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.

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.

Ovarian parameters and fertility of dairy cows selected for one QTL located on BTA3.

Recently, one Quantitative Trait Locus (QTL) of female fertility located on Bos Taurus chromosome 3 (BTA3), QTL-F-Fert-BTA3, has been identified in Holstein breed. It is implied in the success rate after the first AI (AI1) in cow. The failure of pregnancy can be due to several factors involved in the different steps of the reproductive process. The aim of our study was to finely phenotype heifers and primiparous cows selected for their haplotype at the QTL-F-Fert-BTA3. We specifically studied the ovarian follicular dynamic and several fertility parameters. Females carrying the favourable haplotype "fertil+" or unfavourable haplotype "fertil-" were monitored by transrectal ultrasonography during their cycle before the first AI (AI1). Follicular dynamic was similar between the two groups. However, the length of the estrus cycle was shorter in heifers than in primiparous cows and two-wave cycles were shorter than three-wave cycles, regardless of the age and the haplotype. The concentration of plasma anti-Müllerian hormone was correlated with the number of small antral follicles. It was higher in heifers than in primiparous cows, independently of their haplotype. The success rate at the AI1 was significantly higher in "fertil+" than in "fertil-" primiparous cows, 35 d after the AI1 (70% vs 39%). In both haplotypes, pregnancy failure occurred mainly before 21 d after AI1. The commencement of luteal activity after calving was significantly earlier in "fertil+" than in "fertil-" primiparous cows. Calving-AI1 and calving-calving intervals were similar between "fertil+" and "fertil-" primiparous cows. Taken together, "fertil+" and "fertil-" primiparous cows present a difference in the success rate after AI1 that is not explained by variations of ovarian dynamics.

New insights into the influence of breed and time of the year on the response of ewes to the 'ram effect'.

Exposure of anoestrous ewes to rams induces an increase in LH secretion, eventually leading to ovulation. This technique therefore is an effective, low-cost and hormone-free way of mating sheep outside the breeding season. However, the use of this technique is limited by the variability of the ewes' responses. In this study, our objective was to understand more completely the origins of this variability and to determine the relative roles of breed, the point in time during anoestrus and the depth of anoestrus on the response to the 'ram effect'. In the first experiment, the pattern of anoestrus on the basis of the concentration of progesterone determined weekly, was determined in four breeds including two less seasonal (Mérinos d'Arles and Romane), one highly seasonal (Mouton Vendéen) and one intermediate (Île-de-France) breeds. Anoestrus was longer and deeper in Mouton Vendéen and Île-de-France than in Romane or Mérinos d'Arles. In the second experiment, we used the same four breeds and tested their hypophyseal response to a challenge with a single dose of 75 ng gonadotrophin-releasing hormone (GnRH) in early, mid and late anoestrus, and then we examined their endocrine and ovarian responses to the 'ram effect'. Most (97%) ewes responded to GnRH and most (93%) showed a short-term increase in LH pulsatility following the 'ram effect'. The responses in both cases were higher in females that went on to ovulate, suggesting that the magnitude of the hypophyseal response to a GnRH challenge could be a predictor of the response to the 'ram effect'. As previously observed, the best ovarian response was in Mérinos d'Arles at the end of anoestrus. However, there was no relationship between the proportion of females in the flock showing spontaneous ovulation and the response to the 'ram effect' of anoestrous ewes from the same flock.

Sexual experience and temperament affect the response of Merino ewes to the ram effect during the anoestrous season.

