Interactions of orexins/hypocretins with adrenocortical functions

Mutual Effects of Orexin and Bone Morphogenetic Proteins on Catecholamine Regulation Using Adrenomedullary Cells

Orexins are neuronal peptides that play a prominent role in sleep behavior and feeding behavior in the central nervous system, though their receptors also exist in peripheral organs, including the adrenal gland. In this study, the effects of orexins on catecholamine synthesis in the rat adrenomedullary cell line PC12 were investigated by focusing on their interaction with the adrenomedullary bone morphogenetic protein (BMP)-4. Orexin A treatment reduced the mRNA levels of key enzymes for catecholamine synthesis, including tyrosine hydroxylase (Th), 3,4-dihydroxyphenylalanie decarboxylase (Ddc) and dopamine β-hydroxylase (Dbh), in a concentration-dependent manner. On the other hand, treatment with BMP-4 suppressed the expression of Th and Ddc but enhanced that of Dbh with or without co-treatment with orexin A. Of note, orexin A augmented BMP-receptor signaling detected by the phosphorylation of Smad1/5/9 through the suppression of inhibitory Smad6/7 and the upregulation of BMP type-II receptor (BMPRII). Furthermore, treatment with BMP-4 upregulated the mRNA levels of OX1R in PC12 cells. Collectively, the results indicate that orexin and BMP-4 suppress adrenomedullary catecholamine synthesis by mutually upregulating the pathway of each other in adrenomedullary cells.

Interaction of Orexin and Bone Morphogenetic Proteins in Steroidogenesis by Human Adrenocortical Cells

Orexins are neuropeptides that play important roles in sleep-wake regulation and food intake in the central nervous system, but their receptors are also expressed in peripheral tissues, including the endocrine system. In the present study, we investigated the functions of orexin in adrenal steroidogenesis using human adrenocortical H295R cells by focusing on its interaction with adrenocortical bone morphogenetic proteins (BMPs) that induce adrenocortical steroidogenesis. Treatment with orexin A increased the mRNA levels of steroidogenic enzymes including StAR, CYP11B2, CYP17, and HSD3B1, and these effects of orexin A were further enhanced in the presence of forskolin. Interestingly, orexin A treatment suppressed the BMP-receptor signaling detected by Smad1/5/9 phosphorylation and Id-1 expression through upregulation of inhibitory Smad7. Orexin A also suppressed endogenous BMP-6 expression but increased the expression of the type-II receptor of ActRII in H295R cells. Moreover, treatment with BMP-6 downregulated the mRNA level of OX1R, but not that of OX2R, expressed in H295R cells. In conclusion, the results indicate that both orexin and BMP-6 accelerate adrenocortical steroidogenesis in human adrenocortical cells; both pathways mutually inhibit each other, thereby leading to a fine-tuning of adrenocortical steroidogenesis.

Resilience to the effects of social stress on vulnerability to developing drug addiction

We review the still scarce but growing literature on resilience to the effects of social stress on the rewarding properties of drugs of abuse. We define the concept of resilience and how it is applied to the field of drug addiction research. We also describe the internal and external protective factors associated with resilience, such as individual behavioral traits and social support. We then explain the physiological response to stress and how it is modulated by resilience factors. In the subsequent section, we describe the animal models commonly used in the study of resilience to social stress, and we focus on the effects of chronic social defeat (SD), a kind of stress induced by repeated experience of defeat in an agonistic encounter, on different animal behaviors (depression- and anxiety-like behavior, cognitive impairment and addiction-like symptoms). We then summarize the current knowledge on the neurobiological substrates of resilience derived from studies of resilience to the effects of chronic SD stress on depression- and anxiety-related behaviors in rodents. Finally, we focus on the limited studies carried out to explore resilience to the effects of SD stress on the rewarding properties of drugs of abuse, describing the current state of knowledge and suggesting future research directions.

