Short oestrous cycles in sheep during anoestrus involve defects in progesterone biosynthesis and luteal neovascularisation.

Anoestrous ewes can be induced to ovulate by the socio-sexual, 'ram effect'. However, in some ewes, the induced ovulation is followed by an abnormally short luteal phase causing a so-called 'short cycle'. The defect responsible for this luteal dysfunction has not been identified. In this study, we investigated ovarian and uterine factors implicated in male-induced short cycles in anoestrous ewes using a combined endocrine and molecular strategy. Before ovulation, we were able to detect a moderate loss of thecal expression of steroid acute regulatory protein (STAR) in ewes that had not received progesterone priming (which prevents short cycles). At and following ovulation, we were able to identify a significant loss of expression of genes coding key proteins involved in the biosynthesis of progesterone (STAR, CYP11A1 and HSD3B1 (HSD3B)) as well as genes coding proteins critical for vascular development during early luteal development (VEGFA and KDR (VEGFR2)), suggesting dysfunction in at least two pathways critical for normal luteal function. Furthermore, these changes were associated with a significant reduction of progesterone production and luteal weight. Additionally, we cast doubt on the proposed uterus-mediated effect of prostaglandin F2α (PGF2α) as a cause of short cycles by demonstrating the dysregulation of luteal expression of the PGF receptor, which mediates the luteal effects of PGF2α, and by finding no significant changes in the circulating concentrations of PGFM, the principal metabolite of PGF2α in ewes with short cycles. This study is the first of its kind to examine concurrently the endocrine and molecular events in the follicular and early luteal stages of the short cycle.

The pattern of LH secretion and the ovarian response to the 'ram effect' in the anoestrous ewe is influenced by body condition but not by short-term nutritional supplementation.

In sheep, the 'ram effect' induces out-of-season fertility and good nutrition increases prolificacy. This experiment determined if fatness or short-term nutritional supplementation modified the response to the 'ram effect'. A group of 48 Île-de-France ewes were fed diets that produced groups with body-condition scores (BCS) of >3.0 and <2.0. Within each BCS group animals were supplemented daily with 500g of lupins from Day -5 to Day 0 (ram introduction) resulting in four groups: low BCS, supplemented (n=7) and non-supplemented (n=8) and high BCS, supplemented (n=12) and non-supplemented (n=11). The blood concentrations of glucose and insulin and the LH response to gonadotrophin-releasing hormone (GnRH) were determined. After the 'ram effect' the pattern of LH pulsatility, the LH surge and ovarian responses were analysed. Low BCS ewes had lower glucose and insulin (P<0.001) and supplementation increased both (P≤0.001). The increase in LH induced by GnRH was reduced in low BCS ewes (P=0.015) but it was not affected by supplementation. Similarly, LH pulsatility was reduced in low BCS ewes (P<0.05). The LH surge and ovarian cyclicity were not affected but the follow-up cycle was delayed (P=0.034) and progesterone was reduced (P=0.029) in low BCS ewes. There was an effect of BCS on ovulation rate (P<0.05). These results show that the BCS can modify the response to the 'ram effect' and that supplementation has little effect on this response.

Neural pathways involved in the endocrine response of anestrous ewes to the male or its odor.

During the non-breeding season, anestrous ewes do not experience ovarian cycles but exposure to a ram or its odor results in the activation of the luteinizing hormone secretion leading to ovulation. The aim of our work was to identify the neural pathways involved in this phenomenon. Using Fos immunocytochemistry, we examined the brain areas activated by the male or its fleece, in comparison with ewes exposed to the female fleece or the testing room (control group). In comparison with the control group, the male or its odor significantly increases Fos neuronal expression in the main and accessory olfactory bulbs, anterior olfactory nucleus, cortical and basal amygdala, dentate gyrus, ventromedial nucleus of the hypothalamus, piriform and orbitofrontal cortices. The main olfactory bulb, the cortical amygdala and the dentate gyrus are specifically more activated by the male odor than the female odor. Using a procedure of double labeling for Fos and gonadotropin-releasing hormone, we also compared the number of gonadotropin-releasing hormone neurons activated in the four groups of females. The male or its odor significantly increases the number and the proportion of gonadotropin-releasing hormone cells expressing Fos-immunoreactivity in the preoptic area and the organum vasculosum of the lamina terminalis, whereas no such induction of Fos-immunoreactivity was found in gonadotropin-releasing hormone neurons of ewes exposed to the female odor or the testing room. These findings emphasize the role of the main olfactory system in the detection and the integration of the ram odor, and also suggest the participation of the accessory olfactory system. Numerous structures widely distributed seem involved in the processing of the male olfactory cue to reach the gonadotropin-releasing hormone neurons.

