OX1R Receptor Research Articles

Unraveling the function and structure impact of deleterious missense SNPs in the human OX1R receptor by computational analysis

The orexin/hypocretin receptor type 1 (OX1R) plays a crucial role in regulating various physiological functions, especially feeding behavior, addiction, and reward. Genetic variations in the OX1R have been associated with several neurological disorders. In this study, we utilized a combination of sequence and structure-based computational tools to identify the most deleterious missense single nucleotide polymorphisms (SNPs) in the OX1R gene. Our findings revealed four highly conserved and structurally destabilizing missense SNPs, namely R144C, I148N, S172W, and A297D, located in the GTP-binding domain. Molecular dynamics simulations analysis demonstrated that all four most detrimental mutant proteins altered the overall structural flexibility and dynamics of OX1R protein, resulting in significant changes in the structural organization and motion of the protein. These findings provide valuable insights into the impact of missense SNPs on OX1R function loss and their potential contribution to the development of neurological disorders, thereby guiding future research in this field.

Effect of SB-408124, an orexin A OX1R receptor antagonist, on the compulsive behavior and the level of anxiety after the vital stress in rats

The effect of the orexin A OX1R receptor antagonist SB-408124 of on the compulsive behavior and the anxiety in rats after the presentation of vital stress in a number of behavioral tests: marble test, elevated plus maze, in the open field and in the test “resident intruder”. In the buring marble test, the behavioral components of the obsession(obsessiveand obtrusive thoughts) and compulsions (obtrusive behavior), aimed to reduce anxiety, were modeled. Mental trauma was caused by a stressful effect, the essence of which was the experience of the animals of the circumstances of the death of a partner from the actions of a predator. A group of rats were placed once in the terrarium to a tiger python. After the action of vital mental stress in rats, two connected behavioral phenomena were observed: a high level of anxiety and an increase in the number of buried balls. This was accompanied by a decrease in communicability. Intranasal administration (for 7 days) of orexin A antagonist OX1R receptor SB-408124 after presentation of the vital stress reduced the level of anxiety, and also normalized the communicative activity of animals and the number of buried balls, i.e. compulsive behavior. Thus, the orexin system of the brain is an important component of psychotraumatic mechanism. OX1R antagonists of orexin A receptors can potentially be considered as correctors of obsessive-compulsive disorders on the background of posttraumatic stress disorder (PTSD). Use of intranasal administration of OX1R antagonists of orexin A receptors in the clinic will allow the use of small doses of substances and thereby reduce their possible toxic effects. (For citation: Tissen IYu, Yakushina ND, Lebedev AA, et al. Effect of SB-408124, an orexin A OX1R receptor antagonist, on the compulsive behavior and the level of anxiety after the vital stress in rats. Reviews on Clinical Pharmacology and Drug Therapy. 2018;16(1):34-42. doi: 10.17816/RCF16134-42).

INTERACTION BETWEEN OREXIN AND OPIOIDS SYSTEMS OF THE STRUCTURES OF PARAAMYGDALAR COMPLEX IN THE REINFORCING EFFECTS OF SPONTANEOUS AND ACTIVATED SELF-STIMULATION OF THE LATERAL HYPOTHALAMUS

Male Wistar rats were implanted bipolar electrodes into the lateral hypothalamus to study self-stimulation reaction in the Skinner box and microcannulas into the right lateral ventricle and structutes of the paraamygdalar complex (bed nucleus of stria terminalis, central nucleus of amygdala or nucleus accumbens) to study central effects of orexin (5 µg in 5 µl i. v. for an injection) on the reinforcing properties of pharmacological drugs. Intraperitoneal administration of trimeperidine (3 mg/kg), a synthetic opioid, was shown to increase self-stimulation of the lateral hypothalamus in the Skinner box (number of pedal pressings for 10 min) by 51.8%, and sulpiride (5 mg/kg, a small dose), an antagonist of D2 dopamine receptors, did not change but in the large dose (20 mg/kg) decreased self-stimulation by 49.3% (a number of pedal pressings, or self-stimulation frequency within 10 min). At the same time, SB-408124, an antagonist of OX1R receptors and its combination with orexin did not change self-stimulation indexes after intrastructural administration into the bed nucleus of stria terminalis, central nucleus of amygdala or nucleus accumbens. On the background of blockade of OX1R receptors by SB-408124 (1 µg for all structures) trimeperidine reduced their activating action on self-stimulation reaction. Sulpiride (5 mg/kg i. p., a dose not affecting self-stimulation reaction) blocked activating action of trimeperidine after blockade OX1R receptors by SB-408124 (1 µg). The data obtained can suggest that OX1R receptors participate in the reinforcing effects of synthetic opioid trimeperidine and the blockade of them by SB-408124 potentiate antagonist effects of sulpiride on self-stimulation (4 tables, bibliography: 23 refs).

