Melatonin from cerebrospinal fluid but not from blood reaches sheep cerebral tissues under physiological conditions

The pineal gland secretes melatonin (MLT) that circulates in the blood and cerebrospinal fluid (CSF). We provide data to support the hypothesis that, in sheep and possibly in humans, only the CSF MLT, and not the blood MLT, can provide most of MLT to the cerebral tissue in high concentrations, particularly in the periventricular area. The MLT content of sheep brain, our chosen animal model, was found in significant concentration gradients oriented from the ventricle (close to the CSF) to the cerebral tissue, with concentrations varying by a factor of 1-125. The highest concentrations were observed close to the ventricle wall, whereas the lowest concentrations were furthest from the ventricles (407.0 ± 71.5 pg/ml compared to 84.7 ± 5.2 pg/ml around the third ventricle). This concentration gradient was measured in brain tissue collected at mid-day and at the end of the night. Nocturnal concentrations were higher than daytime concentrations, reflecting the diurnal variation in the pineal gland. The concentration gradient was not detected when MLT was delivered to the brain via the bloodstream. The diffusion of MLT to cerebral tissues via CSF was supported by in vivo scintigraphy and autoradiography. 2-[(123)I]-MLT infused into the CSF quickly and efficiently diffused into the brain tissues, whereas [(123)I]-iodine (control) was mostly washed away by the CSF flow and [(123)I]-bovine serum albumin remained mostly in the CSF. Taken together, these data support a critical role of CSF in providing the brain with MLT.

Functional characterization of polymorphic variants for ovine MT1 melatonin receptors: possible implication for seasonal reproduction in sheep

In seasonal breeding species, the gene encoding for the melatonin MT(1) receptor (oMT(1)) is highly polymorphic and numerous data have reported the existence of an association between an allele of the receptor and a marked expression of the seasonality of reproduction in ewes. This allele called "m" (previously named "-" allele) carries a mutation leading to the absence of a MnlI restriction site as opposed to the "M" allele (previously named "+" allele) carrying the MnlI restriction site (previously "+" allele). This allows the determination of the three genotypes "M/M" (+/+), "M/m" (+/-) and "m/m" (-/-). This mutation is conservative and could therefore not be causal. However, it is associated with another mutation introducing the change of a valine to an isoleucine in the fifth transmembrane domain of the receptor. Homozygous "M/M" and "m/m" animals consequently express structurally different receptors respectively named oMT(1) Val(220) and oMT(1) Ile(220). The objective of this study was to test whether these polymorphic variants are functionally different. To achieve this goal, we characterized the binding properties and the transduction pathways associated with both variants of the receptors. Using a pharmacological approach, no variation in binding parameters between the two receptors when transiently expressed in COS-7. In stably transfected HEK293 cells, significant differences were detected in the inhibition of cAMP production whereas receptors internalization processes were not different. In conclusion, the possibility that subtle alterations induced by the non conservative mutation in "m/m" animals might modify the perception of the melatoninergic signal is discussed in the context of melatonin action.

Four hours of daily contact with sexually active males is sufficient to induce fertile ovulation in anestrous goats

The study was conducted on two consecutive years to determine whether ovulatory activity can be induced in anovulatory goats by exposing them to sexually active bucks for 4, 8, 12 or 16 h per day during 15 consecutive days. In experiment 1, females remained continuously in the experimental pens where they were in contact with males. One group remained isolated from males (controls) and four other groups were exposed to sexually active males for 4, 8, 12 or 16 h per day. In experiment 2, females were taken away to "resting" pens free of male odours between the periods of contact with bucks. They were allocated to 5 groups as in experiment 1. Ovulations were determined by progesterone plasma levels and transrectal ultrasonography. Pregnancy was determined by abdominal ultrasonography. In both experiments, more than 90% of females exposed to the bucks had at least one ovulation during the whole experiment whereas only 11 or 0% (experiments 1 and 2, respectively) did so in the control group (P<0.001). Furthermore, the proportion of females ovulating did not differ among groups depending on duration of contact with bucks (P>0.05). In both experiments, pregnancy rates were not affected by the daily duration of contact with males (P>0.05). To conclude, 4h of daily contact with sexually active males is sufficient to stimulate ovulatory activity in anovulatory goats and this effect is not due to the presence of olfactory cues from the males remaining in the pens.

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

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

Effects of photoperiod on kisspeptin neuronal populations of the ewe diencephalon in connection with reproductive function

Kisspeptin (Kiss) is a key regulator of reproductive function in both prepubertal and adult mammals. Its expression appears to vary throughout the year in seasonal species. We aimed to determine the impact of a change of photoperiod on the size of Kiss neuronal populations found in the preoptic area (POA) and arcuate nucleus (ARC) of the ewe brain. Using immunocytochemistry, we first examined the proportion of neurones expressing Kiss, using HuC/D as a neuronal marker, at different time-points after transition from long days (LD; 16 : 8 h light/dark cycle) to short days (SD; 8 : 16 h light/dark cycle). Luteinising hormone (LH) secretion was measured in ovariectomised oestradiol replaced ewes from the month preceding the transition to SD until the sacrifice of the animals at days 0, 45 and 112 from this photoperiodic transition. High LH levels were only observed in animals killed at day 112. The number of Kiss neurones/mm(2) doubled in the caudal ARC at day 112. The percentage of neurones showing Kiss immunoreactivity increased significantly in both the POA and ARC in the day 112 group. In a second experiment, ewes kept in LD received an i.c.v. injection of colchicine 20 h before sacrifice. Colchicine treatment increased the number and the percentage of neurones with Kiss in both the POA and caudal ARC. The data obtained suggest that the increase in Kiss neurones detected in the POA and caudal ARC after transition to SD stemmed from an increase in Kiss synthesis. This up-regulation of Kiss content under the shorter day condition appears to be a late event within the cascade activated by a longer secretion of melatonin, which is a critical factor in switching gonadotrophin-releasing hormone secretion to a breeding season profile.

The end of a myth: cloning and characterization of the ovine melatonin MT(2) receptor

BACKGROUND AND PURPOSE

For many years, it was suspected that sheep expressed only one melatonin receptor (closely resembling MT(1) from other mammal species). Here we report the cloning of another melatonin receptor, MT(2), from sheep.

EXPERIMENTAL APPROACH

Using a thermo-resistant reverse transcriptase and polymerase chain reaction primer set homologous to the bovine MT(2) mRNA sequence, we have cloned and characterized MT(2) receptors from sheep retina.

KEY RESULTS

The ovine MT(2) receptor presents 96%, 72% and 67% identity with cattle, human and rat respectively. This MT(2) receptor stably expressed in CHO-K1 cells showed high-affinity 2[(125)I]-iodomelatonin binding (K(D)= 0.04 nM). The rank order of inhibition of 2[(125)I]-iodomelatonin binding by melatonin, 4-phenyl-2-propionamidotetralin and luzindole was similar to that exhibited by MT(2) receptors of other species (melatonin > 4-phenyl-2-propionamidotetralin > luzindole). However, its pharmacological profile was closer to that of rat, rather than human MT(2) receptors. Functionally, the ovine MT(2) receptors were coupled to G(i) proteins leading to inhibition of adenylyl cyclase, as the other melatonin receptors. In sheep brain, MT(2) mRNA was expressed in pars tuberalis, choroid plexus and retina, and moderately in mammillary bodies. Real-time polymerase chain reaction showed that in sheep pars tuberalis, premammillary hypothalamus and mammillary bodies, the temporal pattern of expression of MT(1) and MT(2) mRNA was not parallel in the three tissues.

