Effects of melatonin treatment on the onset of ovarian activity,reproductive parameters and PRL plasma levels of immature ewes

The aims of this study were to investigate the effect of chronic treatment with melatonin on prolactin plasma profiles, the onset of ovarian activity and the fertility of immature ewes. Beginning in late June, 30 maiden ewes were administered 2.5 mg melatonin (i.m.) daily until mid-September. Progesterone and prolactin plasma concentrations were determined by validated radioimmunoassays on samples collected every 5 days from 17 June until 31 October. Prolactin (PRL) plasma concentrations in the control animals were highest at the beginning of the experiment and lowest towards its conclusion. In melatonin-treated ewes, PRL levels dropped just after treatment began and were similar to those observed in the control animals at the end of the experimental period. The ovarian activity was advanced by approximately one month by the administration of melatonin, while the mean date of lambings was advanced by about two weeks compared with the controls. Fertility in the treated animals was very similar to that of the controls; the prolificacy (lambs/pregnant ewe) was 2 in the treated and 1.62 in the control ewes.     

Influence of exogenous melatonin on seasonality of reproduction in sheep

Two experiments were performed to examine the effects of exogenous melatonin on seasonality of reproduction in ewes. To simulate the secretory pattern observed on the shortest day of the year, 2.5 mg melatonin were administered i.m. at 1600 hr each day. This dosage has been shown to keep serum concentrations of melatonin elevated until the onset of darkness on the longest day of the year (i.e. ～ 6 hr). In experiment 1, ewes were administered melatonin (n=10) or vehicle (n=10) from June 20 until the onset of the breeding season. Jugular blood samples were collected three times weekly and the serum was stored for analysis of progesterone by radioimmunoassay. The interval from the first day of treatment until serum concentrations of progesterone remained above 1 ng/ml in two consecutive samples (indicative of the formation of a corpus luteum) was 39.3 ± 4.1 days in the melatonin-treated ewes and 61.4 ± 4.6 days in the ewes receiving vehicle (P < 0.05). In the second experiment, ewes were given injections of melatonin (n=6) or vehicle (n=6) from December 21 until September 1. Again, jugular blood samples were collected three times weekly for analysis of progesterone. The interval from initiation of treatment until serum concentrations of progesterone remained below 1 ng/ml for at least 5 consecutive samples was 80.2 ± 3.3 days in the melatonin-treated ewes and 37.8 ± 6.5 days in the control ewes (P < 0.05). However, when treatment was begun on January 1 the interval from the initiation of treatment until serum concentrations of progesterone were greater than 1 ng/ml in two consecutive samples after June 1 was not different between melatonin-treated and control ewes. These data indicate that adminis-tration of melatonin to ewes can hasten the onset of the breeding season in the fall or extend the length of the breeding season in the spring, but after prolonged administration the ewes may become refractory to the treatment.     

Seasonal and hormonal control of pulsatile LH secretion in the dairy goat (Capra hircus)

In Exp. 1, the changes in pulsatile LH secretion at the onset of the breeding season were observed in 20 intact, mature Saanen does. Blood was sampled every 20 min for 6 h each week from the beginning of August until the onset of ovulatory activity, as evidenced by cycles in plasma progesterone. The first doe ovulated at the end of August and all were cycling by the end of September. As the first ovulation approached, LH pulse frequency increased by 67% and mean levels of LH increased by 47%. These changes were progressive rather than abrupt. In Exp. 2, seasonal changes in the inhibition of pulsatile LH secretion by ovarian steroids were studied in ovariectomized Saanen does. The animals were untreated (N = 4) or given subcutaneous oestradiol implants (N = 4) and blood was sampled every 10 min for 6 h, twice during the breeding season and twice during the anoestrous season. In each season, the second series of samples was taken after the animals had been treated with progesterone, administered by intravaginal implants. Season did not significantly affect LH secretion in goats not treated with oestradiol, but LH pulse frequency was 54% lower during the anoestrous season than during the breeding season in oestradiol-treated goats. Mean LH concentrations were affected in the same manner as pulse frequency, but pulse amplitude was increased by oestradiol treatment in both seasons. Progesterone had no detectable effect on LH secretion in either season. In Exp. 3, the response to repeated melatonin injections at a set time after dawn was investigated in 11 oestradiol-treated, ovariectomized goats.(ABSTRACT TRUNCATED AT 250 WORDS)     

