Spinal orexin-1 receptors mediate anti-hyperalgesic effects of intrathecally-administered orexins in diabetic neuropathic pain model rats

Supraspinal glycinergic neurotransmission in pain: A scoping review of current literature

The neurotransmitter glycine is an agonist at the strychnine-sensitive glycine receptors. In addition, it has recently been discovered to act at two new receptors, the excitatory glycine receptor and metabotropic glycine receptor. Glycine's neurotransmitter roles have been most extensively investigated in the spinal cord, where it is known to play essential roles in pain, itch, and motor function. In contrast, less is known about supraspinal glycinergic functions, and their contributions to pain circuits are largely unrecognized. As glycinergic neurons are absent from cortical regions, a clearer understanding of how supraspinal glycine modulates pain could reveal new pharmacological targets. This review aims to synthesize the published research on glycine's role in the adult brain, highlighting regions where glycine signaling may modulate pain responses. This was achieved through a scoping review methodology identifying several key regions of supraspinal pain circuitry where glycine signaling is involved. Therefore, this review unveils critical research gaps for supraspinal glycine's potential roles in pain and pain-associated responses, encouraging researchers to consider glycinergic neurotransmission more widely when investigating neural mechanisms of pain.

Orexins and primary headaches: an overview of the neurobiology and clinical impact

INTRODUCTION

Primary headaches, including migraines and cluster headaches, are highly prevalent disorders that significantly impact quality of life. Several factors suggest a key role for the hypothalamus, including neuroimaging studies, attack periodicity, and the presence of altered homeostatic regulation. The orexins are two neuropeptides synthesized almost exclusively in the lateral hypothalamus with widespread projections across the central nervous system. They are involved in an array of functions including homeostatic regulation and nociception, suggesting a potential role in primary headaches.

AREAS COVERED

This review summarizes current knowledge of the neurobiology of orexins, their involvement in sleep-wake regulation, nociception, and functions relevant to the associated symptomology of headache disorders. Preclinical reports of the antinociceptive effects of orexin-A in preclinical models are discussed, as well as clinical evidence for the potential involvement of the orexinergic system in headache.

EXPERT OPINION

Several lines of evidence support the targeted modulation of orexinergic signaling in primary headaches. Critically, orexins A and B, acting differentially via the orexin 1 and 2 receptors, respectively, demonstrate differential effects on trigeminal pain processing, indicating why dual-receptor antagonists failed to show clinical efficacy. The authors propose that orexin 1 receptor agonists or positive allosteric modulators should be the focus of future research.

Orexin neurons play contrasting roles in itch and pain neural processing via projecting to the periaqueductal gray

Pain and itch are recognized as antagonistically regulated sensations; pain suppresses itch, whilst pain inhibition enhances itch. The neural mechanisms at the central nervous system (CNS) underlying these pain-itch interactions still need to be explored. Here, we revealed the contrasting role of orexin-producing neurons (ORX neurons) in the lateral hypothalamus (LH), which suppresses pain while enhancing itch neural processing, by applying optogenetics to the acute pruritus and pain model. We also revealed that the circuit of ORX neurons from LH to periaqueductal gray regions served in the contrasting modulation of itch and pain processing using optogenetic terminal inhibition techniques. Additionally, by using an atopic dermatitis model, we confirmed the involvement of ORX neurons in regulating chronic itch processing, which could lead to a novel therapeutic target for persistent pruritus in clinical settings. Our findings provide new insight into the mechanism of antagonistic regulation between pain and itch in the CNS.

The opposite roles of orexin neurons in pain and itch neural processing

Pain and itch are antagonistically regulated sensations; pain suppresses itch, and inhibition of pain enhances itch. Understanding the central neural circuit of antagonistic regulation between pain and itch is required to develop new therapeutics better to manage these two feelings in a clinical situation. However, evidence of the neural mechanism underlying the pain-itch interaction in the central nervous system (CNS) is still insufficient. To pave the way for this research area, our laboratory has focused on orexin (ORX) producing neurons in the hypothalamus, which is known as a master switch that induces various defense responses when animals face a stressful environment. This review article summarized the previous evidence and our latest findings to argue the neural regulation between pain and itch and the bidirectional roles of ORX neurons in processing these two sensations. i.e., pain relief and itch exacerbation. Further, we discussed the possible neural circuit mechanism for the opposite controlling of pain and itch by ORX neurons. Focusing on the roles of ORX neurons would provide a new perspective to understand the antagonistic regulation of pain and itch in CNS.

Orexinergic descending inhibitory pathway mediates linalool odor-induced analgesia in mice

Linalool odor exposure induces an analgesic effect in mice. This effect disappeared in the anosmic model mice, indicating that olfactory input evoked by linalool odor triggered this effect. Furthermore, hypothalamic orexinergic neurons play a pivotal role in this effect. However, the neuronal circuit mechanisms underlying this effect have not been fully addressed. In this study, we focused on the descending orexinergic projection to the spinal cord and examined whether this pathway contributes to the effect. We assessed the effect of intrathecal administration of orexin receptor antagonists on linalool odor-induced analgesia in the tail capsaicin test. We found that the selective orexin type 1 receptor antagonist, but not the selective orexin type 2 receptor antagonist, prevented the odor-induced analgesic effect. Furthermore, immunohistochemical analyses of c-Fos expression induced by the capsaicin test revealed that neuronal activity of spinal cord neurons was suppressed by linalool odor exposure, which was prevented by intrathecal administration of the orexin 1 receptor antagonist. These results indicate that linalool odor exposure drives the orexinergic descending pathway and suppresses nociceptive information flow at the spinal level.

Antinociceptive effects of oleuropein in experimental models of neuropathic pain in male rats

Background

The present investigation explored the therapeutic actions of oleuropein along with the possible signaling pathway involved in attenuating neuropathic pain in chronic constriction injury (CCI) and vincristine-induced neuropathic pain in male rats.

