Targeting orexin receptors: Recent advances in the development of subtype selective or dual ligands for the treatment of neuropsychiatric disorders

Mirbolouk B., Rohampour K., Rostampour M., Jafari A., Khakpour-Taleghani B.

Depression is a devastating mood disorder affecting more than 300 million people worldwide. Almost 30 % of patients still suffer from treatment resistant depression. Although many reports support the involvement of orexin in the pathophysiology of depression, the precise role of orexin is still unclear. In this study, we evaluated the role of the orexin 1 receptor (Orx1R) on depressive behaviors and the alterations in postsynaptic density-95 (PSD-95) protein in the chronic mild stress (CMS) model of depression. Fifty-four male Wistar rats were randomly allocated to 6 groups; Control, CMS, acute SB-334867 (SB), CMS+SB, chronic SB (CSB) and CMS+CSB. Rats were exposed to one or two unpredictable stressors each day for three weeks for the induction of CMS. Intracerebroventricular (icv) injection of SB-334867, a selective Orx1R antagonist, was performed either 30 min before behavioral tests (acute) or once daily for 14 days (chronic). Behavioral despair was assessed by immobility time in the forced swim test (FST), sucrose consumption in sucrose preference test (SPT), and the number of crosses in the open field test (OFT) on days 1, 11, and 22 of the experiment. Finally, rats were decapitated, and brain tissue of the hippocampus (HPC) and prefrontal cortex (PFC) were collected, and the relative expression of PSD-95 was evaluated by western blotting. The CMS model rats showed a significant increase in FST immobility time (P = 0.001) and a decrease in locomotion (P = 0.04) and sucrose preference (P = 0.039). Chronic application of SB decreased immobility time to the control values (P = 0.001) and diminished locomotion (P = 0.047) and sucrose preference (P = 0.042) in comparison to the CMS group. Acute SB reversed just the immobility time (P ≤ 0.006). Chronic SB treatment increased the relative PSD-95 expression in PFC (P = 0.001). Hence, chronic antagonism of Orx1R alleviates depressive behaviors induced by CMS and improves PSD-95 expression in PFC.

Schuller J., Koch M.

Binge eating disorder (BED) is the most prevalent eating disorder in the adult population. It is characterized by recurrent episodes of uncontrollable overconsumption of palatable food (PF). BED is connected to several comorbidities such as obesity, major depression, and substance use disorder, and was linked to heightened levels of impulsivity. The neurobiological basis of BED is however still vaguely known. Binge eating (BE) occurs without homeostatic needs, and therefore, relates to hedonic consumption of PF. A major brain structure in the control of hedonic feeding, and part of the network regulating impulsive action, is the nucleus accumbens shell (NAcSh). The present study in rats assessed the influence of trait impulsivity on the development of BE-like feeding and the role of the orexigenic neuropeptide orexin (OX) and the anorexigenic neuropeptide cocaine- and amphetamine-regulated transcript (CART) within the NAcSh in a BE-model. The rats were initially ranked for their trait impulsivity based on a screening in the 5-choice serial reaction time task. They were subsequently introduced into a limited access-model to establish BE-like feeding with pure vegetable fat to examine a correlation between trait impulsivity and the development of BE-like feeding. The effect of bilateral infusions of the OX 1-receptor (OX1R) antagonist SB-334867 (SB) and CART-antibodies (CART-ABs) into the NAcSh was examined in relation to trait impulsivity. Further, accumbal OX1R density was evaluated by immunohistochemical staining in rats with normal and BE-like feeding behavior. We found that all animals developed stable BE-like PF intake, independent of trait impulsivity and without differences in the dynamics. The blockade of accumbal OX1Rs effectively reduced PF intake only in the control group that had daily access to PF, with impulsivity trait as a decisive factor, pointing towards alterations in orexinergic transmission in the NAcSh of rats bingeing on pure fat. This was corroborated by a lower density of OX1Rs in the NAcSh of rats with BE-like feeding behavior, precisely, in low-impulsive bingeing rats. Regardless of impulsivity trait, antagonizing CART in the NAcSh did not affect PF intake of control or bingeing animals. This suggests that endogenous accumbal CART does not influence consummatory behavior in ad libitum-fed rats with access to fat, under both normal and BE-like feeding patterns.

