Attenuation of mechanical allodynia by clinically utilized drugs in a rat chemotherapy-induced neuropathic pain model

Functionally-selective inhibition of threshold sodium currents and excitability in dorsal root ganglion neurons by cannabinol

Cannabinol (CBN), an incompletely understood metabolite for ∆9-tetrahydrocannabinol, has been suggested as an analgesic. CBN interacts with endocannabinoid (CB) receptors, but is also reported to interact with non-CB targets, including various ion channels. We assessed CBN effects on voltage-dependent sodium (Nav) channels expressed heterologously and in native dorsal root ganglion (DRG) neurons. Our results indicate that CBN is a functionally-selective, but structurally-non-selective Nav current inhibitor. CBN's main effect is on slow inactivation. CBN slows recovery from slow-inactivated states, and hyperpolarizes steady-state inactivation, as channels enter deeper and slower inactivated states. Multielectrode array recordings indicate that CBN attenuates DRG neuron excitability. Voltage- and current-clamp analysis of freshly isolated DRG neurons via our automated patch-clamp platform confirmed these findings. The inhibitory effects of CBN on Nav currents and on DRG neuron excitability add a new dimension to its actions and suggest that this cannabinoid may be useful for neuropathic pain.

Mechanisms of cancer pain

Personalised and targeted interventions have revolutionised cancer treatment and dramatically improved survival rates in recent decades. Nonetheless, effective pain management remains a problem for patients diagnosed with cancer, who continue to suffer from the painful side effects of cancer itself, as well as treatments for the disease. This problem of cancer pain will continue to grow with an ageing population and the rapid advent of more effective therapeutics to treat the disease. Current pain management guidelines from the World Health Organisation are generalised for different pain severities, but fail to address the heterogeneity of mechanisms in patients with varying cancer types, stages of disease and treatment plans. Pain is the most common complaint leading to emergency unit visits by patients with cancer and over one-third of patients that have been diagnosed with cancer will experience under-treated pain. This review summarises preclinical models of cancer pain states, with a particular focus on cancer-induced bone pain and chemotherapy-associated pain. We provide an overview of how preclinical models can recapitulate aspects of pain and sensory dysfunction that is observed in patients with persistent cancer-induced bone pain or neuropathic pain following chemotherapy. Peripheral and central nervous system mechanisms of cancer pain are discussed, along with key cellular and molecular mediators that have been highlighted in animal models of cancer pain. These include interactions between neuronal cells, cancer cells and non-neuronal cells in the tumour microenvironment. Therapeutic targets beyond opioid-based management are reviewed for the treatment of cancer pain.

Neuropathy, its Profile and Experimental Nerve Injury Neuropathic Pain Models: A Review

Neuropathy is a terrible disorder that has a wide range of etiologies. Drug-induced neuropathy, which happens whenever a chemical agent damages the peripheral nerve system, has been linked here to the iatrogenic creation of some drugs. It is potentially permanent and causes sensory impairments and paresthesia that typically affects the hands, feet, and stockings; motor participation is uncommon. It might appear suddenly or over time, and the long-term outlook varies. The wide range of chronic pain conditions experienced by people has been one of the main obstacles to developing new, more effective medications for the treatment of neuropathic pain. Animal models can be used to examine various neuropathic pain etiologies and symptoms. Several models investigate the peripheral processes of neuropathic pain, whereas some even investigate the central mechanisms, such as drug induce models like vincristine, cisplatin, bortezomib, or thalidomide, etc., and surgical models like sciatic nerve chronic constriction injury (CCI), sciatic nerve ligation through spinal nerve ligation (SNL), sciatic nerve damage caused by a laser, SNI (spared nerve injury), etc. The more popular animal models relying on peripheral nerve ligatures are explained. In contrast to chronic sciatic nerve contraction, which results in behavioral symptoms of less reliable stressful neuropathies, (SNI) spared nerve injury generates behavioral irregularities that are more feasible over a longer period. This review summarizes the latest methods models as well as clinical ideas concerning this mechanism. Every strongest current information on neuropathy is discussed, along with several popular laboratory models for causing neuropathy.

Mu-opioid receptor and receptor tyrosine kinase crosstalk: Implications in mechanisms of opioid tolerance, reduced analgesia to neuropathic pain, dependence, and reward

Despite the prevalence of opioid misuse, opioids remain the frontline treatment regimen for severe pain. However, opioid safety is hampered by side-effects such as analgesic tolerance, reduced analgesia to neuropathic pain, physical dependence, or reward. These side effects promote development of opioid use disorders and ultimately cause overdose deaths due to opioid-induced respiratory depression. The intertwined nature of signaling via μ-opioid receptors (MOR), the primary target of prescription opioids, with signaling pathways responsible for opioid side-effects presents important challenges. Therefore, a critical objective is to uncouple cellular and molecular mechanisms that selectively modulate analgesia from those that mediate side-effects. One such mechanism could be the transactivation of receptor tyrosine kinases (RTKs) via MOR. Notably, MOR-mediated side-effects can be uncoupled from analgesia signaling via targeting RTK family receptors, highlighting physiological relevance of MOR-RTKs crosstalk. This review focuses on the current state of knowledge surrounding the basic pharmacology of RTKs and bidirectional regulation of MOR signaling, as well as how MOR-RTK signaling may modulate undesirable effects of chronic opioid use, including opioid analgesic tolerance, reduced analgesia to neuropathic pain, physical dependence, and reward. Further research is needed to better understand RTK-MOR transactivation signaling pathways, and to determine if RTKs are a plausible therapeutic target for mitigating opioid side effects.

Non-psychotropic phytocannabinoid interactions with voltage-gated sodium channels: An update on cannabidiol and cannabigerol

Phytocannabinoids, found in the plant, Cannabis sativa, are an important class of natural compounds with physiological effects. These compounds can be generally divided into two classes: psychoactive and non-psychoactive. Those which do not impart psychoactivity are assumed to predominantly function via endocannabinoid receptor (CB) -independent pathways and molecular targets, including other receptors and ion channels. Among these targets, the voltage-gated sodium (Nav) channels are particularly interesting due to their well-established role in electrical signalling in the nervous system. The interactions between the main non-psychoactive phytocannabinoid, cannabidiol (CBD), and Nav channels were studied in detail. In addition to CBD, cannabigerol (CBG), is another non-psychoactive molecule implicated as a potential therapeutic for several conditions, including pain via interactions with Nav channels. In this mini review, we provide an update on the interactions of Nav channels with CBD and CBG.

Inhibition of sodium conductance by cannabigerol contributes to a reduction of dorsal root ganglion neuron excitability

BACKGROUND AND PURPOSE

Cannabigerol (CBG), a non-psychotropic phytocannabinoid and a precursor of ∆9 -tetrahydrocannabinol and cannabidiol, has been suggested to act as an analgesic. A previous study reported that CBG (10 μM) blocks voltage-gated sodium (Nav ) currents in CNS neurons, although the underlying mechanism is not well understood. Genetic and functional studies have validated Nav 1.7 channels as an opportune target for analgesic drug development. The effects of CBG on Nav 1.7 channels, which may contribute to its analgesic properties, have not been previously investigated.

EXPERIMENTAL APPROACH

To determine the effects of CBG on Nav channels, we used stably transfected HEK cells and primary dorsal root ganglion (DRG) neurons to characterize compound effects using experimental and computational techniques. These included patch-clamp, multielectrode array, and action potential modelling.

KEY RESULTS

CBG is a ~10-fold state-dependent Nav channel inhibitor (KI -KR : ~2-20 μM) with an average Hill-slope of ~2. We determined that, at lower concentrations, CBG predominantly blocks sodium Gmax and slows recovery from inactivation. However, as the concentration is increased, CBG also induces a hyperpolarizing shift in the half-voltage of inactivation. Our modelling and multielectrode array recordings suggest that CBG attenuates DRG excitability.

CONCLUSIONS AND IMPLICATIONS

Inhibition of Nav 1.7 channels in DRG neurons may underlie CBG-induced neuronal hypoexcitability. As most Nav 1.7 channels are inactivated at the resting membrane potential of DRG neurons, they are more likely to be inhibited by lower CBG concentrations, suggesting functional selectivity against Nav 1.7 channels, compared with other Nav channels (via Gmax block).