Sequential role of e2 and GnRH for the expression of estrous behavior in ewes.

Preovulatory GnRH secretion in ewes, measured in portal blood and cerebrospinal fluid, starts at the time of the LH surge, approximately 4 h after the onset of estrous behavior, and lasts as long as receptivity (36-48 h), which is much longer than the LH surge. This study tested the hypothesis that the extended GnRH secretion is involved in the maintenance of receptive behavior, prolonging the initial triggering effect of E2. Ovariectomized ewes were subjected to artificial estrous cycles and infused intracerebroventricularly either with a water soluble GnRH antagonist (Teverelix, Exp 1 and 2) or GnRH (Exp 3 and 4) after preovulatory E2 challenges of various intensity. The GnRH antagonist infused for 20 h (0.5 mg/ml, flow rate 3 microl/min) following a treatment with 2 x 30-mm E2 implants for 24 h (Exp 1) significantly reduced receptivity 36-48 h post E2 compared with vehicle infusion. By contrast, when the GnRH antagonist was infused with E2 implants still present (Exp 2: E2 for 48 h, GnRH antagonist given 24-44 h after E2 insertion, n = 14) receptivity was not affected. Administration of GnRH (0.5 mg/ml, flow rate 3 microl/min) when receptivity began to decline (Exp 3: 30-48 h after a 6-h 2 x 30-mm E2 implants n = 12) resulted in significantly higher receptivity scores at 48 and 52 h post E2 in GnRH treated animals compared with vehicle treated. GnRH infusion of ewes under subthreshold E2 treatment (Exp 4: GnRH 6-24 h after implantation of 1 x 30-mm E2 for 3 h, n = 12 in a cross-over design) significantly increased their receptivity compared with vehicle administration at 18 and 24 h post E2 insertion, but receptivity remained lower than when induced by high doses of E2. Our results demonstrate for the first time that GnRH is involved in the control of receptivity in a ruminant species and suggest that in the cycling ewe the sustained preovulatory GnRH secretion plays a physiological role in extending the duration of estrous behavior. They also indicate that it is possible to dissociate a direct effect of E2 on estrous behavior from its effect via stimulation of GnRH secretion.

Evidence that the mediobasal hypothalamus is the primary site of action of estradiol in inducing the preovulatory gonadotropin releasing hormone surge in the ewe.

Although a neural site of action for estradiol in inducing a LH surge via a surge of GnRH is now well established in sheep, the precise target(s) for estrogen within the brain is unknown. To address this issue, two experiments were conducted during the breeding season using an artificial model of the follicular phase. In the first experiment, bilateral 17beta-estradiol microimplants were positioned in either the medial preoptic area (MPOA) or the mediobasal hypothalamus (MBH), and LH secretion was monitored. An initial negative feedback inhibition of LH secretion was observed in ewes that had estradiol microimplants located in the MPOA (6 of 6 ewes) or caudal MBH in the vicinity of the arcuate nucleus (4 of 4). In contrast, a normal LH surge was only found in animals bearing estradiol microimplants in the MBH (5 of 10). Detailed analysis of estradiol microimplant location with respect to the estrogen receptor-alpha-immunoreactive cells of the hypothalamus revealed that 4 of the 5 ewes exhibiting a LH surge had microimplants located bilaterally within or adjacent to the area of estrogen receptor-expressing cells of the ventromedial nucleus. Two of these ewes exhibited a LH surge without showing any form of estrogen negative feedback. In the second experiment, we used the technique of hypophyseal portal blood collection to monitor GnRH secretion directly at the time of the LH surge induced by estradiol delivered either centrally or peripherally. Central estradiol implants induced the GnRH surge. The duration and mean plasma concentration of GnRH during the surge were not different between animals given peripheral or central MBH estradiol implants. Cholesterol-filled MBH microimplants did not evoke a GnRH surge. We conclude that the ventromedial nucleus is the primary site of action for estradiol in stimulating the preovulatory GnRH surge of the ewe, whereas the MPOA and possibly the caudal MBH are sites at which estrogen can act to inhibit LH secretion. These data provide evidence for the sites within the ovine hypothalamus responsible for mediating the bimodal influence of estradiol on GnRH secretion and suggest that different, and possibly independent, neuronal cell populations are responsible for the negative and positive feedback actions of estradiol.

