Influence of exogenous melatonin on seasonality of reproduction in sheep

Lack of effect of melatonin on ovarian function and response to estrous synchronization and fixed-time AI during the nonbreeding season in lactating dairy buffalo (Bubalus bubalis)

The present study was conducted to examine whether pretreatment with melatonin would enhance ovarian follicular functions and increase response to estrous synchronization and fixed-time AI (TAI) during the nonbreeding season in lactating dairy buffalo. In Experiment 1, buffalo cows without a detectable corpus luteum (CL) were assigned on Day -20 (D-20) to three groups: control (n = 12); melatonin (n = 13); progesterone (P₄) (n = 15). Cows in the melatonin group were implanted with melatonin on D-20. From D0 to D9, there was imposing of an estrous synchronization treatment regimen using either a standard Ovsynch protocol (control, melatonin) or a P₄-based Ovsynch treatment regimen (P₄). There were no differences (P > 0.05) among groups for the presence of a CL at D0, size of the largest follicle at D0, ovulation to GnRH injection at D0 and D9, or the time to ovulation after injection of GnRH at D9. In Experiment 2, there was imposing of the same treatment regimens as in Experiment 1, with inclusion of TAI. Females of the P₄ group had a greater (P = 0.001) pregnancy/AI percentage (60 %) than those in the control (17 %) and melatonin (23 %) groups. Females of the P₄ group also had a larger (P = 0.005) CL at D20 compared with those in the control and melatonin groups. Findings indicate treatment with melatonin for 20 days did not affect ovarian functions or the response to an estrous synchronization treatment regimen and TAI during the nonbreeding season in lactating dairy buffalo.

Effect of Melatonin Implants during the Non-Breeding Season on the Onset of Ovarian Activity and the Plasma Prolactin in Dromedary Camel

To examine a possible control of reproductive seasonality by melatonin, continual-release subcutaneous melatonin implants were inserted 4.5 months before the natural breeding season (October-April) into female camels (Melatonin-treated group). The animals were exposed to an artificial long photoperiod (16L:8D) for 41 days prior to implant placement to facilitate receptivity to the short-day signal that is expected with melatonin implants. The treated and control groups (untreated females) were maintained separately under outdoor natural conditions. Ovarian follicular development was monitored in both groups by transrectal ultrasonography and by plasma estradiol-17β concentrations performed weekly for 8 weeks and then for 14 weeks following implant insertion. Plasma prolactin concentrations were determined at 45 and 15 days before and 0, 14, 28, 56, and 98 days after implant insertion. Plasma melatonin concentration was determined to validate response to the artificial long photoperiod and to verify the pattern of release from the implants. Results showed that the artificial long photoperiod induced a melatonin secretion peak of significantly (P < 0.05) shorter duration (about 2.5 h). Melatonin release from the implants resulted in higher circulating plasma melatonin levels during daytime and nighttime which persisted for more than 12 weeks following implants insertion. Treatment with melatonin implants advanced the onset of follicular growth activity by 3.5 months compared to untreated animals. Plasma estradiol-17β increased gradually from the second week after the beginning of treatment to reach significantly (P < 0.01) higher concentrations (39.2 ± 6.2 to 46.4 ± 4.5 pg/ml) between the third and the fifth week post insertion of melatonin implants. Treatment with melatonin implants also induced a moderate, but significant (P < 0.05) suppressive effect on plasma prolactin concentration on the 28th day. These results demonstrate that photoperiod appears to be involved in dromedary reproductive seasonality. Melatonin implants may be a useful tool to manipulate seasonality and to improve reproductive performance in this species. Administration of subcutaneous melatonin implants during the transition period to the breeding season following an artificial signal of long photoperiod have the potential to advance the breeding season in camels by about 2.5 months.

The suprachiasmatic nuclei as a seasonal clock

In mammals, the suprachiasmatic nucleus (SCN) contains a central clock that synchronizes daily (i.e., 24-h) rhythms in physiology and behavior. SCN neurons are cell-autonomous oscillators that act synchronously to produce a coherent circadian rhythm. In addition, the SCN helps regulate seasonal rhythmicity. Photic information is perceived by the SCN and transmitted to the pineal gland, where it regulates melatonin production. Within the SCN, adaptations to changing photoperiod are reflected in changes in neurotransmitters and clock gene expression, resulting in waveform changes in rhythmic electrical activity, a major output of the SCN. Efferent pathways regulate the seasonal timing of breeding and hibernation. In humans, seasonal physiology and behavioral rhythms are also present, and the human SCN has seasonally rhythmic neurotransmitter levels and morphology. In summary, the SCN perceives and encodes changes in day length and drives seasonal changes in downstream pathways and structures in order to adapt to the changing seasons.

