Body development and puberty in spontaneously hypertensive rats

The body growth and the onset of puberty in spontaneously hypertensive rats (SHR) and in normotensive controls (WKY) have been studied. In female rats the onset of puberty was determined by both the age and the body weight at which the vaginal opening and first estrus appeared, as well as the ability of estradiol and progesterone to induce pituitary LH release. For this purpose females were injected with estradiol benzoate (0.1 mg/kg) and progesterone (1 mg/rat). Control animals received only oil vehicle. In male rats, puberty was assessed by studying the age and body weight at the time of balano-preputial separation. In another experiment, SH and WKY rats were decapitated on day 30 to determine FSH, LH, PRL, GH and testosterone plasma levels in males and FSH and LH in females. The results obtained show: a) A greater body weight, at all the ages studied (every 4 days between days 28 and 92) in SHR animals. b) A delay in vaginal opening and first estrus presentation in SHR females. c) Absence of spontaneous LH peaks in WKY females. d) Advancement in balano-preputial separation in SHR males and e) Higher plasma FSH levels in SHR males than in WKY males, without differences in other hormones.     

Hormone responses to clomiphene citrate in young chimpanzees

The responses of gonadotropin and gonadal steroids to the administration of clomiphene citrate were studied in male and female chimpanzees, aged 3.6 to 9.9 years. Follicle-stimulating hormone (FSH) was significantly reduced after treatment in the prepubertal females (n = 4) and in early pubertal males (n = 2) but not in prepubertal males (n = 5). FSH was unchanged or increased in early pubertal females (n = 2) and late pubertal males (n = 2). There was no consistent response to treatment with clomiphene citrate by luteinizing hormone (LH) in either males or females, nor by 17 beta-estradiol in the females. Testosterone levels were reduced in the early pubertal males only. These results support the hypothesis that negative feedback by gonadal steroids is operative in prepubertal chimpanzees and that puberty is accompanied by a reduction in the sensitivity to such feedback.     

Prolactin and LH release in response to LH-RH and TRH in ewes during dioestrus, pregnancy and post partum

With advancing pregnancy in the ewe there was a marked decline in plasma LH concentrations and pituitary LH-RH responsiveness (integrated LH release) and a marked increase in plasma prolactin values and pituitary TRH responsiveness (integrated prolactin release). In lactating ewes plasma LH levels and pituitary LH-RH responsiveness had returned to values found in the luteal phase of the normal cycle by 21 days post partum, whereas at 42 days post partum prolactin levels were still high. No interaction between TRH and LH-RH on prolactin and LH release in dioestrous ewes was detected. In non-pregnant ewes plasma prolactin levels were significantly higher in June than in January but TRH responsiveness was similar. It is concluded that, in sheep, pituitary LH secretion recovers more rapidly from the chronic negative feedback effect of oestrogens and progesterone in pregnancy than prolactin secretion recovers from the chronic positive feedback effects of oestrogens. This finding may be a contributory factor in the resistance to resumption of breeding activity.     

Changes in gonadotrophin and ovarian hormone levels during the estrous cycle of black-tailed deer (Odocoileus hemionus columbianus)

Daily blood samples over a fifteen day period were obtained from two adult female black-tailed deer and circulating levels of progesterone, estrogens, luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin were measured by radioimmunoassay. LH levels showed an apogee at the time when there was observed estrous behaviour. The length of the estrous cycle appeared to be 7 days. Progesterone levels peaked just subsequent to the LH peak. High estrogen levels coincided with high progesterone and prolactin levels. FSH reached maximum levels prior to peak estrogen levels. LH and FSH levels reached maxima on different days. There were two steroid peaks between the LH apogees.     

Menstrual disturbances in chronic renal failure.

Twelve female patients undergoing intermittent hemodialysis (HD) and 5 females posttransplantation (PT) were studied. All the HD patients had menstrual disturbances and 5 had galactorrhea. The mean basal LH level was significantly elevated (p less than .05) in patients on HD compared to normal controls, but the mean LH response to luteinizing hormone releasing hormone (LRH) was not significantly different from the control group. Mean basal FSH and the FSH response to LRH was normal. In the PT pateints the LH response to LRH was significantly greater at 120 min when compared to normal females. In the HD group the serum 17B estradiol, progesterone and testosterone levels were significantly lower than in the controls but in the PT group only testosterone levels were significantly lower. These results differ from those previously found in uremic males. Elevated prolactin levels were found in the patients on hemodialysis and correlated well with the presence of galactorrhea. These was no correlation between the elevated prolactin levels and amenorrhea in the patients on hemodialysis but one PT patient with amenorrhea had elevated prolactin levels.     

