Gabapentin: a new tool in the treatment of neuropathic pain

Kaempferol exerts a neuroprotective effect to reduce neuropathic pain through TLR4/NF-ĸB signaling pathway

Switching microglial polarization from the M1 to M2 phenotype is a promising therapeutic strategy for neuropathic pain (NP). Toll‐like receptor 4 (TLR4) is activated by lipopolysaccharide (LPS). Uncontrolled activation of TLR4 has been proven to trigger chronic inflammation. Kaempferol, a dietary flavonoid, is known to have anti‐inflammatory properties. This study is aimed to investigate the analgesic and anti‐inflammatory effects and the underlying mechanisms of kaempferol, which were explored with an NP model in vivo and LPS‐induced injury in microglial BV2 cells in vitro. The levels of proinflammatory cytokines were evaluated. H&E staining and immunohistochemistry were used to assess the sciatic nerve condition after chronic constriction injury surgery. Western blotting and immunofluorescence were used to determine whether TLR4/NF‐ĸB signaling pathway plays a major role in kaempferol‐mediated alleviation of neuroinflammation. Quantitative real‐time polymerase chain reaction and flow cytometry were used to examine the modulator effect of kaempferol on microglial M1/M2 polarization. We found that kaempferol treatment can significantly reduce NP and proinflammatory cytokine production. Kaempferol attenuated the activation of TLR4/NF‐κB pathways in LPS‐activated BV2 cells. The analgesic effects of kaempferol on NP may be due to inhibition of microglia activation and switching the M1 to M2 phenotype.

Exploring the Promise of Flavonoids to Combat Neuropathic Pain: From Molecular Mechanisms to Therapeutic Implications

Neuropathic pain (NP) is the result of irregular processing in the central or peripheral nervous system, which is generally caused by neuronal injury. The management of NP represents a great challenge owing to its heterogeneous profile and the significant undesirable side effects of the frequently prescribed psychoactive agents, including benzodiazepines (BDZ). Currently, several established drugs including antidepressants, anticonvulsants, topical lidocaine, and opioids are used to treat NP, but they exert a wide range of adverse effects. To reduce the burden of adverse effects, we need to investigate alternative therapeutics for the management of NP. Flavonoids are the most common secondary metabolites of plants used in folkloric medicine as tranquilizers, and have been claimed to have a selective affinity to the BDZ binding site. Several studies in animal models have reported that flavonoids can reduce NP. In this paper, we emphasize the potentiality of flavonoids for the management of NP.

In Vitro and In Vivo Effects of Flavonoids on Peripheral Neuropathic Pain

Neuropathic pain is a common symptom and is associated with an impaired quality of life. It is caused by the lesion or disease of the somatosensory system. Neuropathic pain syndromes can be subdivided into two categories: central and peripheral neuropathic pain. The present review highlights the peripheral neuropathic models, including spared nerve injury, spinal nerve ligation, partial sciatic nerve injury, diabetes-induced neuropathy, chemotherapy-induced neuropathy, chronic constriction injury, and related conditions. The drugs which are currently used to attenuate peripheral neuropathy, such as antidepressants, anticonvulsants, baclofen, and clonidine, are associated with adverse side effects. These negative side effects necessitate the investigation of alternative therapeutics for treating neuropathic pain conditions. Flavonoids have been reported to alleviate neuropathic pain in murine models. The present review elucidates that several flavonoids attenuate different peripheral neuropathic pain conditions at behavioral, electrophysiological, biochemical and molecular biological levels in different murine models. Therefore, the flavonoids hold future promise and can be effectively used in treating or mitigating peripheral neuropathic conditions. Thus, future studies should focus on the structure-activity relationships among different categories of flavonoids and develop therapeutic products that enhance their antineuropathic effects.

6-Methoxyflavanone attenuates mechanical allodynia and vulvodynia in the streptozotocin-induced diabetic neuropathic pain

BACKGROUND

Diabetic neuropathy is the most prevalent, persistent and debilitating complication of diabetes mellitus often coupled with vulvodynia that may present as an isolated symptom or as a part of constellation of other neuropathic abnormalities.

OBJECTIVE

Flavonoids have selective affinity for GABA receptors and 6-methoxyflavanone (6-MeOF) is a positive allosteric modulator of GABA responses at human recombinant GABA_A receptors. GABAergic and opioidergic system inhibition have been shown to facilitate neuropathic pain.

