Botulinum toxin type A reduces capsaicin-evoked pain and neurogenic vasodilatation in human skin

Exploring the effects of extracranial injections of botulinum toxin type A on activation and sensitization of central trigeminovascular neurons by cortical spreading depression in male and female rats

BACKGROUND

OnabotulinumtoxinA (onabotA), is assumed to achieve its therapeutic effect in migraine through blocking activation of unmyelinated meningeal nociceptors and their downstream communications with central dura-sensitive trigeminovascular neurons in the spinal trigeminal nucleus (SPV). The present study investigated the mechanism of action of onabotA by assessing its effect on activation and sensitization of dura-sensitive neurons in the SPV by cortical spreading depression (CSD). It is a follow up to our recent study on onabotA effects on activation and sensitization of peripheral trigeminovascular neurons.

METHODS

In anesthetized male and female rats, single-unit recordings were used to assess effects of extracranial injections of onabotA (five injections, one unit each, diluted in 5 μl of saline were made along the lambdoid (two injection sites) and sagittal (two injection sites) suture) vs. vehicle on CSD-induced activation and sensitization of high-threshold (HT) and wide-dynamic range (WDR) dura-sensitive neurons in the SPV.

RESULTS

Single cell analysis of onabotA pretreatment effects on CSD-induced activation and sensitization of central trigeminovascular neurons in the SPV revealed the ability of this neurotoxin to prevent activation and sensitization of WDR neurons (13/20 (65%) vs. 4/16 (25%) activated neurons in the control vs. treated groups, p = 0.022, Fisher's exact). By contrast, onabotA pretreatment effects on CSD-induced activation and sensitization of HT neurons had no effect on their activation (12/18 (67%) vs. 4/7 (36%) activated neurons in the control vs. treated groups, p = 0.14, Fisher's exact). Regarding sensitization, we found that onabotA pretreatment prevented the enhanced responses to mechanical stimulation of the skin (i.e. responses reflecting central sensitization) in both WDR and HT neurons. In control but not treated WDR neurons, responses to brush (p = 0.004 vs. p = 0.007), pressure (p = 0.002 vs. p = 0.79) and pinch (p = 0.007 vs. 0.79) increased significantly two hours after CSD. Similarly, in control but not treated HT neurons, responses to brush (p = 0.002 vs. p = 0.79), pressure (p = 0.002 vs. p = 0.72) and pinch (p = 0.0006 vs. p = 0.28) increased significantly two hours after CSD. Unexpectedly, onabotA pretreatment prevented the enhanced responses of both WDR and HT neurons to mechanical stimulation of the dura (commonly reflecting peripheral sensitization). In control vs. treated WDR and HT neurons, responses to dural stimulation were enhanced in 70 vs. 25% (p = 0.017) and 78 vs. 27% (p = 0.017), respectively.

CONCLUSIONS

The ability of onabotA to prevent activation and sensitization of WDR neurons is attributed to its preferential inhibitory effects on unmyelinated C-fibers. The inability of onabotA to prevent activation of HT neurons is attributed to its less extensive inhibitory effects on the thinly myelinated Aδ-fibers. These findings provide further pre-clinical evidence about differences and potentially complementary mechanisms of action of onabotA and calcitonin gene-related peptide-signaling neutralizing drugs.

Botulinum toxin type A is a potential therapeutic drug for chronic orofacial pain

BACKGROUND

Botulinum toxin type A (BTX-A), produced by the gram-positive anaerobic bacterium Clostridium botulinum, acts by cleaving synaptosome-associated protein-25 (SNAP-25), an essential component of the presynaptic neuronal membrane that is necessary for fusion with the membrane proteins of neurotransmitter-containing vesicles. Recent studies have highlighted the efficacy of BTX-A in treating chronic pain conditions, including lower back pain, chronic neck pain, neuropathic pain, and trigeminal neuralgia, particularly when patients are unresponsive to traditional painkillers. This review focuses on the analgesic effects of BTX-A in various chronic pain conditions, with a particular emphasis on the orofacial region.

HIGHLIGHT

This review focuses on the mechanisms by which BTX-A induces analgesia in patients with inflammatory and temporomandibular joint pain. This review also highlights the fact that BTX-A can effectively manage neuropathic pain and trigeminal neuralgia, which are difficult-to-treat chronic pain conditions. Herein, we present a comprehensive assessment of the central analgesic effects of BTX-A and a discussion of its various applications in clinical dental practice.

