Importance of duration of nocturnal melatonin secretion in determining the reproductive response to inductive photoperiod in the ewe

The pineal gland, through its nocturnal melatonin secretion, mediates the effects of inhibitory (long) and stimulatory (short) photoperiod on reproduction in female sheep. Earlier studies revealed that duration of the nighttime melatonin rise is important in determining the inhibitory effect of day length on reproduction in the ewe. The present study tested whether the duration is also important in mediating the inductive response to short days. Pinealectomized ewes, housed under long days, received a short-day melatonin infusion (16-h duration) for 90 days. Reproductive status was monitored from the response to estradiol negative feedback of luteinizing hormone (LH) secretion. This short-day melatonin pattern led to unambiguous reproductive induction, despite the exposure to inhibitory long days. The increase in serum LH was comparable, in terms of latency and magnitude, to that in pinealectomized controls receiving the same short-day melatonin pattern under short days, and in pineal-intact controls transferred from long to short days. Since the reproductive status conformed to the length of time that melatonin was elevated each day rather than to photoperiod, these results support the conclusion that duration of the nighttime melatonin rise mediates the reproductive response of the ewe to an inductive photoperiod. In all, the melatonin rhythm is considered an integral component of the physiologic mechanism measuring day length; through duration of its nocturnal secretion, melatonin encodes both inhibitory and stimulatory photoperiods.     

Melatonin and photorefractoriness: loss of response to the melatonin signal leads to seasonal reproductive transitions in the ewe

Experiments were conducted to examine whether the refractoriness of the Suffolk ewe to the reproductive effects of day length is associated with a deficit in the generation of the circadian rhythm of melatonin secretion or in the postpineal processing of this photoperiodic message. Using serum luteinizing hormone (LH) concentrations in ovariectomized ewes bearing constant-release estradiol implants as a marker of reproductive induction, ewes with intact pineal glands were found to become unresponsive to fixed artificial photoperiods that initially had been either inductive (short days) or inhibitory (long days). The loss of the photoperiodic response was not associated with notable changes in the 24-h secretory pattern of melatonin, which remained characteristically low throughout the day and rose at night. In pinealectomized ewes, nightly infusion of a stimulatory pattern of melatonin (simulating that seen on short days) initially provoked reproductive induction; this response then lessened over much the same time course that pineal intact ewes became refractory to short days. These results support the hypothesis that photorefractoriness reflects a deficit in the postpineal processing of the photoperiodic message. Further, in view of recent evidence that photorefractoriness normally leads to both onset and cessation of the breeding season in Suffolk ewes maintained outdoors, these findings suggest that the loss of response to the melatonin signal contributes to at least one of these reproductive transitions, the cessation of the breeding season, under natural environmental conditions.     

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

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

Importance of changing photoperiod and melatonin secretory pattern in determining the length of the breeding season in the Suffolk ewe

Three groups of ovariectomized Suffolk ewes bearing s.c. Silastic implants of oestradiol were subjected to a 90-day priming treatment of an inhibitory long photoperiod (16 h light/day; 16L:8D). On Day 0 of the experiment, they were moved to stimulatory photoperiods. One control group was transferred to 12L:12D and a second control group was transferred to 8L:16D; both groups remained in those photoperiods to determine the timing of reproductive induction and refractoriness. The experimental group was transferred to 12L:12D on Day 0 and then to 8L:16D on Day 55 to determine whether the further reduction in daylength could delay the development of refractoriness. Reproductive neuroendocrine condition was monitored by serum concentrations of LH and FSH. Both gonadotrophins remained elevated for a longer period of time in the experimental group receiving the second reduction in daylength than in either control group, indicating that the second photoperiodic drop delayed the onset of photorefractoriness. Measurement of 24-h patterns of circulating melatonin suggests that the prolonged stimulation of reproductive neuroendocrine activity in the experimental group resulted from a lengthening of the nocturnal melatonin rise. These findings indicate that refractoriness to an inductive photoperiod can be temporarily overcome by exposure to a shorter daylength, and that the change in duration of the nocturnal increase in melatonin secretion is important in photoperiodic signalling. Thus, in natural conditions, the decreasing autumnal daylength, and the resulting expansion of the nocturnal elevation in melatonin secretion, may be utilized to produce a breeding season of normal duration.     