METHODS

6-MeOF was evaluated for analgesic effect in the hot plate test and streptozotocin-induced diabetic neuropathic pain in female rats using von Frey hairs. The possible involvement of opioidergic and GABAergic mechanisms was investigated using naloxone and pentylenetetrazole (PTZ) antagonists, respectively. The biodistribution of 6-MeOF in plasma and CNS was examined using a validated HPLC/UV analytical method. The binding affinity of 6-MeOF with opioid and GABA receptors was studied using molecular docking simulation approach.

RESULTS

6-MeOF (10 and 30mg/kg) attenuated the acute phasic thermal nociception in the hot plate test while in the case of streptozotocin-induced diabetic neuropathy model, 6-MeOF (10 and 30mg/kg) produced static/dynamic anti-allodynic (increased paw withdrawal threshold and latency) as well as static/dynamic anti-vulvodynic effects (increased flinching response threshold and latency), when compared to the vehicle and standard gabapentin (75mg/kg). In silico studies depicted the preference of 6-MeOF for the delta- and kappa-opioid and GABA_A receptors. Moreover, the pharmacokinetic profile revealed a quick appearance of 6-MeOF in the systemic circulation and brain areas with maximum concentration observed after 30min in the amygdala, brain stem and cerebral cortex.

CONCLUSION

6-MeOF readily crosses the blood brain barrier and may be effective in attenuating the diabetes-induced allodynia as well as vulvodynia, probably through interactions with the GABAergic and opioidergic systems.

A clinical approach to complex regional pain syndrome

The goal of this article is to provide an introductory look into current concepts regarding chronic regional pain syndrome. Great advances have been made over the last 15 years, but we are far from a complete understanding of this disorder. This article places great emphasis on early clinical recognition and treatment intervention.

Muscle pain syndromes

This article summarizes the evidence for two major clinical syndromes of muscle pain: fibromyalgia and myofascial pain syndrome. The evidence for diagnostic and treatment approaches is reviewed.

Managing pain after shingles: a nursing perspective

Post-herpetic neuralgia (PHN) is the neuropathic pain syndrome that may develop following an attack of shingles. While for many the symptoms subside, there can be long-term pain problems for up to 40% of those affected with PHN, and for 3% of these, symptoms can be severe (Dworkin and Portenoy, 1996). Knowledge and understanding of the symptoms and various treatments and approaches available is important to enable nurses and patients to work together in partnership to achieve the best outcomes. Realizing that more than one approach may be needed can allow for treatments which are complementary and for optimization of both biomedical and self-care approaches.

Pharmacologic Treatment of Central Post-Stroke Pain

OBJECTIVES

Almost 100 years after the first report of the thalamic syndrome, the scientific basis for the treatment of central post-stroke pain (CPSP) is remarkably small. Therefore, the authors aimed to provide evidence-based recommendations for the treatment of CPSP.

METHODS

The authors performed a systematic review of the literature on the pharmacologic treatment of CPSP. All studies and case series were included and evaluated according to their level of evidence. Only CPSP was considered, not other types of central pain.

RESULTS

Amitriptyline and lamotrigine are the only oral drugs proven to be effective in the treatment of CPSP in a placebo-controlled study. IV drugs such as lidocaine, propofol, and ketamine have shown efficacy for short-term control of CPSP, but their application and potential side effects make them unsuitable for long-term treatment. The novel antiepileptic drug gabapentin has been reported to control CPSP in a few patients.

CONCLUSIONS

Amitriptyline, lamotrigine, and gabapentin provide a more favorable efficacy and safety profile than the classic antiepileptic drugs carbamazepine and phenytoin, for which no placebo-controlled evidence of efficacy was found. Clinical trials are urgently needed to optimize pharmacologic treatment of CPSP.

Differential distribution of voltage-gated calcium channel alpha-2 delta (alpha2delta) subunit mRNA-containing cells in the rat central nervous system and the dorsal root ganglia