CONCLUSION

BTX-A is an approved treatment option for various chronic pain conditions. Although there is evidence of axonal transport of BTX-A from peripheral to central endings in motor neurons, the precise mechanism underlying its pain-modulating effects remains unclear. This review discusses the evidence supporting the effectiveness of BTX-A in controlling chronic pain conditions in the orofacial region. BTX-A is a promising therapeutic agent for treating pain conditions that do not respond to conventional analgesics.

Therapeutic Options for Migraines in the Microsurgical Patient: A Scoping Review

BACKGROUND

There exists an increasing array of treatments proposed to prevent, alleviate, and abort symptoms of a migraine; however, for patients who undergo reconstructive microsurgery, caution must be taken to preserve vascular integrity. This study is the first-to-date scoping review of vascular and bleeding risk of current migraine therapies, with the purpose of identifying potential therapeutic agents for postoperative migraine management appropriate for microsurgical patients.

METHODS

Currently available migraine therapeutics were compiled from the UpToDate software system and the American Academy of Family Physicians. A PubMed literature review was performed for each therapeutic's effect on bleeding or vascular involvement. Data were compiled into tables of abortive, symptom-controlling and prophylactic, and nonpharmacologic treatments. Expert microsurgeons reviewed the data to provide recommendations for optimized patient care.

RESULTS

Triptans and other ergot derivatives demonstrated strong evidence of vasoconstriction and were greatly advised against for immediate postmicrosurgical use. Novel pharmaceutical therapies such as lasmiditan and calcitonin gene-related peptide antagonists have no literature indicating potential for vasoconstriction or hematoma and remain an investigational option for abortive medical treatment. For symptom control, acetaminophen appears the safest option, with clinical judgment and further research needed for use of nonsteroidal antiinflammatory drugs. Alternative treatment techniques may include migraine prophylaxis with botulinum toxin injection or nutraceutical treatment by means of magnesium supplementation or coenzyme Q10 administration, minimizing the need for additional medication in the postoperative setting.

CONCLUSIONS

Patients undergoing reconstructive microsurgery have a unique medical profile limiting the therapeutic options available to treat migraines. This review provides preliminary evidence to be considered as a guide for prescribing therapeutics for migraine in the postoperative setting.

Cellular and molecular mechanisms involved in the analgesic effects of botulinum neurotoxin: A literature review

Botulinum neurotoxins (BoNTs), derived from Clostridium botulinum, have been employed to treat a range of central and peripheral neurological disease. Some studies indicate that BoNT may be beneficial for pain conditions as well. It has been hypothesized that BoNTs may exert their analgesic effects by preventing the release of pain-related neurotransmitters and neuroinflammatory agents from sensory nerve endings, suppressing glial activation, and inhibiting the transmission of pain-related receptors to the neuronal cell membrane. In addition, there is evidence to suggest that the central analgesic effects of BoNTs are mediated through their retrograde axonal transport. The purpose of this review is to summarize the experimental evidence of the analgesic functions of BoNTs and discuss the cellular and molecular mechanisms by which they can act on pain conditions. Most of the studies reviewed in this article were conducted using BoNT/A. The PubMed database was searched from 1995 to December 2022 to identify relevant literature.

Botulinum toxin A and neuropathic pain: An update

Botulinum toxin type A is a widely used neurotoxin for the treatment of muscle hyperactivity such as dystonia and spasticity. Several clinical trials have also reported an efficacy of subcutaneous or intradermal administrations of botulinum toxin A on various neuropathic pain conditions including idiopathic trigeminal neuralgia and found that specific sensory phenotypes were predictors of the response. This narrative review summarizes the potential mechanisms of action, efficacy and safety of botulinum toxin A in neuropathic pain as well as its place in the therapeutic algorithm of neuropathic pain.