Modulation of gonadotrophin and prolacrin secretion by daylength and breeding behaviour in free-living starlings, Sturnus vulgaris

Plasma levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin were measured by radioimmunoassay in plasma samples collected from free-living starlings, Sturnus vulgaris , trapped in nest-boxes. By leaving some nest-boxes undisturbed, and repeatedly destroying nests in others, birds from a single-brooded population were trapped whilst nest-building, incubating or feeding nestlings, at different times throughout the normal breeding season. In both males and females trapped whilst nest-building, plasma LH and prolactin levels increased progressively from mid March until late May. In females sampled during incubation, LH and FSH levels were high throughout May but decreased in early June. Prolactin levels were highest in late May. In both males and females trapped during mid May, LH levels were highest in these birds which were nest-building at this time and lowest in those feeding nestlings, FSH did not change significantly, and prolactin was low in those birds which were nest-building and high in those incubating or feeding nestlings. In female starlings from a double-brooded population, levels of LH and FSH were much lower whilst feeding the second brood than when feeding the first brood, whereas prolactin levels were similar. The results suggest that incubation and feeding behaviour inhibit the increase in LH secretion caused by increasing daylength, but stimulate prolactin secretion in excess of that caused by increasing daylength.     

Plasma gonadotropins and prolactin in pseudopregnancy in the rat

Radioimmunoassay presented a method of measuring plasma levels of FSH,LH and prolactin in pseudopregnant rats. Plasma prolactin levels doubled 15 minutes following cervical probing (p .01) on the day of estrus. Plasma LH levels were not significantly elevated. Due to the use of ether anesthesia at blood collecting 3 hr before and 15 minutes after stimulation, only 1 of 16 rats developed pseudopregnancy. On Day 4 of pseudopregnancy in rats mated with vasectomized males; plasma LH was lower (p .05) than in normal rats on the first day of diestrus, perhaps due to the suppressive action of ovarian progesterone and some estrogen. FSH was higher than in normal rats (p .05) perhaps due to the lesser sensitivity of FSH to the inhibitory effect of progesterone. Large decidoumata developed by Day 9 in uterine horns traumatized on Day 4 (153 plus or minus 8 mg uterus weight compared to 1510 plus or minus 204 mg). Thus, the corpora remain functional after LH and prolactin are at normal and subnormal levels. On Day 9 plasma prolactin was lower than at Day 1 of diestrus (p .001). Plasma FSH was elevated (p .01). Plasma LH was unchanged. The degree of rise of LH levels 5 days following ovariectomy on Day 4 of psuedopregnancy or on the first day of diestrus was greater in the former group (p .02), perhaps due to rebound of LH from suppression by ovarian steroids. FSH rose equally in both groups. Prolactin remain about the same. Increased prolactin release by the adenohypophysis was briefer than expected.     

Short-term effects of vasoligation upon plasma follicle-stimulating hormone, luteinizing hormone, and prolactin in the adult rat: further evidence for a direct neural connection between the testes and the central nervous system

The purpose of these experiments was to determine whether bilateral vasoligation of adult male rats had any short-term effects upon plasma levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin. Adult male rats (250-300 g) were either bilaterally vasoligated or sham vasoligated, and blood samples were obtained by cardiac puncture preoperatively and at 24 h and 7 days following surgery. Plasma levels of both FSH and LH were significantly (P less than 0.01) decreased at 24 h following vasoligation compared to preoperative levels and those of sham-operated controls. However, the response was differential since, at 7 days following vasoligation, plasma FSH was still significantly decreased while LH was returning to control levels. Conversely, plasma prolactin levels were significantly (P less than 0.01) increased at 24 h compared to preoperative values and those in sham-operated controls, and at 7 days prolactin had returned to preoperative control levels. Sham vasoligation did not significantly change plasma levels of FSH, LH, or prolactin at any of the time intervals investigated. These results provide further evidence that suggests that there may be a direct connection between the testis and central nervous system that may be involved in the short-term regulation of gonadotropin and prolactin secretion.     