Sex hormones enhance the impact of male sensory cues on both primary and association cortical components of visual and olfactory processing pathways as well as in limbic and hypothalamic regions in female sheep.

Differential activation of neural substrates was investigated in female sheep exposed to a male when they were in oestrus, and sexually receptive and attracted to males, as opposed to anoestrus when they were not. Changes in neuronal activation were visualized in ovariectomized, hormone-treated ewes by quantifying changes in cellular expression of c-fos messenger RNA by in situ hybridization histochemistry. Results showed that, while oestrus induction had no significant effects on c-fos expression per se, a 5-min exposure to a male significantly increased it in a number of primary and association cortical regions (the mitral and granule cell layers of the olfactory bulb, visual, somatosensory, orbitofrontal, piriform, cingulate and temporal cortices), the limbic system (CA1 region of the hippocampus, subiculum, lateral septum, lateral and basolateral amygdala, bed nucleus of the stria terminalis) and hypothalamus (mediobasal hypothalamus, medial preoptic area and paraventricular nucleus) as well as the nucleus accumbens and mediodorsal thalamus. Intromissions did not contribute significantly to these c-fos changes however. In anoestrus females, exposure to a male only produced a small significant increase in c-fos messenger RNA expression in the temporal cortex inspite of receiving similar amounts of visual and olfactory cues from him and a number of mating attempts. These results clearly demonstrate that changes in sexual motivation markedly alter the neural processing of sensory cues from males. They also show that the hormonal induction of sexual attraction to males cues and the resultant stimulation of sexual behaviour is due not only to altered responsiveness of oestrogen-sensitive brain regions involved in mediating behavioural responses towards the male, but also to changes in primary and secondary/tertiary somatosensory, olfactory and visual processing regions which relay sensory information to them.

Roles of progesterone and oestradiol in determining the temporal sequence and quantitative expression of sexual receptivity and the preovulatory LH surge in the ewe.

In a series of experiments using a quantitative method for measuring receptivity and implants that allowed rapid and controlled changes in the blood concentrations of oestradiol-17 beta and progesterone, we have re-examined the roles of these steroids in the induction of sexual behaviour and the LH surge in ovariectomized ewes. Progesterone priming was found to increase the proportion of ewes showing oestrus, reduce the latency to the onset of oestrus, and increase the 'intensity' of the behaviour as measured by the receptivity index, but it did not affect the size of the LH surge. Progesterone was able to facilitate the expression of oestrus even when it was withdrawn 8 days before oestrogen treatment, suggesting that it exerts its effect by restoring the sensitivity of the oestrogen-refractory animal to oestrogen. When it was present at the time oestrogen was administered, progesterone inhibited the stimulatory effect of oestrogen, but this effect disappeared as soon as the progesterone was withdrawn. Thereafter, expression of both the behavioural and endocrine responses was delayed by 24-30 h. These data show that the timing of the preovulatory behavioural and endocrine events is determined primarily by the time of progesterone withdrawal. The amount of oestradiol and the timing of any rise in concentration serve only to modulate these effects.

