Axis In Sheep Research Articles

Effect of Maternal High Fat Diet on the Hypothalamus-Pituitary-Adrenal Gland (HPA) Axis of the Offspring in Sheep

Maternal consumption of a high fat diet (HFD) during pregnancy is associated with metabolic dysfunction and altered HPA axis function in the offspring. These effects are well established in species with short gestation length (e.g., rodents), but less established in species with longer gestation periods more comparable with humans. The objective of the study was to determine the effect of maternal HFD on basal and stimulated cortisol secretion in the offspring at 16 weeks of age in sheep. All experiments were approved by the Institutional Animal Care and Use Committee at Cal Poly Pomona. Pregnant ewes were assigned to control (n=3) and HFD (n=3) groups. The control group was fed farmers best complete ewe and ram pellets. The HFD group was fed the same diet with an additional 30% rumen protected fat (Megalac, Volac Wilmar Feed Ingredients Ltd). After birth lambs were weaned at 12 weeks of age and given free access to control pellet diet with grains. To assess adrenal cortisol secretion at 16 weeks, lambs were infused intravenously with adrenocorticotropic hormone (ACTH; 2 ug/kg cortrosyn in 1 mL saline). Blood samples (1 mL) were taken through the jugular catheter at −30, −15, and 0 min (immediately before) and 5, 15, 30, 60, 90, and 120 min after ACTH administration for measurement of plasma cortisol by ELISA. Data were analyzed by two-way repeated measures ANOVA with a Tukey posthoc test. Basal plasma cortisol tended to be higher in HFD (32±8 ng/mL) than control (13±3 ng/mL) lambs, but this did not reach statistical significance. Plasma cortisol was higher than baseline from 15 to 90 minutes after ACTH administration in all lambs (p<0.05). The increment in plasma cortisol was higher in HFD than control lambs from 15-90 minutes (p<0.05). Accordingly, the maximum increase in plasma cortisol at 60 minutes and was higher in HFD (245±8 ng/mL) than control lambs (121±50 ng/mL p<0.05). These preliminary data suggest that maternal consumption of a high fat diet during pregnancy may increase basal and stimulated activity of the HPA axis in sheep. The study was funded by a Strategic Interdisciplinary Research Grant (SIRG) at California State Polytechnic University, Pomona. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Comprehensive transcriptomic analysis revealing the regulatory dynamics and networks of the pituitary-testis axis in sheep across developmental stages.

Spermatogenesis is a complex process intricately regulated by the hypothalamic-pituitary-testis (HPT) axis. However, research on the regulatory factors governing the HPT axis remains limited. This study addresses this gap by conducting a comprehensive analysis of transcriptomes from the pituitary and testis tissues across various developmental stages, encompassing embryonic day (E120), neonatal period (P0), pre-puberty (P90), and post-puberty day (P270). Utilizing edgeR and WGCNA, we identified stage-specific genes in both the pituitary and testis throughout the four developmental stages. Notably, 380, 242, 34, and 479 stage-specific genes were identified in the pituitary, while 886, 297, 201, and 3,678 genes were identified in the testis. Subsequent analyses unveiled associations between these stage-specific genes and crucial pathways such as the cAMP signaling pathway, GnRH secretion, and male gamete generation. Furthermore, leveraging single-cell data from the pituitary and testis, we identified some signaling pathways involving BMP, HGF, IGF, and TGF-β, highlighting mutual regulation between the pituitary and testis at different developmental stages. This study sheds light on the pivotal role of the pituitary-testis axis in the reproductive process of sheep across four distinct developmental stages. Additionally, it delves into the intricate regulatory networks governing reproduction, offering novel insights into the dynamics of the pituitary-testis axis within the reproductive system.