Orexin A Enhances Pro-Opiomelanocortin Transcription Regulated by BMP-4 in Mouse Corticotrope AtT20 Cells

Orexin is expressed mainly in the hypothalamus and is known to activate the hypothalamic-pituitary-adrenal (HPA) axis that is involved in various stress responses and its resilience. However, the effects of orexin on the endocrine function of pituitary corticotrope cells remain unclear. In this study, we investigated the roles of orexin A in pro-opiomelanocortin (POMC) transcription using mouse corticotrope AtT20 cells, focusing on the bone morphogenetic protein (BMP) system expressed in the pituitary. Regarding the receptors for orexin, type 2 (OXR2) rather than type 1 (OX1R) receptor mRNA was predominantly expressed in AtT20 cells. It was found that orexin A treatment enhanced POMC expression, induced by corticotropin-releasing hormone (CRH) stimulation through upregulation of CRH receptor type-1 (CRHR1). Orexin A had no direct effect on the POMC transcription suppressed by BMP-4 treatment, whereas it suppressed Smad1/5/9 phosphorylation and Id-1 mRNA expression induced by BMP-4. It was further revealed that orexin A had no significant effect on the expression levels of type I and II BMP receptors but upregulated inhibitory Smad6/7 mRNA and protein levels in AtT20 cells. The results demonstrated that orexin A upregulated CRHR signaling and downregulated BMP-Smad signaling, leading to an enhancement of POMC transcription by corticotrope cells.

Hypocretin/orexin modulates body weight and the metabolism of glucose and insulin

The hypocretin/orexin (Hcrt/orexin) unit affects the functions of the nervous, cardiovascular, gastrointestinal, and reproductive systems. Hcrt/orexin ligands and receptors have been localized to different parts of the central and peripheral nervous systems, cerebrospinal fluid and blood, exocrine (pancreas, salivary, lacrimal) as well as endocrine (pancreatic islets, pituitary, adrenal) glands. Several factors including stress, glucagon-like peptide-1 agonists, glutamate, nicotine, glucose, and hypoglycaemia stimulate the expression of Hcrt/orexin system, but it is inhibited by ageing, bone morphogenetic protein, hypoxia/hypercapnia, melanocortin receptor accessory protein 2, and glucagon. Literature reports show that Hcrt/orexin can significantly increase insulin secretion from normal and diabetic rat pancreata. Hcrt/orexin decreases blood glucose concentration and reduces insulin resistance partly via increased tissue expression of glucose transporter type 4. It reduces obesity by increasing browning of fat cells and energy expenditure. Taken together, Hcrt/orexin modulates obesity and the metabolism of glucose and insulin. The Hcrt/orexin system may thus be a target in the development of new therapies for the treatment of diabetes mellitus.

Orexin A modulates prolactin production by regulating BMP-4 activity in rat pituitary lactotorope cells

The impact of orexins on anterior pituitary function has yet to be clarified. We studied the effects of orexin A and its interaction with the bone morphogenetic protein (BMP) system on the regulatory role of prolactin synthesis using rat lactotrope GH3 cells expressing BMP-4. Orexin type 1 receptor (OX1R), but not type 2 receptor (OX2R), was predominantly expressed in GH3 cells. Orexin A suppressed forskolin-induced, but not basal, prolactin mRNA expression without reducing cAMP levels. Of note, orexin A suppressed BMP-4-induced prolactin mRNA and cAMP synthesis. Impairment of the effects of orexin by chemical inhibitors suggested involvement of the P38 pathway in the OX1R activity that suppresses BMP-4-induced PRL expression. Given that inhibition of BMP-receptor signaling reduced prolactin mRNA levels, endogenous BMP action is likely to be linked to the activation of prolactin synthesis by GH3 cells. Orexin A was revealed to suppress Smad1/5/9 phosphorylation and Id-1 transcription induced by BMP-4, which was restored in the presence of orexin-receptor antagonists, suggesting that the inhibitory effect of orexin A occurred via OX1R. Orexin A also reduced ALK-3 expression but increased inhibitory Smad6/7 expression, while BMP-4 treatment downregulated OX1R expression. These results indicated that orexin A plays an inhibitory role in prolactin production through suppression of endogenous BMP activity in GH3 cells, suggesting that a new functional role of the interaction between orexin and BMP-4 is modulation of prolactin levels in lactotrope cells.

