Endogenous alpha-MSH modulates the hypothalamic-pituitary-adrenal response to the cytokine interleukin-1beta

Functional heterogeneity of POMC neurons relies on mTORC1 signaling

Hypothalamic pro-opiomelanocortin (POMC) neurons are known to trigger satiety. However, these neuronal cells encompass heterogeneous subpopulations that release γ-aminobutyric acid (GABA), glutamate, or both neurotransmitters, whose functions are poorly defined. Using conditional mutagenesis and chemogenetics, we show that blockade of the energy sensor mechanistic target of rapamycin complex 1 (mTORC1) in POMC neurons causes hyperphagia by mimicking a cellular negative energy state. This is associated with decreased POMC-derived anorexigenic α-melanocyte-stimulating hormone and recruitment of POMC/GABAergic neurotransmission, which is restrained by cannabinoid type 1 receptor signaling. Electrophysiology and optogenetic studies further reveal that pharmacological blockade of mTORC1 simultaneously activates POMC/GABAergic neurons and inhibits POMC/glutamatergic ones, implying that the functional specificity of these subpopulations relies on mTORC1 activity. Finally, POMC neurons with different neurotransmitter profiles possess specific molecular signatures and spatial distribution. Altogether, these findings suggest that mTORC1 orchestrates the activity of distinct POMC neurons subpopulations to regulate feeding behavior.

Nicotinic Cholinergic System in the Hypothalamus Modulates the Activity of the Hypothalamic Neuropeptides During the Stress Response

BACKGROUND

The hypothalamus harbors high levels of cholinergic neurons and axon terminals. Nicotinic acetylcholine receptors, which play an important role in cholinergic neurotransmission, are expressed abundantly in the hypothalamus. Accumulating evidence reveals a regulatory role for nicotine in the regulation of the stress responses. The present review will discuss the hypothalamic neuropeptides and their interaction with the nicotinic cholinergic system. The anatomical distribution of the cholinergic neurons, axon terminals and nicotinic receptors in discrete hypothalamic nuclei will be described. The effect of nicotinic cholinergic neurotransmission and nicotine exposure on hypothalamic-pituitaryadrenal (HPA) axis regulation at the hypothalamic level will be analyzed in view of the different neuropeptides involved.

METHODS

Published research related to nicotinic cholinergic regulation of the HPA axis activity at the hypothalamic level is reviewed.

RESULTS

The nicotinic cholinergic system is one of the major modulators of the HPA axis activity. There is substantial evidence supporting the regulation of hypothalamic neuropeptides by nicotinic acetylcholine receptors. However, most of the studies showing the nicotinic regulation of hypothalamic neuropeptides have employed systemic administration of nicotine. Additionally, we know little about the nicotinic receptor distribution on neuropeptide-synthesizing neurons in the hypothalamus and the physiological responses they trigger in these neurons.

CONCLUSION

Disturbed functioning of the HPA axis and hypothalamic neuropeptides results in pathologies such as depression, anxiety disorders and obesity, which are common and significant health problems. A better understanding of the nicotinic regulation of hypothalamic neuropeptides will aid in drug development and provide means to cope with these diseases. Considering that nicotine is also an abused substance, a better understanding of the role of the nicotinic cholinergic system on the HPA axis will aid in developing improved therapeutic strategies for smoking cessation.

Altered levels of α-melanocyte stimulating hormone in cerebrospinal fluid and plasma of patients with traumatic brain injury