A novel melatonin antagonist, N-(2,4-dinitrophenyl)-5-methoxytryptamine neutralizes some effects of melatonin in the female Syrian hamster

In this present study we evaluated the ability of a recently synthesized melatonin antagonist, N-(2,4-dinitrophenyl)-5-methoxytryptamine (ML-23), to antagonize the effects of afternoon injections of melatonin on the reproductive and thyroid axes in the female Syrian hamster. Thirty-six animals were divided into four groups and treated daily for 13 weeks with an afternoon injection of melatonin (25 micrograms/injection) or saline diluent. ML-23 was given via the drinking water to both melatonin- and saline-treated groups. The experiment was continued until 78% of melatonin-treated animals exhibited acyclicity. The results show that ML-23 partially reversed the effects of melatonin on pituitary follicle-stimulating hormone concentrations but was without effect on the decreased pituitary and plasma prolactin concentrations induced by melatonin treatment. Furthermore, ML-23 antagonized the effects of melatonin on plasma thyroxine levels and significantly increased plasma triiodothyronine concentrations and the free triiodothyronine index when used in combination with melatonin. The decrease in ovarian weight and plasma estradiol, but not progesterone, obtained with melatonin treatment also was reversed by ML-23. Our data suggest that ML-23 prevents the effects of melatonin treatment on ovarian weight, pituitary follicle-stimulating hormone levels, plasma estradiol, and thyroxine concentrations in the female Syrian hamster. Since ML-23 did not prevent the effects of melatonin on pituitary weight, plasma luteinizing hormone and prolactin, and pituitary prolactin concentrations, the actions of ML-23 may involve only peripheral sites of action of melatonin. Alternatively, the dose of ML-23 may not have been optimal to prevent all of the central effects of the indoleamine.     

Melatonin concentrations in the two jugular veins, and relationship with the seasonal reproductive activity in goats

The authors investigated whether melatonin concentrations vary between the two jugular veins and whether absolute (nocturnal) or relative (nocturnal/diurnal ratio) plasma melatonin concentrations are associated with seasonal reproductive activity measured by oestrus or ovulatory activity in Payoya goats. Thirty-two adult Payoya goats were penned under natural photoperiod. Oestrus activity was tested daily using aproned males—twice a week plasma was sampled for progesterone. Melatonin plasma concentrations were studied at each equinox and solstice of the year in jugular samples taken simultaneously by venipuncture. Nocturnal and diurnal plasma melatonin concentrations from each jugular vein were assessed in 3 and 2 plasma samples per goat, respectively, taken at hourly intervals in each period. No differences in melatonin concentrations between the two veins were observed, but there was a significant interaction (P < 0.001) between jugular vein and animal in nocturnal melatonin concentrations. There was no effect of sampling period on melatonin concentrations and the coefficient of correlation between sampling periods was very high. The analyses performed indicated that neither absolute nor relative melatonin concentrations were related with the dates of onset or end of ovulatory/oestrus activity. Therefore, we concluded that in goats (1) melatonin concentrations are highly variable between jugular veins in the same individual but not in the general population, (2) melatonin concentrations are highly repeatable for each individual, and (3) absolute and relative amplitudes of melatonin concentrations are not linked to the seasonal breeding activity in Mediterranean goats.     

Exogenous melatonin elevates the plasma leptin and thyroxine concentrations of the mink (Mustela vison)

Eight male and eight female minks were given exogenous melatonin as subcutaneous implants. The plasma leptin and thyroxine concentrations were measured. The leptin concentrations showed clear seasonal variations and differences between the experimental groups. In September most of the control females had undetectable plasma leptin concentrations, but the melatonin-treated females had detectable concentrations significantly higher than the leptin levels of the controls. Most of the males had undetectable leptin concentrations, too. In October the plasma leptin levels had increased significantly in all the groups except the control males. The melatonin-treated minks had significantly higher leptin levels than the controls. There was a significant rise in the thyroxine levels from September to October and the melatonin-treated groups had significantly higher thyroxine levels than the controls. The effects of exogenous melatonin are very pronounced in the mink. Melatonin elevates the plasma leptin and thyroxine levels possibly by direct and indirect mechanisms.     

