Photoperiod-pineal-hypothalamic relay in sheep

Hypothalamic-pituitary-gonadal axis response to photoperiod changes in female guanacos (Lama guanicoe)

The guanaco, a wild South American camelid, is renowned for its remarkable resilience to extreme conditions. Despite this, little is known about how reproductive hormones in female camelids are influenced during their seasonal breeding period, which occurs during long photoperiod. To explore this, the study investigated the response of the hypothalamic-pituitary-gonadal axis in female guanacos during short days (10L:14D; July) and long days (16L:8D; December) in the Mediterranean ecosystem (33°38'28″S, 70°34'27″W). Blood samples from 14 adult animals were collected, and measurements of melatonin, 17β-estradiol, FSH, and LH concentrations were taken. The results showed that melatonin concentration was lower (P < 0.05) during long days than short days, whereas 17β-estradiol, FSH, and LH concentrations were higher (P < 0.05) during long days compared to short days. Furthermore, the study detected the expression of the melatonin receptor 1A and kisspeptin in the hypothalamus and pituitary, suggesting that the pineal gland of female guanacos is sensitive to seasonal changes in day length. These findings also indicate a seasonal variation in the concentration of reproductive hormones, likely linked to the distinct modulation of the hypothalamic-pituitary-gonadal axis of female guanacos during short and long days.

Guinea Pig Sperm Morphology and Fertility under Different Photoperiod

Sperm morphology can predict the reproductive male fertilizing potential. This study aimed to determine the morphological and morphometric spermatozoa characteristics from guinea pigs subjected to different photoperiodic stimulation. Thirty F1 guinea pigs were randomly assigned to three photoperiodic treatments: FT1 (photoperiod with 10 Light/14 Dark LED light), FT2 (photoperiod with 10L/14D sunlight), and FT0 (room without direct light source). At 107 ± 9.8 days of age, sperm concentration and motility were higher in the FT0 and FT1 groups (p < 0.05); furthermore, there were no differences in nucleus length and ellipticity between the FT0 and FT1 groups, but the sperm of the FT1 group was higher in perimeter and nuclear area, while that of the FT0 group was higher in roughness, regularity, midpiece length, and tail (p < 0.01). Expanding acrosome (Type 2) was more frequent in the FT2 group, but there was variation in head measurements between all morphological categories. Pregnancy rate, calving age, and mating age were higher in the FT0 group; meanwhile, the FT1 group initiated successful matings earlier (p < 0.01). The FT0 group had a higher fertility rate, and the age of mating and first calving were earlier in the FT1 group than the FT0 group, but no pregnancies were reported for the FT2 group. Photoperiodic stimulation can increase the morphometric dimensions of guinea pig spermatozoa, favoring the reproductive characteristics, but sunlight could reduce their size due to heat stress.

Effect of Photoperiod with Sunlight at Thermal Stress and Sperm Parameters in Guinea Pigs

&lt;b&gt;Background and Objective:&lt;/b&gt; Photoperiod can regulate reproductive physiological processes in mammals, in which improvements in testosterone concentration, testicular volume and seminal quality have been reported. The aim was to evaluate the influence of photoperiod treatments on guinea pigs' spermatic parameters. &lt;b&gt;Materials and Methods:&lt;/b&gt; Thirty guinea pigs, between males and females, were distributed in two rooms with the photoperiodic treatment of 10 hrs light and 14 hrs dark (PT&lt;sub&gt;1&lt;/sub&gt; with artificial photoperiod and PT&lt;sub&gt;2&lt;/sub&gt; photoperiod with sunlight by opening windows from 08:00-18:00) and one without any direct light stimulus (PT&lt;sub&gt;0&lt;/sub&gt;) for 78 days. The temperature and humidity were recorded and the TH index was calculated for each room. The sperms were recovered in Tris base medium from the epididymis of 16 males to determine sperm concentration, motility, kinetic parameters, vitality, HOST, acrosomal integrity and DNA fragmentation. &lt;b&gt;Results:&lt;/b&gt; Sperm values in PT&lt;sub&gt;1&lt;/sub&gt; and PT&lt;sub&gt;0&lt;/sub&gt; were similar but PT&lt;sub&gt;2&lt;/sub&gt; obtained values lower in sperm concentration, non-progressive motility, total motility, VCL, ALH, vitality, HOST+, acrosomal integrity, sperm with non-fragmented DNA and no pregnancies were reported (0/5). A 100% pregnancy was observed in PT&lt;sub&gt;0&lt;/sub&gt; (4/4) and 50% in PT&lt;sub&gt;1&lt;/sub&gt; (2/4). However, precocity was evidenced in PT&lt;sub&gt;1&lt;/sub&gt; compared to PT&lt;sub&gt;0&lt;/sub&gt;. PT&lt;sub&gt;2&lt;/sub&gt; recorded higher peaks in temperature (33.8°C, THI 81, considered as thermal stress) compared to PT&lt;sub&gt;0&lt;/sub&gt; (32.65°C, THI 81.8) and PT&lt;sub&gt;1&lt;/sub&gt; (32.75°C, THI 81.6). &lt;b&gt;Conclusion:&lt;/b&gt; An artificial photoperiod can improve sperm characteristics and reproductive precociousness of guinea pigs, unlike the photoperiod with sunlight, which generated low spermiogram values and absence of pregnancy due to thermal stress.

