Pharmacogenomic study of the role of the nociceptin/orphanin FQ receptor and opioid receptors in diabetic hyperalgesia

Role of Spinal Cholecystokinin Octapeptide, Nociceptin/Orphanin FQ, and Hemokinin-1 in Diabetic Allodynia

A complication of diabetes is neuropathic pain, which is difficult to control with medication. We have confirmed that neuropathic pain due to mechanical allodynia in diabetic mice is mediated by a characteristic neuropeptide in the spinal cord. We evaluated the strength of mechanical allodynia in mice using von Frey filaments. When mice were intravenously injected with streptozotocin, mechanical allodynia appeared 3 days later. Antibodies of representative neuropeptides were intrathecally (i.t.) administered to allodynia-induced mice 7 days after the intravenous administration of streptozotocin, and allodynia was reduced by anti-cholecystokinin octapeptide antibodies, anti-nociceptin/orphanin FQ antibodies, and anti-hemokinin-1 antibodies. In contrast, i.t.-administered anti-substance P antibodies, anti-somatostatin antibodies, and anti-angiotensin II antibodies did not affect streptozotocin-induced diabetic allodynia mice. Mechanical allodynia was attenuated by the i.t. administration of CCK-B receptor antagonists and ORL-1 receptor antagonists. The mRNA level of CCK-B receptors in streptozotocin-induced diabetic allodynia mice increased in the spinal cord, but not in the dorsal root ganglion. These results indicate that diabetic allodynia is caused by cholecystokinin octapeptide, nociceptin/orphanin FQ, and hemokinin-1 released from primary afferent neurons in the spinal cord that transmit pain to the brain via the spinal dorsal horn.

Naturally Inspired Molecules for Neuropathic Pain Inhibition-Effect of Mirogabalin and Cebranopadol on Mechanical and Thermal Nociceptive Threshold in Mice

BACKGROUND

Neuropathic pain is drug-resistant to available analgesics and therefore novel treatment options for this debilitating clinical condition are urgently needed. Recently, two drug candidates, namely mirogabalin and cebranopadol have become a subject of interest because of their potential utility as analgesics for chronic pain treatment. However, they have not been investigated thoroughly in some types of neuropathic pain, both in humans and experimental animals.

METHODS

This study used the von Frey test, the hot plate test and the two-plate thermal place preference test supported by image analysis and machine learning to assess the effect of intraperitoneal mirogabalin and subcutaneous cebranopadol on mechanical and thermal nociceptive threshold in mouse models of neuropathic pain induced by streptozotocin, paclitaxel and oxaliplatin.

RESULTS

Mirogabalin and cebranopadol effectively attenuated tactile allodynia in models of neuropathic pain induced by streptozotocin and paclitaxel. Cebranopadol was more effective than mirogabalin in this respect. Both drugs also elevated the heat nociceptive threshold in mice. In the oxaliplatin model, cebranopadol and mirogabalin reduced cold-exacerbated pain.

CONCLUSIONS

Since mirogabalin and cebranopadol are effective in animal models of neuropathic pain, they seem to be promising novel therapies for various types of neuropathic pain in patients, in particular those who are resistant to available analgesics.

Identification of significant gene biomarkers of low back pain caused by changes in the osmotic pressure of nucleus pulposus cells

The incidence of intervertebral disc (IVD) degeneration disease, caused by changes in the osmotic pressure of nucleus pulposus (NP) cells, increases with age. In general, low back pain is associated with IVD degeneration. However, the mechanism and molecular target of low back pain have not been elucidated, and there are no data suggesting specific biomarkers of low back pain. Therefore, the research aims to identify and verify the significant gene biomarkers of low back pain. The differentially expressed genes (DEGs) were screened in the Gene Expression Omnibus (GEO) database, and the identification and analysis of significant gene biomarkers were also performed with various bioinformatics programs. A total of 120 patients with low back pain were recruited. Before surgery, the degree of pain was measured by the numeric rating scale (NRS), which enables comparison of the pain scores from individuals. After surgery, IVD tissues were obtained, and NP cells were isolated. The NP cells were cultured in two various osmotic media, including iso-osmotic media (293 mOsm/kg H₂O) to account for the morbid environment of NP cells in IVD degeneration disease and hyper-osmotic media (450 mOsm/kg H₂O) to account for the normal condition of NP cells in healthy individuals. The relative mRNA expression levels of CCL5, OPRL1, CXCL13, and SST were measured by quantitative real-time PCR in the in vitro analysis of the osmotic pressure experiments. Finally, correlation analysis and a neural network module were employed to explore the linkage between significant gene biomarkers and pain. A total of 371 DEGs were identified, including 128 downregulated genes and 243 upregulated genes. Furthermore, the four genes (CCL5, OPRL1, SST, and CXCL13) were identified as significant gene biomarkers of low back pain (P < 0.001) based on univariate linear regression, and CCL5 (odds ratio, 34.667; P = 0.003) and OPRL1 (odds ratio, 19.875; P < 0.001) were significantly related to low back pain through multivariate logistic regression. The expression of CCL5 and OPRL1 might be correlated with low back pain in patients with IVD degeneration disease caused by changes in the osmotic pressure of NP cells.