Moreno E., Casajuana-Martin N., Coyle M., Campos B.C., Galaj E., del Torrent C.L., Seyedian A., Rea W., Cai N., Bonifazi A., Florán B., Xi Z., Guitart X., Casadó V., Newman A.H., et. al.

A main rationale for the role of G protein-coupled receptor (GPCR) heteromers as targets for drug development is the putative ability of selective ligands for specific GPCRs to change their pharmacological properties upon GPCR heteromerization. The present study provides a proof of concept for this rationale by demonstrating that heteromerization of dopamine D1 and D3 receptors (D1R and D3R) influences the pharmacological properties of three structurally similar selective dopamine D3R ligands, the phenylpiperazine derivatives PG01042, PG01037 and VK4-116. By using D1R-D3R heteromer-disrupting peptides, it could be demonstrated that the three D3R ligands display different D1R-D3R heteromer-dependent pharmacological properties: PG01042, acting as G protein-biased agonist, counteracted D1R-mediated signaling in the D1R-D3R heteromer; PG01037, acting as a D3R antagonist cross-antagonized D1R-mediated signaling in the D1R-D3R heteromer; and VK4-116 specifically acted as a ß-arrestin-biased agonist in the D1R-D3R heteromer. Molecular dynamics simulations predicted potential molecular mechanisms mediating these qualitatively different pharmacological properties of the selective D3R ligands that are dependent on D1R-D3R heteromerization. The results of in vitro experiments were paralleled by qualitatively different pharmacological properties of the D3R ligands in vivo. The results supported the involvement of D1R-D3R heteromers in the locomotor activation by D1R agonists in reserpinized mice and L-DOPA-induced dyskinesia in rats, highlighting the D1R-D3R heteromer as a main pharmacological target for L-DOPA-induced dyskinesia in Parkinson's disease. More generally, the present study implies that when suspecting its pathogenetic role, a GPCR heteromer, and not its individual GPCR units, should be considered as main target for drug development.

Saitoh T., Amezawa M., Horiuchi J., Nagumo Y., Yamamoto N., Kutsumura N., Ohshita R., Tokuda A., Irukayama-Tomobe Y., Ogawa Y., Ishikawa Y., Hasegawa E., Sakurai T., Uchida Y., Sato T., et. al.

Structurally diverse small compounds are utilized to obtain hit compounds that have suitable pharmacophores in appropriate three-dimensional conformations for the target drug receptors. We have focused on the 1,3,5-trioxazatriquinane skeleton, which has a rigid bowl-like structure enabling the diverse orientation of side chain units, leading to a novel small-scale focused library based on the skeleton. In the library screening for the orexin receptor, some of the compounds showed orexin receptor antagonistic activity with a high hit rate of 7%. By optimizing the hit compounds, we discovered a potent dual orexin receptor antagonist, 38b, and a selective orexin 1 receptor antagonist, 41b carrying the same plane structure. Both compounds showed reasonable brain permeability and beneficial effects when administered intraperitoneally to wild-type mice. Docking simulations of their eutomers, (-)-38b and (+)-41b, with orexin receptors suggested that the interaction between the 1,3,5-trioxazatriquinane core structure and the hydrophobic subpocket in orexin receptors enables a U-shape structure, which causes tight van der Waals interactions with the receptors similar to SB-334867, a selective orexin 1 receptor antagonist. These results indicate that the library approach utilizing the 1,3,5-trioxazatriquinanes bearing multiple effective residues (TriMERs) might be useful for the hit discovery process targeting not only opioid and orexin receptors but other G-protein coupled receptors.

dos Santos J.B., da Silva M.R.

The involvement of the orexin system in the physiopathology of insomnia has been rapidly increasing in understanding. In this sense, daridorexant was the third orexin receptor antagonist approved by the FDA in January 2022. This review aims to summarize the chemistry, pharmacodynamics, pharmacokinetics, efficacy, safety, and tolerability profile of daridorexant for the treatment of insomnia disorder. We performed a review of daridorexant for the treatment of insomnia disorder. The search was carried out in Medline via PubMed, Embase, and clinical trials, up to March 2022. Daridorexant 25 and 50 mg had more significant improvement for the wake after sleep onset (WASO), latency to persistent sleep (LPS), and subjective total sleep time (sTST) than placebo. In addition, daridorexant 50 mg was better for Insomnia Daytime Symptoms and Impacts Questionnaire (IDSIQ) than placebo. The most common adverse events were nasopharyngitis and headache. Daridorexant was efficacious and safe. Studies that evaluate the long-term safety and compare daridorexant with benzodiazepines, benzodiazepine receptor agonists, sedative antidepressants, and other orexin receptor antagonists are required.