CircINSR Inhibits Adipogenic Differentiation of Adipose-Derived Stromal Vascular Fractions through the miR-152/MEOX2 Axis in Sheep

Adipose tissue plays a crucial role in energy metabolism. Several studies have shown that circular RNA (circRNA) is involved in the regulation of fat development and lipid metabolism. However, little is known about their involvement in the adipogenic differentiation of ovine stromal vascular fractions (SVFs). Here, based on previous sequencing data and bioinformatics analysis, a novel circINSR was identified in sheep, which acts as a sponge to promote miR-152 in inhibiting the adipogenic differentiation of ovine SVFs. The interactions between circINSR and miR-152 were examined using bioinformatics, luciferase assays, and RNA immunoprecipitation. Of note, we found that circINSR was involved in adipogenic differentiation via the miR-152/mesenchyme homeobox 2 (MEOX2) pathway. MEOX2 inhibited adipogenic differentiation of ovine SVFs and miR-152 inhibited the expression of MEOX2. In other words, circINSR directly isolates miR-152 in the cytoplasm and inhibits its ability to promote adipogenic differentiation of ovine SVFs. In summary, this study revealed the role of circINSR in the adipogenic differentiation of ovine SVFs and its regulatory mechanisms, providing a reference for further interpretation of the development of ovine fat and its regulatory mechanisms.

Screening of Differentially Expressed Genes and miRNAs in Hypothalamus and Pituitary Gland of Sheep under Different Photoperiods.

The reproduction of sheep is affected by many factors such as light, nutrition and genetics. The Hypothalamic-pituitary-gonadal (HPG) axis is an important pathway for sheep reproduction, and changes in HPG axis-related gene expression can affect sheep reproduction. In this study, a model of bilateral ovarian removal and estrogen supplementation (OVX + E2) was applied to screen differentially expressed genes and miRNAs under different photoperiods using whole transcriptome sequencing and reveal the regulatory effects of the photoperiod on the upstream tissues of the HPG axis in sheep. Whole transcriptome sequencing was performed in ewe hypothalamus (HYP) and distal pituitary (PD) tissues under short photoperiod 21st day (SP21) and long photoperiod 21st day (LP21). Compared to the short photoperiod, a total of 1813 differential genes (up-regulation 966 and down-regulation 847) and 145 differential miRNAs (up-regulation 73 and down-regulation 72) were identified in the hypothalamus of long photoperiod group. Similarly, 2492 differential genes (up-regulation 1829 and down-regulation 663) and 59 differential miRNAs (up-regulation 49 and down-regulation 10) were identified in the pituitary of long photoperiod group. Subsequently, GO and KEGG enrichment analysis revealed that the differential genes and target genes of differential miRNA were enriched in GnRH, Wnt, ErbB and circadian rhythm pathways associated with reproduction. Combined with sequence complementation and gene expression correlation analysis, several miRNA-mRNA target combinations (e.g., LHB regulated by novel-414) were obtained. Taken together, these results will help to understand the regulatory effect of the photoperiod on the upstream tissues of HPG in sheep.

Brain-derived neurotrophic factor (BDNF) affects the activity of the gonadotrophic axis in sheep

This study aimed to examine the hypothesis that BDNF modulates the activity of the gonadotrophic axis in sheep. Central infusions of BDNF were administered to sexually mature Polish Merino sheep. The sheep were randomly divided into three groups: the control group received intracerebroventricular (ICV) infusions of the vehicle, the BDNF 10 group received ICV infusions of BDNF at 10 μg/480 μL/day, and the BDNF 60 group was infused with BDNF at 60 μg/480 μL/day. A series of four infusions on three consecutive days was performed. Blood samples were collected on days 0 and 3 of the infusions. Immediately after the experiment, all the sheep were slaughtered, and selected structures of the hypothalamus and pituitaries were collected for Real Time RT-qPCR analysis. The collected plasma samples, as well as parts of pituitaries were stored for radioimmunoassay analysis of LH and FSH.Central treatment with exogenous BDNF stimulated GnRH mRNA expression in the preoptic area, as well as GnRH-R mRNA in the pituitary. Furthermore, substantial changes in the KNDy mRNA expression in the mediobasal hypothalamus were observed after the ICV BDNF administration. Additionally, central BDNF infusion caused a decrease in LH concentration and a simultaneous increase in FSH concentration in peripheral blood. Neither pulse amplitude nor pulse frequency for any gonadotrophin was affected in both groups of sheep that received BDNF infusion.Our results revealed that exogenous BDNF affects GnRH and KNDy gene expression and changes in the LH and FSH pituitary cell secretory activities. These findings suggest that BDNF may participate in the mechanism modulating the activity of the gonadotrophic axis at the central level in female sheep.