The Expression of Birth Weight is Modulated by the Breeding Season in a Goat Model

Birth weight (BW) is frequently considered as an indicator to detect possible restrictions of intrauterine development. This study evaluated the effect of breeding season (BS) across year: spring (SP), summer (SM), fall (FL) and winter (WT) upon the expression of BW. This meta-analysis considered records (n = 1,084) collected from a commercial herd kept under intensive conditions (22° NL, 1,835 m). The definitive statistical model for BW analysis considered the independent variables kidding year, (KY), genetic group (GT), litter size (LS), gender (GN), breeding season (BS), kidding season (KS) plus the interactions (KY*GT) and (GN*BS). BW expression was affected (P<0.05) by KY, GN, LS, KS, and KY*GT. Interestingly, while BW expression was not affected (P>0.05) by kidding season, it was affected (P<0.05) by breeding season. The highest and lowest BW values were observed in SP and WN (3.34 vs. 3.10 kg; P<0.05), respectively. This physiological scenario could be the result of embryonic-fetal adaptive responses representing homeostatic adaptations due to alterations including doe nutritional status, available quantity and quality of food to both the embryo and the fetus as well as to a changing external environment.

Rat pinealectomy: a modified direct visual approach

PURPOSE: To report a new, direct visual approach for rat pinealectomy. METHODS: Eighty adult female rats (Rattus norvegicus albinus EPM-1 strain) were weighted and anesthetized intraperitoneally with 15 mg/kg xylazine and 30 mg/kg ketamine. The animal was fastened to a dissection table, an incision was made in the skin and the subcutaneous tissue, bringing the lambda into view. The skullcap was opened with a dental drill, bringing the cerebral hemispheres and the superior sagittal sinus into view. The pineal gland, located under the venous sinus, was removed in a single piece using tweezers. Next, the bone fragment was returned to its place and the surgical layers were sutured. RESULTS: This new technique is easy to be done, avoids bleedings and removes only the pineal gland without damage to the remaining encephalon. In addition it makes possible the achievement of a sham surgery, allowing the pineal gland to remain intact. CONCLUSION: The proposed technique intends to facilitate studies aiming to better understanding the complexity and importance of the pineal gland on reproductive and other body systems.

Gonadotropin-releasing hormone stimulates prolactin release from lactotrophs in photoperiodic species through a gonadotropin-independent mechanism

Previous studies have provided evidence for a paracrine interaction between pituitary gonadotrophs and lactotrophs. Here, we show that GnRH is able to stimulate prolactin (PRL) release in ovine primary pituitary cultures. This effect was observed during the breeding season (BS), but not during the nonbreeding season (NBS), and was abolished by the application of bromocriptine, a specific dopamine agonist. Interestingly, GnRH gained the ability to stimulate PRL release in NBS cultures following treatment with bromocriptine. In contrast, thyrotropin-releasing hormone, a potent secretagogue of PRL, stimulated PRL release during both the BS and NBS and significantly enhanced the PRL response to GnRH during the BS. These results provide evidence for a photoperiodically modulated functional interaction between the GnRH/gonadotropic and prolactin axes in the pituitary gland of a short day breeder. Moreover, the stimulation of PRL release by GnRH was shown not to be mediated by the gonadotropins, since immunocytochemical, Western blotting, and PCR studies failed to detect pituitary LH or FSH receptor protein and mRNA expressions. Similarly, no gonadotropin receptor expression was observed in the pituitary gland of the horse, a long day breeder. In contrast, S100 protein, a marker of folliculostellate cells, which are known to participate in paracrine mechanisms within this tissue, was detected throughout the pituitaries of both these seasonal breeders. Therefore, an alternative gonadotroph secretory product, a direct effect of GnRH on the lactotroph, or another cell type, such as the folliculostellate cell, may be involved in the PRL response to GnRH in these species.

