Reproductive refractoriness in the Welsh Mountain ewe induced by a short photoperiod can be overridden by exposure to a shorter photoperiod

The objectives were to determine if relative lengths of photoperiods that induce reproductive cycles in ewes affect the length of the subsequent breeding season, if duration of the refractoriness that terminates breeding is affected by photoperiod length, and if the resulting refractoriness to an inductive photoperiod is absolute. Groups of Welsh Mountain ewes were exposed to either 12L:12D (n = 12) or 8L:16D (n = 6) photoperiods beginning at the summer solstice when daylengths reach a maximum of 17.5 h at Bristol, England. A control group (n = 10) was exposed to natural daylengths. Ovarian cycles in the controls, as judged by monitored plasma progesterone levels, commenced in early October, about 1 mo later (p less than 0.001 in both cases) than in sheep exposed to 12L:12D or 8L:16D. The advancement in cycle onset was similar under 12L:12D and 8L:16D (69 +/- 2 and 77 +/- 4 days after the summer solstice compared with 102 +/- 2 days in the controls). Duration of the breeding season (100 +/- 4 days) in ewes exposed to 12L:12D was significantly shorter (p less than 0.001 in both cases) than in ewes exposed to natural daylengths or 8L:16D (153 +/- 3 and 133 +/- 5 days, respectively). Approximately 70 days after the ending of ovulatory cycles in the 12L:12D group, half of the animals (n = 6) were transferred to 8L:16D. This treatment greatly (p less than 0.001) reduced the duration of anestrus and cycles began again 62 +/- 4 days after transfer to 8L:16D, or about 90 days earlier than in ewes (n = 6) remaining in 12L:12D.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of frontal hypothalamic deafferentation on duration of breeding season and melatonin secretion in the ewe

The objectives of this study were to determine if ewes subjected to frontal hypothalamic deafferentation (FHD) during anestrus remained anestrus or began to have estrous cycles, and if melatonin secretion was disrupted by FHD. Ovary-intact ewes in Group 1 were subjected to either FHD (n = 10) or sham FHD (n = 5) in early July 1983. Estrous cycles were monitored by measuring circulating progesterone concentrations from before FHD until September 1985. Group 2 ewes (n = 4) were subjected to FHD in October 1984. In late April 1985, blood samples were taken from all ewes at 1- to 4-h intervals from 1100 h to 0700 h of the following day to monitor diurnal changes of melatonin. Hypothalami were collected for histological evaluation of lesions. All Group 1 ewes (sham FHD and FHD) initiated normal estrous cycles in August and September 1983, and all ceased cycles by mid-February 1984. All sham FHD and 4 FHD ewes remained anestrus until August or September of 1984 and then resumed normal cycles. In contrast, 5 FHD ewes resumed cycles as early as April 1984 and then cycled intermittently or almost continuously. Two Group 2 ewes cycled continuously after FHD and 2 cycled infrequently. FHD ewes that showed prolonged breeding seasons had cuts that damaged the suprachiasmatic nucleus (SCN) and adjacent structures. Mean nocturnal (2000 h-0500 h) melatonin concentrations did not differ (p greater than 0.05) between sham FHD, FHD "normal season," and FHD "continuous cycle" ewes. In summary, damage to the SCN region by FHD during anestrus had no detectable effect on either onset or cessation of the next breeding season but greatly prolonged subsequent breeding seasons. Thus, the environmental signals that both initiated and terminated the 1983 breeding season apparently had been given before FHD was performed in midsummer. Damage to the SCN region during the breeding season caused some ewes to cycle continuously. The effects of FHD apparently were not due to disruption of melatonin secretion. FHD ewes that showed prolonged breeding seasons had normal seasonal changes of plasma prolactin concentrations. This suggests that different neural structures control seasonal patterns of gonadotropin and prolactin secretion.     