In seasonally anoestrous ewes of many breeds, the introduction of rams triggers an increase in gonadotrophin secretion that induces ovulation, a phenomenon known as the 'ram effect'. The ram effect is a practical method for mating ewes outside the natural breeding season, and also can provide synchronised lambing, but the variability of the response, especially in young animals, reduces its potential for widespread application. The aim of our study was to assess two factors that are thought to contribute to the variability in young ewes: temperament and sexual experience. We used anovulatory ewes from a flock that had been genetically selected for 'calm' or 'nervous' temperament and compared the endocrine and ovarian responses to the ram effect in four groups (each n=15): 'calm' and parous (3-6 years old); 'calm' and nulliparous (2 years old); 'nervous' and parous; and 'nervous' and nulliparous. Parous ewes, independently of their temperament, exhibited a faster endocrine response and a higher proportion of females cycling after ram introduction than nulliparous ewes. 'Nervous' ewes exhibited a higher proportion of females cycling after ram introduction than calm ewes, but only in the nulliparous group. We conclude that temperament exerts little influence on the response to the ram effect in sexually experienced ewes, and that females of 'nervous' temperament appear to respond better when sexually 'naive'. Both sexual experience and temperament need to be taken into consideration when flock management involves the ram effect. Finally, some ewes were cyclic at ram introduction, yet exhibited an increase in LH secretion even in the presence of high concentrations of progesterone. The mechanism by which the inhibitory effect of progesterone on LH secretion was bypassed needs to be clarified.

Early sexual experience and stressful conditions affect the response of young ewes to the male.

Exposure of anoestrous ewes to rams induces oestrous cycles (the 'ram effect'). This response is poor in young-sexually naive females, possibly because they lack sexual experience and are sensitive to stress. Firstly, we assessed the conditions required for the acquisition of sexual experience. We exposed naive females at 5 or 9 months of age to rams and subsequently assessed their response at 1 year of age, to the ram effect. Unexpectedly, 2 weeks pre-exposure at 5 months and under some conditions at 9 months, inhibited the ram effect at 1 year of age, suggesting that early contact with rams was stressful and had long-term negative effects on reproduction. Consequently, we assessed the combined effect of stress and sexual experience on responses to the ram effect. At 9 months of age, we pre-exposed naive females using sexually active rams to achieve 'positive' (voluntary contact) or 'negative' (forced contact) experiences, and assessed responses to the ram effect at a year of age. In parallel, we subjected naive females to stressful conditions at the time of the male effect. During pre-exposure, females with 'negative' contact exhibited more vigilance and stress cues and had higher levels of cortisol than 'positive' contact ewes. Regardless of pre-exposure or stress, the response to the ram effect at a year of age was inhibited. These experiments confirm that contact with sexual partners can be stressful to young ewes and highlight the need to consider their emotional state when conditioning them to the presence of rams and when managing breeding programs.

Sexual behavior in ewes and other domestic ruminants.

Similarities as well as differences across species in the control of sexual behavior are helping to fully understand the subtle relations between physiology and eco-ethological constraints and how the brain integrates such information. We will illustrate this with sexual behavior in domestic ruminants and especially ewes. Females of these species like humans, but unlike rodents, have a long luteal phase. A prolonged exposure to progesterone (Pg) before the preovulatory estradiol rise is necessary for estrous behavior to be displayed. Estradiol action and receptor localization is very similar to that observed in other species. But not too surprisingly, the role of Pg is rather different with a priming effect not observed in rodents. However, as in rodents, Pg also has an inhibitory effect, is necessary for the display of proceptivity and is responsible for the timing of the different periovulatory events. These steroids act on the central nervous system in similar areas across mammalian species to regulate estrous behavior. Steroid fluctuations during the estrous cycle cause changes in catecholaminergic activity in the hypothalamus. Interestingly, these neurotransmitters seem to have very similar effects in ewes and rats as illustrated by the norepinephrine rise after male-female interactions observed in both species. Similar comparisons can be made regarding the action of some neuropeptides, including oxytocin and GnRH, and more integrative processes like sexual differentiation and modulation of reproduction by social interactions. Data on sheep, goats and cows will be compared with those of rodents.

Differential estradiol requirement for the induction of estrus behavior and the luteinizing hormone surge in two breeds of sheep.

