The contribution of Gi/o protein to opioid antinociception in an oxaliplatin-induced neuropathy rat model

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

Review of the Role of the Brain in Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a common, debilitating, and dose-limiting side effect of many chemotherapy regimens yet has limited treatments due to incomplete knowledge of its pathophysiology. Research on the pathophysiology of CIPN has focused on peripheral nerves because CIPN symptoms are felt in the hands and feet. However, better understanding the role of the brain in CIPN may accelerate understanding, diagnosing, and treating CIPN. The goals of this review are to (1) investigate the role of the brain in CIPN, and (2) use this knowledge to inform future research and treatment of CIPN. We identified 16 papers using brain interventions in animal models of CIPN and five papers using brain imaging in humans or monkeys with CIPN. These studies suggest that CIPN is partly caused by (1) brain hyperactivity, (2) reduced GABAergic inhibition, (3) neuroinflammation, and (4) overactivation of GPCR/MAPK pathways. These four features were observed in several brain regions including the thalamus, periaqueductal gray, anterior cingulate cortex, somatosensory cortex, and insula. We discuss how to leverage this knowledge for future preclinical research, clinical research, and brain-based treatments for CIPN.

Therapeutic Agents for Oxaliplatin-Induced Peripheral Neuropathy; Experimental and Clinical Evidence

Oxaliplatin is an essential drug in the chemotherapy of colorectal, gastric, and pancreatic cancers, but it frequently causes peripheral neuropathy as a dose-limiting factor. So far, animal models of oxaliplatin-induced peripheral neuropathy have been established. The mechanisms of development of neuropathy induced by oxaliplatin have been elucidated, and many drugs and agents have been proven to have neuroprotective effects in basic studies. In addition, some of these drugs have been validated in clinical studies for their inhibitory effects on neuropathy. In this review, we summarize the basic and clinical evidence for the therapeutic effects of oxaliplatin. In basic research, there are many reports of neuropathy inhibitors that target oxidative stress, inflammatory response, sodium channel, transient receptor potential (TRP) channel, glutamate nervous system, and monoamine nervous system. Alternatively, very few drugs have clearly demonstrated the efficacy for oxaliplatin-induced peripheral neuropathy in clinical trials. It is important to activate translational research in order to translate basic research into clinical research.

Therapeutic Agents for Oxaliplatin-Induced Peripheral Neuropathy; Experimental and Clinical Evidence

Oxaliplatin is an essential drug in the chemotherapy of colorectal, gastric, and pancreatic cancers, but it frequently causes peripheral neuropathy as a dose-limiting factor. So far, animal models of oxaliplatin-induced peripheral neuropathy have been established. The mechanisms of development of neuropathy induced by oxaliplatin have been elucidated, and many drugs and agents have been proven to have neuroprotective effects in basic studies. In addition, some of these drugs have been validated in clinical studies for their inhibitory effects on neuropathy. In this review, we summarize the basic and clinical evidence for the therapeutic effects of oxaliplatin. In basic research, there are many reports of neuropathy inhibitors that target oxidative stress, inflammatory response, sodium channel, transient receptor potential (TRP) channel, glutamate nervous system, and monoamine nervous system. Alternatively, very few drugs have clearly demonstrated the efficacy for oxaliplatin-induced peripheral neuropathy in clinical trials. It is important to activate translational research in order to translate basic research into clinical research.

Therapeutic Agents for Oxaliplatin-Induced Peripheral Neuropathy; Experimental and Clinical Evidence

Oxaliplatin is an essential drug in the chemotherapy of colorectal, gastric, and pancreatic cancers, but it frequently causes peripheral neuropathy as a dose-limiting factor. So far, animal models of oxaliplatin-induced peripheral neuropathy have been established. The mechanisms of development of neuropathy induced by oxaliplatin have been elucidated, and many drugs and agents have been proven to have neuroprotective effects in basic studies. In addition, some of these drugs have been validated in clinical studies for their inhibitory effects on neuropathy. In this review, we summarize the basic and clinical evidence for the therapeutic effects of oxaliplatin. In basic research, there are many reports of neuropathy inhibitors that target oxidative stress, inflammatory response, sodium channel, transient receptor potential (TRP) channel, glutamate nervous system, and monoamine nervous system. Alternatively, very few drugs have clearly demonstrated the efficacy for oxaliplatin-induced peripheral neuropathy in clinical trials. It is important to activate translational research in order to translate basic research into clinical research.