Effect of scrotal insulation on seminal fructose,peripheral plasma prolactin,and circulating steroids in rams

Peripheral plasma prolactin and seminal fructose levels were compared in rams subjected to insulation of the scrotum. After 7 days of scrotal insulation, prolactin levels were elevated and were higher than control levels for the duration of the experiment (28 days). Seminal fructose levels increased after 14 days of scrotal insulation and also remained significantly greater than control levels until the end of the experiment. No changes were observed in plasma oestradiol and progesterone, whereas after 14 and 28 days of treatment, testosterone levels were depressed. It is suggested from these findings that prolactin may play a role in the regulation of accessory gland activity in the ram, although the proximate cause of high prolactin levels in treated animals is not clear.     

Effect of neonatal melatonin administration on sexual development in the rat

In order to study the mechanisms by which melatonin modulates sexual development, 5-day-old female Wistar rats have been treated with a single s.c. injection of melatonin, 3 h before the darkness onset. Criteria for sexual development were the age of vaginal opening and the circulating levels of prolactin, LH, FSH and estradiol. Also, pineal melatonin content was measured. There was a precocious puberty (P less than 0.01) in melatonin-treated rats measured by the age of the vaginal opening. An increase in the number of estrous smears over the whole period studied was observed in melatonin-treated animals as compared to controls. Along with these modifications, there was decrease in pineal melatonin content and serum prolactin levels, on day 21 of life (P less than 0.05), with an increase in both parameters on day 30 of age, in melatonin-treated rats as compared to controls, with no modifications at any other time studied. No differences were detected for serum LH levels considering the whole period studied for both groups. There was a faster decrease in plasma FSH levels with age in melatonin-treated animals than in controls. Serum estradiol levels were decreased in the peripubertal period in melatonin-treated rats as compared to controls. All these data suggest that the modifications induced by neonatal melatonin administration on prolactin, FSH and estradiol could be responsible for the precocious puberty shown in this study.     

Effect of melatonin implants on sexual activity in Mediterranean goat females without separation from males

This work was designed to determine whether melatonin treatment at the spring equinox can induce reproductive activity in goats without separation from males (separation being the normal practice in Spanish farming systems) and whether this treatment modifies the onset of the natural breeding season. Twenty-nine entire does were distributed into two groups (Group M, n = 14; Group C, n = 15). A third group of ovariectomized, estradiol-treated goats (OVX group, n = 5) was used to study the effect of melatonin on reproductive activity. On March 18, Groups M and OVX received a subcutaneous melatonin implant. In entire females, estrus was tested daily using entire aproned males, and ovulation rate was assessed after identification of estrus. Plasma progesterone in entire goats, plasma luteinizing hormone (LH) in the OVX group, and live weight and body condition score for all animals were recorded once a week. In entire goats, a clear treatment by time interaction was observed for progesterone concentrations (P < 0.001), with a period of high progesterone concentrations during the natural seasonal anestrus in Group M. A similar period of high LH concentrations was observed in the OVX group. Whereas all females of Group M presented ovarian activity during this period, no female of Group C did. The resumption of the natural breeding season was retarded in Group M in comparison with that in Group C (P < 0.05). We can conclude that in Mediterranean goats, melatonin implants can induce reproductive activity without separation from males, and it causes a small retardation in the reactivation of reproductive activity in the natural breeding season.     

Effects of ageing and exogenous melatonin on pituitary responsiveness to GnRH in rats