Transient Receptor Potential Channels and Botulinum Neurotoxins in Chronic Pain

Pain afflicts more than 1.5 billion people worldwide, with hundreds of millions suffering from unrelieved chronic pain. Despite widespread recognition of the importance of developing better interventions for the relief of chronic pain, little is known about the mechanisms underlying this condition. However, transient receptor potential (TRP) ion channels in nociceptors have been shown to be essential players in the generation and progression of pain and have attracted the attention of several pharmaceutical companies as therapeutic targets. Unfortunately, TRP channel inhibitors have failed in clinical trials, at least in part due to their thermoregulatory function. Botulinum neurotoxins (BoNTs) have emerged as novel and safe pain therapeutics because of their regulation of exocytosis and pro-nociceptive neurotransmitters. However, it is becoming evident that BoNTs also regulate the expression and function of TRP channels, which may explain their analgesic effects. Here, we summarize the roles of TRP channels in pain, with a particular focus on TRPV1 and TRPA1, their regulation by BoNTs, and briefly discuss the use of BoNTs for the treatment of chronic pain.

Post-Burn Pruritus

Post-burn pruritus is the pruritus that occurs after burn during the rehabilitation and healing process of burn wounds. The post-burn pruritus is a common and serious complication of burn injury, which severely lowers the quality of life of the patient. Many potential treatments are available for pruritus but there is no consensus of the best single treatment yet. The precise mechanism of post-burn pruritus has not been elucidated, but it appears to have pruritogenic and neuropathic aspects. Clinically, post-burn pruritus tends to be intractable to conventional treatment but rather responds to neuroleptic agents, such as gabapentin and pregabalin. During wound healing, various neuropeptides secreted from the nerves of the skin control epidermal and vascular proliferation and connective tissue cells. When keratinocytes are activated by an itch-inducing substance, they secrete a variety of inflammatory substances that increase the susceptibility of the itch receptor. There are two mechanisms underlying post-burn neuropathic pruritus. The first one is peripheral sensitization. The second one is the intact nociceptor hypothesis. An effective treatment for post-burn pruritus will also be effective in other neuropathic and intractable itching. In this review, we summarized the interaction and mechanism of keratinocytes, immune cells, and nerve fibers related to post-burn pruritus.

Perineural injection of botulinum toxin-A in painful peripheral nerve injury - a case series: pain relief, safety, sensory profile and sample size recommendation

Objectives: Subcutaneous injection of botulinum toxin-A (sBONT-A) is a novel treatment for peripheral neuropathic pain. While its analgesic effects are well documented, this treatment is often not comfortable and fails in patients who show signs of sensory loss but rarely allodynia. There are some case reports about perineural BONT-A injection (pBONT-A) which could be an alternative approach. Here we present a retrospective, open label case series of pBONT-A's efficacy and safety regarding neurological consequences involving changes in somatosensory profiles of both responders and non-responders. Methods: Sixty patients (53 ± 13years, 77% males) with PNI were treated with pBONT-A after a test injection with a local anesthetic, which prompted distinctive pain relief. Quantitative sensory testing (QST; DFNS protocol) and pain intensity were assessed before and ≥7 days post pBONT-A injection. Definition of response: satisfactory pain reduction of ≥30% NRS (numerical rating scale: 0 = no pain, 10 = worst pain) for ≥4 days. Statistics: Paired t-test, Mann-Whitney U-test, χ² test. Results: A temporary weak paresis in one case was clinically verified. The QST -parameters remained unchanged, but patients with more frequent hyperalgesia signs reported less analgesia (p = .04). The pBONT-A injection prompted pain relief by 24.8% (NRS: 6.0 ± 1.7 vs. 4.5 ± 2.1; p < .0001); 57% (n = 34) were responders (NRS: 6.0 ± 1.6 vs. 3.4 ± 1.6, relief of 43.4%; p < .0001). Based on these results, we suggest that future parallel design trials on pBONT-A need to include at least 84 patients. Discussion: Ultrasound-guided pBONT-A injection seems to be a safe treatment leading to a sufficient pain relief for some months without sensory changes. Surprisingly, pBONT-A showed a pronounced analgesic effect also in patients without signs of hyperalgesia.

A Pilot Study to Evaluate the Efficacy and Safety of Treatment with Botulinum Toxin in Patients with Recalcitrant and Persistent Erythematotelangiectatic Rosacea

Background

There are few pharmacologic options to reduce erythema and flushing in patients with recalcitrant erythematotelangiectatic rosacea (ETR). We previously reported two cases of refractory flushing and erythema of rosacea that were successfully treated with intradermal botulinum toxin injection, and additional research is needed to prove the efficacy and safety of this treatment.

