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.     

Pituitary response to LHRH, LH pulsatility and plasma melatonin and prolactin changes in ewe lambs treated with melatonin implants to delay puberty

Prepubertal ewe lambs were treated with empty or filled melatonin implants. The implants were placed s.c. at birth and pituitary responsiveness to various doses of LHRH, LH/FSH pulsatility and prolactin and melatonin secretion were examined at 10, 19, 28, 36 and 45 weeks of age. Control animals (N = 10) showed no consistent alteration in pituitary responsiveness to LHRH during development. Ewes treated with melatonin (N = 10) had puberty onset delayed by 4 weeks (P less than 0.03) but no effect of melatonin on LH or FSH response to LHRH injection was observed at any stage of development. In the control and melatonin-treated ewe lambs the responses to LHRH injection were lower during darkness than during the day at all stages of development. No consistent differences in LH or FSH pulsatility were observed between treatment groups or during development. Prolactin concentrations, however, failed to decrease at the time of puberty (autumn) in the melatonin-treated group. Melatonin-treated ewe lambs maintained normal rhythmic melatonin production which was superimposed on a higher basal concentration and showed the same increase in melatonin output with age as the control ewes. These results indicate that the delayed puberty caused by melatonin implants is not due to decreased pituitary responsiveness to LHRH or to dramatic changes in basal LH or FSH secretion.     

Plasma LH,FSH, testosterone,and age at puberty in ram lambs actively immunized against an inhibin alpha-subunit peptide

Active immunization against inhibin has been shown to advance puberty and increase ovulation rate in ewe lambs; but in ram lambs, effects on puberty and sperm production are equivocal. The objective of the present study was to determine whether active immunization against an inhibin alpha-subunit peptide advances the onset of puberty in ram lambs. St. Croix hair sheep ram lambs were assigned to inhibin-immunized (n = 7) and control (n = 8) treatment groups. Lambs in the inhibin-immunized group were immunized against a synthetic peptide-carrier protein conjugate, alpha-(1-25)-human alpha-globulin (halpha-G), and control lambs were immunized against halpha-G. Lambs were immunized at 3, 7, 13, 19, 25, 31, and 37 weeks of age. On the day of immunization a blood sample was collected and lambs were weighed. Another blood sample was collected 1 week following each immunization. At 20 weeks of age additional blood samples were collected at 20 min intervals for 8h. Beginning at 20 weeks of age and at weekly intervals thereafter, scrotal circumference (SC) was measured and semen was collected using electroejaculation. A subsequent ejaculate was collected 1 week following onset of puberty, which was defined as the week of age when an ejaculate first contained > or =50 x 10(6) sperm cells. In control lambs, plasma alpha-(1-25)-antibody (Ab) was nondetectable. In inhibin-immunized lambs, alpha-(1-25)-Ab titer increased from 7 to 25 weeks of age and then plateaued at a level that varied (P<0.001) among animals. Body weight and SC of control and inhibin-immunized lambs were similar at the onset of puberty. At pubertal onset inhibin-immunized lambs were older than control lambs (31.9+/-0.5 vs. 29.5+/-0.7 weeks of age, P<0.05). Plasma FSH concentrations were similar in control and inhibin-immunized lambs from 3 to 38 weeks of age. Plasma LH levels were lower (P<0.01) in inhibin-immunized than control lambs. During the 8-h blood sampling period at 20 weeks of age, LH and testosterone concentrations were lower (P<0.05) in inhibin-immunized than control ram lambs, and the LH pulse frequency was similar in the two groups of animals. The decreased LH secretion is consistent with the immunoneutralization of a putative inhibin alpha-subunit-related peptide that stimulates LH secretion in ram lambs. Present findings show that active immunization against an inhibin alpha-peptide delays rather than advances puberty in ram lambs.     

