Lactational Anoestrus Research Articles

Kisspeptin and lactational anestrus: Current understanding and future prospects

Lactational anestrus, characterized by the suppression of pulsatile gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) release, would be a strategic adaptation to ensure survival by avoiding pregnancy during lactation in mammals. In the present article, we first provide a current understanding of the central regulation of reproduction in mammals, i.e., a fundamental role of arcuate kisspeptin neurons in mammalian reproduction by driving GnRH/LH pulses. Second, we discuss the central mechanism inhibiting arcuate Kiss1 (encoding kisspeptin) expression and GnRH/LH pulses during lactation with a focus on suckling stimulus, negative energy balance due to milk production, and the role of circulating estrogen in rats. We also discuss upper regulators that control arcuate kisspeptin neurons in rats during the early and late lactation periods based on the findings obtained by a lactating rat model. Finally, we discuss potential reproductive technology for the improvement of reproductive performance in milking cows.

Mechanisms of Lactation-induced Infertility in Female Mice.

Lactation in mammals is associated with a period of infertility, which serves to direct maternal metabolic resources toward caring for the newborn offspring rather than supporting another pregnancy. This lactational infertility is characterized by reduced pulsatile luteinizing hormone (LH) secretion and lack of ovulation. The mechanisms mediating suppression of LH secretion during lactation are unclear. There are potential roles for both hormonal cues such as prolactin and progesterone, and pup-derived cues such as suckling, on the inhibition of reproduction. To enable future studies using transgenic animals to investigate these mechanisms, in the present study our aim was to characterize lactational infertility in mice, and to investigate the effect of removing pup-derived cues on LH secretion, time to ovulation, and kisspeptin immunoreactivity. We first confirmed that C57BL/6J mice experience prolonged anestrus during lactation, which is dependent on establishment of lactation, as removal of pups the day of parturition led to immediate resumption of pulsatile LH secretion and normal estrous cycles. Once lactation is established, however, the lactational anestrus persisted for several days even after premature removal of pups. Pharmacological suppression of prolactin following premature weaning significantly reduced this period of lactational infertility. Progesterone does not appear to play a significant role in the suppression of fertility during lactation in mice, as levels measured during lactation were not different from nonpregnant mice. These data suggest that prolactin plays a key role in mediating anestrus during early lactation in mice, even in the absence of the suckling stimulus.

Neuroendocrinology applied to rabbit breeding

Successful rabbit production relies heavily on the use of adequate practices that enhance specific aspects of reproduction, such as mating, ovulation and lactation. Regardless of the type of production unit or strain of rabbits used, these processes rely on a complex chain of neuroendocrine steps that include particular hormones, peripheral stimuli and activation of discrete brain regions. Such is the case, for instance, of reflex ovulation, which occurs in response to copulation but is inhibited throughout lactation. Little is known about the mechanisms mediating lactational anoestrus and the restoration of oestrus following the cancellation of a single suckling episode (biostimulation). Nevertheless, the latter procedure (adopted worldwide to accelerate reproduction) has unwanted consequences for the doe and her litter. After successive episodes of biostimulation, the former shows a loss of fertility and body mass. In the kits, alterations are observed in their neuroendocrine response to mildly aversive stimulation in adulthood, as well as reductions in sexual behaviour. In addition to milk intake, a good nest is essential for normal litter growth and development. If this is not available, or if it deteriorates, rabbit caretakers can easily (re) build one from hair sheared off other rabbits or using synthetic material. Lactating does will nurse equally well their own or ‘alien’ young, placed inside the nest. It is crucial to have a minimum of six suckling kits in the nest, as the doe relies on this stimulation to maintain a normal nursing behaviour, i.e., only once a day throughout lactation. Recent work is revealing the similarities and differences in the responsiveness to mating among oestrous, lactating and biostimulated does. The relevance of these findings for the likelihood of reflex ovulation and the additional contribution of factors contained in the semen warrant more in-depth research. New insights on these issues, essential to reproductive neuroendocrinology, can emerge by fostering a richer interaction between academic laboratories and rabbit production settings worldwide.

Effects of physiological changes and social life events on adrenal glucocorticoid activity in female zoo-housed Asian elephants (Elephas maximus).

