Blockade of the orexin-2 receptors within the ventral tegmental area facilitates the extinction and prevents the reinstatement of methamphetamine-seeking behavior

Repeated use of methamphetamine (METH) causes severe effects on the central nervous system, associated with an increased relapse rate. The orexinergic system is highly implicated in the reward circuitry and may be a promising target for treating psychostimulant dependency. The present study aimed to investigate the involvement of the orexin system, mainly the orexin-2 receptors (OX2R) in the ventral tegmental area (VTA) in the extinction and reinstatement of METH-seeking behavior using a conditioned place preference (CPP) paradigm. To this end, animals received METH (1 mg/kg; sc) for a 5-day conditioning period. Then, in the first set of experiments, different groups of rats were given intra-VTA TCS OX2 29 (1, 3, 10, or 30 nmol/0.3 μl DMSO) as an OX2R antagonist over a 10-day extinction period. In another experiment, after the extinction period, a different set of animals received a single dose of TCS OX2 29 (1, 3, 10, or 30 nmol) before the priming dose of METH (0.25 mg/kg; sc) on the reinstatement day. The results revealed that TCS OX2 29 (10 and 30 nmol) remarkably facilitated the extinction of rewarding properties of METH (P < 0.001 for both doses). Furthermore, TCS OX2 29 (3, 10, or 30 nmol) significantly suppressed the METH-induced reinstatement (3 nmol; P < 0.05, 10 nmol; P < 0.01, and 30 nmol; P < 0.001). In conclusion, the current study revealed that the orexinergic system, specifically the VTA OX2R, is involved in METH-seeking behaviors and that manipulation of this system can be considered a potential therapeutics in treating METH dependency.

Effect of different doses of almorexant on learning and memory in 8-month-old APP/PS1 (AD) mice

OBJECTIVE

To explore the effects of different doses of almorexant (an dual orexin receptor antagonist) on learning and memory in Alzheimer's disease (AD) mice.

METHODS

Forty-four APP/PS1 (model of Alzheimer's disease; AD) mice were randomly divided into 4 groups: the control group (CON) and those that received 10mg/kg almorexant (low dose; LOW), 30mg/kg almorexant (medium dose; MED) and 60mg/kg almorexant (high dose; HIGH). During the 28-day intervention period, mice received an intraperitoneal injection at the beginning of the light period (6:00 am). The effects of different doses of almorexant on learning and memory and 24-hour sleep-wake behaviour were assessed by immunohistochemical staining. The above continuous variables are expressed as the mean ± standard deviation (SD), and then univariate regression analysis and generalized estimating equations were performed to compare the groups; these results are expressed as the mean difference (MD) and 95% confidence interval (CI). The statistical software used STATA 17.0 MP.

RESULTS

Forty-one mice completed the experiment (3 died: 2 mice in the HIGH group and 1 mouse in the CON group). Compared with the CON group, the LOW group (MD=6,803s, 95% CI: 4,470 to 9,137s), MED group (MD=14,473s, 95% CI: 12,140 to 16,806s) and the HIGH group (MD=24,505s, 95% CI: 22,052 to 26,959s) had significantly longer sleep durations. The Y maze results showed that LOW group (MD=0.14,95%CI: 0.078 to 0.20) and MED group (MD=0.14,95%CI = 0.074 to 0.20) mice compared to the CON group, and the low-medium dose of Almorexant did not damage the short-term learning and memory performance of APP / PS1 (AD) mice.Compared with the CON, LOW, and MED groups, the HIGH group exhibited a significant decrease in the Aβ plaque-positive area in the cortex (MD= -0.030, 95% CI: -0.035 to -0.025; MD=-0.049, 95% CI: -0.054 to -0.044; and MD=-0.07, 95% CI: -0.076 to -0.066, respectively).

CONCLUSION

The moderate dose of almorexant (30mg/kg) prolonged the sleep duration of APP/PS1 (AD) mice to a greater extent than the low dose (10mg/kg) without altering learning and memory. The MED mice showed a good sleep response and a small residual effect on the next day. High-dose (60mg / kg) almorexant impaired behavioral learning and memory performance in mice.Compared to the CON group and the LOW group, the MED group exhibited improved working memory. Thus, treatment with almorexant may reduce β-amyloid deposition in AD, slowing neurodegeneration. Additional studies are needed to determine the mechanism of action.

Distinct roles for orexin-1 and orexin-2 receptors in the dentate gyrus of the hippocampus in the methamphetamine-seeking behavior in the rats

Because of the relapsing properties of psychostimulants such as methamphetamine (Meth), there is no established pharmacotherapy for Meth addiction. The orexinergic system is a promising target for treating psychostimulant use disorders and relapse. However, to the best of our knowledge, no investigation regarding the role of orexin receptors in the dentate gyrus (DG) region of the hippocampus has been conducted in the extinction and reinstatement of Meth-seeking behavior. Two stainless-steel guide cannulae were bilaterally implanted into the DG of the rats' brains. The unbiased conditioned place preference (CPP) procedure was conducted to induce Meth conditioning. Following the five days Meth injections (1 mg/kg; sc), animals received intra-DG microinjection of SB334867 or TCS OX2 29, as orexin 1 (OX1) or orexin 2 (OX2) receptor antagonists, respectively (without Meth administration) during extinction phase to elucidate the role of orexin receptors in the latency of the extinction period in the Meth-conditioned rats. To evaluate the role of orexin receptors in the DG region in the reinstatement of Meth-seeking behavior, the extinguished rats received SB334867 or TCS OX2 29 before injecting a priming dose of Meth (0.25 mg/kg; sc). The results indicated two distinct roles for the OX1 and OX2 receptors in the DG region. TCS OX2 29 attenuated the extinction latency, and SB334867 considerably reduced the reinstatement of Meth-seeking behavior in this region. Therefore, the DG region's orexinergic system might be a potential therapeutic target for psychostimulant use disorders.

In vivo microdialysis reveals that blockade of accumbal orexin OX2 but not OX1 receptors enhances dopamine efflux in the nucleus accumbens of freely moving rats

The nucleus accumbens contains orexinergic neural inputs and orexin OX₁ -and OX₂ -receptors. Behavioural studies suggest that accumbal orexin receptors modulate accumbal dopaminergic activity-dependent locomotion in rats. We studied the effects of intra-accumbal injection of orexin receptor ligands on accumbal extracellular dopamine levels in freely moving rats, using in vivo microdialysis, and analysed the roles of OX₁ - and OX₂ -receptors in the regulation of basal accumbal dopamine efflux. The orexin receptor ligands were applied intra-accumbally though a microinjection needle attached with a dialysis probe. Neither the non-selective OX₁ - and OX₂ -receptor agonist orexin-A nor the preferential OX₂ -receptor agonist orexin-B (500.0 pg and 5.0 ng) altered accumbal dopamine levels. The non-selective OX₁ - and OX₂ -receptor antagonist MK-4305 (suvorexant, 500.0 pg, 2.5 and 5.0 ng) enhanced dopamine efflux. A 2-h tetrodotoxin infusion into nucleus accumbens through the probe or co-administration of orexin-A (500.0 pg) strongly inhibited MK-4305 (5.0 ng)-induced accumbal dopamine efflux. The selective OX₂ -receptor antagonist EMPA (90.0 and 900.0 pg, 9.0 ng) increased dopamine efflux. Intra-accumbal infusion of tetrodotoxin abolished EMPA (9.0 ng)-induced dopamine efflux. The selective OX₁ -receptor antagonist SB-334867 (10.0 and 20.0 ng) failed to alter dopamine efflux. Co-administration of orexin-B (500.0 pg) inhibited both EMPA (9.0 ng)- and MK-4305 (5.0 ng)-induced dopamine efflux. Intraperitoneal injection of MK-4305 (10.0 mg/kg) did not affect accumbal dopamine efflux. The present study provides in vivo neuropharmacological evidence that accumbal OX₂ - but not OX₁ -receptors exert inhibitory regulation of basal accumbal dopamine efflux and that blockade of accumbal OX₂ -receptors enhances dopamine efflux in nucleus accumbens of freely moving rats.

