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Bradykinin is a peptide implicated in inflammatory pain in both humans and rodents. In rodent sensory neurons, activation of B1 and B2 bradykinin receptors induces neuronal hyperexcitability. Recent evidence suggests that human and rodent dorsal root ganglia (DRG), which contain the cell bodies of sensory neurons, differ in the expression and function of key GPCRs and ion channels; whether BK receptor expression and function are conserved across species has not been studied in depth. In this study, we used human DRG tissue from organ donors to provide a detailed characterization of bradykinin receptor expression and bradykinin-induced changes in the excitability of human sensory neurons. We found that B2 and, to a lesser extent, B1 receptors are expressed by human DRG neurons and satellite glial cells. B2 receptors were enriched in the nociceptor subpopulation. Using patch-clamp electrophysiology, we found that acute bradykinin increases the excitability of human sensory neurons, while prolonged exposure to bradykinin decreases neuronal excitability in a subpopulation of human DRG neurons. Finally, our analyses suggest that donor’s history of chronic pain and age may be predictors of higher B1 receptor expression in human DRG neurons. Together, these results indicate that acute BK-induced hyperexcitability, first identified in rodents, is conserved in humans and provide further evidence supporting BK signaling as a potential therapeutic target for treating pain in humans.

Neurotransmitters and neuromodulators mediate communication between neurons and other cell types; knowledge of release dynamics is critical to understanding their physiological role in normal and pathological brain function. Investigation into transient neurotransmitter dynamics has largely been hindered due to electrical and material requirements for electrochemical stimulation and recording. Current systems require complex electronics for biasing and amplification and rely on materials that offer limited sensor selectivity and sensitivity. These restrictions result in bulky, tethered, or battery-powered systems impacting behavior and that require constant care of subjects. To overcome these challenges, we demonstrate a fully implantable, wireless, and battery-free platform that enables optogenetic stimulation and electrochemical recording of catecholamine dynamics in real time. The device is nearly 1/10th the size of previously reported examples and includes a probe that relies on a multilayer electrode architecture featuring a microscale light emitting diode (μ-LED) and a carbon nanotube (CNT)-based sensor with sensitivities among the highest recorded in the literature (1264.1 nA μM

Use of prescription opioids, particularly oxycodone, is an initiating factor driving the current opioid epidemic. There are several challenges with modelling oxycodone abuse. First, prescription opioids including oxycodone are orally self-administered and have different pharmacokinetics and dynamics than morphine or fentanyl, which have been more commonly used in rodent research. This oral route of administration determines the pharmacokinetic profile, which then influences the establishment of drug-reinforcement associations in animals. Moreover, the pattern of intake and the environment in which addictive drugs are self-administered are critical determinants of the levels of drug intake, of behavioural sensitization and of propensity to relapse behaviour. These are all important considerations when modelling prescription opioid use, which is characterized by continuous drug access in familiar environments. Thus, to model features of prescription opioid use and the transition to abuse, we designed an oral, homecage-based oxycodone self-administration paradigm. Mice voluntarily self-administer oxycodone in this paradigm without any taste modification such as sweeteners, and the majority exhibit preference for oxycodone, escalation of intake, physical signs of dependence and reinstatement of seeking after withdrawal. In addition, a subset of animals demonstrate drug taking that is resistant to aversive consequences. This model is therefore translationally relevant and useful for studying the neurobiological substrates of prescription opioid abuse.

Use of prescription opioids, particularly oxycodone is an initiating factor driving the current opioid epidemic. There are several challenges with modeling oxycodone abuse. First, prescription opioids including oxycodone are orally self-administered and have different pharmacokinetics and dynamics than morphine or fentanyl which have been more commonly used in rodent research. This oral route of administration determines the pharmacokinetic profile, which then influences the establishment of drug-reinforcement associations in animals. Moreover, the pattern of intake and the environment in which addictive drugs are self-administered are critical determinants of the levels of drug intake, of behavioral sensitization, and of propensity to relapse behavior. These are all important considerations when modeling prescription opioid use, which is characterized by continuous drug access in familiar environments. Thus, to model features of prescription opioid use and the transition to abuse, we designed an oral, homecage-based oxycodone self-administration paradigm. Mice voluntarily self-administer oxycodone in this paradigm without any taste modification such as sweeteners, and the majority exhibit preference for oxycodone, escalation of intake, physical signs of dependence, and reinstatement of seeking after withdrawal. In addition, a subset of animals demonstrate drug taking that is resistant to aversive consequences. This model is therefore translationally relevant and useful for studying the neurobiological substrates of prescription opioid abuse.

