Factors affecting the secretion of luteinizing hormone in the ewe

Patterns of preoptic-hypothalamic neuronal activation and LH secretion in female sheep following the introduction and withdrawal of novel males

The neuroendocrine response of female sheep to a novel male involves neural activation in the hypothalamus. However, if males are removed, the gonadotrophic signal declines, so the neural activity is likely to change. We examined Fos-immunoreactive (IR) cells in hypothalamic tissues from seasonally anovulatory female sheep exposed to males for 2 or 6h, or for 2h followed by 4h isolation from males. Control females were killed in the absence of male exposure. Male introduction increased LH secretion in all females; male removal was associated with a reduction only in mean and basal LH concentrations. Females exposed to males for 2h had more Fos-IR cells in the arcuate nucleus (ARC), ventromedial nucleus of the hypothalamus (VMH) and organum vasculosum of the lamina terminalis (OVLT) than control females. Fos-IR cells in the preoptic area (POA) were only greater than in control females after 6h exposure to a male. Removal of males decreased the number of Fos-IR cells in the ARC, VMH and OVLT, but not in the POA. Thus, hypothalamic neural activation and LH secretion in female sheep are stimulated by males and decline after male removal. However, activation in the POA persists after removal and may explain the incomplete decline in the LH response.

Hormonal changes, semen quality and variance in reproductive activity outcomes of post pubertal rabbits fed Moringa oleifera Lam. leaf powder

ETHNOPHARMACOLOGICAL RELEVANCE

Moringa oleifera Lam. (Moringaceae) is an important plant based staple food, known for its nutritional and medicinal value and is usually prescribed by herbal practitioners in Nigeria and in other tropical countries for the treatment of male infertility problems and reproductive diseases in females. Although the aphrodisiac properties and fertility enhancement potential in males have been reported, the underlying mechanisms for the activity remain unclear. In this study, we investigated the influence of supplementing the diet with M. oleifera Lam. leaf powder on reproductive hormones and semen quality of New Zealand White (NZW) rabbits.

MATERIALS AND METHODS

Thirty-two (32) NZW rabbits of 50:50 ratio bucks to does, were randomly distributed to four treatment groups (n = 4 bucks, n = 4 does per group). Graded levels (0, 5, 10 and 15 g/kg) of M. oleifera Lam. leaf powder was incorporated into rabbit growers pellet. The does and bucks were housed separately in hutches and sheltered under the same environmental conditions with free access to their respective treatment diets for a period of 12 weeks.

RESULTS

In female rabbits, treatment revealed significant (P < 0.05) dose-dependent reduction in the concentration of serum FSH, LH and oestrogen. While in contrast the highest dose of leaf powder significantly (P < 0.05) increased progesterone and prolactin concentrations remained unaffected. On the other hand, the concentration of FSH and LH in bucks was significantly (P < 0.05) increased in treatment groups compared to the control group. Serum testosterone concentrations were significantly lower in the 5 and 10 g/kg treatment groups. Semen volume, sperm count and motility were significantly improved in a dose dependent manner with increasing amounts of M. oleifera Lam. leaf powder in the diet.

CONCLUSIONS

We conclude that M. oleifera Lam. leaf powder supplementation to the diet was more beneficial to male rabbit fertility than the female, where it tended to have a negative impact through the hypothalamic-pituitary-gonadal axis. However, with the varying impact of M. oleifera Lam. leaf powder on the hypothalamic-pituitary-gonadal axis of male and female animals, further investigation is necessary to determine the mechanism through which it operates.

Modeling the Male Reproductive Endocrine Axis: Potential Role for a Delay Mechanism in the Inhibitory Action of Gonadal Steroids on GnRH Pulse Frequency

We developed a compartmental model so we could test mechanistic concepts in the control of the male reproductive endocrine axis. Using SAAM II computer software and a bank of experimental data from male sheep, we began by modeling GnRH-LH feed-forward and LH-T feedback. A key assumption was that the primary control signal comes from a hypothetical neural network (the PULSAR) that emits a digital (pulsatile) signal of variable frequency that drives GnRH secretion in square wave-like pulses. This model produced endocrine profiles that matched experimental observations for the testis-intact animal and for changes in GnRH pulse frequency after castration and T replacement. In the second stage of the model development, we introduced a delay in the negative feedback caused by the aromatization of T to estradiol at the brain level, a concept supported by empirical observations. The simulations showed how changes in the process of aromatization could affect the response of the pulsatile signal to inhibition by steroid feedback. The sensitivity of the PULSAR to estradiol was a critical factor, but the most striking observation was the effect of time delays. With longer delays, there was a reduction in the rate of aromatization and therefore a decrease in local estradiol concentrations, and the outcome was multiple-pulse events in the secretion of GnRH/LH, reflecting experimental observations. In conclusion, our model successfully emulates the GnRH-LH-T-GnRH loop, accommodates a pivotal role for central aromatization in negative feedback, and suggests that time delays in negative feedback are an important aspect of the control of GnRH pulse frequency.

Domestication affects the structure, development and stability of biobehavioural profiles

Domestication is an evolutionary process during which the biobehavioural profile (comprising e.g. social and emotional behaviour, cognitive abilities, as well as hormonal stress responses) is substantially reshaped. Using a comparative approach, and focusing mainly on the domestic and wild guinea pig, an established model system for the study of domestication, we review (a) how wild and domestic animals of the same species differ in behaviour, emotion, cognition, and hormonal stress responses, (b) during which phases of life differences in biobehavioural profiles emerge and (c) whether or not animal personalities exist in both the wild and domestic form. Concerning (a), typical changes with domestication include increased courtship, sociopositive and maternal behaviours as well as decreased aggression and attentive behaviour. In addition, domestic animals display more anxiety-like and less risk-taking and exploratory behaviour than the wild form and they show distinctly lower endocrine stress responsiveness. There are no indications, however, that domestic animals have diminished cognitive abilities relative to the wild form. The different biobehavioural profiles of the wild and domestic animals can be regarded as adaptations to the different environmental conditions under which they live, i.e., the natural habitat and artificial man-made housing conditions, respectively. Concerning (b), the comparison of infantile, adolescent and adult wild and domestic guinea pigs shows that the typical biobehavioural profile of the domestic form is already present during early phases of life, that is, during early adolescence and weaning. Thus, differences between the domestic and the wild form can be attributed to genetic alterations resulting from artificial selection, and likely to environmental influences during the pre- and perinatal phase. Interestingly, the frequency of play behaviour does not differ between the domestic and wild form early in life, but is significantly higher in domesticated guinea pigs at later ages. Concerning (c), there is some evidence that personalities occur in both wild and domestic animals. However, there may be differences in which behavioural domains – social and sexual behaviour, emotionality, stress-responsiveness – are consistent over time. These differences are probably due to changing selection pressures during domestication.