Paracetamol for multimodal analgesia

Pain and related disability remain a major social and therapeutic problem. Comorbidities and therapies increase drug interactions and side effects making pain management more compounded especially in the elderly who are the fastest-growing pain population. Multimodal analgesia consists of using two or more drugs and/or techniques that target different sites of pain, increasing the level of analgesia and decreasing adverse events from treatment. Paracetamol enhances multimodal analgesia in experimental and clinical pain states. Strong preclinical evidence supports that paracetamol has additive and synergistic interactions with anti-inflammatory, opioid and anti-neuropathic drugs in rodent models of nociceptive and neuropathic pain. Clinical studies in young and adult elderly patients confirm the utility of paracetamol in multimodal, non-opioid or opioid-sparing, therapies for the treatment of acute and chronic pain.

Drug treatment for chemotherapy-induced peripheral neuropathy in patients with pancreatic cancer

Pancreatic cancer (PC) is a lethal disease where most tumors are too advanced at diagnosis for resection, leaving chemotherapy as the mainstay of treatment. Although the prognosis of unresectable PC is poor, it has been dramatically improved by new chemotherapy treatments, such as the combination of 5-fluorouracil, oxaliplatin, irinotecan, and leucovorin (FOLFIRINOX) or gemcitabine plus nab-paclitaxel. However, as oxaliplatin and paclitaxel are common neurotoxic drugs, chemotherapy-induced peripheral neuropathy (CIPN) is a common and severe adverse effect of both treatments. As there are no agents recommended in the ASCO guidelines, we review the methods used to treat CIPN caused by PC treatment. The efficacy of duloxetine was observed in a large randomized controlled trial (RCT). In addition, pregabalin was more effective than duloxetine for CIPN in two RCTs. Although duloxetine and pregabalin can be effective for CIPN, they have several side effects. Therefore, the choice between the two drugs should be determined according to effect and tolerability. Mirogabalin is also used in patients with PC and there is hope it will yield positive outcomes when treating CIPN in the future.

Systematic Review of Systemic and Neuraxial Effects of Acetaminophen in Preclinical Models of Nociceptive Processing

Acetaminophen (APAP) in humans has robust effects with a high therapeutic index in altering postoperative and inflammatory pain states in clinical and experimental pain paradigms with no known abuse potential. This review considers the literature reflecting the preclinical actions of acetaminophen in a variety of pain models. Significant observations arising from this review are as follows: 1) acetaminophen has little effect upon acute nociceptive thresholds; 2) acetaminophen robustly reduces facilitated states as generated by mechanical and thermal hyperalgesic end points in mouse and rat models of carrageenan and complete Freund’s adjuvant evoked inflammation; 3) an antihyperalgesic effect is observed in models of facilitated processing with minimal inflammation (eg, phase II intraplantar formalin); and 4) potent anti-hyperpathic effects on the thermal hyperalgesia, mechanical and cold allodynia, allodynic thresholds in rat and mouse models of polyneuropathy and mononeuropathies and bone cancer pain. These results reflect a surprisingly robust drug effect upon a variety of facilitated states that clearly translate into a wide range of efficacy in preclinical models and to important end points in human therapy. The specific systems upon which acetaminophen may act based on targeted delivery suggest both a spinal and a supraspinal action. Review of current targets for this molecule excludes a role of cyclooxygenase inhibitor but includes effects that may be mediated through metabolites acting on the TRPV1 channel, or by effect upon cannabinoid and serotonin signaling. These findings suggest that the mode of action of acetaminophen, a drug with a long therapeutic history of utilization, has surprisingly robust effects on a variety of pain states in clinical patients and in preclinical models with a good therapeutic index, but in spite of its extensive use, its mechanisms of action are yet poorly understood.

Synergistic interaction between amitriptyline and paracetamol in persistent and neuropathic pain models: An isobolografic analysis

The current study was designed to evaluate the transient antinociceptive interaction between amitriptyline and paracetamol in the formalin test. In addition, considering other long-term neuroprotective mechanisms of these drugs, we hypothesized that this combination might exert some synergistic effects on neuropathic pain linked with its possible ability to prevent Wallerian degeneration (WD). The effects of individual and fixed-ratio of 1:1 combinations of orally administered amitriptyline and paracetamol were assayed in the two phases of the formalin test and in the chronic constriction injury (CCI) model in rats. Isobolographic analysis was employed to characterize the synergism produced by the combinations. Amitriptyline, paracetamol, and fixed-ratio amitriptyline-paracetamol combinations produced dose-dependent antinociceptive effects mainly on the inflammatory tonic phase. Repeated doses of individual drugs and their combination decreased CCI-induced mechanical allodynia in a dose-dependent manner. ED₃₀ (formalin) and ED₅₀ (CCI) values were estimated for the individual drugs, and isobolograms were constructed. Theoretical ED_30/50 values for the combination estimated from the isobolograms were 16.5 ± 3.9 mg/kg and 26.0 ± 7.2 mg/kg for the single and repeated doses in persistent and neuropathic pain models, respectively. These values were significantly higher than the actually observed ED_30/50 values, which were 0.39 ± 0.1 mg/kg and 8.2 ± 0.8 mg/kg in each model, respectively, indicating a synergistic interaction. Remarkably, CCI-induced sciatic nerve WD-related histopathological changes were prevented by this combination compared to either drug administered alone.

Prostaglandin D2 contributes to cisplatin-induced neuropathic pain in rats via DP2 receptor in the spinal cord

Background

Chemotherapy-induced peripheral neuropathy (CIPN) is a major reason for stopping or changing anticancer therapy. Among the proposed pathomechanisms underlying CIPN, proinflammatory processes have attracted increasing attention. Here we assessed the role of prostaglandin D² (PGD²) signaling in cisplatin-induced neuropathic pain.

Methods

CIPN was induced by intraperitoneal administration of cisplatin 2 mg/kg for 4 consecutive days using adult male Sprague-Dawley rats. PGD² receptor DP1 and/or DP2 antagonists were administered intrathecally and the paw withdrawal thresholds were measured using von Frey filaments. Spinal expression of DP1, DP2, hematopoietic PGD synthase (H-PGDS), and lipocalin PGD synthase (L-PGDS) proteins were analyzed by western blotting.

Results

The DP1 and DP2 antagonist AMG 853 and the selective DP2 antagonist CAY10471, but not the DP1 antagonist MK0524, significantly increased the paw withdrawal threshold compared to vehicle controls (P = 0.004 and P < 0.001, respectively). Western blotting analyses revealed comparable protein expression levels in DP1 and DP2 in the spinal cord. In the CIPN group the protein expression level of L-PGDS, but not of H-PGDS, was significantly increased compared to the control group (P < 0.001).

Conclusions

The findings presented here indicate that enhanced PGD² signaling, via upregulation of L-PGDS in the spinal cord, contributes to mechanical allodynia via DP2 receptors in a cisplatin-induced neuropathic pain model in rats, and that a blockade of DP2 receptor activation may present a novel therapeutic target for managing CIPN.

An overview of ongoing clinical trials assessing pharmacological therapeutic strategies to manage chemotherapy-induced peripheral neuropathy, based on preclinical studies in rodent models

Chemotherapy-induced peripheral neuropathy (CIPN) is a major dose-limiting side effect induced by a variety of chemotherapeutic agents. Symptoms are mainly sensory: pain, tingling, numbness, and temperature sensitivity. They may require the tapering of chemotherapy regimens or even their cessation; thus, the prevention/treatment of CIPN is critical to increase effectiveness of cancer treatment. However, CIPN management is mainly based on conventional neuropathic pain treatments, with poor clinical efficacy. Therefore, significant effort is made to identify new pharmacological targets to prevent/treat CIPN. Animal modeling is a key component in predicting human response to drugs and in understanding the pathophysiological mechanisms underlying CIPN. In fact, studies performed in rodents highlighted several pharmacological targets to treat/prevent CIPN. This review provides updated information about ongoing clinical trials testing drugs for the management of CIPN and presents some of their proof-of-concept studies conducted in rodent models. The presented drugs target oxidative stress, renin-angiotensin system, glutamatergic neurotransmission, sphingolipid metabolism, neuronal uptake transporters, nicotinamide adenine dinucleotide metabolism, endocannabinoid system, transient receptor potential channels, and serotoninergic receptors. As some clinical trials focus on the effect of the drugs on pain, others evaluate their efficacy by assessing general neuropathy. Moreover, based on studies conducted in rodent models, it remains unclear if some of the tested drugs act in an antinociceptive fashion or have neuroprotective properties. Thus, further investigations are needed to understand their mechanism of action, as well as a global standardization of the methods used to assess efficacy of new therapeutic strategies in the treatment of CIPN.