Orexin receptors in GtoPdb v.2021.3

Orexin receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Orexin receptors [42]) are activated by the endogenous polypeptides orexin-A and orexin-B (also known as hypocretin-1 and -2; 33 and 28 aa) derived from a common precursor, preproorexin or orexin precursor, by proteolytic cleavage and some typical peptide modifications [109]. Currently the only orexin receptor ligands in clinical use are suvorexant and lemborexant, which are used as hypnotics. Orexin receptor crystal structures have been solved [134, 133, 54, 117, 46].

Synthesis and biological activity evaluation of azacycloheptane sulfonamide derivatives as potential orexin receptor antagonists

As the orexin signaling system is crucial for the regulation of the sleep/wake cycle, inhibitors of orexin-1 and orexin-2 receptors are of significant interest in the treatment of insomnia. Herein, a series of novel azacycloheptane sulfonamide derivatives were designed and synthesized, and all the compounds were evaluated as potential orexin receptor inhibitors by FLIPR Tetra calcium assay. A majority of the tested azacycloheptane sulfonamide derivatives showed OX1R and OX2R inhibitory activity. Chloro-substituted derivatives functionalized at the C5 or C6 position of the benzoxazole group exhibited better inhibitory activity for OX1R and OX2R than unsubstituted derivatives functionalized at C5 or C6. In addition, phenyl group modification had positive effects on the inhibitory activities, and an electron-withdrawing fluorine group at the ortho or meta position of the phenyl ring improved the OX2R inhibitory activity of the derivatives. This suggests that azacycloheptane sulfonamide derivatives are promising scaffolds for the development of OX1R and OX2R antagonists.

Azacycloheptane sulfonamide derivatives are promising scaffolds for the development of OX1R and OX2R antagonists.

Targeting the Orexin System for Prescription Opioid Use Disorder

Prescription opioids are potent analgesics that are used for clinical pain management. However, the nonmedical use of these medications has emerged as a major concern because of dramatic increases in abuse and overdose. Therefore, effective strategies to prevent prescription opioid use disorder are urgently needed. The orexin system has been implicated in the regulation of motivation, arousal, and stress, making this system a promising target for the treatment of substance use disorder. This review discusses recent preclinical studies that suggest that orexin receptor blockade could be beneficial for the treatment of prescription opioid use disorder.

A New Positron Emission Tomography Probe for Orexin Receptors Neuroimaging

The orexin receptor (OX) is critically involved in motivation and sleep−wake regulation and holds promising therapeutic potential in various mood disorders. To further investigate the role of orexin receptors (OXRs) in the living human brain and to evaluate the treatment potential of orexin-targeting therapeutics, we herein report a novel PET probe ([¹¹C]CW24) for OXRs in the brain. CW24 has moderate binding affinity for OXRs (IC₅₀ = 0.253 μM and 1.406 μM for OX₁R and OX₂R, respectively) and shows good selectivity to OXRs over 40 other central nervous system (CNS) targets. [¹¹C]CW24 has high brain uptake in rodents and nonhuman primates, suitable metabolic stability, and appropriate distribution and pharmacokinetics for brain positron emission tomography (PET) imaging. [¹¹C]CW24 warrants further evaluation as a PET imaging probe of OXRs in the brain.

Optical probing of orexin/hypocretin receptor antagonists

Study Objectives

The present study investigated the function of Hypocretin (Hcrt or Orexin/OX) receptor antagonists in sleep modulation and memory function with optical methods in transgenic mice.

Methods

We used Hcrt-IRES-Cre knock-in mice and AAV vectors expressing channelrhodopsin-2 (ChR2) to render Hcrt neurons sensitive to blue light stimulation. We optogenetically stimulated Hcrt neurons and measured latencies to wakefulness in the presence or absence of OX1/2R antagonists and Zolpidem. We also examined endogenous Hcrt neuronal activity with fiber photometry. Changes in memory after optogenetic sleep disruption were evaluated by the novel object recognition test (NOR) and compared for groups treated with vehicle, OX1/2R antagonists, or Zolpidem. We also analyzed electroencephalogram (EEG) power spectra of wakefulness, rapid eye movement (REM) sleep, and non-REM (NREM) sleep following the injections of vehicle, OX1/2R antagonists, and Zolpidem in young adult mice.

Results

Acute optogenetic stimulation of Hcrt neurons at different frequencies resulted in wakefulness. Treatment with dual OX1/2R antagonists (DORAs) DORA12 and MK6096, as well as selective OX2R antagonist MK1064 and Zolpidem, but not selective OX1R antagonist 1SORA1, significantly reduced the bout length of optogenetic stimulation-evoked wakefulness episode. Fiber photometry recordings of GCaMP6f signals showed that Hcrt neurons are active during wakefulness, even in the presence of OXR antagonists. Treatment with dual OX1/2R antagonists improved memory function despite optogenetic sleep fragmentation caused impaired memory function in a NOR test.

Conclusions

Our results show DORAs and selective OX2R antagonists stabilize sleep and improve sleep-dependent cognitive processes even when challenged by optogenetic stimulation mimicking highly arousing stimuli.

Therapeutics development for addiction: Orexin-1 receptor antagonists

The orexin system includes the neuropeptides orexin A and B and the cognate receptors of orexin-1 (OX1) and -2 (OX2) and has been indicated in a number of important physiological processes. It is generally accepted that the OX1 receptor is mainly involved in motivation and reward and the OX2 receptor in the modulation of sleep/wake cycle and energy homeostasis. A variety of OX1 selective antagonists (1-SORAs) have been disclosed in the literature and some of them have been evaluated as potential therapeutics for addiction treatment. In this review we summarize all OX1 antagonists reported thus far based on their core structure. Several dual orexin receptor antagonists (DORAs) and OX2 selective antagonist (2-SORAs) have also been recently evaluated in reward and addiction models. While DORAs may seem pharmacologically advantageous for alcohol addiction given the recent findings on the OX2 receptor in reward and alcohol consumption, 1-SORAs are the better options for other drugs of addiction such as cocaine due to the absence of the sedative effects inherently associated with dual antagonists.

An Update on Dual Orexin Receptor Antagonists and Their Potential Role in Insomnia Therapeutics

STUDY OBJECTIVES

Current pharmacological options for the treatment of insomnia insufficiently meet the needs of all insomnia patients. Approved treatments are not consistently effective in improving sleep onset and sleep maintenance, while also having complicated safety profiles. These limitations highlight the unmet need for additional medications and treatment strategies. Initial research suggests that the dual orexin receptor antagonists (DORAs) may offer an additional pharmaceutical option to treat insomnia in some patients.

METHODS

We reviewed the existing literature on dual orexin receptor antagonists in PubMed databases using the search terms "orexin receptor antagonist," "almorexant" "filorexant," "lembroexant" and "suvorexant"; searches were limited to English language primary research articles, clinical trials, and reviews.

RESULTS

Targeting the orexin receptor system for treatment of insomnia offers an additional and alternative pharmacological approach to more common gamma aminobutyric acid agonist sedative hypnotic treatment. Effectiveness is not well established in the current literature; however, the literature does suggest efficacy. Preclinical reports also suggest the potential for treatment in individuals with comorbid Alzheimer disease and insomnia.

CONCLUSIONS

DORAs offer an additional treatment option for insomnia. More clinical trials are needed to robustly evaluate their safety and effectiveness in several subclasses of individuals with insomnia. Given the published literature, head-to-head comparisons to existing treatment for insomnia are warranted.

