Melatonin Treatment at Dry-off Improves Reproductive Performance Postpartum in High-producing Dairy Cows under Heat Stress Conditions

Melatonin administration during the first half of pregnancy improves physiological response and reproductive performance of rabbits under heat stress conditions

Context Melatonin may have a heat-stress-alleviating role during pregnancy. Aims To investigate the effects of melatonin administration during the first half of pregnancy on heat-tolerance capacity and pregnancy outputs of naturally heat-stressed rabbits. Methods Forty female rabbits were stratified equally into two experimental groups and daily received 1mg melatonin/kg body weight or not (control) for 15 consecutive days post-insemination. Heat tolerance indices, hormone profile, ovarian structures, and fetal loss were determined. Key results Treatment with melatonin significantly decreased respiration rate and rectal temperature, improved concentrations of nitric oxide, and tended to decrease malondialdehyde concentrations (P =0.064) compared to control. Melatonin treatment significantly increased concentrations of high-density lipoprotein, oestradiol, and progesterone compared to control. No significant differences in the numbers of visible ovarian follicles, corpora lutea, and total implantation sites on day 18 of pregnancy were observed between experimental groups. However, melatonin treatment significantly reduced the number of absorbed implantation sites and significantly improved amniotic fluid volume and conception rate compared to control. Conclusions Melatonin administration during the first half of pregnancy can improve reproductive performance of heat-stressed female rabbits. Implications Melatonin can improve fetal survivability via improving heat-tolerance capacity of does and steroidogenesis.

Melatonin in Health and Disease: A Perspective for Livestock Production

Mounting evidence in the literature indicates an important role of endogenous and exogenous melatonin in driving physiological and molecular adaptations in livestock. Melatonin has been extensively studied in seasonally polyestrous animals whereby supplementation studies have been used to adjust circannual rhythms in herds of animals under abnormal photoperiodic conditions. Livestock undergo multiple metabolic and physiological adaptation processes throughout their production cycle which can result in decreased immune response leading to chronic illness, weight loss, or decreased production efficiency; however, melatonin’s antioxidant capacity and immunostimulatory properties could alleviate these effects. The cardiovascular system responds to melatonin and depending on receptor type and localization, melatonin can vasodilate or vasoconstrict several systemic arteries, thereby controlling whole animal nutrient partitioning via vascular resistance. Increased incidences of non-communicable diseases in populations exposed to circadian disruption have uncovered novel pathways of neurohormones, such as melatonin, influence health, and disease. Perturbations in immune function can negatively impact the growth and development of livestock which has been examined following melatonin supplementation. Specifically, melatonin can influence nutrient uptake, circulating nutrient profiles, and endocrine profiles controlling economically important livestock growth and development. This review focuses on the physiological, cellular, and molecular implications of melatonin on the health and disease of domesticated food animals.

Importance of Antioxidant Supplementation during In Vitro Maturation of Mammalian Oocytes

The in vitro embryo production (IVEP) technique is widely used in the field of reproductive biology. In vitro maturation (IVM) is the first and most critical step of IVEP, during which, the oocyte is matured in an artificial maturation medium under strict laboratory conditions. Despite all of the progress in the field of IVEP, the quality of in vitro matured oocytes remains inferior to that of those matured in vivo. The accumulation of substantial amounts of reactive oxygen species (ROS) within oocytes during IVM has been regarded as one of the main factors altering oocyte quality. One of the most promising approaches to overcome ROS accumulation within oocytes is the supplementation of oocyte IVM medium with antioxidants. In this article, we discuss recent advancements depicting the adverse effects of ROS on mammalian oocytes. We also discuss the potential use of antioxidants and their effect on both oocyte quality and IVM rate.

Alleviative effect of melatonin on the decrease of uterine receptivity caused by blood ammonia through ROS/NF-κB pathway in dairy cow