CONCLUSION AND IMPLICATIONS

Co-expression of MT(1) and MT(2) receptors in all analysed sheep brain tissues suggests that MT(2) receptors may participate in melatonin regulation of seasonal anovulatory activity in ewes by modulating MT(1) receptor action.

Nutritional supplementation improves ovulation and pregnancy rates in female goats managed under natural grazing conditions and exposed to the male effect

Two experiments were conducted to determine if nutritional supplementation improved ovulation and pregnancy rates in female goats managed under grazing conditions and submitted to the male effect. In Experiment 1, one group of does did not receive nutritional supplementation, while the other group was supplemented daily for 7 days starting at the time when the males were introduced to the females. The ovulation rate at the second male-induced ovulation was greater (P<0.05) in supplemented (2.0+/-0.1) than in non-supplemented (1.6+/-0.1) does. For Experiment 2, female goats were supplemented for 0, 7, 14 or 28 days, starting 9 days following buck introduction. The proportion of does that were pregnant in the group supplemented for 28 days was greater (P<0.05) than in the non-supplemented group, but did not differ from 14-day and the 7-day supplemented groups. The proportion of pregnant does was greater (P<0.05) in the group supplemented for 14 days compared to the group supplemented for 7 days and the non-supplemented group. These latter two groups did not differ (P>0.05). In conclusion, feed supplementation for 7 days, starting at the time when males were introduced increased ovulation rate and feed supplementation for 14 or 28 days starting 9 days after males were introduced improved pregnancy rates in goats managed under grazing conditions and exposed to males.

Seasonality of reproduction in mammals: intimate regulatory mechanisms and practical implications

Farm mammals generally express seasonal variations in their production traits, thus inducing changing availability of fresh derived animal products (meat, milk and cheese) or performances (horses). This is due to a more or less marked seasonal birth distribution in sheep and goats, in horses but not cattle. Birth peak occurs at the end of winter-early spring, the most favourable period for the progeny to survive. Most species show seasonal variations in their ovulation frequency (presence or absence of ovulation), spermatogenic activity (from moderate decrease to complete absence of sperm production), gamete quality (variations in fertilization rates and embryo survival), and also sexual behaviour. The intimate mechanism involved is a complex combination of endogenous circannual rhythm driven and synchronized by light and melatonin. Profound and long-term neuroendocrine changes involving different neuromediator systems were described to play a role in these processes. In most species artificial photoperiodic treatments consisting of extra-light during natural short days (in sheep and goats and mares) or melatonin during long days (in sheep and goats) are extensively used to either adjust the breeding season to animal producer needs and/or to completely overcome seasonal variations of sperm production in artificial insemination centres. Pure light treatments (without melatonin), especially when applied in open barns, could be considered as non-invasive ones which fully respect animal welfare. Genetic selection could be one of the future ways to decrease seasonality in sheep and goats.

Reproductive seasonality in female goats adapted to a subtropical environment persists independently of food availability

The aim of this study was to determine whether the seasonal change in food availability provides a predictive cue that synchronizes the breeding season in goats adapted to the subtropical conditions of Northern Mexico. Groups of multiparous intact (n=7) and ovariectomized does bearing a subcutaneous implant constantly releasing estradiol-l7 beta (OVX+E; n=12) were allocated in two pens and received a constant amount of feed. Another group of OVX+E does (n=10) was incorporated into a flock raised under natural grazing conditions with seasonal fluctuations in food availability. Blood samples were taken twice weekly. Ovulation of intact goats was inferred from plasma progesterone levels. In OVX+E does plasma levels of LH were measured. Intact does displayed seasonal variations in ovulation and the breeding season lasted from September to February. OVX+E goats displayed large seasonal variations in LH secretion regardless of whether they were fed constantly indoors or kept under natural grazing conditions (P<0.0001). The mean date of increase in LH secretion varied between years (P<0.0001) but did not differ between groups (naturally grazed: August 13+/-10 days; constantly fed: August 26+/-10 days). In contrast, the seasonal decline in LH secretion differed between groups: it was consistently delayed in constantly fed animals compared to those feeding naturally (naturally grazed: January 28+/-4 days; constantly fed: February 24+/-6 days, group effect, P<0.01). These results show that local female goats from subtropical Mexico display large reproductive seasonality independently of food availability.

Melatonin activates brain dopaminergic systems in the eel with an inhibitory impact on reproductive function

In the eel, a deficit in gonadotrophin-releasing hormone (GnRH) and a strong dopaminergic (DA) inhibition are responsible for the blockade of gonad development if silver eels are prevented from their reproductive migration. Environmental factors that eels encounter during their oceanic reproductive migration are thought to play an important role in the stimulation of eel pubertal development. We investigated the potential role of melatonin, a known mediator of the effects of external factors on reproductive function in vertebrates. We demonstrated that a long-term melatonin treatment increased brain tyrosine hydroxylase (TH, the rate limiting enzyme of DA synthesis) mRNA expression in a region-dependent way. Melatonin stimulated the dopaminergic system of the preoptic area, which is involved in the inhibitory control of gonadotrophin [luteinising hormone (LH) and follicle-stimulating hormone (FSH)] synthesis and release. Moreover, we showed that the increased TH expression appeared to be consistent with melatonin binding site distribution as shown by 2[(125)I]-melatonin labelling studies. On the other hand, melatonin had no effects on the two eel native forms of GnRH (mGnRH and cGnRH-II) mRNA expression. Concerning the pituitary-gonad axis, we showed that melatonin treatment decreased both gonadotrophin beta-subunit (LHbeta, FSHbeta) mRNA expression and reduced sexual steroid (11-ketotestosterone, oestradiol) plasma levels. This indicates that melatonin treatment had a negative effect on eel reproductive function. To our knowledge, the results of the present study provide the first evidence that melatonin enhances TH expression in specific brain regions in a non-mammalian species. By this mechanism melatonin could represent one pathway by which environmental factors could modulate reproductive function in the eel.

Stimulation of estrous behavior in grazing female goats by continuous or discontinuous exposure to males1,2

Two experiments were conducted during the anestrous period to determine: (1) whether males rendered sexually active by exposure to artificial long days stimulate estrous activity of female goats under grazed conditions (Exp. 1); and (2) whether continuous presence of the buck is necessary to stimulate this estrous activity (Exp. 2). In Exp. 1, 2 groups of females (n = 20/group), one in confinement and another under grazing conditions, were exposed to 4 bucks subjected to natural photoperiod (2 males/group). Two other groups of females (n = 20/group), in confinement or grazing, were exposed to 4 males treated with artificial long days (2 males/group). All groups were exposed to males for 15 d. The percentage of does detected in estrus during these 15 d was greater (P < 0.001) in the 2 groups exposed to males sexually prepared by long days (confined, 95%; grazed, 90%) than in groups exposed to males in natural photoperiod (confined, 15%; grazed, 45%). Does in Exp. 2 were allowed to graze and were exposed continuously (n = 26) or discontinuously (from 1700 to 0900; n = 26) for 18 d to males that had been stimulated to enter the breeding season by exposure to long days. The proportion of does that displayed estrous behavior in 18 d did not differ (P = 0.55) between groups (96.2 and 92.3% for continuous and discontinuous groups, respectively). The results indicate that anestrous goats managed under grazing conditions can be stimulated to express estrus by joining with males previously exposed to artificial long days. Continuous presence of the male is not necessary for this male effect.