Methods

Four loose ligatures were placed around the sciatic nerve to induce CCI, and vincristine (50 μg/kg) was injected for 10 days to develop neuropathic pain. The development of cold allodynia, mechanical allodynia, and mechanical hyperalgesia was assessed using different pain-related behavioral tests. The levels of H₂S, cystathionine-γ-lyase (CSE), cystathionine-β-synthase (CBS), orexin, and nuclear factor erythroid-2-related factor 2 (Nrf2) were measured in the sciatic nerve.

Results

Treatment with oleuropein for 14 days led to significant amelioration of behavioral manifestations of neuropathic pain in two pain models. Moreover, oleuropein restored both CCI and vincristine-induced decreases in H₂S, CSE, CBS, orexin, and Nrf2 levels. Co-administration of suvorexant, an orexin receptor antagonist, significantly counteracted the pain-attenuating actions of oleuropein and Nrf2 levels without modulating H₂S, CSE and CBS.

Conclusions

Oleuropein has therapeutic potential to attenuate the pain manifestations in CCI and vincristine-induced neuropathic pain, possibly by restoring the CSE, CBS, and H₂S, which may subsequently increase the expression of orexin and Nrf2 to ameliorate behavioral manifestations of pain.

Differential role of specific cardiovascular neuropeptides in pain regulation: Relevance to cardiovascular diseases

In many instances, the perception of pain is disproportionate to the strength of the algesic stimulus. Excessive or inadequate pain sensation is frequently observed in cardiovascular diseases, especially in coronary ischemia. The mechanisms responsible for individual differences in the perception of cardiovascular pain are not well recognized. Cardiovascular disorders may provoke pain in multiple ways engaging molecules released locally in the heart due to tissue ischemia, inflammation or cellular stress, and through neurogenic and endocrine mechanisms brought into action by hemodynamic disturbances. Cardiovascular neuropeptides, namely angiotensin II (Ang II), angiotensin-(1-7) [Ang-(1-7)], vasopressin, oxytocin, and orexins belong to this group. Although participation of these peptides in the regulation of circulation and pain has been firmly established, their mutual interaction in the regulation of pain in cardiovascular diseases has not been profoundly analyzed. In the present review we discuss the regulation of the release, and mechanisms of the central and systemic actions of these peptides on the cardiovascular system in the context of their central and peripheral nociceptive (Ang II) and antinociceptive [Ang-(1-7), vasopressin, oxytocin, orexins] properties. We also consider the possibility that they may play a significant role in the modulation of pain in cardiovascular diseases. The rationale for focusing attention on these very compounds was based on the following premises (1) cardiovascular disturbances influence the release of these peptides (2) they regulate vascular tone and cardiac function and can influence the intensity of ischemia - the factor initiating pain signals in the cardiovascular system, (3) they differentially modulate nociception through peripheral and central mechanisms, and their effect strongly depends on specific receptors and site of action. Accordingly, an altered release of these peptides and/or pharmacological blockade of their receptors may have a significant but different impact on individual sensation of pain and comfort of an individual patient.

The Lateral Hypothalamus: An Uncharted Territory for Processing Peripheral Neurogenic Inflammation

The roles of the hypothalamus and particularly the lateral hypothalamus (LH) in the regulation of inflammation and pain have been widely studied. The LH consists of a parasympathetic area that has connections with all the major parts of the brain. It controls the autonomic nervous system (ANS), regulates feeding behavior and wakeful cycles, and is a part of the reward system. In addition, it contains different types of neurons, most importantly the orexin neurons. These neurons, though few in number, perform critical functions such as inhibiting pain transmission and interfering with the reward system, feeding behavior and the hypothalamic pituitary axis (HPA). Recent evidence has identified a new role for orexin neurons in the modulation of pain transmission associated with several inflammatory diseases, including rheumatoid arthritis and ulcerative colitis. Here, we review recent findings on the various physiological functions of the LH with special emphasis on the orexin/receptor system and its role in mediating inflammatory pain.

OX2 receptors mediate the inhibitory effects of orexin-A on potassium chloride-induced increases in intracellular calcium ion levels in neurons derived from rat dorsal root ganglion in a chronic pain model

AIMS

Orexin-A is known to induce anti-nociceptive effects in animal models of chronic pain. We have found that orexin-A inhibits KCl loading-induced increases in the intracellular calcium ion levels ([Ca²⁺ ]_i ) in C-fiber-like neurons of rats showing inflammatory nociceptive behavior. Here, we examined the effects of orexin-A on the depolarization of C-fiber-like neurons derived from a rat model for another type of chronic pain, namely neuropathic pain. Thus, we analyzed the effects of orexin-A on KCl-induced increases in [Ca²⁺ ]_i in C-fiber-like neurons of rats with sciatic nerve ligation.

METHODS

Paw withdrawal and threshold force in response to tactile stimuli were evaluated using von Frey filaments. Sham-operated rats served as controls. [Ca²⁺ ]_i in neurons were visualized by calcium fluorescent probe. Changes in [Ca²⁺ ]_i were assessed using relative fluorescence intensity.

RESULTS

Seven days after sciatic nerve ligation, paw withdrawal and threshold force for tactile stimuli were increased and reduced, respectively. KCl loading to neurons from either sciatic nerve-ligated or control rats increased relative fluorescence intensity. The KCl-induced increase in relative fluorescence intensity in sciatic nerve-ligated, but not that of control, rats was inhibited by orexin-A. The OX₁ and OX₂ receptor antagonist MK-4305 and OX₂ receptor antagonist EMPA, but not the OX₁ receptor antagonist SB 334867, each counteracted orexin-A-induced inhibition of KCl-provoked increases in relative fluorescence intensity.

CONCLUSION

The present findings constitute neuropharmacological evidence that OX₂ but not OX₁ receptors mediate the inhibitory effects of orexin-A on KCl-induced increases in [Ca²⁺ ]_i in C-fiber-like neurons of rats showing hyperalgesia provoked by sciatic nerve ligation.