Photoperiodic disruption of photorefractoriness in the ewe

The primary objective of this study was to determine the duration of exposure to a long-day or short-day photoperiod required to disrupt photorefractoriness to short-day and long-day photoperiods, respectively. In Experiment 1, 4 groups of Suffolk breed ewes--designated B1, B2, B3, and B4--were placed in photochambers one day before the winter solstice, exposed to a 16L:8D photoperiod for 0, 30, 60, or 90 days, and then exposed to a 10L:14D photoperiod until the time of the summer solstice. Blood samples taken by venipuncture thrice weekly were analyzed for progesterone concentrations. The interval between start of the study and cessation of estrous cycles did not differ significantly between groups (p greater than 0.05). All 6 ewes in Group B1 then remained in anestrus for the duration of the study. Four of the 6 ewes in Group B2, and all ewes in Groups B3 and B4 resumed cycles after exposure to the 10L:14D photoperiod. In Experiment 2, 4 groups of ewes--designated A1, A2, A3, and A4--were placed in photochambers one day before the summer solstice, exposed to a 10L:14D photoperiod for 0, 30, 60, and 90 days, respectively, and then exposed to a 16L:8D photoperiod. Ewes in Group A1 started estrous cycles at a time not significantly different from ewes kept outdoors. However, onset of cycles was significantly advanced (p less than 0.05) in ewes exposed to 10L:14D. After ewes were returned to the 16L:8D photoperiod, estrous cycles were suppressed in 5 of 6 ewes in Group A2 and in all ewes in Groups A3 and A4.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of short days in timing the onset and duration of reproductive activity in ewes under artificial photoperiods

The objective of this work was to evaluate the role of short photoperiod in timing the onset and duration of reproductive activity in ewes. The perception of photoperiod was disrupted by pinealectomy following transfer from long (17L:7D) to short (8.5L:15.5D) photoperiod and the subsequent reproductive response was monitored. Ovariectomized ewes given Silastic implants containing estradiol-17 beta were exposed to long days until Day 0 (May 24) and then were allocated to the following groups (n = 5-6/group): Group 1) short-day control--moved to short days; Groups 2 to 5) pinealectomy after 0, 30, 60, or 90 short days, respectively; Group 6) long-day hold--kept on long days; Group 7) long days after 60 short days--moved to short days on Day 0 and returned to long days on Day 60. Six ewes kept outdoors served as additional controls. Reproductive neuroendocrine activity was assessed from plasma LH concentrations, high values being indicative of the breeding season and low values indicative of anestrus. Time of reproductive neuroendocrine activity onset (LH rise) did not differ among animals in the 7 groups kept indoors, but was advanced (p less than 0.05) relative to that of ewes outdoors. In contrast, duration of the LH elevation differed among ewes in groups kept indoors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Different neuroendocrine systems modulate pulsatile luteinizing hormone secretion in photosuppressed and photorefractory ewes.

The objective of this study was to determine whether two photoperiod regimens that induce anestrus in the ewe-short-day photorefractoriness (SDPR) and long-day photosuppression (LDPS)--act by different neuronal mechanisms. In separate experiments, ovary-intact (INTACT), ovariectomized (OVX), and ovariectomized estradiol-treated (OVX + E) ewes were subjected to three different photoperiodic regimens that resulted in reproductive quiescence: (1) exposure to long days (16L:8D), which caused photosuppression (INTACT, n = 9; OVX, n = 6; OVX + E, n = 5; (2) prolonged exposure to short days (10L:14D)), which caused photorefractoriness (INTACT, n = 10; OVX, n = 6; OVX + E, n = 5); (3) exposure to natural photoperiod, which induced seasonal anestrus (INTACT, n = 11; OVX, n = 6; OVX + E, n = 5). Effect of photoregimen was monitored by measuring progesterone or LH. Drug challenges were made after two sequential estrous cycles were missed in INTACT ewes, after mean LH concentrations dropped below 1 ng/ml in OVX + E ewes, and after LH interpulse intervals increased in OVX ewes. Effects of drug on LH pulse pattern were determined by taking blood samples at 12-min intervals for 8 h after i.v. diluent injection; then for 8 h after i.v. injection of cyproheptadine, a serotonin antagonist (3 mg/kg); and again 7 days later after i.v. injection of diluent or pimozide, a dopamine antagonist (0.25 mg/kg). Cyproheptadine had little effect except to decrease (p = 0.05) mean LH in INTACT anestrous ewes and decrease (p less than 0.01) pulse amplitude in OVX + E SDPR ewes. Pimozide did not affect LH pulse frequency in LDPS ewes. However, pimozide increased LH pulse frequency (p less than 0.005) and mean concentrations (p less than 0.005) in SDPR OVX + E ewes, whereas it suppressed LH pulse frequency (p less than 0.05) and amplitude (p less than 0.03) in SDPR INTACT and SDPR OVX ewes. The results suggest that (1) the role of the dopaminergic system differs in SDPR and LDPS ewes, and that different neuronal systems may effect SDPR and LDPS, (2) the effect of pimozide in SDPR ewes is altered by ovarian steroids, and (3) the serotonergic system has relatively little role in regulating pulsatile LH secretion in any of the three different states of anestrus.     

