Leukemia inhibitory factor as a mediator of lipopolysaccharide effects on appetite and selected hormones and metabolites.

Leukemia inhibitory factor (LIF) has been suggested to function as a potent inhibitor of feed intake in rodents. In sheep, intravenous injection of lipopolysaccharide (LPS) resulted in an increase in gene expression for LIF in the arcuate nucleus ( < 0.01). In the same experiment, agouti related protein (AgRP) expression was elevated ( < 0.05) but there were no effects on proopiomelanocortin expression. Another group of sheep were provided intracerebroventricular (ICV) injections of LIF at 250, 500, 1,000, and 2,500 ng per sheep. Cumulative feed intake was inhibited by the 1,000- and 2,500-ng doses at 8 and 10 h after ICV injection ( < 0.03). All doses of LIF elevated temperature above 40°C, indicating a fever. When AgRP was intracerebroventricularly injected before LIF, there was no effect of LIF to reduce feed intake, suggesting the LIF inhibition of feed intake is consistent with the concept that the effect is mediated by the melanocortin-4 receptor. In an experiment to determine whether endocrine and metabolic effects of LIF were similar to reported effects of LPS, sheep were intracerebroventricularly injected with 2,500 ng LIF, and blood samples were collected at 10-min intervals for 6 h for assay of LH, samples from the first 3 h were assayed for GH, and samples at 30-min intervals were assayed for glucose and free fatty acids. The effect of treatment and treatment × time interaction was significant, indicating elevated plasma free fatty acids ( < 0.03 and < 0.001, respectively) and glucose ( < 0.01 and < 0.0001, respectively). There was also a treatment × time interaction on circulating concentrations of LH such that LIF caused LH to decrease ( < 0.0001). Additionally, there was a tendency for LIF treatment to increase circulating concentrations of GH (P = 0.0874). The effects of LIF on feed intake and other parameters was similar to the effects of LPS and leads to a hypothesis that LIF expression in response to LPS may be a component of the mechanism for feed intake inhibition and perhaps for changes in selected hormone and metabolites in disease models.

Hypothalamic integration of nutrient status and reproduction in the sheep.

Nutrient availability is a determinant of reproductive success. It is well known that inadequate nutrition results in reproductive failure due to a number of factors including delay of puberty or anoestrous in post-pubertal animals. The lack of nutrients is detected primarily by changes in circulating nutrient molecules and hormones and communicated directly or indirectly to the hypothalamus and brain stem for integration. The general effect is that low nutrition leads to increased appetite stimulation and reduced reproductive performance. When nutrition is adequate, the reverse is true. Both aspects will be the focus of this review. One result of the lack of nutrients is a reduction in luteinizing hormone (LH) concentrations and pulse frequency. Nutrient signals, such as glucose availability, hormonal signals, such as insulin and leptin, and neuroendocrine signals, such as neuropeptide Y and corticotropin-releasing hormone, have been clearly demonstrated to interact to produce changes in LH and reproductive success. Other signals, such as fatty acids, ghrelin, agouti-related peptide, melanin-concentrating hormone, orexin, melanocyte-stimulating hormone, kisspeptin, neurokinin, dynorphin and gonadotropin inhibitory hormone may also play a role in integrating nutrition and reproduction. This review will focus on the major features of the reciprocal control of appetite and reproduction in sheep.

Acute regulation of IGF-I by alterations in post-exercise macronutrients.

This investigation sought to examine the contributions of exercise and nutrient replenishment on in vivo regulation of the insulin-like growth factor-I (IGF-I) axis components. Eight college-aged males completed three high-intensity interval training (HIIT) protocols followed by three post-exercise nutritional protocols: (1) placebo (EX); (2) carbohydrate only (CHO); and (3) essential amino acid/carbohydrate (EAA/CHO). Samples were analyzed for growth hormone (GH), free IGF-I, IGFBP-1, IGFBP-2, insulin, hematocrit, hemoglobin, serum leucine, matrix metalloproteinase-9 (MMP-9) proteolytic activity, and presence of IGFBP-3 protease activity. No evidence for IGFBP-3 proteolysis was observed. Significant increases in [free IGF-I] and [leucine] were observed in the EAA/CHO group only. Significant differences were noted in [IGFBP-1] and [IGFBP-2] across conditions. Significant increases in [GH] and MMP-9 activity were observed in all groups. These results indicate that post-exercise macronutrient ratio is a determinant of [free IGF-I], [IGFBP-1 and -2] and may play a role in modulating the IGF-I axis in vivo.

