Orexins alleviate motor deficits via increasing firing activity of pallidal neurons in a mouse model of Parkinson's disease

The role of amphipathic and cationic helical peptides in Parkinson's disease

Peptides are attracting a growing interest for therapeutic applications in biomedicine. In Parkinson's disease (PD), different human endogenous peptides have been associated with beneficial effects, including protein aggregation inhibition, reduced inflammation, or the protection of dopaminergic neurons. Such effects seem to be connected to the spatial arrangement of peptide side chains, and many of these human molecules share common conformational traits, displaying a distinctive amphipathic and cationic helical structure, which is believed to be crucial for their activities. This review delves into the relationship between these structural properties and the current evidence connecting biogenic peptides to the amelioration of PD symptoms. We discuss their implications in the disease, the different mechanisms of action, their state of validation, and their therapeutic potential.

International Union of Basic and Clinical Pharmacology CXIV: Orexin Receptor Function, Nomenclature and Pharmacology

The orexin system consists of the peptide transmitters orexin-A and -B and the G protein-coupled orexin receptors OX1 and OX2 Orexin receptors are capable of coupling to all four families of heterotrimeric G proteins, and there are also other complex features of the orexin receptor signaling. The system was discovered 25 years ago and was immediately identified as a central regulator of sleep and wakefulness; this is exemplified by the symptomatology of the disorder narcolepsy with cataplexy, in which orexinergic neurons degenerate. Subsequent translation of these findings into drug discovery and development has resulted to date in three clinically used orexin receptor antagonists to treat insomnia. In addition to sleep and wakefulness, the orexin system appears to be a central player at least in addiction and reward, and has a role in depression, anxiety and pain gating. Additional antagonists and agonists are in development to treat, for instance, insomnia, narcolepsy with or without cataplexy and other disorders with excessive daytime sleepiness, depression with insomnia, anxiety, schizophrenia, as well as eating and substance use disorders. The orexin system has thus proved an important regulator of numerous neural functions and a valuable drug target. Orexin prepro-peptide and orexin receptors are also expressed outside the central nervous system, but their potential physiological roles there remain unknown. SIGNIFICANCE STATEMENT: The orexin system was discovered 25 years ago and immediately emerged as an essential sleep-wakefulness regulator. This discovery has tremendously increased the understanding of these processes and has thus far resulted in the market approval of three orexin receptor antagonists, which promote more physiological aspects of sleep than previous hypnotics. Further, orexin receptor agonists and antagonists with different pharmacodynamic properties are in development since research has revealed additional potential therapeutic indications. Orexin receptor signaling is complex and may represent novel features.

Orexin B protects dopaminergic neurons from 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurotoxicity associated with reduced extracellular signal-regulated kinase phosphorylation

BACKGROUND

The loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) is a major pathological hallmark of Parkinson's disease (PD). Orexin B (OXB) has been reported to promote the growth of DA neurons. However, the roles of OXB in the degeneration of DA neurons still remained not fully clear.

METHODS

An in vivo PD model was constructed by administrating 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice. Pole test was performed to investigate the motor function of mice and the number of DA neurons was detected by immunofluorescence (IF). A PD cell model was established by treating SH-SY5Y cells with 1-methyl-4-phenylpyridinium (MPP+). OXB was added to the culture medium 2 h after MPP + treatment. Microscopic analysis was carried out to investigate the function of OXB in the cell model of PD 24 h after MPP + challenge. RNA-Seq analysis of the PD cell model was performed to explore the possible mechanisms. Western blot was used to detect the phosphorylation levels of extracellular signal-regulated kinase (ERK).

RESULTS

OXB significantly decreased the DA neurons death caused by MPTP, alleviated MPP+-induced neurotoxicity in SH-SY5Y cells, and robustly enhanced the weight and motor ability of PD mice. Besides, RNA-Seq analysis demonstrated that the mitogen-activated protein kinase (MAPK) pathway was involved in the pathology of PD. Furthermore, MPP + led to increased levels of phosphorylation of ERK (p-ERK), OXB treatment significantly decreased the levels of p-ERK in MPP+-treated SH-SY5Y cells.