Allopregnanolone reduces neuroendocrine response to acute stressful stimuli in sheep.

The verified hypothesis assumed that centrally administered neurosteroid, allopregnanolone (AL), could affect basal and/or stress-induced activity of the hypothalamic-pituitary-adrenal (HPA) axis in sheep. Four groups (n = 6 each) of luteal-phase sheep were intracerebroventricularly infused for 3 days with a vehicle without stress (control); a vehicle treated with stressful stimuli (isolation and partial movement restriction) on the third day; AL (4 × 15 µg/60 µL/30 min, at 30-min intervals) treated with stressful stimuli, and AL alone. Simultaneously, the push-pull perfusion of the infundibular nucleus/median eminence and plasma sample collection were performed. After the experiment, the sheep were killed to collect the hypothalamic and anterior pituitary (AP) tissues. Stressful stimuli evoked an increase in the expression of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) mRNA in the hypothalamic paraventricular nucleus (PVN), and AVP receptor (V1b) and proopiomelanocortin (POMC) mRNA in the AP; the concentrations of perfusate CRH, and plasma adrenocorticotropic hormone (ACTH) and cortisol compared to controls. Conversely, the expression of the CRH receptor (CRHR1) mRNA in the AP was downregulated. AL decreased the expression of CRH and AVP mRNA in the PVN, and AVPRV1b and POMC mRNA in the AP in stressed sheep, compared to only stressed ones. There was also a reduction in perfusate CRH, and plasma ACTH and cortisol concentrations. AL alone decreased the expression of CRHR1 mRNA in the AP, and plasma cortisol concentration at the beginning of the collection period compared to controls. In conclusion, AL may function centrally as a suppressor of HPA axis activity in stressed sheep.

Role of GnRH in the ontogeny and regulation of the fetal hypothalamo-pituitary-gonadal axis in sheep

Adult reproductive ability is to a large extent determined by the appropriate development of the reproductive axis during fetal life. Studies have investigated the role of the fetal hypothalamus in the ontogeny and regulation of pituitary gonadal function during fetal development in sheep. Using immunocytochemistry, we examined the ontogeny of gonadotroph development in the pituitary of female sheep fetuses. At day 70 of gestation (term = 145 days), only immunopositive LH beta cells were present. The number and intensity of staining of these LH beta cells had increased by day 100 but had declined again by day 130. Immunopositive alpha-subunit and FSH beta cells appeared at day 100 of gestation and had further increased in number and staining intensity by day 130 of gestation. Treatment of fetuses with the GnRH agonist buserelin resulted in desensitization of the fetal pituitary gonadotrophs, inhibition of pituitary LH beta and FSH beta mRNA expression and a reduction in the number of immunopositive gonadotrophin-containing cells. Pulsatile GnRH treatment resulted in pituitary-gonadal activation and an increase in LH, FSH and testosterone secretion in males. Thus, the synthesis and secretion of the gonadotrophins during fetal development is critically dependent on the secretion of GnRH from the fetal hypothalamus. Inhibition of fetal gonadotrophins by buserelin treatment from day 70 of gestation resulted in a 40% reduction in the size of the fetal testis at birth, and there were no effects on the fetal ovaries. This reduction in testis size was due to a 45% reduction in the number of Sertoli cells. However, when buserelin was given between day 70 and day 110 of gestation, there were no effects on testis size or morphological development of the testis, suggesting that gonadotrophins regulate testicular development during a 'critical window' late in gestation. Taken together, these studies provide convincing evidence that GnRH plays a central role in the ontogeny and regulation of pituitary-gonadal function during fetal life.