Orexin A in swine corpus luteum

Orexin A (OXA) is a hypothalamic neuropeptide which acts on 2 known G-protein-coupled receptors. It has been demonstrated that OXA is a central molecular link between food intake and reproduction. More recently, its peripheral role has been investigated, and we demonstrated its involvement in regulating ovarian follicle function. The present study was undertaken to explore a potential physiological role of orexin system in swine corpus luteum, a transient ovarian endocrine organ. Our aim was, first, to analyze the localization and eventual colocalization of OXA and its 2 receptors within the different cell types composing the corpus luteum structure. Second, we wanted to explore the effects of OXA on isolated luteal cells, and finally to verify a potential involvement of OXA in angiogenesis, a crucial event in corpus luteum development. Our data demonstrate the local expression of OXA and its receptors in swine corpus luteum. Luteal cell functions were affected by treatment with OXA. In particular, progesterone production was inhibited (P < 0.05) and nonenzymatic scavenging activity was increased (P < 0.05). Moreover, OXA inhibited (P < 0.05) new vessel growth. Our results suggest that OXA could act locally to play a role in corpus luteum demise.

Dietary Manipulations That Induce Ketosis Activate the HPA Axis in Male Rats and Mice: A Potential Role for Fibroblast Growth Factor-21

In response to an acute threat to homeostasis or well-being, the hypothalamic-pituitary-adrenocortical (HPA) axis is engaged. A major outcome of this HPA axis activation is the mobilization of stored energy, to fuel an appropriate behavioral and/or physiological response to the perceived threat. Importantly, the extent of HPA axis activity is thought to be modulated by an individual's nutritional environment. In this study, we report that nutritional manipulations signaling a relative depletion of dietary carbohydrates, thereby inducing nutritional ketosis, acutely and chronically activate the HPA axis. Male rats and mice maintained on a low-carbohydrate high-fat ketogenic diet (KD) exhibited canonical markers of chronic stress, including increased basal and stress-evoked plasma corticosterone, increased adrenal sensitivity to adrenocorticotropin hormone, increased stress-evoked c-Fos immunolabeling in the paraventricular nucleus of the hypothalamus, and thymic atrophy, an indicator of chronic glucocorticoid exposure. Moreover, acutely feeding medium-chain triglycerides (MCTs) to rapidly induce ketosis among chow-fed male rats and mice also acutely increased HPA axis activity. Lastly, and consistent with a growing literature that characterizes the hepatokine fibroblast growth factor-21 (FGF21) as both a marker of the ketotic state and as a key metabolic stress hormone, the HPA response to both KD and MCTs was significantly blunted among mice lacking FGF21. We conclude that dietary manipulations that induce ketosis lead to increased HPA axis tone, and that the hepatokine FGF21 may play an important role to facilitate this effect.

Expression of orexin B and its receptor 2 in rat testis

The peptides orexin A (OxA) and orexin B (OxB) deriving from a common precursor molecule, prepro-orexin, by proteolytic cleavage, bind the two G-coupled OX₁ and OX₂ receptors. While OX₁ selectively binds OxA, OX₂ shows similar affinity for both orexins. Firstly discovered in the hypothalamus, orexins and their receptors have been found in other brain regions as well as in peripheral tissues of mammals, thus resulting involved in the regulation of a broad variety of physiological functions. While the functional localization of OxA and OX₁ in the mammalian genital tract has been already described, the expression of OxB and OX₂ and their potential role in the reproductive functions remain to be explored. Here, we investigated the presence of OxB and OX₂ in the rat testis by immunohistochemical and biochemical analyses. The results definitely demonstrated the localization of OxB and OX₂ in pachytene and second spermatocytes as well as in spermatids at all stages of the cycle of the seminiferous epithelium. The expression of both OX₂ mRNA and protein in the rat testis was also established by RT-PCR and Western blotting, respectively. The analysis of the molecular mechanism of action of OxB in the rat testis showed that OxB, in contrast with OxA, is unable to promote steroidogenesis. These results translate into the regulation of diverse biological actions by OxA and OxB in the male gonad.