Maintaining contact with bucks does not induce refractoriness to the male effect in seasonally anestrous female goats

A study was performed to determine if complete separation between sexes is a pre-requisite to stimulate sexual activity of anovulatory female goats by male exposure in mid-anestrus when reproductive activity is greatly inhibited. The response to the male was studied in anovulatory goats which had either been in contact with bucks before the stimulation (n = 22; CONT), or totally isolated from bucks (n = 22; ISOL). At the time of male exposure, the bucks from the CONT groups were removed and females of both groups (CONT and ISOL) were divided into two subgroups (n = 11) exposed to novel bucks (n = 1 per subgroup), either in sexual rest (sexually inactive; SI) or in which sexual activity had been induced by a photoperiodic treatment (sexually active; SA). The sexual behavior of the SA bucks during the first 5 days of stimulation was greater than that of SI bucks (P < 0.01). Regardless of whether females had been (82%) or not (91%) exposed to bucks before teasing, most goats stimulated by SA males displayed estrous behavior and ovulation in the first 15 days following stimulation, whereas no female from the two subgroups exposed to SI bucks showed estrous behavior and only one of them ovulated (P < 0.02). These results indicate that previous isolation of seasonally anovulatory goats from bucks is not required to stimulate their reproductive activity by the male effect, if sexually active bucks are used.

Differences in reproductive pattern between wild and domestic rams are not associated with inter-specific annual variations in plasma prolactin and melatonin concentrations

This study compares morphological changes, performed with real-time transrectal ultrasonography, of testis and accessory glands and seasonal endocrine changes in a wild (Mouflon) and a domesticated (Spanish Merino) breed of sheep. In Mouflons, the maximum plasma testosterone concentrations, testicular diameter and vesicular gland size occurred synchronously during autumn (P<0.001). In Merino rams the highest circulating levels of testosterone (P<0.05) and maximum testicular diameter (P<0.001) occurred during summer, with no seasonal variations in vesicular and bulbourethral glands. The seasonal changes in plasma concentrations of prolactin were not correlated with annual variations in testicular and glandular size in neither wild nor domestic species. No differences were observed between both species for the seasonal pattern of prolactin secretion and mean amplitude of melatonin. Wide differences in reproductive patterns between wild and domestic types of rams do not appear to be attributed to seasonal changes in prolactin and melatonin secretion.

The endogenous rhythm of plasma melatonin and its regulation by light in the highveld mole-rat (Cryptomys hottentotus pretoriae): a microphthalmic, seasonally breeding rodent

The day- and night-time levels of plasma melatonin were measured in adult male and female highveld mole-rats, Cryptomys hottentotus pretoriae. This study aimed to assess whether melatonin secretion in this nocturnal, strictly subterranean but seasonally breeding rodent has a day-night rhythm and whether that rhythm is circadian and can be modified by photoperiod. In experiment 1, a day-night rhythm of plasma melatonin was found in all animals housed on a 12L:12D schedule, with significantly higher concentrations in the dark (D) compared with the light (L) phase. The increment of plasma melatonin concentration at night was the same on days 1 and 2 for animals in the control group and animals transferred to constant dark. The animals transferred to constant light substantially reduced the amplitude of the melatonin rhythm on day 2. This suggests that the endogenous melatonin rhythm in C. h. pretoriae has a circadian pattern, which can be synchronized by photoperiod and inhibited by exposure to light at night. In experiment 2, the concentration of plasma melatonin in animals kept under 14L:10D (long day, LD) conditions differed significantly from animals on 10L:14D (short day, SD). This finding supports the notion that C. h. pretoriae is sensitive to changes in day length.

The chronobiology of the Natal mole-rat, Cryptomys hottentotus natalensis

The Natal mole-rat, Cryptomys hottentotus natalensis, rarely, if ever, is exposed to external light cues because it occurs in completely sealed tunnel systems. As a result, their classical visual system is regressed, and therefore, their circadian system is expected proportionally to be expanded. Locomotor activity was investigated under a number of different photic regimes. Nine of the 12 mole-rats exhibited endogenous circadian rhythms of locomotor activity under constant darkness, with a mean free run period of 24.13 h (range 23.93-24.13 h), with these animals entrained to a light-dark cycle (12 L:12 D). Because C. hottentotus natalensis are able to entrain their locomotor activity to an external light source, light must reach the suprachiasmatic nucleus (SCN), suggesting a functional circadian clock. A clear day-night rhythm of melatonin secretion in animals housed under a neutral photoperiod (12 L:12 D) was observed, with higher melatonin concentrations in the dark compared with the light phase. The rhythm was maintained after the animals were transferred to either continuous light (LL) or dark (DD), suggesting that the endogenous rhythm was maintained under acute exposure to light and dark. However, under DD, the rhythm appeared to shift slightly, potentially as a result of the rhythm free running. These results show that C. hottentotus natalensis has endogenous rhythms of both locomotor activity and melatonin secretion, which are modulated by light.

Passage of progesterone into the brain changes with photoperiod in the ewe

In this study we tested the hypothesis that photoperiod can modulate steroid access to the brain in a seasonal breeder. To this goal, we compared the passage of exogenous progesterone to the brain of female sheep maintained under short (SD) or long (LD) daylengths. In the first experiment, we studied two groups of ovariectomized females maintained under SD or LD, for three artificial cycles, consisting of bearing a subcutaneous oestradiol implant (E2-treated) and an intravaginal device releasing progesterone (CIDR). During the third cycle, the concentrations of progesterone and of its metabolites 5alpha-dihydroprogesterone and 3alpha-hydroxy-5alpha-pregnan-20-one were measured in the preoptic area (POA). The levels of progesterone in the POA were higher in ewes under LD than under SD while the amounts of metabolites were unchanged. In the second experiment, we compared ovariectomized female sheep equipped with a cannula in the third ventricle to sample the cerebrospinal fluid (CSF) under LD vs. SD. After progesterone (1 mg and 10 mg) was injected into the carotid artery, it was only detectable in the cerebrospinal fluid in sheep under LD. In the third experiment, we compared progesterone concentration in plasma and CSF in two groups of SD vs. LD ovariectomized E2-treated ewes for 2 h under CIDR treatment. Despite similar progesterone plasma concentrations, concentration in the CSF was 2.5 times higher in SD than in LD. Our results suggest a physiological modulation of the passage of progesterone to the brain according to the photoperiod.

Melatonin and seasonal reproduction: understanding the neuroendocrine mechanisms using the sheep as a model

The mechanisms by which melatonin controls seasonal reproduction are poorly understood. The use of a large animal model, namely the sheep, has allowed progress in the understanding of these mechanisms, and is the subject of this review. Firstly, the contribution made by large animal models to demonstrating that melatonin acts in the hypothalamus and the identification of this hypothalamic target is reviewed. Secondly, the way in which large animal models have facilitated the demonstration of a specific mechanism of release of melatonin in the cerebrospinal fluid and, thus, raised the question of the route used by melatonin to reach its central targets is discussed. Finally, the human and agricultural relevance of the data presented is considered.