Hypocretin/Orexin Peptides Excite Rat Neuroendocrine Dopamine Neurons through Orexin 2 Receptor-Mediated Activation of a Mixed Cation Current

Hypocretin/Orexin (H/O) neurons of the lateral hypothalamus are compelling modulator candidates for the chronobiology of neuroendocrine output and, as a consequence, hormone release from the anterior pituitary. Here we investigate the effects of H/O peptides upon tuberoinfundibular dopamine (TIDA) neurons – cells which control, via inhibition, the pituitary secretion of prolactin. In whole cell recordings performed in male rat hypothalamic slices, application of H/O-A, as well as H/O-B, excited oscillating TIDA neurons, inducing a reversible depolarising switch from phasic to tonic discharge. The H/O-induced inward current underpinning this effect was post-synaptic (as it endured in the presence of tetrodotoxin), appeared to be carried by a Na+-dependent transient receptor potential-like channel (as it was blocked by 2-APB and was diminished by removal of extracellular Na+), and was a consequence of OX2R receptor activation (as it was blocked by the OX2R receptor antagonist TCS OX2 29, but not the OX1R receptor antagonist SB 334867). Application of the hormone, melatonin, failed to alter TIDA membrane potential or oscillatory activity. This first description of the electrophysiological effects of H/Os upon the TIDA network identifies cellular mechanisms that may contribute to the circadian rhythmicity of prolactin secretion.

Orexin/hypocretin-1 receptor antagonism reduces ethanol self-administration and reinstatement selectively in highly-motivated rats

The orexin/hypocretin (ORX) system regulates motivation for natural rewards and drugs of abuse such as alcohol. ORX receptor antagonists, most commonly OX1R antagonists including SB-334867 (SB), decrease alcohol drinking, self-administration and reinstatement in both genetically-bred alcohol-preferring and outbred strains of rats. Importantly, levels of alcohol seeking and drinking in outbred rats are variable, as they are in humans. We have shown that OX1R antagonism selectively decreases homecage alcohol drinking in high-, but not low-alcohol-preferring rats. It is unknown, however, whether this effect is selective to homecage drinking or whether it also applies to alcohol seeking paradigms such as self-administration and reinstatement following extinction, in which motivation is high in the absence of alcohol. Here we trained Sprague Dawley rats to self-administer 20% ethanol paired with a light-tone cue on an FR3 regimen. Rats were then extinguished and subjected to cue-induced reinstatement. Rats were segregated into high- and low-ethanol-responding groups (HR and LR) based on self-administration levels. During self-administration and cue-induced reinstatement, rats were given SB or vehicle prior to ethanol seeking. In both conditions, OX1R antagonism decreased responding selectively in HR, but not LR rats. There were no non-specific effects of SB treatment on arousal or general behavior. These data indicate that ORX signaling at the OX1R receptor specifically regulates high levels of motivation for alcohol, even in the absence of direct alcohol reinforcement. This implicates the ORX system in the pathological motivation underlying alcohol abuse and alcoholism and demonstrates that the OX1R may be an important target for treating alcohol abuse.

Orexin-1 receptor antagonist in central nucleus of the amygdala attenuates the acquisition of flavor-taste preference in rats