CONCLUSIONS

This study demonstrated that OXB exerts a neuroprotective role associated with reduced ERK phosphorylation in the PD model. This suggests that OXB may have therapeutic potential for treatment of PD.

The role of orexin in Parkinson's disease

Emerging evidence has implicated the orexin system in non-motor pathogenesis of Parkinson's disease. It has also been suggested the orexin system is involved in the modulation of motor control, further implicating the orexin system in Parkinson's disease. Parkinson's disease is the second most common neurodegenerative disease with millions of people suffering worldwide with motor and non-motor symptoms, significantly affecting their quality of life. Treatments are based solely on symptomatic management and no cure currently exists. The orexin system has the potential to be a treatment target in Parkinson's disease, particularly in the non-motor stage. In this review, the most current evidence on the orexin system in Parkinson's disease and its potential role in motor and non-motor symptoms of the disease is summarized. This review begins with a brief overview of Parkinson's disease, animal models of the disease, and the orexin system. This leads into discussion of the possible roles of orexin neurons in Parkinson's disease and levels of orexin in the cerebral spinal fluid and plasma in Parkinson's disease and animal models of the disease. The role of orexin is then discussed in relation to symptoms of the disease including motor control, sleep, cognitive impairment, psychological behaviors, and the gastrointestinal system. The neuroprotective effects of orexin are also summarized in preclinical models of the disease.

Several neuropeptides involved in parkinsonian neuroprotection modulate the firing properties of nigral dopaminergic neurons

Parkinson's disease is characterized by progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta. The surviving nigral dopaminergic neurons display altered spontaneous firing activity in Parkinson's disease. The firing rate of nigral dopaminergic neurons decreases long before complete neuronal death and the appearance of parkinsonian symptoms. A mild stimulation could rescue dopaminergic neurons from death and in turn play neuroprotective effects. Several neuropeptides, including cholecystokinin (CCK), ghrelin, neurotensin, orexin, tachykinins and apelin, within the substantia nigra pars compacta play important roles in the modulation of spontaneous firing activity of dopaminergic neurons and therefore involve motor control and motor disorders. Here, we review neuropeptide-induced modulation of the firing properties of nigral dopaminergic neurons. This review may provide a background to guide further investigations into the involvement of neuropeptides in movement control by modulating firing activity of nigral dopaminergic neurons in Parkinson's disease.

Targeting the orexin/hypocretin system for the treatment of neuropsychiatric and neurodegenerative diseases: from animal to clinical studies

Orexins (also known as hypocretins) are neuropeptides located exclusively in hypothalamic neurons that have extensive projections throughout the central nervous system and bind two different G protein-coupled receptors (OX1R and OX2R). Since its discovery in 1998, the orexin system has gained the interest of the scientific community as a potential therapeutic target for the treatment of different pathological conditions. Considering previous basic science research, a dual orexin receptor antagonist, suvorexant, was the first orexin agent to be approved by the US Food and Drug Administration to treat insomnia. In this review, we discuss and update the main preclinical and human studies involving the orexin system with several psychiatric and neurodegenerative diseases. This system constitutes a nice example of how basic scientific research driven by curiosity can be the best route to the generation of new and powerful pharmacological treatments.