Voltage-gated calcium channels (VGCCs) play an essential role in controlling neurotransmitter release, neuronal excitability, and gene expression in the nervous system. The distribution of cells that contain mRNAs encoding the auxiliary alpha2delta-1, alpha2delta-2, and alpha2delta-3 subunits of the VGCCs in the central nervous system (CNS) and the dorsal root ganglia (DRG) was examined in rats by using in situ hybridization. Specific labeling of alpha2delta-1, alpha2delta-2, and alpha2delta-3 mRNAs appeared to be largely confined to neurons and was widely, although differentially, distributed in the brain, the spinal cord, and the DRG. Importantly, alpha2delta-2 mRNA was found to be expressed in interneurons in the cortex, the hippocampus, the striatum, and in regions that contain dense cholinergic neurons. Our results suggest that different alpha2delta subunits may exert distinctive functions in the CNS. The alpha2delta-1 subunit mRNA is localized in brain regions known to be involved in cortical processing, learning and memory, defensive behavior, neuroendocrine secretion, autonomic activation, primary sensory transmission, and general arousal. The alpha2delta-2 subunit mRNA is present in brain regions known to modulate the overall activities of the cortex, the hippocampus, and the thalamus. The alpha2delta-2 subunit is also found in brain regions known to be involved in olfaction, somatic motor control, fluid homeostasis, ingestive and defensive behaviors, neuroendocrine functions, and circadian rhythm. In addition to being localized in brain regions that express alpha2delta-1 and alpha2delta-2 subunit mRNAs, alpha2delta-3 subunit mRNA is highly expressed in regions involved in auditory information processing and somatic movement.

A treatment algorithm for neuropathic pain

BACKGROUND

Neuropathic pain is a chronic pain syndrome caused by drug-, disease-, or injury-induced damage or destruction of sensory neurons within the dorsal root ganglia of the peripheral nervous system. Characteristic clinical symptoms include the feeling of pins and needles; burning, shooting, and/or stabbing pain with or without throbbing; and numbness. Neuronal hyperexcitability represents the hallmark cellular mechanism involved in the underlying pathophysiology of neuropathic pain. Although the primary goal is to alleviate pain, clinicians recognize that even the most appropriate treatment strategy may be, at best, only able to reduce pain to a more tolerable level.

OBJECTIVE

The purpose of this review is to propose a treatment algorithm for neuropathic pain that health care professionals can logically follow and adapt to the specific needs of each patient. The algorithm is intended to serve as a general guide to assist clinicians in optimizing available therapeutic options.

METHODS

A comprehensive review of the literature using the PubMed, MEDLINE, Cochrane, and Toxnet databases was conducted to design and develop a novel treatment algorithm for neuropathic pain that encompasses agents from several drug classes, including antidepressants, antiepileptic drugs, topical antineuralgic agents, narcotics, and analgesics, as well as various treatment options for refractory cases.

RESULTS

Any of the agents in the first-line drug classes (tricyclic antidepressants, antiepileptic drugs, topical antineuralgics, analgesics) may be used as a starting point in the treatment of neuropathic pain. If a patient does not respond to treatment with at least 3 different agents within a drug class, agents from a second drug class may be tried. When all first-line options have been exhausted, narcotic analgesics or refractory treatment options may provide some benefit. Patients who do not respond to monotherapy with any of the first- or second-line agents may respond to combination therapy or may be candidates for referral to a pain clinic. Because the techniques used at pain clinics tend to be invasive, referrals to these clinics should be reserved for patients who are truly refractory to all forms of pharmacotherapy.

CONCLUSIONS

Neuropathic pain continues to be one of the most difficult pain conditions to treat. With the proposed algorithm, clinicians will have a framework from which to design a pain treatment protocol appropriate for each patient. The algorithm will also help streamline referrals to specialized pain clinics, thereby reducing waiting list times for patients who are truly refractory to traditional pharmacotherapy.

Anti-allodynic action of the tormentic acid, a triterpene isolated from plant, against neuropathic and inflammatory persistent pain in mice

Experiments were designed to address whether the pentacyclic triterpene tormentic acid isolated from the stem bark of the plant Vochysia divergens exerts oral anti-allodynic properties in two models of chronic pain in mice: neuropathic pain caused by partial ligation of the sciatic nerve and inflammatory pain produced by intraplantar injection of Complete Freund's Adjuvant. Oral administration of tormentic acid (30 mg/kg) twice a day for several consecutive days produced time-dependent and pronounced anti-allodynia effect in both ispsilateral and contralateral paws after plantar injection of Complete Freund's Adjuvant. The inhibition observed was 82+/-9% and 100+/-11%, respectively. Interestingly, tormentic acid did not inhibit paw oedema formation following Complete Freund's Adjuvant plantar injection. Tormentic acid (30 mg/kg, p.o.) and gabapentin (70 mg/kg, p.o.), given twice a day, inhibited markedly the neuropathic allodynia induced by partial ligation of the sciatic nerve, with inhibition of 91+/-19% and 71+/-16%, respectively. The anti-allodynic action of tormentic acid was not associated with impairment of the motor activity of the animals. Together, the present results indicate that tormentic acid or its derivatives might be of potential interest in the development of new clinically relevant drugs for the management of persistent neuropathic and inflammatory allodynia.