Traumatic brain injury (TBI) is closely associated with marked inflammation. Although alpha-Melanocyte-Stimulating Hormone (α-MSH) exerts powerful anti-inflammatory effects, changes in endogenous α-MSH levels following TBI remain poorly understood. We investigated the changes of α-MSH levels in the cerebrospinal fluid (CSF) and plasma of post-TBI patients and the association of these changes with the severity of TBI and inflammation. TBI severity was assessed by the GCS coma scale from which, patients were separated into three groups. Clinical data were collected on days 1, 3, 5, and 7 including levels of α-MSH, tumor necrosis factor (TNF-α), and intracranial pressure (ICP). α-MSH levels in CSF steadily increased for one week (peak at day 5) but plasma α-MSH decreased and remained low. These changes were more substantial in the Severe Group of TBI with lower GCS. TNF-α levels were similarly increased in both CSF and plasma (peak at day 3). In the early phase of TBI elevated TNF-α and ICP dominated, and CSF α-MSH displayed a slow and insufficient increase. In later phases of TBI, TNF-α and ICP levels were alleviated concordantly with sustained increases in central α-MSH, wherein an anti-inflammatory environment might predominate. The relationship between plasma α-MSH and TNF-α showed significant negative correlation, and the relationship between CSF α-MSH and TNF-α showed significant positive correlation with a two-day lag. In conclusion, plasma α-MSH levels decreased, but CSF levels increased slowly following TBI. These changes were more substantial in severe patients with a lower GCS. Increases in central α-MSH paralleled alleviation of inflammation.

Proopiomelanocortin, agouti-related protein, and leptin in human cerebrospinal fluid: correlations with body weight and adiposity

Leptin and its neuronal targets, which produce proopiomelanocortin (POMC) and agouti-related protein (AgRP), regulate energy balance. This study characterized leptin, POMC, and AgRP in the cerebrospinal fluid (CSF) of 47 healthy human subjects, 23 lean and 24 overweight/obese (OW/OB), as related to BMI, adiposity, plasma leptin, soluble leptin receptor (s-OB-R), and insulin. POMC was measured since the POMC prohormone is the predominant POMC peptide in CSF and correlates with hypothalamic POMC in rodents. Plasma AgRP was similarly characterized. CSF leptin was 83-fold lower than in plasma and correlated strongly with BMI, body fat, and insulin. The relative amount of leptin transported into CSF declined with increasing BMI, ranging from 4.5 to 0.52%, consistent with a saturable transport mechanism. CSF sOB-R was 78-fold lower than in plasma and correlated negatively with plasma and CSF leptin. CSF POMC was higher in lean vs. OW/OB subjects (P < 0.001) and correlated negatively with CSF leptin (r = -0.60, P < 0.001) and with plasma leptin, insulin, BMI, and adiposity. CSF AgRP was not different in lean vs. OW/OB; however, plasma AgRP was higher in lean subjects (P = 0.001) and correlated negatively with BMI, adiposity, leptin, insulin, and HOMA (P < 0.005). Thus, CSF measurements may provide useful biomarkers for brain leptin and POMC activity. The striking negative correlation between CSF leptin and POMC could be secondary to leptin resistance and/or neuronal changes associated with obesity but may also indicate that POMC plays a primary role in regulating body weight and adiposity. The role of plasma AgRP as a neuroendocrine biomarker deserves further study.

Central insulin signaling modulates hypothalamus-pituitary-adrenal axis responsiveness

Objective

Obesity is often accompanied by hyperactivity of the neuroendocrine stress axis and has been linked to an increased risk of psychiatric disorders. Insulin is reciprocally regulated with the stress hormone corticosterone (CORT), raising the possibility that insulin normally provides inhibitory tone to the hypothalamus-adrenal-pituitary (HPA) axis. Here we examined whether disrupting signaling via the insulin receptor (InsR) in hypothalamic subpopulations impacts the neuroendocrine response to acute psychological stress.

Methods

We used Nkx2.1-Cre, Sim1-Cre and Agrp-Cre transgenic driver lines to generate conditional knockouts of InsR signaling throughout the hypothalamus, paraventricular nucleus of the hypothalamus (PVH) and in neurons expressing Agouti-related peptide (AgRP) in the arcuate nucleus of the hypothalamus (ARH), respectively. We used a combination of molecular, behavioral and neuroendocrine criteria to evaluate the consequences on HPA axis responsiveness.

