groups. Large oestrogenic follicles were responsible for 90% of the total oestradiol production during culture while only providing 55% of the total inhibin production, with large non-oestrogenic and small follicles contributing 33% and 12% of inhibin production respectively. From the results of this study we conclude that while oestradiol is mainly produced by the large oestrogenic follicles, a considerable amount of inhibin is also produced by large non-oestrogenic and small follicles.(ABSTRACT TRUNCATED AT 400 WORDS)

Pulsatile secretion of inhibin, oestradiol and androstenedione by the ovary of the sheep during the oestrous cycle

The pattern of pulsatile secretion of inhibin, oestradiol and androstenedione by the ovary at different stages of the oestrous cycle in sheep was studied in five Finn-Merino ewes in which the left ovary had been auto-transplanted to the neck. The ewes had jugular venous blood samples collected at 4-hourly intervals from 42 h before the induction of luteolysis by i.m. injection of cloprostenol (100 micrograms) on day 10 of the oestrous cycle, until day 3 of the following cycle. There were five periods of intensive blood sampling, when both ovarian and jugular venous blood samples were collected, as follows: (a) mid-luteal phase, before the second injection of cloprostenol on day 10 (15-min intervals for 6 h); (b) early follicular phase, 24 h after the second injection of cloprostenol (10-min intervals for 4 h); (c) late follicular phase, 48 h after the second injection of cloprostenol (10-min intervals for 4 h); (d) after the LH surge on day 1 of the cycle, 76 h after the second injection of cloprostenol (10-min intervals for 4 h); (e) early luteal phase on day 3 of the cycle, 120 h after the second injection of cloprostenol (10-min intervals for 3 h). Plasma was collected and the samples assayed for LH, FSH, progesterone, oestradiol, androstenedione and inhibin. The ovarian secretion rates for oestradiol, androstenedione and inhibin were calculated. All ewes responded normally to the luteolytic dose of cloprostenol with the preovulatory surge of LH occurring within 56.4 +/- 1.6 h (mean +/- S.E.M.) followed by the establishment of a normal luteal phase.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of a potent gonadotrophin-releasing hormone antagonist on ovarian secretion of oestradiol, inhibin and androstenedione and the concentration of LH and FSH during the follicular phase of the sheep oestrous cycle

By selective removal and replacement of LH stimulation we sought to examine the relative importance of inhibin and oestradiol in controlling FSH secretion, and the role of LH in the control of ovarian hormone secretion, during the follicular phase of the oestrous cycle. Eight Finn-Merino ewes which had one ovary removed and the other autotransplanted to a site in the neck were given two injections of a gonadotrophin-releasing hormone (GnRH) antagonist (50 micrograms/kg s.c.) in the follicular phase of the cycle 27 h and 51 h after luteal regression had been induced by cloprostenol (100 micrograms i.m.). Four of the ewes received, in addition, i.v. injections of 2.5 micrograms LH at hourly intervals for 23 h from 42 to 65 h after GnRH antagonist treatment. Ovarian jugular venous blood samples were taken at 10-min intervals for 3 h before and 5 h after the injection of antagonist (24-32 h after cloprostenol) and from 49 to 53 h after antagonist (74-78 h after cloprostenol). Additional blood samples were taken at 4-h intervals between the periods of intensive blood sampling. The GnRH antagonist completely inhibited endogenous pulsatile LH secretion within 1 h of injection. This resulted in a marked decrease in the ovarian secretion of oestradiol and androstenedione (P less than 0.001), an effect that was reversible by injection of exogenous pulses of LH (P less than 0.001). The pattern of ovarian inhibin secretion was episodic, but removal or replacement of stimulation by LH had no effect on the pattern or level of inhibin secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular composition of utero-ovarian lymph during the exogenous gonadotrophin-stimulated oestrous cycle in sheep

Utero-ovarian peripheral lymph was collected for extended periods from sheep that had received exogenous gonadotrophins. High lymph flow rates and progesterone outputs, over 13 mL h-1 and 65 nmol h-1 respectively, were observed during the subsequent oestrous cycles. A positive correlation was found between the maximum lymph flow rate and the number of corpora lutea in the draining ovary. At periods approximating the beginning and end of the luteal phase of the oestrous cycle there were increases in the output of red blood cells and, towards the end of the luteal phase, eosinophils constituted over 10% of the white blood cells in the lymph. Lymph provides a dynamic record of the changing internal environment of the ovary and as such may yield useful information on intraovarian control mechanisms in sheep.