Activation of cannabinoid receptor type 1 (CB 1 ) produces analgesia in a variety of preclinical models of pain; however, engagement of central CB 1 receptors is accompanied by unwanted side effects, such as psychoactivity, tolerance, and dependence. Therefore, some efforts to develop novel analgesics have focused on targeting peripheral CB 1 receptors to circumvent central CB 1 -related side effects. In the present study, we evaluated the effects of acute and repeated dosing with the peripherally selective CB 1 -preferring agonist CB-13 on nociception and central CB 1 -related phenotypes in a model of inflammatory pain in mice. We also evaluated cellular mechanisms underlying CB-13-induced antinociception in vitro using cultured mouse dorsal root ganglion neurons. CB-13 reduced inflammation-induced mechanical allodynia in male and female mice in a peripheral CB 1 -receptor-dependent manner and relieved inflammatory thermal hyperalgesia. In cultured mouse dorsal root ganglion neurons, CB-13 reduced TRPV1 sensitization and neuronal hyperexcitability induced by the inflammatory mediator prostaglandin E 2 , providing potential mechanistic explanations for the analgesic actions of peripheral CB 1 receptor activation. With acute dosing, phenotypes associated with central CB 1 receptor activation occurred only at a dose of CB-13 approximately 10-fold the ED 50 for reducing allodynia. Strikingly, repeated dosing resulted in both analgesic tolerance and CB 1 receptor dependence, even at a dose that did not produce central CB 1 -receptor-mediated phenotypes on acute dosing. This suggests that repeated CB-13 dosing leads to increased CNS exposure and unwanted engagement of central CB 1 receptors. Thus, caution is warranted regarding therapeutic use of CB-13 with the goal of avoiding CNS side effects. Nonetheless, the clear analgesic effect of acute peripheral CB 1 receptor activation suggests that peripherally restricted cannabinoids are a viable target for novel analgesic development.

Activation of cannabinoid receptor type 1 (CB1) produces analgesia in a variety of preclinical models of pain; however, engagement of central CB1 receptors is accompanied by unwanted side effects, such as tolerance and dependence. Efforts to develop novel analgesics have focused on targeting peripheral CB1 receptors to circumvent central CB1-related side effects. In the present study, we evaluated the effects of acute and repeated dosing with the peripherally selective CB1-preferring agonist CB-13 on nociception and central CB1-related phenotypes in an inflammatory model of pain in mice. We also evaluated cellular mechanisms underlying CB-13-induced antinociception in vitro using cultured mouse dorsal root ganglion (DRG) neurons. CB-13 reduced inflammation-induced mechanical allodynia in a peripheral CB1 receptor-dependent manner and relieved inflammatory thermal hyperalgesia. In cultured mouse DRG neurons, CB-13 reduced TRPV1 sensitization and neuronal hyperexcitability induced by the inflammatory mediator prostaglandin E2, providing potential mechanistic explanations for the analgesic actions of peripheral CB1 receptor activation. With acute dosing, phenotypes associated with central CB1 receptor activation occurred only at a dose of CB-13 approximately 10-fold the ED50 for reducing allodynia. Strikingly, repeated dosing resulted in both analgesic tolerance and CB1 receptor dependence, even at a dose that did not produce central CB1 receptor-mediated phenotypes on acute dosing. This suggests repeated CB-13 dosing leads to increased CNS exposure and unwanted engagement of central CB1 receptors. Thus, caution is warranted regarding therapeutic use of CB-13 with the goal of avoiding CNS side effects. Nonetheless, the clear analgesic effect of acute peripheral CB1 receptor activation suggests that peripherally restricted cannabinoids are a viable target for novel analgesic development.

Prescription opioid use is an initiating factor driving the current opioid epidemic. There are several challenges with modeling prescription opioid addiction. First, prescription opioids such as oxycodone are orally self-administered and have different pharmacokinetics and dynamics than morphine or fentanyl. This oral route of administration determines the pharmacokinetic profile, which is critical for establishing reliable drug-reinforcement associations in animals. Moreover, the pattern of intake and environment in which addictive drugs are self-administered intake are critical determinants of the levels of drug intake, sensitization and relapse behavior. This is an important consideration with prescription opioid use, which is characterized by continuous drug access in familiar environments. Thus, to model features of prescription opioid use and the transition to abuse, we present an oral oxycodone self-administration paradigm that is administered in the home cage. Mice voluntarily self-administer oxycodone in this paradigm without any taste modification such as sweeteners, and exhibit preference for oxycodone, escalation of intake, physical signs of dependence, reinstatement of seeking after withdrawal, and a subset of animals demonstrate drug taking that is resistant to aversive consequences. This model could be useful for studying the neurobiological substrates specifically relevant to prescription opioid abuse.

