Cytosolic calcium elevation induced by orexin/hypocretin in granule cell domain cells of the rat cochlear nucleus in vitro

Orexins/Hypocretins: Gatekeepers of Social Interaction and Motivation

Ever since the discovery of the brain's orexin/hypocretin system, most research was directed toward unveiling its contribution to the normal functioning of individuals. The investigation of reward-seeking behaviors then gained a lot of attention once the distribution of orexinergic neurons was revealed. Here, we discuss findings on the involvement of orexins in social interaction, a natural reward type. While some studies have succeeded in defining the relationship between orexin and social interaction, the controversy regarding its nature (direct or inverse relation) raises questions about what aspects have been overlooked until now. Upon examining the literature, we identified a research gap concerning conditions influencing the impact of orexins on social behavior expression. In this review, we introduce a number of factors (e.g., stress, orexin's source) that must be considered while studying the role of orexins in social interaction. Furthermore, we refer to published research to investigate the stage at which orexins affect social interaction and we highlight the nucleus accumbens (NAc) shell's role in social interaction and other rewarding behaviors. Finally, the underlying orexin molecular pathway influencing social motivation in particular illnesses is proposed. We conclude that orexin's impact on social interaction is multifactorial and depends on specific conditions available at a time.

Orexin receptor antagonists in the pathophysiology and treatment of sleep disorders and epilepsy

The correlation between sleep and epilepsy has been argued over the past decades among scientists. Although the similarities and contrasts between sleep and epilepsy had been considered, their intertwined nature was not revealed until the nineteenth century. Sleep is recognized as a recurring state of mind and body through alternating brain electrical activities. It is documented that sleep disorders are associated with epilepsy. The origin, suppression, and spread of seizures are affected by sleep. As such, in patients with epilepsy, sleep disorders are a frequent comorbidity. Meanwhile, orexin, a wake-promoting neuropeptide, provides a bidirectional effect on both sleep and epilepsy. Orexin and its cognate receptors, orexin receptor type 1 (OX1R) and type 2 (OX2R), orchestrate their effects by activating various downstream signaling pathways. Although orexin was considered a therapeutic target in insomnia shortly after its discovery, its potential usefulness for psychiatric disorders and epileptic seizures has been suggested in the pre-clinical studies. This review aimed to discuss whether the relationship between sleep, epilepsy, and orexin is clearly reciprocal.

A molecular network map of orexin-orexin receptor signaling system

Orexins are excitatory neuropeptides, which are predominantly associated with feeding behavior, sleep-wake cycle and energy homeostasis. The orexinergic system comprises of HCRTR1 and HCRTR2, G-protein-coupled receptors of rhodopsin family and the endogenous ligands processed from HCRT pro-hormone, Orexin A and Orexin B. These neuropeptides are biosynthesized by the orexin neurons present in the lateral hypothalamus area, with dense projections to other brain regions. The orexin-receptor signaling is implicated in various metabolic as well as neurological disorders, making it a promising target for pharmacological interventions. However, there is limited information available on the collective representation of the signal transduction pathways pertaining to the orexin-orexin receptor signaling system. Here, we depict a compendium of the Orexin A/B stimulated reactions in the form of a basic signaling pathway map. This map catalogs the reactions into five categories: molecular association, activation/inhibition, catalysis, transport, and gene regulation. A total of 318 downstream molecules were annotated adhering to the guidelines of NetPath curation. This pathway map can be utilized for further assessment of signaling events associated with orexin-mediated physiological functions and is freely available on WikiPathways, an open-source pathway database ( https://www.wikipathways.org/index.php/Pathway:WP5094 ).

Estradiol-dependent hypocretinergic/orexinergic behaviors throughout the estrous cycle

RATIONALE

The female menstrual or estrous cycle and its associated fluctuations in circulating estradiol (E2), progesterone, and other gonadal hormones alter orexin or hypocretin peptide production and receptor activity. Depending on the estrous cycle phase, the transcription of prepro-orexin mRNA, post-translational modification of orexin peptide, and abundance of orexin receptors change in a brain region-specific manner. The most dramatic changes occur in the hypothalamus, which is considered the starting point of the hypothalamic-pituitary-gonadal axis as well as the hub of orexin-producing neurons. Thus, hypothalamus-regulated behaviors, including arousal, feeding, reward processing, and the stress response depend on coordinated efforts between E2, progesterone, and the orexin system. Given the rise of orexin therapeutics for various neuropsychiatric conditions including insomnia and affective disorders, it is important to delineate the behavioral outcomes of this drug class in both sexes, as well as within different time points of the female reproductive cycle.