Triennial Growth Symposium: neural regulation of feed intake: modification by hormones, fasting, and disease.

Appetite is a complex process that results from the integration of multiple signals at the hypothalamus. The hypothalamus receives neural signals; hormonal signals such as leptin, cholecystokinin, and ghrelin; and nutrient signals such as glucose, FFA, AA, and VFA. This effect is processed by a specific sequence of neurotransmitters beginning with the arcuate nucleus and orexigenic cells containing neuropeptide Y or agouti-related protein and anorexigenic cells containing proopiomelanocortin (yielding the neurotransmitter α-melanocyte-stimulating hormone) or cells expressing cocaine amphetamine-related transcript. These so-called first-order neurons act on second-order orexigenic neurons (containing either melanin-concentrating hormone or orexin) or act on anorexigenic neurons (e.g., expressing corticotropin-releasing hormone) to alter feed intake. In addition, satiety signals from the liver and gastrointestinal tract signal through the vagus nerve to the nucleus tractus solitarius to cause meal termination, and in combination with the hypothalamus, integrate the various signals to determine the feeding response. The activities of these neuronal pathways are also influenced by numerous factors such as nutrients, fasting, and disease to modify appetite and hence affect growth and reproduction. This review will begin with the central nervous system pathways and then discuss the ways in which hormones and metabolites may alter the process to affect feed intake with emphasis on farm animals.

Interaction of kisspeptin and the somatotropic axis.

Kisspeptin, a regulator of gonadotropin-releasing hormone, has been hypothesized as an integrator of nutrition and hormones critical to metabolism and the regulation of reproduction. Growth hormone (GH) is necessary for optimal reproduction and recent evidence suggests that its secretion may be influenced by kisspeptin. The objectives of this study were to determine whether the effect of kisspeptin to stimulate GH release is due to an interaction with growth hormone-releasing hormone (GHRH) or somatostatin (SS), or an effect at the hypothalamus. Intravenous injection and infusion of kisspeptin [500 pmol/kg BW (650 ng/kg)/h × 5 h] to cows (n = 5) increased serum concentrations of luteinizing hormone (LH) but not GH. Pretreatment with kisspeptin injection and infusion in cows (n = 5) reduced the stimulatory effect of GHRH (0.05 µg/kg BW) on GH secretion. However, the magnitude of the GH response to GHRH (assessed by incremental AUC) was not affected by kisspeptin. In these same cows, administration of kisspeptin prevented the increase in GH induced by SS infusion (0.5 µg/kg BW/ h × 1.5 h) withdrawal. Peripheral administration of kisspeptin [200 and 1,000 pmol/kg BW (260 and 1,300 ng/kg)] increased serum concentrations of LH but not GH in ewes (n = 8). However, concentrations of GH were stimulated by central kisspeptin treatment [100 and 200 pmol/kg BW (130 and 260 ng/kg)] in ewes. In addition to activating the gonadotropic axis, kisspeptin can activate the somatotropic axis in ruminants. Present data support the concept of a central site of action for this effect.

Selected hormonal and neurotransmitter mechanisms regulating feed intake in sheep.

Appetite control is a major issue in normal growth and in suboptimal growth performance settings. A number of hormones, in particular leptin, activate or inhibit orexigenic or anorexigenic neurotransmitters within the arcuate nucleus of the hypothalamus, where feed intake regulation is integrated. Examples of appetite regulatory neurotransmitters are the stimulatory neurotransmitters neuropeptide Y (NPY), agouti-related protein (AgRP), orexin and melanin-concentrating hormone and the inhibitory neurotransmitter, melanocyte-stimulating hormone (MSH). Examination of messenger RNA (using in situ hybridization and real-time PCR) and proteins (using immunohistochemistry) for these neurotransmitters in ruminants has indicated that physiological regulation occurs in response to fasting for several of these critical genes and proteins, especially AgRP and NPY. Moreover, intracerebroventricular injection of each of the four stimulatory neurotransmitters can increase feed intake in sheep and may also regulate either growth hormone, luteinizing hormone, cortisol or other hormones. In contrast, both leptin and MSH are inhibitory to feed intake in ruminants. Interestingly, the natural melanocortin-4 receptor (MC4R) antagonist, AgRP, as well as NPY can prevent the inhibition of feed intake after injection of endotoxin (to model disease suppression of appetite). Thus, knowledge of the mechanisms regulating feed intake in the hypothalamus may lead to mechanisms to increase feed intake in normal growing animals and prevent the wasting effects of severe disease in animals.

