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

The structural and functional complexity of the integrative hypothalamus

The hypothalamus ("hypo" meaning below, and "thalamus" meaning bed) consists of regulatory circuits that support basic life functions that ensure survival. Sitting at the interface between peripheral, environmental, and neural inputs, the hypothalamus integrates these sensory inputs to influence a range of physiologies and behaviors. Unlike the neocortex, in which a stereotyped cytoarchitecture mediates complex functions across a comparatively small number of neuronal fates, the hypothalamus comprises upwards of thousands of distinct cell types that form redundant yet functionally discrete circuits. With single-cell RNA sequencing studies revealing further cellular heterogeneity and modern photonic tools enabling high-resolution dissection of complex circuitry, a new era of hypothalamic mapping has begun. Here, we provide a general overview of mammalian hypothalamic organization, development, and connectivity to help welcome newcomers into this exciting field.

Hypothalamic orexinergic neurons modulate pain and itch in an opposite way: pain relief and itch exacerbation

Pain and itch are recognized as antagonistic sensations; pain suppresses itch and inhibition of pain generates itch. There is still a lack of evidence about the neural mechanism of the interaction between pain and itch in the central nervous system. In this study, we focused on the orexin (ORX) neurons in the lateral hypothalamus (LH), which mediate various "defense responses" when animals confront stressors. We found that the scratching behaviors induced by the pruritogen were significantly suppressed in ORX-neuron-ablated (ORX-abl) mice. The exaggerated pain behavior and attenuated itch behavior observed in ORX-abl mice indicated that ORX neurons modulate pain and itch in an opposite way, i.e., pain relief and itch exacerbation. In addition, most of the ORX neurons responded to both pain and itch input. Our results suggest that ORX neurons inversely regulate pain- and itch-related behaviors, which could be understood as a defense response to cope with stress environment.

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.

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.

Significant Quantitative Differences in Orexin Neuronal Activation After Pain Assessments in an Animal Model of Sickle Cell Disease

Sickle cell disease is a hemoglobinopathy that causes sickling of red blood cells, resulting in vessel blockage, stroke, anemia, inflammation, and extreme pain. The development and treatment of pain, in particular, neuropathic pain in sickle cell disease patients is poorly understood and impedes our progress toward the development of novel therapies to treat pain associated with sickle cell disease. The orexin/hypocretin system offers a novel approach to treat chronic pain and hyperalgesia. These neuropeptides are synthesized in three regions: perifornical area (PFA), lateral hypothalamus (LH), and dorsomedial hypothalamus (DMH). Data suggest that orexin-A neuropeptide has an analgesic effect on inflammatory pain and may affect mechanisms underlying the maintenance of neuropathic pain. The purpose of this study was to determine whether there are neuronal activation differences in the orexin system as a result of neuropathic pain testing in a mouse model of sickle cell disease. Female transgenic sickle mice that express exclusively (99%) human sickle hemoglobin (HbSS-BERK) and age-/gender-matched controls (HbAA-BERK mice; n = 10/group, 20-30 g) expressing normal human hemoglobin A were habituated to each test protocol and environment before collecting baseline measurements and testing. Four measures were used to assess pain-related behaviors: thermal/heat hyperalgesia, cold hyperalgesia, mechanical hyperalgesia, and deep-tissue hyperalgesia. Hypothalamic brain sections from HbAA-BERK and HbSS-BERK mice were processed to visualize orexin and c-Fos immunoreactivity and quantified. The percentage of double labeled neurons in the PFA was significantly higher than the percentage of double labeled neurons in the LH orexin field of HbAA-BERK mice (* p < 0.05). The percentages of double labeled neurons in PFA and DMH orexin fields are significantly higher than those neurons in the LH of HbSS-BERK mice (* p < 0.05). These data suggest that DMH orexin neurons were preferentially recruited during neuropathic pain testing and a more diverse distribution of orexin neurons may be required to produce analgesia in response to pain in the HbSS-BERK mice. Identifying specific orexin neuronal populations that are integral in neuropathic pain processing will allow us to elucidate mechanisms that provide a more selective, targeted approach in treating of neuropathic pain in sickle cell disease.