Sexual differentiation of sexual behaviour and preovulatory LH surge in ewes.

Adult ewes were studied after prenatal treatment with androgen. Although seven of the eight androgenized ewes (ANDR) when intact presented signs of cyclic ovarian activity at least once, none of them showed cyclic female receptivity when intact or after ovariectomy and progesterone (P) + estradiol (E2) treatments, whereas all ten control (CONT) ewes did. Receptivity also was induced in CONT, but not in ANDR, ewes by long term E2 or testosterone (T) treatments. Increases in LH levels, with latency and duration similar to the LH surges of the CONT ewes, were observed in six (P + low E2 cycle) and five (P + high E2 cycle) of the eight ANDR ewes. The maximum LH level was lower in the ANDR than in the CONT ewes after high E2 treatment (p less than 0.002). Progesterone had a clear inhibitory effect on the induction of the LH surge by E2 only in the CONT ewes. Male-type mounting and nudging were observed more often in the ANDR than in the CONT ewes when intact (p less than 0.05). However, there was no facilitation of male sexual behaviour induced by long term E2 or T treatment in the ANDR compared to the CONT ewes. Progesterone inhibited male behaviour induced by T in the CONT but not in the ANDR ewes. This study shows that prenatal androgens defeminize LH secretion and sexual behaviour of ewes in a dissociated manner. It also suggests that prenatal androgen does not really masculinize the females but decreases their sensitivity to the inhibitory effect of P, and thus extends the "juvenile state" in which male-like patterns appear independently of the hormone milieu.

Oestrogen receptors in the preoptico-hypothalamic continuum: immunohistochemical study of the distribution and cell density during induced oestrous cycle in ovariectomized ewe.

Oestrogen plays a key role in the regulation of the endocrine and behavioural events associated with the oestrous cycle. It is important, therefore, to know the location of neurones receptive to this steroid and to know whether their distribution varies with the oestrous cycle. We have undertaken experiments to identify the location of oestrogen receptors (ER) within the preoptico-hypothalamic continuum of ovariectomized ewes submitted to a variety of different hormone replacement regimes which mimic the different stages of the oestrous cycle. We used a monoclonal antibody to ER and detected receptors with immunohistological methods in the non-vascular part of the organum vasculosum of the lamina terminalis, the lateral septum, the medial preoptic area, the supraoptic, suprachiasmatic and arcuate (ARC) nuclei, the ventromedial hypothalamus (HVM) and in the region close to the mamillari recess. ER neurones were scarce or absent from the anterior hypothalamus and the paraventricular nucleus. The density of ER staining in the HVM, but in no other localization, was found to be higher, and in a more lateral position, during the induced luteal phase (progesterone treatment) than during the follicular phase (7 days of progesterone treatment followed by oestradiol) or in the ovariectomized female. In all areas studied, except for the ARC, the apparent surface area of the nucleus in ER immunoreactive cells varied with hormonal treatment. These data, and especially those in the HVM, contribute towards our understanding of how steroids may act in the ovine to control sexual behaviour.

Short-term effect of oestradiol on neurokinin B mRNA expression in the infundibular nucleus of ewes.

In sheep, essentially all the neurokinin B (NKB) neurones of the infundibular nucleus express oestradiol receptor alpha, and analysis of female and male brains has revealed an exceptionally marked female-dominant sex difference in the numbers of NKB neurones in the infundibular nucleus. This neuronal population is located in an oestradiol-sensitive brain area involved in the control of gonadotropin-releasing hormone (GnRH) secretion and oestrous behaviour, but its physiological role is poorly documented. The aim of the present study was to analyse NKB mRNA expression at a crucial time when the steroid has stimulated the pathways leading to the induction of these two events. After cloning a specific ovine NKB antisense riboprobe, we examined the effects of a short oestradiol treatment (4 h subcutaneously) on the expression of NKB mRNA in the caudal part of the infundibular nucleus of progesterone-primed ovariectomized ewes. We demonstrated that oestradiol decreased both the level of NKB mRNA expression (34%) and the number of cells containing NKB mRNA (43%). Oestradiol acts strongly on these NKB cells in the short term. We suggest that this early change in NKB mRNA expression during the preovulatory period might be involved in the control of the induction of GnRH secretion or oestrous behaviour.