Melatonin-dependent timing of seasonal reproduction by the pars tuberalis: pivotal roles for long daylengths and thyroid hormones

Most mammals living at temperate latitudes exhibit marked seasonal variations in reproduction. In long-lived species, it is assumed that timely physiological alternations between a breeding season and a period of sexual rest depend upon the ability of day length (photoperiod) to synchronise an endogenous timing mechanism called the circannual clock. The sheep has been extensively used to characterise the time-measurement mechanisms of seasonal reproduction. Melatonin, secreted only during the night, acts as the endocrine transducer of the photoperiodic message. The present review is concerned with the endocrine mechanisms of seasonal reproduction in sheep and the evidence that long day length and thyroid hormones are mandatory to their proper timing. Recent evidence for a circadian-based molecular mechanism within the pars tuberalis of the pituitary, which ties the short duration melatonin signal reflecting long day length to the hypothalamic increase of triiodothyronine (T3) through a thyroid-stimulating hormone/deiodinase2 paracrine mechanism is presented and evaluated in this context. A parallel is also drawn with the golden hamster, a long-day breeder, aiming to demonstrate that features of seasonality appear to be phylogenetically conserved. Finally, potential mechanisms of T3 action within the hypothalamus/median eminence in relationship to seasonal timing are examined.

Effect of constant-release melatonin implants on the onset of oestrous activity and on reproductive performance in the ewe

Various types of constant-release melatonin implants were designed and administered to adult ewes during seasonal anoestrus. Implants were given between May and July in a series of four experiments. Treatment of ewes with melatonin implants resulted in advancement of the onset of oestrous activity. Melatonin treatment also resulted in improved reproductive performance, increased numbers of lambs born per ewe bred and caused more multiple births. Results from this study show, therefore, that constant-release melatonin implants administered during late spring or early summer advance the onset of the breeding season and improve reproductive performance in ewes.

The effect of melatonin implants on breeding activity and litter size in commercial sheep flocks in the UK

Two trials were undertaken to investigate the effects of treating seasonally anoestrous ewes with melatonin implants on date of first oestrus and other aspects of reproductive performance.

Trial 1 involved a total of 368 Mule ewes and 79 Scottish Blackface ewes on five farms, approximately half of which were treated with a single subcutaneous implant of melatonin (Regulin®), containing 18 mg melatonin, between 23 July and 6 August 1986 and the remainder acted as untreated controls. Treatment had no significant effect on the date of first oestrus or conception rate in Mule ewes, although it increased the number of Scottish Blackface ewes mating (92% v. 73%) and the number of mated ewes conceiving (69% v. 54%) in a 5-week mating period, resulting in significantly more treated ewes lambing (63% v. 37%; P < 0·01). Litter size was higher in 4/5 flocks, although this only reached statistical significance in one Mule flock and the Scottish Blackface flock.

A total of 2116 ewes from 17 commercial flocks were used in trial 2, approximately half of which were Suffolk/Suffolk-cross ewes and the remainder Mule/Mule-cross ewes. Implantation with melatonin occurred between 22 June and 24 July 1987. Flocks with over 100 ewes were divided into three equal-sized groups and treated with either 18 mg melatonin (one implant of Regulin, 36 mg melatonin (two implants of Regulin given at the same time) or acted as untreated controls. Flocks with less than 100 ewes contained only the 18 mg melatonin and untreated control groups. Treatment with melatonin significantly advanced the date of first oestrus in most flocks of both breeds (P < 0·05) but the magnitude of this effect was variable. Significant (P < 0·05 at least) increases in ‘potential’ (from scanning) mean litter size (+0·13 to +0·18) and actual mean litter size (+0·11 to +0·14) resulting from treatment with melatonin were apparent in ewes of both breeds when the data were pooled across all flocks, but only in 4/17 of the individual flocks.

These results indicate that treatment with melatonin implants may be a simple and effective way of advancing the breeding season and enhancing litter size of early lambing flocks under commercial farming conditions in the United Kingdom, but treatment must be given >60 days before the start of the natural breeding season for benefits in date of first oestrus to be manifest.