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.

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.

Murine hypothalamic destruction with vascular cell apoptosis subsequent to combined administration of human papilloma virus vaccine and pertussis toxin

Vaccination is the most powerful way to prevent human beings from contracting infectious diseases including viruses. In the case of the human papillomavirus (HPV) vaccine, an unexpectedly novel disease entity, HPV vaccination associated neuro-immunopathetic syndrome (HANS), has been reported and remains to be carefully verified. To elucidate the mechanism of HANS, we applied a strategy similar to the active experimental autoimmune encephalitis (EAE) model - one of the most popular animal models used to induce maximum immunological change in the central nervous system. Surprisingly, mice vaccinated with pertussis toxin showed neurological phenotypes that include low responsiveness of the tail reflex and locomotive mobility. Pathological analyses revealed the damage to the hypothalamus and circumventricular regions around the third ventricle, and these regions contained apoptotic vascular endothelial cells. These data suggested that HPV-vaccinated donners that are susceptible to the HPV vaccine might develop HANS under certain environmental factors. These results will give us the new insight into the murine pathological model of HANS and help us to find a way to treat of patients suffering from HANS.

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.

Involvement of the orexin/hypocretin system in the pharmacological effects induced by Δ(9) -tetrahydrocannabinol

BACKGROUND AND PURPOSE

Anatomical, biochemical and pharmacological evidence suggest the existence of a crosstalk between the orexinergic and endocannabinoid systems. While the orexin receptor 1 (OX1 receptor) modulates the reinforcing properties of cannabinoids, the participation of orexins in the acute pharmacological effects of Δ(9) -tetrahydrocannabinol (THC) remains unexplored.

EXPERIMENTAL APPROACH

We assessed the possible role of orexins in THC-induced hypolocomotion, hypothermia, antinociception, anxiolytic- and anxiogenic-like effects and memory impairment. Selective OX1 and OX2 receptor antagonists and OX1 knockout (KO) mice as well as prepro-orexin (PPO) KO mice were used as pharmacological and genetic approaches. CB1 receptor levels in control and PPO KO mice were evaluated by immunoblot analysis. The expression of c-Fos after THC treatment was analysed in several brain areas in wild-type mice and in mice lacking the PPO gene.

KEY RESULTS

The hypothermia, supraspinal antinociception and anxiolytic-like effects induced by THC were modulated by orexins through OX2 receptor signalling. OX1 receptors did not seem to be involved in these THC responses. No differences in CB1 receptor levels were found between wild-type and PPO KO mice. THC-induced increase in c-Fos expression was reduced in the central amygdala, medial preoptic area and lateral septum in these mutant mice.

CONCLUSIONS AND IMPLICATIONS

Our results provide new findings to further clarify the interaction between orexins and cannabinoids. OX1 and OX2 receptors are differently implicated in the pharmacological effects of cannabinoids.

Intra-accumbal Orexin-1 Receptors are Involved in Antinociception Induced by Stimulation of the Lateral Hypothalamus in the Formalin Test as an Animal Model of Persistent Inflammatory Pain

Orexin, mainly produced by orexin-expressing neurons in the lateral hypothalamus (LH), plays an important role in pain modulation. Moreover, it is shown that the nucleus accumbens (NAc) is one of the important areas involved in this modulation. Orexin-1 (OX1) and orexin-2 (OX2) receptors are densely distributed in the NAc. The study investigated the involvement of OX1 receptors in the NAc on antinociception induced by intra-LH administration of carbachol in formalin test. Rats were unilaterally implanted by two separate cannulae into the LH and NAc. Different doses of SB334867, as an OX1 receptor antagonist, were microinjected into the NAc (1, 3 and 10 nM/0.5 µL DMSO) prior to intra-LH carbachol injection (250 nM/0.5 µL saline). Formalin test was applied as an animal model of persistent inflammatory pain. The animals received a subcutaneous injection of formalin into the hind paw, 5 min after SB334867 administration. Pain scores were calculated at 5-min blocks for a 60-min test period. Results showed that the administration of SB334867 into the NAc decreased LH chemical stimulation-induced antinociception dose-dependently in early and second phase of formalin test. Our findings showed that OX1 receptors in the NAc may be involved in modulation of inflammatory pain.