Photoperiodic history and a changing melatonin pattern can determine the neuroendocrine response of the ewe to daylength

The reproductive neuroendocrine response of Suffolk ewes to the direction of daylength change was determined in animals which were ovariectomized and treated with constant release capsules of oestradiol. Two groups of animals were initially exposed to 16 or 10 h light/day for 74 days. On day zero of the study, when one group of ewes was reproductively stimulated (elevated LH concentrations) and the other reproductively inhibited (undetectable LH concentrations), half the animals from each group were transferred to an intermediate daylength of 13 h light/day. The remaining ewes were maintained on their respective solstice photoperiods to control for photorefractoriness. LH concentrations rose in animals experiencing a 3 h decrease in daylength from 16L:8D to 13L:11D while LH concentrations fell to undetectable values in those that experienced a 3 h increase in daylength from 10L:14D to 13L:11D. The photoperiodic response of the Suffolk ewe, therefore, depends on her daylength history. Such a result could be explained if the 24-h secretory pattern of melatonin secretion, known to transduce photoperiodic information to the reproductive axis, was influenced by the direction of change of daylength. Hourly samples for melatonin were collected for 24 h 17 days before and three times after transfer to 13L:11D. The melatonin secretory profile always conformed to daylength. Therefore, the mechanism by which the same photoperiod can produce opposite neuroendocrine responses must lie downstream from the pineal gland in the processing of the melatonin signal.     

Refractoriness to inhibitory day lengths initiates the breeding season of the Suffolk ewe

Recent evidence indicates that the breeding season of the Suffolk ewe ends because of loss of response to a day length that was previously inductive. This condition of photorefractoriness could potentially also initiate reproduction, as is the case in several long-day breeding rodents. In this study we determined if ewes enter their breeding season because they become refractory to the long ambient photoperiods of late summer. On the summer solstice, 3 groups of ovariectomized ewes (n = 6) bearing s.c. Silastic implants of estradiol (OVX + E) were placed in different day length treatments: 1) natural photoperiod; 2) artificial photoperiod, stimulating natural day lengths; or 3) artificial photoperiod equivalent to that of the summer solstice (16.25L). Entry into the breeding season is associated with a striking (greater than 30-fold) increase in circulating levels of luteinizing hormone (LH). Timing of the onset of the breeding season was not delayed in ewes maintained on the summer solstice photoperiod; LH levels rose simultaneously in all groups. We conclude that ewes normally begin breeding not because they are actively driven to do so by decreasing or short days, but because they become refractory to prevailing long days. Because the pattern of circulating melatonin, which is known to transduce the photoperiodic message, remained entirely appropriate to day length, we believe that the mechanism responsible for photorefractoriness resides in the postpineal processing of the melatonin signal.     

Effects of prolonged artificial photoperiod on circulating prolactin and melatonin levels in seasonal ewes

Crossbred ewes exposed to long days for 46 months prior to photoperiod reversal showed no alteration in the duration or amplitude of the circulating melatonin peak between 24 and 46 months of continuous long day exposure. By 3 months after photoreversal to short days, both the amplitude and duration of the peak had adapted to the new scotophase. In short day treated ewes, the melatonin peak was abolished by 46 but not 24 months of short day exposure, and was not fully restored in all ewes 3 months after photoreversal. Mean prolactin levels over 24 h remained high up to 46 months of long day treatment, and declined 3 months after short day exposure. Conversely, mean prolactin levels remained low up to 46 months of short day treatment, increasing 3 months after exposure to long days. Thus: (i) depletion of the melatonin-synthesizing capability of the ovine pineal gland by prolonged exposure to long nights is not completely reversed after 3 months of continuous long day exposure, and (ii) a nocturnal melatonin peak is not essential for maintenance of plasma prolactin levels under these conditions.Special issue dedicated to Dr. Lawrence Austin.     

Effects of breed, ovarian steroids and season on the pulsatile secretion of LH in ovariectomized ewes

The effects of season and of oestradiol and progesterone on the tonic secretion of LH were studied in ovariectomized Merino and Suffolk ewes, two breeds which differ markedly in the seasonal pattern of their reproductive activity. In the absence of exogenous steroids, the frequency of LH pulses was lower and the amplitude of the pulses was higher in anoestrus than in the breeding season for Merino and Suffolk ewes 30 days after ovariectomy. In long-term (190 days) ovariectomized ewes, this seasonal change in LH secretion was observed in Suffolk ewes only. During seasonal anoestrus, treatment of ewes with subcutaneous oestradiol-17 beta implants (3, 6 or 12 mm in length) decreased the frequency of LH pulses in a dose-dependent manner, with Suffolk ewes being far more sensitive to the inhibitory effects of oestradiol than Merino ewes. The lowest dose of oestradiol (3 mm) had no effect on the secretion of LH in Merino ewes, but reduced secretion in Suffolk ewes. Treatment of ewes with the highest dose of oestradiol (12 mm) completely abolished LH pulses in Suffolk ewes, whereas infrequent pulses remained evident in Merino ewes. During the breeding season, oestradiol alone had no effect on the pulsatile release of LH in either breed, but in combination with progesterone there was a significant reduction in LH pulse frequency. Progesterone effectively decreased LH secretion in both breeds in both seasons. It was concluded that differences between breeds in the 'depth' of anoestrus could be related to differences in the sensitivity of the hypothalamus to both negative feedback by oestradiol and the direct effects of photoperiod.     

