Michel A. Steiner

Discovery and characterization of ACT-335827, an orally available, brain penetrant orexin receptor type 1 selective antagonist.

Stress relief: Orexin neuropeptides regulate arousal and stress processing through orexin receptor type 1 (OXR-1) and 2 (OXR-2) signaling. A selective OXR-1 antagonist, represented by a phenylglycine-amide substituted tetrahydropapaverine derivative (ACT-335827), is described that is orally available, penetrates the brain, and decreases fear, compulsive behaviors and autonomic stress reactions in rats.

Differential Effects of the Dual Orexin Receptor Antagonist Almorexant and the GABAA-α1 Receptor Modulator Zolpidem, Alone or Combined with Ethanol, on Motor Performance in the Rat

Current insomnia treatments such as γ-aminobutyric acid (GABA) receptor modulators are associated with sedative and muscle-relaxant effects, which increase when drug intake is combined with alcohol. This study compared the novel sleep-enabling compound almorexant (ACT-078573-hydrochloride), a dual orexin receptor antagonist, with the positive GABAA-α1 receptor modulator zolpidem. Both compounds were administered alone or in combination with ethanol, and their effects on forced motor performance were determined in Wistar rats upon waking after treatment. To detect substance-induced sedation and myorelaxation, time spent on an accelerating rotating rod (rotarod) and forepaw grip strength were measured. Zolpidem (10, 30, and 100 mg/kg, p.o.) and ethanol (0.32, 1, and 1.5 g/kg, i.p.) dose-dependently decreased rotarod performance and grip strength, whereas almorexant (30, 100, and 300 mg/kg, p.o.) did not. Doses of ethanol (0.32 and 1 g/kg), which were ineffective when administered alone, showed interactions with zolpidem (10 and 30 mg/kg) leading to reduced rotarod performance and grip strength; in contrast, combination of ethanol (0.32 and 1 g/kg) with almorexant (100 and 300 mg/kg) did not reduce performance or grip strength below ethanol alone. We conclude that unlike zolpidem, almorexant does not interfere with forced motor performance or grip strength in the rat, nor does it further increase the sedative effects of ethanol. Our results suggest that the effect of almorexant can be immediately reversed to full alertness like under physiological sleep, and that almorexant is less likely to show strong sedation, excessive myorelaxation, or interaction with alcohol than commonly prescribed hypnotics such as zolpidem.

The dual orexin receptor antagonist almorexant, alone and in combination with morphine, cocaine and amphetamine, on conditioned place preference and locomotor sensitization in the rat

Dual orexin receptor (OXR) antagonists emerge as a novel therapeutic class to treat insomnia that, based on anti-addictive effects of selective OXR type 1 antagonists in rats, might be associated with less abuse liability than commonly used γ-aminobutyric acid (GABA) receptor modulators. Here, we studied the effects of the sleep-enabling dual OXR antagonist almorexant on conditioned place preference (CPP) and locomotor sensitization in rats. First, we compared almorexant to the GABA metabolite γ-hydroxybutyrate (GHB), which is clinically used as a sleep-inducing drug and which is associated with mild abuse liability. Whereas conditioning with GHB induced significant place preference, conditioning with almorexant did not. Second, we tested the potential of almorexant to interfere with the conditioned rewarding or locomotor sensitizing effects related to psychostimulants or opiates. Almorexant attenuated the expression of CPP to high doses of cocaine (15 mg/kg) and d.l-amphetamine (2 mg/kg), but not to high dose of morphine (10 mg/kg). Conversely, almorexant interfered with the expression of locomotor sensitization to morphine, but not with that to cocaine and d.l-amphetamine. Third, we observed that chronic almorexant (12 d) treatment in morphine, cocaine or amphetamine pre-conditioned and locomotor-sensitized rats had no influence on the maintenance of CPP and locomotor sensitization when tested after almorexant washout. Our findings suggest that almorexant itself does not exert conditioned rewarding effects in the rat and that it may acutely interfere with the expression of CPP or locomotor sensitization in a drug-dependent manner (monoaminergic psychostimulants vs. opiates).