Ensuring good health and welfare is an increasingly important consideration for conservation of endangered species and includes breeding of individuals managed under human care. Understanding how factors in the captive environment affect individual animal wellbeing can be aided by long-term monitoring of biological functioning. This study involved longitudinal assessments (4 to 28 years) of reproductive and adrenal hormones in zoo-housed female Asian elephants (Elephas maximus) (age range 4 to ~71 years) to elucidate patterns in adrenal glucocorticoid (GC) activity in association with reproductive and demographic factors, and examine individual response to major social changes. Concentrations of serum and urinary cortisol covaried more consistently with physiological changes (ovarian cycle phase, puberty, pregnancy, lactational anestrus, and age) than with social life events (births, deaths, and facility transfers). Cortisol fluctuated across the ovarian cycle with mean concentrations being higher in the follicular than in the luteal phase, and concentrations were highest in lactational anestrous compared to all other reproductive states. The elephants in this study exhibited substantial individuality in adrenal GC response to major social change, reinforcing the need to assess welfare on an individual basis and to consider factors influencing the impact of perceived stressors, such as social relationships, social support, temperament, and life history. Outcomes from this study deepen our understanding of Asian elephant physiology and highlight the importance of taking intrinsic patterns of hormone secretion into account when evaluating the impact of external factors. Finally, a better understanding of the impact of social change and resiliency in response to real and perceived stressors allows us to improve social management to enhance welfare in both captive settings and free-ranging environments.

Somatostatin-Somatostatin Receptor 2 Signaling Mediates LH Pulse Suppression in Lactating Rats.

Follicular development and ovulation are profoundly suppressed during lactation in mammals. This suppression is suggested to be mainly due to the suckling-induced inhibition of kisspeptin gene (Kiss1) expression in the arcuate nucleus (ARC) and consequent inhibition of pulsatile GnRH/LH release. We examined whether central somatostatin (SST) signaling mediates the suckling-induced suppression of pulsatile LH secretion. SST has been reported to be expressed in the posterior intralaminar thalamic nucleus (PIL), where the suckling stimulus is postulated to be relayed to the hypothalamus during lactation. SST inhibitory receptors (SSTRs) are abundantly expressed in the ARC, where kisspeptin/neurokinin B/dynorphin A (KNDy) neurons are located. Histological and quantitative studies revealed that the suckling stimulus increased the number of SST-expressing cells in the PIL, and Sstr2 expression in the ARC. Furthermore, a central injection of an SSTR2 antagonist caused a significant increase in pulsatile LH release in lactating rats. Double labeling of Sstr2 and the neurokinin B gene, as a marker for ARC KNDy neurons, showed Sstr2 expression was abundantly detected in the ARC, but few KNDy neurons coexpressed Sstr2 in lactating rats. Taken together, these findings suggest the suckling-induced activation of SST-SSTR2 signaling mediates, at least in part, the suppression of pulsatile LH secretion during lactation in rats, probably via the indirect effects of SST on KNDy neurons. These results provide a new aspect on the role of central SST-SSTR signaling in understanding the mechanism underlying lactational anestrus.

Lactational anestrus in Caribbean donkeys

Lactational anestrus is common in many species, but not in equids. This case report documents an unusual finding of eight donkeys (from a group of 25) on the Caribbean island of St. Kitts that remained anestrus after foaling during the physiological breeding season. Twenty-five feral female donkeys in advanced pregnancy were acquired. No breeding history was known and all pregnancies were monitored via weekly transabdominal ultrasonography. All jennies foaled without complications and weekly transrectal ultrasonographic examinations were conducted to monitor uterine involution and cyclicity. Four foals died from septicemia within the first week and all of their dams soon re-established ovarian cyclicity. Thirteen jennies had a foal heat with overt signs of estrus within 2 weeks after foaling, then returned to normal cyclicity. However, the remaining eight jennies did not ovulate or display signs of estrus until their foals were weaned, ~4 months later. Following weaning, all eight jennies promptly returned to normal cyclicity. That these eight jennies returned to normal cyclicity following weaning supported our diagnosis of lactational anestrus during the physiological breeding season, a phenomenon apparently never reported in donkeys.