Analgesic tolerance induced by repeated morphine injections induces cross-tolerance to the analgesic effect of orexin-A in rats

Repeated administration of morphine or orexin-A produces tolerance to their antinociceptive effects. We investigated the possible incidence of cross-tolerance between orexin-A and morphine. Adult male Sprague-Dawley rats (200-250 g) were used. Under deep anesthesia, a stereotaxic apparatus was used to implant a 23 G cannula into the lateral ventricle for an intracerebroventricular (ICV) microinjection. The antinociceptive effect of three different doses of orexin-A (5, 20, and 40 µM; dissolved in 5 µl sterile saline; ICV) was examined using the hot-plate test at 15, 30, 60, and 90 min after infusion. To evaluate tolerance, orexin-A (20 µM; ICV) or morphine (10 mg/kg; intraperitoneal) was administered for 7 consecutive days (twice per day) and the analgesic response was assessed at days 1, 4, and 7. Cross-tolerance was investigated at day 8 with a single injection of morphine (10 mg/kg; intraperitoneal) to the repeated orexin-A group and a single microinjection of orexin-A (20 µM; ICV) to the repeated morphine group. Analgesic responses were then examined. Administration of both orexin-A and morphine produced significant antinociception at day 1 (P<0.001 compared with the saline group). However, a significant reduction in the analgesic effects of both morphine and orexin-A appeared at day 7, following repeated administration (P<0.01). Orexin-A microinjection at day 8 in the repeated morphine group did not result in significant antinociception (P>0.05), whereas morphine injection in the repeated orexin-A group at day 8 showed a significant analgesic effect (P<0.001). These results indicate cross-tolerance to the analgesic effect of orexin-A following morphine tolerance.

Orexin facilitates GABAergic IPSCs via postsynaptic OX1 receptors coupling to the intracellular PKC signalling cascade in the rat cerebral cortex

Orexin has multiple physiological functions including wakefulness, appetite, nicotine intake, and nociception. The cerebral cortex receives abundant orexinergic projections and expresses both orexinergic receptor 1 (OX₁R) and 2 (OX₂R). However, little is known about orexinergic regulation of GABA-mediated inhibitory synaptic transmission. In the cerebral cortex, there are multiple GABAergic neural subtypes, each of which has its own morphological and physiological characteristics. Therefore, identification of presynaptic GABAergic neural subtypes is critical to understand orexinergic effects on GABAergic connections. We focused on inhibitory synapses at pyramidal neurons (PNs) from fast-spiking GABAergic neurons (FSNs) in the insular cortex by a paired whole-cell patch-clamp technique, and elucidated the mechanisms of orexin-induced IPSC regulation. We found that both orexin A and orexin B enhanced unitary IPSC (uIPSC) amplitude in FSN→PN connections without changing the paired-pulse ratio or failure rate. These effects were blocked by SB-334867, an OX₁ receptor (OX₁R) antagonist, but not by TCS-OX2-29, an OX₂R antagonist. [Ala¹¹, D-Leu¹⁵]-orexin B, a selective OX₂R agonist, had little effect on uIPSCs. Variance-mean analysis demonstrated an increase in quantal content without a change in release probability or the number of readily releasable pools. Laser photolysis of caged GABA revealed that orexin A enhanced GABA-mediated currents in PNs. Downstream blockade of G_q/11 protein-coupled OX₁Rs by IP₃ receptor or protein kinase C (PKC) blockers and BAPTA injection into postsynaptic PNs diminished the orexin A-induced uIPSC enhancement. These results suggest that the orexinergic uIPSC enhancement is mediated via postsynaptic OX₁Rs, which potentiate GABA_A receptors through PKC activation.

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.

Orexin receptors mediate long-term depression of excitatory synaptic transmission in the spinal cord dorsal horn

Neuropeptides orexin-A and -B are related to the regulation of sleep/wakefulness and feeding behaviors. Recently, the peptides have also been shown to yield antinociceptive effects in various pain models. However, it is not clear whether orexins are involved in forms of synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD), the increase and the decrease of synaptic efficacy, respectively. In the present study, we examined whether orexin receptor type 1 (OX1) and 2 (OX2) are involved in the induction or maintenance of LTD of excitatory synaptic transmission using transverse spinal cord slices of young rats. Repetitive electrical stimulation of Lissauer's tract zone at 2Hz for 5min (600 pulses), combined with a holding potential of -30mV, induced LTD of the amplitude of excitatory postsynaptic currents (EPSCs) which are evoked by the activation of primary afferent fibers. The maintenance of LTD was significantly prevented by bath application of SB674042 (1μM), an OX1 antagonist, or EMPA (1μM), an OX2 antagonist. In addition, LTD was dependent on the NMDA receptor, as the NMDA receptor antagonist D-AP5 blocked the maintenance of LTD. Our study suggests that orexins, via activation of both OX1 and OX2, play a significant role in the expression of NMDA-dependent LTD, thereby contributing to the spinal modulation of pain transmission.

The analgesic effect of orexin-A in a murine model of chemotherapy-induced neuropathic pain

Orexins are neuropeptides that are localized to neurons in the lateral and dorsal hypothalamus but its receptors are distributed to many different regions of the central nervous system. Orexins are implicated in a variety of physiological functions including sleep regulation, energy homeostats, and stress reactions. Furthermore, orexins administered exogenously have been shown to have analgesic effects in animal models. A type of intractable pain in patients is pain due to chemotherapy-induced peripheral neuropathy (CIPN). Several chemotherapeutic agents used for the treatment of malignant diseases induce dose-limiting neuropathic pain that compromises patients' quality of life. Here, we examined the analgesic effect of orexin-A in a murine model of CIPN, and compared it with the effect of duloxetine, the only drug recommended for the treatment of CIPN pain in patients. CIPN was induced in male BALB/c mice by repeated intraperitoneal injection of oxaliplatin, a platinum chemotherapeutic agent used for the treatment of advanced colorectal cancer. Neuropathic mechanical allodynia was assessed by the von Frey test, and the effect on acute thermal pain was assessed by the tail flick test. Intracerebroventricularly administered orexin-A dose-dependently attenuated oxaliplatin-induced mechanical allodynia and increased tail flick latencies. Oxaliplatin-induced mechanical allodynia was completely reversed by orexin-A at a low dose that did not increase tail flick latency. Duloxetine only partially reversed mechanical allodynia and had no effect on tail flick latency. The analgesic effect of orexin-A on oxaliplatin-induced mechanical allodynia was completely antagonized by prior intraperitoneal injection of SB-408124 (orexin type-1 receptor antagonist), but not by prior intraperitoneal injection of TCS-OX2-29 (orexin type-2 receptor antagonist). Our findings suggest that orexin-A is more potent than duloxetine in relieving pain CIPN pain and its analgesic effect is mediated by orexin type-1 receptors. Orexin type-1 receptor agonists may have potential therapeutic roles in the treatment of CIPN pain in patients.