Results

Endpoints related to body weight and glucose homeostasis were not altered in any of the conditional mutant lines. Consistent with observations in the neuronal Insr knockout mice (NIRKO), baseline levels of serum CORT were similar to controls in all three lines. In male mice with broad disruptions of InsR signals in Nkx2.1-expressing regions of the hypothalamus (IR^Nkx2.1 KO), we observed elevated arginine vasopressin (AVP) levels at baseline and heightened neuroendocrine responses to restraint stress. IR^Nkx2.1 KO males also exhibited increased anxiety-like behaviors in open field, marble burying, and stress-induced hyperthermia testing paradigms. HPA axis responsivity was not altered in IR^Sim1 KO males, in which InsR was disrupted in the PVH. In contrast to observations in the IR^Nkx2.1 KO males, disrupting InsR signals in ARH neurons expressing Agrp (IR^Agrp KO) led to reduced AVP release in the median eminence (ME).

Conclusions

We find that central InsR signals modulate HPA responsivity to restraint stress. InsR signaling in AgRP/NPY neurons appears to promote AVP release, while signaling in other hypothalamic neuron(s) likely acts in an opposing fashion. Alterations in InsR signals in neurons that integrate metabolic and psychiatric information could contribute to the high co-morbidity of obesity and mental disorders.

Regulation of prolactin in mice with altered hypothalamic melanocortin activity

This study used two mouse models with genetic manipulation of the melanocortin system to investigate prolactin regulation. Mice with overexpression of the melanocortin receptor (MC-R) agonist, α-melanocyte-stimulating hormone (Tg-MSH) or deletion of the MC-R antagonist agouti-related protein (AgRP KO) were studied. Male Tg-MSH mice had lower blood prolactin levels at baseline (2.9±0.3 vs. 4.7±0.7ng/ml) and after restraint stress (68±6.5 vs. 117±22ng/ml) vs. WT (p<0.05); however, pituitary prolactin content was not different. Blood prolactin was also decreased in male AgRP KO mice at baseline (4.2±0.5 vs. 7.6±1.3ng/ml) and after stress (60±4.5 vs. 86.1±5.7ng/ml) vs. WT (p<0.001). Pituitary prolactin content was lower in male AgRP KO mice (4.3±0.3 vs. 6.7±0.5μg/pituitary, p<0.001) vs. WT. No differences in blood or pituitary prolactin levels were observed in female AgRP KO mice vs. WT. Hypothalamic dopamine activity was assessed as the potential mechanism responsible for changes in prolactin levels. Hypothalamic tyrosine hydroxylase mRNA was measured in both genetic models vs. WT mice and hypothalamic dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) content were measured in male AgRP KO and WT mice but neither were significantly different. However, these results do not preclude changes in dopamine activity as dopamine turnover was not directly investigated. This is the first study to show that baseline and stress-induced prolactin release and pituitary prolactin content are reduced in mice with genetic alterations of the melanocortin system and suggests that changes in hypothalamic melanocortin activity may be reflected in measurements of serum prolactin levels.

Changes in melanocortin expression and inflammatory pathways in fetal offspring of nonhuman primates fed a high-fat diet

The hypothalamic melanocortin system, which controls appetite and energy expenditure, develops during the third trimester in primates. Thus, maternal nutrition and health may have a profound influence on the development of this system. To study the effects of chronic maternal high-fat diet (HFD) on the development of the melanocortin system in the fetal nonhuman primate, we placed adult female macaques on either a control (CTR) diet or a HFD for up to 4 yr. A subgroup of adult female HFD animals was also switched to CTR diet during the fifth year of the study (diet reversal). Third-trimester fetuses from mothers on HFD showed increases in proopiomelanocortin mRNA expression, whereas agouti-related protein mRNA and peptide levels were decreased in comparison with CTR fetuses. Proinflammatory cytokines, including IL-1beta and IL-1 type 1 receptor, and markers of activated microglia were elevated in the hypothalamus, suggesting an activation of the local inflammatory response. Fetuses of diet-reversal mothers had normal melanocortin levels. These results raise the concern that chronic consumption of a HFD during pregnancy, independent of maternal obesity and diabetes, can lead to widespread activation of proinflammatory cytokines that may alter the development of the melanocortin system. The abnormalities in the fetal POMC system, if maintained into the postnatal period, could impact several systems, including body weight homeostasis, stress responses, and cardiovascular function. Indeed, the HFD offspring develop early-onset excess weight gain. These abnormalities may be prevented by healthful nutrient consumption during pregnancy even in obese and severely insulin-resistant individuals.