Structure–Activity Relationship, Biological, and Pharmacological Characterization of the Proline Sulfonamide ACT-462206: a Potent, Brain-Penetrant Dual Orexin 1/Orexin 2 Receptor Antagonist

The orexin system consists of two G‐protein‐coupled receptors, the orexin 1 and orexin 2 receptors, widely expressed in diverse regions of the brain, and two peptide agonists, orexin A and orexin B, which are produced in a small assembly of neurons in the lateral hypothalamus. The orexin system plays an important role in the maintenance of wakefulness. Several compounds (almorexant, SB‐649868, suvorexant) have been in advanced clinical trials for treating primary insomnia. ACT‐462206 is a new, potent, and selective dual orexin receptor antagonist (DORA) that inhibits the stimulating effects of the orexin peptides at both the orexin 1 and 2 receptors. It decreases wakefulness and increases non‐rapid eye movement (non‐REM) and REM sleep while maintaining natural sleep architectures in rat and dog electroencephalography/electromyography (EEG/EMG) experiments. ACT‐462206 shows anxiolytic‐like properties in rats without affecting cognition and motor function. It is therefore a potential candidate for the treatment of insomnia.

Examining the role of endogenous orexins in hypothalamus-pituitary-adrenal axis endocrine function using transient dual orexin receptor antagonism in the rat.

The orexin neuropeptide system regulates wakefulness and contributes to physiological and behavioral stress responses. Moreover, a role for orexins in modulating hypothalamus-pituitary-adrenal (HPA) axis activity has been proposed. Brain penetrating dual orexin receptor (OXR) antagonists such as almorexant decrease vigilance and have emerged as a novel therapeutic class for the treatment of insomnia. Almorexant was used here as a pharmacological tool to examine the role of endogenous orexin signaling in HPA axis endocrine function under natural conditions. After confirming the expression of prepro-orexin and OXR-1 and OXR-2 mRNA in hypothalamus, pituitary and adrenal glands, the effects of systemic almorexant were investigated on peripheral HPA axis hormone release in the rat under baseline, stress and pharmacological challenge conditions. Almorexant did not alter basal or stress-induced corticosterone release despite affecting wake and sleep stages (detected by radiotelemetric electroencephalography/electromyography) during the stress exposure. Moreover, almorexant did not affect the release of adrenocorticotropin (ACTH) and corticosterone at different time points along the diurnal rhythm, nor corticotrophin-releasing hormone (CRH)- and ACTH-stimulated neuroendocrine responses, measured in vivo under stress-free conditions. These results illustrate that dual OXR antagonists, despite modulating stress-induced wakefulness, do not interfere with endocrine HPA axis function in the rat. They converge to suggest that endogenous orexin signaling plays a minor role in stress hormone release under basal conditions and under challenge.

Favoured genetic background for testing anxiolytics in the fear-potentiated and light-enhanced startle paradigms in the rat

The fear-potentiated startle (FPS) and the light-enhanced startle (LES) paradigms are rodent tests of fear and anxiety, which combine face validity with predictive validity for clinically effective anxiolytic drugs. However, systematic strain comparisons aimed at identifying a rat strain that shows robust and reliable fear and anxiety responses in both models are missing. Here, we investigated four commonly used laboratory rat strains: Wistar, Sprague Dawley, Long-Evans and F344. Following strong cued fear conditioning training [60 conditioned stimulus-unconditioned stimulus (CS-US) pairings], all strains except Wistar exhibited significant FPS responses. F344 rats showed the strongest FPS response. Following milder cued fear conditioning protocols, designed to reduce the underlying component of contextual fear conditioning (by context pre-exposure or less CS-US pairings), also Wistar rats were able to show significant FPS, albeit still to a lesser extent than F344 rats tested under identical conditions. When tested in the LES protocol (light intensity ∼ 1500 lx), all strains except Long-Evans displayed significant light-enhanced startle responses. F344 and Wistar showed the strongest LES responses, which were of similar magnitude. The most sensitive strain in both paradigms, F344, was chosen for further pharmacological validation. The clinically active anxiolytic alprazolam (0.3, 1, 3mg/kg p.o.) dose-dependently reduced both fear-like responses in the FPS paradigm and anxiety-like responses in the LES paradigm at non-myorelaxant dosages. We propose that the F344 rat strain is particularly suited for the predictability assessment of novel anxiolytic drugs in both startle paradigms.

The selective orexin receptor 1 antagonist ACT-335827 in a rat model of diet-induced obesity associated with metabolic syndrome.