Odour-induced analgesia mediated by hypothalamic orexin neurons in mice

Various folk remedies employ certain odorous compounds with analgesic effects. In fact, linalool, a monoterpene alcohol found in lavender extracts, has been found to attenuate pain responses via subcutaneous, intraperitoneal, intrathecal, and oral administration. However, the analgesic effects of odorous compounds mediated by olfaction have not been thoroughly examined. We performed behavioural pain tests under odourant vapour exposure in mice. Among six odourant molecules examined, linalool significantly increased the pain threshold and attenuated pain behaviours. Olfactory bulb or epithelium lesion removed these effects, indicating that olfactory sensory input triggered the effects. Furthermore, immunohistochemical analysis revealed that linalool activated hypothalamic orexin neurons, one of the key mediators for pain processing. Formalin tests in orexin neuron-ablated and orexin peptide-deficient mice showed orexinergic transmission was essential for linalool odour-induced analgesia. Together, these findings reveal central analgesic circuits triggered by olfactory input in the mammalian brain and support a potential therapeutic approach for treating pain with linalool odour stimulation.

Modulation of nociception by medial pre-optic area orexin a receptors and its relation with morphine in male rats

INTRODUCTION

Recent studies have shown that medial pre-optic area (MPOA) of hypothalamus are involved in nociception. Orexin A (hypocretin 1) has been found to have numerous applications including pain modulation. However, the role of orexin A receptors in the MPOA on the nociception has not been yet studied. Therefore, the aim of the present study is to investigate the effect of orexin A microinjection on MPOA on the nociception transmission and morphine induced analgesia in adult male rats.

METHODS

Using stereotaxic surgery, a cannula was implanted at a site 1mm above the MPOA in the anesthetized rats. After the recovery period, tail-flick (TF) latency was measured as 0, 15, 30, 45 and 60min following the onset of two experimental protocols. Two experiments were carried out. Experiment 1: The male rats received intra-MPOA of 25, 100, 1000, 10000pmol/0.5μl orexin A or 0.5μl of aCSF (control, just 5min before the TF assay. Experiment 2: The aim of this experiment was to examine the effect of orexin microinjection into MPOA on morphine analgesia (3mg/kg,s.c). Morphine was administered 30min before orexin A intra-MPOA microinjection (four doses similar to experiment 1) or aCSF, then TF latency was measured.

RESULTS

The results indicated that microinjection of orexin A into the MPOA showed anti-nociceptive effect in a time-dependent manner. Dose response curve results also revealed that the maximum effective dose of orexin A injection into MPOA for pain inhibition is 1000pmol/0.5μl. Co-administration of systemic morphine and orexin into the MPOA has additive analgesia with different time course compared morphine or orexin alone.

CONCLUSION

It can be concluded that MPOA OrexinA receptors play an important role in the modulation of pain in normal and morphine treated male rats.

The integrative role of orexin/hypocretin neurons in nociceptive perception and analgesic regulation

The level of wakefulness is one of the major factors affecting nociception and pain. Stress-induced analgesia supports an animal’s survival via prompt defensive responses against predators or competitors. Previous studies have shown the pharmacological effects of orexin peptides on analgesia. However, orexin neurons contain not only orexin but also other co-transmitters such as dynorphin, neurotensin and glutamate. Thus, the physiological importance of orexin neuronal activity in nociception is unknown. Here we show that adult-stage selective ablation of orexin neurons enhances pain-related behaviors, while pharmacogenetic activation of orexin neurons induces analgesia. Additionally, we found correlative activation of orexin neurons during nociception using fiber photometry recordings of orexin neurons in conscious animals. These findings suggest an integrative role for orexin neurons in nociceptive perception and pain regulation.

Lateral Hypothalamic Stimulation Reduces Hyperalgesia Through Spinally Descending Orexin-A Neurons in Neuropathic Pain

No evidence to date shows that lateral hypothalamic (LH) stimulation produces orexin-A-mediated antinociception in the spinal cord dorsal horn (SCDH) in a model of neuropathic pain. We conducted experiments to examine the effect of orexin-A-mediated LH stimulation in female rats with chronic constriction injury (CCI) on thermal hyperalgesia. Rats receiving carbachol into the LH demonstrated antinociception on both the left CCI and right nonligated paws (p < .05). Rats were given carbachol in the LH followed by intrathecal injection of the orexin-1 (OX1) receptor antagonist SB-334867, which blocked LH-induced antinociception compared with control groups (p < .05) in the left paw, but not in the right paw. These findings support the hypothesis that LH stimulation produces antinociception in rats with thermal hyperalgesia from neuropathic pain via an orexin-A connection between the LH and the SCDH. Identification of this pathway may lead to studies using orexins to manage clinical pain.