OBJECTIVES

Summarize how the menstrual or estrous cycle affects orexin system functionality in animal models in order to predict how orexin pharmacotherapies exert varying degrees of behavioral effects across the dynamic hormonal milieu.

The regulation of PKA signaling in obesity and in the maintenance of metabolic health

The cAMP-dependent protein kinase (PKA) system represents a primary cell-signaling pathway throughout systems and across species. PKA facilitates the actions of hormones, neurotransmitters and other signaling molecules that bind G-protein coupled receptors (GPCR) to modulate cAMP levels. Through its control of synaptic events, exocytosis, transcriptional regulation, and more, PKA signaling regulates cellular metabolism and emotional and stress responses making it integral in the maintenance and dysregulation of energy homeostasis. Neural PKA signaling is regulated by afferent and peripheral efferent signals that link specific neural cell populations to the regulation of metabolic processes in adipose tissue, liver, pancreas, adrenal, skeletal muscle, and gut. Mouse models have provided invaluable information on the roles for PKA subunits in brain and key metabolic organs. While limited, human studies infer differential regulation of the PKA system in obese compared to lean individuals. Variants identified in PKA subunit genes cause Cushing syndrome that is characterized by metabolic dysregulation associated with endogenous glucocorticoid excess. Under healthy physiologic conditions, the PKA system is exquisitely regulated by stimuli that activate GPCRs to alter intracellular cAMP concentrations, and by PKA cellular localization and holoenzyme stability. Adenylate cyclase activity generates cAMP while phosphodiesterase-mediated cAMP degradation to AMP decreases cAMP levels downstream of GPCRs. Chronic perturbations in PKA signaling appear to be capable of resetting PKA regulation at several levels; in addition, sex differences in PKA signaling regulation, while not well understood, impact the physiologic consequences of metabolic dysregulation and obesity. This review explores the roles for PKA signaling in the pathogenesis of metabolic diseases including obesity, type 2 diabetes mellitus and associated co-morbidities through neural-peripheral crosstalk and cAMP/PKA signaling pathway targets that hold therapeutic potential.

Forced ethanol ingestion by Wistar rats from a juvenile age increased voluntary alcohol consumption in adulthood, with the involvement of orexin-A

Human adolescents who drink alcohol are more likely to become alcoholics in adulthood. Alcohol administration (intraperitoneally) or drinking (in a 2-bottle free choice paradigm) during the juvenile/adolescent age of rats promotes voluntary alcohol consumption in adulthood. On the other hand, there is growing evidence that the orexinergic system plays a role in several rewarded behaviors, including alcohol ingestion. Since it is unknown what effect is exerted in adulthood by forced oral ethanol intake and/or administration of orexin-A (OX-A) in juvenile rats, the present study aimed to evaluate this question. A group of male Wistar rats was forced to drink ethanol (10% v/v) as the only liquid in the diet from weaning (postnatal day 21) to postnatal day 67 (46 days), followed by a forced withdrawal period. An age-matched group was raised drinking tap water (control). OX-A or its vehicle was microinjected intracerebroventricularly (i.c.v.) (1 nmol/0.6 μL) to explore its effect as well. Locomotor activity and voluntary ethanol consumption were later assessed in all groups. The rats forced to consume ethanol early in life showed an elevated level of ambulation and alcohol ingestion in adulthood. A single injection of OX-A increased locomotor activity and acute ethanol intake in rats with or without prior exposure to alcohol at the juvenile stage. In conclusion, forced ethanol consumption in juvenile rats led to increased voluntary alcohol drinking behavior during adulthood, an effect likely facilitated by OX-A.

The Orexin/Receptor System: Molecular Mechanism and Therapeutic Potential for Neurological Diseases

Orexins, also known as hypocretins, are two neuropeptides secreted from orexin-containing neurons, mainly in the lateral hypothalamus (LH). Orexins orchestrate their effects by binding and activating two G-protein–coupled receptors (GPCRs), orexin receptor type 1 (OX1R) and type 2 (OX2R). Orexin/receptor pathways play vital regulatory roles in many physiological processes, especially feeding behavior, sleep–wake rhythm, reward and addiction and energy balance. Furthermore several reports showed that orexin/receptor pathways are involved in pathological processes of neurological diseases such as narcolepsy, depression, ischemic stroke, drug addiction and Alzheimer’s disease (AD). This review article summarizes the expression patterns, physiological functions and potential molecular mechanisms of the orexin/receptor system in neurological diseases, providing an overall framework for considering these pathways from the standpoints of basic research and clinical treatment of neurological diseases.