Meanwell N.A., Loiseleur O.

Applications of piperazine and homopiperazine in drug design are well-established, and these heterocycles have found use as both scaffolding and terminal elements and also as a means of introducing a water-solubilizing element into a molecule. In the accompanying review (10.1021/acs.jafc.2c00726), we summarized applications of piperazine and homopiperazine and their fused ring homologues in bioactive compound design along with illustrations of the use of 4-substituted piperidines and a sulfoximine-based mimetic. In this review, we discuss applications of pyrrolidine- and fused-pyrrolidine-based mimetics of piperazine and homopiperazine and illustrate derivatives of azetidine that include stretched and spirocyclic motifs, along with applications of a series of diaminocycloalkanes.

Meanwell N.A., Loiseleur O.

Piperazine and homopiperazine are well-studied heterocycles in drug design that have found gainful application as scaffolds and terminal elements and for enhancing the aqueous solubility of a molecule. The optimization of drug candidates that incorporate these heterocycles in an effort to refine potency, selectivity, and developability properties has stimulated the design and evaluation of a wide range of bioisosteres that can offer advantage. In this review, we summarize the design and application of bioisosteres of piperazine and homopiperazine that have almost exclusively been in the drug design arena. While there are ∼100 approved drugs that incorporate a piperazine ring, only a single marketed agricultural product is built on this heterocycle. As part of the review, we discuss some of the potential reasons underlying the relatively low level of importance of this heterocycle to the design of agrochemicals and highlight the potential opportunities for their use in contemporary research programs.

Bai P., Liu Y., Xu Y., Striar R., Yuan G., Afshar S., Langan A.G., Rattray A.K., Wang C.

The orexin receptors (OXRs) have been involved in multiple physiological and neuropsychiatric functions. Identification of PET imaging probes specifically targeting OXRs enables us to better understand the OX system. Seltorexant (JNJ-42847922) is a potent OX2R antagonist with the potential to be an OX2R PET imaging probe. Here, we describe the synthesis and characterization of [18F]Seltorexant as an OX2R PET probe. The ex vivo autoradiography studies indicated the good binding specificity of [18F]Seltorexant. In vivo PET imaging of [18F]Seltorexant in rodents showed suitable BBB penetration with the highest brain uptake of %ID/cc = 3.4 at 2 min post-injection in mice. The regional brain biodistribution analysis and blocking studies showed that [18F]Seltorexant had good binding selectivity and specificity. However, pretreatment with unlabelled Seltorexant and P-gp competitor CsA observed significantly increased brain uptake of [18F]Seltorexant, indicating [18F]Seltorexant could interact P-gp at the blood-brain barrier. Our findings demonstrated that [18F]Seltorexant is a potential brain OX2R PET imaging probe, which paves the way for new OX2R PET probes development and OX system investigation.

Yin J., Kang Y., McGrath A.P., Chapman K., Sjodt M., Kimura E., Okabe A., Koike T., Miyanohana Y., Shimizu Y., Rallabandi R., Lian P., Bai X., Flinspach M., De Brabander J.K., et. al.