Effects of intrathecal administration of orexin-1 receptor antagonist on antinociceptive responses induced by chemical stimulation of lateral hypothalamus in an animal model of tonic nociception

Orexins are produced in the restricted regions of the lateral hypothalamus (LH). However, orexinergic receptors and projections are localized in wide regions like nucleus accumbens, ventral tegmental area, periaqueductal gray area and spinal cord which are involved in the pain modulation. This study was carried out to investigate the effects of intrathecal administration of orexin-1 receptor antagonist (SB-334867) in the spinal antinociception induced by intra-LH administration of carbachol (cholinergic receptor agonist) in both early and late phases of pain related behaviors in formalin test. In this study, pain-related behaviors (pain scores) were evaluated using the formalin test during 5-min block intervals for a 60-min period in seventy male Wistar rats were given SB-334867 (3, 10, 30 and 100 μM/10 μl) or vehicle (DMSO 12%; 10 μl) intrathecally following intra-LH administration of carbachol (250 nM/rat). Our data showed that intra-LH injection of carbachol attenuated the formalin-induced biphasic pain responses, and intrathecal administration of SB-334867 dose-dependently decreased LH stimulation-induced antinociceptive responses during both phases. Moreover, administration of different doses of SB-334867 during the early phase were more effective than those during the late phase. The antinociceptive role of orexinergic system in the formalin test through a neural pathway from the LH to the spinal cord provides evidence that orexins can be useful in therapeutic targets for pain relief.

Perinatal programming of the orexinergic (hypocretinergic) system in hypothalamus and anterior pituitary by testosterone

Orexins A/B derived from hypothalamic prepro-orexin (PPO) are agonists for orexin receptors 1 (OX1) and 2 (OX2). Previously, we showed clear sex differences in the hypothalamic-pituitary-gonadal orexinergic system in adult rodents. Here, we studied the effect of sexual brain differentiation on the orexinergic system in neuroendocrine structures regulating reproduction. We evaluated: a: proestrous and neonatally androgenized female rats; b: adult males, untreated or gonadectomized in adulthood and injected with oil or estradiol and progesterone (E₂/P₄); c: control and demasculinized males (perinatally treated with flutamide and later castration) injected either with oil or E₂/P₄ in adulthood. Rats were sacrificed at 12:00 and 18:00h; blood samples and brains were collected. Hormones were measured using radioimmunoassay. PPO, OX1 and OX2 mRNAs were quantified by qPCR in medial basal hypothalamus, anterior hypothalamus, adenohypophysis, and cortex. Western blots for OX1 were done in the same structures. In normal females, gonadotropins surged at 18:00h coinciding with significant elevations of PPO, OX1 and OX2 mRNAs and OX1 protein in hypothalamus and pituitary; no increases were observed at noon. Afternoon changes were absent in masculinized females. Demasculinized males when treated with E₂/P₄ showed high PPO, OX1 and OX2 mRNAs and OX1 protein expression in hypothalamus and pituitary at 12:00 and 18:00h compared vehicle-treated controls. The same steroid treatment was ineffective in males with normal brain masculinization. Here we show that neonatal testosterone shapes the sexual differences in the hypothalamic-pituitary orexinergic system in synchronicity to establishing the brain sex differences of the reproductive axis. The female brain controls gonadotropin surges and concurrent elevations of all studied components of the orexinergic system, suggesting its participation as a possible link between food intake, behavior and hormonal control of reproduction.

Role of orexin receptors in the ventral tegmental area on acquisition and expression of morphine-induced conditioned place preference in the rats

The orexins are hypothalamic neuropeptides and their role in reward processing and drug addiction has been demonstrated. The extent of involvement of each orexin receptor in the acquisition and expression of conditioned place preference (CPP) for morphine is still a matter of controversy. We investigated the functional differences between orexin-1 and -2 receptor blockade in the ventral tegmental area (VTA) on the acquisition and expression of morphine CPP. A total of 86 adult male Wistar rats weighing 250±30g (age 7-8weeks) received intra-VTA microinjection of either SB334867 (0.1, 1 and 10nM), a selective orexin-1 receptor (OX1R) antagonist, or TCS-OX2-29 (1, 5 and 25nM), a selective orexin-2 receptor (OX2R) antagonist. To measure the acquisition, the animals received each antagonist (SB334867 or TCS-OX2-29) 5min prior to subcutaneous injection of morphine (5mg/kg) during the conditioning phase. To measure the CPP expression, the animals received each antagonist on the post-conditioning phase. The CPP conditioning score was recorded by Ethovision software. Data showed that intra-VTA microinjection of OX1-R antagonist significantly attenuated morphine CPP acquisition, during the conditioning phase, and expression, during the post-conditioning phase. Intra-VTA microinjection of OX2-R antagonist also significantly attenuated morphine CPP acquisition and expression in the mentioned phases. Our results showed the orexin role in learning and memory and indicate that orexin receptors (OX1R and OX2R) function in the VTA is essential for both acquisition and expression of morphine reward in rats in the CPP model.

Orexin OX2 Receptor Antagonists as Sleep Aids

The discovery of the orexin system represents the single major progress in the sleep field of the last three to four decades. The two orexin peptides and their two receptors play a major role in arousal and sleep/wake cycles. Defects in the orexin system lead to narcolepsy with cataplexy in humans and dogs and can be experimentally reproduced in rodents. At least six orexin receptor antagonists have reached Phase II or Phase III clinical trials in insomnia, five of which are dual orexin receptor antagonists (DORAs) that target both OX1 and OX2 receptors (OX2Rs). All clinically tested DORAs induce and maintain sleep: suvorexant, recently registered in the USA and Japan for insomnia, represents the first hypnotic principle that acts in a completely different manner from the current standard medications. It is clear, however, that in the clinic, all DORAs promote sleep primarily by increasing rapid eye movement (REM) and are almost devoid of effects on slow-wave (SWS) sleep. At present, there is no consensus on whether the sole promotion of REM sleep has a negative impact in patients suffering from insomnia. However, sleep onset REM (SOREM), which has been documented with DORAs, is clearly an undesirable effect, especially for narcoleptic patients and also in fragile populations (e.g. elderly patients) where REM-associated loss of muscle tone may promote an elevated risk of falls. Debate thus remains as to the ideal orexin agent to achieve a balanced increase in REM and non-rapid eye movement (NREM) sleep. Here, we review the evidence that an OX2R antagonist should be at least equivalent, or perhaps superior, to a DORA for the treatment of insomnia. An OX2R antagonist may produce more balanced sleep than a DORA. Rodent sleep experiments show that the OX2R is the primary target of orexin receptor antagonists in sleep modulation. Furthermore, an OX2R antagonist should, in theory, have a lower narcoleptic/cataplexic potential. In the clinic, the situation remains equivocal, since OX2R antagonists are in early stages: MK-1064 has completed Phase I, and MIN202 is currently in clinical Phase II/III trials. However, data from insomnia patients have not yet been released. Promotional material suggests that balanced sleep is indeed induced by MIN-202, whereas in volunteers MK-1064 has been reported to act similarly to DORAs.

The dual orexin receptor antagonist TCS1102 does not affect reinstatement of nicotine-seeking

The orexin/hypocretin system is important for appetitive motivation towards multiple drugs of abuse, including nicotine. Both OX1 and OX2 receptors individually have been shown to influence nicotine self-administration and reinstatement. Due to the increasing clinical use of dual orexin receptor antagonists in the treatment of disorders such as insomnia, we examined whether a dual orexin receptor antagonist may also be effective in reducing nicotine seeking. We tested the effect of intracerebroventricular (i.c.v.) administration of the potent and selective dual orexin receptor antagonist TCS1102 on orexin-A-induced food self-administration, nicotine self-administration and reinstatement of nicotine-seeking in rats. Our results show that 30 μg of TCS1102 i.c.v. abolishes orexin-A-induced increases in food self-administration but does not reduce nicotine self-administration. Neither i.c.v. 10 μg nor 30 μg of TCS1102 reduced compound reinstatement after short-term (15 days) self-administration nicotine, but 30 μg transiently reduced cue/nicotine compound reinstatement after chronic self-administration (29 days). These results indicate that TCS1102 has no substantial effect on motivation for nicotine seeking following chronic self-administration and no effect after shorter periods of intake. Orexin receptor antagonists may therefore have little clinical utility against nicotine addiction.