Orexin-A modulates excitatory synaptic transmission and neuronal excitability in the spinal cord substantia gelatinosa

Although intrathecal orexin-A has been known to be antinociceptive in various pain models, the role of orexin-A in antinociception is not well characterized. In the present study, we examined whether orexin-A modulates primary afferent fiber-mediated or spontaneous excitatory synaptic transmission using transverse spinal cord slices with attached dorsal root. Bath-application of orexin-A (100nM) reduced the amplitude of excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation of Aδ- or C-primary afferent fibers. The magnitude of reduction was much larger for EPSCs evoked by polysynaptic C-fibers than polysynaptic Aδ-fibers, whereas it was similar in EPSCs evoked by monosynaptic Aδ- or C-fibers. SB674042, an orexin-1 receptor antagonist, but not EMPA, an orexin-2 receptor antagonist, significantly inhibited the orexin-A-induced reduction in EPSC amplitude from mono- or polysynaptic Aδ-fibers, as well as from mono- or polysynaptic C-fibers. Furthermore, orexin-A significantly increased the frequency of spontaneous EPSCs but not the amplitude. This increase was almost completely blocked by both SB674042 and EMPA. On the other hand, orexin-A produced membrane oscillations and inward currents in the SG neurons that were partially or completely inhibited by SB674042 or EMPA, respectively. Thus, this study suggests that the spinal actions of orexin-A underlie orexin-A-induced antinociceptive effects via different subtypes of orexin receptors.

Understanding how discrete populations of hypothalamic neurons orchestrate complicated behavioral states

A major question in systems neuroscience is how a single population of neurons can interact with the rest of the brain to orchestrate complex behavioral states. The hypothalamus contains many such discrete neuronal populations that individually regulate arousal, feeding, and drinking. For example, hypothalamic neurons that express hypocretin (Hcrt) neuropeptides can sense homeostatic and metabolic factors affecting wakefulness and orchestrate organismal arousal. Neurons that express agouti-related protein (AgRP) can sense the metabolic needs of the body and orchestrate a state of hunger. The organum vasculosum of the lamina terminalis (OVLT) can detect the hypertonicity of blood and orchestrate a state of thirst. Each hypothalamic population is sufficient to generate complicated behavioral states through the combined efforts of distinct efferent projections. The principal challenge to understanding these brain systems is therefore to determine the individual roles of each downstream projection for each behavioral state. In recent years, the development and application of temporally precise, genetically encoded tools has greatly improved our understanding of the structure and function of these neural systems. This review will survey recent advances in our understanding of how these individual hypothalamic populations can orchestrate complicated behavioral states due to the combined efforts of individual downstream projections.

Effect of orexin-A (hypocretin-1) on hyperalgesic and cachectic manifestations of experimentally induced rheumatoid arthritis in rats

Orexin-A has been shown to modulate pain sensation and increase appetite. Rheumatoid arthritis (RA) is characterized by joint destruction, deformity, hyperalgesia, and weight reduction. Aim: Evaluate the possible effect of orexin-A on hyperalgesic and cachectic manifestations in an adjuvant-induced arthritis (AIA) rat model. Methods: Forty adult male Wistar rats were distributed among 4 groups; I, normal controls; II, rats with AIA induced by intradermal injection of Mycobacterium butyricum, but with no other treatment; III, AIA rats treated daily with an intravenous injection of orexin-A for 8 days; and IV, AIA rats treated orally with dexamethasone for 8 days. The parameters we assessed were pain-associated behavior, body mass, hind paw volume, serum levels of nerve growth factor (NGF) and neuropeptide Y (NPY). Results: Orexin-A caused a significant reduction in pain sensation and NGF levels, and increased body mass and the levels of NPY, whereas treatment with dexamethasone led to significant reductions in paw swelling and pain sensation. Conclusion: Orexin-A has hypoalgesic properties and increases body mass, whereas dexamethasone has a potent anti-inflammatory effect. Therefore, the combination of orexin-A and dexamethasone should have a greater effect with respect to attenuating the manifestations and complications associated with RA.

Posterior hypothalamic modulation of ocular-responsive trigeminal subnucleus caudalis neurons is mediated by Orexin-A and Orexin1 receptors

Orexin-A (OxA) is synthesized in posterior and lateral regions of the hypothalamus and contributes to homeostatic regulation of body functions including pain modulation. To determine if orexinergic mechanisms contribute to posterior hypothalamus (PH)-induced modulation of ocular input to subnucleus caudalis/upper cervical (Vc/C1) neurons, the orexin-1 receptor antagonist SB334867 was applied to the dorsal brainstem surface prior to PH disinhibition, by bicuculline methiodide, in male rats under isoflurane anesthesia. Ocular input to Vc/C1 units by bright light or hypertonic saline was markedly reduced by PH disinhibition and reversed completely by local Vc/C1 application of SB334867. OxA applied to the Vc/C1 surface mimicked the effects of PH disinhibition in a dose-dependent manner. OxA-induced inhibition was prevented by co-application of SB334867, but not by the orexin-2 receptor antagonist TCS Ox2 29. PH disinhibition and local OxA application also reduced the high threshold convergent cutaneous receptive field area of ocular units, suggesting widespread effects on somatic input to Vc/C1 ocular units. Vc/C1 application of OxA or SB334867 alone did not affect the background discharge of ocular units and suggested that the PH-OxA influence on ocular unit activity was not tonically active. Vc/C1 application of OxA or SB334867 alone also did not alter mean arterial pressure, whereas PH disinhibition evoked prompt and sustained increases. These results suggest that stimulus-evoked increases in PH outflow acts through OxA and orexin-1 receptors to alter the encoding properties of trigeminal brainstem neurons responsive to input from the ocular surface and deep tissues of the eye.