Changes in the plasma concentrations of inhibin throughout the normal sheep oestrous cycle and after the infusion of FSH

A radioimmunoassay for inhibin was developed using a peptide containing the 1-26 amino acid sequence of the N-terminus of the alpha-chain of 32 kDa porcine inhibin as immunogen, and 125I-labelled tracer. Evaluation of this assay using Sephadex column chromatography, chromatoelectrophoresis and immunoblotting confirmed that it measured all forms of inhibin present in sheep follicular fluid and was suitable for measurement of inhibin in sheep plasma. There was no evidence of the presence of free alpha-subunit in either sheep follicular fluid or ovarian vein plasma. The concentration of inhibin in jugular plasma throughout the follicular and luteal phases of four ewes with ovarian autotransplants was measured. The ovarian secretion of inhibin and oestradiol were also measured simultaneously throughout the follicular phase in a spontaneous cycle and after infusion of NIH-oFSH-S14 at 10 micrograms/h for 48 h following premature luteal regression induced by prostaglandin. The results showed: (1) no change in the peripheral concentration of inhibin throughout the cycle except an increase related to the periovulatory increase in FSH and LH. (2) Following luteal regression, the concentration of FSH fell as the secretion rate of oestradiol increased. During this time there was no significant change in the peripheral concentration of inhibin or ovarian inhibin secretion rate. (3) Following the infusion of FSH there was a marked increase in the concentration of inhibin in both ovarian and peripheral plasma and an increase in ovarian inhibin secretion rate. (4) The calculated metabolic clearance rate of inhibin, 20.3 ml/min, is similar to that of FSH. We conclude that in the ewe the ovarian inhibin secretion rate is stimulated by FSH and, although inhibin may modulate the basal secretion of FSH, a change in its secretion does not account for the fall in FSH which occurs during the follicular phase of the sheep oestrous cycle.

Role of intrafollicular regulators and FSH in growth and development of large antral follicles in sheep.

Manipulation of circulating concentrations of hormones and ovarian follicle status was carried out on Day 11-12 of the oestrous cycle in sheep. All follicles visible on the ovary were ablated by cautery and ewes were treated with oestradiol or ovine follicular fluid (oFF) to suppress FSH or with PMSG to increase circulating gonadotrophic activity. One group underwent unilateral ovariectomy which greatly increased endogenous FSH and was the only treatment which significantly affected LH pulse frequency. The size distribution of antral follicles, the extent of atresia and the mitotic index of granulosa cells of follicles on Day 15 showed that (a) treatment with oFF inhibited the growth of follicles beyond 2 mm diameter by suppressing the mitotic index of the granulosa cells and (b) the concentration of FSH in peripheral plasma was related to the ability of small antral follicles to grow during the late luteal-early follicular phase of the cycle. Subsequently, it was demonstrated that oFF inhibits, in a dose-dependent manner, folliculogenesis sustained by PMSG in ewes on Days 12-15. Inhibition of folliculogenesis was represented by a decrease in those follicles greater than 4 mm, an increase in the relative proportion of follicles less than 2 mm, and minimal change in the average number of follicles visible on the ovarian surface, and a decrease in the mitotic index of granulosa cells of follicles less than 2 mm. There was no change in the extent of atresia.(ABSTRACT TRUNCATED AT 250 WORDS)

Follicular dynamics throughout the oestrous cycle in sheep. A review.