Study of LH response to GnRH in the young male as a criterion of genetic merit for female reproduction in sheep

A high and a low response line in sheep were selected on the basis of the mean concentration of LH in 10-week-old Finn-Dorset ram lambs after an i.v. injection of 5 micrograms GnRH. After 8 male generations the mean LH response of the high line was more than 5-fold that of the low line and the heritability of the selected trait was estimated at 0.44 +/- 0.015. Highly significant line differences in mean LH response to GnRH were also found in males at 20 weeks of age and females at 10 and 20 weeks of age and the genetic correlations between the four LH response traits appear to be close to unity. Large line differences in the mean FSH response to GnRH were also found in both males and females at 10 and 20 weeks of age. Selection had little effect on the physical characteristics of lambs. High-response line ewes entering their first breeding season at about 7 months of age showed oestrus earlier in the season and had higher ovulation rates and numbers of lambs born per ewe lambing than did low-response line ewes. In the second breeding season, at about 19 months of age, the only line difference was a higher ovulation rate early in the breeding season in high-line ewes. It is suggested that these changes may be mediated by a more rapid response in high-line ewes to increased GnRH stimulation at puberty or at the beginning of the breeding season.     

Effects of shortened daylength and melatonin treatment on plasma prolactin and melatonin levels in pinealectomised and sham-operated ewes

Comparisons have been made between the effects of shortened daylength and melatonin treatment on plasma prolactin and melatonin levels in pinealectomised (Px) and sham-operated (Sh) ewes. Twenty-two anoestrous Merino crossbred ewes, maintained under normal grazing conditions, were assigned to four groups for a period of 9 weeks. Group 1 remained untreated (control), Group 2 was herded into a dark shed at 1600 h each day until dark (approx 4 h), ewes in Group 3 were injected with 100 μg melatonin s.c. at 1600 h each day and ewes in Group 4 were implanted with a melatonin capsule releasing 125–200 μg/day. Another group (Group 5) of 4 Px and 4 Sh ewes from the same flock was maintained in an animal house and subjected to shortened daylength (10. 5 h L : 13. 5 h D, lights off 1600 h). Three weeks after the treatments began, ewes in Groups 1–4 were exposed to a fertile ram and ewes in Group 5 to a vasectomised ram and the day of mating noted. No differences were evident between Groups 1–4 in the ewes' response to the ram, time taken to conceive, duration of gestation or number of lambs born. In untreated Px ewes no plasma melatonin (< 20 pg/ml) was found in either day or night samples, whereas intact animals showed the characteristic night-time rise. The silastic implants produced stable daytime blood levels of 90–120 pg/ml, whereas a single injection of 100 μg melatonin caused a transitory (2–3 h) rise. Shortened daylength (Group 2) or a single daily injection of melatonin (Group 3) lowered prolactin levels but only in ewes with an intact pineal gland, whereas melatonin implants (Group 4) caused a reduction in plasma prolactin in both Px and Sh sheep. The results indicate that light-induced alterations in prolactin production in sheep involve both the pineal gland and melatonin. Continuous melatonin release from implants caused changes in plasma prolactin levels similar to those seen following exposure to short days.     

Reproductive response and LH secretion in ewes treated with melatonin implants and induced to ovulate with the ram effect