Sheep as a mammalian model of genetic variability in melatonin

Large inter-individual variability in plasma melatonin concentration at night is a common mammalian trait. In sheep, it varies from < 50 pg ml(-1) to > 800 pg ml(-1) but is very consistent within individuals. This inter-individual variability is under strong genetic control, which depends on melatonin secretion from the pineal gland, but not on melatonin catabolism. This genetic variability does not originate from differences in the synthetic enzymes or from a difference in melatonin secretion per mg of tissue, but from a difference in pineal size, which is highly variable among lambs of the same age and live weight. The genetic difference among lambs is already expressed at birth and is due to a difference in the number of pinealocytes rather than in their size. Pineal size and number of pinealocytes correlate strongly with plasma melatonin concentrations. The variability in pineal mass is not associated with the variability in any other organ (for example, the pituitary). The identification of genetic markers in the genome associated with the size of the pineal gland may lead to identification of genes involved in development of the mammalian pineal gland. Divergent selection of sheep on the basis of plasma melatonin concentrations could be used to constitute a mammalian model for extreme plasma concentrations.

Origin of cerebrospinal fluid melatonin and possible function in the integration of photoperiod

Melatonin, which is synthesized at night by the pineal gland, is present in the cerebrospinal fluid (CSF), but its entry site and its role in this compartment are not known. Using several approaches, we tested the hypothesis that melatonin enters the CSF through the pineal recess, an evagination of the third ventricle. CSF melatonin concentrations are higher near the pineal gland than in the anterior part of the third ventricle, and decrease markedly (80%) after sealing off the pineal recess. Moreover, ultrastructure and permeability analyses of the pineal-CSF interface showed that melatonin could reach the CSF either via delivery in situ by protruding pinealocytes that make direct contact with the CSF or via extracellular secretion and interstitial fluid draining into the ventricular lumen. These data indicate that melatonin in the CSF probably originates from a few pinealocytes of the basal part of the pineal gland neighbouring the pineal recess. Melatonin carried to the brain by the blood appears to be able to mediate the effects of photoperiod on reproduction, but it is unclear whether melatonin in CSF may fine-tune this response both in terms of timing and amplitude. It is critical to determine which pathway, blood or CSF, allows melatonin to reach its central targets more efficiently.

Induction of sexual activity in lactating anovulatory female goats using male goats treated only with artificially long days

Two experiments were conducted to determine the response of Creole male goats treated with long days and melatonin implants, and the response of the anovulatory does to male effect using males treated only with artificially long days. All animals were allocated to open sheds. In Exp. 1, one group of males was under natural photoperiod (CG; n = 7); the second group was submitted to 2.5 mo of long days followed by the insertion of two s.c. melatonin implants (LD+MEL; n = 7); the third group was subjected only to 2.5 mo of long days (LD; n = 7). Testicular weight was measured every 2 wk. Plasma testosterone concentrations were determined weekly. A treatment x time interaction was detected (P < 0.001) for testicular weight and plasma testosterone concentration. In the LD+MEL and LD groups, testicular size and plasma testosterone levels varied in a similar way, but differed from those observed in CG (P < 0.001). In this latter group, testicular weight displayed seasonal variations and peaked in June, whereas in treated groups this peak occurred in March. In CG, testosterone varied in a seasonal manner and plasma concentrations increased in June and remained elevated throughout the study. In experimental groups, testosterone increased in February and peaked in March. In Exp. 2, one group of males was left under natural photoperiod (CG, n = 5) and the other one was submitted to 2.5 mo of artificially long days (LD, n = 4). On March 16, two control and two treated males were put in contact with 20 and 19 females, respectively. Sexual behavior of the bucks was observed during the 5 d following male introduction. Progesterone assays and estrous behavior were used to determine ovarian and behavioral responses of the females to teasing. The anogenital sniffing, nudging, and mount instances registered in LD-treated males were greater than those observed in CG (P < 0.05). Of the does exposed to CG, none ovulated and only two of 20 females displayed estrous behavior. All does in contact with LD-treated males ovulated and showed at least one estrous behavior during the 15 d following joining (P < 0.001). These results indicate that the sexual activity of male goats from subtropical latitudes can be induced using only artificially long days. In addition, males treated in this way are capable of stimulating sexual activity in anovulatory females by the male effect.

Male effect in seasonally anovulatory lactating goats depends on the presence of sexually active bucks, but not estrous females

A study was conducted in subtropical northern Mexico (26 degrees N) to determine whether the presence of estrous females can improve the response of seasonally anovulatory goats to the introduction of bucks in the group. The induction of estrous activity was studied in three groups of anovulatory lactating goats during seasonal anestrus. These females were of the Mexican Creole breed. In the control group (sexually inactive (SI), n = 20), two control (SI) bucks exposed to normal seasonal daylength variations were used. In the second group (SI + E, n = 20 + 3), two control males were also used, but in addition, three females of the group were in estrus at the time of male introduction. In the third group (sexually active, SA + E, n = 19 + 4), anovulatory females were exposed to two bucks made sexually active by exposure to 2.5 months of long days (16L:8D) followed by two subcutaneous 18 mg melatonin implants, and four estrous females were also present when introducing the bucks. In all groups, males were introduced on 15 March and estrous detection was conducted twice daily for 15 days. The sexual activity of the bucks was observed from 08:00 to 10:00 h during the first five days of exposure to females. More females displayed estrous behavior in the first 15 days following the introduction of the males in the SA + E group (18/19) as compared with the SI or SI + E groups (2/20 and 0/20, respectively; P < 0.001). No difference was observed between the two latter groups. Thirteen females of SA + E group showed a second estrus between days 6 and 11 (short estrous cycle duration: 5.4 +/- 0.4 days). By contrast, in the SI group none showed a second estrus. The sexual behavior of the males in the SA + E group was greater as compared with that of the males in SI and SI + E groups (over 80% of the total sexual activity recorded in the three groups; P < 0.001). By contrast, no differences were found between SI and SI + E males. These results indicate that the presence of estrous females alone at the time of buck introduction is not sufficient to induce an adequate stimulation of seasonally inactive males. The use of sexually active bucks is necessary to induce reproductive activity in anovulatory females, whereas preparation of the bucks with long days followed by melatonin implants allows them to gain such a capacity.

Neuroendocrine interactions and seasonality

Sheep in temperate latitudes are seasonal breeders. Of the different seasonal cues, photoperiod is the most reliable parameter and is used by animals as an indication of the time of the year to synchronize endogenous annual rhythms of reproduction and physiology. The photoperiodic information is transduced into neuroendocrine changes through variations in melatonin secretion from the pineal gland. Melatonin triggers variations in the secretion of luteinizing hormone-releasing hormone, luteinizing hormone and follicle stimulating hormone (LHRH/LH/FSH) responsible for seasonal changes in reproductive activity. In female sheep, the seasonal changes in the hormonal LH pattern mainly reflect an increase in the negative feedback exerted by estradiol under long days on the frequency of pulsatile LH secretion. The resulting seasonal inhibition of LH secretion involves the activation of monoaminergic and especially dopaminergic systems by estradiol. Other types of physiological regulation subject to seasonal changes such as voluntary food intake (VFI), fat metabolism, body mass and pelage growth also occur in sheep, goats or related wild species. Several neuroendocrine intermediates seem to be shared by these different systems and may participate in their synchronization, providing the advantage that this helps mammalian species to adapt to their environment.