OX1 and OX2 orexin/hypocretin receptor pharmacogenetics

Orexin/hypocretin peptide mutations are rare in humans. Even though human narcolepsy is associated with orexin deficiency, this is only extremely rarely due to mutations in the gene coding prepro-orexin, the precursor for both orexin peptides. In contrast, coding and non-coding variants of the OX₁ and OX₂ orexin receptors have been identified in many human populations; sometimes, these have been associated with disease phenotype, although most confer a relatively low risk. In most cases, these studies have been based on a candidate gene hypothesis that predicts the involvement of orexins in the relevant pathophysiological processes. In the current review, the known human OX₁/HCRTR1 and OX₂/HCRTR2 genetic variants/polymorphisms as well as studies concerning their involvement in disorders such as narcolepsy, excessive daytime sleepiness, cluster headache, polydipsia-hyponatremia in schizophrenia, and affective disorders are discussed. In most cases, the functional cellular or pharmacological correlates of orexin variants have not been investigated—with the exception of the possible impact of an amino acid 10 Pro/Ser variant of OX₂ on orexin potency—leaving conclusions on the nature of the receptor variant effects speculative. Nevertheless, we present perspectives that could shape the basis for further studies. The pharmacology and other properties of the orexin receptor variants are discussed in the context of GPCR signaling. Since orexinergic therapeutics are emerging, the impact of receptor variants on the affinity or potency of ligands deserves consideration. This perspective (pharmacogenetics) is also discussed in the review.

Why does sleep stop migraine?

The relationship between sleep and migraine headaches is complex. Changes in sleep patterns can trigger migraine attacks, and sleep disorders may be associated with increased migraine frequency. Furthermore, migraine patients and their doctors very consistently report that sleep relieves already established migraine attacks. Herein we will try to answer the question, "Why does sleep stop migraine?" Since evidence for this relationship is largely based on empirical clinical observation, we will not provide a clinical review of the association. Instead, we will focus on the pathophysiology of migraine attacks and its intersections with sleep biology.

Obesity and headache: Part II--potential mechanism and treatment considerations

Obesity and headache are both associated with a substantial personal and societal impact, and epidemiologic studies have consistently identified a positive association between obesity and headache in general, as well as obesity and migraine specifically (see part I). In the current manuscript, we will discuss the potential mechanisms for the migraine-obesity association, with a focus on the central and peripheral pathophysiological pathways which overlap between migraine and those modulating the drive to feed. We then discuss surgical, behavioral, and pharmacological treatment considerations for overweight and obese migraineurs as well as for those with idiopathic intracranial hypertension. We close by briefly discussing where future research may be headed in light of this data.

Orexin A and orexin receptor 1 axonal traffic in dorsal roots at the CNS/PNS interface

Hypothalamic orexin/hypocretin neurons send long axonal projections through the dorsal spinal cord in lamina I-II of the dorsal horn (DH) at the interface with the peripheral nervous system (PNS). We show that in the DH OXA fibers colocalize with substance P (SP) positive afferents of dorsal root ganglia (DRG) neurons known to mediate sensory processing. Further, OR1 is expressed in p75(NTR) and SP positive DRG neurons, suggesting a potential signaling pathway between orexin and DRG neurons. Interestingly, DRG sensory neurons have a distinctive bifurcating axon where one branch innervates the periphery and the other one the spinal cord (pseudo-unipolar neurons), allowing for potential functional coupling of distinct targets. We observe that OR1 is transported selectively from DRG toward the spinal cord, while OXA is accumulated retrogradely toward the DRG. We hence report a rare situation of asymmetrical neuropeptide receptor distribution between axons projected by a single neuron. These molecular and cellular data are consistent with the role of OXA/OR1 in sensory processing, including DRG neuronal modulation, and support the potential existence of an OX/HCRT circuit between CNS and PNS.

Migraine and obesity, is there a link?

Obesity and migraine are two frequent conditions found in the general population. In the past years, large-scale studies have established epidemiological links between the two conditions. Migraine prevalence appears to be increased in the obese population, and some characteristics of migraine are affected in the overweight population. More recent but limited data point out an improvement of migraine in the obese population after weight loss. Obesity may facilitate migraine progression to chronic daily headache or chronic migraine. Common physiological mechanisms that would be responsible for both conditions are not fully established. Several hypotheses suggest a common etiological factor for obesity and migraine. This work proposes to review the epidemiological data and to highlight the main hypotheses currently discussed.

Dual hypocretin receptor antagonism is more effective for sleep promotion than antagonism of either receptor alone

The hypocretin (orexin) system is involved in sleep/wake regulation, and antagonists of both hypocretin receptor type 1 (HCRTR1) and/or HCRTR2 are considered to be potential hypnotic medications. It is currently unclear whether blockade of either or both receptors is more effective for promoting sleep with minimal side effects. Accordingly, we compared the properties of selective HCRTR1 (SB-408124 and SB-334867) and HCRTR2 (EMPA) antagonists with that of the dual HCRTR1/R2 antagonist almorexant in the rat. All 4 antagonists bound to their respective receptors with high affinity and selectivity in vitro. Since in vivo pharmacokinetic experiments revealed poor brain penetration for SB-408124, SB-334867 was selected for subsequent in vivo studies. When injected in the mid-active phase, SB-334867 produced small increases in rapid-eye-movement (REM) and non-REM (NR) sleep. EMPA produced a significant increase in NR only at the highest dose studied. In contrast, almorexant decreased NR latency and increased both NR and REM proportionally throughout the subsequent 6 h without rebound wakefulness. The increased NR was due to a greater number of NR bouts; NR bout duration was unchanged. At the highest dose tested (100 mg/kg), almorexant fragmented sleep architecture by increasing the number of waking and REM bouts. No evidence of cataplexy was observed. HCRTR1 occupancy by almorexant declined 4–6 h post-administration while HCRTR2 occupancy was still elevated after 12 h, revealing a complex relationship between occupancy of HCRT receptors and sleep promotion. We conclude that dual HCRTR1/R2 blockade is more effective in promoting sleep than blockade of either HCRTR alone. In contrast to GABA receptor agonists which induce sleep by generalized inhibition, HCRTR antagonists seem to facilitate sleep by reducing waking “drive”.