Two or 24 h of daily contact with sexually active males results in different profiles of LH secretion that both lead to ovulation in anestrous goats

Two experiments were conducted to (a) determine whether sexually active males are able to stimulate the sexual activity of anestrous female goats when duration of contact is reduced to an intermittent contact shorter than 4 daily hours and (b) compare the pattern of secretion of LH when anestrous goats are exposed either permanently or intermittently to males. In the first experiment, 4 groups of anovulatory goats were exposed to sexually active males for 24, 4, 2, or 1 h/d during 15 consecutive days, whereas control females remained isolated. More than 89% of females in the groups exposed to the sexually active bucks ovulated, whereas only 5% did so in the control group (P < 0.001). However, the proportion of females ovulating before day 4 was greater in the 2-, 4-, or 24-h contact groups than in the control, whereas it did not differ between the control group and the 1-h contact group (P = 0.02, <0.001, <0.001 and 0.23, respectively). In the second experiment, 3 groups of anovulatory goats were exposed permanently (24 h/d) or intermittently (2 h/d) to bucks during 5 d or remained isolated. We found that pulsatility of luteinizing hormone (LH) increased in the intermittent and permanent contact groups after males were introduced to females (P = 0.05); this pulsatility of LH remained elevated in the permanent-contact group, whereas it decreased in the intermittent-contact group, once the male was removed (P = 0.32 and 0.05, respectively). We conclude that 1 or 2 daily hours of contact with sexually active males is sufficient to stimulate ovulatory activity in anovulatory goats; however, ovulation is obtained through a different pattern of secretion of LH.

Effects of domestication on biobehavioural profiles: a comparison of domestic guinea pigs and wild cavies from early to late adolescence

INTRODUCTION

Domestication can lead to marked alterations in the biobehavioural profile of a species. Furthermore, during ontogeny, the individual phenotype of an animal can be shaped by the environment in important phases such as adolescence. We investigated differences in biobehavioural profiles between domestic guinea pigs and their ancestor, the wild cavy, over the course of adolescence. At this age, comparisons between the two groups have not been conducted yet. Male guinea pigs and cavies were subjected to a series of tests twice: during the early and late phase of adolescence. We analysed emotional and social behaviours as well as cortisol reactivity and testosterone levels.

RESULTS

Concerning emotional behaviour, cavies were more explorative and showed more anxiety-like behaviour in the open field test and dark-light test. They also were more risk-taking when having to jump off an elevated platform. Regarding social behaviour, cavies showed less social activity towards unfamiliar females and infants. Furthermore, while guinea pigs and cavies did not differ in basal cortisol levels, cavies showed distinctly higher and prolonged cortisol responses when exposed to an unfamiliar environment. Cavies also had lower basal testosterone titres. No substantial changes in biobehavioural profiles were revealed over the course of adolescence in both groups.

CONCLUSIONS

Domestication led to a substantial shift in the biobehavioural profile of the guinea pig regarding all investigated domains in early and late adolescence. Hence, the differentiation between guinea pigs and cavies emerges early in ontogeny, well before the attainment of sexual maturity. The young individuals already show adaptations that reflect the differences between the natural habitat of cavies and the man-made housing conditions guinea pigs are exposed to. Higher levels of exploration and risk-taking and lower levels of anxiety-like behaviour are necessary for cavies in order to cope with their challenging environment. Their high cortisol reactivity can be interpreted as an energy provisioning mechanism that is needed to meet these demands. By contrast, guinea pigs are adapted to a less challenging environment with much higher population densities. Hence, their biobehavioural profile is characterised by higher levels of social activity and lower levels of exploration, risk-taking, and cortisol reactivity.

Goats excel at learning and remembering a highly novel cognitive task

Introduction

The computational demands of sociality (maintaining group cohesion, reducing conflict) and ecological problems (extractive foraging, memorizing resource locations) are the main drivers proposed to explain the evolution cognition. Different predictions follow, about whether animals would preferentially learn new tasks socially or not, but the prevalent view today is that intelligent species should excel at social learning. However, the predictions were originally used to explain primate cognition, and studies of species with relatively smaller brains are rare. By contrast, domestication has often led to a decrease in brain size, which could affect cognition. In domestic animals, the relaxed selection pressures compared to a wild environment could have led to reduced social and physical cognition. Goats possess several features commonly associated with advanced cognition, such as successful colonization of new environments and complex fission-fusion societies. Here, we assessed goat social and physical cognition as well as long-term memory of a complex two-step foraging task (food box cognitive challenge), in order to investigate some of the main selection pressures thought to affect the evolution of ungulate cognition.

Results

The majority of trained goats (9/12) successfully learned the task quickly; on average, within 12 trials. After intervals of up to 10 months, they solved the task within two minutes, indicating excellent long-term memory. The goats did not learn the task faster after observing a demonstrator than if they did not have that opportunity. This indicates that they learned through individual rather than social learning.

Conclusions

The individual learning abilities and long-term memory of goats highlighted in our study suggest that domestication has not affected goat physical cognition. However, these cognitive abilities contrast with the apparent lack of social learning, suggesting that relatively intelligent species do not always preferentially learn socially. We propose that goat cognition, and maybe more generally ungulate cognition, is mainly driven by the need to forage efficiently in harsh environments and feed on plants that are difficult to access and to process, more than by the computational demands of sociality. Our results could also explain why goats are so successful at colonizing new environments.