Efficacy-Based Perspective to Overcome Reduced Opioid Analgesia of Advanced Painful Diabetic Neuropathy in Rats

Reduction of the opioid analgesia in diabetic neuropathic pain (DNP) results from μ-opioid receptor (MOR) reserve reduction. Herein, we examined the antinociceptive and antiallodynic actions of a novel opioid agonist 14-O-methymorphine-6-O-sulfate (14-O-MeM6SU), fentanyl and morphine in rats with streptozocin-evoked DNP of 9–12 weeks following their systemic administration. The antinociceptive dose-response curve of morphine but not of 14-O-MeM6SU or fentanyl showed a significant right-shift in diabetic compared to non-diabetic rats. Only 14-O-MeM6SU produced antiallodynic effects in doses matching antinociceptive doses obtained in non-diabetic rats. Co-administered naloxone methiodide (NAL-M), a peripherally acting opioid receptor antagonist failed to alter the antiallodynic effect of test compounds, indicating the contribution of central opioid receptors. Reduction in spinal MOR binding sites and loss in MOR immunoreactivity of nerve terminals in the spinal cord and dorsal root ganglia in diabetic rats were observed. G-protein coupling assay revealed low efficacy character for morphine and high efficacy character for 14-O-MeM6SU or fentanyl at spinal or supraspinal levels (E_max values). Furthermore, at the spinal level only 14-O-MeM6SU showed equal efficacy in G-protein activation in tissues of diabetic- and non-diabetic animals. Altogether, the reduction of spinal opioid receptors concomitant with reduced analgesic effect of morphine may be circumvented by using high efficacy opioids, which provide superior analgesia over morphine. In conclusion, the reduction in the analgesic action of opioids in DNP might be a consequence of MOR reduction, particularly in the spinal cord. Therefore, developing opioids of high efficacy might provide analgesia exceeding that of currently available opioids.

Core Outcome Measures in Preclinical Assessment of Candidate Analgesics

All preclinical procedures for analgesic drug discovery involve two components: 1) a "pain stimulus" (the principal independent variable), which is delivered to an experimental subject with the intention of producing a pain state; and 2) a "pain behavior" (the principal dependent variable), which is measured as evidence of that pain state. Candidate analgesics are then evaluated for their effectiveness to reduce the pain behavior, and results are used to prioritize drugs for advancement to clinical testing. This review describes a taxonomy of preclinical procedures organized into an "antinociception matrix" by reference to their types of pain stimulus (noxious, inflammatory, neuropathic, disease related) and pain behavior (unconditioned, classically conditioned, operant conditioned). Particular emphasis is devoted to pain behaviors and the behavioral principals that govern their expression, pharmacological modulation, and preclinical-to-clinical translation. Strengths and weaknesses are compared and contrasted for procedures using each type of behavioral outcome measure, and the following four recommendations are offered to promote strategic use of these procedures for preclinical-to-clinical analgesic drug testing. First, attend to the degree of homology between preclinical and clinical outcome measures, and use preclinical procedures with behavioral outcome measures homologous to clinically relevant outcomes in humans. Second, use combinations of preclinical procedures with complementary strengths and weaknesses to optimize both sensitivity and selectivity of preclinical testing. Third, take advantage of failed clinical translation to identify drugs that can be back-translated preclinically as active negative controls. Finally, increase precision of procedure labels by indicating both the pain stimulus and the pain behavior in naming preclinical procedures.

