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.     

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

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

The effect of photoperiod and melatonin feeding on reproduction in the ewe

Sixty anestrous ewes were used to determine the effects of artificial photoperiod and/or melatonin feeding on seasonality of reproduction. Treatments included natural daylight (ND), 8 h of light, 16 h of darkness (8L: 16D), natural daylight plus 3.5 mg melatonin fed per ewe daily (ND + MEL), and 8L: 16D plus 3.5 mg melatonin fed per ewe daily (8L: 16D + MEL). The percentage of ewes lambing was lower (P < 0.05) for ND treated ewes (40%) than for ewes in 8L: 16D (100%), ND + MEL (91.7%), or 8L: 16D + MEL (93.3%). The earliest mean conception date was for ewes in the 8L: 16D + MEL treatment. This was 10 days earlier than for ewes in the ND treatment (P < 0.05). ND and ND + MEL treated ewes had fewer lambs (P < 0.05) and lighter litter weight (P < 0.05) per ewe lambing than did 8L: 16D and 8L: 16D + MEL treated ewes. Serum progesterone levels above 1.0 ng/ml were reached and maintained approximately 3 wk earlier in the 8L: 16D, 8L: 16D + MEL, and ND + MEL treated ewes than in the ND treated ewes (P < 0.05). Ewes in ND treatment had higher overall serum prolactin levels (P < 0.05) than did ewes in all other treatments. Results indicate that the 8L: 16D treatment and/or feeding melatonin can hasten cyclicity in ewes and increase the number of ewes conceiving.     

Reproductive and immune responses to photoperiod and melatonin are linked in Peromyscus subspecies

The effects of photoperiod and melatonin treatment on reproductive and immune function were assessed in two subspecies of Peromyscus maniculatus from different latitudes of origin. In experiment 1, P. m. bairidii (latitude = 42°51 N) and P. m. luteus (latitude = 30°37 N) were housed in either long (LD 168) or short days (LD 816) for 8 weeks. Short-day P. m. bairdii displayed reproductive regression and elevated splenocyte proliferation in response to the T-cell mitogen concanavalin A, as compared to long-day mice. In contrast, P. m. luteus did not undergo reproductive regression or exhibit any increase in lymphocyte proliferation in short days. In experiment 2, individuals of both P. m. bairdii and P. m. luteus were implanted with empty capsules or capsules that contained melatonin. Individual P. m. bairdii implanted with melatonin underwent reproductive regression. Individuals of this subspecies also displayed elevated lymphocyte proliferation compared to control mice. Conversely, P. m. luteus implanted with melatonin did not undergo reproductive regression and displayed no significant changes in lymphocyte proliferation. These results suggest that reproductive responsiveness to melatonin mediates short-day enhancement of immune function in deer mice. These data also imply that melatonin may not possess universal immunoenhancing properties. Rather, the effectiveness of melatonin to influence immune responses may be constrained by reproductive responsiveness to this indole-amine.     

Failure of melatonin to influence endogenous opioid effects on LH secretion in the anoestrous ewe

In May mature seasonally anoestrous ewes were implanted with melatonin which advanced the onset of cycles by about 1 month. The LH response to an opioid antagonist, WIN-3, was determined 5, 15, 25 and 60 days after melatonin implantation, by intravenous administration of WIN-3 (12.5 mg/dose) 4 times at 15-min intervals during both the 1st and the 5th hour of an 8-h treatment period. There was no effect of WIN-3 at 5, 15 and 25 days after melatonin implantation. At 60 days LH concentration and pulse frequency were significantly increased (P less than 0.05 and less than 0.01 respectively) in response to WIN-3 treatment, but only in those animals which had begun reproductive cycles, an effect known to be mediated by the presence of progesterone. We were therefore unable to find evidence to support the hypothesis that the influence of melatonin in advancing the breeding season may be via an opioidergic pathway.     

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.     

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

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

The response of luteinizing hormone secretion to photoperiod is modified by the level of nutrition in female Mediterranean goats

This paper reports the influence of nutrition on the photoperiodic control of luteinizing hormone (LH) secretion in female Mediterranean goats (i.e., goats from the Mediterranean area in general). Ovariectomized, oestradiol-treated goats were subjected to two consecutive intervals of 3 months of long days followed by 3 months of short days (group LDSD, N=20), or vice versa (group SDLD, N=20). The LDSD and SDLD does were also randomly assigned to one of two nutrition groups that received either 1.1 (H group, N=10) or 0.7 (L group, N=10) times their maintenance requirements. Live weight and body condition score were determined weekly and LH concentrations twice per week. To establish the pulsatility of secretion of LH, three periods of intensive sampling were undertaken. Melatonin was determined after a period of 45 short or long days. All photoperiod/nutrition groups showed large variations in LH concentrations according to photoperiod, with nutrition having a significant effect (P<0.001). The mean time between the shift from long to short days and the stimulation of LH secretion, and between the shift from short to long days and the inhibition of LH secretion, was different in each nutrition group (at least P<0.05). No differences were seen in the frequency of LH pulses between the nutrition groups, but differences between sampling periods were observed (P<0.001). Melatonin secretion was not affected by food supply. These results confirm: (1) that Mediterranean female goats are sensitive to photoperiod, (2) that this environmental cue may control the timing of pituitary activity under natural conditions, and (3) suggest that nutrition plays an important role in the effect of photoperiod on LH secretion.     