High concentration of blood ammonia can affect the uterus receptivity and decrease fecundity in dairy cow. Melatonin can reduce reactive oxygen species (ROS) level and has antioxidant and anti-inflammatory effects. However, it is not clear whether melatonin can alleviate ammonia-induced apoptosis of endometrial epithelial cell (EEC) and reduced uterus receptivity. The bovine EEC were treated with ammonium chloride and/or melatonin. Cell viability, apoptosis, oxidative stress and mitochondrial membrane potential were measured and the expression of apoptosis-related genes (p53, Cyt-c, Bax, Bcl-2, caspase-8, caspase-9 and caspase-3), uterus receptivity related genes (VEGF, LIF and EGF) and inflammatory factors (TLR-4, IL-6 and NF-κB) were detected. In addition, the expression of VEGF was detected after adding NF-κB inhibitor (40 μM) and IL-6 (1 ng/mL and 50 ng/mL). The results showed that ammonia significantly increased intracellular ROS level, mRNA and protein expression of Bax, p53, Cyt-c, caspase-9, caspase-8, caspase-3, TLR-4, NF-κB and IL-6, promoted cell apoptosis, while decreased mitochondrial membrane potential, the mRNA and protein expression of VEGF and EGF. Interestingly, melatonin significantly mitigated ammonia-induced changes. However, melatonin could not alleviate ammonia-induced changes of IL-6 and VEGF when NF-κB signal pathway was inhibited. The addition of IL-6 significantly reduced mRNA and protein expression of VEGF. In conclusion, ammonia induced EEC apoptosis through ROS production and activation of mitochondrial apoptosis pathway, and induced inflammatory response through TLR4/NF-κB/IL-6 pathway. Melatonin alleviated EEC apoptosis by inhibiting ROS pathway, and reduced IL-6 expression by inhibiting TLR-4/NF-κB signal pathway, which eventually improved VEGF expression and uterus receptivity in dairy cows.

Melatonin and CIDR improved the follicular and luteal haemodynamics, uterine and ovarian arteries vascular perfusion, ovarian hormones and nitric oxide in cyclic cows

This study hypothesizes that melatonin with exogenous progesterone (CIDR) can improve follicular, luteal, ovarian and uterine haemodynamic of heat-stressed cows. Holstein cows (N = 12) studied for two spontaneous oestrous cycles during winter then divided equally during summer into the CIDR group received CIDR for 7 days and the melatonin group (Mel) received three injections of melatonin (75 mg/head) at the CIDR insertion, removal and ovulation days. Blood samples were collected to assay oestradiol (E2), progesterone (P4) and nitric oxide (NO). On day 0 (Ovulation), Mel had more small follicles (p < .05), higher ipsilateral and contralateral ovarian arteries (Ov.A.) peak systolic velocity (PSV), higher ipsilateral uterine artery (Ut.A.) PSV (p = .031) and blood flow volume (BFV), also Mel elevated contralateral Ut.A. PSV and BFV (p < .0001) but lowered contra Ut.A. pulsatility index (PI, p < .0001), E2 (p < .01) and NO (p < .0001). Mel increased the corpus luteum diameter (CL, p < .001), coloured area (p < .007) and P4 (p < .0001) on day 5 and reduced them (p < .05; p < .01) on Day 14. On day 10, Mel obtained CL diameter (p < .03) and coloured area (p < .002) of spontaneous that was higher than CIDR and decreased P4 (p < .003). Mel increased CL diameter, area and coloured area and decreased them thereafter. Mel increased the ipsilateral ovarian and uterine arteries PSV and BFV before ovulation and until day 8. Mel increased P4 and decreased NO until days 6 and 14. In conclusion, the improvement in follicular, luteal, ovarian and uterine haemodynamic and the decrease of NO production proved our hypothesis Melatonin doses higher than 75 mg/head is recommended to improve the heat-stressed cow's fertility.

Molecular, functional, and cellular alterations of oocytes and cumulus cells induced by heat stress and shock in animals

Global warming is considered as the main environmental stress affecting ecosystems as well as physiological and biochemical characteristics, and survivability of living organisms. High temperature induces various stresses and causes reduction of fertility through reducing the oocyte developmental competence and alteration in surrounding cells' functions. This causes major economic loss to livestock creating a selective pressure on animals to the advantage of better adapted genotypes and to the detriment of others. In this review, a search in Science Direct, Google Scholar, PubMed, Web of Science, Scopus, and SID databases until 2020 was conducted. Keywords which include heat stress, shock, high temperature, oocyte, cumulus, and animals were investigated. Studies have exhibited that heat stress can disturb the development and function of oocyte and cumulus cells (CCs) concerning reproductive efficiency. Heat stress has deleterious consequences on oocyte maturation and development via reduced number of polar body extrusion, adenosine monophosphate, and guanosine monophosphate synthesis. Heat stress caused the alteration of cytoplasmic and nuclear features as well as trans-zonal projections and gap junctions. In addition, heat stress is accompanied with reduced mitochondrial activity (copy mDNA number, distribution, and membrane potential) in cumulus-oocyte complexes. This review targets the description of results in the most recent studies that aimed to call attention to the influences of heat stress on molecular, functional, and cellular changes in oocytes and CCs in animals to design evidence on the acting mechanisms as the core of this problem from a comparative review.