Controlling lactation oestrus: The final frontier for breeding herd management.

Lactation anoestrus limits the flexibility of modern pig production systems such that any increase in lactation length reduces farrowing frequency, and thus profit. This review focuses on post-partum development of the sow's reproductive system, the physiology of lactation anoestrus and how it can be overcome, as well as the fertility of sows mated while lactating. The propensity for sows to ovulate spontaneously while lactating is high (24-31%), and a high proportion of sows will ovulate rapidly and synchronously in response to combinations of altered suckling (split weaning, interrupted suckling), daily boar contact, exogenous gonadotrophins, and group housing. The apparent ease with which lactation anoestrus can be overcome represents an opportunity to uncouple sow mating from weaning, thus reducing the impact of lactation length on productivity. This is especially true when considering the benefits of the described stimulation methods on the reproductive performance (i.e., shorter weaning to oestrus intervals and higher litter sizes) of the low proportion of sows that maintain lactation anoestrus.

An opportunity to revolutionise sow management

Research in any area of animal production can provide the opportunity to change how the system operates and is managed. The reliance on having to wean lactating sows to re-mate them has limited the commercial options for sow management. The desire to limit lactation length to maximise the litters per sow per year concurrently creates major challenges for such-aged piglets weaned abruptly. These issues are discussed in the review. This management system also fails to recognise that sows have the potential to spontaneously ovulate in lactation even when housed in farrowing crates. Inhibition of luteinising hormone release is the basis of lactational anoestrus with the suckling stimulus providing the strongest afferent signal to this inhibitory system. Any management strategy that reduces this inhibition has the potential to trigger lactational oestrus. In this review, group housing of sows, boar exposure and intermittent suckling are identified as strong stimuli that can promote lactational oestrus. Removing the need to wean sows to mate them offers further opportunities to change the way lactating sows are managed. One option is a two-stage lactation system in which the sows are housed in farrowing crates for the first 10–14 days and then moved to group accommodation for the remainder of lactation. This system provides welfare benefits for the litter in the early stage of lactation and then the benefits of less confinement for the sows in later lactation. Group lactation would also lend itself to the implementation of stimuli to assist the mating of sows in lactation, such as piglet separation and/or boar exposure. It also accommodates the mating of sows that spontaneously ovulate in lactation. Removing the need to wean sows to re-mate them provides the opportunity to increase weaning age and implement a gradual weaning, helping to attenuate the post-weaning growth check and potentially limiting antimicrobial use in weaner pigs.

Genetics of early and lifetime annual reproductive performance in cows of two tropical beef genotypes in northern Australia

Reproduction records from 2137 cows first mated at 2 years of age and recorded through to 8.5 years of age were used to study the genetics of early and lifetime reproductive performance from two genotypes (1020 Brahman and 1117 Tropical Composite) in tropical Australian production systems. Regular ultrasound scanning of the reproductive tract, coupled with full recording of mating, calving and weaning histories, allowed a comprehensive evaluation of a range of reproductive traits. Results showed components traits of early reproductive performance had moderate to high heritabilities, especially in Brahmans. The heritability of lactation anoestrous interval in 3-year-old cows was 0.51 ± 0.18 and 0.26 ± 0.11 for Brahman and Tropical Composite, respectively. Heritabilities of binary reproductive output traits (conception rate, pregnancy rate, calving rate and weaning rate) from first and second matings were generally moderate to high on the underlying scale. Estimates ranged from 0.15 to 0.69 in Brahman and 0.15 to 0.34 in Tropical Composite, but were considerably lower when expressed on the observed scale, particularly for those traits with high mean levels. Heritabilities of lifetime reproduction traits were low, with estimates of 0.11 ± 0.06 and 0.07 ± 0.06 for lifetime annual weaning rate in Brahman and Tropical Composite, respectively. Significant differences in mean reproductive performance were observed between the two genotypes, especially for traits associated with anoestrus in first-lactation cows. Genetic correlations between early-in-life reproductive measures and lifetime reproduction traits were moderate to high. Genetic correlations between lactation anoestrous interval and lifetime annual weaning rate were –0.62 ± 0.24 in Brahman and –0.87 ± 0.32 in Tropical Composite. The results emphasise the substantial opportunity that exists to genetically improve weaning rates in tropical beef cattle breeds by focusing recording and selection on early-in-life female reproduction traits, particularly in Brahman for traits associated with lactation anoestrus.