Two experiments were conducted to examine the effects of treating seasonally anoestrous ewes with melatonin before ram introduction on reproductive response, and on LH secretion in anoestrous ewes induced to ovulate by rams.In Experiment 1, a total of 667 ewes from three flocks involving Merino (Flock 1, N = 149), Merino entrefino (Flock 2, N = 325) and Rasa Aragonesa (Flock 3, N = 203) breeds were used. Within each flock, ewes isolated from rams since the previous lambing were assigned at random to receive melatonin implants of Regulin (75, 175 and 105 in Merino, Merino entrefino and Rasa Aragonesa flocks, respectively) or to serve as untreated controls (74 in Merino, 150 in Merino entrefino and 98 in Rasa Aragonesa flocks). Fertile rams were introduced into all flocks 5 weeks after implantation in March (Flocks 1 and 2) or April (Flock 3), and remained with the ewes for a 50 day mating period. Percentage of ewes with luteal activity at ram introduction did not differ between melatonin treated and control ewes in any flock. There were no significant differences in either the mean interval from ram introduction to lambing or the distribution of lambing. Implantation with melatonin resulted in an improvement of prolificacy in all three flocks, although this only reached statistical significance in the Merino flock (1.15 vs. 1.03 in treated and control ewes, respectively, P < 0.05). Fertility was increased significantly (P < 0.05) in the Merino entrefino flock (64.5% in treated vs. 51.3% in control ewes).In Experiment 2, two trials were undertaken utilizing a total of 63 ewes. Trial 1 involved 24 mature Manchega ewes and Trial 2 involved 39 Merino ewe lambs. Half of the animals in each trial received a Regulin implant on 28 February (Trial 1) or 12 March (Trial 2) and the remaining half acted as controls. Rams were introduced 5 weeks after implantation and remained with the ewes for a 25 day period. In both trials, anoestrous ewes at ram introduction were bled at 20 min intervals for 3 h before and 5 h after ram introduction and then at 3 h intervals over the next 24 h for assessment of plasma concentrations of LH. Secretion of LH before or following introduction of rams was not affected by melatonin. Both treated and control anoestrous ewes in each trial responded to introduction of rams with an increase in the frequency of the LH pulses (P < 0.05), but no significant changes were detected in pulse amplitude or mean levels of LH. A preovulatory surge of LH was detected between 8 and 26 h after ram introduction, but neither mean interval from ram introduction to the peak of LH surge, nor the magnitude of the LH peak, was influenced by melatonin treatment.Results from this study show that: (1) melatonin implants administered during early seasonal anoestrus have the potential to improve reproductive performance in Spanish breeds of sheep, but the response is conditioned by breed, management system and environmental factors; (2) melatonin did not modify the secretion of LH in anoestrous ewes induced to ovulate by the ram effect under our experimental conditions.     

Nightly duration of pineal melatonin secretion determines the reproductive response to inhibitory day length in the ewe

The pineal controls the reproductive response of ewes to both stimulatory (short) and inhibitory (long) day lengths. Melatonin, a pineal hormone whose nocturnal secretion is entrained by photoperiod, mediates the effect of stimulatory photoperiod. We now report that melatonin also mediates the effect of inhibitory day length, monitored as response to estradiol negative feedback on luteinizing hormone (LH) secretion. Ovariectomized, estradiol-implanted ewes were pinealectomized and intravenously infused with melatonin to restore the nightly melatonin rise. Following transfer from short to long days, and a concurrent switch from short- to long-day melatonin patterns, LH dropped precipitously in pinealectomized ewes, matching the photoinhibitory response of pineal intact controls. LH dropped similarly in pinealectomized ewes when long-day melatonin was infused under short days. Pinealectomized ewes transferred from long to short days displayed a marked LH rise, provided melatonin was also switched to the short-day pattern. LH remained suppressed if long-day melatonin was infused following transfer to short days. These data indicate the nighttime melatonin rise mediates reproductive responses to inhibitory, as well as stimulatory photoperiods; they further suggest the duration of this rise controls suppression of LH under long days. Rather than being strictly pro- or antigonadal, the pineal participates in measuring day length.     

Effect of maternal pinealectomy and reverse photoperiod on the circadian melatonin rhythm in the sheep and fetus during the last trimester of pregnancy

The present study tested the hypothesis that the nocturnal melatonin rhythm in the fetal sheep results from transfer across the placenta of melatonin from maternal circulation. Pregnant ewes were exposed to an artificial reverse photoperiod at about 100 days gestation (n = 6; lights on 10 h, 2200-0800 h PST). This treatment tested for entrainment in the ewe and its fetus of the 24-h pattern of melatonin production from the pineal gland. Other ewes were pinealectomized at 55 days post-breeding (n = 6), and similarly treated. Catheters were implanted and blood samples were collected between 117 and 142 days gestation at two 48-h periods, about every 0.5-4 h, to assess the pattern of melatonin in maternal and fetal circulations. In pineal-intact ewes and their fetuses, melatonin rhythms conformed to the reverse photoperiod, i.e. plasma melatonin concentrations were relatively low during the light period and significantly increased for the duration of darkness. In contrast, maternal pinealectomy abolished the melatonin rhythms in both the ewe and fetus; melatonin concentrations remained at or below the limits of detection. Pineal-intact sheep gave birth about 139 +/- 2 days (mean +/- SE, n = 4) at 1915 +/- 0.7 h and pinealectomized ewes (n = 5 of 6) lambed at 149 +/- 2 days at 0424 +/- 0.5 h. Finally, in lambs (n = 3) born to pinealectomized ewes, typical melatonin rhythms were present within the first week of life. The findings indicate that the maternal pineal gland is responsible for the 24-h pattern of melatonin in the ewe and its fetus during the last trimester of pregnancy.     