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.

The hypocretin/orexin system: an increasingly important role in neuropsychiatry

Hypocretins, also named as orexins, are excitatory neuropeptides secreted by neurons specifically located in lateral hypothalamus and perifornical areas. Orexinergic fibers are extensively distributed in various brain regions and involved in a number of physiological functions, such as arousal, cognition, stress, appetite, and metabolism. Arousal is the most important function of orexin system as dysfunction of orexin signaling leads to narcolepsy. In addition to narcolepsy, orexin dysfunction is associated with serious neural disorders, including addiction, depression, and anxiety. However, some results linking orexin with these disorders are still contradictory, which may result from differences of detection methods or the precision of tools used in measurements; strategies targeted to orexin system (e.g., antagonists to orexin receptors, gene delivery, and cell transplantation) are promising new tools for treatment of neuropsychiatric disorders, though studies are still in a stage of preclinical or clinical research.

Differences in carbachol dose, pain condition, and sex following lateral hypothalamic stimulation

Lateral hypothalamic (LH) stimulation produces antinociception in female rats in acute, nociceptive pain. Whether this effect occurs in neuropathic pain or whether male-female sex differences exist is unknown. We examined the effect of LH stimulation in male and female rats using conditions of nociceptive and neuropathic pain. Neuropathic groups received chronic constriction injury (CCI) to induce thermal hyperalgesia, a sign of neuropathic pain. Nociceptive rats were naive for CCI, but received the same thermal stimulus following LH stimulation. To demonstrate that CCI ligation produced thermal hyperalgesia, males and females received either ligation or sham surgery for control. Both males and females demonstrated significant thermal hyperalgesia following CCI ligation (p<0.05), but male sham surgery rats also showed a significant left-right difference not present in female sham rats. In the second experiment, rats randomly assigned to CCI or nociceptive groups were given one of three doses of the cholinergic agonist carbachol (125, 250, or 500 nmol) or normal saline for control, microinjected into the left LH. Paw withdrawal from a thermal stimulus (paw withdrawal latency; PWL) was measured every 5 min for 45 min. Linear mixed models analysis showed that males and females in both pain conditions demonstrated significant antinociception, with the 500-nmol dose producing the greatest effect across groups compared with controls for the left paw (p<0.05). Female CCI rats showed equivalent responses to the three doses, while male CCI rats showed more variability for dose. However, nociceptive females responded only to the 500-nmol dose, while nociceptive males responded to all doses (p<0.05). For right PWL, only nociceptive males showed a significant carbachol dose response. These findings are suggestive that LH stimulation produces antinociception in male and female rats in both nociceptive and neuropathic pain, but dose response differences exist based on sex and pain condition.

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.

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.

Characterizing activity and muscle atrophy changes in rats with neuropathic pain: a pilot study

The study of neuropathic pain has focused on changes within the nervous system, but little research has described systemic changes that may accompany neuropathic pain.

OBJECTIVE

As part of a larger project characterizing the metabolic, activity, and musculoskeletal changes associated with neuropathic pain, the objective of the current study was to characterize changes in spontaneous activity and skeletal muscle mass using an established animal model of neuropathic pain, the chronic constriction injury (CCI) model.

METHOD

Male Sprague-Dawley rats were used in this pre- and posttest quasi-experimental study. The experimental group (n = 13) received CCI surgery, while age- and weight-matched rats received sham surgery (SHAM; n = 5). Thermal testing verified the presence of neuropathic pain. Spontaneous cage activity was measured gravimetrically prior to and following CCI (n = 4). Animals were euthanized and skeletal muscle was dissected and weighed to determine muscle atrophy.