Genetic variability in melatonin secretion originates in the number of pinealocytes in sheep

Genetic variability in plasma melatonin concentrations in ewes results from variations in pineal weight. This study investigated whether it is due to a difference in the number of pinealocytes, or in their size. Two groups of lambs were assigned before birth to being extremes (18 High and 21 Low) by calculating their genetic value on the basis of the melatonin concentrations of their parents. Lambs were bled from 1 week of age until 14 weeks of age. Pineal gland, brain and pituitary weights, length and width of the brain, and length of the hypothalamus were recorded. A significant effect (ANOVA) of genetic group (P<0.05) and age (P<0.05) was detected on mean nocturnal plasma melatonin concentrations, as soon as the first week after birth (mean +/- s.e.m.; High: 51.7 +/- 10.7 vs Low: 31.9 +/- 3.2 pg/ml). There was no difference between the two genetic groups in any of the brain parameters measured, but the pineal glands of the High group were heavier and contained significantly more pinealocytes (High: 27.8 +/- 2.4 vs Low: 21.0 +/- 2.4 x 10(6); P<0.05) than those in the Low group. The mean size of pinealocytes did not differ between the two genetic groups. Thus, the genetic variability in nocturnal plasma melatonin concentrations in sheep is expressed by 1 week of age and higher levels of secretion are the consequence of larger pineal glands containing a greater number of pinealocytes.

Comparison of immune parameters of sheep with naturally high or low plasma concentrations of melatonin

Studies in rodents and humans have suggested that the pineal gland and its secretory product melatonin play an important role in the modulation of the immune system. In this study, we tested the hypothesis of a difference in immune parameters between ewes with naturally high vs. low circulating melatonin. Thus, two comparable groups of 10 Ile-de-France sheep were selected from a large flock, for their naturally high and low plasma concentrations of melatonin. The mean plasma melatonin concentrations during daytime (09:00 hr) and nighttime (24:00 hr) were, respectively, 9 and 664 pg/mL (high group) and 5 and 169 pg/mL (low group; P<0.01). Animals from both groups were subjected to various in vitro and in vivo measurements of the characteristics of their immune system. The total number of white blood cells (lymphocytes, polymorphonuclears, and monocytes) and the assessment of the sub-populations of blood lymphocytes (T4, T8, T19, B, and monocytes) did not show any significant differences between the two groups, sampled during day or night. The level of blood leukocytes proliferation after in vitro culture with ConA, LPS, or CWF stimulation, before or after experimental immunization, did not reveal any differences. No significant differences were registered in the production of antibodies between the two groups of animals. The results of the present experiment suggest that in natural conditions a high level of circulating melatonin does not modify the activity of the immune system in sheep.

Induction of sexual activity of male creole goats in subtropical northern Mexico using long days and melatonin

The aim of this study was to determine whether the sexual activity of local male Creole goats in subtropical Mexico can be induced during the non-breeding season by a long-day treatment followed by insertion of two melatonin implants. The experiment was carried out in the Laguna region in the State of Coahuila, Mexico (26 degrees N). Fourteen male goats were allocated to two balanced groups (n = 7 each) according to body and testicular weights. Males were kept together in two separate groups and fed lucerne hay for ad libitum intake and 300 g of commercial concentrate and had free access to water and mineral blocks. The control group remained in open sheds under natural photoperiod and ambient temperature conditions. The experimental group was placed in a light-proof building and exposed to 2.5 mo of long days (16 h of light/d) from November 1 to January 15. On January 16, each male received two s.c. melatonin implants and was exposed to natural photoperiodic changes in an open shed. In the control group, testicular weight exhibited seasonal variations; the highest value occurred on May 30 (146 +/- 10 g). Treated males reached maximum testicular weight earlier (March 15; 147 +/- 11 g), and sperm quality from January to March was higher than that observed in the control group (P < 0.05). Treatment caused an increase in LH pulse frequency (2.0 +/- 0.5 vs 0.3 +/- 0.2 pulse/8 h in February, 4.6 +/- 1.1 vs 0.1 +/- 0.1 pulse/8 h in March; experimental vs control group, respectively). In the control group, plasma testosterone remained low until mid-June and increased thereafter to remain elevated until the end of the study. In the experimental group, elevated plasma testosterone was observed from February to April and from July to November. Treating male goats in subtropical latitudes with artificial long days and melatonin can induce an intense sexual activity during the natural nonbreeding season.

Biology of mammalian photoperiodism and the critical role of the pineal gland and melatonin

In mammals, photoperiodic information is transformed into a melatonin secretory rhythm in the pineal gland (high levels at night, low levels during the day). Melatonin exerts its effects in discrete hypothalamic areas, most likely through MT1 melatonin receptors. Whether melatonin is brought to the hypothalamus from the cerebrospinal fluid or the blood is still unclear. The final action of this indoleamine at the level of the central nervous system is a modulation of GnRH secretion but it does not act directly on GnRH neurones; rather, its action involves a complex neural circuit of interneurones that includes at least dopaminergic, serotoninergic and aminoacidergic neurones. In addition, this network appears to undergo morphological changes between seasons.

Annual ovarian cycles in an aseasonal breeder, the springbok (Antidorcas marsupialis)

The springbok is an arid-adapted antelope inhabiting the desert and semidesert regions of southern Africa. Because it thrives in these sparsely vegetated areas, the springbok is of potential agricultural importance and the prospect of domestication has been speculated for many years. However, apart from observational studies on its breeding in the wild, suggesting it is an aseasonal breeder, little is known about the underlying reproductive endocrinology of this species. In this study, biweekly peripheral blood samples were collected from eight captive springbok ewes from October 1995 until September 1998 and analyzed for progesterone. At the start of the study, six ewes were prepubertal and cycling commenced spontaneously between November 1995 and June 1996. Cycling had already commenced in two ewes. At the end of November 1996, estrous cycles ceased abruptly in all ewes and restarted in April 1997. Cycling ceased again between December 1997 and February 1998 and restarted in June 1998 in six ewes; there was no cessation of estrous cycles in two ewes. Thus, although some individuals cycle continuously, there is a clear endocrine anestrus of between 4 and 5 mo in springbok, the timing and duration of which is synchronized between some individuals but the time of onset and cessation is variable from year to year. To ensure that the fluctuations we observed in progesterone levels were reliable indicators of changes in the estrous cycle, blood samples were collected every 6 h for 16 days in August 1998. A surge in LH secretion was observed in all ewes 55 +/- 5 h after the fall in progesterone. Progesterone levels increased again 45 +/- 8 h after the surge. A final study showed that the pattern of melatonin release in springbok exhibits a normal day/night profile, and thus photoperiodic information is transformed into an endocrine code to springbok but does not appear to affect reproduction. Rather, our data raise the possibility that the prevailing ambient temperature may influence the onset of ovarian activity in this species.

Estradiol increases multiunit electrical activity in the A15 area of ewes exposed to inhibitory photoperiods

Seasonal anestrus in ewes results from an increase in response to the negative feedback action of estradiol (E(2)). This increase in the inhibitory effects of E(2) is controlled by photoperiod and appears to be mediated, in part, by dopaminergic neurons in the retrochiasmatic area of the hypothalamus (A15 group). This study was designed to test the hypothesis that E(2) increases multiunit electrical activity (MUA) in the A15 during inhibitory long days. MUA was monitored in the retrochiasmatic area of 14 ovariectomized ewes from 4 h before to 24 h after insertion of an E(2)-containing implant subcutaneously. In six of these ewes, MUA activity was also monitored before and after insertion of blank implants. Three of the 14 ewes were excluded from analysis because E(2) failed to inhibit LH. When MUA was recorded within the A15, E(2) produced a gradual increase in MUA that was sustained for 24 h. Blank implants failed to increase MUA in the A15 area, and E(2) did not alter MUA if recording electrodes were outside the A15. These data demonstrate that E(2) increases MUA in the A15 region of ewes and are consistent with the hypothesis that these neurons mediate E(2) negative feedback during long photoperiods.