Evidence that the onset of the breeding season in the ewe may be independent of decreasing plasma prolactin concentrations

Ten ewes of each of two breeds, Dorset Horn (long breeding season) and Welsh Mountain (short breeding season), were given subcutaneous oestradiol-17 beta implants and then ovariectomized. Another 10 ewes of each breed were left intact. On 3 May 1982, all the ewes were housed in an artificial photoperiod of 16L:8D. After 4 weeks, half of the ewes of each breed and physiological state were abruptly exposed to a short-day (8L:16D) photoperiod while the others remained in long days (16L:8D). The time of onset of the breeding season was significantly (P less than 0.05) advanced in ewes switched to short days (12 August +/- 10 days) compared to those maintained in long days (4 September +/- 14 days). Dorset Horn ewes began to cycle (20 July +/- 7 days) significantly (P less than 0.001) earlier than Welsh Mountain ewes (19 September +/- 6 days). Disparities in the time of onset of cyclic activity in ewes of different breeds and daylength groups were echoed in disparities in the time at which plasma LH and FSH concentrations rose in oestrogen-implanted, ovariectomized ewes of the same light treatment group. Prolactin concentrations showed an immediate decrease in ewes switched to short days, but remained elevated in long-day ewes. Since the breeding season started in the presence of high prolactin concentrations in long-day ewes, it seems unlikely that prolactin is an important factor determining the timing of the onset of cyclic activity.     

What photoperiodic signal is provided by a continuous-release melatonin implant?

The purpose of this study was to evaluate whether the insertion of a continuous-release melatonin implant into ewes provides a short-day photoperiodic signal or acts as a functional pinealectomy (provides no specific photoperiodic signal but renders ewes incapable of responding to changes in photoperiod). Ewes primed with 60 long days (18L:6D) during the spring were moved to intermediate day length (13L:11D) for 66 days and then given one of five treatments: 1) short-day control, second drop in photoperiod to 8L:16D; 2) intermediate-photoperiod control, kept on 13L:11D; 3) pinealectomy and kept on 13L:11D; 4) melatonin implant and kept on 13L:11D; 5) melatonin implant and moved to 8L:16D. Mean number of estrous cycles per group and total duration of reproductive activity were determined. Ewes in all groups began to exhibit estrous cycles after the initial reduction in photoperiod. The number of estrous cycles and duration of reproductive activity differed among groups. The number of estrous cycles and duration of reproductive activity was extended in ewes receiving the second drop in photoperiod compared to that of the intermediate-photoperiod controls. Pinealectomized ewes had a number of estrous cycles and duration of reproductive activity similar to those of ewes maintained on the intermediate photoperiod. Melatonin implants increased the number of estrous cycles and prolonged reproductive activity in ewes maintained on the intermediate photoperiod; melatonin implants did not prevent the extension of reproductive activity in ewes receiving the second photoperiodic drop to the short daylength.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pineal and ovarian response to 22- and 24-h days in the ewe