The effect of age and melatonin on the activity of the neuroendocrine reproductive system was studied in young cyclic (3-5 months-old), and old acyclic (23-25 month-old) female rats. Pituitary responsiveness to a bolus of GnRH (50 ng per 100 g body weight) was assessed at both reproductive stages in control and melatonin-treated (150 micrograms melatonin per 100 g body weight each day for 1 month) groups. After this experiment, female rats were treated for another month to study the influence of ageing and melatonin on the reproductive axis. Plasma LH, FSH, prolactin, oestradiol and progesterone were measured. A positive LH response to GnRH was observed in both control groups (cyclic and acyclic). However, a response of greater magnitude was observed in old acyclic rats. Melatonin treatment reduced this increased response in acyclic rats and produced a pituitary responsiveness similar to that of young cyclic rats. FSH secretion was independent of GnRH administration in all groups, indicating desynchronization between LH and FSH secretion in response to GnRH in young animals and during senescence. No effect on prolactin was observed. Significantly higher LH (3009.11 +/- 1275.08 pg ml(-1); P < 0.05) and FSH concentrations (5879.28 +/- 1631.68 pg ml(-1); P < 0.01) were seen in acyclic control rats. After melatonin treatment, LH (811.11 +/- 89.71 pg ml(-1)) and FSH concentrations (2070 +/- 301.62 pg ml(-1)) decreased to amounts similar to those observed in young cyclic rats. However, plasma concentrations of oestradiol and progesterone were not reduced. In conclusion, the results of the present study indicate that, during ageing, the effect of melatonin is exerted primarily at the hypothalamo-pituitary axis rather than on the ovary. Melatonin restored the basal concentrations of pituitary hormones and pituitary responsiveness to similar values to those observed in young rats.     

Effects of melatonin implants on plasma concentrations of testosterone, thyroxine and prolactin in the male silver fox (Vulpes vulpes)

Melatonin administration in the form of slow-release implants advanced breeding activity in silver fox males when treatment began in June. Plasma testosterone concentrations were significantly higher in treated animals than in controls from September to November, whereas in February and March they were significantly lower. Plasma prolactin concentrations were significantly reduced immediately following melatonin treatment in June but increased to greater levels than control values and 'peaked' after 7 months. This 'peak' was associated with a rapid decrease in testosterone secretion. The normal seasonal spring rise in prolactin secretion was prevented by melatonin administration. Thyroxine values decreased and were significantly lower after 2 months of melatonin treatment.     

Reproductive seasonality in female Iberian red deer (Cervus elaphus hispanicus)

This study characterized the seasonal pattern of luteal cyclicity in Iberian red deer (n=16), by measuring plasma progesterone concentrations in hinds (female red deer) twice per week from calving (May and June) 1996 until May 1997. In eight of these hinds we also examined plasma prolactin profiles to assess seasonal responses to photoperiod. Plasma progesterone concentration in the 16 hinds studied indicated that the reproductive pattern is seasonal, and lasts for 5.73 +/- 0.27 months. After calving, progesterone levels remained basal (no luteal activity) for several months, except in a hind that lost her calf just after calving, and thus did not have to suckle it. This hind showed two consecutive estrus cycles in the month following calving, which suggests that suckling has an inhibiting effect on the resumption of ovarian activity. These results also showed that as long as the hinds do not become pregnant, they show between 5 and 10 estrus cycles per reproductive season (8.06 +/- 0.35), ranging between 105 and 249 days from onset of the first cycle to end of the last one. Uninterrupted cycling lasted for 3.5-6.4 months (mean, 4.6 +/- 0.24). Cyclic luteal activity was found from October to February in all hinds, with a smaller, but notable proportion in September (56.25%) and March (68.8%), whereas it was negligible in the remaining period. Our results show a reproductive season similar to or longer than that recorded by other authors. Prolactin plasma concentrations showed a yearly trend following that of photoperiod, with peak concentrations from April to July, a decrease in August, minimal concentrations from September to February and a sharp increase in March.     

Induction and persistence of pituitary and ovarian activity in the out-of-season lactating dairy goat after a treatment combining a skeleton photoperiod, melatonin and the male effect

Thirty-two out-of-season Saanen dairy goats were involved in a 2 X 2 factorial experiment using (1) extra-light treatment (equivalent to long days) from 18 January to 19 March, at the end of gestation and the onset of lactation and (2) daily melatonin injections (equivalent to short days) from 20 March to 30 June. Hence, there were 4 groups of 8 goats each: CC (control-control), EC (extra-light-control), CM (control-melatonin) and EM (extra-light-melatonin). In each group teasers (males and androgenized females) were introduced on 30 May. Serial blood samples were taken for measurement of LH pulsatility on 30 April and 30 May. Ovarian activity was followed by plasma progesterone assay and laparoscopy. Oestrous behaviour was checked daily from 30 May to 15 July. The number of LH pulses was not modified by the treatments (extra light or melatonin). Melatonin treatment increased the basal concentration of LH (Groups CC + EC = 0.30 ng/ml; Groups CM + EM = 0.38 ng/ml; P less than 0.02) and the amplitude of pulses (Groups CC + EC = 1.24; Groups CM + EM = 2.44 ng/ml; P less than 0.02); the highest amplitude was observed for females in Group EM. Melatonin-treated females ovulated earlier after introduction of teasers than did untreated goats (Groups CM + EM = 2.3 days; Groups CC + EC = 6.1 days; P less than 0.001), but treatments did not modify ovulation rate (mean 2.1) or percentage of goats experiencing a short luteal phase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma prolactin,progesterone and oestradiol-17β concentrations around parturition in the pig