Low doses of estradiol partly inhibit release of GH in sheep without affecting basal levels.

Estradiol increases basal growth hormone (GH) concentrations in sheep and cattle. This study sought to determine the effects of estradiol on GH-releasing hormone (GRH)-stimulated GH release in sheep. Growth hormone secretory characteristics, the GH response to GRH, and steady-state GH mRNA concentrations were determined in castrated male lambs treated with 2 different doses of estradiol 17-beta for a 28-d experimental period. Although no differences between treatments in mean GH, basal GH, or GH pulse number were observed after 28 d of estradiol treatment, GH pulse amplitude was greater (P < 0.05) in the 2.00-cm implant-treated animals than in the control and 0.75-cm implant group. The effect of estradiol treatment on GRH-stimulated GH release revealed differences between the control and estradiol-treated animals (P < 0.05). The 15-min GH responses to 0.075 microg/kg hGRH in the control, 0.75-cm, and 2.00-cm implant groups, respectively, were 76 +/- 10, 22.6 +/- 2.1, and 43.6 +/- 15.0 ng/mL. Growth hormone mRNA content was determined for pituitary glands from the different treatment groups, and no differences in steady-state GH mRNA levels were observed. There were no differences in the mean plasma concentrations of IGF-I, cortisol, T(3), or T(4) from weekly samples. Growth hormone release from cultured ovine pituitary cells from control sheep was not affected by estradiol after 72 h or in a subsequent 3-h incubation with estradiol combined with GRH. These data suggest that estradiol has differing actions on basal and GRH-stimulated GH concentrations in plasma, but the increase in pulse amplitude does not represent an increased pituitary sensitivity to GRH.

Comparative aspects of the endotoxin- and cytokine-induced endocrine cascade influencing neuroendocrine control of growth and reproduction in farm animals.

Disease in animals is a well-known inhibitor of growth and reproduction. Earlier studies were initiated to determine the effects of endotoxin on pituitary hormone secretion. These studies found that in sheep, growth hormone (GH) concentration was elevated, whereas insulin-like growth factor-I (IGF-I) was inhibited, as was luteinizing hormone (LH). Examination of the site of action of endotoxin in sheep determined that somatotropes expressed the endotoxin receptor (CD14) and that both endotoxin and interleukin-I beta activated GH secretion directly from the pituitary. In the face of elevated GH, there is a reduction of IGF-I in all species examined. As GH cannot activate IGF-I release during disease, there appears to be a downregulation of GH signalling at the liver, perhaps related to altered nitration of Janus kinase (JAK). In contrast to GH downregulation, LH release is inhibited at the level of the hypothalamus. New insights have been gained in determining the mechanisms by which disease perturbs growth and reproduction, particularly with regard to nitration of critical control pathways, with this perhaps serving as a novel mechanism central to lipopolysaccharide suppression of all signalling pathways. This pathway-based analysis is critical to the developing novel strategies to reverse the detrimental effect of disease on animal production.

Central role of the melanocortin-4 receptors in appetite regulation after endotoxin.

Melanocortin-4 receptors (MC4R) are key factors in the depression of appetite during disease. This study was designed to determine the role of agouti-related protein (AgRP) in the effect of endotoxin (lipopolysaccharide, LPS) on appetite. Sheep received an intracerebroventricular injection of either saline or AgRP (0.5 nmol/kg of BW) 1 h before intravenous injection of either saline or LPS (0.6 microg/kg of BW) at time 0 and again at 4 h. Agouti-related protein prevented the reduction in feed intake due to LPS (P < 0.05). In a second experiment, AgRP gene expression was unaffected at 3 h and increased (P < 0.01) at 6 h after LPS. Immunohistochemical evidence indicated that there was an increase in the percentage of AgRP neurons with c-Fos immunoreactive nuclei 6 h after sheep were injected with LPS (P < 0.04) and a corresponding decrease in a-melanocyte-stimulating hormone neurons coexpressing c-Fos (P < 0.001). In situ hybridization provided evidence for an increase in AgRP gene expression and a decrease in proopiomelanocortin gene expression 6 h after LPS (P < 0.05). In a final experiment, physiological elevation of orexigenic agents by short-term fasting kept feed intake at the same level as controls, in spite of the presence of LPS, similar to the effects of AgRP in Exp. 1. The AgRP inhibition of the MC4R prevents appetite inhibition in response to LPS and well after LPS inhibition of feed intake, both AgRP and a-melanocyte-stimulating hormone may change in a pattern that favors appetite increases. These studies support the notion of the MC4R as a critical component of the mechanism for appetite suppression due to endotoxin.