Melatonin secretion in ewes was entrained by 22-h light-dark cycles whether of long (16L:6D) or short (6L:16D) photoperiod. In photoperiods of 6L:16D, a phase-delay of melatonin secretion was evident, leading to a dark-phase duration shorter than that found in 8L:16D. Early onset of estrus was induced in anestrous ewes kept in 8L:16D, but not 6L:16D, from 22 July compared to controls in natural light. In photoperiods of 16L:6D, the melatonin profile corresponded precisely to the dark phase. Early offset of estrus was induced in estrous ewes kept in both 18L:6D and 16L:6D from 18 December compared to controls in natural light. Thus, when the duration of melatonin secretion was appropriate to the long photoperiod (16L:6D), but with a constantly changing phase position, a long-day reproductive response was found. Activity-rest cycles were not entrained by 16L:6D; thus the synchronization of melatonin and activity-rest cycles does not appear to be essential for the induction of a long-day reproductive response. These results support the hypothesis that the duration, not the circadian-phase position, of melatonin is critical to the induction of photoperiodic effects.     

Effects of pentobarbital anesthesia on tonic luteinizing hormone secretion in the ewe: evidence for active inhibition of luteinizing hormone in anestrus

In the ewe, seasonal anestrus appears to result from two effects of inhibitory photoperiod: 1) estradiol gains the capacity to suppress luteinizing hormone (LH) pulse frequency and hence becomes a potent inhibitor of tonic LH secretion and 2) a steroid-independent decrease in LH pulse frequency occurs in ovariectomized ewes. In this study, we have obtained evidence, using pentobarbital anesthesia, that both these actions of photoperiod reflect the activation, in anestrus, of an inhibitory neural system. Administration of pentobarbital to intact anestrous ewes produced a dramatic, 3-fold increase in LH pulse frequency during the 6 h of anesthesia. In contrast, during the breeding season, pentobarbital inhibited LH pulse frequency in luteal phase animals. There was also a seasonal variation in the effects of pentobarbital in ovariectomized ewes. During the breeding season this drug again suppressed LH secretion, inhibiting both LH pulse amplitude and frequency. In anestrus, pentobarbital also suppressed pulse amplitude, but it produced a transitory increase (lasting 3 h) in pulse frequency. To account for the stimulatory actions of pentobarbital, we propose that in anestrus, but not the breeding season, LH pulse frequency is held in check by a set of estradiol-sensitive inhibitory neurons. Further, we suggest that these neurons are activated by inhibitory photoperiod and account for both the steroid-dependent and steroid-independent actions of photoperiod.     

Anomalies in ovarian function of peulh ewes

The ovarian activity of 8 Niger Peulh ewes was followed for 2 1 2 years by assaying the levels of progesterone in blood plasma sampled daily and by endoscopic observation. Although the ewes did not experience seasonal anestrus, their cycles were not regular. Most animals had persistent corpora lutea at some stage, but particularly in June. This resulted in cycles averaging 49.9+/-6.8 days in length instead of the normal 16.9+/-0.1 days. Intervals between successive luteal phases lasted 4-15 days as compared with 2.3+/-0.06 days seen in normal cycles. This occurred in most ewes at least once during the period from December to April. In these cases, the preovulatory discharge of LH was delayed until 7.5+/-1.8 days after the fall in the level of progesterone. The incidence of these anomalies suggests that the ewes had 69% of the ovulations and 56% of the behavioral estrus as compared to ewes that cycled regularly.     

Thyroid hormones mediate steroid-independent seasonal changes in luteinizing hormone pulsatility in the ewe

Thyroid hormones permit the increase in response to estradiol negative feedback in ewes at the transition to anestrus. In this study, we tested whether the thyroid hormones are also required for steroid-independent seasonal changes in pulsatile LH secretion. In experiment 1, Suffolk ewes were ovariectomized and thyroidectomized (THX) or ovariectomized only (controls) in late November. LH pulse frequency and amplitude were measured for 4 h in December, April, May, June, and August. Pulse frequency was also measured in the presence of estradiol-containing implants during the breeding (December) and early anestrus (March) seasons. As expected, in the presence of estradiol, pulse frequency declined between December and March in control but not THX ewes. In the absence of estradiol, a seasonal decline in frequency and an increase in amplitude occurred in control ewes, concurrent with lengthening photoperiod. A similar trend was seen in THX ewes, but the seasonal changes were lower in magnitude and not significant. In experiment 2, the same protocol was used (pulse measurements in December, May, and June) with a larger THX group size (n = 7). Results were similar to those of experiment 1 for controls. In THX ewes, pulse frequency did not change over time and was significantly elevated relative to that of controls during the summer. Pulse amplitude in THX ewes tended to increase during summer and did not differ from pulse amplitudes in control ewes. These results demonstrate that thyroid hormones are required for steroid-independent cycles in LH pulse frequency; however, some seasonal changes in amplitude still occur in the absence of thyroid hormones. This finding contrasts with the changes in estradiol negative feedback at the transition to anestrus, which are entirely thyroid hormone dependent.     