Optogenetic identification of hypothalamic orexin neuron projections to paraventricular spinally projecting neurons

Orexin neurons, and activation of orexin receptors, are generally thought to be sympathoexcitatory; however, the functional connectivity between orexin neurons and a likely sympathetic target, the hypothalamic spinally projecting neurons (SPNs) in the paraventricular nucleus of the hypothalamus (PVN) has not been established. To test the hypothesis that orexin neurons project directly to SPNs in the PVN, channelrhodopsin-2 (ChR2) was selectively expressed in orexin neurons to enable photoactivation of ChR2-expressing fibers while examining evoked postsynaptic currents in SPNs in rat hypothalamic slices. Selective photoactivation of orexin fibers elicited short-latency postsynaptic currents in all SPNs tested (n = 34). These light-triggered responses were heterogeneous, with a majority being excitatory glutamatergic responses (59%) and a minority of inhibitory GABAergic (35%) and mixed glutamatergic and GABAergic currents (6%). Both glutamatergic and GABAergic responses were present in the presence of tetrodotoxin and 4-aminopyridine, suggesting a monosynaptic connection between orexin neurons and SPNs. In addition to generating postsynaptic responses, photostimulation facilitated action potential firing in SPNs (current clamp configuration). Glutamatergic, but not GABAergic, postsynaptic currents were diminished by application of the orexin receptor antagonist almorexant, indicating orexin release facilitates glutamatergic neurotransmission in this pathway. This work identifies a neuronal circuit by which orexin neurons likely exert sympathoexcitatory control of cardiovascular function.NEW & NOTEWORTHY This is the first study to establish, using innovative optogenetic approaches in a transgenic rat model, that there are robust heterogeneous projections from orexin neurons to paraventricular spinally projecting neurons, including excitatory glutamatergic and inhibitory GABAergic neurotransmission. Endogenous orexin release modulates glutamatergic, but not GABAergic, neurotransmission in these pathways.

Orexin and Central Modulation of Cardiovascular and Respiratory Function

Orexin makes an important contribution to the regulation of cardiorespiratory function. When injected centrally under anesthesia, orexin increases blood pressure, heart rate, sympathetic nerve activity, and the amplitude and frequency of respiration. This is consistent with the location of orexin neurons in the hypothalamus and the distribution of orexin terminals at all levels of the central autonomic and respiratory network. These cardiorespiratory responses are components of arousal and are necessary to allow the expression of motivated behaviors. Thus, orexin contributes to the cardiorespiratory response to acute stressors, especially those of a psychogenic nature. Consequently, upregulation of orexin signaling, whether it is spontaneous or environmentally induced, can increase blood pressure and lead to hypertension, as is the case for the spontaneously hypertensive rat and the hypertensive BPH/2J Schlager mouse. Blockade of orexin receptors will reduce blood pressure in these animals, which could be a new pharmacological approach for the treatment of some forms of hypertension. Orexin can also magnify the respiratory reflex to hypercapnia in order to maintain respiratory homeostasis, and this may be in part why it is upregulated during obstructive sleep apnea. In this pathological condition, blockade of orexin receptors would make the apnea worse. To summarize, orexin is an important modulator of cardiorespiratory function. Acting on orexin signaling may help in the treatment of some cardiovascular and respiratory disorders.

Morphological and Physiological Interactions Between GnRH3 and Hypocretin/Orexin Neuronal Systems in Zebrafish (Danio rerio)

GnRH neurons integrate internal and external cues to control sexual maturation and fertility. Homeostasis of energy balance and food intake correlates strongly with the status of reproduction. Neuropeptides secreted by the hypothalamus involved in modulating energy balance and feeding may play additional roles in the regulation of reproduction. Hypocretin (Hcrt) (also known as orexin) is one such peptide, primarily controlling sleep/wakefulness, food intake, and reward processing. There is a growing body of evidence indicating that Hcrt/orexin (Hcrt) modulates reproduction through interacting with the hypothalamo-pituitary-gonadal axis in mammals. To explore potential morphological and functional interactions between the GnRH and Hcrt neuronal systems, we employed a variety of experimental approaches including confocal imaging, immunohistochemistry, and electrophysiology in transgenic zebrafish, in which fluorescent proteins are genetically expressed in GnRH3 and Hcrt neurons. Our imaging data revealed close apposition and direct connection between GnRH3 and Hcrt neuronal systems in the hypothalamus during larval development through adulthood. Furthermore, the Hcrt receptor (HcrtR) is expressed in GnRH3 neurons. Electrophysiological data revealed a reversible inhibitory effect of Hcrt on GnRH3 neuron electrical activity, which was blocked by the HcrtR antagonist almorexant. In addition, Hcrt had no effect on the electrical activity of GnRH3 neurons in the HcrtR null mutant zebrafish (HcrtR-/-). Our findings demonstrate a close anatomical and functional relationship between Hcrt and GnRH neuronal systems in zebrafish. It is the first demonstration of a link between neuronal circuits controlling sleeping/arousal/feeding and reproduction in zebrafish, an important animal model for investigating the molecular genetics of development.

Blockade of orexin receptors attenuates the cardiovascular response to air-jet stress in spontaneously hypertensive rats

This study tested the hypothesis that orexin plays a role in the elevated pressor response to acute stress in the spontaneously hypertensive rat (SHR). The pressor response to air jet stress (AJS) (n=11/group) was 2.5 times greater in vehicle treated SHR versus Wistar (WIS) rats. Systemic delivery of 30mg/kg of the dual orexin receptor antagonist almorexant did not significantly change resting mean arterial pressure (MAP) but did attenuate the pressor response elicited by AJS to a greater extent in the SHR compared to the Wistar rats (~65% versus ~33% reduction respectively; n=6/group). Alternatively 100mg/kg almorexant reduced resting MAP in the SHR (~25mm Hg drop) and attenuated both the heart rate (HR; ~50% reduction) and MAP (~62% reduction) response to AJS in both strains (n=6/group). Systemic application of SB-334867 (3mg/kg), an orexin receptor type 1 antagonist (n=5/group), selectively reduced resting MAP and attenuated the HR response to AJS in the SHR but had no effect on the pressor response in either strain. The potential role of endogenous orexin release in cardiovascular control in the SHR was linked to a significant increase in brain-derived neurotrophic factor mRNA expression in the hypothalamus and elevated orexin receptor expression (type 2 only) in the dorsal pons when compared to WIS (n=4/group). These results demonstrate that the exaggerated pressor response in the SHR to stress is linked to increased orexin receptor activation and possibly altered orexin receptor expression in the dorsal pons and BDNF expression in the hypothalamus.

Orexin receptor antagonists--a patent review (2010 to August 2014)

INTRODUCTION

The orexin (hypocretin) system is an evolutionarily conserved neuropeptide-G-protein-coupled receptor system, consisting of two neuropeptides the orexin-A and the orexin-B peptides as well as two receptors the orexin-1 and the orexin-2 receptors. The orexin system is crucially involved in the regulation of the circadian rhythm, states of wakefulness and arousal and the modulation of emotions and has attracted the interest of many researchers which resulted in an enormous amount of insight, mainly in the field of antagonists. Clinical proof of concept was obtained with dual orexin receptor antagonists in primary insomnia. Merck's suvorexant got FDA approval on 13 August 2014 for the treatment of insomnia.

AREAS COVERED

The patent applications from Thomson Reuters Integrity Database (covering 2010-August 2014) are summarized, analyzed and discussed in the review.

EXPERT OPINION

Intense patenting activities have been observed over the past 3 years in the field of orexin antagonists. Several compounds have been investigated in clinical trials mainly for the treatment of primary insomnia. The advantage of orexin antagonists, based on animal pharmacology results, is the promotion and maintenance of physiological sleep which should avoid hangover phenomena reported as side effects of approved treatments. Many other potential treatment options are mentioned for orexin antagonists of different selectivity profiles.