Synchrony of ovulation and follicular dynamics in merino ewes treated with GnRH in the breeding and non-breeding seasons

The effect of gonadotropin releasing hormone (GnRH) treatment on the time of ovulation and the occurrence of follicular dominance during the non-breeding and breeding seasons (experiment 1), and on fertility after artificial insemination (AI) in the non-breeding season (experiment 2), was examined in Merino ewes. Oestrus was synchronized in 40 nulliparous ewes (experiment 1; n = 20, in the non-breeding and breeding seasons) and in 79 multiparous ewes (experiment 2) using intravaginal sponges and pregnant mare serum gonadotropin. Thirty six hours after sponge removal (SR), half the ewes were injected (i.m.) with 40 microg of synthetic GnRH and the remainder used as controls. GnRH improved the synchrony of ovulation compared with the controls in the breeding (SD = 2.8 vs 5.7 days, p = 0.04) but not the non-breeding season (SD = 3.8 vs 4.4 days, p = 0.69), with ewes ovulating from 42 to 54 h (mean 50.4 +/- 4.08 h) and 42-60 h (mean 54.4 +/- 5.47 h) after SR for GnRH and control, respectively. For both treated and control ewes, ovulation occurred earlier in the non-breeding than the breeding season (50.1 vs 54.6 h; p = 0.002). GnRH had no effect on follicular dominance, as assessed by divergence (D: the time the ovulatory follicle exceeded the average size of the other non-ovulating follicles) or on the interval from D to ovulation (IDO). However, follicular dynamics differed between seasons. The mean follicle diameter increased at a faster rate up to 36 h after SR in the non-breeding compared with the breeding season and then rapidly declined, compared with a later peak (42 h after SR) in mean follicular size during the breeding season. IDO was shorter in the non-breeding than in the breeding season (26.7 +/- 4.30 h vs 39.6 +/- 4.53 h; p = 0.05). In experiment 2, ewes (n = 38 GnRH-treated, n = 40 controls) were inseminated in the uterus by laparoscopy 42 h or 48 h after SR with frozen-thawed sperm. The fertility of ewes treated with GnRH (nine of 39, 23%) was not different to the controls (eight of 38, 21%; p = 0.01). In conclusion the application of GnRH improved synchronization of ovulation but did not improve fertility rates after AI.

Effects of melatonin on the endometrial morphology and embryo implantation in rats

OBJECTIVE

To determine the effects of melatonin on rat endometrium morphology and embryo implantation.

DESIGN

Experimental study.

SETTING

Federal University of São Paulo, Brazil.

ANIMAL(S)

Forty female rats.

INTERVENTION(S)

GI: control, GII: sham-operated, GIII: pinealectomized, and GIV: pinealectomized rats that received melatonin during 3 months. The GI, GII, and GIII groups received the vehicle of melatonin (NaCl + ethanol). At the end of the treatment, the animals were killed during the estrous phase; the uterus was removed for morphometric analysis. Urine was collected for 6-sulfatoxymelatonin. Blood was collected for estrogen (E) and progesterone (P) level determinations. In a second experiment, female rats were used to evaluate the endometrial embryo implantation.

MAIN OUTCOME MEASURE(S)

Endometrial morphology and embryo implantation.

RESULT(S)

GIII presented the highest values for endometrial area and thickness index, number of endometrial glands, and eosinophils. The number of vessels of groups I, II, and IV was fewer than that of GIII. The highest number of eosinophils was detected in GIII in comparison to other groups. The implantation rate in GIII was the lowest of all groups. This implantation rate was significantly increased and restored toward normal in GIV.

CONCLUSION(S)

Our data suggested that, in nonphotoperiodic animals such as rats, melatonin may positively affect the endometrial morphology and improve embryo implantation.