Leptin actions on the reproductive neuroendocrine axis in sheep

There is a growing literature on the role of leptin in appetite and neuroendocrine regulation in domestic ruminants. Circulating leptin concentration is higher in fat than in thin sheep, is reduced by chronic underfeeding and is higher in sheep subjected to long-day rather than short-day photoperiods. Leptin is reduced acutely by fasting and increases after meals so that there are long- and short-term components to the systemic leptin signal. Nutritional stimulation of reproductive neuroendocrine output is associated with increased circulating concentrations of leptin; peripheral leptin administration restores LH secretion in fasted sheep, and leptin is permissive (although not a trigger) for puberty. Intracerebroventricular (i.c.v.) pharmacological leptin infusion stimulates LH in underfed but not in well-fed sheep, and reduces food intake in well-fed sheep. A single i.c.v. pharmacological injection or physiological infusion of leptin stimulates LH in well-fed sheep, with or without a concomitant decrease in appetite. Furthermore, these appetite and LH responses are differentially affected by photoperiod, indicating that different neuronal pathways may mediate the two responses. Hypothalamic leptin receptors co-localize with orexigenic and anorexigenic neurones, some of which contact GnRH cells, but the confluence of leptin signalling with photoperiod (melatonin) signalling remains unresolved. Photoperiod-entrained sheep provide potential models of altered central leptin sensitivity, in which downstream mechanisms regulating appetite and GnRH may be dissociated.

Chronic stress influences attentional and judgement bias and the activity of the HPA axis in sheep.

IntroductionEnvironmental challenges are part of everyday life for most domestic animals. However, very little is known about how animals cope emotionally and physiologically with cumulative challenges. This experiment aimed to determine the impact of long-term exposure to environmental challenges on the affective state and hypothalamic-pituitary-adrenal (HPA) axis responses to a subsequent additional acute shearing challenge.MethodsSheep were exposed to either a long-term environmental challenge (rest disruption and individual housing) in order to induce chronic stress (chronic stress group) or control conditions (group housing in a field with low stress handling and daily feed rewards, control group). Judgement and attention bias were assessed as measures of the emotional state following several days of the challenge or control treatment (pre-shearing tests). In addition, the responsiveness of the HPA-axis was evaluated using a combined Corticotropin Releasing Hormone and Arginine Vasopressin (CRH/AVP) challenge. Finally, all animals were exposed to an acute shearing challenge, then judgement bias (post-shearing test), HPA-axis and internal body temperature responses were determined.ResultsIn the pre-shearing judgement bias test, the chronic stress group slightly increased optimism compared to the control treatment. In the attention bias test, the chronic stress group showed reduced vigilance behaviour towards a predator threat and a quicker approach to the food compared to the control treatment. The chronic stress group also had lower plasma ACTH concentrations in response to the CRH/AVP challenge compared to the control group, no differences in cortisol concentrations were found. In the post-shearing judgement bias test, differences in optimism were no longer evident between the chronic stress and control groups. Plasma ACTH concentrations and body temperatures showed a greater increase in response to shearing in the chronic stress group compared to the control group.ConclusionOur results suggest that long-term exposure to challenges biased cognitive measures of the affective state towards an increased expectation of a reward and reduced attention towards a threat. The exaggerated ACTH responses in the chronic stress group may be indicative of HPA-axis dysregulation. Despite a period of challenge exposure in the chronic stress group, judgement bias responses to the shearing challenge were similar in the chronic stress and control groups; the reasons for this need further investigation. The altered affective state together with signs of HPA-axis dysregulation may indicate an increased risk of compromised welfare in animals exposed to long-term environmental challenges.

Do Substance P and Neurokinin A Play Important Roles in the Control of LH Secretion in Ewes?