Effects of estradiol on cerebrospinal fluid levels of agouti-related protein in ovariectomized rhesus monkeys

Hypothalamic proopiomelanocortin (POMC)-derived MSH peptides and the melanocortin receptor antagonist, agouti-related protein (AgRP), interact to regulate energy balance. Both POMC and AgRP neurons express estrogen receptors, but little is known about estrogen regulation of the melanocortin system in the primate. We have therefore examined the effects of physiological doses of estradiol (E2) on POMC and AgRP in lumbar cerebrospinal fluid (CSF) of ovariectomized monkeys. POMC prohormone was measured by ELISA. AgRP was measured by RIA (sensitive for the more biologically active C-terminal AgRP(83-132) but also detects full-length AgRP) and by ELISA (measures primarily full length AgRP). In the first experiment, 14 animals were studied before and after 3 wk of E2. CSF POMC did not change, but AgRP(RIA) decreased from 7.9 +/- 1.2 to 4.7 +/- 1.2 fmol/ml after E2 (P = 0.03) and the POMC/AgRP(RIA) ratio increased from 4.2 +/- 0.89 to 6.8 +/- 1.04 (P = 0.04). AgRP(ELISA) did not change, but the ratio of AgRP(RIA) compared with AgRP(ELISA) was reduced after E2 (P = 0.02). In the second experiment, 11 animals were studied after 6 wk of E2, and similar changes were noted. The degree of AgRP(RIA) suppression with E2 was inversely related to body mass index (r = 0.569; P = 0.03). These results show for the first time that E2 suppresses AgRP(C-terminal) in CSF, increases the POMC to AgRP ratio, and may decrease AgRP processing, thus leading to increased melanocortin signaling. Furthermore, obesity was associated with resistance to the suppressive effects of E2 on AgRP, analogous to what is seen with obesity and leptin resistance.

The obesity susceptibility gene Cpe links FoxO1 signaling in hypothalamic pro-opiomelanocortin neurons with regulation of food intake

Reduced food intake brings about an adaptive decrease in energy expenditure that contributes to the recidivism of obesity following weight loss. Insulin and leptin inhibit food intake through actions in the central nervous system that are partly mediated by FoxO1. We show that FoxO1 ablation in pro–opiomelanocortin (Pomc) neurons (Pomc–Foxo1^−/−) reduces food intake without affecting energy expenditure. Analyses of hypothalamic neuropeptides in Pomc–Foxo1^−/− mice reveal selective increases of α–Msh and COOH–cleaved β–endorphin, the products of Carboxypeptidase E (Cpe)–dependent processing of Pomc. We show that Cpe is decreased in diet–induced obesity, and that FoxO1 deletion offsets the decrease, protecting against weight gain. Moreover, moderate Cpe overexpression in the arcuate nucleus phenocopies features of the FoxO1 mutation. The dissociation of food intake from energy expenditure in Pomc–Foxo1^−/− mice represents a model for therapeutic intervention in obesity, and raises the possibility of targeting Cpe to develop weight loss medications.

Anti-inflammatory influences on behavioral, but not cortisol, responses during maternal separation

The present study examined the effect of anti-inflammatory agents on the behavioral and plasma cortisol responses of guinea pig pups during isolation in a novel environment. In Experiment 1, intracerebroventricular (i.c.v.) administration of interleukin-10 (IL-10), across a range of doses, significantly reduced passive behaviors of isolated pups. In Experiment 2, neither i.c.v. IL-10 nor alpha-melanocyte stimulating hormone affected the plasma cortisol response to the separation procedure. These results provide further evidence that the passive behaviors of isolated guinea pig pups are mediated by proinflammatory cytokines and indicate that cytokines do not affect behavior indirectly by means of an action on hypothalamic-pituitary-adrenal activity.