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)     

Time of the sidereal year affects responsiveness to the phase-resetting effects of photoperiod in the ewe

Two groups of ovary-intact ewes were placed in separate photochambers on the day of the vernal equinox (VE). One group was exposed to a 16 h light:8 h dark (16L:8D) photoperiod and the other to 8L:16D. On the day of the summer solstice (SS) and at 90-91-day intervals thereafter [autumnal equinox (AE), winter solstice (WS), VE and SS], each group was changed to the opposite photoperiod. The latent period between each change and either onset or cessation of cycles, as determined by measuring blood progesterone concentrations, was recorded. The latent period between change to 8L:16D and onset of cycles was shortest after the exposure at AE and longest after exposure at WS (P less than 0.001). The latent period after AE was shorter (P less than 0.001) than after VE. The correlations were small between ambient temperature and interval to onset of cycles. The latent period to cessation of cycles in response to 16L:8D was shorter after SS exposure than after WS exposure (P less than 0.01), but other differences were not significant. There was a strong (r = -0.94, P less than 0.05) negative correlation between interval to cessation of cycles and ambient temperature. Cessation of cycles in response to 16L:8D occurred more rapidly (P less than 0.001) than onset in response to 8L:16D. These results show that responsiveness to the inductive effects of photoperiod varies significantly with time of the sidereal year.     

Amplitude of the plasma melatonin nycthemeral rhythms is not associated with the dates of onset and offset of the seasonal ovulatory activity in the Ile-de-France ewe

We investigated if absolute (nocturnal) or relative (nocturnal/diurnal ratio) plasma melatonin concentrations were associated with the seasonal ovulatory activity in Ile-de-France ewes. Ninety-six and 121 ewes in two different groups of the same flock were used to determine the potential existence of a relationship between melatonin concentrations at the summer and winter solstices, and the dates of onset and offset of the ovulatory activity, respectively. The dates of the first and last ovulations were estimated by assaying progesterone in plasma samples taken once weekly. Mean +/- SEM (1) plasma melatonin concentrations at the summer solstice and at the winter solstice were 302.4 +/- 19.4 and 412.0 +/- 18.7 pg x mL(-1), respectively, (2) date of onset of the breeding season 29 Jul. +/- 1.6 days, (3) date of offset of the breeding season 24 Jan. +/- 2.2 days. In spite of a large variability in the different traits studied here, the analyses of correlation and regression indicated that neither the absolute nor relative melatonin concentrations were significantly related with the dates of onset or offset of ovulatory activity. Therefore, we concluded that absolute or relative plasma melatonin concentrations are not linked to the seasonal breeding activity, in Ile-de-France ewes.     

Leptin, but not immune function, is linked to reproductive responsiveness to photoperiod

Energetic demands are high while energy availability is minimum during winter. To cope with this energetic bottleneck, animals exhibit numerous energy-conserving adaptations during winter, including changes in immune and reproductive functions. A majority of individual rodents within a population inhibits reproductive function (responders) as winter approaches. A substantial proportion of small rodents within a species, however, fails to inhibit reproduction (nonresponders) during winter in the field or in the laboratory when maintained in winter-simulated day lengths. In contrast, immune function is bolstered by short day lengths in some species. The specific mechanisms that link reproductive and immune functions remain unspecified. Leptin is a hormone produced by adipose tissue, and several studies suggest that leptin modulates reproductive and immune functions. The present study sought to determine if photoperiodic alterations in reproductive function and leptin concentrations are linked to photoperiod-modulated changes in immune function. Siberian hamsters (Phodopus sungorus) were housed in either long (LD 16:8) or short (LD 8:16) day lengths for 9 wk. After 9 wk, blood samples were collected during the middle of the light and dark phase to assess leptin concentrations. One week later, animals were injected with keyhole limpet hemocyanin to evaluate humoral immunity. Body mass, body fat content, and serum leptin concentrations were correlated with reproductive responsiveness to photoperiod; short-day animals with regressed gonads exhibited a reduction in these measures, whereas short-day nonresponders resembled long-day animals. In contrast, immune function was influenced by photoperiod but not reproductive status. Taken together, these data suggest that humoral immune function in Siberian hamsters is independent of photoperiod-mediated changes in leptin concentrations.     