Parkinson disease: Protective role and function of neuropeptides

Neuropeptides are bioactive molecules, made up of small chains of amino acids, with many neuromodulatory properties. Several lines of evidence suggest that neuropeptides, mainly expressed in the central nervous system (CNS), play an important role in the onset of Parkinson's Disease (PD) pathology. The wide spread disruption of neuropeptides has been excessively demonstrated to be related to the pathophysiological symptoms in PD where impairment in motor function per example was correlated with neuropeptides dysregulation in the substantia niagra (SN). Moreover, the levels of different neuropeptides have been found modified in the cerebrospinal fluid and blood of PD patients, indicating their potential role in the manifestation of PD symptoms and dysfunctions. In this review, we outlined the neuroprotective effects of neuropeptides on dopaminergic neuronal loss, oxidative stress and neuroinflammation in several models and tissues of PD. Our main focus was to elaborate the role of orexin, pituitary adenylate cyclase activating polypeptide (PACAP), vasoactive intestinal peptide (VIP), opioids, angiotensin, carnosine and many others in the protection and/or involvement in the neurodegeneration of striatal dopaminergic cells. Further studies are required to better assess the mode of action and cellular mechanisms of neuropeptides in order to shift the focus from the in vitro and in vivo testing to applicable clinical testing. This review, allows a support for future use of neuropeptides as therapeutic solution for PA pathophysiology.

Orexin-B exerts excitatory effects on nigral dopaminergic neurons and alleviates motor disorders in MPTP parkinsonian mice

The study aimed to investigate the effects of orexin-B in Parkinson's disease. The present study showed that orexin-B exerted marked excitatory effects via orexin-2 receptor on the nigral dopaminergic neurons in MPTP parkinsonian mice, while blocking orexin-2 receptor decreased the firing rate of dopaminergic neurons significantly. Furthermore, intracerebroventricular application of orexin-B relieved the degeneration of dopaminergic neurons, increased the general spontaneous activity and alleviated motor coordination in MPTP parkinsonian mice. The present study suggests that orexin-B could exert protective effects on dopaminergic neurons and improve motor disorders in parkinsonian mice. Such protective effects of orexin-B on Parkinson's disease may be partially attributed to the excitatory effects on the nigral dopaminergic neurons.

Focus on the Complex Interconnection between Cancer, Narcolepsy and Other Neurodegenerative Diseases: A Possible Case of Orexin-Dependent Inverse Comorbidity

Simple Summary

This narrative review first describes from several points of view the complex interrelationship between cancer and neurodegeneration, with special attention to the mechanisms that might underlie an inverse relationship between them. In particular, the mechanisms that might induce an imbalance between cell apoptotic and proliferative stimuli are discussed. Second, the review summarizes findings on orexins and their involvement in narcolepsy, neurodegenerative diseases, and cancer, starting from epidemiological data then addressing laboratory findings, animal models, and human clinical observational and interventional investigations. Important research efforts are warranted on these topics, as they might lead to novel therapeutic approaches to both neurodegenerative diseases and cancer.

Abstract

Conditions such as Alzheimer’s (AD) and Parkinson’s diseases (PD) are less prevalent in cancer survivors and, overall, cancer is less prevalent in subjects with these neurodegenerative disorders. This seems to suggest that a propensity towards one type of disease may decrease the risk of the other. In addition to epidemiologic data, there is also evidence of a complex biological interconnection, with genes, proteins, and pathways often showing opposite dysregulation in cancer and neurodegenerative diseases. In this narrative review, we focus on the possible role played by orexin signaling, which is altered in patients with narcolepsy type 1 and in those with AD and PD, and which has been linked to β-amyloid brain levels and inflammation in mouse models and to cancer in cell lines. Taken together, these lines of evidence depict a possible case of inverse comorbidity between cancer and neurodegenerative disorders, with a role played by orexins. These considerations suggest a therapeutic potential of orexin modulation in diverse pathologies such as narcolepsy, neurodegenerative disorders, and cancer.

[Orexin-A inhibits γ-aminobutyric acid current of neonatal rat spinal cord ventral horn neurons by activating OX1R, OX2R and Ca2+-independent PKC]

OBJECTIVE

To investigate the effect of orexin-A on the functionality of ionotropic γ-aminobutyric acid (GABA) receptors in spinal cord ventral horn neurons and its mechanisms.