There is now general agreement that neurokinin B (NKB) acts via neurokinin-3-receptor (NK3R) to stimulate secretion of GnRH and LH in several species, including rats, mice, sheep, and humans. However, the roles of two other tachykinins, substance P (SP) and neurokinin A, which act primarily via NK1R and NK2R, respectively, are less clear. In rodents, these signaling pathways can stimulate LH release and substitute for NKB signaling; in humans, SP is colocalized with kisspeptin and NKB in the mediobasal hypothalamus. In this study, we examined the possible role of these tachykinins in control of the reproductive axis in sheep. Immunohistochemistry was used to describe the expression of SP and NK1R in the ovine diencephalon and determine whether these proteins are colocalized in kisspeptin or GnRH neurons. SP-containing cell bodies were largely confined to the arcuate nucleus, but NK1R-immunoreactivity was more widespread. However, there was very low coexpression of SP or NK1R in kisspeptin cells and none in GnRH neurons. We next determined the minimal effective dose of these three tachykinins that would stimulate LH secretion when administered into the third ventricle of ovary-intact anestrous sheep. A much lower dose of NKB (0.2 nmol) than of neurokinin A (2 nmol) or SP (10 nmol) consistently stimulated LH secretion. Moreover, the relative potency of these three neuropeptides parallels the relative selectivity of NK3R. Based on these anatomical and pharmacological data, we conclude that NKB-NK3R signaling is the primary pathway for the control of GnRH secretion by tachykinins in ewes.

Salsolinol—a potential inhibitor of the gonadotropic axis in sheep during lactation

This study tested the hypothesis that salsolinol, a derivative of dopamine, affects GnRH and LH secretion in lactating sheep. In the in vivo experiment, the structural analogue of salsolinol, 1-methyl-3,4-dihydroisoquinoline (1-MeDIQ), was infused into the infundibular nucleus-median eminence of sheep at the fifth wk of lactation to antagonize salsolinol's action. Simultaneously, cerebrospinal fluid from the third brain ventricle, to determine GnRH concentration, and plasma samples, to measure LH concentration, were collected. In the in vitro experiment, the anterior pituitary (AP) explants from weaned sheep were incubated in culture medium containing 2 doses of salsolinol, 20 and 100 μg/mL (S20 and S100, respectively). The concentration of LH in the collected media and relative expression of LHβ subunit messenger RNA in the AP explants were determined. No significant difference was found in mean GnRH concentration in response to 1-MeDIQ infusion, but both mean plasma LH concentration and LH pulse frequency increased significantly (P < 0.001 and P < 0.05, respectively) compared with those in controls. Significantly higher LH concentrations occurred during the first (P < 0.001), second (P < 0.001), and fourth (P < 0.05) h of 1-MeDIQ infusion. In the in vitro study, both the S20 and S100 doses of salsolinol caused a significant decrease in the mean medium LH concentration compared with that in the control (P < 0.01 and P < 0.001, respectively). Salsolinol had no effect on the relative LHβ subunit messenger RNA expression in the incubated tissue. In conclusion, salsolinol is a potential inhibitor of the secretory activity of the gonadotropic axis in lactating sheep, at least at the AP level. Although no significant changes in GnRH release were directly confirmed, an increase in the frequency of LH pulses does not allow to exclude the central action of salsolinol.

Hypothalamic-pituitary GnRH/LH axis activity is affected by salsolinol in sheep during lactation: Effects of intracerebroventricular infusions of salsolinol and its antagonizing analogue

The aim of the study was to test the hypothesis that salsolinol, a derivative of dopamine, is involved in the regulation of hypothalamic-pituitary gonadotropic (GnRH/LH) axis activity in lactating sheep. In the first experiment performed on sheep during the fifth week of lactation, a structural analogue of salsolinol (1-MeDIQ) was infused into the third brain ventricle (IIIv) to antagonize its action within the central nervous system (CNS). A push–pull perfusion of the infundibular nucleus/median eminence was performed simultaneously, and blood samples were collected from the jugular vein. In the second experiment, sheep received infusions of salsolinol into the IIIv, 48 hours after the weaning of their 8-week-old lambs. Blood samples were collected during the experimental periods, and the anterior pituitary (AP) tissue was dissected immediately after the end of the experiment. Perfusate GnRH concentration (experiment 1), plasma LH concentration (experiments 1 and 2), and relative LHβ mRNA levels in the AP tissue (experiment 2) were assayed. Blocking of salsolinol action in the CNS of lactating sheep caused a significant (P < 0.001) decrease in the perfusate GnRH concentrations in comparison with controls. Treatment with 1-MEDIQ also significantly decreased (P < 0.001) the LH concentration in the blood plasma. In turn, salsolinol infused 48 hours after lamb weaning significantly (P < 0.001) increased plasma LH concentration, reflected in the significant (P < 0.05) increase in the amplitude of LH pulses in the treated sheep as compared to the control animals. There was no significant difference in the relative levels of LHβ-subunit mRNA in the AP between control and salsolinol-infused sheep. The results lead to a conclusion that salsolinol affects the secretory activity of the GnRH/LH axis in sheep during lactation. Whether salsolinol infused into the IIIv evokes this stimulatory effect by itself or by modulation of other regulatory systems needs to be clarified.