Sex differences in acute luteinizing hormone responses to gonadectomy remain after progesterone antagonist and dopamine agonist treatment.

In male rats, LH pulse frequency and amplitude increase dramatically by 24 h after gonadectomy; in females they increase only slightly by this time. Mean FSH levels increase significantly in both sexes by 24 h after gonadectomy. The objectives of the present studies were to compare pulsatile LH, FSH, and prolactin (PRL) secretion in intact versus gonadectomized and in male versus female rats, and to determine whether the acute postovariectomy lag in LH rise is due to a lingering effect of the higher PRL and/or progesterone (P) levels seen in intact females. LH pulse amplitude, frequency, and mean levels increased significantly by 24 h after gonadectomy in both sexes, but the increases were greater in the males. FSH mean levels, but not pulse amplitude or frequency, increased similarly in both sexes by 24 h after gonadectomy. PRL did not change with gonadectomy. Treatment with CB-154 (a dopamine agonist), with or without RU486 (a P antagonist), 1 h before gonadectomy significantly suppressed pulsatile PRL secretion 1 day later in both sexes. There was no effect of either treatment on LH secretion. We have demonstrated that there is a sex difference in LH, but not FSH or PRL, pulsatility at 24 h after gonadectomy, and that female rats' higher PRL and P levels do not account for their slow rate of LH rise after ovariectomy.     

Fetal role in the control of parturition in the tammar, Macropus eugenii

When female tammars carrying dormant blastocysts were injected with progesterone at the time of removal of their pouch young the development of the fetus was advanced and parturition occurred 5 days earlier than in the control tammars. In these tammars the prolactin pulse was also advanced by 5 days but the usually concomitant fall in progesterone was not. In non-pregnant tammars similar injections of progesterone did not advance the subsequent fall in progesterone, oestrus, or the LH pulse. In non-pregnant tammars injected with ovine prolactin on Day 26, to mimic the prolactin pulse, plasma progesterone was reduced to basal levels within 12 h, significantly earlier than in controls. Conversely, in 5 pregnant and 1 non-pregnant tammar injected with ovine prolactin on Day 23, to mimic the condition induced by advancing the time of parturition with progesterone, the decline in plasma progesterone was not advanced and the endogenous prolactin pulse, parturition, post-partum ovulation and the LH pulse all occurred after intervals similar to those of controls. The results support the view that the fetus is associated with the pre-partum prolactin pulse in maternal plasma and that a prolactin pulse at this stage is luteolytic in non-pregnant tammars.     

Multiple causes of pregnancy failure in hamsters precociously ovulated by human chorionic gonadotropin

Cycling adult female hamsters can be induced to mate and ovulate 24 h early by the injection of 20 IU human chorionic gonadotropin (hCG) at 1500 h on Day 3 (day before proestrus), but pregnancy is not established. Although there is evidence of decreased sperm transport in precociously ovulated females, this does not appear to be the primary cause of infertility. Reduced size and vascularity of corpora lutea (CL) in treated females suggests incomplete or failed CL activation. Control and hCG-treated females were killed by exsanguination under ether anesthesia at intervals for the first 5 days after mating. Serum luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, estradiol, and progesterone were measured by radioimmunoassay. Luteinizing hormone in treated animals was very high at 2200 h on Day 1 after mating (31 h after the hCG injection), due to endogenous release, and dropped below control levels thereafter. Follicle-stimulating hormone, by contrast, was significantly lower than controls at 2200 h on Day 1 and remained low until 2200 h on Day 3 after mating. Prolactin in treated animals was not different from that in controls, except for 1000 h on Day 4, when it showed a significant dip. Estradiol in treated animals was significantly higher than in controls at 2200 h on Day 1 (when LH was also high and FSH was low), and remained high at 1000 h and 2200 h on Day 2, dropping thereafter to control levels. Progesterone was initially at control levels but had dropped significantly by 1000 h on Day 2 and remained low for the next 24 h. These results suggest that pregnancy failure is due to inadequate activation of corpora lutea. This may be due to: 1) immaturity of follicles at the time of ovulation; 2) inappropriate timing of preovulatory events; 3) the luteolytic effects of high levels of LH or estradiol or both; 4) the low level of FSH in the early stages of corpus luteum development; or 5) a combination of the above. Abnormalities of prolactin secretion were not investigated in detail but cannot be ruled out at this time.     