Influence of lambing date on subsequent ovarian cyclicity and ovulation rate in ewes

The effect of lambing date on the subsequent onset and duration of ovarian cyclicity in Mule (Bluefaced Leicester × Scottish Blackface) ewes was investigated. Nineteen ewes which had lambed in January (16 January 1993 (s.e. 3 days)) and been weaned in February-March and 22 comparable ewes which had lambed in May (14 May 1993 (s.e. 2 days)) and been weaned on 23 August were maintained at pasture as two isolated groups. A raddled vasectomized ram was continually present with each group from 14 July 1993 to 26 May 1994 and marked (oestrous) ewes were recorded twice weekly. Ovarian activity was assessed by measuring peripheral progesterone concentrations in blood samples collected twice weekly and by laparoscopic viewing of the ovaries of all ewes during October, January and March. The onset and duration of ovarian activity were significantly affected by the previous lambing date. For January and May lambing ewes, mean dates of onset were 5 September 1993 (s.e. 2 days) v. 25 September 1993 (s.e. 4 days) (P < 0·001) and of cessation were 5 April 1994 (s.e. 5 days) v. 10 April 1994 (s.e. 3 days). Mean durations of ovarian activity were 212 (s.e. 6) and 195 (s.e. 5) days (P < 0·05) during which 12·4 (s.e. 0·29) and 11·5 (s.e. 0·38) ovarian cycles respectively were recorded. Ovulation rate was not affected by previous lambing date but was significantly lower in March compared with October (January lambing ewes 1·7 (s.e. 0·1) v. 2·3 (s.e. 0·1) (P < 0·001); May lambing ewes 1·6 (s.e. 0·1) v. 2·1 (s.e. 0·1) (P < 0·01)). Results demonstrate that (i) Mule ewes have a potential breeding season of up to 8 months duration; (ii) the onset and duration of ovarian activity can be influenced by previous lambing date; and (Hi) a seasonal decline in ovulation rate may, in practical terms, result in a lower lambing percentage for animals bred towards the end of their natural breeding period.

Melatonin advances and condenses the onset of seasonal breeding in Greek dairy ewes

The effects of melatonin, administered during anoestrus, on the time of onset of oestrus and on conception rate were investigated in two trials carried out on mature ewes of three indigenous and one imported breed. The ewes lambed between December and February, their lambs were weaned in March and they were hand milked twice daily until September.

In the first trial, 12 Chios and 12 Serres ewes were dosed, orally, for 60 days from 19 April with 3 mg melatonin (M) in 10 ml ethanol and water (1: 4 v/v). A similar number of Chios and Serres type ewes in the same flock were nominated as untreated controls (C). Vasectomized rams were used to detect oestrus from 1 May and detected ewes were immediately mated to entire rams of their own breed. The mean dates of first oestrus in Chios ewes were 4 July (s.e. 1·7 days) and 23 July (s.e. 5·4 days) (P < 0·01) for treated and control ewes respectively. Spread of first oestrus was 19 and 61 days, respectively (P < 0·01). Conception rate and ovulation rate were both higher in M (0·90 and 2·3, respectively) than in C (0·55 and 1·8, respectively) but the differences were not statistically significant. The corresponding results in the Serres ewes were 8 July (s.e. 1·6 days) and 31 July (s.e. 6·8 days) (P < 0·01) for date of first oestrus, 19 and 69 days (P < 0·01) for spread of first oestrus, 0·75 and 0·50 (P > 0·05) for conception rate and 1·8 and 1·2 ( P > 0·05) for ovulation rate.

In the second trial, 12 Karagouniko and 12 East Friesland ewes received a single subcutaneous implant of melatonin (Regulin®) on 5 June and a further 12 ewes of each breed were untreated controls. The procedure was as given for trial 1. The mean date of first oestrus was advanced by melatonin in both Karagouniko ewes (25 July (s.e. 3·91 days) v. 11 August (s.e. 6·7 days); P < 0·05) and Friesland ewes (28 July (s.e. 3·2 days) v. 24 August (s.e. 7·3 days); P < 0·01). Spread of oestrus was reduced (43 v. 68 days in Karagouniko, P < 0·05 and 22 v. 87 days in Friesland ewes, P < 0·01). Conception rate of melatonin-treated ewes was higher (0·60 v. 0·45; P > 0·05) in both breeds. Ovulation rate was higher in melatonin-treated Karagouniko ewes (1·8 v. 1·3; P > 0·05 but, in the Friesland breed melatonin-treated ewes had a lower ovulation rate (2·8 v. 3·7; P > 0·05).

In both trials, melatonin treatment significantly advanced and condensed the dates of first oestrus. Conception rate was consistently increased over all groups (P < 0·05) and there was a tendency towards a higher ovulation rate.