Role of orexin-A in experimental autoimmune encephalomyelitis

The aim of this study was to evaluate the effects of orexin-A (OX-A) on behavioral and pathological parameters and on gene expression of some multiple sclerosis-related peptides in a model of experimental autoimmune encephalomyelitis (EAE). EAE was induced by subcutaneous administration of MOG 35-55. Following immunization, the treatment was initiated by using SB.334867 (orexin-1 receptor antagonist) and/or OX-A. Locomotor activity and exploratory behaviors were monitored using open field and T-maze continuous alternation task (T-CAT) respectively. Pain sensitivity was assessed by hot-plate test. Histopathological assessments were performed by H&E staining. The expression of TGF-β, MBP, MMP-9, IL-12, iNOS and MCP-1 were measured using real-time PCR method in lumbar spinal cord. OX-A administration in EAE mice remarkably attenuated the clinical symptoms, increased latency response in hot plate test, inhibited infiltration of inflammatory cells, up-regulated mRNA expression of TGF-β as well as MBP and down-regulated mRNA expression of iNOS, MMP-9 and IL-12. In contrast SB.334867 administration in EAE mice deteriorated the clinical symptoms, decreased the alternation in T-CAT, increased infiltration of inflammatory cells, down-regulated mRNA expression of TGF-β and MBP and up-regulated mRNA expression of iNOS. Results of this study suggest that the orexinergic system might be involved in pathological development of EAE. These findings suggest orexinergic system as a potential target for treatment of multiple sclerosis.

Orexin-A regulates cell apoptosis in human H295R adrenocortical cells via orexin receptor type 1 through the AKT signaling pathway

Numerous studies have demonstrated the ability of orexin-A to regulate adrenocortical cells through the mitogen-activated protein kinase signaling pathway. In the present study, human H295R adrenocortical cells were exposed to orexin‑A (10‑10-10‑6 M), with orexin receptor type 1 (OX1 receptor) antagonist SB334867 or AKT antagonist PF‑04691502. It was found that orexin‑A stimulated H295R cell proliferation, reduced the pro‑apoptotic activity of caspase‑3 to protect against apoptotic cell death and increased cortisol secretion. Furthermore, phospho‑AKT protein was increased by orexin‑A. SB334867 (10‑6 M) and PF‑04691502 (10‑6 M) abolished the effects of orexin‑A (10‑6 M). These results suggested that the orexin‑A/OX1 receptor axis has a significant pro-survival function in adrenal cells, which is mediated by AKT activation. Further studies investigating the effects of orexin-A-upregulation may further elucidate the diverse biological effects of orexin-A in adrenal cells.

Orexin A upregulates the protein expression of OX1R and enhances the proliferation of SGC-7901 gastric cancer cells through the ERK signaling pathway

Orexins are hypothalamic peptides that regulate food intake, wakefulness, the reward system and energy metabolism. Recent studies have demonstrated the ability of orexins to promote a robust apoptosis and subsequent inhibition of cell growth in various types of cancer cells. The present study was conducted to investigate the effects of orexin A on the survival of human gastric cancer cells, SGC‑7901, and the possible mechanisms. SGC‑7901 cells were exposed to various concentrations of orexin A in vitro in the presence or absence of the orexin receptor 1 (OX1R) antagonist (SB334867), extracellular signal‑regulated kinases 1 and 2 (ERK1/2) antagonist (U0126) or a combination of the two antagonists. The amount of cell proliferation, viability and apoptosis, caspase‑8 and caspases‑9 activities, OX1R protein expression and ERK1/2 protein levels were determined. The expression of OX1R in SGC‑7901 cells was observed. Orexin A (10-10 to 10-6 M) stimulated SGC‑7901 cell proliferation and viability, reduced the pro‑apoptotic activity of caspase‑9 and protected the cells from apoptosis in a dose‑dependent manner. Additionally, ERK1/2 phosphorylation was stimulated by orexin A (10-10 to 10-6 M). However, the OX1R antagonist SB334867 (10-6 M), ERK1/2 antagonist U0126 (30 µM) or the combination of antagonists blocked the effects of orexin A to a certain extent. These results suggest that stimulation of OX1R induces the growth of SGC‑7901 gastric cancer cells through activation of ERK1/2 signaling pathway. These findings add a new dimension to the biological activities of orexin, which may have important implications in health and disease, in particular gastric cancer.