Night-time Melatonin Secretion and Seasonally Delayed Puberty in Gilts

This study was conducted to determine whether the seasonal delay in puberty in autumn is driven by individual differences in night-time melatonin secretion in domestic gilts at the attainment of puberty. A group of spring-born gilts (n = 30) were expected to reach puberty in autumn by the age of 7 months. Eighteen of these gilts were selected in pairs on the basis of matched days of birth. By the expected time, half of the animals showed oestrous symptoms (group CYCLING, n = 9) with the rest remaining silent (group SILENT, n = 9). Afterwards, all gilts were fitted with indwelling jugular catheters for frequent blood sampling. Blood samples were collected from all animals three times during the day followed by three times in the night at 2-h intervals for 48 h. The samples were analysed by a commercial radioimmunoassay (RIA). The results show a consistent 25-fold rise (on average) in night-time melatonin concentration in every animal sampled with group averages ranging from 0.28 +/- 0.04 to 0.37 +/- 0.06 pg/ml at day and from 10.20 +/- 2.16 to 10.67 +/- 0.05 pg/ml at night. Night-time group mean values between CYCLING and SILENT gilts did not differ significantly (10.26 +/- 0.67 and 10.38 +/- 0.94 for the CYCLING; 10.67 +/- 0.05 and 10.20 +/- 2.16 for the SILENT). When 10 pg/ml was used as a threshold value, six individuals did not reach it during the night (low responders). Two of these gilts were CYCLING and four were SILENT. In conclusion, the results presented imply no involvement of the level of night-time melatonin concentration in the seasonal delay of puberty in gilts.

Effects of melatonin on the ovarian response to pinealectomy or continuous light in female rats: similarity with polycystic ovary syndrome

The current study was conducted to investigate the relationship between melatonin and chronic anovulation. Adult (3-4 months old) female Wistar rats were submitted to pinealectomy: group I: pinealectomized ovariectomized melatonin-treated (N = 10); group II: pinealectomized ovariectomized placebo-treated (N = 12); group III: pinealectomized light-treated placebo-treated (N = 10) or maintained under continuous light; group IV: maintained under continuous light, ovariectomized melatonin-treated (N = 22); group V: maintained under continuous light, ovariectomized placebo-treated (N = 10); group VI: maintained under continuous light placebo-treated (N = 10). In order to assess ovarian modifications, unilateral ovariectomy was performed during the fourth month in groups I, II, IV, V and the other ovary was removed after 8 months. Ovariectomy was performed in groups III and VI only after eight months. Melatonin (200 micro g/100 g body weight) dissolved in 0.02 ml absolute ethanol was injected intramuscularly daily during the last 4 months into groups I and IV. The other groups were treated with placebo (NaCl). The ovarian cysts were analyzed and their area, perimeter and maximum diameter, as well as the thickness of the ovarian capsule were measured. Daily colpocytological smears were performed throughout the study. Persistent estrous condition and ovarian cysts were observed in all groups. In pinealectomized rats the ovarian and vaginal alterations disappeared at the end of the study and in rats maintained under continuous light the vaginal and ovarian polycystic aspect was reversed only in those treated with melatonin. We conclude that melatonin may act on the ovarian response reverting chronic anovulation induced by pinealectomy or continuous light.

Reproductive performance of gilts and sows in temperate versus subarctic and arctic zones of Norway

The reproductive performance of gilts and sows from two regions in Norway was investigated in a retrospective analysis of data from the litter recording system. In the Northern region (North; between 65 degrees N and 71 degrees N), there are extreme shifts in natural photoperiod between winter and summer. In the Southern region (South; between 59 degrees N and 60 degrees 30'N), photoperiodic changes are less dramatic. Gilts were 8 days older at first mating or insemination in the North than in the South (P<0.01). A significantly lower proportion of sows in the North were mated or inseminated within 5 days post-weaning than in the South, a difference present both among primiparous and multiparous sows (P<0.01). Overall farrowing rate in the North was lower than in the South, but litter size (total number born) among those pigs that farrowed was larger. After correction for year, month, breed and age at first service, there were still lower odds of farrowing for gilts in North than in South. Neither for primiparous nor multiparous sows were regional differences in farrowing probability significant when year, month, breed and weaning to service interval were included in the model. Gilts and primiparous sows had a lower probability of farrowing following insemination during summer or autumn months, but service month was not significantly related to the farrowing probability of multiparous sows. For gilts, litter size was positively related to age at first service. For sows, litter size was lowest at weaning to service intervals between 6 and 10 days. Total numbers of piglets born per litter were estimated to be 0.36, 0.38 and 0.55 larger in the North than in the South (differences in least square means; gilts, primiparous sows and multiparous sows, respectively) (P<0.01). Litter size was lower after service during natural long photoperiod than during the rest of the year.