Objective

To report the efficacy and safety of botulinum toxin injection as an aid in persistent erythema of rosacea patients.

Methods

A total of 20 Korean patients with recalcitrant ETR were enrolled to receive treatment by injection of botulinum toxin. Patients received one treatment of intradermal botulinum toxin injection and were assessed 1, 2, 4, and 8 weeks after treatment. The severity of erythema and telangiectasia was investigated by a non-treating physician, and the Erythema Index (EI) was assessed by mexameter at each visit. Patient satisfaction and any adverse events were also assessed at each visit.

Results

17 patients completed all follow-up visits and were included in the analysis. Intradermal injection of botulinum toxin significantly reduced erythema severity and EI in ETR patients. Patients reported a satisfaction score of 2.94±0.56 at 8 weeks after treatment. Except for three patients who discontinued the study early due to inconvenience of facial muscle paralysis, 17 patients participating in the final analysis did not report side effects except injection pain at the time of the procedure.

Conclusion

Intradermal injection of botulinum toxin can be used as an effective and relatively safe adjuvant agent for recalcitrant and persistent erythema of ETR patients.

Efficacy of Botulinum Toxin A for Treating Cramps in Diabetic Neuropathy

OBJECTIVE

Muscle cramps occur in >50% of diabetic patients and reduce the quality of life. No effective treatment is available. We evaluated the clinical effectiveness of botulinum toxin A (BTX-A) injections for treating cramps in diabetic patients with neuropathy.

METHODS

This single-center, double-blind, placebo-controlled perspective study investigated the efficacy and safety of BTX-A intramuscular injection for treating calf or foot cramps refractory to common pharmacological drugs. Fifty diabetic patients with peripheral neuropathy and cramps were randomly assigned to 2 matched groups. BTX-A (100 or 30 units) or saline was injected on each side into the gastrocnemius or the small flexor foot muscles. Changes in pain intensity (primary outcome) and cramp frequency were evaluated over the course of 20 weeks after BTX-A administration. Cramp interference in daily life and the electrophysiological cramp threshold frequency were also measured. The treatment was repeated 5 months after first injection in 19 responders.

RESULTS

All outcome measures improved significantly after BTX-A compared with placebo. The changes with respect to baseline were already significant after 1 week and persisted up to week 14. Only 5 of 25 (20%) patients were nonresponders (<50% decrease of the primary outcome). The responses to a second BTX-A injection provided results similar to the first administration. Mild pain at the injection site (4/25 cases) was the only adverse event, and it disappeared within 2 to 3 days.

INTERPRETATION

Local BTX-A infiltration is an efficacious and safe procedure for obtaining a sustained amelioration of muscle cramps associated with diabetic neuropathy. Ann Neurol 2018;84:682-690.

Therapeutic use of botulinum toxin in pain treatment

Botulinum toxin is one of the most potent molecule known to mankind. A neurotoxin, with high affinity for cholinergic synapse, is effectively capable of inhibiting the release of acetylcholine. On the other hand, botulinum toxin is therapeutically used for several musculoskeletal disorders. Although most of the therapeutic effect of botulinum toxin is due to temporary skeletal muscle relaxation (mainly due to inhibition of the acetylcholine release), other effects on the nervous system are also investigated. One of the therapeutically investigated areas of the botulinum neurotoxin (BoNT) is the treatment of pain. At present, it is used for several chronic pain diseases, such as myofascial syndrome, headaches, arthritis, and neuropathic pain. Although the effect of botulinum toxin in pain is mainly due to its effect on cholinergic transmission in the somatic and autonomic nervous systems, research suggests that botulinum toxin can also provide benefits related to effects on cholinergic control of cholinergic nociceptive and antinociceptive systems. Furthermore, evidence suggests that botulinum toxin can also affect central nervous system (CNS). In summary, botulinum toxin holds great potential for pain treatments. It may be also useful for the pain treatments where other methods are ineffective with no side effect(s). Further studies will establish the exact analgesic mechanisms, efficacy, and complication of botulinum toxin in chronic pain disorders, and to some extent acute pain disorders.