Male reproductive condition is the limiting factor of efficiency in the male effect during seasonal anestrus in female goats

Two experiments were conducted to determine whether the failure of males to induce sexual activity in goats during seasonal anestrus is due to unresponsiveness of females to male stimulus or insufficient stimulation from males. In the first study, one group of males (sexually inactive, SI; n = 4) was kept under natural photoperiod while the other (sexually active, SA; n = 4) was subjected to 2.5 mo of long days (16L:8D) and received 2 s.c. implants of melatonin. Two mo later, 2 different flocks of anovulatory goats previously separated from bucks were exposed to either SI (n = 34) or SA (n = 40) bucks. Progesterone assays and estrous behavior were used to determine ovarian and behavioral responses of the females to teasing. Of the goats exposed to SI males, only 2 ovulated, and none showed estrous behavior during the 35 days of the study. In contrast, all females (40 of 40) in contact with SA males ovulated and showed at least one estrous behavior during the first 11 days following male introduction (P < 0.001). Overall, 38 of 40 females stimulated with SA bucks were diagnosed pregnant at Day 35, according to progesterone assay (versus 0 in SI-treated group: P < 0.001). To control for a possible difference of responsiveness between flocks, the experiment was repeated 1 yr later using a single flock of goats divided into 2 groups. Again, over the first 14 days, 1 of 33 goats showed estrous behavior in the SI-treated group versus 27 of 33 in the SA-treated group (P < 0.001). Therefore, treating bucks with long days and melatonin increased their teasing capacity to induce sexual activity in females during anestrus. These results indicate that the absence of response to teasing at this time of the year is not due to female unresponsiveness, but to insufficient stimulation from the male.

Association between expression of reproductive seasonality and alleles of the gene for Mel(1a) receptor in the ewe

To determine whether a link exists between reproductive seasonality and the structure of the gene for melatonin receptor Mel(1a), the latter was studied in two groups of Mérinos d'Arles (MA) ewes previously chosen for their genetic value, which took into account their own out-of-season ovulatory activity adjusted by environmental parameters and that of their relatives. The genomic DNA of 36 ewes found regularly cycling in spring (group H) and that of 35 ewes never cycling in spring (group L) during the 2-3 yr before the present study was prepared, and the cDNA corresponding to almost all exon II was amplified and checked for the presence of MnlI restriction sites. The presence (+) or absence (-) of an MnlI site at position 605 led to genotypes "++", "+-", and "--", whose frequencies differed significantly (P < 0.001) between the H and L groups: 52.8%, 47.2%, and 0% vs. 28.5%, 42.9%, and 28.5%, respectively. Sequencing of exon II cDNA in group L ewes with genotype -- showed the presence of only one allele - with 4 mutations, while that in ewes with genotype ++ showed different types of alleles unrelated to the H or L groups. These + alleles exhibited a combination of 1 to 7 of the 8 mutations recorded in the part of exon II studied. The genotyping of 29 ewes from the more seasonal Ile-de-France breed indicated that 38% of animals had a -- genotype and exhibited the same mutations as in the MA ewes. Finally, a comparison of (125)I-melatonin binding to membrane preparations of pars tuberalis showed a lower number of binding sites (P < 0. 0005) in MA ewes with genotype ++ than in those with genotype -- (43. 2 +/- 4.4 vs. 75.4 +/- 8.4 fmol/mg protein in genotype ++ and genotype --, respectively). In conclusion, the data show an association between genotype -- for site MnlI at position 605 and seasonal anovulatory activity in MA ewes.

Implications of recent advances in reproductive physiology for reproductive management of goats

The control of reproduction in goats is interesting for technical reasons (synchronization of kiddings, adjustment to forage availability or to economy), and for genetic reasons (identification and dissemination of improved genotypes). The use of short-light rhythms leads to markedly increased production of semen per buck and prevents occurrence of a 'resting' season. Recent identification of a bulbourethral lipase in goat spermatozoa opens new perspectives in sperm preservation. Light plus 'short day' treatments also allow induction of out-of-season oestrous cycles and ovulations leading to enhanced fertility. Repeated use of eCG provokes the production of antibodies, delays the timing of ovulation and causes a reduction in fertility after fixed-time artificial insemination. All steps of embryo production, freezing and transfer are now controlled and allow the attainment of satisfactory numbers of kids born per donor female, which are compatible with the development of the technique for exchanging genotypes between countries. In vitro production of embryos allows high development rates to be achieved after in vitro maturation and fertilization of oocytes, and will ensure the production of synchronous populations of one-cell zygotes at the stage required by new biotechnologies.

Genetic variability in plasma melatonin in sheep is due to pineal weight, not to variations in enzyme activities

This study was conducted to determine the origin of the high variability in the mean nocturnal plasma melatonin concentration (MC) in sheep. Two extreme groups of 25 lambs each [low (L) and high (H)] were obtained by calculating their genetic value on the basis of the MC of their parents. The MC of lambs was significantly higher in the H group than in the L group (L: 189.7 +/- 24.4 vs. H: 344.1 +/- 33.0 pg/ml, P < 0.001). Within each group, 13 lambs were slaughtered during the day (D) and 12 lambs during the night (N). Pineal weight was significantly higher in the H group than in the L group (L: 83.5 +/- 6.7 vs. H: 119.1 +/- 9.2 mg, P < 0.01) but did not differ between D and N. The amount of melatonin released in vitro per milligram of pineal gland, the arylalkylamine N-acetyltransferase (AANAT) activity, the AANAT protein content, and the level of AANAT mRNA differed significantly between D and N but not with genetic group. Hydroxyindole O-methyltransferase activity did not differ significantly between D and N or between genetic groups. Therefore, the genetic difference in MC between the two groups of lambs was attributed to a difference in pineal size, not in enzymatic activity of the pinealocytes.

Daily and light-at-night induced variations of circulating 5-methoxytryptophol (5-ML) in ewes with respectively high and low nocturnal melatonin secretion

The aim of the present study was to determine whether the genetic differences previously reported in ewe plasma melatonin concentrations were correlated with differences in the synthesis and release of other 5-methoxyindoles. To determine if 5-methoxytryptophol (5-ML), which is known to be present in large amounts in the sheep pineal gland, is released, as is melatonin, into the general circulation, and if some temporal relationships between 5-ML and melatonin release could be observed, two groups of ewes were selected with respect to their endogenous melatonin secretion: in the first experiment, ten ewes from the low melatonin group (low group) and ten ewes from the high melatonin group (high group). 5-ML was measured every hour during a 24-hr period by radioimmunoassay. In all ewes, 5-ML was released during day-time, the rhythm of 5-ML concentrations being inversely related with the melatonin rhythm. Both day-time and night-time 5-ML concentrations were higher in the ewes from the high group than in the ewes from the low group (14.7 +/- 1.0 pg/mL plasma versus 6.4 +/- 0.3 pg/mL plasma during the day, 3.1 +/- 0.2 pg/mL plasma versus 1.9 +/- 0.2 pg/mL plasma during the night). The 5-ML/melatonin ratio appeared much higher during the day than during the night but was very similar in both groups (day-time: 1.03 in the high group versus 1.16 in the low group, night-time: 0.01 in both groups). In a second experiment, six low group and seven high group ewes were submitted to 1 hr of extra light at night. 5-ML increased and melatonin decreased during extra light. Our results clearly show for the first time a daily variation in circulating 5-ML, and that the strong genetic contribution in the variability in melatonin concentrations in sheep are clearly correlated with a similar variability in 5-ML concentrations. Whether 5-ML, like melatonin, plays a physiological role in the different adaptation processes to the environment remains to be determined.