Effect of an inverse subtropical (19 degrees 13'N) photoperiod on ovarian activity, melatonin and prolactin secretion in Pelibuey ewes

Twenty-one Pelibuey ewes were used from December 21, 1996 to December 21, 1998. Fourteen of them had never been exposed to artificial photoperiod, and they were maintained on natural photoperiod until March 21, 1997, when they were assigned to natural photoperiod (control group, n=8) or to inverse photoperiod (n=6). The other seven animals had been kept on a long photoperiod (16L:8D) from October 21, 1996 to December 21, 1996, when they entered the present study and were subjected to a gradual decrease in photoperiod, so that they reached an equinox photoperiod (12L:12 D) on March 21, 1997. At that time, they were assigned to natural photoperiod (n=3) or to inverse photoperiod (n=4). Blood samples for progesterone determination were taken twice a week from all the animals. During the second year of the study, prolactin was measured in the samples from five animals in inverse photoperiod and from five control ewes. Hourly samples were obtained to determine the 24-h melatonin profile of five animals from each group on September 21, 1997, December 21, 1997, March 21, 1997, and June 21, 1997. Exposure to inverse photoperiod resulted in a gradual shift on the annual reproductive cycle, so that the second ovulatory season was advanced by 5 months in the ewes kept on inverse photoperiod as compared to the control ewes (P<0.05). There were wide variations in the dates for the onset and the end of the ovulatory season within the inverse photoperiod groups, and three animals in this groups maintained ovulatory activity for at least 18 consecutive months. The duration of melatonin secretion was directly related to the length of the dark period (P<0.05), and this response was not affected by the calendar date. Prolactin concentrations were directly related to daylength, however, they were also affected by calendar date, being lower in the inverse group as compared to the corresponding time of the annual photoperiodic cycle of ewes on natural photoperiod. It is concluded that reproductive activity, melatonin secretion and prolactin secretion of Pelibuey ewes respond to the small variations in photoperiod that are present at 19 degrees 13'N, and that under natural conditions, photoperiod appears to be the main regulator of ovarian activity at this latitude. However, other factors such as temperature or humidity may act as modulators, and their relative importance could increase at more equatorial latitudes.

Factors effecting reproduction in the pig: seasonal effects and restricted feeding of the pregnant gilt and sow

Recent advances in research on seasonal infertility are discussed with a special focus on implications of the generally recommended restricted post-mating feeding strategy of the early pregnant gilt and sow for the physiology of seasonal infertility. The endocrinological basis of seasonal breeding of the wild and domestic pig is being clarified: as in other seasonal breeders, melatonin is relaying photoperiodic information about season to the pituitary-gonadal axis. Earlier confusion on this matter appears to have been caused by a lack of specificity of the melatonin assays employed. Group housing of the pregnant sow is becoming a common practice and, as an important environmental risk factor for seasonal infertility, may lead to an increase in the incidence of seasonal infertility in the future. After an initial progesterone-mediated beneficial effect on embryonic survival, a restricted post-mating feeding strategy may have a negative effect on maintenance of early pregnancy in the gilt and sow in the summer-autumn period. The endocrinological mechanism of seasonal disruption of pregnancy is yet to be determined. However, it is proposed that LH is reduced in the summer-autumn period and this reduction is amplified by the commonly applied restricted post-mating feeding strategy. These changes in LH secretion, although not as such inducing CL regression, may exert a progesterone-mediated detrimental effect on the capability of embryos to produce adequate embryonic signaling. This may lead to a seasonal disruption of pregnancy and a return to oestrus 25-30 days after mating.

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.