The orexin system regulates feeding, nutrient metabolism and energy homeostasis. Acute pharmacological blockade of orexin receptor 1 (OXR-1) in rodents induces satiety and reduces normal and palatable food intake. Genetic OXR-1 deletion in mice improves hyperglycemia under high-fat (HF) diet conditions. Here we investigated the effects of chronic treatment with the novel selective OXR-1 antagonist ACT-335827 in a rat model of diet-induced obesity (DIO) associated with metabolic syndrome (MetS). Rats were fed either standard chow (SC) or a cafeteria (CAF) diet comprised of intermittent human snacks and a constant free choice between a HF/sweet (HF/S) diet and SC for 13 weeks. Thereafter the SC group was treated with vehicle (for 4 weeks) and the CAF group was divided into a vehicle and an ACT-335827 treatment group. Energy and water intake, food preference, and indicators of MetS (abdominal obesity, glucose homeostasis, plasma lipids, and blood pressure) were monitored. Hippocampus-dependent memory, which can be impaired by DIO, was assessed. CAF diet fed rats treated with ACT-335827 consumed less of the HF/S diet and more of the SC, but did not change their snack or total kcal intake compared to vehicle-treated rats. ACT-335827 increased water intake and the high-density lipoprotein associated cholesterol proportion of total circulating cholesterol. ACT-335827 slightly increased body weight gain (4% vs. controls) and feed efficiency in the absence of hyperphagia. These effects were not associated with significant changes in the elevated fasting glucose and triglyceride (TG) plasma levels, glucose intolerance, elevated blood pressure, and adiposity due to CAF diet consumption. Neither CAF diet consumption alone nor ACT-335827 affected memory. In conclusion, the main metabolic characteristics associated with DIO and MetS in rats remained unaffected by chronic ACT-335827 treatment, suggesting that pharmacological OXR-1 blockade has minimal impact in this model.

Sex comparison on long-lasting behavioral and physiological disturbances induced by single shock experience in rats.

Enhanced female vulnerability and symptom severity are described in post-traumatic stress disorder (PTSD). It remains largely unknown whether females present with more pronounced PTSD-like symptoms than males in rodent models of PTSD. A model of single electric foot-shock followed by situational reminders was used to investigate in rats the impact of sex on potential long-lasting changes in anxiety-like behavior, and in endocrine and physiological responses to stress and fearful situations. Three weeks after single shock exposure (in the dark side of a shuttle box) both male and female rats spent less time in the dark compartment of a dark-light box and in the closed arms of an elevated plus maze than non-shocked controls. Both behaviors were likely due to avoidance of places reminiscent of the initial shock context. The shock exposure had no long-term impact on social interaction behavior or on basal and restraint stress-induced increases in plasma corticosterone. Shock exposure increased sudden silence-induced freezing responses and hyperthermia during novelty stress and fear to a similar extent in both males and females and left heart rate responses unaffected. Non-shocked females generally showed a reduced response or faster recovery to baseline under stress- or fearful test conditions as compared to non-shocked males, which was likely due to differential sex-specific coping strategies. Taken together, our results suggest that, despite some baseline sex differences, both male and female rats are similarly affected in the long-term by the initial foot-shock exposure used in this particular simulation of PTSD.