The OX2 orexin receptor (OX2R) is a highly expressed G protein-coupled receptor (GPCR) in the brain that regulates wakefulness and circadian rhythms in humans. Antagonism of OX2R is a proven therapeutic strategy for insomnia drugs, and agonism of OX2R is a potentially powerful approach for narcolepsy type 1, which is characterized by the death of orexinergic neurons. Until recently, agonism of OX2R had been considered ‘undruggable.’ We harness cryo-electron microscopy of OX2R-G protein complexes to determine how the first clinically tested OX2R agonist TAK-925 can activate OX2R in a highly selective manner. Two structures of TAK-925-bound OX2R with either a Gq mimetic or Gi reveal that TAK-925 binds at the same site occupied by antagonists, yet interacts with the transmembrane helices to trigger activating microswitches. Our structural and mutagenesis data show that TAK-925’s selectivity is mediated by subtle differences between OX1 and OX2 receptor subtypes at the orthosteric pocket. Finally, differences in the polarity of interactions at the G protein binding interfaces help to rationalize OX2R’s coupling selectivity for Gq signaling. The mechanisms of TAK-925’s binding, activation, and selectivity presented herein will aid in understanding the efficacy of small molecule OX2R agonists for narcolepsy and other circadian disorders. The OX2 orexin receptor (OX2R) is a brain GPCR that regulates wakefulness and circadian rhythms in humans, and a potential drug target in insomnia and narcolepsy. Here, the authors use cryo-EM to determine how the first clinically tested OX2R agonist TAK-925 can activate OX2R in a selective manner.

Dale N.C., Hoyer D., Jacobson L.H., Pfleger K.D., Johnstone E.K.

The orexin system comprises two G protein-coupled receptors, OX1 and OX2 receptors (OX1R and OX2R, respectively), along with two endogenous agonists cleaved from a common precursor (prepro-orexin), orexin-A (OX-A) and orexin-B (OX-B). For the receptors, a complex array of signaling behaviors has been reported. In particular, it becomes obvious that orexin receptor coupling is very diverse and can be tissue-, cell- and context-dependent. Here, the early signal transduction interactions of the orexin receptors will be discussed in depth, with particular emphasis on the direct G protein interactions of each receptor. In doing so, it is evident that ligands, additional receptor-protein interactions and cellular environment all play important roles in the G protein coupling profiles of the orexin receptors. This has potential implications for our understanding of the orexin system’s function in vivo in both central and peripheral environments, as well as the development of novel agonists, antagonists and possibly allosteric modulators targeting the orexin system.

Zhang R., Li D., Mao H., Wei X., Xu M., Zhang S., Jiang Y., Wang C., Xin Q., Chen X., Li G., Ji B., Yan M., Cai X., Dong B., et. al.

G protein-coupled receptor (GPCR) heterodimers are new targets for the treatment of depression. Increasing evidence supports the importance of serotonergic and orexin-producing neurons in numerous physiological processes, possibly via a crucial interaction between 5-hydroxytryptamine 1A receptor (5-HT1AR) and orexin receptor 1 (OX1R). However, little is known about the function of 5-HT1AR/OX1R heterodimers. It is unclear how the transmembrane domains (TMs) of the dimer affect its function and whether its modulation mediates antidepressant-like effects. Here, we examined the mechanism of 5-HT1AR/OX1R dimerization and downstream G protein-dependent signaling. We found that 5-HT1AR and OX1R form constitutive heterodimers that induce novel G protein-dependent signaling, and that this heterodimerization does not affect recruitment of β-arrestins to the complex. In addition, we found that the structural interface of the active 5-HT1AR/OX1R dimer transforms from TM4/TM5 in the basal state to TM6 in the active conformation. We also used mutation analyses to identify key residues at the interface (5-HT1AR R1514.40, 5-HT1AR Y1985.41, and OX1R L2305.54). Injection of chronic unpredictable mild stress (CUMS) rats with TM4/TM5 peptides improved their depression-like emotional status and decreased the number of endogenous 5-HT1AR/OX1R heterodimers in the rat brain. These antidepressant effects may be mediated by upregulation of BDNF levels and enhanced phosphorylation and activation of CREB in the hippocampus and medial prefrontal cortex. This study provides evidence that 5-HT1AR/OX1R heterodimers are involved in the pathological process of depression. Peptides including TMs of the 5-HT1AR/OX1R heterodimer interface are candidates for the development of compounds with fast-acting antidepressant-like effects.

Katoh K., Yamamoto N., Ishikawa Y., Irukayama-Tomobe Y., Tanimura R., Saitoh T., Nagumo Y., Kutsumura N., Yanagisawa M., Nagase H.