The role of oxytocin release in the mediobasal hypothalamus of the sheep in relation to female sexual receptivity.

In vivo microdialysis and retrodialysis were used to investigate the role of oxytocin (OXY) release in the mediobasal hypothalamus (MBH) of the ewe in the control of sexual receptivity. Initial experiments showed that OXY release was significantly increased in ovariectomized animals treated with progesterone and oestradiol when they were sexually receptive towards males and received intromissions. No such increases were seen during tests where the ewes were receptive but the males were prevented from achieving intromission. By contrast, OXY release was significantly reduced in tests where the ewes were not receptive to the male. In a second experiment artificial vaginocervical stimulation (VCS) was found to significantly increase OXY release when the animals were treated with oestradiol and this effect was potentiated by progesterone priming. OXY release in the MBH was not significantly altered by VCS in the presence of progesterone priming alone. Plasma OXY concentrations were significantly increased by VCS following all three hormone treatments but no one treatment was significantly more effective than another. Noradrenaline release in the MBH was only significantly increased following VCS when progesterone priming was given before oestradiol treatment. No effects of VCS on release of GABA, glutamate or dopamine were seen but their basal concentrations were significantly increased by the combined steroid treatment compared to oestradiol alone. In a third experiment it was found that OXY (10 microM) infused bilaterally into the MBH of receptive ewes, by retrodialysis, significantly decreased sexual receptivity and increased the release of noradrenaline and GABA.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventromedial hypothalamus as a target for oestradiol action on proceptivity, receptivity and luteinizing hormone surge of the ewe.

Most of the literature suggests that in sheep as in rodents nervous structures involved in female sexual behaviour are not necessarily identical to those involved in the LH surge. In rodents, oestradiol triggers female sexual behaviour by acting on a restricted area of the mediobasal hypothalamus whereas the concomitant induction of the preovulatory LH surge is at least partially under the control of more anterior structures. The central sites of oestradiol action, however, remained poorly defined in sheep. To provide this definition, 37 ovariectomized ewes were stereotaxically implanted unilaterally or bilaterally with a guide cannula in preoptic area (POA), anterior, mediobasal, lateral, or posterior hypothalamus (AH, MBH, LHT, PH). Experiments were made during the breeding season (Br) and the anoestrous period (An: unilat only) and females were primed with a peripheral treatment of progesterone and a dose of 17 beta-oestradiol subthreshold for both the LH surge and sexual behaviour. Intracranial implants (i.d. = 0.45 mm) of crystalline E2 were lowered 16 h after progesterone removal and left in the brain for 48 h. Whereas POA implants never had any significant effects on either the behaviour or the LH surge, all MBH implants caused receptivity (11 bilat, 5 unilat Br and 5 unilat An). Bilateral MBH implants also induced proceptivity in 9 of 11 ewes and increased the LH levels in 7 of them. These proportions do not differ significantly from those observed after a 25 microgram peripheral injection of E2. Unilateral MBH implants had no significant effect on proceptivity and LH increase but oestrous behaviour was induced by some implants placed laterally to the MBH (25 recept and 3/5 procept).(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of sexual behaviour and the luteinizing hormone surge by intracerebral progesterone implants in the female sheep.

In female sheep, progesterone blocks the induction by oestradiol of both sexual behaviour and the pre-ovulatory surges of gonadotrophin releasing hormone (GnRH) and luteinising hormone (LH). However, the central sites of action of progesterone remain poorly defined, so we attempted to locate them by implanting progesterone intracerebrally in ovariectomised ewes treated with exogenous steroids to induce oestrous behaviour and the LH surge. Single bilateral implants or a double bilateral implants filled with progesterone or cholesterol were placed in the ventromedial hypothalamus (VMH) or the preoptic area (POA). Control ewes were not implanted. To determine the inhibitory capacity of the central progesterone implants, ewes received an injection (i.m.) of 8 micrograms or 16 micrograms of oestradiol. The single bilateral implants of progesterone failed to block oestrous behaviour and the LH surge induced by 8 micrograms of oestradiol. Double bilateral progesterone implants in the VMH blocked the sexual behaviour (P < 0.05) and the LH surge (P < 0.05), but implants in the POA blocked only sexual receptivity (P < 0.05). No changes were observed after central implantation of cholesterol. Our results support the hypothesis that progesterone acts centrally in the VMH and the POA to inhibit the induction of LH surge and sexual behaviour by oestradiol.