The hypothalamo-pituitary-adrenal (HPA) axis in sheep is attenuated during lactation in response to psychosocial and predator stress

Activation of the hypothalamo-pituitary-adrenal (HPA) axis by psychosocial stress is attenuated during lactation. We tested the hypothesis that lactating ewes will have attenuated HPA axis responses to isolation and restraint but will have greater responses to predator stress in the form of barking dogs. We imposed two 4 h stressors: psychosocial stress (isolation and restraint of ewes) and predator stress (barking dogs). Blood was collected intravenous every 10 min from nonlactating ewes (n = 6), lactating ewes with lambs present but not able to be suckled (n = 6), and lactating ewes with lambs present and able to be suckled (n = 6). Plasma cortisol and oxytocin were measured. For nonlactating ewes, cortisol increased (P < 0.01) in response to both stressors, and these increases were greater (P < 0.01) than that in the lactating animals. For lactating ewes with lambs present but unable to be suckled, cortisol increased (P < 0.05) in response to both stressors with a greater response to barking dogs (P < 0.05). For lactating ewes with lambs present and able to be suckled, cortisol increased (P < 0.01) in response to barking dogs only. Plasma oxytocin was greater (P < 0.01) in lactating ewes than in nonlactating ewes and did not change in response to the stressors. In conclusion, lactating ewes are likely to have a greater HPA axis response to a stressor that may be perceived to threaten the welfare of themselves and/or their offspring. The role of oxytocin in attenuation of the HPA axis to stress in sheep is unclear from the current research and requires further investigation.

Effects of early- and late-gestational maternal stress and synthetic glucocorticoid on development of the fetal hypothalamus–pituitary–adrenal axis in sheep

Prenatal maternal stress (PMS) programs dysregulation of the hypothalamus–pituitary–adrenal axis (HPAA) in postnatal life, though time periods vulnerable to PMS, are still unclear. We evaluated in pregnant sheep the effect of PMS during early gestation [30–100 days of gestation (dGA); term is 150 dGA] or late gestation (100–120 dGA) on development of fetal HPAA function. We compared the effects of endogenous cortisol with synthetic glucocorticoid (GC) exposure, as used clinically to enhance fetal lung maturation. Pregnant sheep were exposed to repeated isolation stress twice per week for 3 h in a separate box with no visual, tactile, or auditory contact with their flock-mates either during early (n = 7) or late (n = 7) gestation. Additional groups received two courses of betamethasone (BM; n = 7; 2 × 110 μg kg− 1 body weight, 24 h apart) during late gestation (106/107 and 112/113 dGA, n = 7) or acted as controls (n = 7). Fetal cortisol responses to hypotensive challenge, a physiological fetal stressor, were measured at 112 and 129 dGA, i.e. before and during maturation of the HPAA. Hypotension was induced by fetal infusion of sodium nitroprusside, a potent vasodilator. At 112 dGA, neither PMS nor BM altered fetal cortisol responses. PMS, during early or late gestation, and BM treatment increased fetal cortisol responses at 129 dGA with the greatest increase achieved in stressed early pregnant sheep. Thus, development of the HPAA is vulnerable to inappropriate levels of GCs during long periods of fetal life, whereas early gestation is most vulnerable to PMS.