Endocrinology of pregnancy in the dog: a review

Pregnancy regulation in the dog is not yet fully elucidated. Since plasma progesterone concentrations are similar in pregnant versus non-pregnant animals, it is a poor reflection on CL function and progesterone metabolism. Increased progesterone secretion by the CL in pregnant animals follows implantation and relaxin secretion by the feto-placental units. Progesterone is absolutely required to maintain pregnancy and no placental sources of progesterone have been identified. Pregnancy can be artificially maintained by progesterone administration. Prolactin secretion appears to be increased in response to the increase in relaxin production and occurs independent of estrogen production by the CL. The respective roles of LH, FSH and prolactin are still unclear, with considerable conflicting evidence among studies. However, it appears that prolactin is absolutely required, whereas LH is either permissive or facilitates CL function during pregnancy. Pre-implantation events are still poorly defined in the bitch, and no embryonic factors have been isolated or purified, preventing early pregnancy diagnosis. Parturition occurs following luteolysis, which results from the release of prostaglandin F(2alpha), which begins 36h prepartum in a process similar to that observed in other species. The role of estrogens at the time of parturition remains undefined.     

Prenatal melatonin exposure influences the maturation of gonadotropin and prolactin estradiol-benzoate feedback system.

Gonadotropin and prolactin response to estrogen feedback in female rat offspring of control and melatonin treated (150 microg/100 g BW) mother rats during pregnancy (MEL-offspring) were studied at these periods: infantile, prepubertal and pubertal. In controls negative or absent LH feedback developed after estradiol benzoate (EB) injection up to 30 days of age indicating that the onset of puberty had not occurred. The positive feedback was established from day 33 on. However, in MEL-offspring the first activation of gonadotropin secretion during afternoon, 31 h after EB, was observed at 25 days of age, representing the first neuroendocrine sign of the onset of puberty. This positive response disappeared on day 30 in MEL-offspring. At 33 days of age, the LH positive response to EB was found in both groups, indicating a more advanced sexual development. In controls, this response increased at 35 days of age while in MEL-offspring it was highly depressed. FSH secretion in response to EB showed a negative feedback effect from infantile to the end of prepubertal period in both groups. The positive feedback was observed earlier in MEL-offspring (at 33 days of age) than in controls (at 35 days of age), but at this age it was absent in MEL-offspring. A positive prolactin response to EB at all ages in controls was observed. The typical pulsatility with higher values in the afternoon appeared by the first time at 30 days of age. However, in MEL-offspring no pulsatile response was observed throughout any age. These data suggest that prenatal melatonin administration altered gonadotropin and prolactin response to EB inducing precocious sensitivity during prepubertal period but depressed response during the pubertal period.     

Serum concentrations of gonadotropins and steroid hormones of Neophocaena phocaenoides asiaeorientalis in middle and lower regions of the Yangtze River

As a relatively isolated and unique freshwater population, the Yangtze finless porpoise (Neophocaena phocaenoides asiaeorientalis) is the most endangered subpopulation of this species. The objective of this study was to improve our understanding of their reproductive endocrinology by measuring (radioimmunoassay) serum concentrations of FSH, LH, estradiol (E(2)), progesterone (P(4)) and testosterone (T(2)) in free-ranging animals. Blood samples were collected from 66 Yangtze finless porpoises (41 males and 25 females) captured in the middle and lower reaches of the Yangtze River. Based on significant variation of serum T(2) concentrations in males with body length (BL) >138 cm, we inferred that they were mature; in these animals, there were significant seasonal variations in serum T(2) concentrations, with the highest concentrations in March and April (502.0+/-319.8 ng/dL, mean+/-S.D.) and the lowest in December (79.4+/-83.2 ng/dL). Serum T(2) concentrations positively correlated with serum concentrations of LH and weakly correlated with serum concentrations of E(2), whereas there was a significant negative correlation between serum LH and FSH concentrations in males >138 cm. The smallest apparently pregnant female porpoise had a BL of 130 cm. Serum P(4) concentrations ranged from 13.2 to 112.4 ng/mL (43.9+/-28.3 ng/mL) in pregnant females, and fluctuated under 1.0 ng/mL in non-pregnant females with BL>130 cm. Serum LH concentrations were significantly higher in non-pregnant females >130 cm versus those females <130 cm. To our knowledge, this is the first study of endocrine-related maturity and seasonal breeding characteristics of the Yangtze finless porpoise.     