Genetic correlations of young bull reproductive traits and heifer puberty traits with female reproductive performance in two tropical beef genotypes in northern Australia

Genetic correlations of young bull and heifer puberty traits with measures of early and lifetime female reproductive performance were estimated in two tropical beef cattle genotypes. Heifer age at puberty was highly (rg = –0.71 ± 0.11) and moderately (rg = –0.40 ± 0.20) genetically correlated with pregnancy rate at first annual mating (mating 1) and lifetime annual calving rate, respectively in Brahman (BRAH). In Tropical Composite (TCOMP), heifer age at puberty was highly correlated with reproductive outcomes from the first re-breed (mating 2), mainly due to its association with lactation anoestrous interval (rg = 0.72 ± 0.17). Scrotal circumference were correlated with heifer age at puberty (rg = –0.41 ± 0.11 at 12 months in BRAH; –0.30 ± 0.13 at 6 months in TCOMP) but correlations were lower with later female reproduction traits. Bull insulin-like growth factor-I was correlated with heifer age at puberty (rg = –0.56 ± 0.11 in BRAH; –0.43 ± 0.11 in TCOMP) and blood luteinising hormone concentration was moderately correlated with lactation anoestrous interval (rg = 0.59 ± 0.23) in TCOMP. Semen quality traits, including mass activity, motility and percent normal sperm were genetically correlated with lactation anoestrus and female lifetime female reproductive traits in both genotypes, but the magnitudes of the relationships differed with bull age at measurement. Preputial eversion and sheath scores were genetically associated with lifetime calving and weaning rates in both genotypes. Several of the early-in-life male and female measures examined were moderately to highly genetically correlated with early and lifetime female reproduction traits and may be useful as indirect selection criteria for improving female reproduction in tropical breeds in northern Australia.

Boar contact is an effective stimulant of ovulation during early lactation

The efficacy of 400IU equine chorionic gonadotrophin (eCG) and 200IU human chorionic gonadotrophin (hCG) (PG600; Intervet, Australia) and boar exposure as stimuli of oestrus and ovulation have been established in pre-pubertal gilts and weaned sows, but there is little published data on their use to overcome lactation anoestrus. Designed as a 2×2 factorial, the current study used 40 Large White/Landrace sows (parity 1.9±0.14), to determine the effects of boar exposure (BE) versus NoBE and PG600 versus NoPG600 on the incidence of lactation oestrus and ovulation. On day 1 of lactation, PG600 was administered and BE commenced. Throughout lactation all sows were housed individually in farrowing crates (0.6×2.4m2). BE consisted of 15min of daily, full physical boar exposure in a detection mating area. For all sows, litter size suckled was standardized to 10 piglets on day 1 of lactation, and maintained at this level until weaning on day 26 post-partum. Oestrus detection was performed daily, with oestrus defined as the exhibition of a standing reflex, and ovulation confirmed when progesterone concentrations exceeded 1.0ng/ml on day 3 and 4.0ng/ml on day 10 post-oestrus detection. There was no effect of PG600 on any reproductive measures, and no interactions with the BE treatment. A higher proportion of BE compared to non-BE sows exhibited a behavioural oestrus without ovulation within 5 days of parturition (0.83 versus 0.09; P<0.05). The interval from parturition to lactation ovulation was unaffected by treatment (15.4±1.21 days). However, BE increased the proportion of sows ovulating during lactation (0.61 versus 0.09; P<0.05), and resulted in an earlier ovulation relative to parturition (20.6±1.30 versus 30.1±1.35 days) compared to non-BE sows. In conclusion, this is the first study to demonstrate that taking lactating sows to a detection mating area for 15min of daily, physical boar contact resulted in a high incidence of lactation ovulation within 22 days of parturition.