Synergistic antiallodynic and antihyperalgesic interaction between L-DOPA and celecoxib in parkinsonian rats is mediated by NO-cGMP-ATP-sensitive K+ channel

Pain is a usual and troublesome non-motor symptom of Parkinson's disease, with a prevalence of 29-82%. Therefore, it's vital to find pharmacological treatments for managing PD-associated pain symptoms, to improve patients' quality of life. For this reason, we tested the possible synergy between L-DOPA and celecoxib in decreasing allodynia and hyperalgesia induced by unilateral lesioning with 6-OHDA into the SNpc in rats. We also tested whether the antiallodynic and antihyperalgesic effect induced by combination of L-DOPA and celecoxib is mediated by the NO-cGMP-ATP-sensitive K⁺ channel pathway. Tactile allodynia and mechanical hyperalgesia were evaluated using von Frey filament. Isobolographic analyses were employed to define the nature of the drug interaction using a fixed dose ratio (0.5: 0.5). We found that acute and sub-acute (10-day) treatment with a single dose of L-DOPA (3-25 mg/kg, i. p.) or celecoxib (2.5-20 mg/kg, i. p.) induced a dose-dependent antiallodynic and antihyperalgesic effect in parkinsonian rats. Isobolographic analysis revealed that the ED₅₀ values obtained by L-DOPA + celecoxib combination was significantly less than calculated additive values, indicating that co-administration of L-DOPA with celecoxib produces synergistic interactions in its antiallodynic and antihyperalgesic effect in animals with nigrostriatal lesions. Moreover, the antiallodynic and antihyperalgesic effects induced by L-DOPA + celecoxib combination were blocked by intrathecal pre-treatment with L-NAME, ODQ, and glibenclamide. Taken together, the data suggest that L-DOPA + celecoxib combination produces an antiallodynic and antihyperalgesic synergistic interaction at the systemic level, and these effects are mediated, at the central level, through activation of the NO-cGMP-ATP-sensitive K⁺ channel pathway.

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 Antiallodynic Effect of Nefopam on Vincristine-Induced Neuropathy in Mice

Background

Chemotherapy-induced neuropathic pain is a disabling condition following cancer treatment. Vincristine has more neurotoxicity than other vinca alkaloid agents. This study evaluated the correlation of different doses of nefopam with antiallodynic effects in a mouse vincristine neuropathy model.

Methods

A peripheral neuropathic mouse model was made by intraperitoneal injection of vincristine (0.1 mg/kg/day; 5-day-on, 2-day-off schedule over 12 days). After the development of allodynia, mice were injected intraperitoneally with 0.9% normal saline (NS group) or various doses (10, 30, 60 mg/kg) of nefopam (Nefopam group). We examined allodynia using von Frey hairs pre-administration and at 30, 60, 90, 120, 180, 240 mins, and 24 hrs after drug administration. We also measured the neurokinin-1 receptor concentrations in the spinal cord to confirm the antiallodynic effect of nefopam after drug administration.

Results

The peripheral neuropathic mouse model showed prominent mechanical allodynia. Intraperitoneal nefopam produced a clear dose-dependent increase in paw withdrawal threshold compared with pre-administration values and versus the NS group. The concentration of neurokinin-1 receptor was significantly decreased in the Nefopam group (P<0.05).

Conclusion

Intraperitoneally administered nefopam yielded a dose-dependent attenuation of mechanical allodynia and decreased neurokinin-1 receptor concentration, suggesting that the neurokinin-1 receptor is involved in the antiallodynic effects of nefopam in vincristine neuropathy.

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.

Somatosensory predictors of response to pregabalin in painful chemotherapy-induced peripheral neuropathy: a randomized, placebo-controlled, crossover study

Painful chemotherapy-induced peripheral neuropathy (CIPN) is a debilitating and treatment-resistant sequela of many chemotherapeutic medications. Ligands of α2δ subunits of voltage-gated Ca channels, such as pregabalin, have shown efficacy in reducing mechanical sensitivity in animal models of neuropathic pain. In addition, some data suggest that pregabalin may be more efficacious in relieving neuropathic pain in subjects with increased sensitivity to pinprick. We hypothesized that greater mechanical sensitivity, as quantified by decreased mechanical pain threshold at the feet, would be predictive of a greater reduction in average daily pain in response to pregabalin vs placebo. In a prospective, randomized, double-blinded study, 26 patients with painful CIPN from oxaliplatin, docetaxel, or paclitaxel received 28-day treatment with pregabalin (titrated to maximum dose 600 mg per day) and placebo in crossover design. Twenty-three participants were eligible for efficacy analysis. Mechanical pain threshold was not significantly correlated with reduction in average pain (P = 0.97) or worst pain (P = 0.60) in response to pregabalin. There was no significant difference between pregabalin and placebo in reducing average daily pain (22.5% vs 10.7%, P = 0.23) or worst pain (29.2% vs 16.0%, P = 0.13) from baseline. Post hoc analysis of patients with CIPN caused by oxaliplatin (n = 18) demonstrated a larger reduction in worst pain with pregabalin than with placebo (35.4% vs 14.6%, P = 0.04). In summary, baseline mechanical pain threshold tested on dorsal feet did not meaningfully predict the analgesic response to pregabalin in painful CIPN.

Investigating the possible pain attenuating mechanisms of pregabalin in chronic constriction injury-induced neuropathic pain in rats

Aim of the study: The current study was aimed to investigate the neuropathic pain attenuating mechanism of pregabalin using chronic constriction injury (CCI) model in rats. Material and Methods: The sciatic nerve was ligated by placing four loose ligatures around it to induce neuropathic pain. The pain development in terms of cold allodynia, mechanical hyperalgesia, and heat hyperalgesia was assessed on the 7th and 14th day after surgery, using acetone drop, pinprick, and hot plate tests. On the 14th day after the injury, pain parameters were assessed 30 minutes after administration of pregabalin (30 mg/kg) and sodium nitroprusside (5 mg/kg) in CCI-subjected rats. Results: CCI led to induction of neuropathic pain, which was more prominent on 14th day in comparison to 7th day. A single administration of pregabalin and sodium nitroprusside on 14th day, markedly reduced pain parameters and increased serum nitrite levels. Pretreatment with L-NAME abolished neuropathic pain attenuating effects of pregabalin suggesting that pregabalin may increase the levels of nitric oxide to mitigate neuropathic pain. Pretreatment with naloxone significantly abrogated pain attenuating effects of pregabalin and sodium nitroprusside in CCI-subjected rats suggesting that pregabalin and nitric oxide-mediated analgesic action are mediated through release of endogenous opioids. Moreover, naloxone failed to modulate pregabalin-induced increase in nitric oxide levels suggesting that the opioid system does not control the nitric oxide levels, and opioids may be downstream modulators of nitric oxide. Conclusion: Pregabalin may increase the release of nitric oxide, which may increase the release of endogenous opioids to attenuate neuropathic pain in CCI subjected rats.

Attenuation of vincristine-induced neuropathy by synthetic cyclohexenone-functionalized derivative in mice model

Vincristine (VCR) is a well-known anticancer drug which frequently induced painful neuropathy and impairs the quality of life of patients. The present study was designed to investigate the alleviative potential of a novel cyclohexenone derivative (CHD), i.e., ethyl 6-(4-methoxyphenyl)-2-oxo-4-phenylcyclohexe-3-enecarboxylate, against VCR-induced neuropathic pain in mice model. VCR was administered intraperitoneally for 10 days in two cycles to induce neuropathic pain. Static and dynamic mechanical allodynia was evaluated using von Frey hair filaments and cotton buds, respectively. Paw thermal hyperalgesia was determined through a hot plate analgesiometer. The tail cold immersion hyperalgesia and paw cold allodynia were determined by available standard protocols. The formalin nociception was induced via subplantar injection of formalin. The antioxidant potential was evaluated via 2,2-diphenyl-1-picrylhydrazyl free radical scavenging activity. The outcome of this study revealed that CHD (30-45 mg/kg) and gabapentin (75 mg/kg) significantly enhanced the paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) in static and dynamic allodynia, respectively, and increased the PWL in thermal hyperalgesia and tail withdrawal latency (TWL) as compared to the VCR-treated group. CHD significantly augmented the paw withdrawal duration (PWD) in paw cold allodynia, while the same compound only increased the paw elevation and paw licking in the delayed phase of formalin nociception. Moreover, CHD significantly inhibited the DPPH free radical scavenging action (IC₅₀ = 56), butylated hydroxytoluene (BHT) (IC₅₀ = 39), and ascorbic acid (IC₅₀ = 2.93). In conclusion, CHD exhibited a profile of potential attenuative effect against the VCR-induced neuropathic pain which might be attributed to its possible antinociceptive and antioxidant effect.