Opportunities and perspectives for developing orexin receptor antagonists

The current special topics issue of Frontiers in Neuroscience “Insomnia and Beyond—Exploring the Therapeutic Potential of Orexin Receptor Antagonists” comprises 20 papers from leaders in the orexin/hypocretin field presenting their latest findings and thorough reviews, and will undoubtedly serve as an important reference for others engaged in this field of study. Since the independent identification of the orexin/hypocretin system in 1998 by two teams investigating hypothalamic signaling (de Lecea et al., 1998; Sakurai et al., 1998), the field has rapidly expanded to include potential roles of orexin in processes as diverse as sleep/wake, addiction, feeding, pain and anxiety. Orexin neuropeptides (OX-A, OX-B) are cleaved from a common precursor and secreted from orexinergic neurons localized in the lateral hypothalamus. Orexins (also passionately referred to as hypocretins by many investigators) bind and activate two G-protein coupled receptors (OX1R and OX2R), with OX-A binding equally, and OX-B having ~10 fold selectivity for OX2R. Although orexinergic neurons are localized to less than 100,000 hypothalamic neurons in the human brain, orexin receptors exhibit overlapping and distinct expression throughout the CNS, with significant expression in brain regions associated with arousal and vigilance state, as well as reward and stress processing. Following their seminal discovery of this novel neuropeptide system 16 years ago, authors of the foundational Proc. Natl. Acad. Sci. and Cell papers joined by other exceptional researchers provide their latest insights into the function of orexin signaling and its therapeutic potential. Drs. de Lecea and Huerta (2014) explain their current view on the interaction of orexin neurons with other neuromodulatory systems based on recent optogenetic experiments. A paper from Kohlmeier et al. (2013) presents an original investigation on the interaction of the orexin system with cholinergic and monoaminergic neurons using OX1 and OX2 receptor knockout mice. Two separate papers from Etori et al. (2014) and Tortorella et al. (2013) provide insight into potential synaptic interactions between the hypothalamus, locus coeruleus and oral pontine reticular nucleus, suggesting that orexin signaling within this circuit modulates stage succession during sleep-wakefulness cycles. Expanding upon the first observations of genetic disruptions of orexin receptors in dogs, Thompson et al. (2014) provide a pharmacogenetic overview of orexin peptides and their receptors, and describe the potential impact of polymorphisms observed in human genetic studies. Finally, work described in Flores et al. (2013) illustrates cannabinoid-hypocretin cross-talk within the CNS, an emerging area of interest for the field. The identification of a neuropeptide system with restricted expression, key functions in fundamental CNS processes and approachability of discrete GPCR subtypes has made orexin receptors valuable targets for the next generation of CNS therapeutics, with multiple teams actively characterizing small molecules to understand the pharmacology and potential therapeutic applications of targeting the orexin system. Equihua et al. (2013) takes the reader on a journey comparing the merits and challenges of orexin receptor antagonists as potential sleep medications relative to currently used hypnotics. A paper from Callander et al. (2013) representing a team formerly at Novartis, provides substantial in vitro characterization and describes differential kinetic properties of dual orexin receptor antagonists (DORAs) that have undergone clinical evaluation. Two pharmacological papers from Morairty et al. (2014) (work by groups UCSF and SRI), and Ramirez et al. (2013) (Merck Research Laboratories), demonstrate that DORAs lack the impairment on motor and cognitive function in rodents exhibited by positive GABA-A receptor modulators. Etori et al. (2014) (Kanazawa University), Dugovic et al. (2014) (Janssen), and Hoyer et al. (2013) (team formerly at Novartis) explore the potential differential impact of DORAs as well as antagonists selective for OX1R and OX2R on non-REM and REM sleep architecture. With regard to other potential indications for OXR antagonists beyond insomnia, two manuscripts from teams at Lilly Research Laboratories and MUSC (Anderson et al., 2014; Fitch et al., 2014) describe a novel OX2R selective antagonist, and its effects on rodent models of ethanol addiction and depression relative to a DORA and OX1R selective antagonist. In addition, (Steiner et al., 2013) (Actelion), investigated the impact of chronic pharmacological OX1R blockade in a rat model of diet-induced obesity. A further area of orexin research which is being actively investigated focuses on the impact of orexin signaling on sympathetic autonomous nervous system activation, both during sleep and wake stages, as well as during heightened arousal and stress. Two excellent reviews from leaders in the field (Carrive, 2013; Li and Nattie, 2014) provide an overview of the current perspectives regarding cardiovascular and respiratory control by orexins. (Colas et al., 2014) present evidence supporting the existence of an orexin circuit between central and peripheral nervous system, where orexin signaling potentially modulates sensory processing at dorsal root ganglia. Together with the increasing availability of pharmacological tools, investigations of the role of orexin in limbic emotional processing have also intensified during the last 3–4 years. Two detailed reviews shed light on this topic, from both academic and industry perspectives (Yeoh et al., 2014 from the University of Newcastle and the Hunter Medical Research Institute; Merlo Pich and Melotto, 2014, formerly of Glaxo Smith Kline). These two papers outline recent findings from rodent studies identifying involvement of orexins in mediating compulsive behavior, as well as panic, depression and anxiety-like reactions during withdrawal from drugs of abuse. Importantly, this special issue highlights the exquisite foundational and translational work being conducted by teams at leading academic and biopharmaceutical laboratories, focused on investigating a range of therapeutic opportunities. Although it has only been 16 years since the identification of the orexin/hypocretin system, the wealth of knowledge obtained thus far has set the stage for the next decade of orexin research. It is with great appreciation to the contributing authors and with much enthusiasm that we present this special topics issue of Frontiers in Neuroscience.