Orexin-A stimulates 3β-hydroxysteroid dehydrogenase expression and cortisol production in H295R human adrenocortical cells through the AKT pathway

Orexin-A is a regulatory peptide involved in the regulation of food intake, sleep-wakefulness, and it has various endocrine and metabolic functions. It orchestrates diverse central and peripheral processes through the stimulation of two G-protein coupled receptors, orexin receptor type 1 (OX1 receptor) and orexin receptor type 2 (OX2 receptor). In this study, human adrenocortical cells (NCI-H295R cells) were incubated with various concentrations of orexin-A (10-10 to 10-6 M) in vitro, and the mRNA and protein expression of OX1 receptor was determined in the cells. In addition, NCI-H295R cells treated with 10-6 M orexin-A were then treated with or without OX1 receptor specific antagonist (SB334867), AKT antagonist (PF-04691502), or a combination of both. Subsequently, cell proliferation, the cortisol content in the medium and the mRNA and protein expression expression of 3β-hydroxysteroid dehydrogenase (3β-HSD) were analyzed. The activity of the AKT signaling pathway was also determined in the NCI-H295R cells. We observed that the increase in the mRNA and protein expression of OX1 receptor was orexin-A concentration-dependent, with 10-6 M orexin-A exerting the most potent effect. Orexin-A enhanced cell proliferation and cortisol production, and increased the mRNA and protein expression of 3β-HSD in the NCI-H295R cells; however, these effects were partly blocked by the OX1 receptor antagonist, the AKT antagonist and the combination of both. Furthermore, orexin-A significantly increased the phosphorylation of AKT, with the levels of total AKT protein remaining unaltered. This effect was blocked in the presence of PF-04691502 (10-6 M), SB334867 (10-6 M) and the combination of both. On the whole, our data demonstrate that the effects of orexin-A on the survival and function of human adrenocortical cells are mediated through the AKT signaling pathway.

Effects of sex and reproductive experience on the number of orexin A-immunoreactive cells in the prairie vole brain

Large populations of cells synthesizing the neuropeptide orexin (OX) exist in the caudal hypothalamus of all species examined and are implicated in physiological and behavioral processes including arousal, stress, anxiety and depression, reproduction, and goal-directed behaviors. Hypothalamic OX expression is sexually dimorphic in different directions in laboratory rats (F>M) and mice (M>F), suggesting different roles in male and female physiology and behavior that are species-specific. We here examined if the number of hypothalamic cells immunoreactive for orexin A (OXA) differs between male and female prairie voles (Microtus ochrogaster), a socially monogamous species that pairbonds after mating and in which both sexes care for offspring, and if reproductive experience influences their number of OXA-immunoreactive (OXA-ir) cells. It was found that the total number of OXA-ir cells did not differ between the sexes, but females had more OXA-ir cells than males in anterior levels of the caudal hypothalamus, while males had more OXA-ir cells posteriorly. Sexually experienced females sacrificed 12 days after the birth of their first litter, or one day after birth of a second litter, had more OXA-ir cells in anterior levels but not posterior levels of the caudal hypothalamus compared to females housed with a brother (incest avoidance prevents sibling mating). Male prairie voles showed no effect of reproductive experience but showed an unexpected effect of cohabitation duration regardless of mating. The sex difference in the distribution of OXA-ir cells, and their increased number in anterior levels of the caudal hypothalamus of reproductively experienced female prairie voles, may reflect a sex-specific mechanism involved in pairbonding, parenting, or lactation in this species.

The orexigenic effect of orexin-A revisited: dependence of an intact growth hormone axis