Reproductive cycles in pigs

The oestrous cycle in pigs spans a period of 18–24 days. It consists of a follicular phase of 5–7 days and a luteal phase of 13–15 days. During the follicular phase, small antral follicles develop into large, pre-ovulatory follicles. Being a polytocous species, the pig may ovulate from 15 to 30 follicles, depending on age, nutritional status and other factors. During the luteal phase, follicle development is less pronounced, although there is probably a considerable turnover of primordial to early antral follicles that fail to further develop due to progesterone inhibition of gonadotrophic hormones. Nevertheless, formation of the early antral follicle pool during this stage probably has a major impact on follicle dynamics in the follicular phase in terms of number and quality of follicles. Generally, gilts are mated at their second or third estrous cycle after puberty. After farrowing, pigs experience a lactational anoestrus period, until they are weaned and the follicular phase is initiated, resulting in oestrus and ovulation 4–7 days after weaning. This paper describes the major endocrine processes during the follicular and luteal phases that precede and follow ovulation. The role of nutrition and metabolic status on these processes are briefly discussed.

Do cyclic female goats respond to males with an increase in LH secretion during the breeding season?

The male effect is currently only used during seasonal or lactational anoestrus because the response is thought to be blocked in cyclic females by periods of elevated progesterone. In this study, we tested whether cyclic, female goats would respond to male exposure with an increase in pulsatile LH secretion. During May (breeding season; Southern Hemisphere) the cycles of 16 Australian Cashmere goats were synchronised using intravaginal progesterone pessaries. Pessary insertion was staggered to produce groups in their early luteal (EL; n = 8) and late luteal phases (LL; n = 8). The LL group was retrospectively subdivided into mid-luteal (ML; n = 4) and late luteal (LL; n = 4) groups due to differences in oestrous cycle length that emerged during the study. Male exposure stimulated an increase in LH pulse frequency in the EL and LL groups ( P < 0.01) but not in the ML group ( P > 0.1). This increase was accompanied by an increase in basal and mean concentrations of LH in the LL group ( P < 0.05) but not in EL ( P < 0.1) or ML ( P > 0.1) group. There was no effect of male exposure on LH pulse amplitude ( P > 0.1). Progesterone concentrations differed among all groups on the day of male exposure ( P < 0.05) and declined significantly over the 12-h sampling period in the LL group ( P < 0.05). Prolactin concentrations declined in the EL group but did not change significantly in the ML or LL group. In conclusion, male exposure induced an increase in pulsatile LH in goats in the early and late luteal phases of the oestrous cycle. The high concentrations of progesterone in females in the mid-luteal phase appeared to block the male effect.

Synchrony and timing of breeding influences sexual segregation in western grey and red kangaroos (Macropus fuliginosus and M. rufus)

AbstractWe examined whether synchrony and timing of breeding influenced magnitude and timing of sexual segregation in sympatric western grey kangaroos Macropus fuliginosus and red kangaroos M. rufus, at Hattah‐Kulkyne National Park, in south‐eastern Australia. These species vary in the synchrony and timing of their mating activity: western grey kangaroos mate during a relatively synchronous period, predominantly during spring, while red kangaroos mate throughout the year. Surveys were conducted during autumn and spring 2000 to examine seasonal patterns of mating activity, group composition, spatial distribution, and habitat selection of adults of both species. Habitat selection was also examined by analysing data from culls conducted from 1998 to 2002. Mating activity occurred synchronously in western grey kangaroos, thus, like in many ungulate species, there was a time of peak segregation, during autumn, when females were in lactational anoestrus, and a time of peak aggregation, during the period of peak mating activity in spring. In red kangaroos, mating activity occurred during both seasons, but showed a slight peak during autumn. Consequently, segregation was weaker than in western grey kangaroos but occurred through‐out the year, with a slight peak during spring. The degree of social and spatial segregation in western grey and red kangaroos was comparable to that in ungulates, while habitat segregation was considerably lower than is typically seen in ungulates. These results indicate that breeding phenology is a major factor involved in sexual segregation, and highlight the importance of directing attention at the general conditions underlying this phenomenon.