Role of alpha-melanocyte stimulating hormone and melanocortin 4 receptor in brain inflammation

Inflammatory processes contribute widely to the development of neurodegenerative diseases. The expression of many inflammatory mediators was found to be increased in central nervous system (CNS) disorders suggesting that these molecules are major contributors to neuronal damage. Melanocortins are neuropeptides that have been implicated in a wide range of physiological processes. The melanocortin alpha-melanocyte stimulating hormone (alpha-MSH) has pleiotropic functions and exerts potent anti-inflammatory actions by antagonizing the effects of pro-inflammatory cytokines and by decreasing important inflammatory mediators. Five subtypes of melanocortin receptors (MC1R-MC5R) have been identified. Of these, the MC4 receptor is expressed predominantly throughout the CNS. Evidence of effectiveness of selective MC4R agonists in modulating inflammatory processes and their low toxicity suggest that these molecules may be useful in the treatment of CNS disorders with an inflammatory component. This review describes the involvement of the MC4R in central anti-inflammatory effects of melanocortins and discusses the potential value of MC4R agonists for the treatment of inflammatory-related disorders.

Effects of nutrition on neuro-endocrine stress responses

PURPOSE OF REVIEW

Past studies in humans and animals have shown that low blood glucose concentrations due to fasting negatively interfere with the ability to mount a hypothalamus-pituitary-adrenal (HPA) axis response to psychological stress or to pharmacological activation, respectively. This contradicts the classical view of the proposed primary glucocorticoid function of providing the individual with energy in fight-or-flight situations.

RECENT FINDINGS

Not many studies have followed up on this phenomenon in recent years, but our understanding of how appetite and satiety is regulated has significantly improved. Many of the neuropeptides involved in regulation of energy homeostasis interact with key areas of the HPA axis. The majority of orexigenic peptides have been shown to activate the HPA axis, while some anorexic peptides negatively modulate HPA axis activation and others also stimulate it.

SUMMARY

The effects of orexigenic peptides on the HPA axis are incompatible with the phenomenon of blunted HPA axis activity in states of low energy available, while the fact that some anorexigenic peptides activate the HPA axis might point to a permissive role. In conclusion, current data insufficiently explain negative HPA axis modulation by low glucose levels.

Effects of swim stress and α-MSH acute pre-treatment on brain 5-HT transporter and corticosterone receptor

The forced swim test (FST) can lead to stress-related diseases such as depression, through activation of hypothalamic-pituitary-adrenal axis (HPAA) and corticosteroid disregulation. Among the proopiomelanocortin (POMC)-derived peptides, alpha-melanocyte-stimulating hormone (alpha-MSH) has been shown to regulate long-lasting behavioral responses. Moreover, serotonergic pathways in various brain areas are activated by stressors, a feature that suggests a role for serotonin in both stress-induced HPAA disregulation and depressive physiopathology. Taking all together these data, we investigated the effects of the FST exposure and the effects of pre-treatment with alpha-MSH on cortical synaptosomal serotonin transporter (SERT) activity, corticosterone (CORT) plasma levels and on glucocorticoid receptor (GR) occupancy and expression in rat hippocampus. Young male rats were divided into three groups treated with saline or with alpha-MSH at doses of 1 or 4 microg/rat, 15 min prior to FST. Our data show that FST increased CORT secretion; GR levels in hippocampus decreased in density after stress without variations in affinity; GR redistributed from the cytosolic to the nuclear tissue fraction; finally, SERT activity strongly increased. All these effects were blocked by pre-treatment with alpha-MSH at the higher dose.

The Neuroendocrine–Immune Interactions in Systemic Lupus Erythematosus: A Basis for Understanding Disease Pathogenesis and Complexity

Much progress has been made in the understanding of the impact of the neuroendocrine immune interactions and the pathogenic role in systemic lupus erythematosus, clinically and at the molecular level. This article focuses on the intertwining networks that involve the hypothalamic-pituitary-adrenal axis, cytokines within the central nervous system, and the sympathetic system. Hormones (estrogen, prolactin, gonadotropin-releasing hormone, and leptin) play an important role as immunomodulatory agents.