Interactions between nutrition and reproduction in the management of the mature male ruminant

In mature male sheep and goats, changes in feed intake seem to have little effect on gonadal endocrine function but induce profound changes on sperm production. These outcomes are due to changes in size of the seminiferous tubules and in spermatogenic efficiency. Except with severe underfeeding, there are only minor changes in the endocrine function of the testis (testosterone production) unless season-long treatments are imposed. For cattle, nutrition clearly affects testicular development and the production of spermatozoa in young bulls, as it does in other species but, after the period of rapid growth has ended, there appears to be little or no response to nutrition. We are developing a clear picture of the metabolic signals, neuroendocrine processes and hormonal control systems that are involved, particularly for the mature male sheep. The energetic components of the diet, rather than protein, seem to be responsible, so we have envisaged a model of the relationship between energy balance and reproduction that has 4 'dimensions': genotype, structure (organs), communication (chemical and neural signals, nutrient sensing) and time (dynamics, metabolic memory, programming). We have linked these perspectives to 'resource allocation theory' and incorporated them into strategies for 'clean, green and ethical animal production'. In contrast to the clear outcomes with respect to spermatogenesis, the effects of nutrition on sexual behaviour are more difficult to define, perhaps because the behaviour is affected by a complex mix of physiological factors and because of flawed methods for quantifying male behaviour. For example, sexual behaviour is compromised by severe feed restriction, but male sexual behaviour requires intensive motor activity so a decline in libido could be caused by general weakness rather than specific nutritional limitations. The interaction between sexual activity and feeding behaviour also complicates the issue under field conditions. At the other end of the scale, overweight males can show reduced sexual success because they have difficulty courting and mounting. For this reason, exercise can enhance the fertilising capacity of rams. This will be important in extensive mating systems where males need to assemble and guard a harem and then mate many times for several weeks. For artificial insemination centres, there seems to be very few data on the nutritional management of males, but problems with overfed animals appear to be a risk. Future research should concentrate on the intra-testicular systems mediating the effects of nutrition on the production of spermatozoa.

Exceptional longevity in the tephritid, Ceratitis rosa, a close relative of the Mediterranean fruit fly

This study shows that the fruit fly, Ceratitis rosa (Karsch), has a significantly longer life span than the medfly, C. capitata (Wiedemann); the species used as a model organism for the demographics of insect aging. This was somewhat surprising given that both have similar distributions and overlapping niches. We postulate that the greater longevity of C. rosa is related to the fact that it can occupy colder habitats where the availability of suitable host plants may be very unpredictable in both time and space.

The interaction between photoperiod and nutrition and its effects on seasonal rhythms of reproduction in the ewe

In sheep, the seasonal patterns of reproductive activity are driven primarily by the annual photoperiodic cycle, but can also respond to other environmental factors, such as nutrition, yet little is known about the mechanisms underlying this interaction. This study was designed to define the interaction between photoperiodic and nutritional cues on seasonal patterns of ovarian activity, and to determine if there is a central interaction between these cues. Groups of Ile-de-France ewes were maintained in two nutritional states (restricted and well fed) under a simulated annual photoperiod of 8-16 h of light per day over two breeding seasons. At the end of the first breeding season, half of the animals of each group were ovariectomized (OVX) and fitted subcutaneously with estradiol implants. Low nutritional status shortened the season of ovarian activity, determined from the pattern of progesterone concentrations, by modifying the timing of seasonal transitions between periods of ovarian activity and anestrus. The same results were observed for the seasonal rhythm of neuroendocrine activity, assessed in the OVX ewes, from the pattern of luteinizing hormone concentrations. These results were then confirmed for neuroendocrine activity induced by a photoperiodic treatment. We conclude that nutrition centrally modulates the interpretation of photoperiod to affect seasonal reproductive transitions. The mechanisms of this interaction are discussed in the paper.

Role of male novelty and familiarity in male-induced LH secretion in female sheep

Ewes supposedly need to be separated from rams before male stimuli can increase gonadotrophin secretion and induce ovulation. In the present study, we investigated the LH response of ewes to ‘novel’ and ‘familiar’ rams after varying periods of separation. In Experiment 1, ewes (n = 8 per treatment) were separated from familiar rams for 15 min or 1 month and then exposed to either familiar rams, novel rams or novel wethers. After 15 min or 1 month of separation, exposure to novel rams increased pulsatile LH secretion (P < 0.05) and induced an LH surge in all ewes whereas exposure to familiar rams or novel wethers had no effect on LH secretion (P > 0.1). After 1 month of separation, re-exposure to the same familiar rams increased pulsatile LH secretion (P < 0.05) in six of eight ewes, but only induced an LH surge in two of eight ewes. In Experiment 2, familiar rams were removed and returned after 15 min, 1 day or 17 days (n = 5 per treatment). None of these treatments affected LH secretion. We conclude that separation of ewes from rams is a prerequisite for familiar rams to increase LH secretion, but is not necessary if the rams are novel.

Controlling reproduction in sheep

Intravaginal progestagen pessaries + pregnant mare's serum gonadotrophin (PMSG) for controlling the oestrous cycle and for inducing out-of-season breeding have been commercially available for many years. However, extremely good mating management is required if acceptable results are to be achieved. More recently the role of the pineal hormone, melatonin, in the regulation of seasonal breeding has been recognized, and this has led to the development of slow-release formulations for use under farm conditions. Early indications are that such preparations can advance the breeding season by about 4 to 6 weeks, with lambing percentages in early (January) lambing flocks comparable with those of traditional (March) lambing flocks.

Recent developments in the use of laparoscopy as an aid to intrauterine insemination and multiple ovulation and embryo transfer (MOET) should play a major role in breed improvement. Intrauterine insemination can markedly improve conception rates following the use of frozen semen compared with the conventional cervical technique, as well as eliminating the incidence of fertilization failure in embryo transfer donors. In addition, laparoscopic collection and transfer of embryos permit repeated collections from the same donor ewes and are much more acceptable than surgical techniques from the welfare point of view.

Estrogenicity of bisphenol a: a concentration-effect relationship on luteinizing hormone secretion in a sensitive model of prepubertal lamb

The model of the prepubertal ovariectomized lamb was selected as a sensitive model to characterize the estrogenic effects of bisphenol A (BPA) on the hypothalamo-pituitary axis (HPA). In a first experiment, the disrupting effect of BPA and of 17-beta estradiol (E2), administered as a constant 54-h iv infusion, on luteinizing hormone (LH) pulsatility was quantified. The results showed that the inhibitory effect of BPA and E2 on LH secretion appeared to follow a dual mechanism: a rapid (about 1 h) suppressive effect for high exposure and an effect observed with a period of latency (about 48 h) probably of genomic origin and observed for lower E2 and BPA levels. For E2, the disrupting dose was 0.14 microg/(kg x d), corresponding to a plasma concentration of 2 pg/ml; for BPA, the lowest observed disrupting plasma concentration was 38 ng/ml, a value only 10-fold higher than the human plasma concentration routinely reported in biomonitoring surveys. In a second experiment, we showed that after 7 weeks of BPA treatment, there was no BPA accumulation and no evidence of an alteration in the HPA responsiveness to BPA. Finally, our results showed that directly considering plasma concentrations, the ratio of the BPA disrupting plasma concentration in lambs over the observed human plasma concentration is only 10, whereas if the dose is considered, it could be concluded that the BPA disrupting dose in lamb is conservatively 50-fold higher than the currently recommended Tolerable Daily Intake of 50 microg/(kg x d).