Melatonin slightly alleviates the effect of heat shock on bovine oocytes and resulting blastocysts

Heat stress is associated with increased production of reactive oxygen species (ROS) and disruption of bovine oocyte function. Here, we examined whether the antioxidant melatonin can alleviate the deleterious effects of heat stress on oocyte developmental competence. Cumulus-oocyte complexes were matured for 22 h at 38.5 °C (control) or for 22 h at 41.5 °C (heat shock) with or without 1.0 × 10^-7 M melatonin. At the end of maturation, a subgroup of oocytes was examined for nuclear and cytoplasmic maturation, ROS level and mitochondrial membrane potential. A second subgroup of oocytes underwent fertilization (18 h), and putative zygotes were cultured in an incubator equipped with a time-lapse system for ∼190 h. Cleavage rate and the proportion of blastocysts, as well as embryo kinetics were recorded. Expanded blastocysts were collected and their transcript abundance was evaluated. Heat shock increased ROS and reduced the proportion of oocytes that resumed meiosis and reached the metaphase-II stage. Exposing oocytes to heat shock with melatonin alleviated these effects to some extent, expressed by a marginal reduction in ROS level and increased proportion of metaphase-II stage oocytes. Neither the distribution of oocyte cortical granules nor polarization of the mitochondrial membrane differed between control and heat-shocked oocytes cultured with or without melatonin. Heat shock reduced the proportion of embryos that cleaved and developed to blastocysts, characterized by alterations in kinetics of the developed embryos expressed by a delay in the first cleavage, second cleavage and blastocyst formation for heat-shock vs. control groups. Melatonin did not restore the competence or kinetics of embryos developed from heat-shocked oocytes. However, expanded blastocysts developed from heat-shocked oocytes treated with melatonin expressed a higher transcript abundance of genes associated with mitochondrial function, relative to the control and heat-shock group. In summary, melatonin improved the oxidative status of heat-shocked oocytes to some extent and had a beneficial effect on maternal mitochondrial transcripts in the developed blastocysts.

To What Extent Does Photoperiod Affect Cattle Reproduction? Clinical Perspectives of Melatonin Administration – A Review

The seasonality of reproduction in most mammals is dictated by photoperiod, temperature and nutrition. Melatonin, mainly synthesized in the pineal gland, is generally accepted as the active mediator of photoperiod responses including reproduction. While non-pregnant heifers and cows show continuous sexual activity and are therefore not seasonal breeders, it has been suggested that photo-periodicity may influence the appearance of puberty in heifers and the onset of parturition. Further, the light/dark ratio may influence endocrine patterns of gestation and a shorter light period correlates with the incidence of twin pregnancies. This review considers specific aspects of the effects of photoperiod and melatonin on reproduction in dairy cattle and discusses the clinical applications of melatonin.

Thermoprotective molecules to improve oocyte competence under elevated temperature

Heat stress is an environmental factor that challenges livestock by disturbing animal homeostasis. Despite the broad detrimental effects of heat stress on reproductive function, the germline and the early preimplantation embryo are particularly prone. There is extensive evidence that elevated temperature reduces oocyte developmental competence through a series of cellular and molecular damages. Further research revealed that the oocyte respond to stress by activating cellular mechanisms such as heat shock response, unfolded protein response and autophagy to improve survival under heat shock. Such knowledge paved the way for the identification of thermoprotective molecules that alleviate heat-induced oocyte oxidative stress, organelle damage, and apoptosis. Therefore, this review depicts the deleterious effects of heat shock on oocyte developmental competence, heat-induced cellular and molecular changes, outlines pro-survival cellular mechanisms and explores thermoprotective molecules to improve oocyte competence.