Prolactin replacement fails to inhibit reactivation of gonadotropin secretion in rams treated with melatonin under long days

This study tested the hypothesis that prolactin (PRL) inhibits gonadotropin secretion in rams maintained under long days and that treatment with melatonin (s.c. continuous-release implant; MEL-IMP) reactivates the reproductive axis by suppressing PRL secretion. Adult Soay rams were maintained under long days (16L:8D) and received 1) no further treatment (control, C); 2) MEL-IMP for 16 wk and injections of saline/vehicle for the first 8 wk (M); 3) MEL-IMP for 16 wk and exogenous PRL (s.c. 5 mg ovine PRL 3x daily) for the first 8 wk (M+P). The treatment with melatonin induced a rapid increase in the blood concentrations of FSH and testosterone, rapid growth of the testes, an increase in the frequency of LH pulses, and a decrease in the LH response to N-methyl-D,L-aspartic acid. The concomitant treatment with exogenous PRL had no effect on these reproductive responses but caused a significant delay in the timing of the sexual skin color and growth of the winter pelage. These results do not support the hypothesis and suggest that PRL at physiological long-day concentrations, while being totally ineffective as an inhibitor of gonadotropin secretion, acts in the peripheral tissues and skin to maintain summer characteristics.

Diurnal rhythm of melatonin in bovine milk: pharmacokinetics of exogenous melatonin in lactating cows and goats

We investigated whether the melatonin levels in bovine milk exhibit a similar daily rhythm as serum levels. In 4 Ayrshire cows at the beginning of the lactation period in May the nocturnal rise in milk melatonin was moderate (from 7 +/- 2 pg/ml at noon to 15 +/- 1 pg/ml at night; mean +/- SEM) and did not correlate well with the melatonin level in serum (from 7 +/- 2 pg/ml to 27 +/- 7 pg/ml, respectively). On the other hand, 6 cows in a later phase of lactation, studied in February, showed a clear long-lasting nocturnal melatonin increase both in serum (from 9 +/- 1 pg/ml at noon to 26 +/- 3 pg/ml at night) and in milk (from 12 +/- 5 pg/ml to 26 +/- 7 pg/ml, respectively). Melatonin kinetics during lactation was studied in more detail in 4 Ayrshire cows and 4 dairy goats by giving an intravenous bolus injection of melatonin. A 3-compartment model with melatonin elimination from the central compartment was used to describe the data. The values (mean +/- SD) for the cows and the goats were: elimination half-life 27 +/- 4 min and 27 +/- 1 min, mean residence time 24 +/- 4 min and 18 +/- 4 min, steady state distribution volume 1.0 +/- 0.3 l/kg and 0.6 +/- 0.1 1/kg (p < 0.05), and plasma clearance 0.044 +/- 0.004 l/kg/min and 0.035 +/- 0.011 l/kg/min, respectively. Following injection, the melatonin concentration in milk increased rapidly and exceeded the corresponding serum value 15-30 min later, remaining thereafter above the serum level. Our results suggest that milk melatonin levels reflect blood concentrations of melatonin with a short delay.

Photo-neuroendocrine control of seasonal cycles in body weight, pelage growth and reproduction: lessons from the HPD sheep model

This chapter summarises the results of a recent study which investigated the role of the hypothalamo-pituitary relay system in mediating the effects of photoperiod on seasonal cycles in: (a) body weight; (b) pelage growth; and (c) reproduction in Soay rams. Hypothalamo-pituitary disconnected (HPD) and the control rams were housed indoors under an artificial lighting regimen of alternating 16-weekly periods. These periods consisted of long (16L:8D) and short days (8L:16D) and lasted for more than 2 years. The: (i) body weight; (ii) voluntary food intake; (iii) pelage and horn growth; and (iv) variations in testicular diameter were measured routinely every 2-4 weeks. Twice-weekly blood samples were collected to monitor long-term changes in the blood concentrations of: (1) pituitary; (2) metabolic; and (3) reproductive hormones (prolactin, GH, alpha-MSH, beta-endorphin, ACTH, TSH, LH, FSH, cortisol, insulin, IGF1 and testosterone). In control rams there were clearly defined photoperiod-induced cycles in blood concentrations of prolactin, alpha-MSH, beta-endorphin, LH, FSH, insulin and testosterone and associated morphological changes consistent with causal relationships (e.g. prolactin versus wool and horn growth, alpha-MSH, beta-endorphin and insulin versus body weight/food intake, LH and FSH versus testis size). In the HPD rams there were no photoperiod-induced cycles in the concentrations of any of the pituitary hormones with the exception of prolactin which varied as in controls (10-fold higher under long days). There was a permanent increase in blood concentrations of alpha-MSH, beta-endorphin and insulin in the HPD animals and a decrease in the concentrations of GH (loss of pulsatility) and IGF1. These changes were associated with the development of obesity. The reproductive axis was inactivated (basal LH, FSH and testosterone) although there was residual cyclicity in the size of the testis associated with the changes in prolactin secretion. Overall, the results support the view that the melatonin signal which encodes photoperiod, acts in the hypothalamus to regulate some photoperiodic responses (alpha-MSH and beta-endorphin-body weight axis, gonadotrophin-gonadal axis) but acts in the pituitary gland to regulate other responses (prolactin-pelage axis). However, a functional hypothalamus is required to generate normal seasonal cycles in: (a) body weight; (b) food intake; (c) growth; (d) fattening; and (e) reproduction, to provide the internal coordination between different systems and to facilitate the temporal entrainment to environmental cues.