Synergistic interaction between the agonism of cebranopadol at nociceptin/orphanin FQ and classical opioid receptors in the rat spinal nerve ligation model

Cebranopadol (trans-6'-fluoro-4',9'-dihydro-N,N-dimethyl-4-phenyl-spiro[cyclohexane-1,1'(3'H)-pyrano[3,4-b]indol]-4-amine) is a novel analgesic nociceptin/orphanin FQ opioid peptide (NOP) and classical opioid receptor (MOP, DOP, and KOP) agonist with highly efficacious and potent activity in a broad range of rodent models of nociceptive, inflammatory, and neuropathic pain as well as limited opioid-type side effects such as respiratory depression. This study was designed to explore contribution and interaction of NOP and classical opioid receptor agonist components to cebranopadol analgesia in the rat spinal nerve ligation (SNL) model. Assessing antihypersensitive activity in SNL rats intraperitoneal (IP) administration of cebranopadol resulted in ED 50 values of 3.3 and 3.58 μg/kg in two independent experiments. Pretreatment (IP) with J-113397 (4.64 mg/kg) a selective antagonist for the NOP receptor or naloxone (1 mg/kg), naltrindole (10 mg/kg), or nor-BNI (10 mg/kg), selective antagonists for MOP, DOP, and KOP receptors, yielded ED 50 values of 14.1, 16.9, 17.3, and 15 μg/kg, respectively. This 4-5 fold rightward shift of the dose-response curves suggested agonistic contribution of all four receptors to the analgesic activity of cebranopadol. Combined pretreatment with a mixture of the antagonists for the three classical opioid receptors resulted in an 18-fold potency shift with an ED 50 of 65.5 μg/kg. The concept of dose equivalence was used to calculate the expected additive effects of the parent compound for NOP and opioid receptor contribution and to compare them with the observed effects, respectively. This analysis revealed a statistically significant difference between the expected additive and the observed effects suggesting intrinsic synergistic analgesic interaction of the NOP and the classical opioid receptor components of cebranopadol. Together with the observation of limited respiratory depression in rats and humans the synergistic interaction of NOP and classical opioid receptor components in analgesia described in the current study may contribute to the favorable therapeutic index of cebranopadol observed in clinical trials.

Usefulness of knockout mice to clarify the role of the opioid system in chronic pain

Several lines of knockout mice deficient in the genes encoding each component of the endogenous opioid system have been used for decades to clarify the specific role of the different opioid receptors and peptide precursors in many physiopathological conditions. The use of these genetically modified mice has improved our knowledge of the specific involvement of each endogenous opioid component in nociceptive transmission during acute and chronic pain conditions. The present review summarizes the recent advances obtained using these genetic tools in understanding the role of the opioid system in the pathophysiological mechanisms underlying chronic pain. Behavioural data obtained in these chronic pain models are discussed considering the peculiarities of the behavioural phenotype of each line of knockout mice. These studies have identified the crucial role of specific components of the opioid system in different manifestations of chronic pain and have also opened new possible therapeutic approaches, such as the development of opioid compounds simultaneously targeting several opioid receptors. However, several questions still remain open and require further experimental effort to be clarified. The novel genetic tools now available to manipulate specific neuronal populations and precise genome editing in mice will facilitate in a near future the elucidation of the role of each component of the endogenous opioid system in chronic pain.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Bifunctional opioid/nociceptin hybrid KGNOP1 effectively attenuates pain-related behaviour in a rat model of neuropathy

A bifunctional peptide containing an opioid and nociceptin receptor-binding pharmacophore, H-Dmt-D-Arg-Aba-β-Ala-Arg-Tyr-Tyr-Arg-Ile-Lys-NH2 (KGNOP1), was tested for its analgesic properties when administered intrathecally in naïve and chronic constriction injury (CCI)-exposed rats with neuropathy-like symptoms. KGNOP1 significantly increased the acute pain threshold, as measured by the tail-flick test, and also increased the threshold of a painful reaction to mechanical and thermal stimuli in CCI-exposed rats. Both of the effects could be blocked by pre-administration of [Nphe1]-Nociceptin (1-13)-NH2 (NPhe) or naloxone, antagonists for nociceptin and opioid receptors, respectively. This led us to conclude that KGNOP1 acts as a dual opioid and nociceptin receptor agonist in vivo. The analgesic effect of KGNOP1 proved to be more powerful than clinical drugs such as morphine and buprenorphine. Repeated daily intrathecal injections of KGNOP1 led to the development of analgesic tolerance, with the antiallodynic action being completely abolished on day 6. Nevertheless, the development of tolerance to the antihyperalgesic effect was delayed in comparison to morphine, which lost its efficacy as measured by the cold plate test after 3days of daily intrathecal administration, whereas KGNOP1 was efficient up to day 6. A single intrathecal injection of morphine to KGNOP1-tolerant rats did not raise the pain threshold in any of the behavioural tests; in contrast, a single intrathecal dose of KGNOP1 significantly suppressed allodynia and hyperalgesia in morphine-tolerant rats.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).