Reproductive physiology and endocrinology responses of cows exposed to environmental heat stress - Experiences from the past and lessons for the present

The effects of environmental heat-stress on production and reproduction in dairy cows have been intensively studied throughout the past few decades. In light of climate changes and global warming, this issue has gained attention worldwide. So far, most of the documentations are related to warmer-climate regions, however, environmental thermal stress has recently been reported in cooler regions, such as Europe. The review attempts to present the experiences from the past years and lessons for the present. The review highlights some of the environmental characterizations and provides some practical approaches to estimate the level of heat load on farms. For instance, the intensity of heat stress can be evaluated by the temperature humidity index (THI). Other environmental parameters, such as the increased number of consecutive hot days or the increased frequency of extremely hot days, can be also used to estimate the level of heat load on farms. Exposure of dairy cows to environmental thermal stress results in multiple behavioral changes, physiological responses and endocrinological alterations, which in sequence, lead to reduced reproductive performance. Multiple in-vitro studies have been performed for better understanding the mechanism by which heat stress impairs reproductive processes. However, the current review focuses mainly on animal reactions and on the limitations of physiological and behavioral responses in maintaining normothermia, without human intervention. The review provides evidence that thermal stress induces alterations in the hypothalamus-pituitary-ovarian axis. For instance, impaired gonadotropin secretion, attenuation of follicular development, reduced steroid production and progesterone concentration in the plasma. These were found to be associated with impaired estrus behavior, reduced oocyte developmental competence and embryo survival. Heat stress also has direct and indirect effects on the preimplantation embryo. The review summarizes the thermo-sensitivity of the embryo and the acquisition of its thermotolerance through early developmental stages. Understanding the effects of environmentally elevated temperature on the reproductive physiology of lactating cows is extremely important for the development of new strategies in order to mitigate the effects of heat stress on farms. The review also provides various types of management and practical tools, in order to alleviate the effects of thermal stress. It introduces some approaches that have been developed during recent years, ones that have been practically used to alleviate the effect of the environmental heat load and suggested to be implanted. Cooling is the predominant strategy used nowadays in order to alleviate the effects of heat stress. It includes indirect cooling of the environment surrounding the animal, by providing shed and ventilation (with or without water) or direct evaporative cooling of the cow with water and fans. Using an efficient cooling system can improve milk production during the hot season, but it cannot eliminate the decline in reproduction. The review also discusses some additional approaches such as timed artificial insemination, hormonal treatment and embryo transfer, which have already been developed. These are suggested to be examined, adapted and implemented in dairy farms located in new regions that have recently suffered from environmental heat stress. The review also discusses unclear points and open questions some of which might lead new research directions.

Effects of Melatonin Administration to Pregnant Ewes under Heat-Stress Conditions, in Redox Status and Reproductive Outcome

Heat stress is a known promoter of reactive oxygen species generation, which may compromise pregnancy and foetal development. Melatonin is a pleiotropic molecule that regulates various processes including pregnancy. Thus, it could be used to ameliorate the redox status of pregnant heat-stressed ewes and the outcome of their pregnancy. Sixty-eight ewes participated in the study, which were allocated into two equal groups, i.e., Melatonin (M) and Control (C) group. All ewes were exposed to heat stress from D0 to D120. In both groups, after oestrus synchronization of ewes, rams were introduced to them for mating (D16). In M group, starting with sponges’ insertion (D0), melatonin implants were administered four-fold every 40 days. Pregnancy diagnosis was performed by means of ultrasonography. Daily evaluation of temperature humidity index (THI), rectal temperature, and breathing rate were performed throughout the study. Blood samples were collected repeatedly from D0 until weaning for assaying redox biomarkers. Milk yield was measured thrice during puerperium. The results showed that melatonin administration throughout pregnancy improved the redox status of heat-stressed ewes and increased the mean number and bodyweight of lambs born per ewe, as well as the milk production. Therefore, melatonin may be used as antioxidant regimen in heat-stressed ewes for improving their reproductive traits.

Reproductive physiology of the heat-stressed dairy cow: implications for fertility and assisted reproduction

Heat stress causes a large decline in pregnancy success per insemination during warm times of the year. Improvements in fertility are possible by exploiting knowledge about how heat stress affects the reproductive process. The oocyte can be damaged by heat stress at the earliest stages of folliculogenesis and remains sensitive to heat stress in the peri-ovulatory period. Changes in oocyte quality due to heat stress are the result of altered patterns of folliculogenesis and, possibly, direct effects of elevated body temperature on the oocyte. While adverse effects of elevated temperature on the oocyte have been observed in vitro, local cooling of the ovary and protective effects of follicular fluid may limit these actions in vivo. Heat stress can also compromise fertilization rate. The first seven days of embryonic development are very susceptible to disruption by heat stress. During these seven days, the embryo undergoes a rapid change in sensitivity to heat stress from being very sensitive (2- to 4-cell stage) to largely resistant (by the morulae stage). Direct actions of elevated temperature on the embryo are likely to be an important mechanism for reduction in embryonic survival caused by heat stress. An effective way to avoid effects of heat stress on the oocyte, fertilization, and early embryo is to bypass the effects through embryo transfer because embryos are typically transferred into females after acquisition of thermal resistance. There may be some opportunity to mitigate effects of heat stress by feeding antioxidants or regulating the endocrine environment of the cow but neither approach has been reduced to practice. The best long-term solution to the problem of heat stress may be to increase genetic resistance of cows to heat stress. Thermotolerance genes exist within dairy breeds and additional genes can be introgressed from other breeds by traditional means or gene editing.