LH pulse frequency and the emergence and growth of ovarian antral follicular waves in the ewe during the luteal phase of the estrous cycle

BACKGROUND

In the ewe, ovarian antral follicles emerge or grow from a pool of 2-3 mm follicles in a wave like pattern, reaching greater than or equal to 5 mm in diameter before regression or ovulation. There are 3 or 4 such follicular waves during each estrous cycle. Each wave is preceded by a peak in serum FSH concentrations. The role of pulsatile LH in ovarian antral follicular emergence and growth is unclear; therefore, the purpose of the present study was to further define this role.

METHODS

Ewes (n = 7) were given 200 ng of GnRH (IV) every hour for 96 h from Day 7 of the estrous cycle, to increase LH pulse frequency. Controls (n = 6) received saline. In a second study, ewes (n = 6) received subcutaneous progesterone-releasing implants for 10 days starting on Day 4 of the cycle, to decrease LH pulse frequency. Controls (n = 6) underwent sham surgery. Daily transrectal ovarian ultrasonography and blood sampling was performed on all ewes from the day of estrus to the day of ovulation at the end of the cycle of the study. At appropriate times, additional blood samples were taken every 12 minutes for 6 h and 36 min or 6 h in studies 1 and 2 respectively.

RESULTS

The largest follicle of the follicular wave growing when GnRH treatment started, grew to a larger diameter than the equivalent wave in control ewes (P < 0.05). Mean serum estradiol and progesterone concentrations were higher but mean serum FSH concentrations were lower during GnRH treatment compared to control ewes (P < 0.05). The increased serum concentrations of estradiol and progesterone, in GnRH treated ewes, suppressed a peak in serum concentrations of FSH, causing a follicular wave to be missed. Treatment with progesterone decreased the frequency of LH pulses but did not have any influence on serum FSH concentrations or follicular waves.

CONCLUSION

We concluded that waves of ovarian follicular growth can occur at LH pulse frequencies lower than those seen in the luteal phase of the estrous cycle but frequencies seen in the follicular phase, when applied during the mid-luteal phase, in the presence of progesterone, do enhance follicular growth to resemble an ovulatory follicle, blocking the emergence of the next wave.

The role of rhythms in the response to melatonin

Rhythmicity of melatonin secretion is critical to the regulation of mammalian reproduction by daylength. In the ewe, photoperiod determines ovarian function by modulating the ability of oestradiol to suppress pituitary secretion of luteinizing hormone (LH). This influence of daylength depends in turn upon the pineal gland, which mediates photic control of the frequency at which the brain stimulates the pituitary to secrete gonadotropin. Photoperiod determines the pattern of melatonin secretion, most notably setting the duration of the nightly elevation in serum concentrations. Replacement of melatonin in pinealectomized ewes drives responsiveness to oestradiol negative feedback; LH levels are determined by the duration of the nightly melatonin infusion rather than by the photoperiod to which ovariectomized, oestradiol-implanted pinealectomized ewes are exposed. Refractoriness to stimulatory and inhibitory photoperiods may reflect circannual modulation of the responsiveness of neuroendocrine mechanisms to melatonin signals of a given duration.

Efeitos da progesterona exógena sobre o desenvolvimento folicular em ovelhas

Avaliaram-se os efeitos da progesterona (P4) sobre o crescimento folicular e na endocrinologia reprodutiva em ovelhas Bergamácia. Quatorze ovelhas sincronizadas com prostaglandinas (PGF2alfa ) foram distribuídas em dois grupos (n=7/grupo): grupo-controle e grupo tratado com progesterona (CIDR) depois da ovulação (dia zero). Desde o dia anterior à aplicação de PG até o dia 10, realizaram-se monitoramentos ultra-sonográficos para estabelecer o crescimento folicular. Amostras de sangue foram colhidas para a determinação de P4 desde o dia anterior à aplicação de PG até o dia 10 depois da ovulação. Para o perfil dos pulsos de hormônio luteinizante (LH), as colheitas de sangue ocorreram em intervalos de 30 minutos por um período de oito horas, nos dias um e seis. As taxas de crescimento diferiram (P<0,001) entre os grupos, 0,91±0,15 e 0,70±0,16mm/dia para os grupos controle e tratado, respectivamente. Os dias do platô dos animais controle e tratados foram de 1,9±0,72 e 2,9±0,45 (P<0,05), respectivamente. As concentrações médias de progesterona (P<0,001) foram diferentes entre os tratamentos. A freqüência dos pulsos diferiu no primeiro dia do ciclo (P<0,01), com valores de 2,55±0,09 pulsos/8 horas no grupo-controle e de 1,49±0,11 pulsos/8 horas no grupo tratado. No sexto dia, o grupo-controle 2,20±0,09 pulsos/8 horas apresentou maior número de pulsos (P<0,05) que o grupo tratado, 1,22±0,11 pulsos/8 horas. Os efeitos inibitórios da progesterona exógena no diâmetro do folículo dominante foram mediados pela redução na freqüência dos pulsos de LH.

Twin efficiency for reproductive variables in monozygotic twin sheep

The aim of this study was to determine whether the number of animals used in experiments examining reproductive variables could be reduced without loss of statistical efficiency by using monozygotic twin (MT) sheep. In a series of four experiments, we measured the reproductive responses to changes in nutritional, opioidergic, and calcium status and calculated values for twin efficiency (TE) for each variable. In Experiment 1, we monitored the changes in gonadotrophin and testosterone secretion, scrotal circumference and live weight, of MT rams after an acute change in nutritional regime. In Experiment 2, we examined the changes in ovulation rate and gonadotrophin secretion in MT ewes following treatment with bovine follicular fluid. In Experiment 3, we determined responses to naloxone and exogenous calcium on gonadotrophin secretion in MT rams. In Experiment 4, we investigated the effects of naloxone and exogenous calcium on the hypothalamus-pituitary-ovarian axis of MT ewes. The TE values were high only for live weight and scrotal circumference; the other reproductive traits had less variation between than within MT pairs, suggesting that randomly selected animals were just as efficient as genetically identical twins in experiments examining physiological reproductive traits.