Effects of Superior Cervical Ganglionectomy and Melatonin Replacement on Intra-hypothalamic LHRH Content and Pulsatile Luteinizing Hormone Release in the Mink

Long-term effects of subcutaneous melatonin implants on intrahypothalamic LHRH content and on pulsatile luteinizing hormone release have been investigated in ganglionectomized male mink. Animals were submitted to bilateral removal of the superior cervical ganglion in mid-April. A preliminary study revealed that plasma LH concentrations remain at a basal level throughout the year following ganglionectomy. In a second experiment, one month after ganglionectomy and transfer from the natural photoperiod environment to short daylengths (LD 4:20), melatonin pellets were subcutaneously implanted to overcome deafferentation of the pineal. Progressive effects of treatment were studied 7 days, 15 days, and one, two and three months after insertion of the melatonin implants. The intra-hypothalamic LHRH content in ganglionectomized mink was at a basal level similar to that observed during seasonally sexual quiescence, or after exposure to inhibitory long days (LD 20:4). A significant and transient elevation in LHRH content was observed already after fifteen days, and also one month after insertion of melatonin implants. This resulted in mean values similar to those observed during the breeding season, or after exposure to stimulatory short days (LD 4:20). A decrease in hypothalamic LHRH content started after two months. No pattern of pulsatile LH secretion was recorded in ganglionectomized untreated mink. A significant increase in all parameters of pulsatile LH secretion was observed fifteen days after the elevation of LHRH content induced by melatonin treatment, and maximum values were reached after two months. Pituitary activity tended to decrease after three months, characterized in particular by a significant decrease in the mean frequency of LH pulses. In addition, the increase in pulsatile characteristics of LH release occurred two months before the peripheral renewal of testicular activity. Apparently, the reproductive endocrine function in ganglionectomized mink treated with melatonin implants is restored more rapidly at the hypothalamic level than at the pituitary or testicular levels.     

Effects of Superior Cervical Ganglionectomy and Melatonin Replacement on Intra-hypothalamic LHRH Content and Pulsatile Luteinizing Hormone Release in the Mink

Long-term effects of subcutaneous melatonin implants on intrahypothalamic LHRH content and on pulsatile luteinizing hormone release have been investigated in ganglionectomized male mink. Animals were submitted to bilateral removal of the superior cervical ganglion in mid-April. A preliminary study revealed that plasma LH concentrations remain at a basal level throughout the year following ganglionectomy. In a second experiment, one month after ganglionectomy and transfer from the natural photoperiod environment to short daylengths (LD 4:20), melatonin pellets were subcutaneously implanted to overcome deafferentation of the pineal. Progressive effects of treatment were studied 7 days, 15 days, and one, two and three months after insertion of the melatonin implants. The intra-hypothalamic LHRH content in ganglionectomized mink was at a basal level similar to that observed during seasonally sexual quiescence, or after exposure to inhibitory long days (LD 20:4). A significant and transient elevation in LHRH content was observed already after fifteen days, and also one month after insertion of melatonin implants. This resulted in mean values similar to those observed during the breeding season, or after exposure to stimulatory short days (LD 4:20). A decrease in hypothalamic LHRH content started after two months. No pattern of pulsatile LH secretion was recorded in ganglionectomized untreated mink. A significant increase in all parameters of pulsatile LH secretion was observed fifteen days after the elevation of LHRH content induced by melatonin treatment, and maximum values were reached after two months. Pituitary activity tended to decrease after three months, characterized in particular by a significant decrease in the mean frequency of LH pulses. In addition, the increase in pulsatile characteristics of LH release occurred two months before the peripheral renewal of testicular activity. Apparently, the reproductive endocrine function in ganglionectomized mink treated with melatonin implants is restored more rapidly at the hypothalamic level than at the pituitary or testicular levels.     