Melatonin administration during the dry period stimulates subsequent milk yield and weight gain of offspring in subtropical does kidding in summer

In the present experiment, we tested the hypothesis that in does kidding in summer, melatonin administration during dry period is galactopoietic for the subsequent lactation and results in improved growth of their suckling kids. Twenty-five multiparous pregnant creole does were enrolled into a randomized complete block design during their dry period in the 49 d prepartum, and under natural long photoperiods around the summer solstice, pregnant does either received 2 subcutaneous ear implants (18 mg) of melatonin (MEL, n = 10) or served as nonimplanted controls (CONT, n = 15). During the first 14 wk of subsequent lactation (suckling and milking periods), MEL does yielded more milk than CONT does. Throughout subsequent lactation, milk composition was not affected by treatment. In MEL does, peripheral triiodothyronine levels peaked at 2 wk of lactation, remaining higher than in CONT does. The mean daily weight gain was higher in MEL compared with CONT kids and was also higher in males than females, and for males, was positively correlated with milk yield. The current data support our hypothesis that melatonin during the prepartum period is galactopoietic in suckling does.

Invited review: Management strategies capable of improving the reproductive performance of heat-stressed dairy cattle

Impaired fertility during periods of heat stress is the culmination of numerous physiological responses to heat stress, ranging from reduced estrus expression and altered follicular function to early embryonic death. Furthermore, heat-stressed dairy cattle exhibit a unique metabolic status that likely contributes to the observed reduction in fertility. An understanding of this unique physiological response can be used as a basis for improving cow management strategies, thereby reducing the negative effects of heat stress on reproduction. Potential opportunities for improving the management of dairy cattle during heat stress vary greatly and include feed additives, targeted cooling, genetic selection, embryo transfer and, potentially, crossbreeding. Previous studies indicate that dietary interventions such as melatonin and chromium supplementation could alleviate some of the detrimental effects of heat stress on fertility, and that factors involved in the methionine cycle would likely do the same. These supplements, particularly chromium, may improve reproductive performance during heat stress by alleviating insulin-mediated damage to the follicle and its enclosed cumulus-oocyte complex. Beyond feed additives, some of the simplest, yet most effective strategies involve altering the timing of feeding and cooling to take advantage of comparatively low nighttime temperatures. Likewise, expansion of cooling systems to include breeding-age heifers and dry cows has significant benefits for dams and their offspring. More complicated but promising strategies involve the calculation of breeding values for thermotolerance, the identification of genomic markers for heat tolerance, and the development of bedding-based conductive cooling systems. Unfortunately, no single approach can completely rescue the fertility of lactating dairy cows during heat stress. That said, region-appropriate combinations of strategies can improve reproductive measures to reasonable levels.

Effects of melatonin on production of reactive oxygen species and developmental competence of bovine oocytes exposed to heat shock and oxidative stress during in vitro maturation

SummaryHeat shock may disrupt oocyte function by increasing the generation of reactive oxygen species (ROS). We evaluated the capacity of the antioxidant melatonin to protect oocytes using two models of oxidative stress - heat shock and the pro-oxidant menadione. Bovine cumulus-oocyte complexes (COC) were exposed in the presence or absence of 1 µM melatonin to the following treatments during maturation: 38.5°C, 41°C and 38.5°C+5 µM menadione. In the first experiment, COC were matured for 3 h with 5 µM CellROX® and analyzed by epifluorescence microscopy to quantify production of ROS. The intensity of ROS was greater for oocytes exposed to heat shock and menadione than for control oocytes. Melatonin reduced ROS intensity for heat-shocked oocytes and oocytes exposed to menadione, but not for control oocytes. In the second experiment, COC were matured for 22 h. After maturation, oocytes were fertilized and the embryos cultured for 7.5 days. The proportion of oocytes that cleaved after fertilization was lower for oocytes exposed to heat shock and menadione than for control oocytes. Melatonin increased cleavage for heat-shocked oocytes and oocytes exposed to menadione, but not for control oocytes. Melatonin tended to increase the developmental competence of embryos from heat-shocked oocytes but not for embryos from oocytes exposed to menadione or from control oocytes. In conclusion, melatonin reduced production of ROS of maturing oocytes and protected oocytes from deleterious effects of both stresses on competence of the oocyte to cleave after coincubation with sperm. These results suggest that excessive production of ROS compromises oocyte function.