For a better understanding of the mechanisms that lead to the preovulatory GnRH/LH surge and estrus behavior, the minimum estradiol (E) requirements (dose and duration) to induce each of these events were determined and compared between two breeds of ewes having either single (Ile de France) or multiple (Romanov) ovulations. The ewes were initially studied during a natural estrus cycle, and were then ovariectomized and run through successive artificial estrus cycles. For these artificial cycles the duration and amplitude of the follucular phase E increase were manipulated by E implants. Under all conditions, the onset of estrus behavior was similar in the two breeds, although its duration was longer in Romanov ewes. While a moderate E signal (6 cm for 12 h) induced an LH surge in 10/10 Ile de France ewes, a larger E signal (12 cm for 12 h) was minimally effective in Romanov ewes (4/10). Additional studies revealed that a small E signal (3 cm for 6 h) induced full estrus behavior in all Romanov ewes but was completely ineffective in Ile de France animals (0/10). Higher doses and mostly longer durations of the E signal (12 cm for 24 h) were required to induce a surge in all the Romanov ewes. These results demonstrate a clear difference in the E requirement for the induction of estrus behavior and the LH surge between breeds of ewes that have different ovulation rates. These data provide compelling evidence that, in one breed, the neuronal systems that regulate both events require different estrogen signals.

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.

Importance of learning in the response of ewes to male odor.

Exposure of anestrous ewes to a ram or its odor results in the activation of the luteinizing hormone (LH) secretion leading to reinstatement of cyclicity in most females. Sexual experience and learning have been suggested as important factors to explain the variability of the female responses. In experiment 1, we compared the behavioral and endocrine responses of four groups of anestrous females that differed in age (young or adult) and previous exposure to males [naive (no exposure) or experienced (courtship behavior for young and numerous mating for adults)]. Age did not seem to affect the LH response to males or their odor. In contrast, sexual experience was a critical factor: the proportion of females exhibiting an LH response to male odor was significantly higher in experienced than in naive ewes. Sexual experience affected the response to male odor, but did not have an effect on responses to the male himself. A second experiment investigated whether the LH response to male odor could result from an associative learning process. Accordingly, we tested the effectiveness of a conditioned stimulus (lavender odor) previously associated with the male, in inducing the endocrine response. The results indicate that the odor of lavender activated LH secretion only in ewes that have been previously exposed to scented males. This demonstrates that ewes are able to learn the association between a neutral odor and their sexual partner.

The "male effect" in sheep and goats: a review of the respective roles of the two olfactory systems.

In sheep and goats, exposure of seasonally anestrous females to sexually active males results in activation of luteinizing hormone (LH) secretion and synchronized ovulation. This phenomenon is named "the male effect" and seems to constitute a major factor in the control of reproductive events. This effect depends mostly on olfactory cues and is largely mimicked by exposure to male fleece only. In sheep, preventing the vomeronasal organ (VNO) from functioning does not affect the female responses to male odor suggesting that, unlike in rodents, the accessory olfactory system does not play the major role in the perception of this pheromonal cue. Female responses also seem to depend on previous experience, an effect that is not common for pheromones and renders this model of special interest. The aim of the present report is to summarize our current knowledge concerning the "male effect" and in particular to clarify the respective roles of the two olfactory systems in the processes involved in this effect.

Inactivation of the olfactory amygdala prevents the endocrine response to male odour in anoestrus ewes.

Our aim was to study the role of the olfactory amygdala (medial and cortical nuclei) and the ventromedial nucleus of the hypothalamus (VMN) in the ability of the male odour or live males to induce a release of luteinizing hormone in anoestrus ewes. To achieve this, we temporarily blocked the activity of these structures by localized retrodialysis administration of the anaesthetic lidocaine. The effect of ram odour on the secretion of luteinizing hormone was completely blocked by inactivation of the cortical nucleus of the amygdala. In contrast, inactivation of part of the accessory olfactory system (the medial nucleus of the amygdala or the VMN) had no effect. In the presence of the male, lidocaine never impaired the endocrine response of the ewes. These results show that modulation of reproduction by the sexual partner even through pheromonal cues does not occur via the direct circuit of the accessory system. On the contrary, the cortical nucleus of the amygdala is absolutely necessary for the treatment of and/or the response to the male olfactory signal but this structure can be bypassed when other sensory cues are available.