Evidence that melatonin acts in the pituitary gland through a dopamine-independent mechanism to mediate effects of daylength on the secretion of prolactin in the ram

A previous study provided evidence that melatonin acts in the pituitary gland to mediate the effects of daylength on the secretion of prolactin in sheep. This was based on the observation that hypothalamo-pituitary disconnected (HPD) Soay rams showed normal patterns in the changes in the peripheral blood concentrations of prolactin in response to alterations in photoperiod (10-fold higher concentrations under long than short days), and in response to exogenous melatonin (rapid decline following the administration of a constant-release implant of melatonin). The purpose of this study was to establish whether dopamine (DA) might be involved in mediating the effects of melatonin on the secretion of prolactin. Groups of HPD (n = 7) and control Soay rams (n = 8) were treated with vehicle (control, 2.0 ml 0.1 M tartaric acid/saline sc), bromocriptine (DA agonist, 0.06 mg/kg sc) or sulpiride (DA antagonist, 0.6 mg/kg sc), and the acute prolactin responses were measured over the next 4 h. Treatments were carried out under short days (8L: 16D, low prolactin), long days (16L: 8D), high prolactin), and under long days in the presence of a constant-release implant of melatonin (low prolactin). The prolactin response to TRH (1.25 micrograms/kg iv) was also measured. Bromocriptine caused a decrease in the plasma concentrations of prolactin in both HPD and control rams under short and long days. Sulpiride had no effect in the HPD rams on any occasion, but caused a very marked increase in the plasma concentrations of prolactin in the control rams under short days, long days, and under long days + melatonin. TRH caused an acute increase in the plasma concentrations of prolactin in the HPD rams under both long and short days although the responses were notably reduced compared with the controls especially under long days + melatonin. Overall, the inhibitory response to the DA agonist in HPD rams indicates the presence of DA D2 receptors linked to functional lactotrophs in the isolated pituitary gland. However, the total lack of a response to the DA antagonist indicates the absence of endogenous DA mechanisms regulating the secretion of prolactin in the HPD rams. The conclusion is that melatonin acts directly on the pituitary gland to mediate effects of photoperiod through a DA-independent mechanism.

Light control of the duration of the daily melatonin signal under long and short days in the Soay ram. Role of inhibition and entrainment

Light acts in two ways to control the duration of the nocturnal melatonin rhythm. It inhibits the production of melatonin from the pineal gland and it entrains the underlying circadian rhythm generators located in the suprachiasmatic nuclei. To investigate the role of these two mechanisms under long and short days, four experiments were carried out using groups of adult Soay rams (n = 6-8). The animals were housed in individual pens in light-controlled rooms and entrained to long (LD 16:8) or short (LD 8:16) days for at least 8 wk. The treatments were as follows: (i) dark period extended by 4 h under long days (L dark-delay), (ii) dark period advanced by 4 h under long days (L dark-advance), (iii) dark period extended by 4 h under short days (S dark-delay), and (iv) dark period advanced by 4 h under short days (S dark-advance). Each treatment was given on a single day and the animals were subsequently maintained in, or transferred to, constant dim red light (DD) for 24 h. A control group (C) was run in parallel with each treatment group. Blood samples were collected every 30 min for 6-9 h during the dark-shift to monitor the light-induced changes in the secretion of melatonin, and during DD to monitor any phase shift in the endogenous rhythm (phase markers provided by onset or offset of melatonin secretion). L dark-delay resulted in a significantly (p < 0.01, ANOVA) later offset of the melatonin peak (3.4-h delay) with no phase shift of the onset of the rhythm under DD.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoperiodically-induced cycles in the secretion of prolactin in hypothalamo-pituitary disconnected rams: evidence for translation of the melatonin signal in the pituitary gland