Whatever the stage of the cycle, follicles of various sizes are apparent on the surface of the ovary of sheep. No evidence of waves of follicular growth at fixed-time intervals has been found during the luteal phase, and growth probably occurs at random until the follicle attains a size of 4 to 7 mm. This shows that follicles can enlarge under a wide range of hormonal levels. The recruitment of a crop of follicles, including the one which will ovulate, occurs at variable times around luteolysis due to the interaction of endocrine and follicular factors (FSH priming, sensitizing the follicle to increased LH pulsatility). All healthy follicles greater than 2 mm in diameter are recruited. Selection of the follicle due to ovulate can be defined by morphological criteria (size) or by its "killing" ability. In all cases, the timing of follicle selection is highly variable, and it is due to this variability that the mechanisms of selection are not fully known. They are presumably intraovarian, one of the follicles actively inhibiting the growth of the others. The dominant follicle is probably maintained because of its high oestradiol content, while the others undergo atresia. The main feature of terminal follicular development is its flexibility in terms of hormonal requirements, follicle size and the timing of the main events.

Prostaglandin F 2α and PGE 2 in uterine lymph during the oestrous cycle in sheep

The concentration of prostaglandin F 2α (PGF 2α) in uterine lymph remained low (<1 ng/ml) during the first two-thirds of the oestrous cycle in the non-pregnant sheep. However increased amounts of PGF 2α were present from day 12 onwards. The concentration of prostaglandin E 2 in uterine lymph remained low (< 0.43 ng/ml) throughout the cycle.

Sodium appetite and the oestrous cycle in sheep: effect of oestrogen, progesterone and changes in food intake.

Previous studies of voluntary sodium intake and electrolyte excretion in sheep suggested that the pattern of intake associated with the oestrous cycle was not a response to alterations in sodium balance. The pattern thus seemed likely to be dictated by changes in either hormonal levels or food intake. These possibilities were examined in experiments with ovariectomized sheep. Physiological doses of oestrogen or progesterone had no effect on sodium preference, whether given in isolation or succession. Depression of food intake by 25% (as at oestrus) did affect preference slightly (though significantly) but in the opposite direction to the change at oestrus. It is concluded that none of these 3 factors is responsible for the varying sodium preference during the oestrous cycle and that direct hypothalamic control of this pattern is an interesting possibility.

Water and Electrolyte Excretion During the Oestrous Cycle in Sheep

Electrolyte excretion was observed during 24 oestrous cycles in housed sheep, together with mixed salivary Na/K ratio during 10 additional cycles. 1. The sharp fall in food and fluid intake at oestrus accompanied a peak of sodium excretion which changed to peak retention 3 days later, both in faeces and urine. 2. Potassium excretion declined with food intake at oestrus but subsequently failed to recover to pre-oestrous levels dispite full recovery of dietary intake. 3. Curiously, water intake also recovered completely whereas urinary and faecal water retention continued; faecal loss actually exceeded renal excretion on these liberal water intakes. 4. Changes in salivary, urinary and faecal Na/K indicated an aldosterone peak neither during the luteal phase nor at oestrus but three days later. The data raise questions concerning the regulation of water and electrolyte balance within the normal cycle. They also provide a baseline for the investigation of renal effects of gonadal steroids. Possible roles for aldosterone, ADH and progesterone in maintaining fluid and electrolyte balance are discussed, emphasising problems confronting species which have evolved with heavy obligatory potassium excretion but undependable supplies of sodium and water.

The influence of hyperprolactinaemia on pulsatile LH and ovarian oestradiol secretion during the follicular phase of the sheep oestrous cycle

The influence of hyperprolactinaemia on pulsatile LH and ovarian oestradiol secretion during the follicular phase of the sheep oestrous cycle

Levels of progesterone, LH and FSH in the plasma of sheep during the oestrous cycle.