Calcium affects OX1 orexin (hypocretin) receptor responses by modifying both orexin binding and the signal transduction machinery

BACKGROUND AND PURPOSE

One of the major responses upon orexin receptor activation is Ca(2+) influx, and this influx seems to amplify the other responses mediated by orexin receptors. However, the reduction in Ca(2+) , often used to assess the importance of Ca(2+) influx, might affect other properties, like ligand-receptor interactions, as suggested for some GPCR systems. Hence, we investigated the role of the ligand-receptor interaction and Ca(2+) signal cascades in the apparent Ca(2+) requirement of orexin-A signalling.

EXPERIMENTAL APPROACH

Receptor binding was assessed in CHO cells expressing human OX1 receptors with [(125) I]-orexin-A by conventional ligand binding as well as scintillation proximity assays. PLC activity was determined by chromatography.

KEY RESULTS

Both orexin receptor binding and PLC activation were strongly dependent on the extracellular Ca(2+) concentration. The relationship between Ca(2+) concentration and receptor binding was the same as that for PLC activation. However, when Ca(2+) entry was reduced by depolarizing the cells or by inhibiting the receptor-operated Ca(2+) channels, orexin-A-stimulated PLC activity was much more strongly inhibited than orexin-A binding.

CONCLUSIONS AND IMPLICATIONS

Ca(2+) plays a dual role in orexin signalling by being a prerequisite for both ligand-receptor interaction and amplifying orexin signals via Ca(2+) influx. Some previous results obtained utilizing Ca(2+) chelators have to be re-evaluated based on the results of the current study. From a drug discovery perspective, further experiments need to identify the target for Ca(2+) in orexin-A-OX1 receptor interaction and its mechanism of action.

The hypocretin/orexin system: an increasingly important role in neuropsychiatry

Hypocretins, also named as orexins, are excitatory neuropeptides secreted by neurons specifically located in lateral hypothalamus and perifornical areas. Orexinergic fibers are extensively distributed in various brain regions and involved in a number of physiological functions, such as arousal, cognition, stress, appetite, and metabolism. Arousal is the most important function of orexin system as dysfunction of orexin signaling leads to narcolepsy. In addition to narcolepsy, orexin dysfunction is associated with serious neural disorders, including addiction, depression, and anxiety. However, some results linking orexin with these disorders are still contradictory, which may result from differences of detection methods or the precision of tools used in measurements; strategies targeted to orexin system (e.g., antagonists to orexin receptors, gene delivery, and cell transplantation) are promising new tools for treatment of neuropsychiatric disorders, though studies are still in a stage of preclinical or clinical research.

Orexin/hypocretin receptor signalling: a functional perspective

Multiple homeostatic systems are regulated by orexin (hypocretin) peptides and their two known GPCRs. Activation of orexin receptors promotes waking and is essential for expression of normal sleep and waking behaviour, with the sleep disorder narcolepsy resulting from the absence of orexin signalling. Orexin receptors also influence systems regulating appetite/metabolism, stress and reward, and are found in several peripheral tissues. Nevertheless, much remains unknown about the signalling pathways and targets engaged by native receptors. In this review, we integrate knowledge about the orexin receptor signalling capabilities obtained from studies in expression systems and various native cell types (as presented in Kukkonen and Leonard, this issue of British Journal of Pharmacology) with knowledge of orexin signalling in different tissues. The tissues reviewed include the CNS, the gastrointestinal tract, the pituitary gland, pancreas, adrenal gland, adipose tissue and the male reproductive system. We also summarize the findings in different native and recombinant cell lines, especially focusing on the different cascades in CHO cells, which is the most investigated cell line. This reveals that while a substantial gap exists between what is known about orexin receptor signalling and effectors in recombinant systems and native systems, mounting evidence suggests that orexin receptor signalling is more diverse than originally thought. Moreover, rather than being restricted to orexin receptor 'overexpressing' cells, this signalling diversity may be utilized by native receptors in a site-specific manner.

Orexins cause epileptic activity

Orexins have been implicated in the regulation of sleep-wake cycle, energy homeostasis, drinking behavior, analgesia, attention, learning and memory but their effects on epileptic activity are controversial. We investigated whether intracortical injections of orexin A (100 pmol) and B (100 pmol) cause epileptic activity in rats. We observed epileptic seizure findings on these two groups rats. Orexin A and B also significantly increased total EEG power spectrum. Our findings indicate that orexins cause epileptic activity.