The importance of the 6- and 7-positions of tetrahydroisoquinolines as selective antagonists for the orexin 1 receptor

Selective antagonism of the orexin 1 (OX1) receptor has been proposed as a potential mechanism for treatment of drug addiction. We have previously reported studies on the structure-activity relationships of tetrahydroisoquinoline-based antagonists. In this report, we elucidated the respective role of the 6- and 7-substitutions by preparation of a series of either 6-substituted tetrahydroisoquinolines (with no 7-substituents) or vice versa. We found that 7-substituted tetrahydroisoquinolines showed potent antagonism of OX1, indicating that the 7-position is important for OX1 antagonism (10 c, Ke = 23.7 nM). While the 6-substituted analogs were generally inactive, several 6-amino compounds bearing ester groups showed reasonable potency (26 a, Ke = 427 nM). Further, we show evidence that suggests several compounds initially displaying insurmountable antagonism at the OX1 receptor are competitive antagonists with slow dissociation rates.

Both Ox1r and Ox2r orexin receptors contribute to the cardiovascular and locomotor components of the novelty stress response in the rat

Orexin contributes to the expression of the cardiovascular and behavioural response of some forms of stress, including novelty stress. Thus, Almorexant, a dual receptor antagonist that blocks the two known orexin receptors, Ox1R and Ox2R, reduces these responses. However, it is not known if the reduction results from blockade of one receptor only or both. To answer this question, the selective Ox1R antagonist ACT335827 and the selective Ox2R antagonist EMPA were injected intragastrically (300 mg/kg) or intraperitoneally (30 and 100 mg/kg) either alone or as a cocktail and compared to Almorexant in rats exposed to novelty stress. Cardiovascular and locomotor responses were recorded by radio-telemetry. Triple immunolabelling was also conducted to establish the distribution of Ox1R and Ox2R in sympathetic preganglionic neurons and orexin neurons. Intraperitoneal injections of ACT335827 (100 mg/kg) reduced the pressor and tachycardic but not the locomotor response of novelty (by 32% and 48%, respectively). Intraperitoneal injections of EMPA (100 mg/kg) only reduced the pressor response (42%). However when given together, ACT335827 and EMPA reduced all 3 components (65%, 60% and 57% of the tachycardic, pressor and locomotor responses, respectively) as Almorexant (100 mg/kg) did (69%, 87% and 72%, respectively). Triple immunolabelling revealed that sympathetic preganglionic neurons were mainly Ox1R positive only while orexin neurons were both Ox1R and Ox2R positive. This study shows that orexin's contribution to the cardiovascular and locomotor components of the novelty stress response is not mediated by one receptor alone, but by both receptors and at different levels of the neuraxis.

Binding kinetics differentiates functional antagonism of orexin-2 receptor ligands

Orexin receptor antagonism represents a novel approach for the treatment of insomnia that directly targets sleep/wake regulation. Several such compounds have entered into clinical development, including the dual orexin receptor antagonists, suvorexant and almorexant. In this study, we have used equilibrium and kinetic binding studies with the orexin-2 (OX₂) selective antagonist radioligand, [³H]-EMPA, to profile several orexin receptor antagonists. Furthermore, selected compounds were studied in cell-based assays of inositol phosphate accumulation and ERK-1/2 phosphorylation in CHO cells stably expressing the OX2 receptor that employ different agonist incubation times (30 and 5 min, respectively). EMPA, suvorexant, almorexant and TCS-OX-29 all bind to the OX₂ receptor with moderate to high affinity (pk(I) values ≥ 7.5), whereas the primarily OX1 selective antagonists SB-334867 and SB-408124 displayed low affinity (pK(I) values ca. 6). Competition kinetic analysis showed that the compounds displayed a range of dissociation rates from very fast (TCS-OX2-29, k(off) = 0.22 min⁻¹) to very slow (almorexant, k(off) = 0.005 min⁻¹). Notably, there was a clear correlation between association rate and affinity. In the cell-based assays, fast-offset antagonists EMPA and TCS-OX2-29 displayed surmountable antagonism of orexin-A agonist activity. However, both suvorexant and particularly almorexant cause concentration-dependent depression in the maximal orexin-A response, a profile that is more evident with a shorter agonist incubation time. Analysis according to a hemi-equilibrium model suggests that antagonist dissociation is slower in a cellular system than in membrane binding; under these conditions, almorexant effectively acts as a pseudo-irreversible antagonist.

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.

Toward the Development of Bivalent Ligand Probes of Cannabinoid CB1 and Orexin OX1 Receptor Heterodimers

Cannabinoid CB1 and orexin OX1 receptors have been suggested to form heterodimers and oligomers. Aimed at studying these complexes, a series of bivalent CB1 and OX1 ligands combining SR141716 and ACT-078573 pharmacophores were designed, synthesized, and tested for activity against CB1 and OX1 individually and in cell lines that coexpress both receptors. Compound 20 showed a robust enhancement in potency at both receptors when coexpressed as compared to individually expressed, suggesting possible interaction with CB1-OX1 dimers. Bivalent ligands targeting CB1-OX1 receptor dimers could be potentially useful as a tool for further exploring the roles of such heterodimers in vitro and in vivo.

Selective orexin 2 receptor antagonism blocks cue-induced reinstatement, but not nicotine self-administration or nicotine-induced reinstatement

The orexinergic system has been implicated in a number of behaviors, including reward and incentive motivation. Orexin 1 receptor antagonism has been reported to reduce drug self-administration, conditioned place preference, and reinstatement in rodents, but the role of the orexin 2 receptor is unclear. Here we evaluated the impact of the novel and selective orexin 2 receptor antagonist, 2-SORA 18, on motivation for nicotine as measured by responding on a progressive ratio schedule, as well as cue-induced reinstatement of a response previously associated with nicotine reward, and nicotine-induced reinstatement. 2-SORA 18 demonstrated selective effects on these behaviors. Specifically, doses up to 60 mg/kg 2-SORA 18 were without significant effect on nicotine self-administration or nicotine-induced reinstatement, but doses as low as 15 mg/kg 2-SORA 18 completely blocked cue-induced reinstatement. These findings indicate that orexin 2 receptor antagonism might have utility for attenuating relapse, particularly for patients sensitive to environmental stimuli associated with drug taking.

Orexin antagonists for neuropsychiatric disease: progress and potential pitfalls

The tight regulation of sleep/wake states is critical for mental and physiological wellbeing. For example, dysregulation of sleep/wake systems predisposes individuals to metabolic disorders such as obesity and psychiatric problems, including depression. Contributing to this understanding, the last decade has seen significant advances in our appreciation of the complex interactions between brain systems that control the transition between sleep and wake states. Pivotal to our increased understanding of this pathway was the description of a group of neurons in the lateral hypothalamus (LH) that express the neuropeptides orexin A and B (hypocretin, Hcrt-1 and Hcrt-2). Orexin neurons were quickly placed at center stage with the demonstration that loss of normal orexin function is associated with the development of narcolepsy—a condition in which sufferers fail to maintain normal levels of daytime wakefulness. Since these initial seminal findings, much progress has been made in our understanding of the physiology and function of the orexin system. For example, the orexin system has been identified as a key modulator of autonomic and neuroendocrine function, arousal, reward and attention. Notably, studies in animals suggest that dysregulation of orexin function is associated with neuropsychiatric states such as addiction and mood disorders including depression and anxiety. This review discusses the progress associated with therapeutic attempts to restore orexin system function and treat neuropsychiatric conditions such as addiction, depression and anxiety. We also highlight potential pitfalls and challenges associated with targeting this system to treat these neuropsychiatric states.