α-MSH and γ-MSH inhibit IL-1β induced activation of the hypothalamic–pituitary–adrenal axis through central melanocortin receptors

Alpha-melanocyte-stimulating hormone (alpha-MSH) is a neuroimmunomodulatory peptide that is involved in the control of host responses trough modulation of production and action of proinflammatory cytokines in inflammatory cells in the periphery and within the central nervous system (CNS). However, little is known about the receptors that mediate the modulatory effects of alpha-MSH in the CNS. The objective of the present study was to establish the specific melanocortin receptors involved in the inhibition by MSH peptides of IL-1beta-induced activation of the HPA. i.c.v. injection of 12.5 ng of IL-1beta caused significant changes in plasma corticosterone, as compared to basal levels. The treatment with gamma-MSH (1 microg), an MC3 receptor agonist, resulted in significant reduction of the IL-1beta-induced plasma corticosterone levels. Administration of the MC3/MC4 receptor antagonist SHU9119 blocked this effect. Besides, treatment with a high dose of alpha-MSH (1 microg) increased plasma corticosterone. When alpha-MSH was given at a lower dose (0.1 microg), it did not modify corticosterone levels but caused an inhibitory effect on the corticosterone release induced by IL-1beta. The administration of SHU9119 or a more selective MC4 receptor antagonist like HS014 blocked the effects of alpha-MSH. In conclusion, our results suggest that both alpha-MSH and gamma-MSH are capable of inhibiting the effect of the IL-1beta on the activation of HPA axis acting at the CNS, and that this effect is mediated by specific central melanocortin receptors.

Neuronal expression of caspase-1 immunoreactivity in the rat central nervous system

Caspase-1/interleukin-1beta (IL-1beta)-converting enzyme (ICE) cleaves IL-1beta and IL-18 precursor proteins to the active forms of these proinflammatory cytokines. Since both cytokines are constitutively expressed in the brain, we investigated whether this is also the case for caspase-1. Using an antibody raised against the p10-subunit of the active enzyme, constitutive expression of caspase-1 immunoreactivity was found in nerve cells in the arcuate nucleus and in nerve fibres throughout the brain. Co-localisation with alpha-melanocyte stimulating hormone was demonstrated. The distribution pattern of caspase-1 immunoreactive structures is consistent with a role to produce mature IL-1beta in regions where IL-1beta mediates fever and sleep.

Circuitries Involved in the Control of Energy Homeostasis and the Hypothalamic-Pituitary-Adrenal Axis Activity

The regulation of bodyweight is a complex process involving the interplay of neuronal circuitries controlling food intake and energy expenditure (thermogenesis) with endocrine secretions modulating the activity of the neurons making up those circuitries. The neurons controlling food intake and thermogenesis also modulate the hypothalamic-pituitary-adrenal axis, the role of which in the regulation of energy balance has been acknowledged for some time. These neurons secrete various neuromolecules or neuropeptides including endocannabinoids, neuropeptide Y, agouti-related protein, melanin-concentrating hormone, orexins (hypocretins), melanocortins, cocaine- and amphetamine-regulated transcript, thyrotropin-releasing hormone, corticotropin-releasing hormone, and urocortins. Among those peptides, neuropeptide Y, agouti-related peptide, melanin-concentrating hormone, orexins, and endocannabinoids have been classified as being anabolic molecules whereas melanocortins, cocaine- and amphetamine-regulated transcript, thyrotropin-releasing hormone, and corticotropin-releasing hormone are referred to as catabolic peptides. The expression and secretion of these neuromolecules are known to be affected by the anabolic (corticosteroids and ghrelin) and catabolic (leptin, insulin, and glucagon-like peptide 1) peripheral hormones. A link is made between the pathways regulating energy balance and those modulating the activity of the hypothalamic-pituitary-adrenal axis.