Reproductive and Endocrine Characteristics of Delayed Pubertal Ewe-Lambs after Melatonin and l-Tyrosine Administration

The investigation was carried out to study the impact of melatonin and L-tyrosine administration on the onset of cyclicity in delayed pubertal ewe-lambs. Fifteen delayed pubertal ewe-lambs (age >16 months) were used in this study after being assigned randomly into three groups. In the first group (melatonin treated group, n = 5), each lamb was administered 3 mg melatonin orally at 16:00 hours from 1 July to 15 September while in the second group (L-tyrosine treated group, n = 5), each lamb was administered L-tyrosine at the level of 100 mg/kg b.w. as a single oral dose. The third group (n = 4) served as control. Lambs were exposed to mature, fertile rams daily and blood samples were collected twice weekly. The progesterone concentrations (P4 evaluations) were significantly higher (p < 0.05) in treated groups than the control group. Ovarian activity, assessed by P4 evaluations, showed that all animals in the first group came in oestrus and from them four got pregnant (80%) while in the second group, only three lambs came in heat and from them two became pregnant (40%). On the contrary, none of the control lambs showed oestrus. In conclusion, this study confirmed that the oral administration of melatonin and L-tyrosine played an essentially physiological roles to induce cyclicity in delayed pubertal ewe-lambs and improved their reproduction.

Melatonin supplementation does not prevent photostimulatory effects of night interruption lighting in Japanese quail

Male Japanese quail (Coturnix japonica) exposed to night interruption lighting were given melatonin either as single daily injections (10-100 mg/kg), or as continuous release implants. Birds were exposed to a base photoperiod of 8 hr of light, plus a 15 min night interruption pulse of light 14 hr after the beginning of the base photoperiod. Daily injections of melatonin were given 20 min prior to the night interruption pulse. Control birds either received no exogenous melatonin or received injections during the last 2 min of the night interruption pulse. Exogenous melatonin was able to induce sleep-like behavior at higher doses but did not prevent gonadal recrudescence as measured by the cloacal protrusion area. The role of melatonin in avian reproduction appears to be different from that in mammals. Perhaps melatonin's role in birds is to regulate circadian activities, not circannual ones such as the onset of reproduction.

Failure of melatonin implants to alter onset of puberty in gilts

Forty crossbred gilts (Hampshire x Yorkshire x Landrace x Duroc) weighing 55.4 +/- 0.5 kg at 4.2 +/- 0.1 months of age received either melatonin implants (N = 20) or served as controls (N = 20). Gilts were housed in a temperature controlled room (21 degrees C) with a light (L):dark (D) cycle of 10L:14D. Initially, four implants (12 mg melatonin each, Wildlife Pharmaceuticals, Ft. Collins, CO) were placed in one ear subcutaneously. At 2-week intervals, five, six, seven or eight implants were placed in ears of the gilts (one implant per 11.8 kg body weight). Throughout the trial, blood samples were collected twice weekly by jugular venipuncture. All gilts were exposed to mature boars for 15 min on alternate days. Age of puberty was determined from serum concentrations of progesterone and visual observations of estrus. Serum concentrations of melatonin were elevated (5-10 fold) throughout the trial in those gilts that received melatonin implants. Average daily gain (P = 0.51) and age at puberty (P = 0.57) were similar between gilts that received melatonin implants or no implants. Even though gilts continued to receive melatonin after they attained puberty, elevated serum concentrations of melatonin did not alter the length of subsequent estrous cycles (P = 0.90). These data indicate that elevation of serum concentrations of melatonin via implants did not alter the onset of puberty or length of subsequent estrous cycles.

Is a critical interval of the circadian pacemaker at dusk responsive to light and melatonin responsible for the timing of estrus in the Romney Marsh ewe?

Two experiments, using Romney Marsh ewes, tested for the existence and role of a critical interval of the circadian pacemaker located near dusk that may be integrally involved in the precise timing of the breeding season. Groups of Romney Marsh ewes (n = 6) were provided with exogenous melatonin by injection at dusk (Experiment 1) or by infusion at dawn or subjected to extended darkness at dawn (Experiment 2) from the winter to the summer solstice before being exposed to natural photoperiod at latitude 35 degrees S. Other than the experimental protocols, all animals were held in natural photoperiod. The onset of the breeding season (defined as cyclic ovarian activity as indicated by plasma progesterone monitoring) was normal in those animals treated with morning melatonin but was delayed in those animals treated with melatonin at dusk or extended darkness at dawn compared to controls in natural photoperiod (p < .01). Exogenous melatonin at dusk was associated with a phase advance of the onset of the circadian pacemaker (as measured by endogenous melatonin in acutely extended darkness); additional darkness at dawn was associated with a phase delay of both the onset and the offset of the circadian pacemaker. Exogenous morning melatonin did not change the phase of the circadian pacemaker relative to the controls. The results are consistent with an external coincidence model of seasonal breeding in which a critical interval of the circadian pacemaker requires exposure to light during spring/summer to time estrus correctly. The proposed critical interval appears to be located near dusk in this model and is phase locked to the circadian pacemaker. The effect of the exogenous melatonin on the timing of the breeding season is similar to darkness when administered at dusk but is not equivalent to darkness at dawn. The timing of anestrus was not affected by any of the experimental treatments and may reflect a common response to an environmental influence.