A Study and Review of Effects of Botulinum Toxins on Mast Cell Dependent and Independent Pruritus

Pruriceptive itch originates following activation of peripheral sensory nerve terminals when pruritogens come in contact with the skin. The ability of botulinum neurotoxins (BoNTs) to attenuate transmitter release from afferent terminals provides a rationale for studying its effect on pruritus. This study investigated the effects of BoNT/A1 and BoNT/B1 on mast cell dependent (Compound 48/80:48/80) and independent (Chloroquine:CQ) scratching. C57Bl/6 male mice received intradermal injection of 1.5 U of BoNT/A1, BoNT/B1 or saline 2, 7, 14 and 21 days prior to ipsilateral 48/80 or CQ at the nape of the neck. Ipsilateral hind paw scratching was determined using an automated recording device. The effect of BoNTs on 48/80 mediated mast cell degranulation was analyzed in human and murine mast cells and the presence of SNAREs was determined using qPCR, immunostaining and Western blot. Pre-treatment with BoNT/A1 and BoNT/B1 reduced 48/80 and CQ induced scratching behavior starting on day 2 with reversal by day 21. Both serotypes inhibited 48/80 induced mast cell degranulation. qPCR and immunostaining detected SNAP-25 mRNA and protein, respectively, in mast cells, however, Western blots did not. This study demonstrates the long-lasting anti-pruritic effects of two BoNT serotypes, in a murine pruritus model using two different mechanistically driven pruritogens. These data also indicate that BoNTs may have a direct effect upon mast cell degranulation.

Neurogenic inflammation and its role in migraine

The etiology of migraine pain involves sensitized meningeal afferents that densely innervate the dural vasculature. These afferents, with their cell bodies located in the trigeminal ganglion, project to the nucleus caudalis, which in turn transmits signals to higher brain centers. Factors such as chronic stress, diet, hormonal fluctuations, or events like cortical spreading depression can generate a state of "sterile inflammation" in the intracranial meninges resulting in the sensitization and activation of trigeminal meningeal nociceptors. This sterile inflammatory phenotype also referred to as neurogenic inflammation is characterized by the release of neuropeptides (such as substance P, calcitonin gene related peptide) from the trigeminal innervation. This release leads to vasodilation, plasma extravasation secondary to capillary leakage, edema, and mast cell degranulation. Although neurogenic inflammation has been observed and extensively studied in peripheral tissues, its role has been primarily investigated in the genesis and maintenance of migraine pain. While some aspects of neurogenic inflammation has been disregarded in the occurrence of migraine pain, targeted analysis of factors have opened up the possibilities of a dialogue between the neurons and immune cells in driving such a sterile neuroinflammatory state in migraine pathophysiology.

Botulinum Toxin Type A Injection for Neuropathic Pain in a Patient With a Brain Tumor: A Case Report

Neuropathic pain is usually managed pharmacologically, rather than with botulinum toxin type A (BTX-A). However, medications commonly fail to relieve pain effectively or have intolerable side effects. We present the case of a 62-year-old man diagnosed with an intracranial chondrosarcoma, which was removed surgically and treated with radiation therapy. He suffered from neuropathic pain despite combined pharmacological therapy with gabapentin, amitriptyline, tramadol, diazepam, and duloxetine because of adverse effects. BTX-A (100 units) was injected subcutaneously in the most painful area in the posterior left thigh. Immediately after the injection, his pain decreased significantly from 6/10 to 2/10 on a visual analogue scale. Pain relief lasted for 12 weeks. This case report describes intractable neuropathic pain caused by a brain tumor that was treated with subcutaneous BTX-A, which is a useful addition for the management of neuropathic pain related to a brain tumor.

Differential effect of Incobotulinumtoxin A on pain, neurogenic flare and hyperalgesia in human surrogate models of neurogenic pain

BACKGROUND

The effectiveness of Botulinum-neurotoxin A (BoNT/A) to treat pain in human pain models is very divergent. This study was conducted to clarify if the pain models or the route of BoNT/A application might be responsible for these divergent findings.