Effects of prostaglandin and prolactin on luteolysis and parturient behaviour in the non-pregnant tammar, Macropus eugenii

In Exp. 1 non-pregnant female tammars were injected, on Day 26 (the day parturition would normally occur) after removal of pouch young, with saline, 200 micrograms ovine prolactin or 5 mg PG and changes in plasma concentrations of progesterone, prolactin, PGF-2 alpha metabolite (PGFM), oestradiol-17 beta and LH were determined. Luteolysis occurred in females treated with prolactin alone, while treatment with PG first induced a rapid rise in prolactin and subsequently a significant decrease in plasma progesterone. After prolactin treatment the oestradiol peak, oestrus and the LH surge were advanced significantly compared to the saline-treated females. In Exp. 2 the effects of the same treatments as used in Exp. 1 were determined on Day 23 and again on Day 26 after removal of pouch young in non-pregnant females. On Day 23 both prolactin and PG induced significant elevations in plasma progesterone, but luteolysis did not occur. On Day 26 the treatments initially induced significant elevations in plasma progesterone but these were followed by luteolysis within 8-12 h after treatment. PG treatment induced parturient behaviour in the non-pregnant females within 3-21 min and this persisted during the period that plasma concentrations of PGFM were elevated. The results show that PG induces birth behaviour and the release of prolactin, while prolactin first induces an elevation of plasma progesterone concentrations and, in the mature CL on Day 26, subsequently induces luteolysis.     

Opioidergic control of luteinizing hormone secretion in the female rabbit: influence of age on the response to naloxone

To examine the role of opioid neurons on luteinizing hormone (LH) secretion in the female rabbit, we determined LH release at timed intervals after naloxone administration to rabbits aged 25-150 days. The LH response to naloxone (10 mg/kg) was not significantly elevated until day 43 when LH rose 76-113% above basal levels at 40-80 min. In 56-day-old females the corresponding increase was 160% at 15 min and in 65- to 67-day-olds it was 154%. From 70 to 80 days of age the LH response was blunted and no significant elevations could be elicited. By contrast, naloxone-induced LH increases were again evident when rabbits were older than 100 days. At all ages no significant change in FSH concentrations was observed. In the adult females, naloxone at 2.5, 5, and 10 mg/kg caused increases in LH secretion which occasionally were high enough to induce ovulation as exemplified by elevated serum progesterone 4 days later. These data suggest that opioid peptides may be involved in the prepubertal rise in LH and in the normal inhibition of adult secretion in the female rabbit.     

Gonadotropin pulsatile secretion in girls with premature menarche

Five prepubertal girls (2.3-8.1 years old) were studied for isolated or recurrent vaginal bleeding in the absence of other signs of precocious puberty (premature menarche). Four of these girls with recurrent vaginal bleeding were studied for pulsatile gonadotropin secretory patterns. During sleep 3 girls showed luteinizing hormone (LH) pulses with low amplitude and a pubertal pattern of frequency whereas follicle-stimulating hormone (FSH) increased without demonstrable episodic secretion. Luteinizing hormone-releasing hormone (LHRH) tests demonstrated that FSH responses are greater than the LH responses, as in prepuberty. In 3 cases estradiol levels had augmented above normal prepubertal range. The menses spontaneously stopped during the follow-up. A reevaluation of the gonadotropin pattern, having the menses stopped for 6 months, in one of the girls with pulsatile LH secretion showed an apulsatile prepubertal LH pattern. Also estradiol levels returned to prepubertal range. A follow-up of 10-66 months of these patients did not show any growth and bone acceleration or signs of precocious puberty. Our data suggest that in premature menarche a partial and transient activation of hypothalamo-pituitary axis could be present. Premature menarche seems to be a benign and self-limiting condition and one of the girls had a normal onset of puberty during follow-up.     