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

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

Role of the pituitary gland in the development of photorefractoriness and generation of long-term changes in prolactin secretion in rams

Hypothalamo-pituitary disconnected Soay rams were exposed to two photoperiodic treatments: 1) constant long days (16L:8D) for 48 wk after pretreatment under short days (LD group), and 2) constant short days (8L:16D) for 48 wk after pretreatment under long days (SD group). In the LD group, plasma prolactin (PRL) concentrations increased from 0 to 8 wk (maximum: 143.3 +/- 8.4 microg/l; 8.8 +/- 1. 2 wk), decreased from 9 to 34 wk (minimum: 15.6 +/- 1.6 microg/l; 34. 5 +/- 1.5 wk), and finally increased again under the constant conditions, with a similar cyclical pattern for all individuals. In the SD group, PRL concentrations showed an inverse pattern (minimum: 8.6 +/- 2.6 microg/l; 17.1 +/- 2.0 wk; maximum: 46.4 +/- 5.5 microg/l; 30.2 +/- 3.2 wk), with more variability. Plasma concentrations of FSH were basal in both groups. The duration of the daily nocturnal melatonin peak (measured at 10, 24, and 44 wk) remained close to 8 h under long days (high-fidelity melatonin signal) but decreased significantly (13.8 h to 9.3 h) under short days (low-fidelity melatonin signal). The results support the conclusion that the melatonin signal encoding photoperiod acts within the pituitary gland to induce both acute (inductive) and chronic (refractory) effects photoperiod on PRL secretion.     

How does melatonin code for day length in the ewe: duration of nocturnal melatonin release or coincidence of melatonin with a light-entrained sensitive period?

The main objective of the study was to test the hypothesis that the phase of melatonin release with respect to the light-dark cycle mediates the effects of photoperiod on the reproductive response of the ewe. To test the phase hypothesis, we eliminated endogenous melatonin secretion by pinealectomy and then restored physiological levels of serum melatonin with rises of the same duration but at different phases of the light-dark cycle (either at night or in the middle of the day). Serum melatonin patterns were determined by radioimmunoassay in samples taken hourly for 24 h. The reproductive state was monitored by measuring serum luteinizing hormone (LH) in ovariectomized ewes treated with constant-release estradiol implants. Infusion of a long-day pattern of melatonin was equally effective in maintaining reproductive suppression when given during the night or the middle of the day. LH remained low for approximately 150 days and then rose as ewes became refractory to the inhibitory melatonin signal. These results do not support the phase hypothesis. Rather, they are consistent with the hypothesis that the duration of the nocturnal secretion of melatonin codes for day length.     

Effect of constant-release implants of melatonin on seasonal cycles in reproduction, prolactin secretion and moulting in rams

Seasonal cycles in the size of the testes, blood plasma concentration of testosterone, FSH and prolactin, intensity of the sexual skin flush, timing of rutting behaviour and moulting of the body coat were recorded in Soay rams after s.c. implantation of melatonin contained in a Silastic envelope which increased the circulating blood levels of melatonin to 200-600 pg/ml for many months. Two groups of 8 adult rams were held under alternating periods of short days (8L:16D) and long days (16L:8D) to drive the seasonal cycles and the treatments with melatonin were initiated during the long or short days, and one group of 8 ram lambs was kept out of doors and given implants during the long days of summer (4 melatonin-implanted and 4 control (empty implants) rams per group). The treatments demonstrated that melatonin implants during exposure to long days resulted in a rapid 'switch on' of reproductive redevelopment similar to that produced by exposure to short days melatonin implants prevented the rams from showing the normal responses to changes in the prevailing photoperiod rendering them nonphotoperiodic; and long-term cyclic changes in testicular activity, prolactin secretion and other characteristics occurred in the melatonin-implanted rams; the pattern was similar to that previously observed in rams exposed to prolonged periods of short days. The overall results are consistent with the view that melatonin is the physiological hormone that relays the effects of changing photoperiod on reproduction and other seasonal features, and that continuous exogenous melatonin from an implant interferes with the normal 'signal' and produces an over-riding short-day response.     

Synchronization of the circannual reproductive rhythm of the ewe by discrete photoperiodic signals