Fifteen years ago orexins were identified as central regulators of energy homeostasis. Since then, that concept has evolved considerably and orexins are currently considered, besides orexigenic neuropeptides, key modulators of sleep-wake cycle and neuroendocrine function. Little is known, however, about the effect of the neuroendocrine milieu on orexins' effects on energy balance. We therefore investigated whether hypothalamic-pituitary axes have a role in the central orexigenic action of orexin A (OX-A) by centrally injecting hypophysectomized, adrenalectomized, gonadectomized (male and female), hypothyroid, and GH-deficient dwarf rats with OX-A. Our data showed that the orexigenic effect of OX-A is fully maintained in adrenalectomized and gonadectomized (females and males) rats, slightly reduced in hypothyroid rats, and totally abolished in hypophysectomized and dwarf rats when compared with their respective vehicle-treated controls. Of note, loss of the OX-A effect on feeding was associated with a blunted OX-A-induced increase in the expression of either neuropeptide Y or its putative regulator, the transcription factor cAMP response-element binding protein, as well as its phosphorylated form, in the arcuate nucleus of the hypothalamus of hypophysectomized and dwarf rats. Overall, this evidence suggests that the orexigenic action of OX-A depends on an intact GH axis and that this neuroendocrine feedback loop may be of interest in the understanding of orexins action on energy balance and GH deficiency.

A unified survival theory of the functioning of the hypocretinergic system

This article advances the theory that the hypocretinergic (orexinergic) system initiates, coordinates, and maintains survival behaviors and survival-related processes (i.e., the Unified Survival Theory of the Functioning of the Hypocretinergic System or "Unified Hypocretinergic Survival Theory"). A priori presumptive support for the Unified Hypocretinergic Survival Theory emanates from the fact that neurons that contain hypocretin are located in the key executive central nervous system (CNS) site, the lateral hypothalamus, that for decades has been well-documented to govern core survival behaviors such as fight, flight, and food consumption. In addition, the hypocretinergic system exhibits the requisite morphological and electrophysiological capabilities to control survival behaviors and related processes. Complementary behavioral data demonstrate that all facets of "survival" are coordinated by the hypocretinergic system and that hypocretinergic directives are not promulgated except during survival behaviors. Importantly, it has been shown that survival behaviors are selectively impacted when the hypocretinergic system is impaired or rendered nonfunctional, whereas other behaviors are relatively unaffected. The Unified Hypocretinergic Survival Theory resolves the disparate, perplexing, and often paradoxical-appearing results of previous studies; it also provides a foundation for future hypothesis-driven basic science and clinical explorations of the hypocretinergic system.

The role of the glucocorticoids in developing resilience to stress and addiction

There is emerging evidence that individuals have the capacity to learn to be resilient by developing protective mechanisms that prevent them from the maladaptive effects of stress that can contribute to addiction. The emerging field of the neuroscience of resilience is beginning to uncover the circuits and molecules that protect against stress-related neuropsychiatric diseases, such as addiction. Glucocorticoids (GCs) are important regulators of basal and stress-related homeostasis in all higher organisms and influence a wide array of genes in almost every organ and tissue. GCs, therefore, are ideally situated to either promote or prevent adaptation to stress. In this review, we will focus on the role of GCs in the hypothalamic-pituitary adrenocortical axis and extra-hypothalamic regions in regulating basal and chronic stress responses. GCs interact with a large number of neurotransmitter and neuropeptide systems that are associated with the development of addiction. Additionally, the review will focus on the orexinergic and cholinergic pathways and highlight their role in stress and addiction. GCs play a key role in promoting the development of resilience or susceptibility and represent important pharmacotherapeutic targets that can reduce the impact of a maladapted stress system for the treatment of stress-induced addiction.

Orexin A suppresses the growth of rat C6 glioma cells via a caspase-dependent mechanism

Orexin A and orexin B (also known as hypocretins) are closely related peptides synthesized by hypothalamic neurons. They orchestrate diverse central and peripheral processes by stimulation of two G-protein coupled receptors, OX₁R and OX₂R. Recent studies have demonstrated the ability of orexins to promote a robust apoptosis in different cancer cells in culture and a potent growth reduction of human colon tumors in mice xenografts. Here we report effects of orexins on survival of rat C6 glioma cells, an experimental model for studies on glioblastoma multiforme (GBM). Quantitative real-time PCR demonstrated the expression of both types of orexin receptors in C6 cells. Orexin A and orexin B did not affect rat C6 glioma cell proliferation as assessed by [³H]thymidine incorporation assay. Incubation of the cells with orexin A (0.001–1 μM) resulted in a marked decrease of cell viability. The observed effect was caspase-dependent, as it was blocked by Z-VAD-fmk, a pan caspase inhibitor. In addition to that, a parallel increase in caspase-3 activity was observed. It is suggested that stimulation of orexin receptors induces death of rat C6 glioma cells through activation of caspase pathway.