The use of the reinstatement model to study relapse to palatable food seeking during dieting

Excessive consumption of unhealthy foods is a major public health problem. While many people attempt to control their food intake through dieting, many relapse to unhealthy eating habits within a few months. We have begun to study this clinical condition in rats by adapting the reinstatement model, which has been used extensively to study relapse to drug seeking. In our adaptation of the relapse model, reinstatement of palatable food seeking by exposure to food-pellet priming, food-associated cues, or stress is assessed in food-restricted (to mimic dieting) rats after operant food-pellet self-administration training and subsequent extinction of the food-reinforced responding. In this review, we first outline the clinical problem and discuss a recent study in which we assessed the predictive validity of the reinstatement model for studying relapse to food seeking during dieting by using the anorexigenic drug fenfluramine. Next, we summarize results from our initial studies on the role of several stress- and feeding-related peptides (corticotropin-releasing factor, hypocretin, melanin-concentrating hormone, peptide YY3-36) in reinstatement of palatable food seeking. We then present results from our studies on the role of dopamine and medial prefrontal cortex in stress-induced reinstatement of food seeking. We conclude by discussing potential clinical implications. We offer two main conclusions: (1) the food reinstatement model is a simple, reliable, and valid model to study mechanisms of relapse to palatable food seeking during dieting, and to identify medications to prevent this relapse; (2) mechanisms of relapse to food seeking are often dissociable from mechanisms of ongoing food intake. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Dual orexin receptor antagonists - promising agents in the treatment of sleep disorders

Insomnia is a serious medical and social problem, its prevalence in the general population ranges from 9 to 35% depending on the country and assessment method. Often, patients are subject to inappropriate and therefore dangerous pharmacotherapies that include prolonged administration of hypnotic drugs, benzodiazepines and other GABAA receptor modulators. This usually does not lead to a satisfactory improvement in patients' clinical states and may cause lifelong drug dependence. Brain state transitions require the coordinated activity of numerous neuronal pathways and brain structures. It is thought that orexin-expressing neurons play a crucial role in this process. Due to their interaction with the sleep-wake-regulating neuronal population, they can activate vigilance-promoting regions and prevent unwanted sleep intrusions. Understanding the multiple orexin modulatory effects is crucial in the context of pathogenesis of insomnia and should lead to the development of novel treatments. An important step in this process was the synthesis of dual antagonists of orexin receptors. Crucially, these drugs, as opposed to benzodiazepines, do not change the sleep architecture and have limited side-effects. This new pharmacological approach might be the most appropriate to treat insomnia.

Orexin A and B in vitro modify orexins receptors expression and gonadotropins secretion of anterior pituitary cells of proestrous rats

AIM

Orexin A and orexin B (hypocretins) are neuropeptides synthesized mainly by neurons located in the lateral hypothalamus and projections throughout the brain. They are agonists at both the orexin 1 and orexin 2G protein-coupled receptors. They have been related to arousal, sleep and feeding, autonomic and neuroendocrine functions. Their role in the brain control of gonadotropins secretion was postulated in rodents and humans. Previously, we demonstrated the participation of the orexinergic system in attaining successful reproduction in in vivo studies.

METHODS

We studied in vitro the effects of both neuropeptides, in the presence or absence of selective antagonists, on the mRNA expression of orexin 1 and orexin 2 receptors in anterior pituitary cells of proestrous rats, as well as the direct effects on FSH and LH secretion.

RESULTS

Both orexin A and orexin B increased FSH and LH secretion; these effects were suppressed by the orexin 1 receptor blocking agent SB-334867 and the orexin 2 receptor antagonists JNJ-10397049. Orexin A and orexin B decreased OX1 receptor mRNA expression and this effect was modified only when both blocking agents were present. Neither orexin A nor the blocking drugs by themselves modified OX2 receptor mRNA expression. Orexin B treatment increased the mRNA expression of OX2 receptor. The effect was abolished only by the OX2 receptor antagonist.

CONCLUSION

In an in vitro model, we demonstrated a direct effect of orexins on gonadotropins release and orexins receptors expression, underlining the hypothesis that orexins participate in the brain control of pituitary functions.

Distinct effects of IPSU and suvorexant on mouse sleep architecture

Dual orexin receptor (OXR) antagonists (DORAs) such as almorexant, SB-649868, suvorexant (MK-4305), and filorexant (MK-6096), have shown promise for the treatment of insomnias and sleep disorders. Whether antagonism of both OX₁R and OX₂R is necessary for sleep induction has been a matter of some debate. Experiments using knockout mice suggest that it may be sufficient to antagonize only OX₂R. The recent identification of an orally bioavailable, brain penetrant OX₂R preferring antagonist 2-((1H-Indol-3-yl)methyl)-9-(4-methoxypyrimidin-2-yl)-2,9-diazaspiro[5.5]undecan-1-one (IPSU) has allowed us to test whether selective antagonism of OX₂R may also be a viable strategy for induction of sleep. We previously demonstrated that IPSU and suvorexant increase sleep when dosed during the mouse active phase (lights off); IPSU inducing sleep primarily by increasing NREM sleep, suvorexant primarily by increasing REM sleep. Here, our goal was to determine whether suvorexant and IPSU affect sleep architecture independently of overall sleep induction. We therefore tested suvorexant (25 mg/kg) and IPSU (50 mg/kg) in mice during the inactive phase (lights on) when sleep is naturally more prevalent and when orexin levels are normally low. Whereas IPSU was devoid of effects on the time spent in NREM or REM, suvorexant substantially disturbed the sleep architecture by selectively increasing REM during the first 4 h after dosing. At the doses tested, suvorexant significantly decreased wake only during the first hour and IPSU did not affect wake time. These data suggest that OX₂R preferring antagonists may have a reduced tendency for perturbing NREM/REM architecture in comparison with DORAs. Whether this effect will prove to be a general feature of OX₂R antagonists vs. DORAs remains to be seen.

Kinetic properties of "dual" orexin receptor antagonists at OX1R and OX2R orexin receptors

Orexin receptor antagonists represent attractive targets for the development of drugs for the treatment of insomnia. Both efficacy and safety are crucial in clinical settings and thorough investigations of pharmacokinetics and pharmacodynamics can predict contributing factors such as duration of action and undesirable effects. To this end, we studied the interactions between various “dual” orexin receptor antagonists and the orexin receptors, OX₁R and OX₂R, over time using saturation and competition radioligand binding with [³H]-BBAC ((S)-N-([1,1′-biphenyl]-2-yl)-1-(2-((1-methyl-1H-benzo[d]imidazol-2-yl)thio)acetyl)pyrrolidine-2-carboxamide). In addition, the kinetics of these compounds were investigated in cells expressing human, mouse and rat OX₁R and OX₂R using FLIPR® assays for calcium accumulation. We demonstrate that almorexant reaches equilibrium very slowly at OX₂R, whereas SB-649868, suvorexant, and filorexant may take hours to reach steady state at both orexin receptors. By contrast, compounds such as BBAC or the selective OX₂R antagonist IPSU ((2-((1H-Indol-3-yl)methyl)-9-(4-methoxypyrimidin-2-yl)-2,9-diazaspiro[5.5]undecan-1-one) bind rapidly and reach equilibrium very quickly in binding and/or functional assays. Overall, the “dual” antagonists tested here tend to be rather unselective under non-equilibrium conditions and reach equilibrium very slowly. Once equilibrium is reached, each ligand demonstrates a selectivity profile that is however, distinct from the non-equilibrium condition. The slow kinetics of the “dual” antagonists tested suggest that in vitro receptor occupancy may be longer lasting than would be predicted. This raises questions as to whether pharmacokinetic studies measuring plasma or brain levels of these antagonists are accurate reflections of receptor occupancy in vivo.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Orexin in sleep, addiction and more: is the perfect insomnia drug at hand?