The Analgesic Effect of Venlafaxine and Its Mechanism on Oxaliplatin-Induced Neuropathic Pain in Mice

The analgesic effect of venlafaxine (VLX), which is a selective serotonin and noradrenaline reuptake inhibitor (SNRI), has been observed on oxaliplatin-induced neuropathic pain in mice. Significant allodynia was shown after oxaliplatin treatment (6 mg/kg, i.p.); acetone and von Frey hair tests were used to assess cold and mechanical allodynia, respectively. Intraperitoneal administration of VLX at 40 and 60 mg/kg, but not 10 mg/kg, significantly alleviated these allodynia. Noradrenaline depletion by pretreatment of N-(2-Chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, 50 mg/kg, i.p.) blocked the relieving effect of VLX (40 mg/kg, i.p.) on cold and mechanical allodynia. However, serotonin depletion by three consecutive pretreatments of para-chlorophenylalanine (PCPA, 150 mg/kg/day, i.p.) only blocked the effect of VLX on mechanical allodynia. In cold allodynia, the α₂-adrenergic antagonist idazoxan (10 μg, i.t.), but not the α₁-adrenergic antagonist prazosin (10 μg, i.t.), abolished VLX-induced analgesia. Furthermore, idazoxan and 5-HT₃ receptor antagonist bemesetron (MDL-72222, 15 μg, i.t.), but not prazosin or mixed 5-HT_{1, 2} receptor antagonist methysergide (10 μg, i.t.), abolished VLX-induced analgesia in mechanical allodynia. In conclusion, 40 mg/kg of VLX treatment has a potent relieving effect against oxaliplatin-induced neuropathic pain, and α₂-adrenergic receptor, and both α₂-adrenergic and 5-HT₃ receptors are involved in this effect of VLX on cold and mechanical allodynia, respectively.

High-Intensity Ultrasound Treatment for Vincristine-Induced Neuropathic Pain

BACKGROUND

Vincristine is a commonly used chemotherapeutic agent that results in debilitating untreatable peripheral neuropathy.

OBJECTIVE

To determine the effects of pulsed high-intensity focused ultrasound (HIFU) on sensory thresholds in a validated vincristine-induced neuropathy (VIN) rodent model.

METHODS

VIN was induced and mechanical allodynia was confirmed by nociceptive testing. von Frey fibers and Randall-Sellito test were used as measures of innocuous and noxious mechanical thresholds, respectively, and the hot plate test for thermal thresholds. Tests were performed before VIN, after 2 wk of vincristine, at 24, 48, 72, and 120 h after HIFU applied to the left L5 dorsal root ganglia at 3 Watts for 3 min. Comparisons were made between a VIN cohort who underwent HIFU, a VIN cohort who underwent sham HIFU, and naïve rodents who underwent HIFU.

RESULTS

VIN HIFU rats had significantly increased mechanical thresholds at 24 h (P &lt; .001), 48 h (P = .008), 72 h (P = .003), and 120 h (P = .03) after treatment, when compared to pre-HIFU thresholds. Furthermore, at 24 and 48 h following treatment, VIN HIFU rats had significantly higher innocuous and noxious mechanical thresholds and thermal thresholds than VIN sham HIFU rats (P &lt; .001). Thresholds were not altered in naïve rodents who underwent HIFU. Histological data of L5 dorsal root ganglia of VIN HIFU rats suggest that transient cellular edema resolves by 48 h.

CONCLUSION

Our data suggest that HIFU increases mechanical and thermal thresholds in VIN rodents. Whether HIFU can preclude the development of reduced thresholds in the VIN model warrants further study.

A novel endpoint for the assessment of chemotherapy-induced peripheral neuropathy in rodents: biomechanical properties of peripheral nerve

Chemotherapy-induced peripheral neuropathy (CiPN) is a frequent adverse effect in patients and a leading safety consideration in oncology drug development. Although behavioral assessment and microscopic examination of the nerves and dorsal root ganglia can be incorporated into toxicity studies to assess CiPN risk, more sensitive and less labor-intensive endpoints are often lacking. In this study, rats and mice administered vincristine (75 μg kg^-1  day^-1 , i.p., for 10 days in rats and 100 μg kg^-1  day^-1 , i.p., for 11 days in mice, respectively) were employed as the CiPN models. Behavioral changes were assessed during the dosing phase. At necropsy, the sural or sciatic nerve was harvested from the rats and mice, respectively, and assessed for mechanical and histopathological endpoints. It was found that the maximal load and the load/extension ratio were significantly decreased in the nerves collected from the animals dosed with vincristine compared with the vehicle-treated animals (P < 0.05). Additionally, the gait analysis revealed that the paw print areas were significantly increased in mice (P < 0.01), but not in rats following vincristine administration. Light microscopic histopathology of the nerves and dorsal root ganglia were unaffected by vincristine administration. We concluded that ex vivo mechanical properties of the nerves is a sensitive endpoint, providing a new method to predict CiPN in rodent. Gait analysis may also be a useful tool in these pre-clinical animal models.

Single and combined effects of Δ9 -tetrahydrocannabinol and cannabidiol in a mouse model of chemotherapy-induced neuropathic pain

BACKGROUND AND PURPOSE

The non-psychoactive phytocannabinoid cannabidiol (CBD) can affect the pharmacological effects of Δ⁹ -tetrahydrocannabinol (THC). We tested the possible synergy between CBD and THC in decreasing mechanical sensitivity in a mouse model of paclitaxel-induced neuropathic pain. We also tested the effects of CBD on oxaliplatin- and vincristine-induced mechanical sensitivity.

EXPERIMENTAL APPROACH

Paclitaxel-treated mice (8.0 mg·kg^-1 i.p., days 1, 3, 5 and 7) were pretreated with CBD (0.625-20.0 mg·kg^-1 i.p.), THC (0.625-20.0 mg·kg^-1 i.p.) or CBD + THC (0.04 + 0.04-20.0 + 20.0 mg·kg^-1 i.p.), and mechanical sensitivity was assessed on days 9, 14 and 21. Oxaliplatin-treated (6.0 mg·kg^-1 i.p., day 1) or vincristine-treated mice (0.1 mg·kg^-1 i.p. days 1-7) were pretreated with CBD (1.25-10.0 mg·kg^-1 i.p.), THC (10.0 mg·kg^-1 i.p.) or THC + CBD (0.16 mg·kg^-1 THC + 0.16 mg·kg^-1 CBD i.p.).

KEY RESULTS

Both CBD and THC alone attenuated mechanical allodynia in mice treated with paclitaxel. Very low ineffective doses of CBD and THC were synergistic when given in combination. CBD also attenuated oxaliplatin- but not vincristine-induced mechanical sensitivity, while THC significantly attenuated vincristine- but not oxaliplatin-induced mechanical sensitivity. The low dose combination significantly attenuated oxaliplatin- but not vincristine-induced mechanical sensitivity.

CONCLUSIONS AND IMPLICATIONS

CBD may be potent and effective at preventing the development of chemotherapy-induced peripheral neuropathy, and its clinical use may be enhanced by co-administration of low doses of THC. These treatment strategies would increase the therapeutic window of cannabis-based pharmacotherapies.

Noradrenergic inhibition of spinal hyperexcitation elicited by cutaneous cold stimuli in rats with oxaliplatin-induced allodynia: electrophysiological and behavioral assessments

We investigated the spinal action of noradrenaline on cold-elicited hyperexcitation detected in dorsal horn neurons of rats with allodynia induced by an oxaliplatin (6 mg/kg, i.p.) injection. In vivo extracellular recordings from the spinal dorsal horn showed that wide dynamic range neurons responded to cutaneous acetone (10 μl) stimulation in normal rats, and cold-elicited firings in oxaliplatin-administered rats were increased with a longer duration, correlated with behavioral responses. These responses were significantly attenuated by spinal administration (50 μM) of noradrenaline or its agonists, clonidine (α2), phenylephrine (α1) and isoprenaline (β), in descending order of efficacy. Thus, the inhibitory effect of noradrenaline on spinal oxaliplatin-induced cold hyperexcitation is mediated mainly by activation of α2- and/or α1-adrenoceptors.