The timed infusion paradigm for melatonin delivery: what has it taught us about the melatonin signal, its reception, and the photoperiodic control of seasonal responses?

This review summarizes the evidence showing that the duration of the nocturnal secretory profile of pineal melatonin (MEL) is critical for eliciting seasonally appropriate reproductive physiological and behavioral responses in mammals. We review experiments using the timed infusion paradigm (TIP) to deliver MEL either systemically or centrally to pinealectomized hamsters and sheep. In this paradigm, MEL is infused, usually once daily, for a specific number of hours and at a predetermined time of day. This experimental strategy tests most directly those features of the MEL signal that are necessary to trigger photoperiodic responses. The data suggest that the duration of the MEL stimulation is the critical feature of the MEL signal for both inhibitory and stimulatory effects of the hormone on the photoperiodic control of reproductive development in juvenile Siberian hamsters, and for the photoperiodic control of reproductive and metabolic responses in adult Siberian and Syrian hamsters and sheep. The use of the TIP reveals the importance of the frequency of the signal presentation of MEL and suggests the importance of a period of low-to-absent circulating concentrations of the hormone. The TIP also reveals that the characteristics of the MEL signal that regulate male sexual behavior are similar to those that are critical for reproductive and metabolic responses in Syrian hamsters. We summarize the locations of possible functional MEL target sites identified by combining the TIP with traditional brain lesion techniques. Evidence from such studies suggests that the integrity of the suprachiasmatic nucleus (SCN) region in Siberian hamsters and the anterior hypothalamus in Syrian hamsters is necessary for the response to short-day MEL signals. The TIP has been used to deliver MEL to putative target sites for the hormone in the brain of juvenile and adult Siberian hamsters. The results of these preliminary experiments suggest that the regions of specific MEL binding in this species, especially the SCN, are effective sites where MEL may stimulate short-day-type responses. In contrast, results from intracranial application of MEL in sheep suggest the medial basal hypothalamus as a critical site of action. Finally, we also discuss potential applications of the TIP for identification of brain MEL target sites, understanding of other photoperiodic phenomena and responses, and resolution of the cellular/molecular basis underlying the reception and interpretation of MEL signals.(ABSTRACT TRUNCATED AT 400 WORDS)

Melatonin and photoperiodic time measurement: seasonal breeding in the sheep

Well-established circadian physiology supports the view that photoperiodic time measurement utilizes the coincidence between the presence of light and a photosensitive phase of a 'biological clock' to alter reproductive status--the so-called external coincidence model of seasonal breeding. In this review, we examine the mechanism whereby photoperiod interacts with presumed suprachiasmatic nuclei activity to allow endogenous melatonin to normally synchronize reproductive activity to the optimal time of year. The Romney Marsh sheep is particularly explored as an experimental model. It is suggested that the on/off activity of seasonal reproduction may be a robust mechanism able to be predictably manipulated by the judicious use of the light/dark cycle and exogenous melatonin, but firmly based on circadian principles.

Plasma melatonin levels in Japanese quail exposed to dim light are determined by subjective interpretation of day and night, not light intensity

Plasma melatonin levels were measured in male Japanese quail exposed to lighting schedules consisting of combinations of bright light (2000 or 1500 lx), darkness, or dim light (2 lx) or to constant dim light. Melatonin levels in dim light were dependent upon the relative intensity of accompanying phases, being significantly higher when dim light was subjective night than when it was subjective day. There was no significant melatonin rhythm in constant dim light, even on the first day of constant dim light exposure. Melatonin levels were intermediate when dim light was accompanied by both bright light and darkness. These results indicate that melatonin secretion in birds does not depend solely on light intensity. Furthermore, these results suggest that the avian circadian system may be more sensitive to environmental cues than its mammalian counterpart.