Maturational changes in opioidergic control of luteinizing hormone and follicle-stimulating hormone in ram lambs

Stimulation by naloxone, an opioid antagonist, of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion was examined in spring-born crossbred ram lambs raised under natural photoperiod. Vehicle (n = 6) or 1 mg naloxone/kg vehicle (n = 6) was injected (i.m.) 3 times at 2-h intervals at 5, 10 and 15 weeks of age and 4 times at 2-h intervals at 20, 25, 30 and 35 weeks of age. Blood samples were taken every 12 min for 6 h at 5, 10 and 15 weeks of age and for 8 h at 20, 25, 30 and 35 weeks of age. Naloxone had no effect on age at sexual maturity (controls 239 +/- 23 days; naloxone 232 +/- 33 days). The only significant (P less than 0.05) effect of naloxone on FSH was a greater pulse amplitude in 10-week-old treated lambs than in control lambs. Naloxone treatment resulted in greater LH pulse amplitude at 5 and 10 weeks of age (P less than 0.05), lower basal serum concentration of LH at 10 weeks of age (P less than 0.05), greater LH pulse frequency at 25 weeks of age (P less than 0.05), and greater mean serum concentrations of LH, basal LH and LH pulse amplitude at 35 weeks of age (P less than 0.01) than in the controls. In both groups of lambs, mean and basal FSH, and LH and FSH pulse amplitude were highest at 5 weeks of age and fell with age. LH pulse amplitude was lowest at 35 weeks of age (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of naloxone on gonadotrophin secretion at various stages of development in the ewe lamb

Spring-born crossbred ewe lambs were raised in a natural photoperiod and saline (N = 6) or naloxone (1 mg/kg) in saline (N = 6) was injected (i.m.) every 2 h for 6 h at 5, 10 and 15 weeks of age and for 8 h at 20, 25 and 30 weeks of age. Blood samples were taken every 12 min during treatment periods. Naloxone had no effect on time to first oestrus (controls 235 +/- 6 days, naloxone 242 +/- 7 days). Mean serum LH concentrations and LH pulse frequency were elevated by naloxone in ewe lambs at 20, 25, and 30 weeks of age (P less than 0.05). The only FSH response to naloxone was a depression of mean serum concentrations at 30 weeks of age (P less than 0.05). LH pulse amplitude was elevated at 5 weeks of age in all ewe lambs and declined thereafter to a nadir at 30 weeks of age in control, but not in naloxone-treated animals (P less than 0.05). LH pulse frequency was elevated at 10 weeks of age in control ewe lambs and in all animals at 30 weeks of age (P less than 0.05). FSH pulse frequency declined from 5 weeks of age in control ewe lambs (P less than 0.05), with very few pulses noted in 25- and 30-week-old animals. We conclude that (1) opioidergic suppression of LH, but not FSH, secretion developed at 20 weeks of age in the growing ewe lambs used in the present study, with no obvious change in suppression before the onset of first oestrus: (2) pulsatile FSH secretion occurred in the young ewe lamb but was lost as the lamb matured: (3) attainment of sexual maturity was preceded by an elevation in LH pulse frequency.     

Role of gonadal negative feedback on the gonadotrophin responses to gonadotrophin-releasing hormone (GnRH) in ram lambs from two lines of sheep selected for their luteinizing hormone response to GnRH.