Plasma concentrations of prolactin, progesterone and oestradiol-17β were measured by radioimmunoassay in samples taken from 2–15 days before until 1–4 days after spontaneous parturition in four sows and in one sow around prostaglandin F2α-induced parturition.Between Days ?15 and ?2 (Day 0 = parturition), prolactin concentrations in daily samples fluctuated somewhat, but exceeded 10 ng/ml only exceptionally. Plasma progesterone levels gradually declined or remained high until about 2 days before parturition. A final decrease of the progesterone concentrations coincided with distinct increases of the prolactin levels during the last 40 h of pregnancy.Maximal prolactin concentrations were measured before the onset of delivery of the piglets. Oestradiol-17β reached peak values around delivery. Prostaglandin F2α injection caused an immediate sharp increase of the prolactin concentration which lasted for about 6 h. During this period progesterone and oestradiol-17β concentrations did not change. A second elevation of prolactin levels was measured when progesterone finally decreased.Changes of prolactin concentrations found in this study were compared with those found in other domestic animals at the same reproductive stage.     

Seasonal effects of melatonin on ovary and plasma gonadotropin and vitellogenin levels in intact and pinealectomized catfish, Clarias batrachus (Linn)

Effects of daily administration of melatonin for 15 days were evaluated with respect to ovarian activities and plasma gonadotropin (GtH II) and vitellogenin (Vg) levels in intact (INT) and pinealectomized (Px) female catfish, C. batrachus, during preparatory (April), prespawning (May and June), spawning (July) and post-spawning (September) periods. Px (saline control groups) caused a stimulatory effect during preparatory (with respect to Vg synthesis and incorporation) and prespawning (with respect to Vg synthesis) periods whereas no effect was observed during spawning and post-spawning periods with respect to the reproductive parameters studied. During April, melatonin-treatment significantly decreased plasma GtH II levels and percentage of vitellogenic oocytes without any significant changes in plasma Vg levels and gonadosomatic index (GSI). During early prespawning period, in May, 50microg melatonin brought about a significant reduction in plasma GtH II levels in INT group, whereas 100microg caused a decrease in all parameters; on the other hand, in Px groups both dose levels proved to be inhibitory. In June (late prespawning period) melatonin-treatment could not bring about any change in GSI and plasma Vg levels compared to the control groups regardless of Px but plasma GtH II and mean number of yolky oocytes were significantly reduced in melatonin-treated INT group. During spawning period (July) melatonin inhibited the GSI, mean number of yolky oocytes and plasma GtH II levels without affecting plasma Vg levels. In September (post-spawning period), melatonin did inhibit both GSI and plasma GtH II levels. The results, thus, indicate that melatonin showed variable effects (inhibitory and/or no effect) to GSI, mean number of yolky oocytes and plasma Vg levels but a consistent inhibiton of plasma GtH II levels indicating that melatonin may control the reproduction by blocking the GtH II release from the pituitary via affecting the hypothalamo-hypophysial axis.     

Effect of melatonin implants on reproduction in the male silver fox (Vulpes vulpes)

In June 1987, when the testes were fully regressed, 5 males were given s.c. implants of 40 mg melatonin. The same treatment was repeated in August and October 1987. Five males served as controls. Plasma concentrations of melatonin increased significantly in treated males and were still elevated at the end of the study in April 1988. The changes in testicular volume and blood plasma concentrations of testosterone in response to GnRH indicated that melatonin administration promoted testicular development. However, testicular regression was observed earlier in treated than control animals, perhaps because of refractoriness to melatonin or to a down-regulation of melatonin receptors. Semen was collected and frozen in November 1987, 2 months ahead of the natural breeding season, from the melatonin-treated males, and 10 blue fox vixens were inseminated the following breeding season: 9 vixens conceived, and the average litter size was 7.6 +/- 0.5. The results demonstrate that melatonin treatment initiated during exposure to naturally long days (a) promotes testicular development in a way similar to an artificial short photoperiod and (b) may induce a refractory condition after an extended period of treatment.     