Lack of pubertal influences on female dispersal in muriqui monkeys, Brachyteles arachnoides

The hormonal mediation of dispersal in female mammals is poorly understood, in part because of the difficulties of detecting the onset of ovarian cycling and puberty in dispersing individuals. We used noninvasive methods of faecal steroid assays to determine the timing of dispersal relative to puberty and ovarian cycling in wild female muriqui monkeys, a species in which males are philopatric and nearly all females transfer from their natal groups. Natal females had a mean+/-SE age of 73.4+/-7.2 months (N=18) at the time of their transfers. Intergroup transfers occurred when one or more sexually active adult females were present, but did not show any seasonal patterns. Faecal progesterone and oestradiol profiles from nine natal females prior to transfer and four non-natal females that transferred into our study group demonstrate unequivocally that dispersal occurs prior to puberty in this species. All females showed baseline oestradiol levels and low progesterone levels compared with cycling adult females. Immigrants were first observed to copulate at 11.2+/-2.2 months of age (N=4), prior to the onset of normal ovarian cycles, and gave birth to their first offspring at 33.8+/-7.3 months (N=4) after transferring. Mean cortisol levels did not differ between natal emigrants or recent immigrants, and were within the range of those of adult males during the nonbreeding season in 10 of the 11 prepubertal females sampled. These results indicate that female dispersal is not triggered by activational hormones associated with puberty or escape from reproductive suppression in this species. Copyright 2000 The Association for the Study of Animal Behaviour.     

Estradiol influences on pattern of gonadotropin secretion in bovine males during the period of changed responses to estradiol feedback in age-matched females

When ovaries are removed prior to puberty, administration of exogenous 17 beta-estradiol (E2) decreases concentrations of luteinizing hormone (LH) below that of ovariectomized heifers receiving no E2. Subsequent to the time age-matched intact heifers reach puberty, exogenous E2 increases secretion of LH in ovariectomized heifers above that of ovariectomized heifers receiving no E2. The hypothesis that E2 would inhibit gonadotropin secretion in bovine males during the time E2 no longer inhibited gonadotropin secretion in age-matched bovine females was tested. Males (n = 12) and females (n = 12) were gonadectomized at 241 +/- 3 days of age, and half of each sex (6 males and 6 females) were administered a 27-cm E2 implant. An additional group of males (n = 6) and females (n = 6) remained intact and served as controls. Blood samples were collected (to quantify LH and follicle-stimulating hormone [FSH]) from all animals at 15-min intervals for 24 h at 1, 7, 13, 17, 21, 25, 29, 33, 37, and 43 wk after gonadectomy. Additional blood samples were collected twice weekly from control females to monitor progesterone and onset of corpus luteum function (451 days of age). E2 inhibited frequency of pulses of LH (p less than 0.01) and decreased mean concentration of LH and FSH (p less than 0.01) at Week 1 in gonadectomized males treated with E2 compared to gonadectomized males not administered E2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Seasonal variations in prolactin, growth hormone and thyroid hormones and the prolactin surge at ovulation do not affect litter size of ewes during pregnancy in the oestrous or the anoestrous season

Injection of bromocriptine from 5 days before until 5 days after mating clearly suppressed the periovulatory prolactin surge in ewes in the anoestrous and oestrous season but did not change the litter size significantly. Progesterone, GH, TSH or thyroid hormone concentrations were not influenced by the bromocriptine treatment. The progesterone concentrations were lower during the first weeks after mating in the anoestrous season compared to the oestrous season, while there was no difference between pregnant and non-pregnant ewes. During later gestation this seasonal difference was only observed in the non-pregnant ewes. At the same time there was a clear difference between pregnancy and non-pregnancy in both seasons. The prolactin, GH and thyroid hormone values also varied significantly during gestation. Since these patterns are identical in pregnant and non-pregnant ewes, the fluctuations are due to environmental factors and not to pregnancy or altered progesterone concentrations. In the anoestrous season prolactin, GH, T4 and T3 levels were higher than in the breeding season, while rT3 showed the opposite pattern. The TSH concentration did not differ between the two seasons. These results suggest that seasonal variations in prolactin, GH and thyroid hormones or the periovulatory prolactin surge do not affect litter size of ewes during pregnancy in the oestrous or the anoestrous season.