Divergent selection has resulted in two lines of lambs (high and low) that have a 5-fold difference in their ability to release luteinizing hormone (LH) in response to 5 micrograms of gonadotrophin-releasing hormone (GnRH). Baseline gonadotrophin concentrations, the gonadotrophin responses to a GnRH challenge and the concentrations of testosterone and oestradiol were compared in lambs which were castrated at birth and intact lambs from both selection lines at 2, 6, 10 and 20 weeks of age. The pattern of LH and follicle-stimulating hormone (FSH) secretion was similar in the two lines, but differed between the intact and the castrated lambs. Basal LH and FSH secretion were significantly higher in the castrates than in the intact lambs from both selection lines. The high-line lambs had significantly higher basal FSH concentrations at all ages tested and significantly higher basal LH concentrations during the early postnatal period. The magnitude of the gonadotrophin responses to GnRH differed significantly between the intact and the castrated lambs within each line, the amount of gonadotrophins secreted by the castrated lambs being significantly greater. The removal of gonadal negative feedback by castration did not alter the between-line difference in either LH or the FSH response to the GnRH challenge. Throughout the experimental period, the concentration of testosterone in the intact lambs was significantly greater than in the castrated lambs in both selection lines, but no significant difference was seen in the concentrations of oestradiol. No significant between-line differences were found in the peripheral concentrations of testosterone or oestradiol in the intact lambs from the two selection lines. Therefore, despite similar amounts of gonadal negative feedback in the selection lines, there were significant between-line differences in basal gonadotrophin concentrations, at 2 and 6 weeks of age, and in the LH and FSH responses to an exogenous GnRH challenge, at all ages tested. Removal of gonadal negative feedback did not affect the magnitude of the between-line difference in the response of the lines to GnRH stimulation. The results indicate that the effects of selection on gonadotrophin secretion are primarily at the level of the hypothalamo-pituitary complex.     

Effect of intravaginal implants of melatonin on the onset of ovarian activity in adult and prepubertal ewes

Melatonin was administered intravaginally in Silastic tubing to adult and prepubertal ewes. In Exp. 1, ewe lambs (born early March) were given intravaginal melatonin implants at a mean age (+/- s.e.m.) of 7.5 +/- 0.1 weeks (Group E, N = 10) or 19.4 +/- 0.2 weeks (Group L, N = 10). The third group (Group C, N = 10) received empty implants. In Exp. 2 mature ewes were given implants on 13 May (Group E, N = 10) or 18 July (Group L, N = 10) or received empty implants (Group C, N = 10) on one of these two dates. Blood samples were taken twice weekly for progesterone assay. In Exp. 1 the mean age (+/- s.e.m.) at puberty (progesterone greater than 2 nmol/l for two consecutive samples) was 35.4 +/- 0.8 weeks. Puberty was advanced by 5.2 weeks in Group L lambs, occurring at a mean age of 30.2 +/- 0.7 weeks (P less than 0.001). In Group E lambs the timing of puberty was unaltered, occurring at a mean age of 34.8 +/- 0.6 weeks. Mature ewes in Group L (Exp. 2) showed increased incidence of ovarian activity (9/10 ewes cycling by 26 September) compared with the control ewes (1/10) (P less than 0.001), but there was no effect in Group E ewes (3/10). The results demonstrate that continuous melatonin administration to adult and prepubertal ewes can mimic the effect of short days in terms of the reproductive response, and that the present and previous exposure to melatonin is critical in determining the response.     

Patterns of variations in FSH, LH and 17beta-estradiol during the postnatal development of sheep

The aim of the present study was to estimate the onset of sexual maturity of F(2) lambs born to crossbred ewes (East-Friesian x Black-Head Pleven breeds) x East-Friesian rams based on measurments of plasma FSH, LH and 17beta-estradiol levels during postnatal development. The hormonal levels were measured by radioimmunoassay in blood samples taken from 107 ewe lambs at the age of 0 to 10 days, 1, 2, 3, 4, 5, 5.5, 6 months and at 1 year from anestrous ewes (birth - Day 0). Starting at a baseline concentration during Days 0-10, FSH rose to a peak at Month 2 and declined after Month 3 to levels equivalent to those seen in yearling, sexually mature ewes. Mean LH concentrations rose from baseline to the highest level in samples taken at 5.5 months and stabilized at 6 months to the level seen in yearling ewes. The preovulatory LH peak was recorded in 5.5 month-old lambs. Neither FSH nor LH declined to baseline concentrations in lambs after the initial 10 days of life. 17beta-estradiol fluctuated, showing an initial rise in samples taken between Days 0-10 and Month 2, followed by insignificant variations between different ages and were near to those in yearling ewes. The data suggest that the sexual maturity in lambs is attained at 5.5-6 months of age. The findings allow us to suggest that these crossbred ewes might be fertilized at an earlier age (11-12 months) if they had reached the neccessary body development (body weight: 75-80% of that of adult ewes). They also might be included earlier in estrous synchronization programs in order to give birth to 3 lambs in 2 years.     