Life after shingles: the management of postherpetic neuralgia

Chronic pain may have devastating effects on the physical and psychological well being of many patients (Harden, 1999). Most community nurses are in contact with a number of patients with chronic pain and will be asked for advice and recommendations with regards to its management. Chronic neuropathic pain is a complex and sometimes intractable condition that patients will seek help for, from either GPs or from the community nursing teams. This article will examine one neuropathic pain syndrome - post-herpetic neuralgia - and review the evidence base in relation to treatment strategies, in an attempt to support community staff in the management of this difficult to treat pain syndrome.

Treatment of painful sensory neuropathy with tiagabine: a pilot study

To evaluate the effect of tiagabine hydrochloride in painful neuropathy in a pilot, open-label study. Painful neuropathy is characterized by preferential involvement of small sensory and autonomic fibers. Tiagabine increases gamma-aminobutyric acid and might enhance the central pain-control mechanisms. Seventeen patients (10 men, 7 women; mean age 51.4 +/- 7.7 y) with chronic painful neuropathy (>6 months) were enrolled in this study. Week 0: All pain medications were discontinued. Weeks 1-4: Dose of tiagabine was increased weekly by 4 mg orally up to 16 mg in week 4. Quantitative sensory testing for vibration, cooling, and heat-pain, and quantitative sudomotor axon reflex test (QSART) were done at week 0 and week 4. The McGill Pain Questionnaire was administered weekly. Nine patients completed the study; 8 patients discontinued the treatment. Baseline pain intensity was 6.2 +/- 3.1 on the McGill Pain Questionnaire scale (0-10 range). Low doses (4-8 mg) of tiagabine reduced pain symptoms by 16-38%, improving surface pain (37.5%), skin sensitivity (32.8%), burning (38.6%), cold (25.4%) and pain sharpness (29%; p <0.03). Dull and deep pain did not improve. Quantitative sensory testing abnormalities diminished with treatment (p <0.02). Autonomic test results did not change. This pilot study evaluated the potential of tiagabine hydrochloride (Gabitril) in treatment of painful sensory neuropathy. Pain symptoms and quantitative sensory test results improved with treatment, especially at low doses of tiagabine (4-8 mg). Higher doses (12-16 mg) were associated with increased number of adverse events. Tiagabine may have potential benefits for treatment of painful neuropathy; however, assessment of its efficacy in a larger study is needed.

Evidence for the use of gabapentin in the treatment of diabetic peripheral neuropathy

BACKGROUND

One of the most common peripheral nerve complications of diabetes is painful diabetic peripheral neuropathy (DPN). Although tricyclic antidepressants (TCAs) have traditionally been used to relieve the pain of this condition, gabapentin's reported efficacy in various neuropathic pain states and its favorable side-effect profile compared with other available agents have led to interest in the use of this agent for the treatment of DPN.

OBJECTIVES

This paper reviews the current clinical literature on the effectiveness and tolerability of gabapentin in the treatment of DPN. It also considers whether the evidence favors gabapentin's use as an alternative or first-line agent.

METHODS

A search of the English- and French-language literature for the years 1990 through 2000 was performed using MEDLINE, Current Contents/Clinical Medicine, and International Pharmaceutical Abstracts, plus the reference lists of the articles identified through this search. The search terms used were gabapentin, anticonvulsant, diabetic peripheral neuropathy, and neuropathy. Included studies were limited to trials in human subjects.

RESULTS

The literature search identified several case reports and case series, as well as 3 small placebo-controlled studies (2 complete, 1 brief report) and 1 comparative trial against the TCA amitriptyline. The designs and dosing regimens differed between studies.

CONCLUSIONS

Many clinicians consider gabapentin an alternative treatment option in patients with DPN who are unable to tolerate traditional agents or in whom traditional agents are contraindicated. To date, gabapentin has been well tolerated, superior to placebo, and equivalent to amitriptyline in small clinical trials of short duration. Although overall efficacy and safety profiles appear to be favorable, larger long-term studies are needed to determine the place of gabapentin in relation to other treatment options. There is currently insufficient evidence from controlled trials to support the use of gabapentin as first-line therapy for DPN.