Critical control points in the impact of the proinflammatory immune response on growth and metabolism.

Intrinsic in the equation for successful animal production is the efficiency of nutrient use for assimilation into useful animal-derived products. However, when young growing animals encounter various stressors that activate the proinflammatory response (PR), the biochemical effects of the resulting cascade of PR mediators [cytokines, prostaglandin and prosta-cyclin derivatives, nitric oxide (NO), superoxide anion (O2(.-)), etc.] override the regulatory signals normally ascribed to anabolic tissue accretion and growth. The efficiency of energy and nutrient use will proportionally decrease for growth rate due to the redirection of nutrient use to support immune defense processes. These proinflammatory events can develop in association with infectious disease but also are apparent in and a part of the natural response to birth, parturition, and weaning. If growth patterns are tracked during the PR, growth deficits are often apparent. Some growth deficits are relatively transient in duration, whereas others are quite long lasting, persisting although traditional clinical markers of PR are no longer evident. Recent evidence indicates that the PR cascades initiated by cytokines like tumor necrosis factor-alpha play a major role in these growth deficits. Perturbations in mitochondrial energetics and NO and O2(.-) interactions further affect metabolic balance. Free radicals and reactive nitrogen intermediates interact with select molecular targets in proteins (i.e., enzymes, histone proteins, and signal transduction proteins), causing the nitration and nitrosylation of select amino acids. If these posttranslational modifications occur in proteins associated with control points critical in metabolic stability, the resulting altered protein structure blocks its functionality. Attenuation of these overt posttranslational protein modifications at their site of production offers a strategy to minimize their detrimental impact while preserving needed cytokine, NO, and O2(.-) functions.

Long-term feed intake regulation in sheep is mediated by opioid receptors.

These experiments were conducted to determine if 1) syndyphalin-33 (SD33), a mu-opioid receptor ligand, affects feed intake; 2) SD33 effects on feed intake are mediated by actions on opioid receptors; and 3) its activity can counteract the reduction in feed intake associated with administration of bacterial endotoxin. In Exp. 1, 5 mixed-breed, castrate male sheep were housed indoors in individual pens. Animals had ad libitum access to water and concentrate feed. Saline (SAL; 0.9% NaCl) or SD33 (0.05 or 0.1 micromol/kg of BW) was injected i.v., and feed intake was determined at 2, 4, 6, 8, 24, and 48 h after the i.v. injections. Both doses of SD33 increased (at least P < 0.01) feed intake at 48 h relative to saline. In Exp. 2, SAL + SAL, SAL + SD33 (0.1 micromol/kg of BW), naloxone (NAL; 1 mg/kg of BW) + SAL, and NAL + SD33 were injected i.v. Food intake was determined as in Exp. 1. The SAL + SD33 treatment increased (P = 0.022) feed intake at 48 h relative to SAL + SAL. The NAL + SAL treatment reduced (at least P < 0.01) feed intake at 4, 6, 8, 24, and 48 h, whereas the combination of NAL and SD33 did not reduce feed intake at 24 (P = 0.969) or 48 h (P = 0.076) relative to the saline-treated sheep. In Exp. 3, sheep received 1 of 4 treatments: SAL + SAL, SAL + 0.1 micromol of SD33/kg of BW, 0.1 microg of lipopolysaccharide (LPS)/kg of BW + SAL, or LPS + SD33, and feed intake was monitored as in Exp. 1. Lipopolysaccharide suppressed cumulative feed intake for 48 h (P < 0.01) relative to saline control, but SD33 failed to reverse the reduction in feed intake during this period. These data indicate that SD33 increases feed intake in sheep after i.v. injection, and its effects are mediated via opioid receptors. However, the LPS-induced suppression in feed intake cannot be overcome by the opioid receptor ligand, SD33.