Long term changes in the secretion of prolactin were monitored in groups of hypothalamo-pituitary disconnected rams (HPD rams, n = 8) and control rams (HPD sham-operated and unoperated, n = 8) while exposed to an artificial lighting regimen of alternating 16-weekly periods of long days (16L:8D) and short days (8L:16D) for 72 weeks, and during a treatment with subcutaneous constant-release implants of melatonin under long days. The HPD rams showed all the clinical characteristics of complete pituitary disconnection (diabetes insipidus, gonadal regression and slight obesity), and were unresponsive to a range of provocation tests (exposure to a barking sheep dog, cannulation of the jugular vein, injection of serotonin and NMDA) which caused acute changes in the blood plasma concentrations of prolactin in the controls. Nevertheless, there was a clearly defined cycle in the blood concentrations of prolactin in the HPD rams related to the imposed lighting regimen with values 10-fold higher under long days compared to short days (HPD mean +/- SEM: 90.1 +/- 24.7 vs 9.4 +/- 2.0 micrograms/l, long vs short day respectively, P < 0.001). The temporal pattern was very similar to that observed in the controls, although the concentrations of prolactin were higher in the HPD rams and more variable (control mean +/- SEM: 55.6 +/- 3.6 vs 3.0 +/- 0.5 micrograms/l, long vs short day, P < 0.001). There was a corresponding cycle in the growth and moulting of the wool in the HPD rams consistent with a biological response to the photoperiodically-induced changes in the secretion of prolactin. The diurnal rhythm in the blood concentrations of prolactin was absent in the HPD rams, but there was a normal rhythm in the secretion of melatonin. The treatment of the animals with constant-release implants of melatonin under long days caused a marked decrease in the blood concentrations of prolactin in both the HPD and control rams. The overall conclusion is that the endogenously generated daily melatonin signal which encodes daylength acts directly in the pituitary gland to mediate the effects of photo-period on the secretion of prolactin. The photo-period transduction pathway thus by-passes the hypothalamus.

Effects of excitatory amino acid receptor agonists and antagonists on the secretion of melatonin, luteinizing hormone and prolactin in the ram

To assess the role of excitatory amino acids (EAA) as neurotransmitters in the transmission of light information from the retina to the pineal gland, we have determined whether the systemic injection of EAA agonists in Soay rams will mimic the suppressive effect of light on the secretion of melatonin, and whether pretreatment of rams with EAA antagonists will block this effect. In addition, the efficacy of the drugs in affecting neuroendocrine systems was investigated by measuring the changes in the secretion of luteinizing hormone (LH) and prolactin. Injections fo the EAA receptor agonist, NMDA (N-methyl-D,L-aspartate: 4.0 mg/kg iv), and the non-NMDA type EAA receptor agonist, AMPA (DL-alpha-amino-3-hydroxy-5-methylisoxazole-propionic acid: 0.2 mg/kg iv) given at night to rams exposed to long days (16 h light: 8 h darkness), caused no change in the blood plasma concentrations of melatonin. The treatments induced an acute increase in the concentrations of LH, and NMDA, but not AMPA, caused a sustained increase in the concentrations of prolactin. Injections of the specific NMDA-type receptor antagonist, CGP (CGP 37849: 1.0 mg/kg iv) and the non-NMDA-type receptor antagonist, DNQX (6,7 Dinitroquinoxaline-2,3-dione: 0.5 mg/kg iv), given prior to a 1-h light period at night, in rams under long days, caused no change in the light-induced decrease in blood plasma concentrations of melatonin. The drug treatments had no effect on the plasma concentrations of LH, but CGP, and not DNQX, stimulated an acute increase in the plasma concentrations of prolactin. These results provide support for the hypothesis that EAA mechanisms operate in the hypothalamus to regulate the release of peptides and catecholamines which control the secretion of LH and prolactin from the pituitary gland; different sub-types of EAA receptors are involved in the control of the two pituitary hormones. The failure of the treatments to affect the secretion of melatonin may indicate that EAA receptor activation is not involved in the photic relay to the pineal gland, or may merely reflect the inability of the drugs to penetrate into the retina/SCN/pineal neural circuits to produce a response.