Genotype of MTNR1A gene regulates the conception rate following melatonin treatment in water buffalo

Buffaloes have tendency to show seasonal reproduction and remain in anestrus due to limited ovarian activity during summer. The seasonal reproductive behavior is ascribed the effect of melatonin related to photoperiod. Treating animals with melatonin could be a possible strategy to overcome the problem. The role of MTNR1A gene has not been fully explained in the buffalo. Therefore, we conducted a study on 114 buffalo heifers to detect the polymorphic site in MTNR1A gene and further treated them with melatonin implants to investigate the role of most frequent genotype following melatonin treatment on pregnancy. The present investigation is the first to investigate the association between melatonin treated different MTNR1A genotype buffalo and pregnancy. We confirmed SNP at position 72 in 812 bp fragment exon II of MTNR1A gene. RFLP of PCR products with Hpa I enzyme resulted in three genotypes: TT (812bp), CT (812, 743, 69bp) and CC (743, 69bp). Next, buffaloes of each genotype (TT, CC, CT; n = 28 for each) were treated with melatonin implants to compare the conception rate with their corresponding untreated control (n = 10 for each genotype). Melatonin concentrations were higher (P < 0.05) for the treatment groups of all genotypes compared to their respective untreated control from day 1-28. The pregnancy rate was significantly associated with the MTNR1A genotype. The conception rate was higher (P < 0.05) for TT genotype than for the other genotypes of buffaloes treated with melatonin. Furthermore, buffaloes of TT genotype treated with melatonin started exhibiting estrus activity soon from second week of melatonin treatment (14.1 ± 2.1; range: 10-17 days) and were found to be 7.8 times more likely to become pregnant compared to other genotypes following melatonin treatment. In conclusion, TT genotype of MTNR1A gene is more sensitive to melatonin treatment that favours pregnancy in buffaloes during summer.

Serum metabolomic profile in genetically modified cows carrying human α‑lactalbumin gene

The present study aimed to investigate the serum metabolomic profiles in genetically modified cows carrying and expressing human lactalbumin α (LALBA) and non‑LALBA cows, and identify altered metabolic characteristics following the genetic modification. Serum biochemistry indexes were measured according to protocols recommended by International Federation of Clinical Chemistry. The metabolomic profiles were determined using the serum samples collected from LALBA (n=6) and non‑LALBA cows (n=6). Welch's two‑sample t‑test was used to identify the metabolites that significantly differed between the LALBA and non‑LALBA groups (fold‑change ≠ 1 and P<0.05), followed by random forest and pathway analysis. The serum biochemistry indexes of LALBA and non‑LALBA cows were within the normal ranges of healthy cows. A total of 273 metabolites were detected, among which 79 metabolites, including 46 increased and 33 decreased metabolites, differed significantly between the LALBA and non‑LALBA groups. Random forest analysis identified 30 potential key metabolites, including 14 elevated and 16 reduced metabolites. These metabolites were primarily involved in pathways concerning the metabolism of leucine, isoleucine, valine, tryptophan and lipids, such as myristate and eicosapentaenoate. However, the serum in LALBA cow had unique metabolomic signature compared with non‑LALBA cows. The accumulation of polyunsaturated fatty acids and amino acids, and the reduced levels of long chain saturated fatty acids in serum may benefit LALBA cows. However, further investigations are required to validate these benefits and the corresponding mechanisms.