Microdialysis measurement of neurochemical changes in the mediobasal hypothalamus of ovariectomized ewes during oestrus.

Oestrus behaviour and the luteinizing hormone (LH) surge are induced in ovariectomized ewes by oestradiol (E2) after a period of progesterone priming with a low level of E2 (Pge2) and we have previously shown that these effects are primarily mediated through their action on the mediobasal hypothalamus (MBH). The aim of the present study was to assess what neurochemical changes in the MBH are induced by these steroids that might mediate their action on oestrus behaviour and LH release. Eight ovariectomized ewes were implanted with microdialysis probes in the MBH and submitted to three artificial cycles, so that they exhibited either both oestrus behaviour and an LH surge (Pge2 + E2), an LH surge alone (E2 alone) or neither oestrus behaviour nor an LH surge (Pge2 alone). Microdialysis and blood samples were collected every 30 min from 4 h before the end of Pge2 treatment until the end of oestrus. Behavioural tests with a ram were made to assess receptivity. Dopamine (DA) levels were found to increase significantly at the termination of Pge2 treatment after both Pge2 + E2 and Pge2 treatments. When the ewes received E2 after a Pge2 + low estradiol priming (Pge2 + E2), DA levels decreased 16 h later (4 h after E2) whereas they did not change after E2 or Pge2 alone. By contrast, serotonin (5HT) levels did not change significantly during the first 24 h but then increased when ewes received E2 alone and decreased when they were treated with Pge2 + E2. gamma-Aminobutyric acid (GABA) concentrations decreased significantly at the beginning of the sampling period after all treatments but this decrease lasted longer after Pge2 + E2 and was most pronounced at the beginning of receptivity. No significant long term effects of these steroid treatments were found on noradrenaline (NA), aspartate, glutamate, glycine and taurine levels. However, E2 administration was followed during the next few hours by a significant increase in glycine and to a smaller extent in glutamate and GABA. More importantly, when ewes were treated with Pge2 + E2, NA levels increased significantly following the behavioural interactions with a ram when the ewes were sexually receptive. In contrast to this, DA levels only increased during interactions with the ram when the ewes were not receptive. 5HT levels increased after tests where the ewe was either receptive or unreceptive to the male. GABA, aspartate and glycine levels increased in the sample just preceding the test and then decreased during it.(ABSTRACT TRUNCATED AT 400 WORDS)

The importance of olfactory bulb noradrenalin for maternal recognition in sheep.

Herds of grazing mammals characteristically produce precocial offspring in synchrony, and it is therefore important for the mother to form a rapid recognition of her own offspring to distinguish them from others. In sheep, the ewe forms such a selective bond with her lamb within 2-4 hours of parturition, a bond which is primarily dependent on olfactory sensory recognition. Here we report that the neuronal mechanism whereby the olfactory "imprint" is made is dependent on the centrifugal noradrenergic projections to the olfactory bulbs. Lesioning of this neural pathway prevents the ewe from forming a selective bond with her own lamb, enabling her to adopt alien lambs.

Effect of cyproterone acetate on steroid-induced sexual behavior in adult ewes.

The inhibitory effect of cyproterone acetate on sexual behavior has been investigated in adult ovariectomized ewes treated with either testosterone propionate (10 mg/day) or estradiol benzoate (200 micrograms/day) to induce male activity and female receptivity simultaneously. Intra-muscular injections of 100 mg cyproterone acetate/day resulted in a rapid decrease in some of the male sexual responses after both testosterone and estradiol treatments, whereas the female reactions were eliminated by cyproterone acetate only when they had been induced by testosterone.