METHODS

Sixteen healthy subjects (8 males, mean age 27 ± 5 years) were included in a first set of experiments consisting of three visits: (1) Visit: Quantitative sensory testing (QST) was performed before and after intradermal capsaicin injection (CAPS, 15 μg) on one thigh and electrical current stimulation (ES, 1 Hz) on the contralateral thigh. During stimulation pain and the neurogenic flare response (laser-Doppler imaging) were assessed. (2) Four weeks later, BoNT/A (Xeomin^® , 25 MU) was injected intracutaneously on both sides. (3) Seven days later, the area of BoNT/A application was determined by the iodine-starch staining and the procedure of the (1) visit was exactly repeated. In consequence of these results, 8 healthy subjects (4 males, mean age 26 ± 3 years) were included into a second set of experiments. The experimental setting was exactly the same with the exception that stimulation frequency of ES was increased to 4 Hz and BoNT/A was injected subcutaneously into the thigh, which was stimulated by capsaicin.

RESULTS

BoNT/A reduced the 1 Hz ES flare size (p < 0.001) and pain ratings (p < 0.01), but had no effect on 4 Hz ES and capsaicin-induced pain, hyperalgesia, or flare size, regardless of the depth of BoNT/A injection (i.c./s.c). Moreover, i.c. BoNT/A injection significantly increased warm detection and heat pain thresholds in naive skin (WDT, Δ 2.2 °C, p < 0.001; HPT Δ 1.8 °C, p < 0.005).

CONCLUSION

BoNT/A has a moderate inhibitory effect on peptidergic and thermal C-fibers in healthy human skin.

SIGNIFICANCE

The study demonstrates that BoNT/A (Incobotulinumtoxin A) has differential effects in human pain models: It reduces the neurogenic flare and had a moderate analgesic effects in low frequency but not high frequency current stimulation of cutaneous afferent fibers at C-fiber strength; BoNT/A had no effect in capsaicin-induced (CAPS) neurogenic flare or pain, or on hyperalgesia to mechanical or heat stimuli in both pain models. Intracutaneous BoNT/A increases warm and heat pain thresholds on naïve skin.

Nonparalytic botulinum molecules for the control of pain

BiTox attenuated A-nociceptor-mediated mechanosensitivity in rat models of chronic pain. Plasma extravasation and keratinocyte proliferation were also inhibited but C-fiber nociception was not impaired.

Local injections of botulinum toxins have been reported to be useful not only for the treatment of peripheral neuropathic pain and migraine but also to cause long-lasting muscle paralysis, a potentially serious side effect. Recently, a botulinum A-based molecule (“BiTox”) has been synthesized that retains neuronal silencing capacity without triggering muscle paralysis. In this study, we examined whether BiTox delivered peripherally was able to reduce or prevent the increased nociceptive sensitivity found in animal models of inflammatory, surgical, and neuropathic pain. Plasma extravasation and edema were also measured as well as keratinocyte proliferation. No motor deficits were seen and acute thermal and mechanical nociceptive thresholds were unimpaired by BiTox injections. We found reduced plasma extravasation and inflammatory edema as well as lower levels of keratinocyte proliferation in cutaneous tissue after local BiTox injection. However, we found no evidence that BiTox was transported to the dorsal root ganglia or dorsal horn and no deficits in formalin-elicited behaviors or capsaicin or formalin-induced c-Fos expression within the dorsal horn. In contrast, Bitox treatment strongly reduced A-nociceptor-mediated secondary mechanical hyperalgesia associated with either complete Freund’s adjuvant (CFA)-induced joint inflammation or capsaicin injection and the hypersensitivity associated with spared nerve injury. These results imply that although local release of neuromodulators from C-fibers was inhibited by BiTox injection, C-nociceptive signaling function was not impaired. Taken together with recent clinical data the results suggest that BiTox should be considered for treatment of pain conditions in which A-nociceptors are thought to play a significant role.