Ministry of Agriculture, Fisheries and Food, Central Veterinary Laboratory, New Haw, Weybridge, Surrey KT15 3NB (Received 15th February 1975) Although the changes occurring in the plasma levels of progesterone and LH during the oestrous cycle of sheep have been well documented, there is some doubt about the pattern of plasma FSH concentrations. We present some data on the levels of all three hormones in a limited number of plasma samples taken from ewes experiencing normal oestrous cycles. Eight 3-year-old Cheviot ewes were kept in a paddock with a vasectomized ram during October and November. The ram was fitted with a marking harness and the coloured crayon on the harness was changed every 15 days. The ewes were examined daily and the day that a ewe was first marked by the ram was designated Day 0 of the cycle. Blood samples were taken from the jugular vein into heparinized 10 ml

Some Endocrinological Aspects of corpora lutea and of Follicles during the Oestrous Cycle in Sheep.

Inhalt Die Konzentration von 17-β-Ostradiol im liquor folliculi und Pregnenolon im lutealen Gewebe wurde wahrend des Brunstzyklus bei 63 Schafen bestimmt. Die Konzentration von 17- β-Ostradiol erreichte ein Maximum am Tag 16. Bei Eintritt des Ostrus fiel die Konzentration um mehr als 50 %. Wahrend der progestativen Phase wurden in der Regel geringe Konzentrationen gemessen, obwohl einige Follikel um Tag 8 hohe Konzentrationen zeigten. Bei Eintritt der Brunst war die Pregnenolon-Konzentration 20mal hoher als die von Progesteron. Nach der Ovulation fiel die Pregnenolon-Konzentration, wahrend die Progesteron-Konzentration am dritten Tag ein Maximum erreichte, um dann bis zum Tag 6 abzufallen und erneut am Tag 12 einen Hohepunkt zu erreichen. Die Pregnenolon-Konzentration begann ab Tag 6 anzusteigen, um am Tag 14 einen Hohepunkt zu erreichen. Die Ergebnisse Lassen vermuten, daβ wahrend der ersten 4 Zyklustage Pregnenolon der limitierende Faktor ist in der Progesteron-Synthese, wabrend vom Tag 12 an die Konversion von Pregnenolon in Progesteron allmablich gehemmt wird. Contents The concentration of oestradiol-17 β in follicular fluid and of progesterone and pregnenolone in luteal tissue were determined throughout the oestrous cycle in 63 ewes. The concentration of oestradiol-17 β reached a maximum on day 16. At oestrus the concentration was reduced by more than 50%. During dioestrous generally low concentrations were determined although some follicles showed high concentrations around day 8. At oestrous the concentration of pregnenolone was 20 times higher than that of progesterone. After ovulation the concentration of pregnenolone dropped whereas the concentration of progesterone increased to a maximum on day 3, then decreased until day 6 when it again increased to a peak on day 12. The concentration of pregnenolone gradually increased from day 6 reaching a peak on day 14. The results suggest that during the first four days of the cycle pregnenolone may be the limiting factor in the progesterone synthesis while from day 12 onwards the conversion of pregnenolone in progesterone is gradually inhibited.

Effect of hysterectomy on the life-span of corpora lutea induced artificially in progesterone-treated ewes.

The normal periodicity of the oestrous cycle in sheep is not significantly altered when the formation of additional corpora lutea is induced during the luteal phase by the administration of gonadotrophic hormones (Casida, Dutt & Meyer, 1945). It has been established by Inskeep, Oloufa, Pope & Casida (1963) that such artificially induced corpora lutea become fully functional, but regress prematurely at the time of involution of the naturally formed corpus luteum. The life-span of induced corpora lutea is even more restricted, however, when they are formed in ewes that have previously been treated with exogenous progesterone for an extended period of time (Inskeep, Howland, Pope & Casida, 1964a). As a result of their studies Inskeep et al. (1963, 1964a, b) concluded that the life-span of artificially induced corpora lutea is dependent upon the level of

The Oestrous Cycle in the Sheep

The Oestrous Cycle in the Sheep