Variation in anti-predator behavior among five strains of inbred guppies, Poecilia reticulata

Quantitative genetic studies frequently utilize inbred strains of animals as tools for partitioning the direct and indirect effects of genes from environmental effects in generating an observed phenotype, however, this approach is rarely applied to behavioral studies. Guppies, Poecilia reticulata, perform a set of anti-predator behaviors that may provide an ideal system to study how complex behavioral traits are generated. To assess the utility of ornamental guppies in quantitative genetics studies of behavior, we assayed five morphologically distinct strains of ornamental guppies for response to predator cues and for variation in response among strains. Despite individual variation, all five strains responded to predator cues and differences among strains were found for all assayed behaviors, including measures of boldness and predator avoidance.

"Clean, green and ethical" animal production. Case study: reproductive efficiency in small ruminants

In response to changes in society and thus the marketplace, we need a vision for the future of our animal industries, including both on-farm and off-farm activities, that is "clean, green and ethical". Using small ruminants as a case study, we describe three "clean, green and ethical" strategies that farmers could use to improve reproductive performance. The first allows control of the timing of reproductive events by using socio-sexual signals (the "male effect") to induce synchronised ovulation in females. The second strategy, "focus feeding", is based on using short periods of nutritional supplements that are precisely timed and specifically designed for each event in the reproductive process (eg, gamete production, embryo survival, fetal programming, colostrum production). The third strategy aims to maximize offspring survival by a combination of management, nutrition and genetic selection for behaviour (temperament). All of these approaches involve non-pharmacological manipulation of the endogenous control systems of the animals and complement the detailed information from ultrasound that is now becoming available. Importantly, these approaches all have a solid foundation in reproductive biology. In several cases, they are currently used in commercial practice, but there is still room for improvement through both basic and applied research. Ultimately, these "clean, green and ethical" tools can be cost-effective, increase productivity and, at the same time, greatly improve the image of meat and milk industries in society and the marketplace.

The temporal relationship between patterns of LH and FSH secretion, and development of ovulatory-sized follicles during the mid- to late-luteal phase of sheep

The aim of the present study was to investigate the temporal relationship between the secretory pattern of serum LH and FSH concentrations and waves of ovarian antral follicles during the luteal phase of the estrous cycle in sheep. The growth pattern of ovarian antral follicles and CL were monitored by transrectal ultrasonography and gonadotropin concentrations were measured in blood samples collected every 12 min for 6 h/d from 7 to 14 d after ovulation. There were two follicular waves (penultimate and final waves of the cycle) emerging and growing during the period of intensive blood sampling. Mean and basal LH concentrations and LH pulse frequency increased (P < 0.001) with decreasing progesterone concentration at the end of the cycle. Mean and basal FSH concentrations reached a peak (P < 0.01) on the day of follicular wave emergence before declining to a nadir by 2 d after emergence. None of the parameters of pulsatile LH secretion varied significantly with either the emergence of the final follicular wave or with the end of the growth phase of the largest follicle of the penultimate wave of the cycle. However, mean and basal LH concentrations did increase (P < 0.05) after the end of the growth phase of the largest follicle of the final follicular wave of the cycle. Furthermore, the end of the growth phase of the largest follicle of the final wave coincided with functional luteolysis. In summary, there was no abrupt or short-term change in pulsatile LH secretion in association with the emergence or growth of the largest follicle of a wave. We concluded that the emergence and growth of ovarian antral follicles in follicular waves do not require changes in LH secretion, but may involve changes in sensitivity of ovarian follicles to serum LH concentrations.

Nutritional and environmental effects on reproduction in small ruminants

Animals live in environments that are both complex and continually changing, so they have to respond to short- and long-term variations in a wide range of factors, such as photoperiod, nutrition and sociosexual signals. Before they were domesticated, animals developed reproductive strategies that coped with these changes and often took advantage of them. The physiological processes that implement these strategies have been modified to some extent during several millennia of controlled breeding, but most persist. Thus, many genotypes still exhibit profound responses to external inputs, such as the induction of ovulation by sociosexual signals and the doubling of litter size by a change in nutrition. The complexity in these responses is now becoming clearer. For example, with sociosexual signals, we now need to consider the stimulatory effects of males on females, of females on males and of females on females. Similarly, the impact of nutrition has been extended beyond the control of puberty and the production of gametes to include phenomena such as ‘fetal programming’, with its potentially profound effects on the life-long performance of the animals. Fortunately, our capacity to research these phenomena has been greatly enhanced by technical improvements in hormone assays, molecular and cellular biology, and real-time ultrasound. This has brought us a better understanding of several of the environmental influences on reproduction, including: the cellular processes within ovarian follicles that mediate the effect of nutrition on ovulation rate; the neuroendocrine pathways through which nutritional inputs affect the brain centres that control appetite and reproduction; and the intracerebral pathways through which sociosexual signals (olfactory and non-olfactory) stimulate the reproductive axis. Importantly, we are now beginning to realise that, as well as considering interactions between environmental inputs and genotype, we need to take into account interactions between the environmental factors themselves, just as the animals do. We still have a long way to go for a complete understanding, but we are nevertheless in a position where we can begin to use this information to develop new management systems for our animals to improve their productivity.

Seasonal plasticity within the gonadotropin-releasing hormone (GnRH) system of the ewe: changes in identified GnRH inputs and glial association

The annual reproductive cycle in sheep may reflect a functional remodeling within the GnRH system. Specifically, changes in total synaptic input and association with the polysialylated form of neural cell adhesion molecule have been observed. Whether seasonal changes in a specific subset(s) of GnRH inputs occur or whether glial cells specifically play a role in this remodeling is not clear. We therefore examined GnRH neurons of breeding season (BS) and nonbreeding season (anestrus) ewes and tested the hypotheses that specific (i.e. gamma-aminobutyric acid, catecholamine, neuropeptide Y, or beta-endorphin) inputs to GnRH neurons change seasonally, and concomitant with any changes in neural inputs is a change in glial apposition. Using triple-label immunofluorescent visualization of GnRH, glial acidic fibrillary protein and neuromodulator/neural terminal markers combined with confocal microscopy and optical sectioning techniques, we confirmed that total numbers of neural inputs to GnRH neurons vary with season and demonstrated that specific inputs contribute to these overall changes. Specifically, neuropeptide Y and gamma-aminobutyric acid inputs to GnRH neurons increased during BS and beta-endorphin inputs were greater during either anestrus (GnRH somas) or BS (GnRH dendrites). Associated with the changes in GnRH inputs were seasonal changes in glial apposition, glial acidic fibrillary protein density, and the thickness of glial fibrils. These findings are interpreted to suggest an increase in net stimulatory inputs to GnRH neurons during the BS contributes to the seasonal changes in GnRH neurosecretion and that this increased innervation is perhaps stabilized by glial processes.