Prophylactic treatment with the tricyclic antidepressant desipramine prevents development of paclitaxel-induced neuropathic pain through activation of endogenous analgesic systems

Neuropathic pain impacts approximately 3-4.5% of the global population and remains an unresolved health problem. The management of neuropathic pain has two distinct goals-prevention of development and control of established neuropathic pain. We examined the impact of both prophylactic and therapeutic treatments with the tricyclic antidepressant desipramine on the development and maintenance of toxic neuropathic pain induced by the chemotherapeutic agent paclitaxel. We also investigated the involvement of endogenous analgesic (i.e., endogenous opioid and endocannabinoid) systems in the antinociceptive actions of desipramine in these two distinct phases of neuropathic pain. Chronic subcutaneous infusion of desipramine via osmotic pumps suppressed both the development and maintenance of paclitaxel-induced neuropathic pain. However, only prophylactic desipramine treatment blocked the development of neuropathic pain throughout the three month observation interval; neuropathic pain did not return. The opioid receptor antagonist naloxone blocked the antinociceptive effects of both prophylactic and therapeutic desipramine treatments throughout the entire timecourse of desipramine-induced antinociception. By contrast, cannabinoid CB₁ and CB₂ receptor antagonists partially attenuated the antinociceptive actions of desipramine in a manner that was restricted to the development phase of paclitaxel-induced neuropathic pain only. Paclitaxel decreased cell viability in TMD231 tumor cells in an MTT assay in vitro. Notably, desipramine (1nM-1μM) alone did not alter tumor cell viability and did not prevent the cytotoxic effects of paclitaxel under identical conditions. The highest concentration of desipramine (10μM) reduced tumor cell viability alone and enhanced the cytotoxic effects of paclitaxel. Our study identifies a previously unrecognized preemptive analgesic strategy that prevents development of paclitaxel-induced neuropathic pain, and also dissects receptor mechanisms underlying desipramine-induced antinociceptive effects. This information may be applied to improve current therapeutic strategies with the goal of preventing and managing neuropathic pain induced by chemotherapeutic treatment.

Neuroprotective Effect of Matrine in Mouse Model of Vincristine-Induced Neuropathic Pain

Chemotherapy drugs such as vincristine (VCR) can cause neuropathic pain, and there is still lack of ideal strategy to treat it. The current study was designed to investigate effect of matrine (MT) on VCR-induced neuropathic pain in animal model. VCR (75 μg/kg, i.p. for 10 consecutive days) was administered to induce painful neuropathy model in mice. MT (15, 30 and 60 mg/kg, i.p.) and pregabalin (10 mg/kg, i.p.) were administered for 11 consecutive days. Various tests were performed to assess the degree of pain at different days (1, 6, 11, 16, and 21). Von Frey hair, hot plate, cold-plate and paw pressure tests were conducted to assess the degree of mechanical allodynia, thermal hyperalgesia, cold allodynia and mechanical hyperalgesia in the hind paw respectively. The electrophysiological and histopathological changes were also analyzed. Furthermore, tissue malondialdehyde (MDA), total antioxidant capacity (T-AOC),superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), total calcium (TCA), myeloperoxidase (MPO), tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), interleukin-10 (IL-10) were measured to investigate possible involvement of MT in inflammation and oxidative stress. Administration of MT attenuated the VCR-induced behavioral alterations as well as electrophysiological and histopathological changes in a dose dependent manner. Further, MT also attenuated the VCR-induced oxidative stress (MDA, T-AOC, GSH-Px, SOD and TCA) and inflammation (MPO, TNF-α, IL-6 and IL-10). Taken together, MT ameliorated VCR-induced painful neuropathy, which might be attributed to neuroprotective effects by subsequent reduction in oxidative stress and anti-inflammatory actions.

Anti-allodynic effect of theoesberiven F in a vincristine-induced neuropathy model

Allodynia is a main symptom of the peripheral neuropathy induced by vincristine treatment. Theoesberiven F, a combination of Melilotus extract and proxyphylline, is currently used for the treatment of inflammatory conditions due to its potent anti-inflammatory and analgesic properties. The anti-allodynic effect of theoesberiven F on mechanical and cold allodynia in a rat model of vincristine-induced neuropathy was investigated in the present study. Intraperitoneal vincristine injections were administered to male Sprague-Dawley rats at a dose of 0.1 mg/kg/day over 12 days (5 days of injection, a 2-day cessation and 5 days of injection). Rats that were allodynic following the vincristine injections were randomly allocated into four groups. Normal saline was injected into rats in the control group. Theoesberiven F (0.1, 0.25 and 0.5 mg/kg) was administered to rats in the three experimental groups, respectively. Mechanical and cold allodynia were measured at preadministration and at 15, 30, 60, 90, 120, 150 and 180 min following the intraperitoneal administration of normal saline or theoesberiven F. The decreased paw withdrawal threshold induced by vincristine injection was increased by theoesberiven F administration. The increased withdrawal frequency to cold stimuli developed by vincristine was reduced by theoesberiven F administration. The results of this study suggest that the administration of theoesberiven F may be beneficial in reducing the mechanical and cold allodynia developed during vincristine treatment.

Dextromethorphan: An update on its utility for neurological and neuropsychiatric disorders

Dextromethorphan (DM) is a commonly used antitussive and is currently the only FDA-approved pharmaceutical treatment for pseudobulbar affect. Its safety profile and diverse pharmacologic actions in the central nervous system have stimulated new interest for repurposing it. Numerous preclinical investigations and many open-label or blinded clinical studies have demonstrated its beneficial effects across a variety of neurological and psychiatric disorders. However, the optimal dose and safety of chronic dosing are not fully known. This review summarizes the preclinical and clinical effects of DM and its putative mechanisms of action, focusing on depression, stroke, traumatic brain injury, seizure, pain, methotrexate neurotoxicity, Parkinson's disease and autism. Moreover, we offer suggestions for future research with DM to advance the treatment for these and other neurological and psychiatric disorders.

New Insights into Potential Prevention and Management Options for Chemotherapy-Induced Peripheral Neuropathy

OBJECTIVE

Neurological complications such as chemotherapy-induced peripheral neuropathy (CIPN) and neuropathic pain are frequent side effects of neurotoxic chemotherapy agents. An increasing survival rate and frequent administration of adjuvant chemotherapy treatments involving neurotoxic agents makes it imperative that accurate diagnosis, prevention, and treatment of these neurological complications be implemented.

METHODS

A consideration was undertaken of the current options regarding protective and treatment interventions for patients undergoing chemotherapy with neurotoxic chemotherapy agent or experience with CIPN. Current knowledge on the mechanism of action has also been identified. The following databases PubMed, the Cochrane Library, Science Direct, Scopus, EMBASE, MEDLINE, CINAHL, CNKI, and Google Scholar were searched for relevant article retrieval.

RESULTS

A range of pharmaceutical, nutraceutical, and herbal medicine treatments were identified that either showed efficacy or had some evidence of efficacy. Duloxetine was the most effective pharmaceutical agent for the treatment of CIPN. Vitamin E demonstrated potential for the prevention of cisplatin-IPN. Intravenous glutathione for oxaliplatin, Vitamin B6 for both oxaliplatin and cisplatin, and omega 3 fatty acids for paclitaxel have shown protection for CIPN. Acetyl-L-carnitine may provide some relief as a treatment option. Acupuncture may be of benefit for some patients and Gosha-jinki-gan may be of benefit for protection from adverse effects of oxaliplatin induced peripheral neuropathy.

CONCLUSIONS

Clinicians and researchers acknowledge that there are numerous challenges involved in understanding, preventing, and treating peripheral neuropathy caused by chemotherapeutic agents. New insights into mechanisms of action from chemotherapy agents may facilitate the development of novel preventative and treatment options, thereby enabling medical staff to better support patients by reducing this debilitating side effect.

The effect of thioctic acid on allodynia in a rat vincristine-induced neuropathy model

Objective

To investigate the antiallodynic effects of thioctic acid in vincristine-induced neuropathy in rats.

Methods

Neuropathy was induced in Sprague–Dawley rats via vincristine intraperitoneal injection. After 15 days, rats were investigated for the presence of mechanical and cold allodynia, and those with allodynia received intraperitoneal injection with normal saline or 1, 5, or 10 mg/kg thioctic acid. Mechanical and cold allodynia were assessed before treatment and at 15, 30, 60, 90, 150 and 180 min after treatment.