Implication of D2-like dopaminergic receptors in the median eminence during the establishment of long-day inhibition of LH secretion in the ewe

In ewes, photoperiod modulates LH release and dopaminergic terminals in the median eminence (ME) have a critical role in the establishment of long-day inhibition of LH secretion. This study was undertaken to determine the type of dopaminergic receptors, D1-like or D2-like, that mediate the action of dopamine on LH secretion at the ME level in this situation. This was assessed, in ovariectomized and estradiol-treated ewes, with the use of reverse microdialysis in the ME in three experiments: first, when LH secretion was stimulated by short days, by determining the response to three doses (0.01, 0.1 or 1 mg/ml) of a D1-like (SKF38393) and a D2-like (quinpirole) agonist; secondly, during early long-day inhibition of LH secretion, by determining the ability of SKF38393 and quinpirole (1 mg/ml) to mimic the inhibitory effects of dopamine, after a blockade of its synthesis with alpha-methyl-para-tyrosine (alphaMPT; 2 mg/ml); and thirdly, during early long-day inhibition of LH secretion, by determining the response to three doses (0.009, 0.09 or 0.9 mg/ml) of a D1-like (SCH23390) and a D2-like (sulpiride) antagonist. In none of the conditions was effect of the D1-like analogs on LH secretion found, compared with the control treatments. In contrast, the D2-like analogs caused changes in LH secretion. First, with short days, quinpirole in the highest dose significantly reduced mean LH concentration (P<0.05) and LH pulse frequency (P<0.01). Secondly, with long days, addition of quinpirole to alphaMPT caused a significant decrease in LH secretion relative to alphaMPT alone (P<0.05). Thirdly, with long days, sulpiride at the highest dose significantly increased mean LH concentration (during the first 3 h of treatment, P<0.05) and LH pulse frequency (P<0.05). Prolactin secretion was also determined in these experiments, and D2-like agonist and antagonist caused an inhibition and a stimulation of prolactin secretion, respectively. These results demonstrate that, in the ME, inhibitory action of dopamine on LH secretion, critical for the initiation of long-day-induced inhibition, is mediated by D2-like, not D1-like, dopaminergic receptors.

High melatonin concentrations in third ventricular cerebrospinal fluid are not due to Galen vein blood recirculating through the choroid plexus

Melatonin has been implicated in several neurotropic effects, but few studies have investigated the bioavailability of melatonin in the brain. The discovery of periventricular sites of action adjacent to the third ventricle forced us to investigate the dynamics of cerebrospinal fluid (CSF) melatonin release and the source of this melatonin. Our first study demonstrated unequivocally that third ventricle CSF melatonin, like jugular plasma melatonin, accurately reflects the duration of the night and is rapidly suppressed by light. However, third ventricle CSF melatonin levels are 20-fold higher than nocturnal plasma concentrations. A further study showed that melatonin increased in plasma before third ventricle CSF, raising the possibility that melatonin is taken up from the blood after recirculation through the Galen vein. However, a final experiment suggested strongly that CSF melatonin is released directly into the third ventricle, as melatonin levels in the lateral ventricle were 7-fold lower than those in the third ventricle. Our study raises the possibility that there may be two compartments of melatonin affecting physiological functioning: the first in plasma acting on peripheral organs, and the second in the CSF affecting neurally mediated functions at a much higher concentration of this pineal indoleamine.

Evidence for an annual reproductive rhythm independent of food availability in male Creole goats in subtropical northern Mexico

The aim of this study was to determine if there is a seasonal pattern of sexual activity dependent on food availability in male Creole goats in subtropical Mexico. The study was conducted in the Laguna Region in the State of Coahuila, Mexico (26 degrees N). Male Creole goats (n = 8) were kept in a shed, fed alfalfa ad libitum and given 200 g of concentrate daily throughout the study. Live weight and testicular weight were determined every 2 wk. Sexual behavior and sperm production were determined monthly. Blood samples were obtained weekly to determine testosterone plasma concentrations. All variables were subjected to sinusoidal modeling procedures and showed important seasonal variations (P < 0.0001) with different phase angles for body weight, testicular weight and testosterone plasma concentrations. The nadir of live weight occurred in November and the peak in May. The lowest testicular weight (90 g) and testosterone plasma concentrations (0.1 ng/mL) were observed in January and February, respectively, while the peaks were observed in July and August (145 g and 10 ng/mL, respectively). Ejaculation latency also varied during the study, being low between May and November (96 sec) and reaching a peak in April (183 sec). Minimum number of spermatozoa per ejaculate occurred between February and April (1.4 x 10(9) cells/ejaculate) while the maximum number was observed between May and September (2.8 x 10(9) spermatozoa/ejaculate). Progressive sperm motility was low between January and April (3.04 on average) and high between May and November (about 3.55 on average). The percentage of live spermatozoa diminished between January and April (68% in April) and then increased to values around 80% between May and November. These results lead us to conclude that male Creole goats in Northern Mexico, fed constantly throughout the year, exhibit seasonality in their reproductive activity. Intense sexual activity occurred between May and December.

Melatonin and the seasonal control of reproduction

Many mammalian species from temperate latitudes exhibit seasonal variations in breeding activity which are controlled by the annual photoperiodic cycle. Photoperiodic information is conveyed through several neural relays from the retina to the pineal gland where the light signal is translated into a daily cycle of melatonin secretion: high at night, low in the day. The length of the nocturnal secretion of melatonin reflects the duration of the night and it regulates the pulsatile secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus. Changes in GnRH release induce corresponding changes in luteinising hormone secretion which are responsible for the alternating presence or absence of ovulation in the female, and varying sperm production in the male. It is not yet known where and how this pineal indoleamine acts to exert this effect. Although melatonin binding sites are preferentially localised in the pars tuberalis (PT) of the adenohypophysis, the hypothalamus contains the physiological target sites of melatonin for its action on reproduction. Melatonin does not seem to act directly on GnRH neurons; rather it appears to involve a complex neural circuit of interneurons that includes at least dopaminergic, serotoninergic and excitatory aminoacidergic neurons.

[Melatonin and reproduction in domestic animals]

Melatonin, synthetized by the pineal gland, is the chemical messenger which allows seasonal animals to perceive day length changes. In the ewe, the nervous message, transformed into a hormonal one, triggers pulsatile activity of the LHRH neurons. About 40 days are necessary for melatonin to centrally stimulate the pulsatile LHRH activity. Its sites and mode of action are not completely elucidated, but a precise hypothalamic zone has been defined in which radioactive melatonin binds specifically and where cold melatonin delivered locally stimulates LHRH activity. In veterinary clinic, the most frequent mode of distribution is the sub-cutaneous implant, which induces an advance of the cyclical ovulatory activity of ewes and goats. The date of fertilization is advanced and fecundity of females is improved. It can be used alone, or in association with other hormonal treatments, or after an artificial photoperiodic treatment. Under these conditions, it allows a quantitative and qualitative increase in out-of-season sperm production in rams and he-goats. Such an implant is registered and marketed in France, UK, Greece, Australia and New-Zealand.