Orexins A and B (hypocretins 1 and 2) and their two receptors (OX1R and OX2R) were discovered in 1998 by two different groups. Orexin A and B are derived from the differential processing of a common precursor, the prepro-orexin peptide. The neuropeptides are expressed in a few thousand cells located in the lateral hypothalamus (LH), but their projections and receptor distribution are widespread throughout the brain. Remarkably, prepro peptide and double (OX1R/OX2R) receptor knock out (KO) mice reproduce a sleep phenotype known in humans and dogs as narcolepsy/cataplexy. In humans, this disease is characterized by the absence of orexin producing cells in the LH, and severely depleted levels of orexin the cerebrospinal fluid. Null mutation of the individual OX1R or OX2R in mice substantially ameliorates the narcolepsy/cataplexy phenotype compared to the OX1R/OX2R KO, and highlights specific roles of the individual receptors in sleep architecture, the OX1R KO demonstrating an a attenuated sleep phenotype relative to the OX2R KO. It has therefore been suggested that orexin is a master regulator of the sleep-wake cycle, with high activity of the LH orexin cells during wake and almost none during sleep. Less than 10years later, the first orexin antagonist, almorexant, a dual orexin receptor antagonist (DORA), was reported to be effective in inducing sleep in volunteers and insomnia patients. Although development was stopped for almorexant and for Glaxo's DORA SB-649868, no less than 4 orexin receptor antagonists have reached phase II for insomnia, including Filorexant (MK-6096) and Suvorexant (MK-4305) from Merck. Suvorexant has since progressed to Phase III and dossier submission to the FDA. These four compounds are reported as DORAs, however, they equilibrate very slowly at one and/or the other orexin receptor, and thus at equilibrium may show more or less selectivity for OX1R or OX2R. The appropriate balance of antagonism of the two receptors for sleep is a point of debate, although in rodent models OX2R antagonism alone appears sufficient to induce sleep, whereas OX1R antagonism is largely devoid of this effect. Orexin is involved in a number of other functions including reward and feeding, where OX1R (possibly OX2R) antagonists display anti-addictive properties in rodent models of alcohol, smoking, and drug self-administration. However, despite early findings in feeding and appetite control, orexin receptor antagonists have not produced the anticipated effects in models of increased food intake or obesity in rodents, nor have they shown marked effects on weight in the existing clinical trials. The role of orexin in a number of other domains such as pain, mood, anxiety, migraine and neurodegenerative diseases is an active area of research. The progress of the orexin field is thus extraordinary, and the community awaits the clinical testing of more receptor selective antagonists in sleep and other disorders, as well as that of orexin agonists, with the latter expected to produce positive outcomes in narcolepsy/cataplexy and other conditions.

Identification of a novel series of orexin receptor antagonists with a distinct effect on sleep architecture for the treatment of insomnia

Dual orexin receptor (OXR) antagonists (DORAs) such as almorexant, 1 (SB-649868), or suvorexant have shown promise for the treatment of insomnias and sleep disorders in several recent clinical trials in volunteers and primary insomnia patients. The relative contribution of antagonism of OX1R and OX2R for sleep induction is still a matter of debate. We therefore initiated a drug discovery project with the aim of creating both OX2R selective antagonists and DORAs. Here we report that the OX2R selective antagonist 26 induced sleep in mice primarily by increasing NREM sleep, whereas the DORA suvorexant induced sleep largely by increasing REM sleep. Thus, OX2R selective antagonists may also be beneficial for the treatment of insomnia.

The duration of sleep promoting efficacy by dual orexin receptor antagonists is dependent upon receptor occupancy threshold

Background

Drugs targeting insomnia ideally promote sleep throughout the night, maintain normal sleep architecture, and are devoid of residual effects associated with morning sedation. These features of an ideal compound are not only dependent upon pharmacokinetics, receptor binding kinetics, potency and pharmacodynamic activity, but also upon a compound’s mechanism of action.

Results

Dual orexin receptor antagonists (DORAs) block the arousal-promoting activity of orexin peptides and, as demonstrated in the current work, exhibit an efficacy signal window dependent upon oscillating levels of endogenous orexin neuropeptide. Sleep efficacy of structurally diverse DORAs in rat and dog was achieved at plasma exposures corresponding to orexin 2 receptor (OX₂R) occupancies in the range of 65 to 80%. In rats, the time course of OX₂R occupancy was dependent upon receptor binding kinetics and was tightly correlated with the timing of active wake reduction. In rhesus monkeys, direct comparison of DORA-22 with GABA-A modulators at similar sleep-inducing doses revealed that diazepam produced next-day residual sleep and both diazepam and eszopiclone induced next-day cognitive deficits. In stark contrast, DORA-22 did not produce residual effects. Furthermore, DORA-22 evoked only minimal changes in quantitative electroencephalogram (qEEG) activity during the normal resting phase in contrast to GABA-A modulators which induced substantial qEEG changes.

Conclusion

The higher levels of receptor occupancy necessary for DORA efficacy require a plasma concentration profile sufficient to maintain sleep for the duration of the resting period. DORAs, with a half-life exceeding 8 h in humans, are expected to fulfill this requirement as exposures drop to sub-threshold receptor occupancy levels prior to the wake period, potentially avoiding next-day residual effects at therapeutic doses.

Antagonism of orexin receptors significantly lowers blood pressure in spontaneously hypertensive rats

In normal rats, central administration of orexin or exposure to certain forms of stress can induce significant increases in blood pressure and sympathetic nerve activity, which can be blocked by orexin receptor antagonists. The resting blood pressure is, however, unaffected by such antagonists, but is significantly lower in rodents with total loss of orexin, such as prepro-orexin knockout mice and orexin neuron-ablated orexin/ataxin-3 transgenic rats. We hypothesize that orexin is involved in the pathophysiology and maintenance of high blood pressure in the spontaneously hypertensive rat (SHR), a model of primary hypertension. To test this hypothesis, we measured orexin-A mRNA expression in the rostral ventrolateral medulla and antagonized both orexin receptors using an orally administered potent dual orexin receptor antagonist, almorexant, in SHRs and normotensive Wistar-Kyoto rats. In SHRs, there was a strong trend towards an increased orexin-A mRNA expression in the rostral ventrolateral medulla, and blocking orexin receptors markedly lowered blood pressure (from 182/152 ± 5/6 to 149/119 ± 9/8 mmHg; P < 0.001), heart rate (P < 0.001), sympathetic vasomotor tone (P < 0.001) and the noradrenaline levels in cerebrospinal fluid and plasma (P < 0.002). The significant antihypertensive effects of almorexant were observed in wakefulness and non-rapid eye movement sleep during both dark and light phases of the diurnal cycle only in SHRs. Blocking orexin receptors had no effect on blood pressure and sympathetic tone in normotensive Wistar-Kyoto rats. Our study links the orexin system to the pathogenesis of high blood pressure in SHRs and suggests that modulation of the orexin system could be a potential target in treating some forms of hypertension.

Radiosynthesis and evaluation of [11C]EMPA as a potential PET tracer for orexin 2 receptors

EMPA is a selective antagonist of orexin 2 (OX2) receptors. Previous literature with [(3)H]-EMPA suggest that it may be used as an imaging agent for OX2 receptors; however, brain penetration is known to be modest. To evaluate the potential of EMPA as a PET radiotracer in non-human primate (as a step to imaging in man), we radiolabeled EMPA with carbon-11. Radiosynthesis of [(11)C]N-ethyl-2-(N-(6-methoxypyridin-3-yl)-2-methylphenylsulfonamido)-N-(pyridin-3-ylmethyl)acetamide ([(11)C]EMPA), and evaluation as a potential PET tracer for OX2 receptors is described. Synthesis of an appropriate non-radioactive O-desmethyl precursor was achieved from EMPA with sodium iodide and chlorotrimethylsilane. Selective O-methylation using [(11)C]CH3I in the presence of cesium carbonate in DMSO at room temp afforded [(11)C]EMPA in 1.5-2.5% yield (non-decay corrected relative to trapped [(11)C]CH3I at EOS) with ≥95% chemical and radiochemical purities. The total synthesis time was 34-36min from EOB. Studies in rodent suggested that uptake in tissue was dominated by nonspecific binding. However, [(11)C]EMPA also showed poor uptake in both rats and baboon as measured with PET imaging.