A comparison of the efficiency of melatonin treatments in advancing oestrus in ewes

Early oestrous cycles were induced in adult, maiden, 18-month-old Suffolk-cross ewes, maintained from birth in natural photoperiod by the following treatments applied from mid-June: subcutaneous implantation of melatonin (1 g) in Silastic packets, daily, oral, melatonin administration (3 mg/ewe) at 15:30 h, an artificial photoperiod of 8L:16D (lights on 07:30 h). Ovarian cycles began 5-10 weeks before those of control ewes maintained in a natural photoperiod. In contrast, the onset of ovarian cycles in ewes given s.c. implants of melatonin (1 g) in April, and a further group in May, was highly variable, and not significantly different from that of the control ewes. Plasma melatonin profiles in sheep with implants showed a night-time rise super-imposed on a constant level, which was itself within the physiological night-time range. Implant-derived melatonin declined with time but remained at or above physiological night-time levels for at least 3 1/2 months. These results indicate that melatonin implants in June, but not in April or May, advance onset of oestrus in the non-lactating, adult ewe. The effects of melatonin implants in June on onset of ovarian cycles were indistinguishable from those of melatonin feeding or artificial short photoperiod initiated at this time of year.     

Effects of follicle stimulating hormone (FSH) on follicular development, oocyte retrieval, and in vitro fertilization (IVF) in ewes during breeding season and seasonal anestrus

Administration of FSH increases the number of developing follicles, and affects oocyte health and cleavage rate. To determine the optimal level of FSH treatment, studies were conducted during the normal breeding season and seasonal anestrus. In Experiment 1, ewes were implanted with SyncroMate-B (SMB; norgestomet) for 14 days during the breeding season. Beginning on day 12 or 13 after SMB implantation, ewes were treated with saline (control; n=10), or treated with FSH for two days (2D; n=9) or three days (3D; n=10). In Experiment 2, conducted during seasonal anestrus, ewes were implanted with SMB for 14 days (n=23) or were not implanted (n=26). The SMB-implanted and nonimplanted ewes were assigned to one of three treatments as in Experiment 1: control (n=13), 2D (n=21) or 3D (n=15). In Experiments 1 and 2, ewes were laparotomized to count the number of follicles < or = 3 mm and > 3 mm and to retrieve oocytes. Healthy oocytes from each treatment were used for IVF. In Experiment 3, ewes (n=6) were implanted twice with SMB for 14 days during seasonal anestrus. Ewes were injected with FSH for 2 days, and the oocytes were collected and fertilized as in Experiments 1 and 2. In Experiment 1, FSH-treatment increased (P < 0.05) the number of follicles > 3 mm, the number of oocytes retrieved from follicles < or = 3 mm and > 3 mm, the proportion of healthy oocytes, and the number of oocytes used for IVF. Oocytes from control and 2D ewes had greater (P < 0.01) cleavage rates than 3D ewes (68% and 71% vs. 42%). In Experiment 2, implanted and nonimplanted ewes had similar (P > 0.05) numbers of follicles, total oocytes, and healthy oocytes; therefore, data were combined. The FSH treatment increased (P < 0.01) the number of follicles > 3 mm, and the number of oocytes recovered from follicles > 3 mm. The recovery rate of oocytes and the percentage of healthy oocytes were similar for control and FSH-treated ewes. The cleavage rate in Experiment 2 ranged from 4 to 16%. In Experiment 3, the cleavage rate for ewes treated twice with SMB was 27% which tended to be greater (P < 0.07) than for the 2D ewes that received one SMB implant in Experiment 2. These data indicate that FSH increased the number of developing follicles and the number of healthy oocytes retrieved from ewes during the breeding season and seasonal anestrus. However, cleavage rates during seasonal anestrus were lower than during the normal breeding season in both FSH-treated and control ewes. Treatment of ewes for 2 days with FSH resulted in a greater cleavage rate than treatment of ewes for 3 days.     