Agouti-related protein stimulates the hypothalamic-pituitary-adrenal (HPA) axis and enhances the HPA response to interleukin-1 in the primate

alpha-MSH antagonizes many of the immune and neuroendocrine effects induced by inflammatory cytokines. Studies have shown that alpha-MSH attenuates the stimulatory effect of IL-1 on the hypothalamic-pituitary-adrenal (HPA) axis and plays a physiological role in limiting the HPA response to IL-1. Recently an alpha-MSH antagonist, agouti-related protein (AGRP), has been identified in the hypothalamus, which stimulates food intake by antagonizing the effects of alpha-MSH at specific melanocortin receptors. It is unknown whether AGRP can also modulate neuroendocrine responses to inflammatory cytokines. We have therefore examined the effects of AGRP on the HPA axis and on prolactin (PRL) at baseline and in response to stimulation by IL-1 beta in nine ovariectomized rhesus monkeys. In the first study, the effects of intracerebroventricular (i.c.v) infusion of 20 microg (n = 6) and 50 micro g (n = 4) of human AGRP (83-132)-NH(2) were compared with icv saline infusion. There was a significant stimulatory effect of 20 microg AGRP on cortisol release over time (P < 0.001). The area under the hormone response curve (AUC) for cortisol increased by 29% after 20 microg AGRP vs. saline; the AUC for ACTH increased by 166% (P = 0.028); the AUC for PRL increased by 108% (P = 0.046). There was a significant stimulatory effect of 50 microg AGRP on ACTH (P < 0.001), cortisol (P < 0.001), and PRL (P < 0.001) release over time. The AUC for ACTH after 50 microg AGRP increased by 98%; the AUC for cortisol increased by 37%; the AUC for PRL increased by 161%. The effects of AGRP on ACTH, cortisol, and PRL release were prevented by alpha-MSH infusion. In the second study, animals received icv either 50 ng of human IL-1 beta or 20 microg of AGRP followed by 50 ng IL-1 beta. AGRP significantly enhanced the ACTH (P < 0.05) response to IL-1 beta. The peak ACTH response to IL-1 beta alone was 124 +/- 55 pg/ml vs. 430 +/- 198 pg/ml after IL-1 beta plus AGRP; the peak cortisol response was 70 +/- 8.2 microg/dl vs. 77 +/- 6.2 microg/dl, but this was not significantly different. In conclusion, AGRP stimulated ACTH, cortisol, and PRL release in the monkey and enhanced the ACTH response to IL-1 beta. These studies suggest that, in addition to its known orexigenic effects, AGRP may play a role in neuroendocrine regulation and specifically that AGRP may interact with alpha-MSH to modulate neuroendocrine responses to inflammation.

Cytokines and neuro-immune-endocrine interactions: a role for the hypothalamic-pituitary-adrenal revolving axis

Cytokines, peptide hormones and neurotransmitters, as well as their receptors/ligands, are endogenous to the brain, endocrine and immune systems. These shared ligands and receptors are used as a common chemical language for communication within and between the immune and neuroendocrine systems. Such communication suggests an immunoregulatory role for the brain and a sensory function for the immune system. Interplay between the immune, nervous and endocrine systems is most commonly associated with the pronounced effects of stress on immunity. The hypothalamic-pituitary-adrenal (HPA) axis is the key player in stress responses; it is well established that both external and internal stressors activate the HPA axis. Cytokines are chemical messengers that stimulate the HPA axis when the body is under stress or experiencing an infection. This review discusses current knowledge of cytokine signaling pathways in neuro-immune-endocrine interactions as viewed through the triplet HPA axis. In addition, we elaborate on HPA/cytokine interactions in oxidative stress within the context of nuclear factor-kappaB transcriptional regulation and the role of oxidative markers and related gaseous transmitters.

Regulation of the hypothalamo-pituitary-adrenal axis by cytokines

Many of the pro-inflammatory cytokines which are released in response to immune/inflammatory insults exert marked stimulatory influences on the hypothalamo-pituitary-adrenocortical (HPA) axis; they thus provoke the release of glucocorticoids which, in turn, temper the ensuing immune-inflammatory response and thereby complete a homeostatic neuroendocrine-immune regulatory loop. This article reviews the putative mechanisms by which cytokines, released acutely in response to such insults, activate the HPA axis, placing particular emphasis on the actions and interactions of tumour necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1) and interleukin-6 (IL-6) and on the counter-regulatory mechanisms that are in place.