Role of experience in the neuroendocrine control of ewes' sexual behavior.

We assessed the role of learning in the expression of female sexual behavior and evaluated the relative importance of age versus experience. Two studies were conducted with ovariectomized ewes submitted to steroid treatment that mimicked an estrus cycle. We compared behavioral (experiments 1 and 2), neurochemical (experiment 1), and endocrine (experiment 2) responses of sexually naive young and adult ewes versus sexually experienced adults when exposed to males. In a third study, we compared their performance in an instrumental learning test and the extent to which it was affected by stress. These experiments showed that proceptivity is affected both by age and sexual experience. In experiment 1 only experienced adults were proceptive and displayed an increase in hypothalamic norepinephrine. By the second estrus cycle (experiment 2) naive adults performed similarly to experienced adults but proceptive behavior was still inferior in young ewes. Receptivity was also different between groups but affected more by age than by sexual experience. All ewes mated during the first interaction with a male, although males' latency to ejaculation was shorter with experienced females than naive adults or naive young. Young ewes found food as readily as adults in experiment 3 but were more affected by stress. Together, these experiments show that both experience and age influence sexual activity and that sensitivity to stress may also be involved. This may contribute to the deficient reproductive performance that is often observed in young female mammals.

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.

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.

Steroid control of monoamines in relation to sexual behaviour.

Monoamines are widely distributed in the brain and are involved in arousal and motivational processes as well as motor activity and neuroendocrine control. Interactions between these central monoaminergic systems and steroid hormones play a major role in the integration of reproductive behaviour and gonadal function. This review describes the development of understanding of the relationship between steroids and monoamines, particularly in the control of sexual behaviour.

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.

Male faces and odours evoke differential patterns of neurochemical release in the mediobasal hypothalamus of the ewe during oestrus: an insight into sexual motivation?

During behavioural oestrus female sheep, like females of many species, become both attracted to and sexually receptive towards males, whereas at other times they will avoid them. The mediobasal hypothalamus is the main site for the feedback action of sex steroids to induce sexual behaviour in the sheep and in previous studies we have shown that noradrenaline and serotonin are released in this region during sexual interactions with males. The current study investigated whether such changes are specific to interactions with males and if visual or olfactory cues or somatosensory stimulation during mating are critical. In vivo microdialysis sampling was carried out in the mediobasal hypothalamus of ovariectomized ewes submitted to artificial oestrous cycles. Release of monoamines and amino acid transmitters was first measured in animals during and after oestrus when they were exposed to interactions with either males or females or presentation of food. Noradrenaline concentrations only increased significantly when the females were in oestrus and interacted with males irrespective of whether intromissions were permitted. Females were then exposed to visual (faces) or odour (a home pen) cues from males or to the males themselves. Slide images of male faces increased concentrations of amines, glutamate and GABA during early oestrus, when females spent most time looking at them. During late oestrus noradrenaline, glutamate and GABA concentrations also increased in response to the male faces but no transmitter changes were seen during the luteal phase or at any time where the females were exposed to female faces, or inverted male faces. Exposure to male odour produced a lower increase in noradrenaline concentrations when females were in early oestrus but marked increases 20 and 30 min after exposure to male odours in late oestrus. No other transmitters were affected. Exposure to a male and mating with him when females were in early or late oestrus produced increased noradrenaline concentrations similar to those seen with face stimuli alone although other neurotransmitters were unaffected. These results show that noradrenaline, and to a lesser extent dopamine, serotonin, glutamate and GABA release in the mediobasal hypothalamus, can be modulated specifically in the oestrous female by sensory information coming from the male during oestrus. The differential effects of male cues during early and late oestrus suggest their involvement in (i) proceptive or anticipatory sexual responses shown by the female to male cues, and (ii) receptive sexual responses, and suggest that the mediobasal hypothalamus plays a key role in the integration of hormonal action on sexual motivation and processing of sensory information during oestrus.

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.