Stressor Specificity of Sex Differences in Hypothalamo-Pituitary-Adrenal Axis Activity: Cortisol Responses to Exercise, Endotoxin, Wetting, and Isolation/Restraint Stress in Gonadectomized Male and Female Sheep

Abstract Sex differences in the stress-induced activity of the hypothalamo-pituitary-adrenal axis in sheep appear to be dependent on the stressor encountered and occur irrespective of the presence of gonadal steroids. We tested the hypotheses that cortisol responses to exercise, endotoxin, wetting (experiment 1), and isolation/restraint (experiment 2) stress differ between gonadectomized male and female sheep. At weekly intervals (in experiment 1), we subjected gonadectomized rams and ewes (n = 6/group) to control conditions, to exercise stress, to iv injection of endotoxin, and to wetting stress. In a second experiment (experiment 2), we subjected gonadectomized rams and ewes (n = 5/group) to control conditions or to isolation/restraint stress. In both experiments, we measured plasma concentrations of cortisol before, during, and after stress at a frequency of at least 15 min with samples collected (from an indwelling jugular catheter) at a greater frequency around the time of the stressor. Cortisol responses to wetting (experiment 1) and isolation/restraint (experiment 2) stress were significantly higher in females compared with males but in response to exercise (experiment 1) and endotoxin (experiment 1) stress, there were no differences between the sexes. For some stressors, there are sex differences in sheep in the stress-induced activity of the hypothalamo-pituitary-adrenal axis that are independent of the presence of the sex steroids, but the existence of these sex differences and the direction of these sex differences differs, depending on the stressor imposed.

Stressor Specificity of Sex Differences in Hypothalamo-Pituitary-Adrenal Axis Activity: Cortisol Responses to Exercise, Endotoxin, Wetting, and Isolation/Restraint Stress in Gonadectomized Male and Female Sheep

Sex differences in the stress-induced activity of the hypothalamo-pituitary-adrenal axis in sheep appear to be dependent on the stressor encountered and occur irrespective of the presence of gonadal steroids. We tested the hypotheses that cortisol responses to exercise, endotoxin, wetting (experiment 1), and isolation/restraint (experiment 2) stress differ between gonadectomized male and female sheep. At weekly intervals (in experiment 1), we subjected gonadectomized rams and ewes (n = 6/group) to control conditions, to exercise stress, to iv injection of endotoxin, and to wetting stress. In a second experiment (experiment 2), we subjected gonadectomized rams and ewes (n = 5/group) to control conditions or to isolation/restraint stress. In both experiments, we measured plasma concentrations of cortisol before, during, and after stress at a frequency of at least 15 min with samples collected (from an indwelling jugular catheter) at a greater frequency around the time of the stressor. Cortisol responses to wetting (experiment 1) and isolation/restraint (experiment 2) stress were significantly higher in females compared with males but in response to exercise (experiment 1) and endotoxin (experiment 1) stress, there were no differences between the sexes. For some stressors, there are sex differences in sheep in the stress-induced activity of the hypothalamo-pituitary-adrenal axis that are independent of the presence of the sex steroids, but the existence of these sex differences and the direction of these sex differences differs, depending on the stressor imposed.

Nutritional models of the developmental programming of adult health and disease

The ability to not only replicate but also extend the findings from both historical epidemiological studies and contemporary cohorts of the developmental programming of later disease are critical if the mechanisms by which early diet impacts on later disease are to be fully understood. To date, a plethora of models have been established, with the range including global changes in dietary input, imbalanced diets and diets deficient in single nutrients. Key factors in translating these findings to the human situation are the pronounced differences in the relative growth and development between large and small mammals from the time of conception through pregnancy, lactation and weaning. This disparity is reflected in the very different nutritional requirements between species and the substantial divergence between rodents and large animals in the ontogeny of many of the organ systems that are nutritionally regulated. For example, hypothalamic circuitry is much more developed in species with a long gestation and offspring are born with a mature hypothalamic-pituitary axis in sheep and man compared with mice and rats. Similarly, nephron number is established towards the end of gestation in large mammals compared with the lactational period in rats. These types of differences will impact on the ability of individual and combined nutritional interventions to reset developmental processes, and may be further compounded by the gender of a fetus. The challenge for future work in this exciting and dynamic area of research is to utilise these marked comparative differences to generate imaginative nutritional interventions in order to improve the viability, health and well-being of the offspring.

