The physiological responses of anovulatory ewes to the introduction of rams: a review.

Summary

(1) Luteinizing hormone (LH) is secreted as discrete pulses throughout all stages of the reproductive cycle of the ewe, including pre-pubertal, seasonal and lactational anoestrus, and the luteal and follicular phases of the oestrous cycle. Secretion is probably also pulsatile during the preovulatory surge of LH.



(2) The secretion of LH is affected by the ovarian steroids, oestradiol and progesterone, both of which act principally to reduce the frequency of the pulses. During the luteal phase the two steroids act synergistically to exert this effect, and during anoestrus oestradiol acts independently of progesterone. Androstenedione secreted by the ovary apparently has no role in the control of LH secretion.



(3) The amplitude of the pulses may also be affected by the steroids but there are conflicting reports on these effects, some showing that amplitude is lowered by the presence of oestrogen and others showing increases in amplitude in the presence of oestrogen and progesterone.



(4) The secretion of LH pulses is affected by photoperiod, social environment and nutrition. Under the influence of decreasing day-length, oestradiol alone cannot reduce the frequency of pulses and the ewe experiences oestrous cycles. When day-length is increasing, the hypothalamus becomes more responsive to oestradiol which reduces the frequency of the pulses.



(5) A hypothetical pheromone secreted by rams can increase the frequency of the LH pulses in anoestrous ewes and thereby induce ovulation, possibly by inhibiting the negative feedback exerted by oestradiol.



(6) The relationships between nutrition and reproduction are poorly understood, but it seems likely that the effects of nutrition are mediated partly through the hypothalamus and its control of the secretion of LH pulses.



(7) The pulses of LH secreted by the anterior pituitary gland are evoked by pulses of GnRH secreted by the hypothalamus. The location of the centre controlling the GnRH pulses and the neurotransmitter involved are not known.

Factors affecting the secretion of luteinizing hormone in the ewe.

The ovarian secretion of estradiol, androstenedione, testosterone, and progesterone and the concentration of LH, FSH, and prolactin were measured in the periovulatoiy period of five ewes with ovarian or utero-ovarian autotransplants Samples of jugular and ovarian venous blood were collected every 1 or 2 h before and for 96 h after induction of luteal regression on Day 10 of the cycle by injection of 100 �zg cloprostenol Progesterone concentration in jugular vein plasma fell to less than 1 ng/ml in all ewes by 24 h, and coincidental to this decline there was a significant increase in the secretion of LH and prolactin and an associated rise in the ovarian venous concentration of estradiol, testosterone, and androstenedione The rise in LH secretion was associated with a twofold increase in the frequency of episodic pulses each of which stimulated an increase in the secretion of estradiol In contrast the concentration of FSH declined significantly in the 48 h after luteal regression Shortly after the onset of estrus (48 ± 25 h), there was a marked rise in the concentration of prolactin, FSH, and LH which reached a peak at 61 ± 4 h At the start of this preovulating LH surge, there was a further substantial stimulation in the secretion of estradiol, testosterone, and androstenedione, followed by a sharp fall within 3, 5, and 7 h, respectively In the 24 h after the preovulatory surge, there was a marked decline in the concentration of LH and prolactin as well as all ovarian steroids However, within 12 h there was a further rise in the concentration of FSH which reached a second peak at 23 h after the LH peak These results suggest that the increased secretion of estradiol from the preovulatory follicle is due to stimulation by episodic pulses of LH which occur with increased frequency as the concentration of progesterone falls during luteal regression The sustained rise in LH which occurs during the preovulatory surge stimulates and then markedly inhibits aromatase activity and eventually all steroid secretion from the follicle Thus the final stages of development of the preovulatory follicle are determined not by FSH but by the pattern of secretion of LH

/pdf/relationship-between-lh-fsh-and-prolactin-concentration-and-418j3yijg5.pdf

Relationship Between LH, FSH, and Prolactin Concentration and the Secretion of Androgens and Estrogens by the Preovulatory Follicle in the Ewe

In the previous chapter, several experimental manipulations designed to demonstrate that the secretions of the hypothalamus, the anterior pituitary, and the ovary can have stimulatory and inhibitory effects on one another were discussed. These experimental approaches were basically analytic. They served to dissect the effects of a particular hormone on one tissue from its effects on another tissue. From this analytic approach, an appreciation was gained of the range of potential reactions to hormonal stimuli of individual components of the hypothalamo-hypophyseal-ovarian axis. However, the analytic approach does not indicate which of the potential reactions are actually utilized during the course of normal reproductive cycles. Nor does the analytic method deal with the question of how the various stimulatory and inhibitory actions of hormones become linked together to form a repeatable endocrine pattern with a consistent periodicity. To resolve these problems, one must adopt a more synthetic outlook.

Estrous Cyclicity in Mammals

Publisher Summary This chapter focuses on factors affecting the secretion of steroids from the transplanted ovary of sheep. The investigation of the endocrine changes in vivo associated with control of the ovarian cycle is complicated by multiple variables present in the intact animal. A considerable advantage would be obtained by moving the ovarian vein to a more accessible location in vivo . During a study it was seen that by occluding the jugular vein distal and proximal to the ovary, the ovarian venous blood flowed into the jugular vein between the occlusion points where it would be easily collected by means of a wide-bore catheter inserted percutaneously. For work involving the measurement of steroid secretion from the utero-ovarian transplant preparation, the anastomoses are made with the carotid artery and the jugular vein enclosed in preformed skin loops as for the ovarian transplant procedure.

Factors affecting the secretion of steroids from the transplanted ovary in the sheep.

To investigate the time relationships between the preovulatory luteinizing hormone (LH) peak and ovulation in the ewe a "short" radioimmunoassay for measuring plasma LH concentration was developed. The technique is described. This short assay permitted the determination of LH concentration within 6 hours of sampling. The interval from LH peak to ovulation was then defined by a single laparotomy on each ewe at a known interval after the start of the LH surge. Blood samples were taken immediately after onset of estrus and then at hourly intervals until the LH peak had been identified by 2 consecutive samples. An indwelling silicate cannula had been placed in the jugular vein. Vasectomized rams were used in determining estrus. Using this method the time interval was investigated in 47 ewes experiencing normal estrous cycles in 11 young and 16 old ewes experiencing their second estrus after withdrawal of Cronolone-impregnated sponges and in 19 ewes experiencing their first estrus after withdrawl of such sponges. The results indicated that normally cycling ewes ovulated between 21-26 hours after the LH peak while the young and the old ewes and the Cronolone-treated ewes ovulated 22-26 hours after. At 23 hours 16.6% of ewes from all four groups had ovulated whereas by 24 hours 66.6% of ewes had ovulated indicating that a high proportion of ewes ovulated between 23-24 hours after the start of the LH peak. No correlation was noted between the onset of LH release or the first signs of estrus and the time of day. It is concluded that the constancy of the interval between LH release and ovulation in these experiments confirms the importance of the role of LH in the ovulatory physiology. The present procedure is too cumbersome and too costly for commercial use.

/pdf/constancy-of-interval-between-luteinizing-hormone-release-1mnrxqygf1.pdf

Baxter Rw

Papers

Constancy of Interval Between Luteinizing Hormone Release and Ovulation in the Ewe

Constancy of interval between luteinizing hormone release and ovulation in the ewe.

Proceedings: Control of FSH secretion in the ewe.

Proceedings: Variation in LH release in response to gonadotrophin-releasing hormone (GnRH) during the ovine oestrous cycle.