Modeling growth factor activity during proinflammatory stress: methodological considerations in assessing cytokine modulation of IGF binding proteins released by cultured bovine kidney epithelial cells.

The present research was conducted to model potential mechanisms through which IGFBPs might be affected by a key proinflammatory response initiating cytokine tumor necrosis factor (TNF-)-alpha. Madin-Darby bovine kidney epithelial (MDBK) cells, known to release IGFBPs in response to several stimuli, were grown under several conditions and challenged with forskolin (F) or recombinant TNF-alpha for 24h. Forskolin increased IGFBP-3 gene expression and media content of BP-3 protein. TNF-alpha increased basal and augmented F-mediated IGFBP-3 gene expression. However, TNF-alpha effects on the measurable media content of IGFBPs were influenced by culture conditions; in the absence of added protease inhibitors (PIs) or sufficient media albumin concentration (high BSA, 1mg/ml), the effect of TNF-alpha was to decrease (P<0.02) measurable IGFBPs. In the presence of PI and high BSA, media IGFBP-3 levels were shown to be increased by TNF-alpha consistent with the gene expression data. Changes in media IGFBP-3 protease activity were examined further to explain the observed effects of TNF-alpha on production and destruction of IGFBPs in media. When recombinant human IGFBP-3 (500 ng/ml) was added to PI-free, low BSA 100 microg/ml) media from TNF-treated MDBK cells, less than 10% of the BP-3 was recognizable by Western blot in 30 min; conversely, inclusion of High BSA and PI in media resulted in attenuation of the protease effect on the IGFBPs. The data suggest that the MDBK model of cellular response to proinflammatory stimulus is affected by culture conditions and that TNF-alpha affects media content of IGFBPs through effects on IGFBP gene expression coupled with degradation of IGFBPs via enhanced proteolytic enzyme release.

Interleukin-1beta and tumor necrosis factor-alpha mediation of endotoxin action on growth hormone.

In humans and sheep, endotoxin (LPS) administration results in increased growth hormone (GH) concentrations. To determine the role of cytokines in the effect of LPS on GH, sheep were challenged with IL-1beta or TNF-alpha. GH data were compared with results with LH, where the major effects of LPS are known to act via the hypothalamus. Intracerebroventricular (icv) administration of IL-1beta or TNF-alpha did not alter plasma concentrations of GH. Endotoxin was then administered intravenously (iv) in combination with icv injection of IL-1 receptor antagonist (IL-1RA), TNF antagonist (sTNF-R1), or saline. Administration of LPS increased GH (P < 0.0001), although coadministration of IL-1ra or sTNF-R1 icv did not alter GH response to LPS. In contrast, plasma concentrations of LH were profoundly inhibited by icv administration of either cytokine (P < 0.03), but the LH response to LPS was not altered by cytokine antagonists. Intravenous administration of either IL-1beta or TNF-alpha increased plasma concentrations of GH (P < 0.0001). Administration of IL-1RA and sTNF-R1 iv prevented LPS-induced increases in GH. Although LH was suppressed by high iv doses of IL-1beta (P = 0.0063), the antagonists did not alter the LH response to LPS. To determine whether LPS might directly activate GH release, confocal microscopy revealed colocalization of CD14, the LPS receptor, with GH and, to a lesser extent, LH and some prolactin (PRL)-containing cells, but not ACTH or TSH. These data are consistent with the effects of LPS on GH secretion originating through peripheral cytokine presentation to the pituitary, as well as a potential to act directly on selective populations of pituitary cells via CD14.

Melanocortin-4 receptor in sheep: a potential site for therapeutic intervention in disease models.

Reduced appetite combined with increased metabolic rate and decreased lean body mass is a major consequence of disease and other stressors. Studies in rodent species suggest that an understanding of appetite regulation may provide methodologies for intervention to prevent the deterioration of body mass such as observed with cancer or infectious diseases. For example, melanocortin-4 receptor (MC4-R) antagonists have shown a remarkable ability to reverse or prevent cachexia in rodents with sarcoma or treated with endotoxin. Studies in sheep have indicated that a number of peptide neurotransmitters may have a role in regulating appetite in this species. For example, agouti related protein mRNA and protein levels are dramatically altered with fasting in sheep. Moreover, agouti related protein, neuropeptide Y, melanin concentrating hormone and orexin are potent stimuli to increase feed intake in sheep. Recent studies have indicated that one of these neurotransmitters, NPY, can work in principal to improve appetite in endotoxin-treated sheep. Current studies are examining the role that MC4-R antagonists may have in the prevention or correction of body mass wasting diseases as well as practical applications in animal production.