Effects of active immunization of ram lambs and bull calves against synthetic luteinizing hormone releasing hormone

Ram lambs and bull calves were immunized against LH-RH by injections given in weeks 0, 6, 12 and 28 (ram lambs, week 0 = 16 to 20 weeks of age) and weeks 0, 6, 12 and 18 (bull calves, week 0 = approximately 4 weeks of age). The testis size of LH-RH-immunized animals was significantly less than that of controls from week 13 onwards in ram lambs and from week 15 onwards in bull calves. When ram lambs were sampled in week 17 and bull calves in week 20, mean plasma gonadotrophin and testosterone concentrations were consistently lower in LH-RH-immunized animals than in controls. Single intravenous injection of synthetic LH-RH or an analogue of LH-RH in week 27 failed to induce LH or testosterone responses in LH-RH-immunized ram lambs. Motile semen samples could not be obtained from any of the LH-RH-immunized ram lambs in weeks 24, 25 and 26 or from 7 of 10 in week 72, but samples of moderate motility were obtained in week 72 from three rams in which LH-RH antibody titres had fallen. No attempt was made to obtain semen from bull calves. After castration there was no increase in plasma LH in LH-RH-immunized rams and only a small increase in LH-RH-immunized bull calves. Mean testis weight was significantly lower in LH-RH-immunized animals than in controls of both species. Thus the normal development of the reproductive system in ram lambs and bull calves was blocked by active immunization against LH-RH. Some evidence was obtained for natural reversal of the effects with time and falling antibody titres. These findings demonstrate the potential of LH-RH immunization as an alternative to castration.     

Gonadotrophin secretion in prepubertal bull calves born in spring and autumn

The reproductive development of bull calves born in spring and autumn was compared. Mean serum LH concentrations in calves born in spring increased from week 4 to week 18 after birth and decreased by week 24. In bull calves born in autumn, mean LH concentrations increased from week 4 to week 8 after birth and remained steady until week 44. LH pulse amplitude was lower in bull calves born in autumn than in calves born in spring until week 24 of age (P < 0.05). There was a negative correlation between LH pulse frequency at week 12 after birth and age at puberty in bull calves, irrespective of season of birth, and LH pulse frequency at week 18 also tended to correlate negatively with age at puberty. Mean serum FSH concentrations, age at puberty, bodyweight, scrotal circumference, testes, prostate and vesicular gland dimensions, and ultrasonographic grey scale (pixel units) were not significantly different between bull calves born in autumn and spring. However, age and body-weight at puberty were more variable for bull calves born in autumn (P < 0.05). In a second study, bull calves born in spring received either a melatonin or sham implant immediately after birth and at weeks 6 and 11 after birth. Implants were removed at week 20. Mean LH concentrations, LH pulse frequency and amplitude, mean FSH concentrations and age at puberty did not differ between the two groups. No significant differences between groups in the growth and pixel units of the reproductive tract were observed by ultrasonography. In conclusion, although there were differences in the pattern of LH secretion in the prepubertal period between bull calves born in autumn and spring, the postnatal changes in gonadotrophin secretion were not disrupted by melatonin treatment in bull calves born in spring. Reproductive tract development did not differ between calves born in spring and autumn but age at puberty was more variable in bull calves born in autumn. LH pulse frequency during the early prepubertal period may be a vital factor in determining the age of bull calves at puberty.     