Median eminence dopaminergic activation is critical for the early long-day inhibition of luteinizing hormone secretion in the ewe

In ewes, photoperiod modulates LH release. The median eminence (ME) dopaminergic activity seems to be implicated in the inhibition of LH secretion by photoperiod. This study investigated the functional importance of ME dopaminergic activity for LH secretion inhibition in three inhibitory photoperiodic treatments: after 33 long days (LD) (LD1 treatment), after 72 LD (LD2 treatment), and after 34 short days. Using reverse microdialysis on three groups of seven ewes, a solution of alpha-methyl-paratyrosine [alphaMPT, an inhibitor of tyrosine hydroxylase (TH); 10 mM in Ringer's lactate] was infused into the ME for 5 h, preceded by a 5-h control period during which only vehicle was infused, in each of the three photoperiodic treatments. AlphaMPT dramatically decreased the 3,4-dihydroxyphenylacetic acid concentration, similarly in all three photoperiodic treatments, suggesting a similar inhibition of TH activity. In the LD1 treatment, alphaMPT significantly increased LH pulse frequency (+1.22 +/- 0.46 pulse/5 h from control period, mean +/- SEM, n = 9; P < 0.05) and mean concentration (+51 +/- 28%; P < 0.001). In the other two photoperiodic treatments, alphaMPT had no significant effect on LH release. Thus, blockade of dopamine synthesis in the ME seems to stimulate LH secretion in early, but not long-term, inhibition by LD nor after the transition to short days. Therefore, dopaminergic activity of the ME seems to be critical for LH secretion inhibition in some photoperiodic inhibitory treatments but not in others.

Periparturient rise in fecal egg counts associated with prolactin concentration increase in French Alpine dairy goats

Previous data on periparturient relaxation of immunity during gastrointestinal nematode infection in goats are scarce and conflicting; one study carried out in fiber (Angora) goats showed a positive association of fecal egg counts with prolactin concentrations around parturition, whereas the two other available studies dealing with dairy goats, gave divergent results. The objectives of the study were thus to assess the occurrence of a periparturient rise in fecal egg counts in dairy goats and to examine a possible relationship between the level of milk production and the intensity of the periparturient rise. A total of 28 French Alpine grazing dairy goats naturally infected with Teladorsagia, Trichostrongylus, and Oesophagostomum were allocated into two groups according to their reproductive status; group 1 (n = 7) consisted of nonpregnant lactating animals in the 3rd month of lactation, whereas group 2 (n = 21) was composed of dry goats at 6 weeks before term. Fecal egg counts, pepsinogen and phosphate blood concentrations, blood eosinophil counts, and prolactin concentrations were individually monitored at weekly intervals for 12 weeks (from midwinter to early spring). The mean fecal egg counts were significantly higher in pregnant goats during the 2 weeks before (668 versus 242 eggs per gram of feces (epg), P < 0.05) and the 2 weeks after (962 versus 279 epg, P < 0.01) parturition as compared with nonpregnant lactating animals. No significant difference was seen in the composition of larval cultures between the two groups of animals, with Oesophagostomum infective larvae being found predominantly, particularly at the time of parturition. Pepsinogen and phosphate concentrations as well as blood eosinophil counts were similar between the two groups throughout the survey and indicated a moderate larval challenge. The mean prolactin concentration measured in pregnant goats was significantly higher (P < 0.01) at the time of parturition (298 versus 130 ng ml(-1)) and at 4 weeks after parturition (387 versus 193 ng ml(-1)) than that determined in nonpregnant animals. Furthermore, a significant correlation (rs = 0.30, df = 79; P < 0.01) between fecal egg counts and prolactin concentrations was recorded for the pregnant goats during the 4-weeks period around parturition.

[Melatonin and reproduction in domestic farm animals]

Melatonin, synthetized by the pineal gland, is the chemical messenger which allows seasonally reproductive animals to perceive day length changes. In the ewe, the neural message, transformed into a hormonal one, triggers pulsatile activity of the LHRH neurons. About 40 days are necessary for melatonin to centrally stimulate the pulsatile LHRH activity. Its sites and mode of action are not yet known completely, but a precise hypothalamic zone has been defined in which radioactive melatonin binds specifically and where cold melatonin delivered locally stimulates LHRH activity. In the veterinary clinic, the most frequent mode of distribution is a sub-cutaneous implant, which induces an advancement of the cyclical ovulatory activity of ewes and goats. The date of fertilization is advanced and fecundity of females is improved. It can be used alone, or in association with other hormonal treatments, or after an artificial photoperiodic treatment. Under these conditions, it allows a quantitative and qualitative increase in out-of-season sperm production in rams and billy-goats. Such an implant is registered and marketed in France, the UK, Greece, Australia and New Zealand.

Blockade of tyrosine hydroxylase activity in the median eminence partially reverses the long day-induced inhibition of pulsatile LH secretion in the ewe

The photoperiod-induced stimulation of LH secretion is associated with a decrease in dopamine content, as well as in the activity of its rate limiting enzyme, tyrosine hydroxylase (TH), in the median eminence (ME) of the ewe. We therefore hypothesize that ME-TH activity can constitute a limiting factor of photoperiod-induced inhibition of LH pulsatile secretion. To test this hypothesis, we studied whether the inhibition of ME-TH activity can reverse the long day-induced inhibition of LH. Using microdialysis, a 3 mM solution of alpha methyl-p-tyrosine (alpha MPT; a competitive inhibitor of TH), was administered in the ME of ovariectomized ewes bearing a 0.5 cm oestradiol implant at the beginning of a LD-induced inhibition of LH secretion. The vehicle solution was infused for 4 h followed by a 3 mM alpha MPT solution infused for an additional 4 h. LH pulsatile secretory patterns within the same animal were compared between the control period and the alpha MPT period. alpha MPT infusion in the ME was associated with an increase in LH pulse frequency whereas it did not affect prolactin secretion. In conclusion, our results suggest that the inhibition of TH activity in the ME causes a stimulation of LH secretion in long-day inhibited ewes.

Genetic variability in melatonin concentrations in ewes originates in its synthesis, not in its catabolism

We investigated whether the genetic difference in plasma melatonin concentration in ewes was due to differences in the synthesis pathway from the pineal gland or in the catabolism of the hormone. Two groups of ewes [9 low (L) and 10 high (H)] were selected according to the breeding value of their mean nighttime plasma melatonin concentrations estimated at winter and summer solstices. In response to an identical dose of melatonin administered intravenously at 9:00 AM, no differences between groups were observed for any of the kinetic parameters: clearance rate, steady-state volume of distribution, terminal half-lives, and mean residence times. In the second experiment, two series of frequent blood samples were performed, one in the middle of the dark phase with samples taken every 5 min, and the other over 24 h with hourly samples. Highly significant differences between groups in nocturnal melatonin production rate were observed (L: 25.7 +/- 2.8 vs. H: 63.1 +/- 8.9 microg . kg-1 . h-1, P < 0.01). Thus the genetic differences in plasma melatonin concentrations in ewes originate in the synthesis pathway of the melatonin from the pineal gland rather than from differences in the catabolism of the hormone.