Effects of photoperiod modulation and melatonin feeding around drying-off on bovine mammary gland involution

The risk for a dairy cow to acquire new intramammary infections is high during the transition from lactation to the dry period, because of udder engorgement and altered immune functions. Once the gland is fully involuted, it becomes much more resistant to intramammary infections. Therefore, strategies to depress milk yield before drying-off and accelerate the involution process after drying-off could be beneficial for udder health. The objective of this study was to assess the effect of photoperiod manipulation and melatonin feeding from 14 d before to 14 d after drying-off on the speed of the involution process. Thirty Holstein cows in late lactation were randomly allocated to one of the following treatments: (1) a long-day photoperiod (16 h of light: 8 h of darkness), (2) a short-day photoperiod (8 h of light: 16 h of darkness), and (3) a long-day photoperiod supplemented by melatonin feeding (4 mg/kg of body weight). Milk and blood samples were collected on d -26, -19, -12, -5, -1, 1, 3, 5, 7, 10, and 14 relative to the last milking to determine concentrations of mammary gland involution markers and serum prolactin. Additional blood samples were taken around milking on d -15, before the start of the treatments, and on d -1, before drying-off, to evaluate the treatment effects on milking-induced prolactin release. The short-day photoperiod slightly decreased milk production and basal prolactin secretion during the dry period. The milking-induced prolactin surge was smaller on d -1 than on d -15 regardless of the treatments. Lactoferrin concentration, somatic cell count, and BSA concentration as well as matrix metalloproteinase-2 and -9 activities increased in mammary secretions during the first 2 wk of the dry period, whereas milk citrate concentration and the citrate:lactoferrin molar ratio decreased. The rates of change of these parameters were not significantly affected by the treatments. The long-day photoperiod supplemented by melatonin feeding did not affect milk production, prolactin secretion, or mammary gland involution. Under the conditions in this study, photoperiod modulation and melatonin feeding did not appear to affect the rate of mammary gland involution.

Causes of declining fertility in dairy cows during the warm season

In the Northern Hemisphere, from June to September and in the Southern Hemisphere from December to March, there are periods of reduced fertility (sub-fertility) in dairy cows that are described as summer infertility. Several factors contribute to sub-fertility during this time, such as ambient temperature, humidity and photoperiod. During the warm season there is a reduction in feed intake that may compromise the energy balance of the cow and/or induce an imbalance in the activity of the hypothalamo-hypophyseal-ovarian axis. These factors reduce the reproductive performance of the cow and compromise the quality of oocytes, embryos and corpora lutea. This paper reviews current knowledge on the metabolic and endocrine mechanisms that induce summer infertility and describe their effects on follicle, oocyte and embryo development in dairy cows.

Effect of Heat Stress on Reproduction in Dairy Cows: Insights into the Cellular and Molecular Responses of the Oocyte

Among the components of the female reproductive tract, the ovarian pool of follicles and their enclosed oocytes are highly sensitive to hyperthermia. Heat-induced alterations in small antral follicles can be expressed later as compromised maturation and developmental capacity of the ovulating oocyte. This review summarizes the most up-to-date information on the effects of heat stress on the oocyte with an emphasis on unclear points and open questions, some of which might involve new research directions, for instance, whether preantral follicles are heat resistant. The review focuses on the follicle-enclosed oocytes, provides new insights into the cellular and molecular responses of the oocyte to elevated temperature, points out the role of the follicle microenvironment, and discusses some mechanisms that might underlie oocyte impairment. Mechanisms include nuclear and cytoplasmic maturation, mitochondrial function, apoptotic pathways, and oxidative stress. Understanding the mechanism by which heat stress compromises fertility might enable development of new strategies to mitigate its effects.

Modification of embryonic resistance to heat shock in cattle by melatonin and genetic variation in HSPA1L

The objectives were to test whether (1) melatonin blocks inhibition of embryonic development caused by heat shock at the zygote stage, and (2) the frequency of a thermoprotective allele for HSPA1L is increased in blastocysts formed from heat-shocked zygotes as compared with blastocysts from control zygotes. It was hypothesized that melatonin prevents effects of heat shock on development by reducing accumulation of reactive oxygen species (ROS) and that embryos inheriting the thermoprotective allele of HSPA1L would be more likely to survive heat shock. Effects of 1 µM melatonin on ROS were determined in experiments 1 and 2. Zygotes were cultured at 38.5 or 40°C for 3 h in the presence of CellROX reagent (ThermoFisher Scientific, Waltham, MA). Culture was in a low [5% (vol/vol)] oxygen (experiment 1) or low or high [21% (vol/vol)] oxygen environment (experiment 2). Heat shock and high oxygen increased ROS; melatonin decreased ROS. Development was assessed in experiments 3 and 4. In experiment 3, zygotes were cultured in low oxygen ± 1 µM melatonin and exposed to 38.5 or 40°C for 12 h (experiment 1) beginning 8 h after fertilization. Melatonin did not protect the embryo from heat shock. Experiment 4 was performed similarly except that temperature treatments (38.5 or 40°C, 24 h) were performed in a low or high oxygen environment (2×2 × 2 factorial design with temperature, melatonin, and oxygen concentration as main effects), and blastocysts were genotyped for a deletion (D) mutation (C→D) in the promoter region of HSPA1L associated with thermotolerance. Heat shock decreased percent of zygotes developing to the blastocyst stage independent of melatonin or oxygen concentration. Frequency of genotypes for HSPA1L was affected by oxygen concentration and temperature, with an increase in the D allele for blastocysts that developed in high oxygen and following heat shock. It was concluded that (1) lack of effect of melatonin or oxygen concentration on embryonic development means that the negative effects of heat shock on the zygote are not mediated by ROS, (2) previously reported effect of melatonin on fertility of heat-stressed cows might involve actions independent of the antioxidant properties of melatonin, and (3) the deletion mutation in the promoter of HSPA1L confers protection to the zygote from heat shock and high oxygen. Perhaps, embryonic survival during heat stress could be improved by selecting for thermotolerant genotypes.