Photoperiod and ovulatory menstrual cycles in female macaque monkeys

Macaques (Macaca mulatta and M. assamensis) which had been maintained on a 12L :12D light cycle for the previous 4 years and had 25-35-day menstrual cycles were randomly assigned to two groups. Those in Group 1 were kept in 12L :12D for 13 months. Those in Group 2 were subjected to three successive 5-month periods of 20L :4D, 4L :20D and 20L :4D. There were no significant differences between the two groups in the frequency, duration and percentage of ovulatory menstrual cycles, suggesting that photoperiod is not the sole regulator of seasonal breeding in these animals.     

Immunoreactive luteinizing hormone, estradiol, progesterone, testosterone, and androstenedione levels during the breeding season and anestrus in Siberian tigers

Seasonal analysis of 1239 captive births of Siberian tigers (Panthera tigris altaica) indicated a peak in April to June (P less than 0.001). Studies on seven animals in Minnesota indicated that behavioral heat cycles and ovarian follicular phase cycles began in late January and ceased in early June. Behavioral observation of 12 heat cycles in four tigers yielded an estrous length of 5.3 +/- 0.2 days and an interestrous interval of 25.0 +/- 1.3 days. Hormone assays on weekly blood samples (N = 180) from three female tigers indicated 16 cycles in two breeding seasons. Peak estradiol-17 beta levels were 46.7 +/- 6.0 pg/ml (N = 17) and interestrous concentrations were 8.7 +/- 0.66 pg/ml (N = 28) during the breeding season. Anestrous estradiol levels were 4.2 +/- 0.5 pg/ml (N = 70). The interestrous interval between estradiol peaks was 24.9 +/- 1.3 days (N = 9) with two outliers of 42 days. Serum progesterone concentrations from February to June were 1.2 +/- 0.15 ng/ml (N = 32), providing no evidence for ovulation or corpus luteum formation. Luteinizing hormone (LH) levels were 0.56 +/- 0.04 ng/ml (N = 180). Serum testosterone (r=0.71, P less than 0.001) and androstenedione levels (r=0.75, P less than 0.001) were correlated with estradiol during the breeding season. The duration of anestrus was 8 mo in two of these tigers. The interval was shortened in one tiger by exposure to a 16L:8D photoperiod. The Siberian tiger appears to be a polyestrous seasonal breeder and an induced ovulator whose breeding season may be synchronized by photoperiod.     

Inter-breed variation in the postpartum ewe response to continuous low dose infusion of GnRH