OBJECTIVE

The spinal cord containing the lumbosacral enlargement was isolated from neonatal SD rats (7-12 days old) and sliced. The slices were digested with papain (in 0.18 g/30 mL artificial cerebrospinal fluid) for 40-60 min, and the ventral horn neurons were separated acutely using fire-polished Pasteur pipettes. After the cells adhered to the bottom of Petri dishes, patch-clamp experiments combined with pharmacological methods were performed to test the effects of orexin-A on GABA currents of the neurons treated with SB334867 (a selective OX1R antagonist), TCSOX229 (a selective OX2R antagonist), Bis-Ⅳ (a PKC inhibitor), PMA (a PKC agonist), Rp-cAMP (a PKA inhibitor), or BAPTA (Ca2+ chelator).

OBJECTIVE

The isolated neurons maintained good morphologies with diverse shapes of cell body and long protrusions. Treatment with orexin-A significantly inhibited the amplitude of GABA-induced current (P < 0.001, n=49) with an inhibition rate of (67.48±12.50)%. SB334867 and TCSOX229, when applied simultaneously, completely abolished the suppressive effect of orexin-A on the GABA currents (P=0.93, n=6), and their separate use partially relieved the suppressive effect of orexin-A (P=0.001, n=8; P=0.02, n=8). The application of Bis-Ⅳ also abolished the suppressive effect of orexin-A on GABA currents (P=0.31, n=5). PMA mimicked the effect of orexin-A in these neurons and significantly inhibited GABA currents with an inhibition rate of (60.79±10.94)%, and the application of orexin-A did not cause further suppression of GABA currents in PMA-treated neurons (P=0.15, n=6). Orexin-A was still capable of suppressing GABA currents in Rp-cAMP-treated neurons (P=0.001, n=5). The extracellular Ca2+-free solution (P=0.004, n=8) or the presence of BAPTA (P=0.04, n=7) did not significantly affect the inhibitory effect of orexin-A on GABA currents.

OBJECTIVE

Orexin-A inhibits GABA currents in the ventral horn neurons of rat spinal cord probably by activating OX1R, OX2R and Ca2+-independent PKC.

Apelin-13 regulates electrical activity in the globus pallidus and induces postural changes in rats

The globus pallidus is the relay nucleus of the basal ganglia, and changes in its electrical activity can cause motor impairment. Apelin-13 is widely distributed in the central and peripheral nervous systems. It has been demonstrated that apelin-13 plays important roles in the regulation of blood pressure and other non-motor functions. However, its role in motor function has rarely been reported. In the present study, apelin-13 (10 μM/100 μM) was injected into the globus pallidus of rats. The results showed that apelin-13 increased the spontaneous discharges in the majority of pallidal neurons. However, an apelin-13-induced inhibitory effect on the firing rate was also observed in a few pallidal neurons. In postural tests, after the systemic administration of haloperidol, unilateral pallidal injection of apelin-13 caused a contralateral deflection. Together, these findings suggest that apelin-13 regulates the electrical activity of pallidal neurons and thus participates in central motor control in rats. The study was approved by the Animal Ethics Committee of Qingdao University (approval No. 20200615Wistar0451003020) on June 15, 2020.

Hypocretin (Orexin) Replacement Therapies

Twenty-two years after their discovery, the hypocretins (Hcrts), also known as orexins, are two of the most studied peptidergic systems, involved in myriad physiological systems that range from sleep, arousal, motivation, homeostatic regulation, fear, anxiety and learning. A causal relationship between activity of Hcrt and arousal stability was established shortly after their discovery and have led to the development of a new class of drugs to treat insomnia. In this review we discuss the many faces of the Hcrt system and examine recent findings that implicate decreased Hcrt function in the pathogenesis of a number of neuropsychiatric conditions. We also discuss future therapeutic strategies to replace or enhance Hcrt function as a treatment option for these neuropsychiatric conditions.