Opiate antagonists stimulate affiliative behaviour in monkeys.

The effects of treating captive talapoin monkeys acutely (twice daily for 7 days) with naltrexone hydrochloride (0.25 mg 0.5 mg and 1 mg/kg intramuscular injections twice daily), naloxone hydrochloride (0.5 mg/kg IM twice daily) and sulpiride (1.5 mg/kg IM twice daily) was studied in social pairs and singly caged animals. The behaviour of social pairs and endocrine changes in all treated monkeys were monitored before, during and after withdrawal of the course of drug treatment. Naltrexone and naloxone, but not sulpiride, significant increased grooming and grooming invitations while aggressive behaviour, self grooming, scratching and general locomotor activity were unaffected. There was an overall increase in LH, testosterone and cortisol in plasma samples taken 60 mins after opiate receptor blockade. Prolactin was unchanged but increased dramatically in animals treated with sulpiride. No significant endocrine changes were observed to precede the increased grooming behaviour which opiate receptor blockade induced. The behavioural changes reported for this primate support the view that positive affect arising from social bonds may be mediated by cerebral endorphin containing systems.

Onset of the preovulatory LH surge and of oestrus in intact ewes: Night is a preferred period.

Oestrous cycles were induced in seasonally anoestrous ewes by introducing rams into the flock and giving to the ewes one intramuscular injection of 20 mg progesterone. At the second ovulation the onset of oestrus and the preovulatory surge of luteinizing hormone (LH) were recorded. It was found that the LH surge began in significantly more ewes during the night (79%) than during the day (21%). The onset of oestrus tended to follow a similar pattern. This temporal pattern was not related to the time of ram introduction, but may be the result of daily rhythms in ovarian activity. Furthermore, a preferred period for the LH surge indicates a preferred period for ovulation and this may be important in deciding when to begin artificial insemination.

[Hormones and development of sexual behaviour in the domestic sheep (author's transl)]. / Hormones et développement du comportement sexuel chez les ovins.

All motor patterns of male sexual behaviour can be observed in young lambs from the first days of the life. They considerably increase on weeks 2 to 5 when they decrease dramatically and remain stable up to over 6 months of age. This evolution has no direct relation with that of the physiological events. The adult female has a complete bisexual potentiality. The development of the sexual behaviour during ontogeny appears as a continuous process rather than to be limited to the period of puberty or sexual differentiation.

[Neuroendocrine control of ovulation in sheep]. / Le contrôle neuroendocrinien de l'ovulation chez la brebis.

The preovulatory surge of LH, or positive feedback response to oestradiol, is related to the steroid's direct stimulating action on the pituitary gland, but we have demonstrated that, in sheep, the central nervous system plays an essential role in its action: a preovulatory GnRH surge. Since GnRH neurons appear to contain few, if any receptors for oestradiol the question is raised: Where in the central nervous system does oestradiol act to stimulate GnRH secretion? A series of experiments were conducted with a combination of techniques for sampling pituitary portal blood and stereotaxic brain micro-implantations of oestradiol. Castrated Ile-de-France breed ewes were treated during the pseudo follicular phase of successive artificial cycles, receiving either peripheral oestradiol implants or brain implants containing oestradiol or cholesterol. Administration of oestradiol implants into the ventromedial region of the hypothalamus induced a preovulatory GnRH surge in 5 out of 10 animals. The interval between estradiol and the GnRH surge was comparable with that observed in animals treated with a peripheral implant although the amplitude of the surge was smaller. No GnRH surges were observed in animals treated with cholesterol. Finally, there was an inverse correlation between the response intensity (amplitude of the GnRH/LH surges) and the distance separating implants from cells in the region carrying oestradiol receptors. These results show that the mediobasal hypothalamus is a site of action for the oestradiol-induced preovulatory GnRH surge in the ewe. Immunohistochemical tests should identify the nature of the cells forming the relay for the steroid action.