Effect of rumen degradable bolus containing melatonin or progestagen pessary plus pregnant mare serum gonadotropin on estrus response and lambing rate in ewes

Two practical regimens designed to induce estrus and ovulation in ewes in late anestrus were compared. Forty ewes were given a soluble glass rumen bolus containing 150 mg melatonin on July 9 and were joined with two vasectomized rams on July 23 and with three fertile rams on August 6. A second group of 40 ewes was treated with an intravaginal progestagen pessary (60mg medroxy-progesterone acetate) on July 23. Following pessary removal after 12 d, ewes were given 750 IU of pregnant mare serum gonadotropin (PMSG). Five fertile rams were joined with these ewes 48 h after progestagen removal. Melatonin concentrations were determined in single blood samples collected in early afternoon of July 21. Mating dates, lambing dates and litter sizes were recorded. Date of mating was significantly later in ewes treated with melatonin compared with those treated with progestagen plus PMSG (P<0.0001). All ewes given melatonin were mated within 4 wk, and those on progestagen plus PMSG treatment within one day of fertile ram introduction. Thirty-four ewes (85%) allocated to melatonin treatment and 36 (90%) allocated to progestagen plus PMSG treatment lambed (P>0.05). Mean (+/-SEM) lambing date was later in melatonin-treated ewes (January 17+/-1.2 d) compared to those given progestagen plus PMSG (December 30+/-0.6 d; P<0.0001). Mean litter size was lower in melatonin-treated ewes (1.5+/-0.1) compared with those given progestagen plus PMSG (2.0+/-0.1; P<0.001). Plasma melatonin concentrations indicated that 9 of 40 ewes treated with melatonin had circulating melatonin concentrations of less than 16 pg/ml. It is concluded that under conditions that existed in this experiment, treatment with progestagen plus PMSG in late anestrus resulted in more synchronous mating and lambing and a higher litter size than that following administration of a soluble glass rumen-degradable bolus containing melatonin.

Out-of-season breeding in goats

Maintenance of winter milk production and the production of meat kids for seasonal specialty markets are two major reasons for out-of-season breeding. Strategies include the manipulation of light cycles and the administration of exogenous gonadotrophins following progestogen priming with either vaginal sponges or subcutaneous implants. A number of does in excess of expected need should be prepared. Excess numbers of bucks also are advisable, because libido and fertility often are lower out of season.

Effect of prolonged exposure to exogenous melatonin on the onset and end of the breeding season and on the growth rate of ewe lambs

The effect of prolonged exposure to exogenous melatonin on the reproductive status and growth rate of ewe lambs was investigated. Ewe lambs (born late March) were given intravaginal melatonin implants on 4 July (group J, n = 10) during anoestrus, or 20 December (group D, n = 10) during the breeding season. A third group (group C, n = 7) received empty implants on 4 July. Plasma progesterone concentrations were used to assess reproductive status. In the control group cyclic ovarian activity began on November 15 +/- 4 days, and ceased on January 28 +/- 8 days. In group J the onset of the breeding season was advanced by 4.9 weeks (occurring on October 12 +/- 4 days; P less than 0.001). The onset of anoestrus was also advanced in this group with 9 out of 10 of the ewes ceasing ovarian activity at least 3.4 weeks in advance of the control ewes (P less than 0.01). In contrast the timing of anoestrus was unchanged in group D, occurring on February 14 +/- 6 days. Melatonin treatment of ewe lambs from early July had no effect on growth rate, whereas treatment from mid-December had a depressive effect. Ewes therefore become refractory to the inductive effects of exogenous melatonin after long-term exposure. The results are consistent with the view that photorefractoriness is due to changes in the processing rather than the generation of the melatonin signal.

Failure of prazosin to mimic the effects of melatonin under in vivo and in vitro conditions

The gonads of male hamsters exposed to short photoperiod (LD, 10:14) or treated with melatonin in the late afternoon under long photoperiod (LD, 14:10) had undergone complete regression by the end of the treatments (8 weeks). Animals treated for the same period of time with prazosin (a putative melatonin analogue) under the same conditions failed to show a difference in their gonadal status as compared to the long photoperiod controls. Prazosin was unable to prevent melatonin-induced gonadal atrophy when injected either in the morning or 30 min before melatonin. Moreover, prazosin was without any effect on (and unable to prevent the melatonin-stimulated) progesterone production by rat adrenals under in vitro conditions. These data demonstrate that prazosin, which reportedly inhibits 2-[125I]iodomelatonin binding in the hamster brain in an affinity-related manner, does not possess properties of a biological melatonin analogue under conditions of two different model systems in two species.