During the nonbreeding season the pituitary and ovarian responses to a subcutaneous GnRH infusion were investigated in acyclic, lactating Mule ewes which exhibit a deep seasonal anestrus and in Finn x Dorset ewes in which seasonal anestrus is ill-defined. Each breed received 10 d of progestagen priming before being subdivided into 3 groups. In Group L + G, 5 lactating ewes received GnRH (250 ng/h sc) for 96 h; in Group D + G, 5 dry ewes received GnRH (250 ng/h sc) for 96 h; in Group L, 5 lactating ewes received saline vehicle for 96 h. The infusions began when lactating and dry ewes were approximately 28 d and 120 d post partum, respectively. Blood samples were collected for LH, progesterone and estradiol analysis. Estrous behavior was monitored between Day -4 and Day +7. On Day +7 the reproductive tract was also examined. In the Mule ewes the mean plasma LH concentration increased (P < 0.05) following minipump insertion in each treatment group, although mean LH levels were greater (P < 0.05) in Group D + G, than in either Group L + G or Group L. Following the GnRH infusion, mean plasma estradiol levels increased (P < 0.05) in Group D + G but not in Group L + G. A preovulatory LH surge and subsequent ovulation occurred in 5 5 , 2 5 and 0 5 ewes from Group D + G, L + G and L, respectively, and estrus was recorded in 5 5 , 1 5 and 0 5 of these ewes, respectively. The LH surges began earlier (P < 0.05) (43.2 +/- 6.8 h vs 77.0 +/- 1.0 h) and the ovulation rate was greater (2.2 +/- 0.37 vs 1.00 +/- 0.00) in Group D + G than Group L + G. In the Finn x Dorset ewes mean LH concentrations increased (P < 0.05), to a similar level following minipump insertion in Groups D + G and L + G, but not Group L. The elevated LH levels were accompanied by increased (P < 0.05) plasma estradiol levels in Group D + G, but not in Group L + G. The GnRH infusion culminated in an LH surge and estrous behavior in 5 5 , 1 5 and 0 5 ewes from Groups D + G, L + D and L, respectively. The interval to the LH surge was similar between Group D + G (48.4 +/- 6.6 h) and Group L + G (46.0 h). Ovulation was evident in those ewes which exhibited an LH surge plus one additional ewe from Group L + G. The mean ovulation rate was greater in Group D + G (4.00 +/- 1.05) than in Group L + G (1.5 +/- 0.50). These data show that continuous GnRH infusion can consistently induce out of season breeding in the nonlactating Mule and Finn x Dorset ewe but can not break combined seasonal and lactational anestrous in these breeds. Further, between-breed differences are evident in the site along the hypothalamic-pituitary-ovarian axis at which reproduction is compromised in ewes at the same chronological stage post partum.     

Do endogenous opioid peptides mediate the effects of photoperiod on release of luteinizing hormone and prolactin in ovariectomized ewes?

Three experiments were done to determine if endogenous opioid peptides (EOPs) mediate the effects of photoperiod on release of luteinizing hormone (LH) and prolactin (Prl) in ovariectomized (OVX) ewes. Intravenous infusions of 0.5 naloxone X h-1 X kg body weight-1 for 3.5 h increased (P less than 0.01) mean plasma concentrations of LH and decreased (P less than 0.025) mean interpulse interval (period) of LH pulses in OVX ewes exposed to long day lengths (16L:8D). Infusions of either 1.0 or 2.5 mg morphine-SO4 X h-1 X kg-1 for 3 h increased (P less than 0.005) the period of LH pulses and increased (P less than 0.005) concentrations of Prl in OVX ewes during the breeding season. In OVX ewes exposed to long (16L:8D) or short (8L:16D) day lengths infusions of naloxone increased (P less than 0.05) mean concentrations of LH, whereas morphine decreased (P less than 0.01) mean concentrations of LH. These effects were attributed to changes in period of LH pulses (P less than 0.001). The drug X photoperiod interactions were not significant for LH parameters. Naloxone did not affect Prl release in either long- or short-day groups, but morphine increased (P less than 0.001) Prl release during long and short day lengths. The effect of morphine on Prl release was more pronounced in ewes exposed to long day lengths than in those exposed to short day lengths. In conclusion, EOPs inhibit the LH pulse generator in OVX ewes. However, it is doubtful that the EOPs mediate the steroid-independent effects of photoperiod on LH release. The results also suggest that photoperiod may influence Prl release via opiate neurons.     

Importance of retinal photoreceptors to the photoperiodic control of seasonal breeding in the ewe

Two experiments were performed to determine whether the eyes are necessary for photoperiodic control of reproduction in ewes. In the first, intact and estradiol-treated ovariectomized (OVX + E) ewes were housed in each of 2 photoperiod-controlled rooms with a vasectomized ram and subjected to 90-day alternations between long and short days. Prior to blinding, long days initiated anestrus in intact ewes and a suppression of serum luteinizing hormone (LH) levels in OVX + E ewes; short days caused onset of estrous cycles and an increase in LH levels in the intact and OVX + E ewes, respectively. After 1.5 years of such photoperiodic control, all ewes were blinded by bilateral orbital enucleation. Photoperiodic control was lost following blinding, but circannual alternations between cyclicity and anestrus or high and low LH levels, were maintained in most ewes for the remaining 2.5 years of the study. In one group of OVX + E ewes, serum LH levels remained synchronized to the 90-day shifts in photoperiod for about 1 year after blinding. Once the sighted ram was removed from the room, however, the 90-day rhythm in LH disappeared and a circannual pattern of LH became evident, suggesting that blind ewes may receive photoperiodic information from a sighted ram. This possibility was supported by the results of the second experiment in which 12 additional OVX + E ewes were blinded and exposed to 90 long days and 90 short days in the absence of a sighted ram. In these ewes, serum LH levels were not controlled by the changes in photoperiod. These results are consistent with the following conclusions: 1) the eyes are necessary for perception of photoperiod in the ewe and 2) ewes have an endogenous circannual rhythm of reproduction and/or they can be controlled by other environmental signals in the absence of photoperiodic input. Further, the results lead to the hypothesis that blind ewes can receive photoperiodic information indirectly from a sighted ram.     