To investigate the contribution of hydrogen bonding between the 14-hydroxy group and the 6-amide chain on the binding affinity of nalfurafine toward KOR and OX1R, we prepared the 14-H and 14-dehydrated nalfurafine and their five-membered D-ring nalfurafine (D-nor-nalfurafine) derivatives. The 14-H and 14-dehydrated nalfurafine derivatives showed almost the same affinity for KOR as nalfurafine and more potent affinity for OX1R. On the other hand, 14-H and 14-dehydrated D-nor-nalfurafine derivatives showed weak affinity for KOR and almost no affinity for OX1R. The conformational analyses suggested that the 6-amide chains of the nalfurafine derivatives are mainly oriented just at or downward from the C-ring, while those of the D-nor-nalfurafine derivatives were mainly oriented toward the upper side of the C-ring even in the absence of the 14-hydroxy group. We postulated that the ion-dipole interaction between the 6-amide and the 16-nitrogen might stabilize the upwardly oriented 6-amide group. These results suggested that the 14-hydroxy group and the ion-dipole interaction would play important roles in the orientation of the 6-amide group, which might control the affinity between KOR and OX1R.

Hino T., Saitoh T., Nagumo Y., Yamamoto N., Kutsumura N., Irukayama-Tomobe Y., Ishikawa Y., Tanimura R., Yanagisawa M., Nagase H.

A novel series of naphthalene derivatives were designed and synthesized based on the strategy focusing on the restriction of the flexible bond rotation of OX2R selective agonist YNT-185 (1) and their agonist activities against orexin receptors were evaluated. The 1,7-naphthalene derivatives showed superior agonist activity than 2,7-naphthalene derivatives, suggesting that the bent form of 1 would be favorable for the agonist activity. The conformational analysis of 1,7-naphthalene derivatives indicated that the twisting of the amide unit out from the naphthalene plane is important for the enhancement of activity. The introduction of a methyl group on the 2-position of 1,7-naphthalene ring effectively increased the activity, which led to the discovery of the potent OX2R agonist 28c (EC50 = 9.21 nM for OX2R, 148 nM for OX1R). The structure-activity relationship results were well supported by a comparison of the docking simulation results of the most potent derivative 28c with an active state of agonist-bound OX2R cryo-EM SPA structure. These results suggested important information for understanding the active conformation and orientation of pharmacophores in the orexin receptor agonists, which is expected as a chemotherapeutic agent for the treatment of narcolepsy.

Iio K., Saitoh T., Ohshita R., Hino T., Amezawa M., Takayama Y., Nagumo Y., Yamamoto N., Kutsumura N., Irukayama-Tomobe Y., Ishikawa Y., Tanimura R., Yanagisawa M., Nagase H.

A novel series of 1-amino-tetralin derivatives were designed and synthesized based on the putative binding mode of the naphthalene-type orexin receptor agonist 5 and their agonist activities against orexin receptors were evaluated. The introduction of N-methyl-(3-methoxyphenyl)acetamide unit onto the 1-amino-tetralin skeleton remarkably enhanced the potency of the agonist. The asymmetric synthesis of 6 revealed that (-)-6 having a (S)-1-amino-tetralin skeleton showed a OX2R selective agonist activity (EC50 = 2.69 nM for OX2R, OX1R/OX2R = 461) yet its enantiomer (R)-(+)-6 showed a potent OX1/2R dual agonist activity (EC50 = 13.5 nM for OX1R, 0.579 nM for OX2R, OX1R/OX2R = 23.3). These results suggested that upward orientation of the amide side chain against the tetralin scaffold (S-configuration) would be selective for OX2R activation, and the downward orientation (R-configuration) would be significant for dual agonist activity. To our best knowledge, there have been no reports thus far that the stereochemistry of one carbon center on the agonist structure regulates the orexin receptor selectivity. Our results would provide important information for the development of OX1R selective agonists.

Katoh K., Kutsumura N., Yamamoto N., Nagumo Y., Saitoh T., Ishikawa Y., Irukayama-Tomobe Y., Tanimura R., Yanagisawa M., Nagase H.