Although many species display endogenous circannual rhythms of biological activity that are synchronized by day length, the specific photoperiodic requirements for synchronizing such rhythms are not established for any species. We tested the hypothesis that the circannual reproductive rhythm of sheep can be synchronized by exposure to just one or two discrete blocks of photoperiodic information each year. Ewes were pinealectomized to prevent their ability to transduce photoperiodic information into altered reproductive neuroendocrine activity. During the 53/4 yr following pinealectomy, specific photoperiodic signals were restored for discrete periods of time via replacement of 24-h patterns of melatonin, the pineal hormone that transmits photic information to the reproductive neuroendocrine axis. The ewes were kept in a 12-mo photoycycle that alternated between short (8L:16D) and long (16L:8D) days every 6 mo and that was 6 mo out of phase with the geophysical year. Pineal-intact control ewes exhibited synchronous annual reproductive cycles. Noninfused pinealectomized control ewes did not exhibit synchronous cycles. Pinealectomized ewes infused with alternating 70-day blocks of short- and long-day patterns of melatonin every 6 mo for the first 21/2 yr of the experiment exhibited synchronous annual reproductive cycles that were 6 mo out of phase with those of ewes maintained outdoors. This synchrony persisted when the frequency of the melatonin treatment was reduced to just one 70-day block of a long-day pattern of melatonin each 365 days. Cycle period was 368 +/- 3 days; standard deviation of the date of onset of reproductive induction averaged only 3 days. Our study provides the first direct evidence that a single block of photoperiodic information a year can synchronize a circannual rhythm.     

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)     

Importance of photoperiodic signal quality to entrainment of the circannual reproductive rhythm of the ewe

An endogenous circannual rhythm drives the seasonal reproductive cycle of a broad spectrum of species. This rhythm is synchronized to the seasons (i.e., entrained) by photoperiod, which acts by regulating the circadian pattern of melatonin secretion from the pineal gland. Prior work has revealed that melatonin patterns secreted in spring/summer entrain the circannual rhythm of reproductive neuroendocrine activity in sheep, whereas secretions in winter do not. The goal of this study was to determine if inability of the winter-melatonin pattern to entrain the rhythm is due to the specific melatonin pattern secreted in winter or to the stage of the circannual rhythm at that time of year. Either a summer- or a winter-melatonin pattern was infused for 70 days into pinealectomized ewes, centered around the summer solstice, when an effective stimulus readily entrains the rhythm. The ewes were ovariectomized and treated with constant-release estradiol implants, and circannual cycles of reproductive neuroendocrine activity were monitored by serum LH concentrations. Only the summer-melatonin pattern entrained the circannual reproductive rhythm. The inability of the winter pattern to do so indicates that the mere presence of a circadian melatonin pattern, in itself, is insufficient for entrainment. Rather, the characteristics of the melatonin pattern, in particular a pattern that mimics the photoperiodic signals of summer, determines entrainment of the circannual rhythm of reproductive neuroendocrine activity in the ewe.     

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)     

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.     

Reproductive photorefractoriness in rams and accompanying changes in the patterns of melatonin and prolactin secretion

Exposure of rams to alternating 16-week cycles of long and short days (16L:8D and 8L:16D) results in periods of testicular regression followed by testicular redevelopment, and there is an inverse relationship between the blood levels of prolactin and testis activity. In this study, two groups of rams were held under long or short day lengths for a period of 94 weeks. When held under either long or short days for more than 16 weeks, the animals showed spontaneous changes in gonadal activity and in the secretion of prolactin, both of which were no longer correlated with the prevailing photoperiod, i.e., they became photorefractory. The photorefractoriness was characterized by cyclical changes in testis function which were independent of day length. The period of these spontaneous cycles was similar during both treatment regimens (long days: 40.9 +/- 1.5 weeks; short days: 38.1 +/- 0.33 weeks), suggesting the presence of an endogenous pacemaker for the reproductive system. The changes in blood prolactin levels during photorefractoriness were no longer correlated with testis activity, and though cyclical, the period lengths differed under the two regimens (long days: 31.8 +/- 1.4 weeks; short days: 48.6 +/- 0.3 weeks). The rates of change in testis function and prolactin secretion were slower during the refractory state than during the photosensitive state. Upon switching the rams to a different photoperiod after the 94 weeks of exposure to fixed day lengths, the rams showed relatively rapid testicular and prolactin responses. Photoperiodic information appears to be relayed to the endocrine system through the daily pattern of melatonin secretion by the pineal. We measured the daily blood levels of melatonin on several occasions during phases of photosensitivity and photorefractoriness in the same group of rams. During the first 21 weeks under both lighting treatments, the rams showed synchronized daily patterns in their blood levels of melatonin, with elevated levels occurring mainly during the daily period of darkness. Similar synchronized daily rhythms were also seen when the rams were switched to a different photoperiod following 94 weeks of exposure to either long or short days. Between Weeks 21 and 94, the daily rhythms of melatonin did not occur consistently in all rams; often, the patterns differed markedly between individual rams held under the same day length and peak levels of melatonin were not always confined to periods of darkness.(ABSTRACT TRUNCATED AT 400 WORDS)