Intracerebroventricular melanin-concentrating hormone stimulates food intake in sheep.

Melanin-concentrating hormone (MCH) stimulates feeding when injected intracerebroventricularly (ICV) in rats. At present it is not clear whether the function of MCH is similar in ruminants, which are species with a continuous delivery of nutrients. Therefore the current investigation sought to determine the role of MCH in sheep. In the first experiment, six, castrate male sheep were satiated and received one of four treatments [saline, 0.1, or 1.0 nmol/kg MCH, and NPY (0.1 nmol/kg)] injected ICV over 30s, then infused ICV for 6 h ( approximately 500 microl/h). Food intake was measured for 2 h before and at 2, 4, 6, 8, 12 and 24 h. In this experiment, feed intake was increased (P

Leptin, tumor necrosis factor-alpha (TNF), and CD14 in ovine adipose tissue and changes in circulating TNF in lean and fat sheep.

Four studies were designed to determine whether 1) tumor necrosis factor-alpha (TNF) and the Lipopolysaccharide (LPS) binding ligand, CD14, are produced by sheep adipose tissue; 2) nutritional reserves and/or short-term fasting affect circulating concentrations of TNF; 3) there is a relationship between TNF and metabolic factors in sheep; and 4) inflammation alters circulating concentrations of leptin. In Exp. 1 and 2, ewes were assigned, based on ultrasonic assessments of last-rib subcutaneous fat measurements to fat (fat thickness > 1 cm; mean = 1.52 +/- 0.03 cm) or thin (fat thickness < 1 cm; mean = 0.25 +/- 0.03 cm) groups. Fat and thin ewes were assigned to fed or fasted groups for a total of four groups (fed-fat; fasted-fat; fed-thin; fasted-thin). Fed-ewes had ad libitum access to feed, and fasted-ewes were prohibited feed 48 h before initiation of sample collection. In Exp. 1, subcutaneous fat samples were collected from just above the last rib for detection of TNF and CD14 mRNA, and immunoreactivity. Tumor necrosis factor-alpha-like immunoreactivity in adipocytes was sparse, more pronounced in cells in fed-ewes than fasted-ewes, and localized to membranes between adjacent cells in nucleated regions. Immunoreactivity for CD14 was minimally observed but present in adipocytes and widely expressed in infiltrating monocytes and epithelial vascular cells. Leptin was detected in adipocytes. In Exp. 2, plasma samples collected every 6 h for 24 h were analyzed for plasma concentrations of TNF. Fat ewes had greater plasma concentrations of TNF than thin ewes (P = 0.039). In Exp. 3, wethers were injected i.v. with interleukin-1beta or TNF. Blood samples were collected every 15 min for 8 h following injection. Plasma concentration of leptin was not affected by treatment (P > 0.39). In Exp. 4, wethers were injected with LPS. Blood samples were collected every 15 min for 8 h following injection. Plasma concentration of leptin was not altered by LPS (P > 0.20). These results provide evidence: 1) of TNF-like immunoreactivity within fat tissue; 2) that elements within fatty tissues have CD14 that may allow adipocyte function to be directly affected by LPS; 3) that plasma concentrations of leptin are not altered by LPS treatment; and 4) that circulating concentrations of TNF are elevated with obesity in sheep.

Endotoxin inhibition of luteinizing hormone in sheep.

Administration of endotoxin suppresses circulating concentration of luteinizing hormone (LH) in a number of species, including rats, sheep, cattle, and non-human primates. Specifically, endotoxin administration decreases circulating concentration of LH and LH pulses frequency in castrated male sheep. Endotoxin could alter circulating concentrations of LH via actions at the hypothalamus through altered GnRH production and/or release, or endotoxin could alter circulating concentrations of LH at the level of the pituitary via inhibition of LH production and release or inhibition of LH in response to GnRH. The site of endotoxin suppression of circulating concentrations of LH as well as possible mediators of endotoxin suppression of circulating concentrations of LH, including cortiocotropin-releasing hormone, arginine vasopressin, glucocorticoids, inflammatory cytokines, prostaglandins, and opioids, are discussed.