The opioid crisis has underscored the urgent need to identify safe and effective therapeutic strategies to overcome opioid-induced liabilities. We recently reported that LY2828360, a slowly signaling G protein-biased cannabinoid CB(2) receptor agonist, suppresses neuropathic nociception and attenuates the development of tolerance to the opioid analgesic morphine in paclitaxel-treated mice. Whether beneficial effects of LY2828360 are dependent upon the presence of a pathological pain state are unknown and its impact on unwanted opioid-induced side-effects have never been investigated. Here, we asked whether LY2828360 would produce synergistic anti-allodynic effects with morphine in a paclitaxel model of chemotherapy-induced neuropathic pain and characterized its impact on opioid-induced reward and other unwanted side-effects associated with chronic opioid administration. Isobolographic analysis revealed that combinations of LY2828360 and morphine produced synergistic anti-allodynic effects in suppressing paclitaxel-induced mechanical allodynia. In wildtype (WT) mice, LY2828360 blocked morphine-induced reward in a conditioned place preference assay without producing reward or aversion when administered alone. The LY2828360-induced attenuation of morphine-induced reward was absent in CB(2) knockout (CB(2)KO) mice. In the absence of a neuropathic pain state, LY2828360 partially attenuated naloxone-precipitated opioid withdrawal in morphine-dependent WT mice, and this withdrawal was itself markedly exacerbated in CB(2)KO mice. Moreover, LY2828360 did not reliably alter morphine-induced slowing of colonic transit or attenuate tolerance to morphine antinociceptive efficacy in the hot plate test of acute nociception. Our results suggest that cannabinoid CB(2) receptor activation enhances the therapeutic properties of opioids while attenuating unwanted side-effects such as reward and dependence that occur with sustained opioid treatment.

Background Activation of cannabinoid CB1 receptors suppresses pathological pain but also produces unwanted central side effects. We hypothesized that a positive allosteric modulator of CB1 signaling would suppress inflammatory and neuropathic pain without producing cannabimimetic effects or physical dependence. We also asked whether a CB1 positive allosteric modulator would synergize with inhibitors of endocannabinoid deactivation and/or an orthosteric cannabinoid agonist. Methods GAT211, a novel CB1 positive allosteric modulator, was evaluated for antinociceptive efficacy and tolerance in models of neuropathic and/or inflammatory pain. Cardinal signs of direct CB1-receptor activation were evaluated together with the propensity to induce reward or aversion and physical dependence. Comparisons were made with inhibitors of endocannabinoid deactivation (JZL184, URB597) or an orthosteric cannabinoid agonist (WIN55,212-2). All studies used 4 to 11 subjects per group. Results GAT211 suppressed allodynia induced by complete Freund’s adjuvant and the chemotherapeutic agent paclitaxel in wild-type but not CB1 knockout mice. GAT211 did not impede paclitaxel-induced tumor cell line toxicity. GAT211 did not produce cardinal signs of direct CB1-receptor activation in the presence or absence of pathological pain. GAT211 produced synergistic antiallodynic effects with fatty acid amide hydrolase and monoacylglycerol lipase inhibitors in paclitaxel-treated mice. Therapeutic efficacy was preserved over 19 days of chronic dosing with GAT211, but it was not preserved with the monoacylglycerol lipase inhibitor JZL184. The CB1 antagonist rimonabant precipitated withdrawal in mice treated chronically with WIN55,212-2 but not in mice treated with GAT211. GAT211 did not induce conditioned place preference or aversion. Conclusions Positive allosteric modulation of CB1-receptor signaling shows promise as a safe and effective analgesic strategy that lacks tolerance, dependence, and abuse liability.