OX2R activation induces PKC-mediated ERK and CREB phosphorylation

Deficiencies in brain orexins and components of mitogen activated protein kinase (MAPK) signaling pathway have been reported in either human depression or animal model of depression. Brain administration of orexins affects behaviors toward improvement of depressive symptoms. However, the documentation of endogenous linkage between orexin receptor activation and MAPK signaling pathway remains to be insufficient. In this study, we report the effects of orexin 2 receptor (OX2R) activation on cell signaling in CHO cells over-expressing OX2R and in mouse hypothalamus cell line CLU172. Short-term extracellular signal-regulated kinase (ERK) phosphorylation and long-term cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) phosphorylation were subsequently observed in CHO cells that over-express OX2R while 20 min of ERK phosphorylation was significantly detected in mouse adult hypothalamus neuron cell line CLU172. Orexin A, which can also activate OX2R, mediated ERK phosphorylation was as the same as orexin B in CHO cells. A MAPK inhibitor eliminated ERK phosphorylation but not CREB phosphorylation in CHO cells. Also, ERK and CREB phosphorylation was not mediated by protein kinase A (PKA) or calmodulin kinase (CaMK). However, inhibition of protein kinase C (PKC) by GF 109203X eliminated the phosphorylation of ERK and CREB in CHO cells. A significant decrease in ERK and CREB phosphorylation was observed with 1 μM GF 109203X pre-treatment indicating that the conventional and novel isoforms of PKC are responsible for CREB phosphorylation after OX2R activation. In contrast, ERK phosphorylation induced by orexin B in CLU172 cells cannot be inhibited by 1 μM of protein kinase C inhibitor. From above observation we conclude that OX2R activation by orexin B induces ERK and CREB phosphorylation and orexin A played the same role as orexin B. Several isoforms of PKC may be involved in prolonged CREB phosphorylation. Orexin B induced ERK phosphorylation in mouse hypothalamus neuron cells differs from CHO cell line and cannot be inhibited by PKC inhibitor GF 109203X. And hypothalamus neuron cells may use different downsteam pathway for orexin B induced ERK phosphorylation. This result supports findings that orexins might have anti-depressive roles.

Orexins increase penicillin-induced epileptic activity

Orexins have been implicated with physiological function including sleep-wake cycle, energy homeostasis, drinking behavior, analgesia, attention, learning and memory but their effects on excitability are controversial. We investigated the effects of intracortical injections of orexin A (100 pmol) and B (100 pmol) on the electrophysiological manifestation of epileptic seizures induced by cortical penicillin application in adult male rats. In comparison to saline, orexin A and B enhanced significantly the spike number, spike amplitude and spectral power values induced by cortical penicillin. Our findings indicates that orexins enhances the hyperexcitable and hypersyncronic cortical epileptic activity induced by focal application of penicillin-G.

Electrophysiological effects of orexin-B and dopamine on rat nucleus accumbens shell neurons in vitro

Orexin (ORX) plays a critical role in reward-seeking behavior for natural rewards and drugs of abuse. The mesolimbic dopamine (DA) pathway that projects into the nucleus accumbens (NAc) from the ventral tegmental area is deeply involved in the neural mechanisms underlying reward, drug abuse and motivation. A recent study demonstrated that ORX-immunopositive fibers densely project into the shell of the NAc (NAcSh), suggesting that the NAcSh might be a site of the interaction between the ORXergic and DAergic systems for reward-seeking behavior. Therefore, the electrophysiological effects of ORX-B and DA on NAcSh neurons were examined extracellularly in rat brain slice preparations. ORX-B excited approximately 78% of neurons tested and inhibited 4%, whereas DA excited 50% and inhibited 22% of NAcSh neurons. These excitations and inhibitions persisted during synaptic blockade in a low-Ca(2+)/high-Mg(2+) solution. DA-induced excitation was attenuated by SCH23390 or sulpiride, whereas DA-induced inhibition was suppressed by sulpiride. Of the neurons that were excited by ORX-B, 71% and 18% were excited and inhibited by DA, respectively. In 63% of neurons that were excited by ORX-B, the simultaneous application of ORX-B and DA increased the firing rate to two times greater than ORX-B alone, whereas, the simultaneous application significantly decreased the neuronal firing rate by 73% in the remaining 37% compared to ORX-B. These results suggest that an interaction between the ORXergic and DAergic systems occurs in the NAcSh and that the NAcSh is involved in the neural mechanisms in which ORX participates in the regulation of reward-seeking behavior.