The role of pheromones and biostimulation in animal reproduction

It is now known that pheromonal communication plays an important role in mammalian behaviour and reproductive processes. Chemical communication with pheromones is one means of transmitting such information. In mammals, signalling and priming pheromones are thought to act either singly or in combination through olfaction, auditory, visual (sight) or tactile stimuli. Pheromones are air-borne chemical substances ("signals") released in the urine or feces of animals or secreted from cutaneous glands that are perceived by the olfactory system and that elicit both behavioural and endocrine responses in conspecifics. Extensive studies in insects, rodents, swine, sheep, goats and cattle have established the importance of pheromones in the strong influence exerted by the male on reproductive activity in the female. There is a pheromone produced by the queen honey bee, which has two functions: inhibition of queen rearing and suppression of oogenesis in workers and in addition attracts drones during nuptial flight. It has also been demonstrated that the urine of male mice, rats, feral species and other wild rodents contains a priming pheromone that is responsible for hastening puberty in the females. Pheromones in the wool, wax and urine of a ram are sufficient to stimulate ewes to ovulate, while the buck has a strong characteristic seasonal odor and a buck jar containing the odor of the buck can be used as an aid in the detection of oestrus in does. The mere presence of the boar at the time of insemination of the sow improves sperm transport and ovulation, while the presence of the vasectomised bull has been reported to hasten the onset of puberty in heifers and also early resumption of ovarian activity in cattle following parturition. The role of pheromones in bovine reproduction is not as clearly defined as in sheep, goats and swine. Pheromones and other allelomimetic cues can exert profound effects on reproductive activity via the hypothalamic system that generates pulses of gonadotropin-releasing hormone. Manipulations of these factors and other pathways linking environmental inputs to reproductive output can lead to developing the concept of "control systems technologies", aimed at controlling reproductive performance. The knowledge acquired on the effectiveness of biostimulation; the factor which conditions it and the biological mechanism which produces it in livestock species, allows its use as a breeding management tool. The understanding of the role of pheromones could be of potential economic importance in addressing some of the problems associated with livestock production in the tropics. The biostimulation technique offers a potentially useful and practical way to improve reproductive efficiency in livestock species in the tropics. The exact nature of the cues and the role of biostimulation in livestock species especially swine, sheep, goats and cattle in developing countries require more attention.

Inhibition of hypothalamic GnRH secretion in the ewe by antigonadotropic decapeptide during the estrous cycle and nonbreeding season

Previous experiments from our laboratory and others have shown that the peptide antigonadotropic decapeptide (AGD) has marked inhibitory effects on luteinizing hormone (LH) secretion in rats and ewes. The first objective of this study was to determine whether AGD inhibits LH secretion by regulating hypothalamic release of gonadotropin hormone (GnRH). AGD (200 microg in 200 microL of 0.3% bovine serum albumin [BSA] saline) or vehicle was infused into the lateral ventricle of ovariectomized (OVX) ewes with hypophyseal-portal cannulae, and GnRH secretion was monitored. The frequency of GnRH and LH pulses in AGD-treated ewes was significantly decreased (p < 0.05) but did not change in the control ewes. The second objective of this investigation was to evaluate changes in hypothalamic sensitivity to AGD in the ewe during the estrous cycle and nonbreeding season. During the estrous cycle, the effects of AGD on LH secretion were assessed following ovariectomy, during the metestrous, diestrous, and proestrous phases of the estrous cycle. The response to AGD during the estrous cycle was compared to its effect during the anestrous season. LH, cortisol, and prolactin (PRL) concentrations were assayed in peripheral blood samples obtained at 10-min intervals over a 6-h period prior to injection of either vehicle (200 microL of 0.3% BSA in 0.9% saline) or AGD (200 microg in 200 microL of vehicle), and for an additional 10 h following treatment. LH pulse frequency decreased after treatment with AGD (p < 0.05) at all times in OVX and intact ewes compared to vehicle-treated controls. During the anestrous season, AGD treatment was more effective in inhibiting LH pulse frequency than during the breeding season (p < 0.05). Furthermore, there was a significant increase (p < 0.05) in mean cortisol concentrations after AGD infusion in all AGD-treated groups compared to controls independent of season or reproductive status. PRL concentrations were also increased (p < 0.05) following treatment with AGD. These results suggest that inhibition of pulsatile LH release induced by AGD is modulated by alterations in frequency of hypothalamic discharges of GnRH. Furthermore, changes in the inhibitory actions of AGD may contribute to the seasonal regulation of hypothalamic GnRH secretion in the ewe.

Physiological perspectives on leptin as a regulator of reproduction: role in timing puberty

How nutrition regulates reproductive activity remains a major unsolved question of reproductive biology. Reducing the level of nutrition during adulthood can lead to infertility, primarily through reduction of GnRH secretion. Inquiry about such a mechanism has its roots in the search for cues timing the onset of fertility, because the tempo of sexual maturation is much more closely associated with body growth than with chronological age. Growth depends on the quantity and quality of food intake. When food availability is low, small, short-lived species with high metabolism and reduced growth may not even attain puberty before they die. In longer-lived species, puberty is delayed for months or even years until more food becomes available. To appreciate fully how the pubertal progression is timed will require understanding how peripheral signals relating information about energy metabolism are sensed by the brain and how such information is routed through pathways controlling GnRH secretion. Here, we provide some background and physiologic perspective on the question of whether the fat-derived hormone leptin is the unique peripheral signal, is an important signal, is but one of a constellation of signals, or is not a signal timing puberty.

Effects of superovulation on endogenous LH secretion in cattle, and consequences for embryo production

Stimulation of follicular growth during superovulation is achieved by the injection of FSH or compounds with high FSH-bioactivities. However, some LH-activity is required for follicle maturation. It is of relevance to evaluate, therefore, the effect of superovulatory treatments on endogenous LH secretion. Luteinizing hormone is secreted in discrete pulses, and the pattern of pulsatile LH secretion during superovulation is reviewed. Four of five published studies have shown that LH pulse frequency is significantly reduced by injection of eCG or FSH preparations. This suppression appears within 8 h of treatment Effects of superovulation on LH pulse amplitude are less consistent. The reasons for the decrease in pulse frequency have been investigated, and although the answer is not definitive, it would seem that increased follicular estradiol, acting perhaps in synergism with progesterone, may play a role. Changes in plasma progesterone concentrations are not related to changes in LH pulse frequency. What is the significance of decreased LH pulse frequency? We attempted to investigate this by inducing LH pulses during superovulation, but the result was a major reduction in ovulation rate. More research is required to determine if modification of endogenous LH secretion can improve superovulatory responses.