Opioid receptors mRNAs expression and opioids agonist-dependent G-protein activation in the rat brain following neuropathy

Potent opioid-based therapies are often unsuccessful in promoting satisfactory analgesia in neuropathic pain. Moreover, the side effects associated with opioid therapy are still manifested in neuropathy-like diseases, including tolerance, abuse, addiction and hyperalgesia, although the mechanisms underlying these effects remain unclear. Studies in the spinal cord and periphery indicate that neuropathy alters the expression of mu-[MOP], delta-[DOP] or kappa-[KOP] opioid receptors, interfering with their activity. However, there is no consensus as to the supraspinal opioidergic modulation provoked by neuropathy, the structures where the sensory and affective-related pain components are processed. In this study we explored the effect of chronic constriction of the sciatic nerve (CCI) over 7 and 30 days (CCI-7d and CCI-30d, respectively) on MOP, DOP and KOP mRNAs expression, using in situ hybridization, and the efficacy of G-protein stimulation by DAMGO, DPDPE and U-69593 (MOP, DOP and KOP specific agonists, respectively), using [35S]GTPγS binding, within opioid-sensitive brain structures. After CCI-7d, CCI-30d or both, opioid receptor mRNAs expression was altered throughout the brain: MOP - in the paracentral/centrolateral thalamic nuclei, ventral posteromedial thalamic nuclei, superior olivary complex, parabrachial nucleus [PB] and posterodorsal tegmental nucleus; DOP - in the somatosensory cortex [SSC], ventral tegmental area, caudate putamen [CPu], nucleus accumbens [NAcc], raphe magnus [RMg] and PB; and KOP - in the locus coeruleus. Agonist-stimulated [35S]GTPγS binding was altered following CCI: MOP - CPu and RMg; DOP - prefrontal cortex [PFC], SSC, RMg and NAcc; and KOP - PFC and SSC. Thus, this study shows that several opioidergic circuits in the brain are recruited and modified following neuropathy.

Efficacy of the World Health Organization analgesic ladder in the paclitaxel-induced pain syndrome in rats

Paclitaxel use in cancer treatment is limited by a painful syndrome that has no effective treatment. Despite new therapies, drugs of the World Health Organization (WHO) analgesic ladder remain a useful therapeutic tool for cancer pain relief. Since cancer pain is caused by both tumor and chemotherapy, we assessed the efficacy of drugs from the WHO analgesic ladder for cancer pain relief in a paclitaxel-induced pain syndrome (P-IPS) model. P-IPS was induced in rats by one or four injections of paclitaxel on alternate days. The acute and chronic phases were assessed 24 h and 15 days after the first paclitaxel injection, respectively. The mechanical allodynia was evaluated after (step 1 of the ladder) paracetamol, (step 2) codeine alone or plus paracetamol and (step 3) morphine treatment in the acute or chronic phase of P-IPS. Paracetamol, codeine and morphine were equally efficacious in reducing the acute phase of the P-IPS. Codeine plus paracetamol had similar efficacy and potency when administered together in the acute phase of the P-IPS, but produced a longer-lasting effect than when separately managed. Moreover, paracetamol, codeine and morphine partially reduced the chronic phase of P-IPS, losing their efficacy and, in the case of codeine, potency when compared to the acute phase. However, paracetamol plus codeine increased the potency and efficacy of the codeine when compared to codeine administered alone in the chronic phase of P-IPS, producing a long-lasting anti-allodynic effect. Together, analgesics of WHO analgesic ladder reduce both acute and chronic phases of P-IPS, with codeine plus paracetamol presenting more potent, efficacious and long-lasting effect. Thus, in addition to tumor pain, drugs of WHO analgesics ladder could also be useful to treat P-IPS.