Postpartum anovulation in non-nursing monkeys: Hypothalamic-pituitary-ovarian refractoriness is not induced by the milieu of early pregnancy

To evaluate the role of the early pregnancy milieu in inducing postpartum refractoriness of the hypothalmic-pituitary-ovarian axis, pregnancies in rhesus monkeys were terminated on either day 35 of gestation or near term at 162 days. Non-nursing mothers with gestations interrupted near term resumed ovulation at a mean of 56 ± 12 (X? ± SE) days postpartum, in contrast to the 17 ± 2 days required for the females having had abortions at day 35. These results demonstrate that the early pregnancy milieu is not determinant of the hypothalamic-pituitary-ovarian refractoriness observed in non-nursing mothers delivering at term.     

Elimination of short-day melatonin signaling accelerates gonadal recrudescence but does not break refractoriness in male Turkish hamsters

Long days stimulate and short days (SDs) inhibit the reproductive axis of photoperiodic rodents. In long-day Turkish hamsters, unlike most other rodents, elimination of pineal melatonin secretion by constant light or pinealectomy initiates a cycle of gonadal involution and recrudescence outwardly similar to that induced by short days. The present study assessed whether short days and constant light induce the seasonal reproductive cycle via common or different interval timing mechanisms. Male hamsters that had undergone gonadal involution in SDs for 8 or 14 weeks were treated with LL for 14 and 8 weeks, respectively. If SDs and LL act via independent mechanisms, then gonadal quiescence of SD-regressed males, which normally lasts 10 weeks, might be extended by LL treatment; alternatively, if SDs and LL act on the same timer, or the timer cannot be retriggered, then LL will not extend the duration of reproductive quiescence. Neither of these outcomes materialized. Instead, male hamsters exposed to LL while reproductively quiescent exhibited accelerated gonadal recrudescence. Extended LL treatment did not restore responsiveness to SDs in photorefractory hamsters. In Turkish hamsters, photoperiodic history determines whether constant light inhibits or stimulates the hypothalamic-pituitary-testicular axis.     

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

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

Opioid modulation of LH secretion in the ewe

Administration of opioid agonists and antagonists and measurement of resulting hormone changes were used to study the possible effects of opioids on reproductive function in the ewe. Intravenous administration of the long-acting methionine-enkephalin analogue FK33-824 (250 micrograms/h for 12 h) to 3 ewes during the follicular phase of the oestrous cycle depressed episodic LH secretion. This effect was reversed by administration of the opiate antagonist naloxone (25 mg/h) in combination with the FK33-824 treatment; in fact LH secretion was enhanced by the combined regimen. Naloxone (25 mg/h for 12 h) administered alone to 3 ewes in the follicular phase also enhanced LH secretion. In 3 animals treated with FK33-824 during the follicular phase, progesterone remained basal for 14 days after treatment, suggesting that ovulation was blocked. Jugular venous infusion of naloxone (25, 50 or 100 mg/h for 8h) into 5 ewes during the early and mid-luteal phase of the cycle resulted overall in a significant increase in mean plasma LH concentrations and LH episode frequency. To investigate whether endogenous opioids suppress LH release in seasonally anoestrous sheep, naloxone was infused intravenously into mature (25, 50 or 100 mg/h for 8 h) and yearling ewes (12 . 5, 25 or 50 mg/h for 8 h) during early, mid- and late anoestrus and plasma LH concentrations were measured. In the mature ewes, there was a trend for naloxone to increase LH values during the early anoestrous period but naloxone was without effect during mid- and late anoestrus. In the yearlings, naloxone infusion consistently increased plasma LH concentrations as a result of a significant increase in LH episode frequency. These experiments indicate that endogenous opioid peptides probably modulate gonadotrophin secretion during both the follicular and luteal phases of the oestrous cycle. However, the follicular phase of the sheep cycle is of short duration, and there may be residual effects of luteal-phase progesterone during this period. Secondly, there may be an age-dependent effect of naloxone on LH secretion during seasonal anoestrus in the ewe, with opioids playing a part in the suppression of LH in young but not in mature animals.     

Suppression of sympathetic nervous system by short photoperiod and melatonin in the Syrian hamster

Exposure to short photoperiod or melatonin treatment brings about gonadal regression in Syrian hamsters. The possible influence of these treatments on the sympathetic nervous system (SNS) in these animals was investigated. Male Syrian hamsters were exposed to either long or short photoperiod or subjected to administration of melatonin or its vehicle solution. Exposure of hamsters to 10 weeks of short photoperiod, significantly reduced the noradrenaline (NA) turnover in the heart. Daily administration of melatonin for 8 weeks also resulted in a similar suppression of NA turnover in the heart. Hamsters that were treated with melatonin maintained a lowered metabolic rate as well, at and below thermoneutral temperature. These findings suggest that in a deep hibernator, short photoperiod could suppress the peripheral sympathetic activity and that melatonin may act as the endogenous mediator.