Almorexant promotes sleep and exacerbates cataplexy in a murine model of narcolepsy

STUDY OBJECTIVES

Humans with narcolepsy and orexin/ataxin-3 transgenic (TG) mice exhibit extensive, but incomplete, degeneration of hypo-cretin (Hcrt) neurons. Partial Hcrt cell loss also occurs in Parkinson disease and other neurologic conditions. Whether Hcrt antagonists such as almorexant (ALM) can exert an effect on the Hcrt that remains after Hcrt neurodegeneration has not yet been determined. The current study was designed to evaluate the hypnotic and cataplexy-inducing efficacy of a Hcrt antagonist in an animal model with low Hcrt tone and compare the ALM efficacy profile in the disease model to that produced in wild-type (WT) control animals.

DESIGN

Counterbalanced crossover study.

SETTING

Home cage.

PATIENTS OR PARTICIPANTS

Nine TG mice and 10 WT mice.

INTERVENTIONS

ALM (30, 100, 300 mg/kg), vehicle and positive control injections, dark/active phase onset.

MEASUREMENTS AND RESULTS

During the 12-h dark period after dosing, ALM exacerbated cataplexy in TG mice and increased nonrapid eye movement sleep with heightened sleep/wake fragmentation in both genotypes. ALM showed greater hypnotic potency in WT mice than in TG mice. The 100 mg/kg dose conferred maximal promotion of cataplexy in TG mice and maximal promotion of REM sleep in WT mice. In TG mice, ALM (30 mg/ kg) paradoxically induced a transient increase in active wakefulness. Core body temperature (Tb) decreased after acute Hcrt receptor blockade, but the reduction in Tb that normally accompanies the wake-to-sleep transition was blunted in TG mice.

CONCLUSIONS

These complex dose- and genotype-dependent interactions underscore the importance of effector mechanisms downstream from Hcrt receptors that regulate arousal state. Cataplexy promotion by ALM warrants cautious use of Hcrt antagonists in patient populations with Hcrt neurodegeneration, but may also facilitate the discovery of anticataplectic medications.

CITATION

Black SW; Morairty SR; Fisher SP; Chen TM; Warrier DR; Kilduff TS. Almorexant promotes sleep and exacerbates cataplexy in a murine model of narcolepsy. SLEEP 2013;36(3):325-336.

Almorexant promotes sleep and exacerbates cataplexy in a murine model of narcolepsy

STUDY OBJECTIVES

Humans with narcolepsy and orexin/ataxin-3 transgenic (TG) mice exhibit extensive, but incomplete, degeneration of hypo-cretin (Hcrt) neurons. Partial Hcrt cell loss also occurs in Parkinson disease and other neurologic conditions. Whether Hcrt antagonists such as almorexant (ALM) can exert an effect on the Hcrt that remains after Hcrt neurodegeneration has not yet been determined. The current study was designed to evaluate the hypnotic and cataplexy-inducing efficacy of a Hcrt antagonist in an animal model with low Hcrt tone and compare the ALM efficacy profile in the disease model to that produced in wild-type (WT) control animals.

DESIGN

Counterbalanced crossover study.

SETTING

Home cage.

PATIENTS OR PARTICIPANTS

Nine TG mice and 10 WT mice.

INTERVENTIONS

ALM (30, 100, 300 mg/kg), vehicle and positive control injections, dark/active phase onset.

MEASUREMENTS AND RESULTS

During the 12-h dark period after dosing, ALM exacerbated cataplexy in TG mice and increased nonrapid eye movement sleep with heightened sleep/wake fragmentation in both genotypes. ALM showed greater hypnotic potency in WT mice than in TG mice. The 100 mg/kg dose conferred maximal promotion of cataplexy in TG mice and maximal promotion of REM sleep in WT mice. In TG mice, ALM (30 mg/ kg) paradoxically induced a transient increase in active wakefulness. Core body temperature (Tb) decreased after acute Hcrt receptor blockade, but the reduction in Tb that normally accompanies the wake-to-sleep transition was blunted in TG mice.

CONCLUSIONS

These complex dose- and genotype-dependent interactions underscore the importance of effector mechanisms downstream from Hcrt receptors that regulate arousal state. Cataplexy promotion by ALM warrants cautious use of Hcrt antagonists in patient populations with Hcrt neurodegeneration, but may also facilitate the discovery of anticataplectic medications.

CITATION

Black SW; Morairty SR; Fisher SP; Chen TM; Warrier DR; Kilduff TS. Almorexant promotes sleep and exacerbates cataplexy in a murine model of narcolepsy. SLEEP 2013;36(3):325-336.

The dual orexin receptor antagonist almorexant induces sleep and decreases orexin-induced locomotion by blocking orexin 2 receptors

STUDY OBJECTIVES

Orexin peptides activate orexin 1 and orexin 2 receptors (OX(1)R and OX(2)R), regulate locomotion and sleep-wake. The dual OX(1)R/OX(2)R antagonist almorexant reduces activity and promotes sleep in multiple species, including man. The relative contributions of the two receptors in locomotion and sleep/wake regulation were investigated in mice.

DESIGN

Mice lacking orexin receptors were used to determine the contribution of OX(1)R and OX(2)R to orexin A-induced locomotion and to almorexant-induced sleep.

SETTING

N/A.

PATIENTS OR PARTICIPANTS

C57BL/6J mice and OX(1)R(+/+), OX(1)R(-/-), OX(2)R(+/+), OX(2)R(-/-) and OX(1)R(-/-)/OX(2)R(-/-) mice.

INTERVENTIONS

Intracerebroventricular orexin A; oral dosing of almorexant.

MEASUREMENTS AND RESULTS

Almorexant attenuated orexin A-induced locomotion. As in other species, almorexant dose-dependently increased rapid eye movement sleep (REM) and nonREM sleep in mice. Almorexant and orexin A were ineffective in OX(1)R(-/-)/OX(2)R(-/-) mice. Both orexin A-induced locomotion and sleep induction by almorexant were absent in OX(2)R(-/-) mice. Interestingly, almorexant did not induce cataplexy in wild-type mice under conditions where cataplexy was seen in mice lacking orexins and in OX(1)R(-/-)/OX(2)R(-/-) mice. Almorexant dissociates very slowly from OX(2)R as measured functionally and in radioligand binding. Under non equilibrium conditions in vitro, almorexant was a dual antagonist whereas at equilibrium, almorexant became OX(2)R selective.

CONCLUSIONS

In vivo, almorexant specifically inhibits the actions of orexin A. The two known orexin receptors mediate sleep induction by almorexant and orexin A-induced locomotion. However, OX(2)R activation mediates locomotion induction by orexin A and antagonism of OX(2)R is sufficient to promote sleep in mice.

The role of orexins/hypocretins in alcohol use and abuse: an appetitive-reward relationship

Orexins (hypocretins) are neuropeptides synthesized in neurons located in the lateral (LH), perifornical, and dorsomedial (DMH) hypothalamus. These neurons innervate many regions in the brain and modulate multiple other neurotransmitter systems. As a result of these extensive projections and interactions orexins are involved in numerous functions, such as feeding behavior, neuroendocrine regulation, the sleep-wake cycle, and reward-seeking. This review will summarize the literature to date which has evaluated a role of orexins in the behavioral effects of alcohol, with a focus on understanding the importance of this peptide and its potential as a clinical therapeutic target for alcohol use disorders.

Physiology of the orexinergic/hypocretinergic system: a revisit in 2012

The neuropeptides orexins and their G protein-coupled receptors, OX(1) and OX(2), were discovered in 1998, and since then, their role has been investigated in many functions mediated by the central nervous system, including sleep and wakefulness, appetite/metabolism, stress response, reward/addiction, and analgesia. Orexins also have peripheral actions of less clear physiological significance still. Cellular responses to the orexin receptor activity are highly diverse. The receptors couple to at least three families of heterotrimeric G proteins and other proteins that ultimately regulate entities such as phospholipases and kinases, which impact on neuronal excitation, synaptic plasticity, and cell death. This article is a 10-year update of my previous review on the physiology of the orexinergic/hypocretinergic system. I seek to provide a comprehensive update of orexin physiology that spans from the molecular players in orexin receptor signaling to the systemic responses yet emphasizing the cellular physiological aspects of this system.