Patterns of estrous cycles, estrous behavior, and circulating prolactin in spring and summer in ewes selected for autumn lambing and exposed to ambient or long-day photoperiods

Estrous behavior in response to ambient and long-day photoperiods was evaluated in ewes developed by 10 years of selection for ability to lamb in autumn. Following October lambing, 67 ewes were moved indoors and exposed to long-day (16L:8D) or ambient photoperiods from February 2 until July 6. Two vasectomized rams with marking harnesses were housed with each group. Estrous behavior was monitored twice weekly. Ewes from the selection line were unresponsive to long days, with no effects on estrous behavior, frequency of ovulation, or circulating prolactin. Adult ewes were anestrus for only 34±3 d, but 2- and 3-years-old ewes were anestrus for 72±7 and 57±10 d, respectively. Frequencies of ovulation based on circulating progesterone concentrations in March, May, and June were 97%, 95% and 52%, respectively, indicating that many ewes that did not exhibit estrus still ovulated. Prolactin concentrations increased from 10 ng/ml in February to 27 ng/ml in March and 173 ng/ml in June but were not affected by light treatment. Ten ewes that failed to exhibit estrus behavior for at most 24 d during the main study were then monitored for 74 additional long days. Nine of 10 ewes did not exhibit estrus for periods similar to 1 or 2 estrus cycles during this period, but eight ewes re-initiated cycles by the end of the study on September 18. Selection for ability to lamb in autumn thus resulted in ewes with an abbreviated seasonal anestrus and reduced sensitivity to long days.     

The role of nutrition in the regulation of LH secretion during anestrus by the serotoninergic and dopaminergic systems in Mediterranean ewes treated with melatonin

The steroid-dependent inhibition of LH secretion involves dopaminergic and serotoninergic systems but it is unclear how the plane of nutrition affects this inhibition during anestrus in melatonin treated ewes. Melatonin implants (18 mg) were inserted (Day 0) into ovariectomized, estradiol treated adult Rasa Aragonesa ewes on a high (H; n = 8) or low energy diet (L; n = 6) which were applied in early anestrus (Day 29-57) and late anestrus (Day 90-104). Cyproheptadine (0.1 mg/ kg), a serotoninergic (SHT2) receptor antagonist, was administered in early and late anestrus (Day 50 and 107) followed by pimozide (0.08 mg/kg), a dopaminergic2 receptor antagonist (Day 57 and 114). The H ewes had significantly higher LH concentrations (P < 0.05) before cyproheptadine treatment in early anestrus. The H and L ewes responded in a similar way to the antagonists in both early and late anestrus, except for L ewes who had a higher LH pulse amplitude after pimozide treatment in both periods (P < 0.05). During early anestrus, cyproheptadine tended to increase (P = 0.06) LH pulse frequency in L ewes and LH concentrations in H ewes. The LH secretion also increased in L ewes after pimozide administration during early anestrus (P < 0.05 for mean LH concentrations and LH pulse frequency and amplitude). However, pimozide dramatically increased LH secretion during late anestrus (Day 114) irrespective of the plane of nutrition (P = 0.06-0.08 for LH pulse frequency and P < 0.05 for LH concentrations and pulse amplitude). In melatonin treated Mediterranean ewes, the plane of nutrition appeared to modify the effect of dopaminergic and serotoninergic systems on the steroid-dependent inhibition of LH secretion throughout anestrus.