Hypothalamic orexin--a neurons are involved in the response of the brain stress system to morphine withdrawal

Both the hypothalamus-pituitary-adrenal (HPA) axis and the extrahypothalamic brain stress system are key elements of the neural circuitry that regulates the negative states during abstinence from chronic drug exposure. Orexins have recently been hypothesized to modulate the extended amygdala and to contribute to the negative emotional state associated with dependence. This study examined the impact of chronic morphine and withdrawal on the lateral hypothalamic (LH) orexin A (OXA) gene expression and activity as well as OXA involvement in the brain stress response to morphine abstinence. Male Wistar rats received chronic morphine followed by naloxone to precipitate withdrawal. The selective OX1R antagonist SB334867 was used to examine whether orexins' activity is related to somatic symptoms of opiate withdrawal and alterations in HPA axis and extended amygdala in rats dependent on morphine. OXA mRNA was induced in the hypothalamus during morphine withdrawal, which was accompanied by activation of OXA neurons in the LH. Importantly, SB334867 attenuated the somatic symptoms of withdrawal, and reduced morphine withdrawal-induced c-Fos expression in the nucleus accumbens (NAc) shell, bed nucleus of stria terminalis, central amygdala and hypothalamic paraventricular nucleus, but did not modify the HPA axis activity. These results highlight a critical role of OXA signalling, via OX1R, in activation of brain stress system to morphine withdrawal and suggest that all orexinergic subpopulations in the lateral hypothalamic area contribute in this response.

Orexin-1 receptor antagonism does not reduce the rewarding potency of cocaine in Swiss-Webster mice

The orexin family of hypothalamic neuropeptides has been implicated in reinforcement mechanisms relevant to both food and drug reward. Previous behavioral studies with antagonists at the orexin A-selective receptor, OX(1), have demonstrated its involvement in behavioral sensitization, conditioned place-preference, and self-administration of drugs of abuse. Adult male Swiss-Webster mice were implanted with stimulating electrodes to the lateral hypothalamus and trained to perform intracranial self-stimulation (ICSS). The effects of the OX(1)-selective antagonist SB 334867 on brain stimulation-reward (BSR) and cocaine potentiation of BSR were measured. SB 334867 (10-30mg/kg, i.p.) alone had no effect on ICSS performance or BSR threshold. Cocaine (1.0-30mg/kgi.p.) dose-dependently potentiated BSR, measured as lowering of BSR threshold. This effect was not blocked by 30mg/kg SB 334867 at any cocaine dose tested. In agreement with previous reports, SB 334867 resulted in a reduction of body weight 24h after acute administration. Based on these data, it is concluded that orexins acting at OX(1) do not contribute to BSR; and are not involved in the reward-potentiating actions of cocaine on BSR. The data are discussed in the context of prior findings of SB 334867 effects on drug-seeking and drug-consuming behaviors.

Activation of orexin/hypocretin type 1 receptors stimulates cAMP synthesis in primary cultures of rat astrocytes

The effects of orexins, which are also named hypocretins, on cAMP formation were examined in primary cultures of rat astrocytes. Orexin A, an agonist of OX₁ and OX₂ receptors, stimulated cAMP production with an EC₅₀ value of 0.68 μM and potentiated the forskolin-induced increase in the nucleotide synthesis. [Ala¹¹-D-Leu¹⁵]orexin B, an agonist of OX₂ receptors, was inactive. The effects of orexin A were antagonized by SB 408124, a selective blocker of OX₁ receptors, but were not affected by TCS OX2 29, a selective antagonist of OX₃ receptors. We hypothesized that the activation of OX₁ receptors stimulated cAMP synthesis in primary rat astrocyte cultures.