Comparing the effects of heat stress and mastitis on ovarian function in lactating cows: basic and applied aspects

Reduced reproductive performance of lactating cows is strongly associated with environmental and pathogenic stressors. This review summarizes the most recent knowledge on the effects of acute or chronic heat stress (HS) and acute or chronic intramammary infection (IMI) on ovarian function. It also offers various approaches for improving the fertility of cows under chronic HS or IMI. Comparing the 2 stressors reveals a few similarities in the mode of alteration in the hypothalamus-pituitary-ovarian axis, in particular, in the follicle and its enclosed oocyte. Both HS and IMI cause a reduction in the preovulatory LH surge, with a pronounced effect in cows with IMI, and consequently, ovulation is being delayed or inhibited. Both stresses induce changes in follicular growth dynamics, reduce follicular steroidogenesis, and disrupt follicular dominance. Unlike their effects on follicular function, the effects of mastitis and HS on corpus luteum (CL) function are debatable. Under chronic summer thermal stress, several, but not all, studies show reduced progesterone secretion by the CL. Subclinical mastitis does not affect CL function, whereas the effect of clinical mastitis is controversial; some show a reduction in progesterone, whereas others do not. Both stresses have been found to impair cytoplasmic and nuclear maturation of oocytes, associated with reduced embryonic development. These findings have provided insights into the mechanism by which HS and IMI compromise fertility, which enable developing new strategies to mitigate these effects. For instance, treatment with GnRH and PGF2α to induce follicular turnover successfully improved conception rate in subpopulations of HS cows during the summer, in particular, primiparous cows and cows with high BCS. The "Ovsynch" program, also based on the use of GnRH and PGF2α, has been shown to improve conception rate of subclinical mastitic cows, most likely due to better synchronization of timing of ovulation with that of AI. Supplementing progesterone after AI improves conception rate of HS cows, particularly those with postpartum uterine disease and low BCS. It should be noted that similarities between the 2 stressors do not necessarily suggest a shared mechanism. Although not clear enough, an additive deleterious effects of HS and IMI on reproduction is suggested.

Effects of melatonin on the proliferation and apoptosis of sheep granulosa cells under thermal stress

The cross-talk between oocyte and somatic cells plays a crucial role in the regulation of follicular development and oocyte maturation. As a result, granulosa cell apoptosis causes follicular atresia. In this study, sheep granulosa cells were cultured under thermal stress to induce apoptosis, and melatonin (MT) was examined to evaluate its potential effects on heat-induced granulosa cell injury. The results demonstrated that the Colony Forming Efficiency (CFE) of granulosa cells was significantly decreased (heat 19.70% ± 1.29% vs. control 26.96% ± 1.81%, p < 0.05) and the apoptosis rate was significantly increased (heat 56.16% ± 13.95%vs. control 22.80% ± 12.16%, p < 0.05) in granulosa cells with thermal stress compared with the control group. Melatonin (10⁻⁷ M) remarkably reduced the negative effects caused by thermal stress in the granulosa cells. This reduction was indicated by the improved CFE and decreased apoptotic rate of these cells. The beneficial effects of melatonin on thermal stressed granulosa cells were not inhibited by its membrane receptor antagonist luzindole. A mechanistic exploration indicated that melatonin (10⁻⁷ M) down-regulated p53 and up-regulated Bcl-2 and LHR gene expression of granulosa cells under thermal stress. This study provides evidence for the molecular mechanisms of the protective effects of melatonin on granulosa cells during thermal stress.