Reproductive physiology of the nonpregnant mare. An overview and update

This article reviews the reproductive events in the nonpregnant mare with emphasis on recent advances. The discussion is restricted to the salient features of puberty (prenatal and prepubertal events), seasonality (gonadotropins, photoperiod, and other modifying factors), and the estrous cycle (hormones, estrus, diestrus, and the control of cyclicity) in the nonpregnant mare.

Effects of exogenous melatonin prior to the breeding season on testis weight and epididymal androgen receptors in rams

Rams were randomly assigned to an experiment that evaluated effects of treatment (2.5 mg melatonin/d for 45 d vs control; MEL vs CON) in mid-May through June on testis weight and concentration of epididymal androgen receptor in July or September (Sept). Mean testis weight of MEL and CON rams was not different in July (332 vs 283 g), but in Sept was less (P less than .05) for MEL rams than CON rams (268 vs 382 g). Testis weight of MEL rams was less (P less than .05) in Sept than in July. Caput, corpus and cauda epididymal tissues were used to prepare extracts which were analyzed for concentration of dihydrotestosterone (DHT) receptor using a new standard curve method. A standard extract was characterized using four independent 8-point Scatchard analyses and found to contain 6.0 fmol DHT receptor/mg wet tissue (Ka = 3.5 X 10(8).M-1); this extract was used to establish standard curves for assays of unknown samples. Data on concentration of DHT receptors measured by Scatchard analysis and the standard curve method were highly correlated (r = 0.99; P less than .01; n = 8). Concentrations of DHT receptor were not affected by treatment, month of castration, or their interaction. However, for data pooled across treatment and month, concentration (fmol/mg protein) of DHT receptor was greater (P less than .05) in caput or corpus (125 or 122) than in cauda (92) epididymidis. The regional distribution of epididymal DHT receptors in this study confirmed our previous findings.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of pregnancy in sheep by radioimmunoassay of sera for pregnancy-specific protein B

A radioimmunoassay (RIA) for bovine pregnancy-specific protein B (bPSPB) has been shown to be a reliable test for pregnancy in cows. Pregnant ewes have a blood antigen that cross-reacts in this RIA. Two studies were conducted to determine the accuracy of detection of pregnancy in sheep using the bPSPB RIA. In Study 1, 33 ewe lambs were bred over a 70-d period in late fall. At 26, 56, and 83 d after the end of the breeding period, blood samples were collected for assay in the bPSPB RIA, and the Pregmatic 3 ultrasonic device was used to detect pregnancy. Pregmatic 3 detected pregnancy in 14, 27 and 28 ewes and nonpregnancy in 19, 6 and 3 ewes at Days 26, 56 and 83 past the breeding period, respectively. The bPSPB assay detected pregnancy in 32, 31 and 30 ewes and nonpregnancy in 1, 2 and 2 ewes at Days 26, 56 and 83 past breeding, respectively, Thirty ewes lambed and three did not. In Study 2, 180 multiparous ewes were bred over a 60-d period in summer. At 35 d after the end of the breeding period, blood samples were collected for assay in the RIA, and a real-time ultrasonic scan was done to detect pregnancy. Real-time ultrasonic testing detected pregnancy in 163 ewes and nonpregnancy in 17 ewes; whereas, the RIA detected pregnancy in 161 ewes and nonpregnancy in 19 ewes. One hundred fifty-nine ewes lambed and 21 did not. The bPSPB RIA detected pregnancy earlier and more accurately than the Pregmatic 3 ultrasonic device and was equally as accurate as the real-time scanning instrument. These studies demonstrate an accurate serological test for a pregnancy-specific antigen in sheep.

Influence of exogenous melatonin and altered day length on reproductive performance of Polypay ewes

Two experiments were designed to evaluate postlight treatment use of melatonin as a method of overcoming photorefractory response by measuring the lambing percentage of early-postpartum Polypay ewes bred either late in the breeding season or during anestrus. In Trial 1, pregnant ewes (n = 140) were assigned to one of three groups: 1) ambient photoperiod (control), 2) extended light (20 h) from October 21 to December 27, and 3) extended light as in Group 2, followed by supplemental feeding of 10 mg melatonin/head (hd) daily until April 6. Breeding started on February 3 and ended April 9. A greater percentage of ewes given extended light plus melatonin (54%) and extended light alone (45%) lambed than controls (24%) (P = 0.06). In Trial 2, pregnant ewes (n = 158) were assigned to groups as in Trial 1, except extended light was given to Groups 2 and 3 from January 1 to March 11, and Group 3 ewes were supplemented with 10 mg melatonin daily from March 12 to June 10. Breeding started April 18 and ended June 10. Lambing percentage was increased (P < 0.01) by extended light plus melatonin (54%) compared to controls (6%) or ewes given only extended light (10%). The shift from artificially extended light to the shorter ambient light with or without melatonin enhanced the lambing percentage of early postpartum ewes on an accelerated lambing program during breeding late in the season. However, only the more pronounced shift from long days to short days, accomplished with extended light plus melatonin, was effective in inducing out-of-season breeding in Polypay ewes.