Previous studies demonstrated that the intracerebroventricular administration of SB-334867-A, a selective antagonist of orexin OX1R receptors, blocks the acquisition of saccharin-induced conditioned flavor preference (CFP) but not LiCl-induced taste aversion learning (TAL). Orexinergic fibers from the lateral hypothalamus end in the central nucleus of the amygdala (CeA), which expresses orexin OX1R receptors. Taste and sensory inputs also are present in CeA, which may contribute to the development of taste learning. This study analyzed the effect of two doses (1.5 and 6μg/0.5μl) of SB-334867-A administered into the CeA on flavor–taste preference induced by saccharin and on TAL induced by a single administration of LiCl (0.15M, 20ml/kg, i.p.). Outcomes indicate that inactivation of orexinergic receptors in the CeA attenuates flavor–taste preference in a two-bottle test (saccharin vs. water). Intra-amygdalar SB-334867-A does not affect gustatory processing or the preference for the sweet taste of saccharin given that SB-334867-A- and DMSO-treated groups (control animals) increased the intake of the saccharin-associated flavor across training acquisition sessions. Furthermore, SB-334867-A in the CeA does not block TAL acquisition ruling out the possibility that functional inactivation of OX1R receptors interferes with taste processing. Orexin receptors in the CeA appear to intervene in the association of a flavor with orosensory stimuli, e.g., a sweet and pleasant taste, but could be unnecessary when the association is established with visceral stimuli, e.g., lithium chloride. These data suggest that orexinergic projections to the CeA may contribute to the reinforcing signals facilitating the acquisition of taste learning and the change in hedonic evaluation of the taste, which would have important implications for the OX1R-targeted pharmacological treatment of eating disorders.

Antagonism of orexin-1 receptors attenuates swim- and restraint stress-induced antinociceptive behaviors in formalin test

Orexin (ORX) plays an important role in pain modulation. ORX receptors have been found in many brain structures and are known to be involved in pain processing. It is well-established that the acute and chronic forms of stress could induce hormonal and neuronal changes that affect both pain threshold and nociceptive behaviors. The role of OX1R receptors in stress-induced analgesia (SIA) has not been fully elucidated. In the present study, using the formalin test, attempts were made to evaluate the effects of acute immobilization restraint stress and swimming stress on pain behavioral responses following OX1R antagonist administration in rats. Animals received OX1R antagonist (SB-334867), vehicle, or naloxone before exposure to acute restraint stress (30min) or swimming stress test (6min, 20±1°C), and immediately submitted to hind paw formalin injection (50μl, 2%). Acute 30-min exposure to restraint stress as well as 6-min exposure to swim stress could significantly reduce the formalin-induced nociceptive behaviors in rats. This antinociceptive effect with either restraint stress or swim stress was fully prevented by OX1R antagonist (SB-334867), while the SB-334867 alone had no effect. However, the opioid receptor antagonist naloxone could not totally reverse the antinociception effect with either form of stress. It is suggested that OX1R might be involved in antinociception behaviors induced by these two forms of stress. These data highlight the significant role of OX1R as a novel target for treatment of stress-related disorders.

Short food deprivation inhibits orexin receptor 1 expression and orexin-A induced intracellular calcium signaling in acutely isolated duodenal enterocytes

Close intra-arterial infusion of the appetite regulating peptide orexin-A stimulates bicarbonate secretion from the duodenal mucosa. The aim of the present study was to elucidate the ability of orexin-A to induce intracellular calcium signaling in acutely isolated duodenal enterocytes. Freshly isolated clusters of enterocytes, obtained from rat duodenal mucosa or human duodenal biopsies, were loaded with fura 2-AM and mounted in a perfusion chamber. Cryptlike enterocytes were selected (caged), and changes in intracellular calcium concentration ([Ca2+]i) were evaluated by fluorescence imaging. Total RNA was extracted from pellets of enterocytes and reverse transcribed to cDNA, and expression of orexin receptors 1 and 2 (OX1R and OX2R) was measured by quantitative real-time PCR. Orexin-A at all concentrations tested (1-100 nM) increased [Ca2+]i in enterocytes isolated from continuously fed rats, and the OX1R-antagonist SB-334867 (10 nM) attenuated the response. The primary [Ca2+]i response was a slow increase to a sustained plateau persisting after orexin-A removal, and a similar response was observed in enterocytes from human biopsies. In contrast to orexin-A, the OX2R agonist (Ala11,D-Leu15)-orexin-B (1-10 nM) did not induce calcium signaling. There were no significant [Ca2+]i responses in enterocytes from animals food deprived overnight, and overnight fasting decreased (P<0.01) enterocyte OX1R as well as OX2R mRNA. Induction of intracellular calcium signaling in isolated duodenal enterocytes is thus mediated primarily by OX1R receptors. Short (overnight) food deprivation markedly depresses receptor expression and inhibits orexin-A induced increases in [Ca2+]i. Studies of enterocyte signaling and intestinal secretion requires particular evaluation regarding feeding status.