Orexins/hypocretins acting at Gi protein-coupled OX 2 receptors inhibit cyclic AMP synthesis in the primary neuronal cultures

Orexins A and B are newly discovered neuropeptides with pleiotropic activity. They signal through two G protein-coupled receptors: OX₁ and OX₂. In this study, we examined the expression of orexin receptors and effects of the receptors’ activation on cyclic AMP formation in the primary neuronal cell cultures from rat cerebral cortex. Both types of orexin receptors were expressed in rat cortical neurons; the level of OX₂R was markedly higher compared to OX₁R. Orexin A (an agonist of OX₁R and OX₂R) and [Ala¹¹-D-Leu¹⁵]orexin B (a selective agonist of OX₂R) did not affect basal cyclic AMP formation in the primary neuronal cell cultures. Both peptides (0.001–1 μM) inhibited, in a concentration-dependent manner and IC₅₀ values in low nanomolar range, the increase in the nucleotide production evoked by forskolin (1 μM; a direct activator of adenylyl cyclase), pituitary adenylate cyclase-activating polypeptide (PACAP27; 0.1 μM), and vasoactive intestinal peptide (VIP; 3 μM). Effects of orexin A on forskolin-, PACAP27-, and VIP-stimulated cyclic AMP synthesis were blocked by TCS OX2 29 (a selective antagonist of OX₂R), and unaffected by SB 408124 (a selective antagonist of OX₁R). Pretreatment of neuronal cell cultures with pertussis toxin (PTX) abolished the inhibitory action of orexin A on forskolin- and PACAP-stimulated cyclic AMP accumulation. It is suggested that in cultured rat cortical neurons orexins, acting at OX₂ receptors coupled to PTX-sensitive G_i protein, inhibit cyclic AMP synthesis.

Orexins/hypocretins increase the promoter activity of selective steroidogenic enzymes

Orexins (hypocretins) regulate multiple physiological functions, including central regulation of energy homeostasis and sleep-wake behavior but also peripheral hormonal actions. Recent data suggest specific effects of orexins at adrenal glands. To further assess the mechanism by which orexins regulate steroidogenesis we analyzed the effect of orexin A and B on the transcriptional activity of the luciferase reporter gene driven by the human steroid 21-hydroxylase (CYP21), 3β-hydroxysteroid dehydrogenase (HSD3B2), 11β-hydroxylase (CYP11B1), and aldosterone synthase (CYP11B2) gene promoter regions. After transient transfection of the reporter gene constructs into human NCI H295R cells, treatment with orexin A and B for 6 and 12h increased the promoter activity of the CYP11B2, HSD3B2 and, to a lesser extend, CYP21 genes. The activity of the CYP11B1 was increased by both orexins after 3h of treatment. Compared to the effects of forskolin or angiotensin II, however, the effect of orexins on the transcriptional activity of the steroidogenic enzyme genes was moderate. Our results suggest that orexins increase the expression of steroidogenic enzymes at the transcriptional level and that orexins play a role in the long term regulation of adrenal steroid production.

Wake-promoting effects of orexin: Its independent actions against the background of an impaired corticotropine-releasing hormone receptor system

It is widely accepted that orexin (hypocretin) bears wake-promoting effects. While under normal conditions the circadian rhythm of orexin release has a clear circadian distribution, the amplitude of orexin fluctuation is dampened in depression. Interestingly, clinical symptoms of depression include several sleep disturbances. In this disease, corticotropin-releasing hormone (CRH) seems to be another factor influencing sleep. As neurophysiological interactions and anatomical connections between the orexinergic and the CRH system point to mutual influences of these two neuropeptides, we examined whether a dysfunctional CRH-receptor system in two different CRH receptor knock out models alters general wake-promoting effects of orexin applied exogenously. Orexin was injected intracerebroventricularlly into CNS-restricted CRH-receptor type 1 knockout mice (CRH-R1 KO) and CRH-receptor type 2 knockout mice (CRH-R2 KO) and baseline sleep was recorded from the freely behaving mice. A third experiment included antisauvagine-30 injections (CRH-R2 antagonist) into CRH-R1 KO animals. Orexin had similar wake-promoting effects in CRH-R1KO mice, in CRH-R2 KO animals and in CRH-R1KO mice treated with antisauvagine-30. Consistent results were obtained from all corresponding control littermate experiments. According to our results we conclude that the wake-promoting effects of orexin are not influenced by a possible contribution of CRH.