Obesity and chronic daily headache

Obesity may be the greatest epidemic of modern times. It leads to diabetes and heart disease and shortens lifespan. Although not a risk factor for migraine, it is associated with an increased frequency and intensity of migraine. Obesity is also comorbid with chronic daily headache and is a major risk factor for chronification of episodic migraine in adults and children. Although obesity is not a factor in the effectiveness of migraine treatment, it does increase the peripheral and central events in migraine, ultimately increasing the neurologic potential for migraine. Although evidence suggests that obesity is a modifiable risk factor for migraine progression, it is unknown if weight loss is related to decrease in headache frequency. Recent surgical results suggest that this is true. We suggest all possible effective techniques aimed at weight loss be undertaken for migraineurs, especially obese migraineurs, and that carefully monitoring weight changes should be routinely done as part of their migraine care.

Acute pain and a motivational pathway in adult rats: influence of early life pain experience

Background

The importance of neonatal experience upon behaviour in later life is increasingly recognised. The overlap between pain and reward pathways led us to hypothesise that neonatal pain experience influences reward-related pathways and behaviours in adulthood.

Methodology/Principal Findings

Rat pups received repeat plantar skin incisions (neonatal IN) or control procedures (neonatal anesthesia only, AN) at postnatal days (P)3, 10 and 17. When adult, rats with neonatal ‘pain history’ showed greater sensory sensitivity than control rats following acute plantar skin incision. Motivational behaviour in the two groups of rats was tested in a novelty-induced hypophagia (NIH) paradigm. The sensitivity of this paradigm to pain-induced changes in motivational behaviour was shown by significant increases in the time spent in the central zone of the arena (43.7±5.9% vs. 22.5±6.7%, p<0.05), close to centrally placed food treats, and decreased number of rears (9.5±1.4 vs. 19.2±2.3, p<0.001) in rats with acute plantar skin incision compared to naive, uninjured animals. Rats with a neonatal ‘pain history’ showed the same pain-induced behaviour in the novelty-induced hypophagia paradigm as controls. However, differences were observed in reward-related neural activity between the two groups. Two hours after behavioural testing, brains were harvested and neuronal activity mapped using c-Fos expression in lateral hypothalamic orexin neurons, part of a specific reward seeking pathway. Pain-induced activity in orexin neurons of control rats (18.4±2.8%) was the same as in uninjured naive animals (15.5±2.6%), but in those rats with a ‘pain history’, orexinergic activity was significantly increased (27.2±4.1%, p<0.01). Furthermore the extent of orexin neuron activation in individual rats with a ‘pain history’ was highly correlated with their motivational behaviour (r = −0.86, p = 0.01).

Conclusions/Significance

These results show that acute pain alters motivational behaviour and that neonatal pain experience causes long-term changes in brain motivational orexinergic pathways, known to modulate mesolimbic dopaminergic reward circuitry.

Orexin receptors as therapeutic drug targets

Orexin (hypocretin) receptor antagonists stand as a model for the development of targeted CNS small-molecule therapeutics. The identification of mutations in the gene for the orexin 2 receptor responsible for canine narcolepsy, the demonstration of a hypersomnolence phenotype in hypocretin knockout mice and the disruption in orexin signaling in narcoleptic patients provides clear genetic proof of concept for targeting orexin-induced arousal for the treatment of insomnia. The full characterization of the genes encoding orexin and its two cognate receptors enabled the rapid development of in vitro and ex vivo assays with which to identify lead compound structures and to optimize potency and pharmacokinetic properties. Polysomnographic measures with cross-species translatability capable of measuring the sleep-promoting effects of orexin receptor antagonists from mice to man, and the existence of knockout models not only allow efficacy assessment but also the demonstration of mechanism of action. Focused efforts by a number of groups have identified potent compounds of diverse chemical structure with differential orexin receptor selectivity for either the orexin 1 receptor (OX₁R) or the orexin 2 receptor (OX₂R), or both. This work has yielded tool compounds that, along with genetic models, have been used to specifically define the role these receptors in mediating orexin-induced arousal and vigilance state control. Optimized dual receptor antagonists with favorable pharmacokinetic and safety profiles have now demonstrated efficacy in clinical development and represent a distinct mechanism of action for the treatment of insomnia relative to current standard of care.

High pain frequency in narcolepsy with cataplexy

BACKGROUND

Narcolepsy with cataplexy (NC) is caused by the loss of hypocretin neurons. Recent studies highlighted the roles for hypocretins in the modulation of nociceptive transmission. The aims of the present multicenter case-control study were to look at the frequency of pain in NC and to study the determinants and impact of pain on narcolepsy symptoms and quality of life (QoL).

METHODS

Sixty-seven adult patients with NC, together with their physician, partner/friend, and sex- and age-matched normal controls underwent a face-to-face interview and completed questionnaires on the presence and frequency of pain, narcolepsy symptoms and QoL (Short-Form 36-item score, Functional Outcomes of Sleep Questionnaire, Medical Outcomes Study, Fatigue Severity Scale, and Beck Depression Inventory).

RESULTS

One-third (32.8%) of NC patients experienced pain at least monthly, with a significantly higher frequency and impact than controls (17.9%) and independent of the patients' narcolepsy medication. The reporting of pain was well matched between patients and partners/friends but significant differences were observed between patients and physicians, with physicians significantly underestimating its frequency and impact. The location of chronic pain varies within subjects with differences between NC and controls. We pinpointed that sleep quantity and depression were determinants for pain, and chronic pain had significant impact on sleep quantity, depression and QoL in NC.

CONCLUSION

We report, for the first time, evidence that chronic pain is significantly more common and disabling in NC compared to the general population. The findings call for improved attention to assessment and treatment of pain in the follow-up of NC.