Endocrine differences in rams after genetic selection for testis size

Testis diameter and body weight were recorded from 6 to 76 weeks of age in ram lambs from two established lines selected for high (H) and low (L) testis size. While testis growth was greater in the H line up to 14 weeks of age (P less than 0.001), body weight was significantly lower, with the L line rams being 10 kg heavier by 76 weeks. There were no differences in plasma LH up to 20 weeks of age, but FSH concentrations were significantly lower at 14 and 20 weeks in the H line. Testosterone concentrations were not significantly higher in the H line from 6 to 20 weeks. In lambs castrated at birth, significantly higher FSH values were recorded from 6 to 20 weeks of age in the H line (P less than 0.001) whereas there was no difference in LH concentration at 6 and 10 weeks of age between the lines. At 14 and 20 weeks, however, the concentrations of LH were greater in the H than L line lambs (P less than 0.05). After hemicastration at 6 weeks of age, the rate of growth of the remaining testis in the L line lambs was significantly faster than in entire lambs of that line from 10 to 20 weeks (P less than 0.05 at 10 weeks to P less than 0.001 at 20 weeks). There was no difference in the rate of testis growth between the the entire and hemicastrated lambs from the H line from 6 to 12 weeks of age. It can be concluded that there is an underlying genetic difference in pituitary gland and/or hypothalamic activity in ram lambs from the two selected lines.     

Effects of hemicastration at various ages and of oestradiol-17 beta on plasma concentrations of gonadotrophins and androgens, testicular growth and interstitial cell responses in prepubertal lambs

The effect of the removal of one testis from cross-bred lambs at 1, 4, 8 or 12 weeks of age on plasma FSH, LH and testosterone was studied until 16 weeks of age. Hemicastration at all ages elicited a significant increase in plasma FSH compared to controls without a corresponding change in plasma LH or testosterone. The raised FSH after hemicastration at 1 or 4 weeks of age was suppressed to control levels between weeks 7 and 8; such a suppression was not observed in the 4 weeks following hemicastration at 8 or 12 weeks of age. The weight of the remaining testis had increased compared with the control by 12 weeks of age after hemicastration at 1 week (+ 69%), 4 weeks (+ 13%) and 8 weeks (+ 40%); hemicastration at 12 weeks of age also resulted in growth of the remaining testis at 16 weeks (+ 82%). The total androgen production of interstitial cells in response to ovine LH stimulation in vitro did not differ significantly between lambs of 1 and 12 weeks of age, or in animals of 4, 8 and 12 weeks of age after hemicastration at 1 week of age. Subdermal implantation of oestradiol-17 beta into 1-week hemicastrated lambs at the time of operation or at 6 weeks of age increased plasma oestradiol concentrations by approximately 2-4-fold, prevented the FSH and testicular growth responses to hemicastration and suppressed plasma LH and testosterone to levels lower than those in control lambs. The total androgen response of interstitial cells from the remaining testis of oestradiol-implanted lambs at 12 weeks of age was significantly reduced. We suggest that the pituitary-testis axis varies in sensitivity during the prepubertal period although the interstitial cellular response of the testis to LH stimulation remains constant.     

Short-term pituitary-testicular responses of prepubertal ram lambs to hemicastration

To examine the short-term effects of hemicastration on pituitary-gonadal responses, 12 ram lambs were anesthetized and hemicastrated at 4 mo of age and killed (n = 4) at 2 (HC2), 7 (HC7), or 14 (HC14) days following surgery. Four intact (INT) rams were killed 14 days following anesthesia. Testis and pituitary weights were similar between HC and INT rams. Serum follicle-stimulating hormone (FSH) in HC rams increased within 6 h, peaked at 12 h, and remained elevated above INT levels throughout the study. Overall mean serum testosterone levels in HC rams were lower than in INT rams for the first 48 h, but were similar by 3 days post-surgery. Pulsatile luteinizing hormone (LH) and testosterone secretion was suppressed for the first 9.5 h following anesthesia and/or surgery in both HC and INT animals. A single LH pulse and succeeding testosterone pulse occurred in 10/12 HC and 4/4 INT rams between 10 and 14 h post-surgery, both of which were lower in amplitude in HC than INT animals. However, on Day 7, pulsatile secretory patterns of LH and testosterone were similar, suggesting compensatory androgen secretion had occurred in HC rams. Pituitary LH content was unaffected by hemicastration. In contrast, pituitary FSH content was greater in HC7 and HC14 compared to HC2 and INT animals. Pituitary gonadotropin hormone-releasing hormone (GnRH) receptor concentrations were similar in INT, HC7, and HC14 rams, but were slightly reduced in HC2 rams. Neither testicular LH nor FSH receptor concentrations were altered by hemicastration at any time.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reproductive characteristics of lambs actively immunized early in life with inhibin-enriched preparations from follicular fluid of cows