Artificial long days in addition to exogenous melatonin and daily contact with bucks stimulate the ovarian and oestrous activity in Mediterranean goat females

One experiment was conducted to determine whether the treatment with artificial long days and exogenous melatonin can induce reproductive activity during spring (seasonal anoestrus) in Mediterranean goats that are in daily contact with bucks and whether this treatment causes a variation in the reactivation of the reproductive activity in the normal breeding season. The experiment started on 4 November 2005 and finished on 27 October 2006. Thirty-four adult and barren does were used, distributed into two groups balanced according to their live weight (LW) and body condition score (BCS). Seventeen females were exposed to long days (16 h of light/day) from 14 November 2005 to 20 February 2006. On 20 February, they received one s.c. melatonin implant (LD-M group) and were exposed to natural photoperiodic changes in an open shed. The other females during the experiment were placed in an open shed under natural photoperiod and remained as the control group (C group). The C and LD-M groups were keeping in contact with males during the whole experiment. During the experiment, the LW, BCS and plasma progesterone concentrations were measured weekly, oestrous activity was tested daily using entire aproned bucks, and ovulation rate was evaluated by laparoscopy 7 days after positive identification of the oestrus. A clear treatment-time interaction was observed for plasma progesterone concentrations (P < 0.001), with a period of high progesterone concentrations during the natural seasonal anoestrus in the LD-M group. Although 94.1% of females in the LD-M group presented ovarian activity during this period, no female in group C did. Resumption of ovarian activity in the subsequent natural breeding season was 2 weeks later in the LD-M group in comparison with group C (P < 0.05). We can conclude that in Mediterranean goat breeding systems, when females are in daily contact with bucks, the treatment with 3 months of long days and melatonin implant at the end of the light photoperiodic treatment can induce ovarian and oestrous activity during the seasonal anoestrus. Finally, this treatment causes a short delay in the subsequent reactivation of ovarian activity in the natural breeding season.     

Melatonin treatment of embryo donor and recipient ewes during anestrus affects their endocrine status, but not ovulation rate, embryo survival or pregnancy

Thirty-two Border Leicester x Scottish Blackface ewes that lambed in March were individually penned with their lambs from April 16th and given daily an oral dose of 3 mg melatonin at 1500 h (Group M). A further 32 acted as controls (Group C). Within each group half were used as embryo donors (Group D) following superovulation and half received embryos (Group R) following an induced estrus. Prior to weaning on 21 May ewes received ad libitum a complete diet providing 9 megajoules (MJ) of metabolizable energy and 125 g/kg crude protein. Thereafter each received 1.6 kg of the diet daily. In early June each ewe received an intravaginal device (300 mg progesterone) inserted for 12 d. Donors were superovulated with 4 i.m. injections of porcine FSH 12 h apart, commencing 24 h before progesterone withdrawal. Ovulation in recipients was induced with 800 IU PMSG injected i.m. at progesterone removal. Donor ewes were inseminated 52 h after progesterone withdrawal. Embryos were collected 4 d later and transferred to recipients. Melatonin suppressed plasma prolactin (P < 0.001) and advanced estrus (P < 0.05) and timing of the LH peak (P < 0.05). These events also occurred earlier in donors than in recipients (P < 0.01). Mean (+/- SEM) ovulation rates for melatonin-treated and control donors were 5.5 +/- 0.71 and 4.7 +/- 0.66, respectively (NS). Corresponding recipient values were 3.3 +/- 0.40 and 3.4 +/- 0.39 (NS). Mean (+/- SEM) embryo yields were 2.9 +/- 0.64 and 2.6 +/- 0.73 for melatonin-treated (n = 15) and control (n = 16) donors, respectively, and for the 12 ewes per treatment that supplied embryos, corresponding numbers classified as viable were 2.7 +/- 0.47 and 2.3 +/- 0.61 (NS). Following transfer, 57% of embryos developed to lambs when both donor and recipient received melatonin, 86% when only the donor received melatonin, 91% when only the recipient received melatonin, and 67% when neither received melatonin (NS). Thus, embryo survival following transfer was not improved by treating recipients with melatonin. Gestation length and lamb birthweights were unaffected by melatonin. Unlike nonpregnant control ewes, melatonin-treated recipients that failed to remain pregnant sustained estrous cyclicity following embryo transfer.     