Lactation and lactational effects on metabolism and reproduction in the horse mare

Prolactin plays a major role for lactogenesis and for the initiation but not the maintenance of lactation in the horse. While experimentally supplemented or naturally occurring dopamine agonists inhibit the onset of lactation in the mare via a reduction in prolactin release, treatment with dopamine antagonists stimulates lactation. In lactating mares, follicular growth and ovulation are resumed early postpartum and are not prevented by suckling or by the presence of a foal. Lactational anoestrus does not exist as a physiological condition in the horse. Mares are able to meet the increased energy expenditure during lactation mainly by increasing feed intake while mobilisation of body reserves is limited. Plasma leptin concentrations are reduced after foaling. Plasma IGF-1 levels increase in late gestation, reach a maximum at foaling and decrease gradually thereafter. After foaling, LH release increases and reaches levels comparable to non-lactating mares. This increase, together with simultaneously high IGF-1 concentrations, may stimulate the rapid resumption of cyclic ovarian activity within less than 2 weeks after foaling.

Interaction between lactation, photoperiodism and male effect in German Merino ewes

A study with 93 German Merino ewes was performed from January until the end of March to clarify the relative importance of lactation, photoperiodism and ram effect on cyclic activity and lambing data. Ovarian activity was registered by progesterone concentrations in blood plasma three times weekly. Half of the ewes were kept under supplemental light (20 h/day) for the last 6 weeks of lactation and additionally 3 weeks post-weaning, the other half were kept under natural photoperiod but were weaned simultaneously. Thereafter, light was reduced to natural photoperiod and rams were introduced to half of the ewes, of both light reduced and photoperiod group. Ewes entered cyclicity during lactation gradually, but at weaning 56% of photoperiod ewes and 53% of supplemental light ewes were still acyclic. After weaning, resumption of cyclic activity before ram introduction was more pronounced ( P < 0.05) in the photoperiod group (75% cyclic) than in the supplemental light group (51% cyclic). Ram introduction led to cyclicity in all ewes. Light reduction without ram slightly increased cyclicity but 57% were still acyclic. In the photoperiod group without ram no ewe entered cyclicity and two ewes even ceased cycling again. Data show that German Merinos still have a remarkable lactational anoestrus but are extremely sensitive to ram. Light reduction has no direct effect on cyclicity but is likely to contribute to the elevated ovulation rate so that a combination with the ram effect led to a higher lambing rate (1.94) compared to photoperiod and ram (1.55).

Influence of nutrition and lactation on ovulation and conception in red deer

Reproduction incurs a high energetic cost and failure to meet nutritional needs results in reproductive failure. In red deer hinds, the interaction between nutrition, summer lactation and subsequent ovulation/conception is of particular interest for farmed deer production in NZ due to climatic constraints to summer pasture production in many areas. However, few studies have focussed on the potential perturbing effects of poor nutrition on lactational outputs and potential flow-on effects on ovulation/conception. As lacta tion in red deer naturally persists well beyond the autumn mating period, lactational anoestrus (through hormonal feedback mechanisms) is not a normal feature of the species reproduction. However, lactation is of high nutritional cost to the hind and can potentially exert considerable influence on the incidence and timing of conception via changes in hind liveweight, lipid depots and general body condition. Supporting evidence for this hypothesis is derived from studies of wild red deer within habitats marginal to species survival (e.g. Isle of Rhum, Scotland), where climatic variables greatly influence annual reproductive success of hinds. Under such conditions, even the differences in maternal investment (via lactation) required to raise male and female calves have been demonstrated to influence the subsequent success and timing of conceptions. Studies of farmed red deer in Scotland also demonstrated that hinds on different levels of pasture nutrition during lactation exhibited significant differences in conception date, with earlier conceptions favouring hinds on higher levels of nutrition. There is a growing body of evidence from monitoring of farmed red deer in New Zealand that the interaction between lactation and hind body condition score (BCS)/ liveweight influences subsequent conception rate and date. Recent studies on pre-rut versus post-rut calf weaning practices demonstrated effects of lactation on hind BCS and liveweight, calf growth rates and hind conception dates. These studies highlight the need to better understand the "costs" of lactation on reproductive performance, particularly under conditions of nutritional constraints. Keywords: lactation, nutrition, red deer, reproduction