Results

Mechanical and cold allodynia were reduced by thioctic acid injection. The duration of effect increased with thioctic acid dose.

Conclusion

Thioctic acid may be an effective treatment for vincristine-induced neuropathy.

Cannabidiol inhibits paclitaxel-induced neuropathic pain through 5-HT(1A) receptors without diminishing nervous system function or chemotherapy efficacy

BACKGROUND AND PURPOSE

Paclitaxel (PAC) is associated with chemotherapy-induced neuropathic pain (CIPN) that can lead to the cessation of treatment in cancer patients even in the absence of alternate therapies. We previously reported that chronic administration of the non-psychoactive cannabinoid cannabidiol (CBD) prevents PAC-induced mechanical and thermal sensitivity in mice. Hence, we sought to determine receptor mechanisms by which CBD inhibits CIPN and whether CBD negatively effects nervous system function or chemotherapy efficacy.

EXPERIMENTAL APPROACH

The ability of acute CBD pretreatment to prevent PAC-induced mechanical sensitivity was assessed, as was the effect of CBD on place conditioning and on an operant-conditioned learning and memory task. The potential interaction of CBD and PAC on breast cancer cell viability was determined using the MTT assay.

KEY RESULTS

PAC-induced mechanical sensitivity was prevented by administration of CBD (2.5 - 10 mg·kg⁻¹) in female C57Bl/6 mice. This effect was reversed by co-administration of the 5-HT(1A) antagonist WAY 100635, but not the CB₁ antagonist SR141716 or the CB₂ antagonist SR144528. CBD produced no conditioned rewarding effects and did not affect conditioned learning and memory. Also, CBD + PAC combinations produce additive to synergistic inhibition of breast cancer cell viability.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that CBD is protective against PAC-induced neurotoxicity mediated in part by the 5-HT(1A) receptor system. Furthermore, CBD treatment was devoid of conditioned rewarding effects or cognitive impairment and did not attenuate PAC-induced inhibition of breast cancer cell viability. Hence, adjunct treatment with CBD during PAC chemotherapy may be safe and effective in the prevention or attenuation of CIPN.

Development and pharmacological characterization of a model of sleep disruption-induced hypersensitivity in the rat

BACKGROUND

Sleep disturbance is a commonly reported co-morbidity in chronic pain patients, and conversely, disruption of sleep can cause acute and long-lasting hypersensitivity to painful stimuli. The underlying mechanisms of sleep disruption-induced pain hypersensitivity are poorly understood. Confounding factors of previous studies have been the sleep disruption protocols, such as the 'pedestal over water' or 'inverted flower pot' methods, that can cause large stress responses and therefore may significantly affect pain outcome measures.

METHODS

Sleep disruption was induced by placing rats for 8 h in a slowly rotating cylindrical cage causing arousal via the righting reflex. Mechanical (Von Frey filaments) and thermal (Hargreaves) nociceptive thresholds were assessed, and plasma corticosterone levels were measured (mass spectroscopy). Sleep disruption-induced hypersensitivity was pharmacologically characterized with drugs relevant for pain treatment, including gabapentin (30 mg/kg and 50 mg/kg), Ica-6p (Kv7.2/7.3 potassium channel opener; 10 mg/kg), ibuprofen (30 mg/kg and 100 mg/kg) and amitriptyline (10 mg/kg).

RESULTS

Eight hours of sleep disruption caused robust mechanical and heat hypersensitivity in the absence of a measurable change in plasma corticosterone levels. Gabapentin had no effect on reduced nociceptive thresholds. Ibuprofen attenuated mechanical thresholds, while Ica-6p and amitriptyline attenuated only reduced thermal nociceptive thresholds.

CONCLUSIONS

These results show that acute and low-stress sleep disruption causes mechanical and heat hypersensitivity in rats. Mechanical and heat hypersensitivity exhibited differential sensitivity to pharmacological agents, thus suggesting dissociable mechanisms for those two modalities. Ultimately, this model could help identify underlying mechanisms linking sleep disruption and hypersensitivity.

Low doses of dextromethorphan have a beneficial effect in the treatment of neuropathic pain

N-methyl-D-aspartate receptor (NMDAR) antagonists may be given in persistent neuropathic pain, but adverse events especially with ketamine may limit their clinical use. Less central and cognitive adverse events are described with dextromethorphan and memantine. These molecules have been explored in many preclinical and clinical studies, but data are conflicting as regards neuropathic pain alleviation. Dextromethorphan and memantine have been administered to animals after spinal nerve ligation (SNL) to evaluate their antinociceptive/cognitive effects and associated molecular events, including the phosphorylation of several tyrosine (pTyr(1336), pTyr(1472)) residues in the NR2B NMDAR subunit. Spinal nerve ligation and sham animals received dextromethorphan (10 mg/kg, i.p.), memantine (20 mg/kg, i.p.) or saline (1 mL/kg, i.p.). These drugs were administered once symptoms of allodynia and hyperalgesia had developed. Tests were carried out before and after surgery. Tactile allodynia, mechanical hyperalgesia and spatial memory were, respectively, evaluated by von Frey, Randall & Selitto and Y-maze tests and molecular events by Western blot analysis. Spinal nerve-ligated animals displayed nociception and impaired spatial memory. Dextromethorphan, but not memantine, reversed neuropathic pain (NP) symptoms, restored spatial memory integrity and decreased the expression of pTyr(1336)NR2B. Following postoperative administration of dextromethorphan, this study has demonstrated for the first time a concordance between behaviour, cognitive function and molecular events via pTyr(1336)NR2B for neuropathic pain alleviation. Confirmation of these findings in patients would constitute a major step forward in the treatment of neuropathic pain and in the improvement of cognitive function and quality of life.

Gabapentin inhibits bortezomib-induced mechanical allodynia through supraspinal action in mice

Bortezomib, an inhibitor of proteasome holoenzyme, is used to treat relapsed and refractory multiple myeloma. Peripheral neuropathy is a treatment-limiting adverse effect of bortezomib and is very difficult to control. In this study, we examined the efficacy of gabapentin in inhibiting bortezomib-induced peripheral neuropathy. Single intravenous injections of bortezomib (0.03 - 0.3 mg/kg) dose-dependently induced mechanical allodynia with a peak effect 12 days after injection. Bortezomib (0.3 mg/kg) also caused mechanical hyperalgesia, but neither affected thermal nociception nor induced cold allodynia. Bortezomib increased the response of the saphenous nerve to weak punctate stimulation but not response to cool stimulation of the skin. When administered 12 days after bortezomib injection, oral and intracisternal gabapentin markedly inhibited mechanical allodynia. Intrathecal, but not intraplantar, gabapentin had a tendency to reduce mechanical allodynia. The antiallodynic activity of orally administered gabapentin was suppressed by noradrenaline, but not serotonin, depletion in the spinal cord. Bortezomib did not affect the expression levels of the calcium channel α₂δ-1 subunit, a high-affinity binding site of gabapentin, in the plantar skin, spinal cord, medulla oblongata, and pons. These results suggest that gabapentin inhibits bortezomib-induced mechanical allodynia, most likely through the activation of the descending noradrenergic system.

Spinal changes of a newly isolated neuropeptide endomorphin-2 concomitant with vincristine-induced allodynia

Chemotherapy-induced neuropathic pain (CNP) is the major dose-limiting factor in cancer chemotherapy. However, the neural mechanisms underlying CNP remain unclear. There is increasing evidence implicating the involvement of spinal endomorphin-2 (EM2) in neuropathic pain. In this study, we used a vincristine-evoked rat CNP model displaying mechanical allodynia and central sensitization, and observed a significant decrease in the expression of spinal EM2 in CNP. Also, while intrathecal administration of exogenous EM2 attenuated allodynia and central sensitization, the mu-opioid receptor antagonist β-funaltrexamine facilitated these events. We found that the reduction in spinal EM2 was mediated by increased activity of dipeptidylpeptidase IV, possibly as a consequence of chemotherapy-induced oxidative stress. Taken together, our findings suggest that a decrease in spinal EM2 expression causes the loss of endogenous analgesia and leads to enhanced pain sensation in CNP.