Orexin/hypocretin: a neuropeptide at the interface of sleep, energy homeostasis, and reward system

Recent studies have implicated the orexin system as a critical regulator of sleep/wake states as well as feeding behavior and reward processes. Orexin deficiency results in narcolepsy in humans, dogs, and rodents, suggesting that the orexin system is particularly important for maintenance of wakefulness. In addition, orexin deficiency also cause abnormalities in energy homeostasis and reward systems. Orexin activates waking active monoaminergic and cholinergic neurons in the hypothalamus and brainstem regions to maintain a long, consolidated waking period. Orexin neurons receive abundant input from the limbic system. Orexin neurons also have reciprocal links with the hypothalamic arcuate nucleus, which regulates feeding. Moreover, the responsiveness of orexin neurons to peripheral metabolic cues, such as leptin and glucose, suggest that these neurons have important role as a link between the energy homeostasis and vigilance states. Orexin neurons also have a link with the dopaminergic reward system in the ventral tegmental nucleus. These findings suggest that the orexin system interacts with systems that regulate emotion, reward, and energy homeostasis to maintain proper vigilance states. Therefore, this system may be a potentially important therapeutic target for treatment of sleep disorder, obesity, emotional stress, and addiction.

Putative mechanisms of the relationship between obesity and migraine progression

Studies suggest that obesity is associated with migraine progression from an episodic into a chronic form. We discuss putative mechanisms to justify this relationship. Several of the inflammatory mediators that are increased in obese individuals are important in migraine pathophysiology, including interleukins and calcitonin gene-related peptide. Both migraine and obesity are prothrombotic states. Substances that are important in metabolic control are nociceptive at certain levels. Hypothalamic dysfunction in the orexin pathways seems to be a risk factor for both conditions. In addition, we discuss the importance of metabolic syndrome and autonomic dysfunction in modulating the obesity/migraine progression relationship.

The role of spinal orexin-1 receptors in posterior hypothalamic modulation of neuropathic pain

The posterior hypothalamus (PH) is known to reduce nociceptive pain, but the effect of PH stimulation on neuropathic pain is not known. Because neurons containing the neurotransmitter orexin-A are located in the PH in some strains of rat and intrathecal injection of orexin-A produces antinociception in a neuropathic pain model, we hypothesized that orexin-A from neurons in the PH modifies nociception in the spinal cord dorsal horn. To test this hypothesis, the cholinergic agonist carbachol or normal saline was microinjected into the PH of lightly anesthetized female Sprague-Dawley rats with chronic constriction injury (CCI) and foot withdrawal latencies (FWL) were measured. Carbachol-induced PH stimulation produced dose dependent antinociception as shown by significantly increased FWL compared to saline controls. To investigate the role of orexin-A in PH-induced antinociception, the orexin-1 receptor antagonist SB-334867 or dimethyl sulfoxide (DMSO) for control, was given intrathecally following carbachol-induced PH stimulation. SB-334867 decreased FWL compared to DMSO controls. These data are suggestive that stimulating the PH produces antinociception in a neuropathic pain model and that the antinociceptive effect is mediated in part by orexin-1 receptors in the spinal cord dorsal horn.

Orexin-A modulates glutamatergic NMDA-dependent spinal reflex potentiation via inhibition of NR2B subunit

Glucose-sensitive neurons in the lateral hypothalamic area produce orexin-A (OxA) as well as orexin-B (OxB) and send their axons to the spinal dorsal horn, which predominantly expresses orexin receptor-1 (OX-1), showing a higher sensitivity to OxA. The purpose of the present study was to assess the effects of OxA on the induction of a novel form of activity-dependent reflex potentiation, spinal reflex potentiation (SRP), in the pelvic-urethral reflex activity. External urethra sphincter electromyogram in response to pelvic afferent nerve test stimulation (TS; 1/30 Hz) or repetitive stimulation (RS; 1 Hz) was recorded in anesthetized rats. TS evoked a baseline reflex activity, whereas RS produced SRP, which was abolished by intrathecal OxA (30 nM, 10 mul). Intrathecal SB-408124 (10 muM, 10 mul), an OX-1 antagonist, reversed the abolition on SRP caused by OxA. Although there is, so far, no NR2A- and NR2B-specific agonist available, N-methyl-d-aspartate (NMDA) reversed the abolition on the RS-induced SRP caused by the co-administration of OxA and Co-101244 (30 nM, 10 mul; an NMDA NR2B subunit antagonist), but it did not reverse the abolition by the co-administration of OxA and PPPA (300 nM, 10 mul; an NMDA NR2A subunit antagonist). In conclusion, the activation of descending orexinergic fibers may inhibit the repetitive afferent input-induced central sensitization of pelvic-urethral reflex activity and urethra hyperactivity, indicating that spinal orexinergic neural transmission may be a novel target for the treatment of patients with neuropathetic or postinflammatory pain of pelvic origin.

Hypocretin/orexin and nociceptin/orphanin FQ coordinately regulate analgesia in a mouse model of stress-induced analgesia

Stress-induced analgesia (SIA) is a key component of the defensive behavioral "fight-or-flight" response. Although the neural substrates of SIA are incompletely understood, previous studies have implicated the hypocretin/orexin (Hcrt) and nociceptin/orphanin FQ (N/OFQ) peptidergic systems in the regulation of SIA. Using immunohistochemistry in brain tissue from wild-type mice, we identified N/OFQ-containing fibers forming synaptic contacts with Hcrt neurons at both the light and electron microscopic levels. Patch clamp recordings in GFP-tagged mouse Hcrt neurons revealed that N/OFQ hyperpolarized, decreased input resistance, and blocked the firing of action potentials in Hcrt neurons. N/OFQ postsynaptic effects were consistent with opening of a G protein-regulated inwardly rectifying K+ (GIRK) channel. N/OFQ also modulated presynaptic release of GABA and glutamate onto Hcrt neurons in mouse hypothalamic slices. Orexin/ataxin-3 mice, in which the Hcrt neurons degenerate, did not exhibit SIA, although analgesia was induced by i.c.v. administration of Hcrt-1. N/OFQ blocked SIA in wild-type mice, while coadministration of Hcrt-1 overcame N/OFQ inhibition of SIA. These results establish what is, to our knowledge, a novel interaction between the N/OFQ and Hcrt systems in which the corticotropin-releasing factor and N/OFQ systems coordinately modulate the Hcrt neurons to regulate SIA.