Orexin and Parkinson's disease: A protective neuropeptide with therapeutic potential

Parkinson's disease (PD) is the second most common neurodegenerative disease caused by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta. PD is characterized by motor dysfunctions as well as non-motor disorders. Orexin (also known as hypocretin) is a kind of neuropeptide involved in the regulation of motor control, the sleep/wake cycle, learning and memory, gastric motility and respiratory function. Several lines of evidence suggest that the orexinergic system is involved in the manifestations of PD, especially the non-motor disorders. Recent studies have revealed the protective actions and potential therapeutic applications of orexin in both cellular and animal models of PD. Here we present a brief overview of the involvement of the orexinergic system in PD, including the pathological changes in the lateral hypothalamus, the loss of orexinergic neurons and the fluctuation of orexin levels in CSF. Furthermore, we also review the neuroprotective effects of orexin in cellular and animal models of PD.

Involvement of orexin-A in the regulation of neuronal activity and emotional behaviors in central amygdala in rats

The amygdala is a complex structure involved in the regulation of emotional behaviors including fear and anxiety. The central amygdala is the main output of the amygdala and plays an important role in emotional processing. Recent studies indicate that orexin, a kind of neuropeptides responsible for maintaining wakefulness, is also associated with emotion-related behaviors, such as depression- and anxiety-like behaviors. Central amygdala receives orexinergic fibers originating from the lateral hypothalamus and expresses OX₁ receptors in rats. To test the electrophysiological and behavioral effects of orexins in the central amygdala, single unit in vivo extracellular recordings, open field and elevated plus maze tests were performed in rats. Micro-pressure administration of orexin-A (0.01 mmol/L) increased the firing rate in 18 out of the 31 central amygdala neurons, while the other 13 neurons were not excited by orexin-A. The excitatory effects of orexin-A on central amygdala neurons were mainly mediated by OX₁ receptors rather than OX₂ receptors. Orexin-B (0.01 mmol/L) did not change the firing activity in all recorded central amygdala neurons. Selectively blocking OX₁ receptors by SB-334867 (0.01 mmol/L) significantly decreased the spontaneous firing rate in 14 out of the 33 central amygdala neurons, leaving the remaining 19 neurons were not affected. However, blocking OX₂ receptors by TCS-OX2-29 (0.01 mmol/L) did not change the firing activity. Finally, both open field test and elevated plus maze test showed that bilateral microinjection of orexin-A into the central amygdala induced significantly anxiolytic-like behaviors. The specific OX₁ receptor antagonist tended to produce opposite effects although there was no statistical difference. The present electrophysiological and behavioral studies suggested that orexin-A participates in anxiety-like behaviors by modulating the spontaneous firing activity of central amygdala neurons.

The globus pallidus as a target for neuropeptides and endocannabinoids participating in central activities

The globus pallidus in the basal ganglia plays an important role in movement regulation. Neuropeptides and endocannabinoids are neuronal signalling molecules that influence the functions of the whole brain. Endocannabinoids, enkephalin, substance P, neurotensin, orexin, somatostatin and pituitary adenylate cyclase-activating polypeptides are richly concentrated in the globus pallidus. Neuropeptides and endocannabinoids exert excitatory or inhibitory effects in the globus pallidus mainly by modulating GABAergic, glutamatergic and dopaminergic neurotransmission, as well as many ionic mechanisms. Pallidal neuropeptides and endocannabinoids are associated with the pathophysiology of a number of neurological disorders, such as Parkinson's disease, Huntington's disease, schizophrenia, and depression. The levels of neuropeptides and endocannabinoids and their receptors in the globus pallidus change in neurological diseases. It has been demonstrated that spontaneous firing activity of globus pallidus neurons is closely related to the manifestations of Parkinson's disease. Therefore, the neuropeptides and endocannabinoids in the globus pallidus may function as potential targets for treatment in some neurological diseases. In this review, we highlight the morphology and function of neuropeptides and endocannabinoids in the globus pallidus and their involvement in neurological diseases.