The effect of analgesics on stimulus evoked pain-like behaviour in animal models for chemotherapy induced peripheral neuropathy- a meta-analysis

Chemotherapy induced painful peripheral neuropathy (CIPN) is a common dose-limiting side effect of several chemotherapeutic agents. Despite large amounts of human and animal studies, there is no sufficiently effective pharmacological treatment for CIPN. Although reducing pain is often a focus of CIPN treatment, remarkably few analgesics have been tested for this indication in clinical trials. We conducted a systematic review and meta-analyses regarding the effects of analgesics on stimulus evoked pain-like behaviour during CIPN in animal models. This will form a scientific basis for the development of prospective human clinical trials. A comprehensive search identified forty-six studies. Risk of bias (RoB) analyses revealed that the design and conduct of the included experiments were poorly reported, and therefore RoB was unclear in most studies. Meta-analyses showed that administration of analgesics significantly increases pain threshold for mechanical (SMD: 1.68 [1.41; 1.82]) and cold (SMD: 1. 41 [0.99; 1.83]) evoked pain. Subgroup analyses revealed that dexmedetomidine, celecoxib, fentanyl, morphine, oxycodone and tramadol increased the pain threshold for mechanically evoked pain, and lidocaine and morphine for cold evoked pain. Altogether, this meta-analysis shows that there is ground to investigate the use of morphine in clinical trials. Lidocaine, dexmedetomidine, celecoxib, fentanyl, oxycodone and tramadol might be good alternatives, but more animal-based research is necessary.

DNMT3a contributes to the development and maintenance of bone cancer pain by silencing Kv1.2 expression in spinal cord dorsal horn

Metastatic bone tumor-induced changes in gene transcription and translation in pain-related regions of the nervous system may participate in the development and maintenance of bone cancer pain. Epigenetic modifications including DNA methylation regulate gene transcription. Here, we report that intrathecal injection of decitabine, a DNA methyltransferase (DNMT) inhibitor, dose dependently attenuated the development and maintenance of bone cancer pain induced by injecting prostate cancer cells into the tibia. The level of the de novo DNMT3a, but not DNMT3b, time dependently increased in the ipsilateral L4/5 dorsal horn (not L4/5 dorsal root ganglion) after prostate cancer cells injection. Blocking this increase through microinjection of recombinant adeno-associated virus 5 (AAV5) expressing Dnmt3a shRNA into dorsal horn rescued prostate cancer cells-induced downregulation of dorsal horn Kv1.2 expression and impaired prostate cancer cells-induced pain hypersensitivity. In turn, mimicking this increase through microinjection of AAV5 expressing full-length Dnmt3a into dorsal horn reduced dorsal horn Kv1.2 expression and produced pain hypersensitivity in the absence of prostate cancer cells injection. Administration of neither decitabine nor virus affected locomotor function and acute responses to mechanical, thermal, or cold stimuli. Given that Dnmt3a mRNA is co-expressed with Kcna2 mRNA (encoding Kv1.2) in individual dorsal horn neurons, our findings suggest that increased dorsal horn DNMT3a contributes to bone cancer pain through silencing dorsal horn Kv1.2 expression. DNMT3a may represent a potential new target for cancer pain management.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 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, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Antagonizing the different stages of kappa opioid receptor activation selectively and independently attenuates acquisition and consolidation of associative memories