Regulation of follicular activity and ovulation in ewes by exogenous progestagen

The success of estrus synchronization programs using progestagen sponges, particularly for fixed-time AI, varies considerably. In view of the recent evidence in cattle that exogenous progestins alter follicular dynamics, it may be that the stage of the estrous cycle at which the synchronization protocol is begun affects the synchrony of ovulation. The goal of this study was to evaluate the effect of medroxyprogesterone acetate (MAP) intravaginal sponges on follicular dynamics, luteal function and interval to ovulation when inserted at 3 stages of the estrous cycle. Sponges were inserted for 12 d beginning on either Day 0, 6 or 12 (n = 5) following ovulation. Ovarian activity was monitored using real-time ultrasound imaging during the treatment and the post-treatment estrous cycles. Information from the post-treatment cycle was used as a baseline to compare with the treatment cycle. Most ewes (79%) in the post-treatment cycle exhibited 3 follicular waves in an estrous cycle of 16 d, with the second wave follicles having smaller diameter (P < 0.001). Treatment with MAP increased the number of follicular waves from 3 to 4 or 5 when sponges were inserted on Days 6 and 12, respectively. Size of the largest follicle was smaller (P > 0.01) in waves in the early and middle of the 12-d MAP treatment period when compared with the last 4 days. This effect was most pronounced when endogenous progesterone concentrations were elevated concurrently with the presence of the sponge. Persistence of the ovulatory follicle was increased (P < 0.001) when sponges were inserted on Day 12, the only treatment where these follicles were under the influence of MAP in the absence of functional corpora lutea. Follicles were regressing at sponge removal in the Day 6 treatment, which resulted in a delay in emergence of ovulatory follicles, the LH surge and ovulation (P < 0.08) in relation to Day 0 and Day 12. Treatment with MAP sponges does not adequately synchronize estrus and ovulation among cyclic ewes due to the different follicular patterns that result depending on the stage of cycle at the time of sponge insertion.

Gonadotrophin and prolactin secretion in castrated male sheep following subcutaneous or intracranial treatment with testicular hormones

Interactions between testosterone, estradiol, and inhibin in the control of gonadotrophin secretion in males are poorly understood. Castrated rams were treated with steroid-free bovine follicular fluid (bFF), testosterone, or estradiol and for 7 d (2 x 2 x 2 factorial design). Given independently, none of the exogenous hormones affected follicle-stimulating hormone (FSH) concentrations, but the combination of one or both steroids with bFF reduced FSH secretion. Testosterone and estradiol reduced luteinizing hormone (LH) pulse frequency (there was no synergism), and bFF had no effect. Plasma prolactin concentrations were not affected by any treatment. To locate the central sites of steroid action, castrated rams were bilaterally implanted in the preoptic area (POA), ventromedial nucleus (VMH), or arcuate nucleus (ARC). These implants did not affect FSH or prolactin concentrations, or LH pulse amplitude. The frequency of the LH pulses was not affected by testosterone in any site. Estradiol located in the ARC, but not the POA or VMH, decreased LH pulse frequency. In summary, FSH secretion is controlled by synergistic interactions between inhibin and estradiol or testosterone, whereas GnRH/LH pulse frequency is controlled by testicular steroids. Estradiol acts partly, at least, in the ARC, but the central site of action, testosterone remains unknown.

Anastomosis between ovarian and mesenteric veins in sheep: induction of a physiological hyperstimulation of the ovary

Progesterone (P4) and estradiol 17 beta are essential for the dialogue between the ovary and the hypothalamohypophysial system. Immunization against steroids, although a valuable tool, displays limits. We propose a straightforward method to suppress steroids from the peripheral circulation by using the liver to catabolize steroids, through an anastomosis of the ovary to the mesenteric vein. Twelve mature cycling Ile-de-France ewes were unilaterally ovariectomized during the breeding season and subsequently randomly assigned to be anastomosed (n = 6; A) or sham-operated (n = 6; SO) on Day 10 of a synchronized estrous cycle. The ovarian vein was anastomosed to the superior mesenteric vein and all collateral veins to the ovarian vein were ligatured. Sham-operated ewes had only their collateral vein ligatured without anastomosis. Four days following surgery, ewes from both treatments were injected with PGF-2 alpha and autopsied between 9 and 14 days following surgery. Blood progesterone and estradiol 17 beta were measured daily from Day 4 before surgery until 7 days after castration. Estradiol levels measured at the level of the ovarian vein indicated that anastomosis allowed normal ovarian activity in all but 1 ewe in which collateral veins to the ovarian duct had developed. In the 5 A ewes, progesterone in peripheral blood decreased to low levels the day following surgery but this fall was not accompanied by an increase in estradiol. Estradiol levels measured at the output of the liver from hepatic vein were lower in A ewes (46.6 pg/ml) than in the ovarian vein of SO ewes (334.1 pg/ml). Ovarian hypertrophy was observed in 5 A ewes in which numerous large follicles were observed. These results indicate that the experimental model is functional and permits the study of the stimulation and hyperstimulation of the ovary and the control of the terminal follicular growth in the endocrine environment of the animal itself.

Stimulation of the production of progesterone by the corpus luteum of the ewe by the perfusion of melatonin in vivo and by treatment of granulosa cells with melatonin in vitro

The ability of melatonin to stimulate luteal steroidogenesis directly in the ewe was investigated by using a perfusion cannula system in vivo and luteinised granulosa cells in vitro. Exposure to human chorionic gonodotrophin (hCG) or ovine luteinising hormone (oLH), as positive controls, significantly stimulated progesterone secretion by the perfused corpus luteum within 20 minutes and by granulosa cells after two days of culture. Melatonin perfused through the corpus luteum at a physiological concentration, significantly stimulated progesterone secretion but the response was not as rapid as that to hCG. The significant stimulation of progesterone by melatonin in the perfusion system was maintained for between 30 and 60 minutes. Melatonin significantly stimulated progesterone production by granulosa cells in vitro after two days of culture, but the degree of response depended on the size of the follicles from which the cells were obtained. These results demonstrate that melatonin can act directly on the corpus luteum to increase progesterone production, and that its mechanism of action appears to be different from that of hCG or oLH in the ewe. This action of melatonin may be related to the reported improved luteal function late in the breeding season after the prolonged exposure of ewes to melatonin.