Crocin improved locomotor function and mechanical behavior in the rat model of contused spinal cord injury through decreasing calcitonin gene related peptide (CGRP)

Various approaches have been offered to alleviate chronic pain resulting from spinal cord injuries (SCIs). Application of herbs and natural products, with potentially lower adverse effects, to cure diseases has been recommended in both traditional and modern medicines. Here, the effect of crocin on chronic pain induced by spinal cord contusion was investigated in an animal model. Female Wistar rats were randomly divided into five groups (5 rats in each); three groups were contused at the L1 level. One group was treated with crocin (150mg/kg) two weeks after spinal cord injury; the second group, control, was treated with vehicle only; and the third group was treated with ketoprofen. Two normal groups were also considered with or without crocin treatment. The mechanical behavioral test, the locomotor recovery test and the thermal behavioral test were applied weekly to evaluate the injury and recovery of rats. Significant improvements (p<0.05) in mechanical behavioral and locomotor recovery tests were seen in the rats treated with crocin. Thermal behavioral test did not show any significant changes due to crocin treatment. Plasma concentration of calcitonin-gene related peptide (CGRP) changed from 780.2±2.3 to 1140.3±4.5pg/ml due to SCI and reached 789.1±2.7pg/ml after crocin treatment. These changes were significant at the level of p<0.05. The present study shows the beneficial effects of crocin treatment on chronic pain induced by SCI, through decreasing CGRP as an important mediator of inflammation and pain.

PARP inhibitors attenuate chemotherapy-induced painful neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a major toxicity of chemotherapy treatment for which no therapy is approved. Poly(ADP-ribose) polymerase (PARP)1/2 are nuclear enzymes activated upon DNA damage, and PARP1/2 inhibition provides resistance against DNA damage. A role for PARP inhibition in sensory neurotransmission has also been established. PARP inhibitors attenuate pain-like behaviors and neuropathy-associated decreased peripheral nerve function in diabetic models. The hypothesis tested was that PARP inhibition protects against painful neuropathy. The objective of this study was to investigate whether the novel, selective PARP1/2 inhibitors (ABT-888 and related analogues) would attenuate development of mechanical allodynia in vincristine-treated rats. PARP inhibitors were dosed for 2 days, and then co-administered with vincristine for 12 days. Mechanical allodynia was observed in rats treated with vincristine. PARP1/2 inhibition significantly attenuated development of mechanical allodynia and reduced poly ADP-ribose (PAR) activation in rat skin. The data presented here show that PARP inhibition attenuates vincristine-induced mechanical allodynia in rats, and supports that PARP inhibition may represent a novel therapeutic approach for CIPN.

Spinal astrocytic activation contributes to mechanical allodynia in a rat chemotherapy-induced neuropathic pain model

Chemotherapy-induced neuropathic pain (CNP) is the major dose-limiting factor in cancer chemotherapy. However, the neural mechanisms underlying CNP remain enigmatic. Accumulating evidence implicates the involvement of spinal glia in some neuropathic pain models. In this study, using a vincristine-evoked CNP rat model with obvious mechanical allodynia, we found that spinal astrocyte rather than microglia was dramatically activated. The mechanical allodynia was dose-dependently attenuated by intrathecal administratration of L-α-aminoadipate (astrocytic specific inhibitor); whereas minocycline (microglial specific inhibitor) had no such effect, indicating that spinal astrocytic activation contributes to allodynia in CNP rat. Furthermore, oxidative stress mediated the development of spinal astrocytic activation, and activated astrocytes dramatically increased interleukin-1β expression which induced N-methyl-D-aspartic acid receptor (NMDAR) phosphorylation in spinal neurons to strengthen pain transmission. Taken together, our findings suggest that spinal activated astrocytes may be a crucial component of the pathophysiology of CNP and “Astrocyte-Cytokine-NMDAR-neuron” pathway may be one detailed neural mechanisms underlying CNP. Thus, inhibiting spinal astrocytic activation may represent a novel therapeutic strategy for treating CNP.

Targeting voltage-gated calcium channels: developments in peptide and small-molecule inhibitors for the treatment of neuropathic pain

Chronic pain affects approximately 20% of people worldwide and places a large economic and social burden on society. Despite the availability of a range of analgesics, this condition is inadequately treated, with complete alleviation of symptoms rarely occurring. In the past 30 years, the voltage-gated calcium channels (VGCCs) have been recognized as potential targets for analgesic development. Although the majority of the research has been focused on Ca(v) 2.2 in particular, other VGCC subtypes such as Ca(v) 3.2 have recently come to the forefront of analgesic research. Venom peptides from marine cone snails have been proven to be a valuable tool in neuroscience, playing a major role in the identification and characterization of VGCC subtypes and producing the first conotoxin-based drug on the market, the ω-conotoxin, ziconotide. This peptide potently and selectively inhibits Ca(v) 2.2, resulting in analgesia in chronic pain states. However, this drug is only available via intrathecal administration, and adverse effects and a narrow therapeutic window have limited its use in the clinic. Other Ca(v) 2.2 inhibitors are currently in development and offer the promise of an improved route of administration and safety profile. This review assesses the potential of targeting VGCCs for analgesic development, with a main focus on conotoxins that block Ca(v) 2.2 and the developments made to transform them into therapeutics.

The role of potassium channel activation in celecoxib-induced analgesic action

BACKGROUND AND PURPOSE

Celecoxib (CXB) is a widely prescribed COX-2 inhibitor used clinically to treat pain and inflammation. Recently, COX-2 independent mechanisms have been described to be the targets of CXB. For instance, ion channels such as the voltage-gated sodium channel, L-type calcium channel, Kv2.1, Kv1.5, Kv4.3 and HERG potassium channel were all reported to be inhibited by CXB. Our recent study revealed that CXB is a potent activator of Kv7/M channels. M currents expressed in dorsal root ganglia play an important role in nociception. Our study was aimed at establishing the role of COX-2 independent M current activation in the analgesic action of CXB.

METHODS AND RESULTS

We compared the effects of CXB and its two structural analogues, unmethylated CXB (UMC) and 2,5-dimethyl-CXB (DMC), on Kv7/M currents and pain behavior in animal models. UMC is a more potent inhibitor of COX-2 than CXB while DMC has no COX-2 inhibiting activity. We found that CXB, UMC and DMC concentration-dependently activated Kv7.2/7.3 channels expressed in HEK293 cells and the M-type current in dorsal root ganglia neurons, negatively shifted I-V curve of Kv7.2/7.3 channels, with a potency and efficiency inverse to their COX-2 inhibitory potential. Furthermore, CXB, UMC and DMC greatly reduced inflammatory pain behavior induced by bradykinin, mechanical pain behavior induced by stimulation with von Frey filaments and thermal pain behavior in the Hargreaves test. CXB and DMC also significantly attenuated hyperalgesia in chronic constriction injury neuropathic pain.

CONCLUSION

CXB, DMC and UMC are openers of Kv7/M K(+) channels with effects independent of COX-2 inhibition. The analgesic effects of CXBs on pain behaviors, especially those of DMC, suggest that activation of Kv7/M K(+) channels may play an important role in the analgesic action of CXB. This study strengthens the notion that Kv7/M K(+) channels are a potential target for pain treatment.

Alterations in endocannabinoid tone following chemotherapy-induced peripheral neuropathy: effects of endocannabinoid deactivation inhibitors targeting fatty-acid amide hydrolase and monoacylglycerol lipase in comparison to reference analgesics following cisplatin treatment