RESULTS

Shorter foot withdrawal latency of the ipsilateral hind limb confirmed the presence of thermal hyperalgesia in CCI rats, a sign of neuropathic pain. Weight increased in both CCI and SHAM rats. Spontaneous activity decreased following CCI ligation. Muscles of the ipsilateral hind limb weighed significantly less than contralateral hind limb muscles in CCI rats 2 and 6 weeks after surgery. In addition, CCI rats had smaller ipsilateral hind limb muscles than SHAM rats.

CONCLUSION

Neuropathic pain contributes to skeletal muscle atrophy and decreases in activity in rats.

The posterior hypothalamus exerts opposing effects on nociception via the A7 catecholamine cell group in rats

Stimulation of the posterior hypothalamic area (PH) produces antinociception in rats and humans, but the precise mechanisms are unknown. The PH forms anatomical connections with the parabrachial area, which contains the pontine A7 catecholamine cell group, a group of spinally projecting noradrenergic neurons known to produce antinociception in the dorsal horn. The aim of the present study was to determine whether PH-induced antinociception is mediated in part through connections with the A7 cell group in female Sprague-Dawley rats, as measured by the tail flick and foot withdrawal latency. Stimulation of the PH with the cholinergic agonist carbachol (125 nmol) produced antinociception that was blocked by pretreatment with atropine sulfate. Intrathecal injection of the α(2)-adrenoceptor antagonist yohimbine reversed PH-induced antinociception, but the α(1)-adrenoceptor antagonist WB4101 facilitated antinociception. Intrathecal injection of normal saline had no effect. In a separate experiment, cobalt chloride, which reversibly arrests synaptic activity, was microinjected into the A7 cell group and blocked PH-induced antinociception. These findings provide evidence that the PH modulates nociception in part through connections with the A7 catecholamine cell group through opposing effects. Antinociception occurs from actions at α(2)-adrenoceptors in the dorsal horn, while concurrent hyperalgesia occurs from actions of norepinephrine at α(1)-adrenoceptors. This hyperalgesic response likely attenuates antinociception from PH stimulation.

Spinal cord stimulation modulates cerebral function: an fMRI study

INTRODUCTION

Although spinal cord stimulation (SCS) is widely used for chronic neuropathic pain after failed spinal surgery, little is known about the underlying physiological mechanisms. This study aims to investigate the neural substrate underlying short-term (30 s) SCS by means of functional magnetic resonance imaging in 20 patients with failed back surgery syndrome (FBSS).

METHODS

Twenty patients with FBSS, treated with externalized SCS, participated in a blocked functional magnetic resonance imaging design with stimulation and rest phases of 30 s each, repeated eight times in a row. During scanning, patients rated pain intensity over time using an 11-point numerical rating scale with verbal anchors (0 = no pain at all to 10 = worst pain imaginable) by pushing buttons (left hand, lesser pain; right hand, more pain). This scale was back projected to the patients on a flat screen allowing them to manually direct the pain indicator. To increase the signal-to-noise ratio, the 8-min block measurements were repeated three times.

RESULTS

Marked deactivation of the bilateral medial thalamus and its connections to the rostral and caudal cingulate cortex and the insula was found; the study also showed immediate pain relief obtained by short-term SCS correlated negatively with activity in the inferior olivary nucleus, the cerebellum, and the rostral anterior cingulate cortex.

CONCLUSIONS

Results indicate the key role of the medial thalamus as a mediator and the involvement of a corticocerebellar network implicating the modulation and regulation of averse and negative affect related to pain. The observation of a deactivation of the ipsilateral antero-medial thalamus might be used as a region of interest for further response SCS studies.

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.

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.

Putting the bio in biobehavioral: animal models

Animal models are useful in research that examines physiological mechanisms and, as such, are invaluable in developing therapies to alleviate illness and promote health. Ethical considerations are essential for proper animal use and include replacement by nonanimal models where possible, reduction in the numbers of animals used, and refinement of experimental protocols to reduce animal suffering. Choosing the optimum model depends on the long- and short-term goals of the project, and the choice of a model goes hand in hand with appropriate study design. Five key features to think about when choosing a model are as follows: model asymmetry, necessary differences, specificity to the study, model validity, and model improvement. Appropriate use of both male and female animals has also become an important issue in recent times. These considerations will assist in understanding animal model use.