Opioid-based therapies remain a mainstay for chronic pain management, but unwanted side effects limit therapeutic use. We compared efficacies of brain-permeant and -impermeant inhibitors of fatty acid amide hydrolase (FAAH) in suppressing neuropathic pain induced by the chemotherapeutic agent paclitaxel. Paclitaxel produced mechanical and cold allodynia without altering nestlet shredding or marble burying behaviors. We compared FAAH inhibitors that differ in their ability to penetrate the central nervous system for antiallodynic efficacy, pharmacological specificity, and synergism with the opioid analgesic morphine. (3′-(aminocarbonyl)[1,1′-biphenyl]- 3-yl)-cyclohexylcarbamate (URB597), a brain-permeant FAAH inhibitor, attenuated paclitaxel-induced allodynia via cannabinoid receptor 1 (CB(1)) and cannabinoid receptor 2 (CB(2)) mechanisms. URB937, a brain-impermeant FAAH inhibitor, suppressed paclitaxel-induced allodynia through a CB(1) mechanism only. 5-[4-(4-cyano-1-butyn-1-yl)phenyl]-1-(2,4-dichlorophenyl)-N-(1,1-dioxido-4-thiomorpholinyl)-4-methyl-1H-pyrazole-3-carboxamide (AM6545), a peripherally restricted CB(1) antagonist, fully reversed the antiallodynic efficacy of N-cyclohexyl-carbamic acid, 3′-(aminocarbonyl)-6-hydroxy[1,1′- biphenyl]-3-yl ester (URB937) but only partially reversed that of URB597. Thus, URB937 suppressed paclitaxel-induced allodynia through a mechanism that was dependent upon peripheral CB(1) receptor activation only. Antiallodynic effects of both FAAH inhibitors were reversed by N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251). Antiallodynic effects of URB597, but not URB937, were reversed by 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl](4-methoxyphenyl)methanone (AM630). Isobolographic analysis revealed synergistic interactions between morphine and either URB597 or URB937 in reducing paclitaxel-induced allodynia. A leftward shift in the dose-response curve of morphine antinociception was observed when morphine was coadministered with either URB597 or URB937, consistent with morphine sparing. However, neither URB937 nor URB597 enhanced morphine-induced deficits in colonic transit. Thus, our findings suggest that FAAH inhibition may represent a therapeutic avenue to reduce the overall amount of opioid needed for treating neuropathic pain with potential to reduce unwanted side effects that accompany opioid administration.

Highlights • Paclitaxel treatment did not alter voluntary running activity. • Voluntary running reduced mechanical and cold allodynia induced by paclitaxel. • Voluntary running reduced paclitaxel-induced deficits in hippocampal cellular proliferation., Chemotherapy-induced peripheral neuropathy (CIPN) is a common dose-limiting side-effect of all major chemotherapeutic agents. Here, we explored efficacy of voluntary exercise as a nonpharmacological strategy for suppressing two distinct adverse side effects of chemotherapy treatment. We evaluated whether voluntary running would suppress both neuropathic pain and deficits in hippocampal cell proliferation in a mouse model of CIPN induced by the taxane chemotherapeutic agent paclitaxel. Mice were given free access to running wheels or were housed without running wheels during one of three different intervention phases: 1) during the onset (i.e. development phase) of paclitaxel-induced neuropathy, 2) prior to dosing with paclitaxel or its vehicle, or 3) following the establishment (i.e. maintenance phase) of paclitaxel-induced neuropathy. Paclitaxel treatment did not alter running wheel behavior relative to vehicle-treated animals in any study. Animals that engaged in voluntary running during the development phase of paclitaxel-induced neuropathy failed to display mechanical or cold hypersensitivities relative to sedentary control animals that did not have access to running wheels. A prior history of voluntary running delayed the onset of, but did not fully prevent, development of paclitaxel-induced neuropathic pain behavior. Voluntary running reduced already established mechanical and cold allodynia induced by paclitaxel. Importantly, voluntary running did not alter mechanical or cold responsivity in vehicle-treated animals, suggesting that the observed antinociceptive effect of exercise was dependent upon the presence of the pathological pain state. In the same animals evaluated for nociceptive responding, paclitaxel also reduced cellular proliferation but not cellular survival in the dentate gyrus of the hippocampus, as measured by immunohistochemistry for Ki67 and BrdU expression, respectively. Voluntary running abrogated paclitaxel-induced reductions in cellular proliferation to levels observed in vehicle-treated mice and also increased BrdU expression levels irrespective of chemotherapy treatment. Our studies support the hypothesis that voluntary exercise may be beneficial in suppressing both neuropathic pain and markers of hippocampal cellular function that are impacted by toxic challenge with chemotherapeutic agents.