The circulating concentrations of FSH, LH and prolactin in the oestradiol-implanted ovariectomized ewe treated with caffeine

Caffeine, a trimethylxanthine alkaloid, is a psycho-active drug that effects a wide range of physiological systems, including the reproductive system. Reports of infants with intra-uterine growth retardation and lowered birth weight as a result of in utero exposure to caffeine, are increasing. The drug is also known to alter steroidogenesis but it is not certain whether this is a direct and/or an indirect effect with the involvement of the central nervous system. Thus, an experiment was designed to determine the effect of acute caffeine administration on the circulating concentrations of gonadotrophins and prolactin in the ovariectomized oestradiol-implanted ewe. A single intravenous dose of caffeine (20 mg kg-1 bodyweight) did not affect circulating gonadotrophin concentrations with the parameters for the pulsatile secretion of luteinizing hormone (LH) and the mean concentration of follicle stimulating hormone (FSH) being similar in both experimental and control groups. Circulating prolactin levels, on the other hand, were significantly (P < 0.01) elevated following intravenous treatment with caffeine. The effect was immediate following caffeine administration with elevated concentrations being maintained over the next 3 h before their return to pre-treatment concentrations. The response was bi-phasic with peaks of prolactin concentrations at 1 and 3 h. The results of this experiment show that acute caffeine exposure does not affect the secretion of gonadotrophins from the anterior pituitary gland. Furthermore, they show that acute administration of caffeine stimulates prolactin secretion via an action that is independent of oestradiol feedback and which we suggest, may involve the ACTH/adrenal axis.

A subset of estrogen receptor-containing neurons project to the median eminence in the ewe

The neural pathways responsible for conveying the steroid feedback signals that ultimately affect reproductive neuroendocrine function remain largely undefined. One possibility involves a direct projection from estrogen receptor (ER)-containing neurons to the median eminence (ME), a site of neuroendocrine peptide release. To examine this possibility, 8 ewes received stereotaxic injections of the retrograde neuronal tract-tracing compound cholera toxin-beta subunit (CT beta) into the ME. Neurons sending projections to the ME and containing ER were identified using a dual-label immunoperoxidase method. Double-labeled cells were found in distinct regions: (1) the ER-rich arcuate nucleus (ARC) that contained the greatest number of double-labeled cells, and (2) the organum vasculosum of the lamina terminalis (OVLT) which contained a very consistent, but low, number of double-labeled cells. While a fairly large number of retrogradely-labeled ARC neurons containing ER were identified, the majority of ER-containing ARC neurons were unlabeled and thus send projections elsewhere. Other regions containing high concentrations of ER-positive cells such as the medial preoptic area (MPOA), anterior hypothalamic area, and ventrolateral portion of the ventromedial hypothalamic nucleus, were devoid of double-labeled cells. Similarly, regions rich in neuroendocrine neurons such as the periventricular hypothalamus and paraventricular and supraoptic hypothalamic nuclei contained no double-labeled cells. These results suggest that modulation of neuroendocrine secretory activity may occur directly at the level of the ME by ER-containing neurons located within restricted regions of the hypothalamus and forebrain. However, the relatively low proportion of ER-containing neurons projecting to the ME suggests that the influence of estradiol upon neuroendocrine function also may include target sites other than the ME.

Evidence that testosterone and follicular fluid do not interact in the control of FSH secretion in rams

The hypothesis that testosterone and inhibin interact in the control of FSH secretion in rams was tested. Adult rams were castrated and were simultaneously given testosterone implants and 3-times daily sc injections of 0, 0.4, 0.8 or 1.6 ml charcoal-treated bovine follicular fluid (bFF). After 1 wk, the implants were removed, and the bFF injections continued as before. Blood samples were taken daily for mean LH, FSH and testosterone concentrations, and every 10 min for 12 h in the presence and in the absence of testosterone for assessment of pulsatile LH release. The bFF specifically inhibited FSH secretion from rat pituitary cells in culture. In the presence of testosterone, there were no main effects of bFF on mean plasma FSH or LH concentrations, nor were these values different from their pre-treatment means (P>0.05). Treatment with bFF did not affect LH pulse frequency or amplitude, but the number of rams showing LH pulses was reduced in the 0.8 and 1.6-ml dose groups (P<0.05). Removal of testosterone increased (P<0.05) both gonadotropins. In the absence of testosterone, no main effect of bFF on mean LH or FSH concentrations was observed, although the 1.6-ml dose suppressed the postcastration rise of both LH and FSH. These data suggest that inhibin does not interact with testosterone and that a physiological level of testosterone is sufficient for the regulation of FSH secretion in adult rams.

Effects of ovariectomy, gonadal steroid replacement and photoperiod on plasma β-endorphin in the ewe

The effects of gonadectomy, photoperiod and ovarian steroid replacement on plasma beta-endorphin concentrations were studied in 2 experiments using mature ewes. In Experiment 1 ovariectomy resulted in an increased plasma beta-endorphin concentration. Changing the photoperiod from long to short daylength had little effect on beta-endorphin, but transfer from short to long photoperiod caused a fall in the beta-endorphin level. In Experiment 2 ovariectomy again caused an increase in plasma beta-endorphin. Treatment with progesterone, estradiol or a combination of both failed to reduce the level to preovariectomy values, although LH concentrations were reduced. No diurnal rhythmicity in beta-endorphin secretion was observed.

Immunological approaches to fertility regulation in domestic livestock

Antibodies to hormones have been used extensively to study endocrine regulation of reproduction and in recent times they have been commercially exploited as therapeutic agents to manipulate reproduction. Despite the release of two commercial products, little is known about the mechanism of action of immunoneutralization. The presence of hormone-binding antibodies in the circulation and the detection of hormone antibody complexes in blood has led investigators to suggest a vascular site of action for immunoneutralization. However, a vascular site of action is difficult to reconcile with some of the known effects of immunoneutralization to the boar taint steroid (5 alpha-androst-16-en-3-one) to androstenedione and to gonadotrophin-releasing hormone. In this paper we review some of our recent research into the mechanism of action of androstenedione immunoneutralization. We suggest that hormonal immunoneutralization of androstenedione may take place in the extracellular compartment or within the plasma membrane where antibodies to androstenedione may bind to a specific steroid-binding protein in the plasma membrane of granulosa cells.