Effects of orexins/hypocretins on neuronal activity in the paraventricular nucleus of the thalamus in rats in vitro

Orexin-A (ORX-A) and orexin-B (ORX-B), also called hypocretin-1 and hypocretin-2, respectively, act upon orexin 1 (OX 1R) and orexin 2 (OX 2R) receptors, and are involved in the regulation of sleep-wakefulness and energy homeostasis. Orexin neurons in the lateral hypothalamic perifornical region project heavily to the paraventricular nucleus of the thalamus (PVT), which is deeply involved in the control of motivated behaviors. In the present study, electrophysiological and cytosolic Ca 2+ concentration ([Ca 2+] i ) imaging studies on the effects of ORX-A and ORX-B on neurons in the PVT were carried out in rat brain slice preparations. ORX-A and/or ORX-B were applied extracellularly in the perfusate. Extracellular recordings showed that about 80% of the PVT neurons were excited dose-dependently by both ORX-A and ORX-B at concentrations of 10 −8 to 10 −6 M, and the increase in firing rate was about three times larger for ORX-B than for ORX-A at 10 −7 M. When both ORX-A and ORX-B were applied simultaneously at 10 −7 M, the increase in firing rate was almost equal to that of ORX-B at 10 −7 M, suggesting that the PVT neurons do not show a high affinity to ORX-A which is expected if they have OX 1R receptors. The excitatory effect of ORX-B was seen in low Ca 2+ and high Mg 2+ ACSF as well as in normal ACSF, and the increase in firing rate was greater in low Ca 2+ and high Mg 2+ ACSF than in normal ACSF. [Ca 2+] i imaging studies demonstrated that [Ca 2+] i was increased in about 50% of the PVT neurons by both 10 −7 M ORX-A and ORX-B with a stronger effect for ORX-B, and the increase in [Ca 2+] i induced by ORX-B was abolished in Ca 2+-free ACSF, suggesting that ORX-B does not release Ca 2+ from intracellular Ca 2+ stores. Subsequent whole cell patch clamp recordings revealed that an after hyperpolarization seen following each action potential in normal ACSF disappeared in Ca 2+-free ACSF, and the mean magnitude of the depolarization induced by ORX-B was same in normal, Ca 2+-free and TTX-containing Ca 2+-free ACSFs. Furthermore, ORX-B-induced depolarization was reversed to hyperpolarization when membrane potential was lowered to about −97 mV, and an increase of extracellular K + concentration from 4.25 to 13.25 mM abolished the ORX-B-induced depolarization, indicating that the ORX-B-induced depolarization is associated with an increase in the membrane resistance resulting from a closure of K + channels. These results suggest that orexins depolarize and excite post-synaptically PVT neurons via OX 2R receptors, and that orexin-activated PVT neurons play a role in the integration of sleep-wakefulness and energy homeostasis, and in the control of motivated behaviors.

The NMR-Derived Conformation of Orexin-A: An Orphan G-Protein Coupled Receptor Agonist Involved in Appetite Regulation and Sleep

The conformation of orexin-A, an orphan G-protein coupled receptor agonist (the human sequence is: has been determined when bound to sodium dodecylsulphate-d25 (SDS) micelles by 1H and 13C NMR and molecular modeling. Orexin-A has been implicated in sleep-wakefulness and feeding regulation. The conformational preference of orexin-A consists of a short helical section, involving Asp5 to Gln9 that makes up helix I, followed by a bend from Lys10 to Ser13. Residues Leu16 to Gly22 make up helix II. The conformation of orexin-A can now be used to explain the results of earlier Ala substitution mutagenesis experiments (J. G. Darker, et al. Bioorg. Med. Chem. Lett. 11, 737–740 (2001); S. Ammoun, et al. J. Pharmacol. Expt. Ther. 305, 507–514 (2003)). Darker et al., working with orexin-A (15–33) amide, observed a significant drop in functional potency at the OX1R receptor when Leu16, Leu19, Leu20, His26, Gly29, Ile30, Leu31, Thr32, and Leu33 were replaced by Ala. Ammoun et al. identified three areas of interest, which were the same for OX1R and OX2 R receptors, as amino acids 15–17, 20 and 25–26 with the most marked reduction in activity being produced by the replacement of Leu20 by Ala. We suggest that Leu16, Leu19, and Leu20, which are in helix II, are likely responsible for binding orexin-A to the surface of the micelle.