Chapter 13 Non-metabolic and metabolic factors causing lactational anestrus: rat models uncovering the neuroendocrine mechanism underlying the suckling-induced changes in the mother

Follicular development and ovulation are strongly inhibited during lactation. Administration of a high dose of estrogen induces luteinizing hormone (LH) surges in ovariectomized lactating rats, suggesting that brain mechanisms regulating cyclic LH release remain intact in lactating mothers. On the other hand, tonic LH release is profoundly suppressed in lactating rats. This suggests that lactational anestrus is mainly due to suppression of the mechanism regulating pulsatile gonadotropin-releasing hormone secretion in the hypothalamus, which is responsible for follicular development and steroid production. Both metabolic and non-metabolic factors are involved in suppressing pulsatile LH secretion throughout lactation in rats. During the first half of lactation, pulsatile LH secretion is strongly suppressed, even if milk production is attenuated by pharmacological blockade of prolactin secretion in ovariectomized lactating rats. Pulsatile LH release quickly recovers by removing pups or blocking neuronal input by hypothalamic deafferentation during the period. These data suggest that the suckling stimulus itself is responsible for suppression of LH release during the first half of lactation. During the second half of lactation, negative energy balance, which is caused by the milk production, appears to play a dominant role in suppressing LH secretion. Blockade of milk production by inhibiting prolactin release causes a gradual increase in LH release even if the vigorous suckling stimulus by foster pups remains. In conclusion, the suckling stimulus itself predominantly suppresses LH pulses during the first half of lactation and metabolic factors take over the role of the suckling stimulus during the second half of lactation.

Effect of acute food deprivation on lactational infertility in rats is reduced by leptin administration.

The goals of these experiments were to determine whether lactational anestrus would be prolonged by a 48-h fast at days 13 and 14 postpartum (pp) and, if so, to determine whether this effect could be reversed by treatment with the Ob protein leptin. We found that food deprivation on days 13 and 14 pp prolonged lactational infertility by 7 days and that the nutritional experience of both the dam and her litter contributed to this effect. Leptin administration (2.5 mg . kg-1 . day-1) during food deprivation was sufficient to reduce the length of lactational infertility compared with vehicle-treated food-deprived rats (P < 0.05). Similar leptin treatment in ad libitum-fed animals reduced food intake (P < 0.05) and litter growth (P < 0.05) but had no statistically significant effect on maternal weight gain or length of lactational infertility. Food-deprived lactating animals had lower circulating leptin levels than ad libitum-fed lactating animals on day 15 pp (P < 0.05), as determined by RIA. Levels in nonlactating rats were higher than in either lactating group (P < 0.05).

Bromocriptine given orally to periparturient of lactating sows inhibits milk production.

Gilts were assigned as controls (CTL, n = 6) or received orally 10 mg of bromocriptine thrice daily from d 110 of gestation until farrowing, (BRPP, n = 7), from d 1 to 6 postpartum (PP) (BRL1, n = 6), from d 7 to 13 PP (BRL2, n = 7), from d 14 to 20 PP (BRL3, n = 6), or from d 21 to 27 PP (BRL4, n = 6). Weights of pigs were recorded at birth, 24 h later, and on d 7, 14, 21, 28, 31 (weaning), 42, and 56 PP. Jugular blood samples were collected from sows on d 109 of gestation and every other day until farrowing, as well as on d 1, 6, 13, 20, and 27 PP. Behavioral observations of sows and litters were taken every 3 min for a 24-h period beginning 48 h after the onset of the treatments. In experimental sows, bromocriptine induced marked reductions in prolactin levels during treatment (P < .001). Compared to CTL sows, concentrations of IGF-I were higher at d 21 (P = .01) and 28 (P = .003) PP in BRL3 and BRL4 sows, respectively. In bromocriptine-treated sows, weight gain of litters was either drastically reduced or abolished (P < .001) during the week of treatment. Treatments also altered significantly the suckling behavior of pigs at all stages studied. Therefore, the present results strongly suggest that prolactin is essential for the initiation and the maintenance of milk production in sows. Results also indicate that prolactin does not seem to be involved in the maintenance of lactational anestrus during a 4-wk lactation.