Melatonin advances june breeding and fall lambing in range and farm-flock type ewes

Influence of melatonin on reproductive performance was evaluated by randomly allotting Polypay-type ewes to four treatments: controls in drylot, melatonin-treated ewes in drylot, controls on range, and melatonin-treated ewes on range. An additional group of Targhees and Rambouillets was randomized by breed as control ewes or melatonin-treated ewes to test effect of melatonin in range ewes. All ewes were supplemented with 0.34 kg/head alfalfa-barley (1:1) pellets from 1600 to 1630 h daily, from April 15 to June 30. Ewes maintained on range received no further supplementation, while ewes in drylot received alfalfa pellets for maintenance. Melatonin was fed at 10 mg hd(-1) d(-1) in the pellet to designated ewes in drylot or on range. Three rams were put with each group of ewes on June 1 and rotated among groups at 7-d intervals during the first 30 d of breeding to reduce sire differences. After the first 30 d, ewe groups were combined with all rams. Melatonin induced (P<0.01) an earlier onset of estrus (approximately 17 d) as indicated by earlier lambing dates in Polypay-type, Rambouillet, and Targhee ewes managed on spring range. Melatonin also increased (P<0.01) the number of ewes that conceived during the first 30 d of breeding (June) for both management treatments (drylot and range) and for all breeds.

The effect of exogenous melatonin administration on gonadotropin and prolactin patterns in ovariectomized estradiol-treated heifers exposed to increasing photoperiod

Eight nulliparous Angus and Angus crossbred heifers, which had been ovariectomized and treated with estradiol-17beta (E(2)) S.Q. implants for 6 months, were used to determine the effects of exogenous melatonin on serum gonadotropin and prolactin concentrations. Melatonin (15 mg) or corn oil (vehicle) was administered as a single i.m. injection at 1600 h daily for 12 weeks (March 19 to June 4, 1982). Blood samples taken weekly via jugular venipuncture at approximately 1100 h were assayed for luteinizing hormone (LH), follicle stimulating hormone (FSH), prolactin and E(2). At 4-week intervals, animals were fitted with indwelling jugular cannulae at 1100 h and samples were taken for 4 h at 15-min intervals. These samples were used to estimate pulsatile patterns of LH, FSH and prolactin. On the day of the first 15-min sampling, additional blood samples were collected at 30 min intervals from 1500 to 2200 h to determine the acute effect of melatonin injection on concentrations of LH, FSH and prolactin. Melatonin did not affect concentrations of FSH collected at weekly intervals (P=0.03) but tended to inhibit the decrease in concentrations of LH seen in the heifers treated with vehicle (P=0.12). There was a melatonin x time interaction for FSH (P=0.04) and a tendency for this interaction for LH (P=0.11). Circulating concentrations of prolactin were not different between treatment groups (P=0.83) nor was there a melatonin x time interaction (P=0.03). Estradiol was higher in the melatonin treated group (P=0.03) (15.58 +/- 4.17 versus 8.25 +/- 1.25 pg/ml) (X +/- SEM) and the melatonin x time interaction was significant (P=0.001). There was a tendency for a melatonin x time interaction for FSH pulse frequency (P=0.10). Prolactin pulse duration tended to decrease in response to melatonin treatment (P=0.14) (15.92 +/- 9.29 versus 11.04 +/- 4.57 min). These data do not support the hypothesis that melatonin decreases prolactin concentrations in cattle and indicates that other factors must mediate photoperiod regulation of this hormone. However, the interpretation of these data is less clear concerning the hypothesis that melatonin may maintain elevated concentrations of gonadotropins in the presence of increasing photoperiod. Concentrations of FSH appeared to be more affected by melatonin than LH; consistent with previous observations that FSH may be more affected than LH by changes in photoperiod (2). But neither LH or FSH concentrations were clearly shown to be consistantly elevated in the melatonin treatment group.