Previous work from our laboratory has shown that nonspecific kappa opioid receptor (KOR) antagonism in primary somatosensory cortex (S1) can inhibit acquisition for the forebrain-dependent associative task, Whisker-Trace Eyeblink conditioning (WTEB). Although studies have demonstrated that KOR activation can alter stimuli salience, our studies controlled for these factors, demonstrating that KOR also plays a role in facilitating learning. KOR has two distinct phases of activation followed by internalization/downregulation, that each independently activate kinases and transcription factors known to mediate task acquisition and memory consolidation respectively. The current study demonstrated that antagonism of the initial phase of KOR activation in S1 via local injections of the g-protein inhibitor, pertussis toxin (PTX), blocked initial WTEB acquisition without affecting retention of the association. In contrast, KOR late phase antagonism in S1 via local injections of the GRK3-specific antagonist, guanidinonaltrindole (GNTI), blocked retention of the WTEB association without affecting task acquisition. Consistent with the known mechanism for KOR activation, KOR protein expression in S1 was found to be decreased following WTEB training, further supporting the involvement of neocortical KOR activation with learning. Prior studies have shown that task acquisition and memory consolidation are mediated by distinct molecular processes; however, little is known regarding a potential mechanism driving these processes. The current study suggests that neocortical KOR activation mediates activation of these processes with learning. This study provides the first evidence for a time- and learning-dependent property of neocortical KOR in facilitating acquisition and consolidation of associative memories, while elucidating an unexplored neocortical learning mechanism.

Chemotherapy-induced painful neuropathy: pain-like behaviours in rodent models and their response to commonly used analgesics

PURPOSE OF REVIEW

Chemotherapy-induced painful neuropathy (CIPN) is a major dose-limiting side-effect of several widely used chemotherapeutics. Rodent models of CIPN have been developed using a range of dosing regimens to reproduce pain-like behaviours akin to patient-reported symptoms. This review aims to connect recent evidence-based suggestions for clinical treatment to preclinical data.

RECENT FINDINGS

We will discuss CIPN models evoked by systemic administration of taxanes (paclitaxel and docetaxel), platinum-based agents (oxaliplatin and cisplatin), and the proteasome-inhibitor - bortezomib. We present an overview of dosing regimens to produce CIPN models and their phenotype of pain-like behaviours. In addition, we will discuss how potential, clinically available treatments affect pain-like behaviours in these rodent models, relating those effects to clinical trial data wherever possible. We have focussed on antidepressants, opioids, and gabapentinoids given their broad usage.

SUMMARY

The review outlines the latest description of the most-relevant rodent models of CIPN enabling comparison between chemotherapeutics, dosing regimen, rodent strain, and sex. Preclinical data support many of the recent suggestions for clinical management of established CIPN and provides evidence for potential treatments warranting clinical investigation. Continued research using rodent CIPN models will provide much needed understanding of the causal mechanisms of CIPN, leading to new treatments for this major clinical problem.

The inhibition of high-voltage-activated calcium current by activation of MrgC11 involves phospholipase C-dependent mechanisms

High-voltage-activated (HVA) calcium channels play an important role in synaptic transmission. Activation of Mas-related G-protein-coupled receptor subtype C (MrgC; mouse MrgC11, rat homolog rMrgC) inhibits HVA calcium current (ICa) in small-diameter dorsal root ganglion (DRG) neurons, but the intracellular signaling cascade underlying MrgC agonist-induced inhibition of HVA ICa in native DRG neurons remains unclear. To address this question, we conducted patch-clamp recordings in MrgA3-eGFP-wild-type mice, in which most MrgA3-eGFP(+) DRG neurons co-express MrgC11 and can be identified for recording. We found that the inhibition of HVA ICa by JHU58 (0.001-100nM, a dipeptide, MrgC-selective agonist) was significantly reduced by pretreatment with a phospholipase C (PLC) inhibitor (U73122, 1μM), but not by its inactive analog (U73343) or vehicle. Further, in rats that had undergone spinal nerve injury, pretreatment with intrathecal U73122 nearly abolished the inhibition of mechanical hypersensitivity by intrathecal JHU58. The inhibition of HVA ICa in MrgA3-eGFP(+) neurons by JHU58 (100nM) was partially reduced by pretreatment with a Gβγ blocker (gallein, 100μM). However, applying a depolarizing prepulse and blocking the Gαi and Gαs pathways with pertussis toxin (PTX) (0.5μg/mL) and cholera toxin (CTX) (0.5μg/mL), respectively, had no effect. These findings suggest that activation of MrgC11 may inhibit HVA ICa in mouse DRG neurons through a voltage-independent mechanism that involves activation of the PLC, but not Gαi or Gαs, pathway.