The five-membered D-ring nalfurafine (d-nor-nalfurafine) derivatives with a 16-sulfonamide group were synthesized. Conversion of the 16-cyclopropylmethyl group to the 16-benzenesulfonamide group in the d-nor-nalfurafine derivatives drastically improved the orexin 1 receptor (OX1R) antagonist activities. The intramolecular hydrogen bond between the 14-hydroxy and the 16-sulfonamide groups may play an important role in increasing the probability that the 6-amide group would be located at the lower side of the C-ring, leading to an active conformation for OX1R. The assay results and the conformational analyses of the 14-OH, 14-H, and 14-dehydrated d-nor-nalfurafine derivatives suggested that the 14- and 16-substituents of the d-nor-nalfurafine derivatives had a greater effect on the affinities for the OX1R than did the 14- and 17-substituents of nalfurafine derivatives.

Valentino K., Teopiz K.M., Wong S., Zhang M.C., Le G.H., Choi H., Ballum H., Dri C., Cheung W., McIntyre R.S.

Al-Qahtani S.D., Al-Senani G.M.

Oliveira G.V., Hernandes P.M., Santos F.H., Soares V.P., Falconi‑Sobrinho L.L., Coimbra N.C., Wotjak C.T., Almada R.C.

Despite the existing anatomical and physiological evidence pointing to the involvement of orexinergic projections from the lateral hypothalamus (LH) in regulating fear-related responses, little is known regarding the contribution of the orexin system in the prelimbic cortex (PL) on contextual fear. We investigated the role of orexin-A (OrxA) and orexin type 1 receptors (Orx1R) in the PL during the expression of contextual conditioned fear in mice. Neural tract tracing of the LH-PL pathway and Orx1R immunoreactivity in the PL of C57BL/6 male mice were performed. In a pharmacological approach, the animals were treated with either the Orx1R selective antagonist SB 334,867 (3, 30, and 300 nM/0.1 µL) or OrxA (28, 70, and 140 pmol/0.1 µL) in the PL before the test session of contextual fear conditioning. Neural tract tracing deposits in the LH showed some perikarya, mainly axons and terminal buttons in the PL, suggesting LH-PL reciprocate pathways. Furthermore, we showed a profuse network comprised of Orx1R-labeled thin varicose fibers widely distributed in the same field of LH-PL pathways projection. The selective blockade of Orx1R with SB 334,867 at 30 and 300 nM in the PL caused a decrease in freezing response, whereas the treatment with OrxA at 140 pmol promoted an increase in freezing response. In summary, these data confirmed an anatomical link between LH and PL, established the presence of Orx1R in the PL, and a modulatory role of the orexin system in such structure, possibly mainly via Orx1R, during contextual fear conditioning.

Bjorness T.E., Greene R.W.

The neuromodulator orexin has been identified as a key factor for motivated arousal including recent evidence that sleep deprivation-induced enhancement of reward behavior is modulated by orexin. While orexin is not necessary for either reward or arousal behavior, orexin neurons' broad projections, ability to sense the internal state of the animal, and high plasticity of signaling in response to natural rewards and drugs of abuse may underlie heightened drug seeking, particularly in a subset of highly motivated reward seekers. As such, orexin receptor antagonists have gained deserved attention for putative use in addiction treatments. Ongoing and future clinical trials are expected to identify individuals most likely to benefit from orexin receptor antagonist treatment to promote abstinence, such as those with concurrent sleep disorders or high craving, while attention to methodological considerations will aid interpretation of the numerous preclinical studies investigating disparate aspects of the role of orexin in reward and arousal.

Mun C.J., Reid M.J., Sarandos S., Elam K.K., Li C.M., Strickland J.C.

The present review investigates the potential of the orexin system as a clinical target for co-morbid substance use disorder (SUD) and chronic pain, focusing on improving sleep disturbances, an important shared risk factor. We synthesize current evidence from both human and animal studies and proposes viable future research directions to address existing knowledge gaps. Sleep disturbances significantly contribute to both SUD and chronic pain. The orexin system plays a vital role in sleep–wake regulation, and emerging evidence suggests the orexin system's unique involvement in drug-related reward functioning and pain modulation. Hence, the orexin system presents a unique opportunity to address the complex interplay between SUD, chronic pain, and sleep disturbances. Orexin receptor antagonists, particularly Dual Orexin Receptor Antagonists (DORAs) that are FDA-approved for treating insomnia, show considerable promise as novel treatments for both SUD and chronic pain. The current review highlights orexin system’s complex role in SUD and chronic pain. While preliminary evidence for DORAs in treating SUD and chronic pain is promising, critical gaps remain in limited human clinical trials, long-term effects, safety, abuse liability, and understanding of the orexin system’s underlying mechanisms in both problematic substance use and pain. Future studies should explore appropriate dosing regimens, the potential of different types of orexin antagonists, including Selective Orexin Receptor Antagonists (SORAs) and combined SORA and DORA therapies, and consider individual differences (e.g., sex differences) to realize their full therapeutic potential for co-morbid SUD and chronic pain.