Nocturnal variation of prolactin secretion in the Mouflon (Ovis gmelini musimon) and domestic sheep (Ovis aries): seasonal changes

Seasonal changes in nocturnal prolactin secretion and their relationship with melatonin secretion were monitored in wild (Mouflon, Ovis gmelini musimon) and domesticated sheep (breed Manchega, Ovis aries). Two groups of eleven adult females each, were maintained outdoors under natural photoperiod. Plasma concentrations of prolactin and melatonin were determined during the summer and winter solstices and the autumn and spring equinoxes. Blood samples were collected every 3h during the night hours, and 1h before and after the onset of darkness and sunrise. Maximum mean plasma concentrations of prolactin during the dark-phase in Mouflons were observed in the summer solstice, (P<0.001) and in the summer solstice and spring equinox in Manchega ewes (P<0.001). Mean plasma concentrations of prolactin were higher in the wild species (P<0.001) during the summer solstice. In contrast, during the spring equinox, mean levels of prolactin were higher in Manchega ewes than in Mouflons (P<0.05). Plasma prolactin concentrations showed a nocturnal rhythm in both breeds, with seasonal variations (P<0.001). The increase in plasma melatonin levels during the first hour after sunset was accompanied to increasing concentrations of PRL 1h after the onset of darkness, only in the autumn and spring equinox for the Mouflon, and in the summer solstice and spring equinox for the Manchega ewes. In Mouflons, the fall of plasma PRL concentrations about the middle dark-phase in all the periods studied, coincided with high levels of melatonin. A similar relation was observed in Manchega ewes only in the winter solstice and spring equinox. The current study shows that the nocturnal rhythm of prolactin secretion exhibits seasonal variation; differences in the patterns of prolactin secretion between Mouflon and Manchega sheep are taken to represent the effects of genotype.     

Effect of plane of nutrition on seasonality of reproduction in Spanish Payoya goats

The aim of this study was to determine if there is a seasonal pattern of sexual activity in female Payoya goats and if this seasonality could be modulated by nutrition. During the experimental period of 20 months, 43 non-pregnant adults goats were penned under natural photoperiod at latitude 37 degrees 15'N. At the onset of the experiment, the animals were allocated to three experimental groups differing in the level of nutrition and whether the animals were entire or ovariectomized does. The high nutrition group (H, n = 16 entire does) receiving 1.5 times maintenance requirements. The low nutrition group (L, n = 16 entire does) and an ovariectomized and oestradiol treated group (OVX, n = 11 ovariectomized does) received a diet supporting their maintenance requirements. The groups were balanced for live weight (LW) and body condition score (BCS) at the beginning of the study. In entire goats, oestrus was tested daily using aproned males, ovulation rate was assessed by laparoscopy 7 days after identification of oestrus and plasma samples were obtained twice per week for progesterone assay. OVX goats were isolated from the other groups and bucks, plasma samples were assayed twice per week for LH and there were four intensive sampling periods during the year to determine LH pulsatility. LW and BCS were recorded for all animals once a week. A clear circannual cycle in live weight change was observed in all experimental groups, being relatively stable or slightly decreasing in summer and autumn and increasing during winter and spring. The effect of exposure to high (H) rather than low (L) nutrition was to cause earlier onset of ovarian activity (5 versus 17 August; P < 0.05), and expression of oestrous (16 August versus 2 September; P < 0.01) and later cessation of reproductive activity (ovulation 11 February versus 17 January; P < 0.01). Consequently, seasonal anoestrus was 32 days shorter in does on the higher plane of nutrition (P < 0.01). The seasonality of reproductive activity was confirmed in the OVX does, with reduced LH concentrations during spring and summer, and increased LH concentrations in autumn and winter. There was no effect of nutrition on ovulation rate. These results demonstrate that the female Payoya goat exhibits marked reproductive seasonality which is modulated by nutrition but possibly not ovulation rate.