The opioid crisis has underscored the urgent need to identify safe and effective therapeutic strategies to overcome opioid-induced liabilities. We recently reported that LY2828360, a slowly signaling G protein-biased cannabinoid CB2 receptor agonist, suppresses neuropathic nociception and attenuates the development of tolerance to the opioid analgesic morphine in paclitaxel-treated mice. Whether beneficial effects of LY2828360 are dependent upon the presence of a pathological pain state are unknown and its impact on unwanted opioid-induced side-effects have never been investigated. Here, we asked whether LY2828360 would produce synergistic anti-allodynic effects with morphine in a paclitaxel model of chemotherapy-induced neuropathic pain and characterized its impact on opioid-induced reward and other unwanted side-effects associated with chronic opioid administration. Isobolographic analysis revealed that combinations of LY2828360 and morphine produced synergistic anti-allodynic effects in suppressing paclitaxel-induced mechanical allodynia. In wildtype (WT) mice, LY2828360 blocked morphine-induced reward in a conditioned place preference assay without producing reward or aversion when administered alone. The LY2828360-induced attenuation of morphine-induced reward was absent in CB2 knockout (CB2KO) mice. In the absence of a neuropathic pain state, LY2828360 partially attenuated naloxone-precipitated opioid withdrawal in morphine-dependent WT mice, and this withdrawal was itself markedly exacerbated in CB2KO mice. Moreover, LY2828360 did not reliably alter morphine-induced slowing of colonic transit or attenuate tolerance to morphine antinociceptive efficacy in the hot plate test of acute nociception. Our results suggest that cannabinoid CB2 receptor activation enhances the therapeutic properties of opioids while attenuating unwanted side-effects such as reward and dependence that occur with sustained opioid treatment.

Activation of cannabinoid CB(1) receptors suppresses pathological pain but also produces unwanted side effects, including tolerance and physical dependence. Inhibition of fatty-acid amide hydrolase (FAAH), the major enzyme catalyzing the degradation of anandamide (AEA), an endocannabinoid, and other fatty-acid amides, suppresses pain without unwanted side effects typical of direct CB(1) agonists. However, FAAH inhibitors have failed to show efficacy in several clinical trials suggesting that the right partnership of FAAH inhibition and pathology has yet to be identified. We compared efficacy of chronic treatments with a centrally penetrant FAAH inhibitor (URB597), a peripherally restricted FAAH inhibitor (URB937) and an orthosteric pan-cannabinoid agonist (WIN55,212–2) in suppressing neuropathic pain induced by the chemotherapeutic agent paclitaxel. Each FAAH inhibitor suppressed the development of paclitaxel-induced neuropathic pain and reduced the maintenance of already established allodynia with sustained efficacy. Tolerance developed to the anti-allodynic efficacy of WIN55,212–2, but not to that of URB597 or URB937, in each dosing paradigm. Challenge with the CB(1) antagonist rimonabant precipitated CB(1)-dependent withdrawal in paclitaxel-treated mice receiving WIN55,212–2 but not URB597 or URB937. When dosing with either URB597 or URB937 was restricted to the development of neuropathy, paclitaxel-induced allodynia emerged following termination of drug delivery. These observations suggest that both FAAH inhibitors were anti-allodynic rather than curative. Moreover, neither URB597 nor URB937 impeded the ability of paclitaxel to reduce breast (4T1) or ovarian (HeyA8) tumor cell line viability. In fact, URB597 and URB937 alone reduced 4T1 tumor cell line viability, albeit with low potency, and the dose matrix of each combination with paclitaxel was synergistic in reducing 4T1 and HeyA8 tumor cell line viability according to Bliss, Highest Single Agent (HSA) and Loewe additivity models. Both FAAH inhibitors synergized with paclitaxel to reduce 4T1 and HeyA8 tumor cell line viability without reducing viability of non-tumor HEK293 cells. Neither FAAH inhibitor reduced viability of non-tumor HEK293 cells in either the presence or absence of paclitaxel, suggesting that nonspecific cytotoxic effects were not produced by the same treatments. Our results suggest that FAAH inhibitors reduce paclitaxel-induced allodynia without the occurrence of CB(1)-dependence in vivo and may, in fact, enhance the anti-tumor actions of paclitaxel in vitro.

Chemotherapeutic agents are widely used to treat patients with systemic cancer. The efficacy of these therapies is undermined by their adverse side-effect profiles such as cognitive deficits that have a negative impact on the quality of life of cancer survivors. Cognitive side effects occur across a variety of domains, including memory, executive function, and processing speed. Such impairments are exacerbated under cognitive challenges and a subgroup of patients experience long-term impairments. Episodic memory in rats can be examined using a source memory task. In the current study, rats received paclitaxel, a taxane-derived chemotherapeutic agent, and learning and memory functioning was examined using the source memory task. Treatment with paclitaxel did not impair spatial and episodic memory, and paclitaxel treated rats were not more susceptible to cognitive challenges. Under conditions in which memory was not impaired, paclitaxel treatment impaired learning of new rules, documenting a decreased sensitivity to changes in experimental contingencies. These findings provide new information on the nature of cancer chemotherapy-induced cognitive impairments, particularly regarding the incongruent vulnerability of episodic memory and new learning following treatment with paclitaxel.