Cisplatin, a platinum-derived chemotherapeutic agent, produces mechanical and coldallodynia reminiscent of chemotherapy-induced neuropathy in humans. The endocannabinoid system represents a novel target for analgesic drug development. The endocannabinoid signaling system consists of endocannabinoids (e.g. anandamide (AEA) and 2-arachidonoylglycerol (2-AG)), cannabinoid receptors (e.g. CB(1) and CB(2)) and the enzymes controlling endocannabinoid synthesis and degradation. AEA is hydrolyzed by fatty-acid amide hydrolase (FAAH) whereas 2-AG is hydrolyzed primarily by monoacylglycerol lipase (MGL). We compared effects of brain permeant (URB597) and impermeant (URB937) inhibitors of FAAH with an irreversible inhibitor of MGL (JZL184) on cisplatin-evoked behavioral hypersensitivities. Endocannabinoid modulators were compared with agents used clinically to treat neuropathy (i.e. the opioid analgesic morphine, the anticonvulsant gabapentin and the tricyclic antidepressant amitriptyline). Cisplatin produced robust mechanical and cold allodynia but did not alter responsiveness to heat. After neuropathy was fully established, groups received acute intraperitoneal (i.p.) injections of vehicle, amitriptyline (30 mg/kg), gabapentin (100 mg/kg), morphine (6 mg/kg), URB597 (0.1 or 1 mg/kg), URB937 (0.1 or 1 mg/kg) or JZL184 (1, 3 or 8 mg/kg). Pharmacological specificity was assessed by coadministering each endocannabinoid modulator with either a CB(1) (AM251 3 mg/kg), CB(2) (AM630 3 mg/kg), TRPV1 (AMG9810 3 mg/kg) or TRPA1 (HC030031 8 mg/kg) antagonist. Effects of cisplatin on endocannabinoid levels and transcription of receptors (CB(1), CB(2), TRPV1, TRPA1) and enzymes (FAAH, MGL) linked to the endocannabinoid system were also assessed. URB597, URB937, JZL184 and morphine reversed cisplatin-evoked mechanical and cold allodynia to pre-cisplatin levels. By contrast, gabapentin only partially reversed the observed allodynia while amitriptyline, administered acutely, was ineffective. CB(1) or CB(2) antagonists completely blocked the anti-allodynic effects of both FAAH (URB597, URB937) and MGL (JZL184) inhibitors to mechanical and cold stimulation. By contrast, the TRPV1 antagonist AMG9810 blocked the anti-allodynic efficacy of both FAAH inhibitors, but not the MGL inhibitor. By contrast, the TRPA1 antagonist HC30031 did not attenuate anti-allodynic efficacy of any endocannabinoid modulator. When the levels of endocannabinoids were examined, cisplatin increased both anandamide (AEA) and 2-arachidonoylglycerol (2-AG) levels in the lumbar spinal cord and decreased 2-AG levels (but not AEA) in dorsal hind paw skin. RT-PCR showed that mRNA for FAAH, but not other markers, was upregulated by cisplatin treatment in lumbar spinal cord. The present studies demonstrate that cisplatin alters endocannabinoid tone and that inhibition of endocannabinoid hydrolysis alleviates chemotherapy-induced mechanical and cold allodynia. The anti-allodynic effects of FAAH and MGL inhibitors are mediated by CB(1) and CB(2) cannabinoid receptors, whereas TRPV1, but not TRPA1, -dependent mechanisms contribute to the anti-allodynic efficacy of FAAH (but not MGL) inhibitors. Strikingly, endocannabinoid modulators potently suppressed cisplatin-evoked allodynia with a rapid onset and showed efficacy that equaled or exceeded that of major classes of anti-neuropathic pain medications used clinically. Thus, inhibition of endocannabinoid hydrolysis, via FAAH or MGL inhibitors, represents an efficacious pharmacological approach for suppressing chemotherapy-induced neuropathic pain.

Analgesic effects of dexmedetomidine in vincristine-evoked painful neuropathic rats

Dexmedetomidine, which is a selective α2-adrenoceptor agonist, was recently introduced into clinical practice for its analgesic properties. The purpose of this study was to evaluate the effects of dexmedetomidine in a vincristine-evoked neuropathic rat models. Sprague-Dawley rats were injected intraperitoneally with vincristine or saline (0.1 mg/kg/day) using a 5-day-on, 2-day-off schedule for 2 weeks. Saline and dexmedetomidine (12.5, 25, 50, and 100 µg/kg) were injected to rats developed allodynia 14 days after vincristine injection, respectively. We evaluated allodynia at before, 15, 30, 60, 90, 120, 180, and 240 min, and 24 hr after intraperitoneal drug (normal saline or dexmedetomidine) injection. Saline treatment did not show any differences for all the allodynia. Maximal paw withdrawal thresholds to mechanical stimuli were 3.0 ± 0.4, 9.1 ± 1.9, 13.0 ± 3.6, 16.6 ± 2.4, and 24.4 ± 1.6 g at saline, 12.5, 25, 50, and 100 µg/kg dexmedetomidine injection, respectively. Minimal withdrawal frequency to cold stimuli were 73.3 ± 4.2, 57.1 ± 6.8, 34.3 ± 5.7, 20.0 ± 6.2, and 14.3 ± 9.5 g at saline, 12.5, 25, 50, and 100 µg/kg dexmedetomidine injection, respectively. Dexmedetomidine shows a dose-dependent antiallodynic effect on mechanical and cold stimuli in vincristine-evoked neuropathic rat models (P < 0.05).

Ginkgo biloba extract attenuates hyperalgesia in a rat model of vincristine-induced peripheral neuropathy

BACKGROUND

Chemotherapy-induced peripheral neuropathy is a common, dose-limiting side effect of cancer chemotherapeutic drugs. Hyperalgesia is a common component of neuropathic pain. Ginkgo biloba extract (GBE) is an oriental herbal medicine that has various pharmacological actions. In this study, we evaluated the effects of oral GBE on hyperalgesia in a rat model of vincristine-induced neuropathy.

METHODS

Male Sprague-Dawley rats (200-250 g) were injected intraperitoneally with vincristine or saline (0.1 mg/kg/d) using a 5-day-on, 2-day-off schedule over 12 days. All the behavioral tests for mechanical, cold, and heat hyperalgesia were conducted before the daily injection during the course of vincristine treatment. Rats that developed hyperalgesia 14 days after vincristine injection were randomly assigned into 4 groups. Distilled water and GBE (50, 100, and 150 mg/kg) were administered, respectively, to the individual groups. We examined the hyperalgesia at preadministration and at 15, 30, 60, 90, 120, 150, and 180 minutes after oral drug administration.

RESULTS

Saline injection did not have any significant effect on mechanical, cold, and heat hyperalgesia. Vincristine injection produced mechanical and cold hyperalgesia. For the GBE groups, the paw withdrawal threshold to mechanical stimuli was significantly increased and withdrawal frequency to cold stimuli was significantly reduced versus the control group dose-dependently (P < 0.05).

CONCLUSIONS

This study demonstrates that oral administration of GBE is associated with a dose-dependent antihyperalgesic effect on mechanical and cold stimuli in a rat model of vincristine-induced neuropathy.

Novel small molecule α9α10 nicotinic receptor antagonist prevents and reverses chemotherapy-evoked neuropathic pain in rats

BACKGROUND

Peripheral neuropathy is a common dose-limiting side effect of chemotherapy. There are no clinically proven analgesics for the treatment of this condition. Drugs from different classes have been tested with mixed results. Identification of novel molecular targets for analgesic(s) is important. Antagonism of the α9α10 nicotinic acetylcholine receptor (nAChR) subtype (absent in brain) is thought to underlie analgesic efficacy of peptide α-conotoxins. We found novel nonpeptide small molecule analogs from a family of tetrakis-, tris-, and bis-azaaromatic quaternary ammonium salts (high potency with selectivity as antagonists at the α9α10 nAChRs) to produce dose-related analgesia in rat models of nerve injury-evoked neuropathy and persistent inflammatory pain. No tests were done in a model of neuropathy induced by drug administration (ie, chemotherapy).

METHODS

In this study, a lead bis-analog, ZZ1-61c, was characterized in a rat model of vincristine-evoked neuropathy. Male Sprague-Dawley rats were repeatedly dosed with the vinca-alkaloid, vincristine (100 μg/kg/day IP, days 1 to 5 and 8 to 12). ZZ1-61c (100 μg/kg/day IP) was given either along with or after completion of vincristine (commencing by day 15 when neuropathy was maximum). Responsiveness was assessed with von Frey hairs and the paw-pressure test. The effects of ZZ1-61c on motor function (rotarod) and muscle strength (grip test) were characterized in naïve rats.

RESULTS

The development of neuropathy was demonstrated with repeated dosing of vincristine (pain hypersensitivity in response to mechanical stimulation). ZZ1-61c showed both preventive and restorative effects on this condition: (1) vincristine-evoked sensitivity to pressure was reduced by coadministration of ZZ1-61c; (2) established neuropathy was diminished by ZZ1-61c after cessation of chemotherapy. ZZ1-61c did not cause motor dysfunction (rotarod) or muscular weakness (the grip test).

CONCLUSIONS

This study suggests that ZZ1-61c, a novel compound with a unique mechanism of antagonistic action at the α9α10 nAChR, may be a potential drug candidate for prevention and attenuation of neuropathic pain resulting from chemotherapy. Such a strategy may provide effective treatment that circumvents toxicity of centrally acting agonists at nAChR.