Farmani D., Moteshakereh S.M., Nikoohemmat M., Askari R., Salehi S., Haghparast A.

The complicated relevance between stress and pain has been identified. Neurotransmitters and neuropeptides of various brain areas play a role in this communication. Pain inhibitory response is known as stress-induced analgesia (SIA). The studies demonstrated that the nucleus accumbens (NAc) is critical in modulating pain. As a neuropeptide, orexin is crucially involved in initiating behavioral and physiological responses to threatening and unfeeling stimuli. However, the role of the orexin receptors of the NAc area after exposure to restraint stress (RS) as acute physical stress in the modulation of acute pain is unclear. One hundered twenty adult male albino Wistar rats (230-250g) were used. Animals were unilaterally implanted with cannulae above the NAc. The SB334867 and TCS OX2 29 were used as antagonists for OX1r and OX2r, respectively. Different doses of the antagonists (1, 3, 10, and 30 nmol/0.5µl DMSO) were microinjected intra-NAc five minutes before exposure to RS (3hours). Then, the tail-flick test as a model of acute pain was performed, and the nociceptive threshold (Tail-flick latency; TFL) was measured in 60-minute time set intervals. According to this study's findings, the antinociceptive effects of RS in the tail-flick test were blocked during intra-NAc administration of SB334867 or TCS OX2 29. The RS as acute stress increased TFL and deceased pain-like behavior responses. The 50% effective dose values of the OX1r and OX2r antagonists were 12.82 and 21.64 nmol, respectively. The result demonstrated contribution of the OX1r into the NAc was more remarkable than that of the OX2r on antinociceptive responses induced by the RS. Besides, in the absence of RS, the TFL was attenuated. The current study's data indicated that OX1r and OX2r into the NAc induced pain modulation responses during RS in acute pain. In conclusion, the findings revealed the involvement of intra-NAc orexin receptors in improving SIA.

Rebassa J.B., Capó T., Lillo J., Raïch I., Reyes-Resina I., Navarro G.

Alzheimer’s disease (AD) remains a significant health challenge, with an increasing prevalence globally. Recent research has aimed to deepen the understanding of the disease pathophysiology and to find potential therapeutic interventions. In this regard, G protein-coupled receptors (GPCRs) have emerged as novel potential therapeutic targets to palliate the progression of neurodegenerative diseases such as AD. Orexin and cannabinoid receptors are GPCRs capable of forming heteromeric complexes with a relevant role in the development of this disease. On the one hand, the hyperactivation of the orexins system has been associated with sleep–wake cycle disruption and Aβ peptide accumulation. On the other hand, cannabinoid receptor overexpression takes place in a neuroinflammatory environment, favoring neuroprotective effects. Considering the high number of interactions between cannabinoid and orexin systems that have been described, regulation of this interplay emerges as a new focus of research. In fact, in microglial primary cultures of APPSw/Ind mice model of AD there is an important increase in CB2R–OX1R complex expression, while OX1R antagonism potentiates the neuroprotective effects of CB2R. Specifically, pretreatment with the OX1R antagonist has been shown to strongly potentiate CB2R signaling in the cAMP pathway. Furthermore, the blockade of OX1R can also abolish the detrimental effects of OX1R overactivation in AD. In this sense, CB2R–OX1R becomes a new potential therapeutic target to combat AD.

Hu X., Yu K., Li X., Zhang J., Jiao J., Wang Z., Cai H., Wang L., He Y., Wu M.