Active immunization of Merino and crossbred ewe lambs early in life with a partly purified inhibin derived from bovine follicular fluid (bFF) advanced their puberty. This occurred whether the lambs were immunized from 3 weeks of age or from 9 weeks of age or just 3 times between 3 and 9 weeks. However, the effect was more obvious with multiple injections starting at 3 weeks of age. The ovulation rate of the immunized lambs was significantly higher than that of the control ewes. When lambs were subjected to multiple immunizations the increased ovulation rate persisted in Merinos for at least 1 year after immunization ceased. The lambs injected 3 times only (at 3, 6 and 9 weeks of age) did not show any increase in ovulation rate. Plasma concentrations of FSH and LH measured in blood samples taken 1 week after immunization were not significantly different between the treatment groups. Daily sperm output in the urine and testis diameter were measured in ram lambs from the same breeds and flocks as the ewe lambs. Immunization with the inhibin preparation from 3 weeks of age resulted in a significant increase in daily sperm output (4255 +/- 701 vs 2344 +/- 344 x 10(6), P less than 0.01) and testis diameter (5.21 +/- 0.3 vs 4.33 +/- 0.2 cm, P less than 0.05) in Merino rams examined in the non-breeding season when the rams were aged 23 months. Although the same trend was seen in the following year the differences were not significant. Immunization from 9 weeks of age had no effect. A similar increase in daily sperm output was seen (at 20 and 25 months of age) in crossbred rams immunized from 3 weeks of age but the difference was not significant. Plasma concentrations of FSH (but not LH) in samples from rams immunized from 3 weeks of age were significantly higher than those of control rams at 7 and 60 weeks of age in Merino rams and at 23 and 31 weeks of age in crossbred rams. Plasma FSH concentrations at 21 and 26 weeks of age in the Merino rams immunized from 3 weeks of age were higher than those of control lambs, and these increases preceded a significant (P less than 0.05) increase in testicular diameter at 30 weeks of age (4.99 +/- 0.2 vs 4.37 +/- 0.2 cm). These results suggest that active immunization early in life with an inhibin-enriched fraction from bFF advances puberty in ewe lambs.(ABSTRACT TRUNCATED AT 400 WORDS)     

Does seasonal reproductive state affect the neuroendocrine response of the ewe to a long-day pattern of melatonin?

This study examined whether or not the reproductive response of female sheep to photoperiod varies with seasonal reproductive state. The specific objective was to test the hypothesis that the reproductive response to a long-day pattern of melatonin varies with the reproductive state of the ewe. The response examined was the synchronization of reproductive neuroendocrine induction (rise in serum luteinizing hormone, or LH) following nocturnal infusion of melatonin into pinealectomized ewes for 35 consecutive nights. This infusion restored a pattern of circulating melatonin similar to that in pineal-intact ewes maintained in a long photoperiod (LD 16:8). The ewes had been pinealectomized and without melatonin replacement for 16-25 months prior to the study. They were in differing reproductive states at the start of the infusion, as their endogenous reproductive rhythm had become desynchronized among individuals and with respect to time of year. Noninfused pinealectomized ewes served as controls. Regardless of the reproductive state at the start of the 35-day infusion of the long-day pattern of melatonin, all treated ewes exhibited the same reproductive neuroendocrine response after the infusion was ended. This consisted of a synchronized rise in LH some 6-8 weeks after the infusion was terminated, the maintenance of a high level of serum LH for some 15 weeks, and a subsequent precipitous fall in LH to a very low level. These results provide evidence that a long-day pattern of melatonin can synchronize reproductive neuroendocrine induction in the ewe, regardless of reproductive condition, and thus do not support the hypothesis that this response differs with seasonal reproductive state.