Central Administration of Neuropeptide B, But not Prolactin-Releasing Peptide, Stimulates Cortisol Secretion in Sheep

Two neuropeptides, neuropeptide B (NPB) and prolactin-releasing peptide (PrRP), have been suggested to play important roles in control of the hypothalamic-pituitary-adrenal (HPA) axis in rodents. The aim of the present study was to clarify the central actions of NPB or PrRP in sheep. Ovariectomized ewes were surgically implanted with a cannula directed to the lateral ventricle. They received intracerebroventricular (icv) administration of 400 mul of artificial cerebrospinal fluid, NPB (0.05, 0.5 or 5 nmol), PrRP (0.5, 5 or 50 nmol) or corticotropin-releasing hormone (CRH, 0.5 or 5 nmol) through the cannula, and blood samples were taken 30 and 0 min prior to and 15, 30, 60 and 90 min after the injection. Cortisol concentrations in plasma were determined by enzyme immunoassay. Administration of 0.5 nmol NPB resulted in a significant increase in the cortisol concentration compared with the vehicle control, whereas the cortisol concentration after lower or higher doses of NPB did not differ from the control value. Thus, an icv injection of NPB produced a bell-shaped dose-response of cortisol concentration. Administration of PrRP had no significant effect on the cortisol concentrations at any dose examined. Icv injection of CRH dose-dependently increased plasma cortisol concentrations. These results demonstrate that central NPB stimulates cortisol secretion, suggesting that this neuropeptide plays some roles in control of the HPA axis in sheep. On the other hand, unlike its role in rodents, PrRP is unlikely to be involved in control of the HPA axis in this species.

Bidirectional communication: Growth and immunity in domestic livestock1,2,3,4

Evidence continues to mount supporting the existence of a bidirectional communication network between the immune system and the somatotropic axis in a variety of species. For more than 4 decades, researchers have sought and identified linkages between the growth axis and the immune system. Although significant advances have been made with regard to elucidation of various bidirectional communication pathways between the immune system and growth axis in humans and rodents, the current paper focuses on the relationships between the immune system and somatotropic axis in sheep, cattle, and swine. Aspects from historical and current research associated with changes in somatotropic function following immune challenges with endotoxin, parasites, viruses, and bacteria have been provided. Collectively, these studies demonstrate that a bidirectional communication network, similar to that described in humans and rodents, also exists in a variety of domestic livestock. Identifying and understanding this bidirectional communication network could have significant economic benefits if it leads to intervention strategies to prevent production losses associated with sickness and disease.

Carcass and meat characteristics of sheep with an additional growth hormone gene

Effects of high growth hormone (GH) activity on body composition and some aspects of meat quality were examined in sheep transgenic for an additional copy of the ovine GH gene, as a tool to explore the biological importance of the GH axis in sheep selected commercially for meat production. Carcasses of 16 GH and 25 control mixed-sex sheep aged 45 months, and 6 GH and 6 control ewes aged 20 months, were measured. The dressing percentage was lower in the GH sheep (P &lt; 0.001). The GH sheep had similar muscle mass to controls, but the weight of their fat depots was reduced (P &lt; 0.001) to approximately 40% of controls, whereas limb-bone mass was 43% greater (P &lt; 0.001) than controls. Fore and hind limbs were equally affected. Skin and most internal organs were heavier, particularly the pancreas, kidney, alimentary canal, and the liver. The concentration of intramuscular fat in the GH sheep was only 27% that of controls (P &lt; 0.001), whereas the average pH of muscle 24 h after slaughter and the melting point of subcutaneous fat were both increased (P &lt; 0.05). Similar changes in organ weights and body composition have been observed in sheep selectively bred to enhance lamb growth rate or to decrease fatness, suggesting that relative GH activity contributed to the outcomes of those experiments. This study indicates the importance of a multi-trait breeding objective to ensure that mechanisms associated with GH do not impair meat quality.