Abstract The orexin system is closely related to the pathogenesis of Alzheimer’s disease (AD). Orexin-A aggravates cognitive dysfunction and increases amyloid β (Aβ) deposition in AD model mice, but studies of different dual orexin receptor (OXR) antagonists in AD have shown inconsistent results. Our previous study revealed that OX1R blockade aggravates cognitive deficits and pathological progression in 3xTg-AD mice, but the effects of OX2R and its potential mechanism in AD have not been reported. In the present study, OX2R was blocked by oral administration of the selective OX2R antagonist MK-1064, and the effects of OX2R blockade on cognitive dysfunction and neuropsychiatric symptoms in 3xTg-AD mice were evaluated via behavioral tests. Then, immunohistochemistry, western blotting, and ELISA were used to detect Aβ deposition, tau phosphorylation, and neuroinflammation, and electrophysiological and wheel-running activity recording were recorded to observe hippocampal synaptic plasticity and circadian rhythm. The results showed that OX2R blockade ameliorated cognitive dysfunction, improved LTP depression, increased the expression of PSD-95, alleviated anxiety- and depression-like behaviors and circadian rhythm disturbances in 3xTg-AD mice, and reduced Aβ pathology, tau phosphorylation, and neuroinflammation in the brains of 3xTg-AD mice. These results indicated that chronic OX2R blockade exerts neuroprotective effects in 3xTg-AD mice by reducing AD pathology at least partly through improving circadian rhythm disturbance and the sleep-wake cycle and that OX2R might be a potential target for the prevention and treatment of AD; however, the potential mechanism by which OX2R exerts neuroprotective effects on AD needs to be further investigated.

Jászberényi M., Thurzó B., Bagosi Z., Vécsei L., Tanaka M.

The orexin/hypocretin neuropeptide family has emerged as a focal point of neuroscientific research following the discovery that this family plays a crucial role in a variety of physiological and behavioral processes. These neuropeptides serve as powerful neuromodulators, intricately shaping autonomic, endocrine, and behavioral responses across species. Notably, they serve as master regulators of vigilance and stress responses; however, their roles in food intake, metabolism, and thermoregulation appear complementary and warrant further investigation. This narrative review provides a journey through the evolution of our understanding of the orexin system, from its initial discovery to the promising progress made in developing orexin derivatives. It goes beyond conventional boundaries, striving to synthesize the multifaceted activities of orexins. Special emphasis is placed on domains such as stress response, fear, anxiety, and learning, in which the authors have contributed to the literature with original publications. This paper also overviews the advancement of orexin pharmacology, which has already yielded some promising successes, particularly in the treatment of sleep disorders.

Brotschi C., Bolli M.H., Gatfield J., Roch C., Sifferlen T., Treiber A., Williams J.T., Boss C.

Selective orexin 2 receptor antagonists (2-SORA) such as seltorexant (15) are in clinical development for the treatment of insomnia and other conditions such as depression. Herein, we report our structure-activity-relationship...

Botticelli L., Di Bonaventura E.M., Bello F.D., Giorgioni G., Piergentili A., Quaglia W., Bonifazi A., Cifani C., Di Bonaventura M.V.

Neuromedin U (NMU) is a bioactive peptide produced in the gut and in the brain, with a role in multiple physiological processes. NMU acts by binding and activating two G protein coupled receptors (GPCR), the NMU receptor 1 (NMU-R1), which is predominantly expressed in the periphery, and the NMU receptor 2 (NMU-R2), mainly expressed in the central nervous system (CNS). In the brain, NMU and NMU-R2 are consistently present in the hypothalamus, commonly recognized as the main "feeding center". Considering its distribution pattern, NMU revealed to be an important neuropeptide involved in the regulation of food intake, with a powerful anorexigenic ability. This has been observed through direct administration of NMU and by studies using genetically modified animals, which revealed an obesity phenotype when the NMU gene is deleted. Thus, the development of NMU analogs or NMU-R2 agonists might represent a promising pharmacological strategy to treat obese individuals. Furthermore, NMU has been demonstrated to influence the non-homeostatic aspect of food intake, playing a potential role in binge eating behavior. This review aims to discuss and summarize the current literature linking the NMU system with obesity and binge eating behavior, focusing on the influence of NMU on food intake and the neuronal mechanisms underlying its anti-obesity properties. Pharmacological strategies to improve the pharmacokinetic profile of NMU will also be reported.