Abobotulinum Toxin A in the Treatment of Chronic Low Back Pain

Chronic low back pain is a debilitating condition with a complex and multifactorial pathophysiology. Botulinum neurotoxins (BoNTs) have strong analgesic effects, as shown in both animal models of pain and in human beings. A randomized, double-blind, placebo-controlled, parallel format study to investigate the efficacy of abobotulinum toxin A (aboA) in chronic low back pain was conducted. The study cohort consisted of 18 patients who received 100 units of aboA into each of the five lumbar extensor spinae muscles unilaterally or bilaterally (total dose 500 to 1000 units), and 19 who received normal saline of the same volume. The level of pain and quality of life were assessed using the visual analogue scale (VAS) and three questionnaires including the Oswestry Low Back Pain Disability Questionnaire (OLBPDQ). Patients’ perception of improvement was recorded via patient global impression of change (PGIC). The primary outcome measure, the proportion of responders with VAS of <4 at 6 weeks, was not met, but the data was significantly in favor of aboA at 4 weeks (p = 0.008). The total Oswestry score representing quality of life improved in the aboA group compared to the placebo group (p = 0.0448). Moreover, significantly more patients reported their low back pain as “much improved” in the abobotulinum toxin A group (0.0293).

Understanding migraine: Potential role of neurogenic inflammation

Neurogenic inflammation, a well-defined pathophysiologial process is characterized by the release of potent vasoactive neuropeptides, predominantly calcitonin gene-related peptide (CGRP), substance P (SP), and neurokinin A from activated peripheral nociceptive sensory nerve terminals (usually C and A delta-fibers). These peptides lead to a cascade of inflammatory tissue responses including arteriolar vasodilation, plasma protein extravasation, and degranulation of mast cells in their peripheral target tissue. Neurogenic inflammatory processes have long been implicated as a possible mechanism involved in the pathophysiology of various human diseases of the nervous system, respiratory system, gastrointestinal tract, urogenital tract, and skin. The recent development of several innovative experimental migraine models has provided evidence suggestive of the involvement of neuropeptides (SP, neurokinin A, and CGRP) in migraine headache. Antidromic stimulation of nociceptive fibers of the trigeminal nerve resulted in a neurogenic inflammatory response with marked increase in plasma protein extravasation from dural blood vessels by the release of various sensory neuropeptides. Several clinically effective abortive antimigraine medications, such as ergots and triptans, have been shown to attenuate the release of neuropeptide and neurogenic plasma protein extravasation. These findings provide support for the validity of using animal models to investigate mechanisms of neurogenic inflammation in migraine. These also further strengthen the notion of migraine being a neuroinflammatory disease. In the clinical context, there is a paucity of knowledge and awareness among physicians regarding the role of neurogenic inflammation in migraine. Improved understanding of the molecular biology, pharmacology, and pathophysiology of neurogenic inflammation may provide the practitioner the context-specific feedback to identify the novel and most effective therapeutic approach to treatment. With this objective, the present review summarizes the evidence supporting the involvement of neurogenic inflammation and neuropeptides in the pathophysiology and pharmacology of migraine headache as well as its potential significance in better tailoring therapeutic interventions in migraine or other neurological disorders. In addition, we have briefly highlighted the pathophysiological role of neurogenic inflammation in various other neurological disorders.

Effects of intraplantar botulinum toxin-B on carrageenan-induced changes in nociception and spinal phosphorylation of GluA1 and Akt

Increasing evidence suggests that botulinum neurotoxins (BoNTs) delivered into the skin and muscle in certain human and animal pain states may exert antinociceptive efficacy though their uptake and transport to central afferent terminals. Cleavage of soluble N-methylaleimide-sensitive attachment protein receptor by BoNTs can impede vesicular mediated neurotransmitter release as well as transport/insertion of channel/receptor subunits into plasma membranes, an effect that can reduce activity-evoked facilitation. Here, we explored the effects of intraplantar botulinum toxin- B (BoNT-B) on peripheral inflammation and spinal nociceptive processing in an inflammatory model of pain. C57BL/6 mice (male) received unilateral intraplantar BoNT (1 U, 30 μL) or saline prior to intraplantar carrageenan (20 μL, 2%) or intrathecal N-methyl-D-aspartate (NMDA), substance P or saline (5 μL). Intraplantar carrageenan resulted in edema and mechanical allodynia in the injected paw and increased phosphorylation of a glutamate subunit (pGluA1ser845) and a serine/threonine-specific protein kinase (pAktser473) in spinal dorsal horn along with an increased incidence of spinal c-Fos positive cells. Pre-treatment with intraplantar BoNT-B reduced carrageenan evoked: (i) allodynia, but not edema; (ii) pGluA1 and pAkt and (iii) c-Fos expression. Further, intrathecal NMDA and substance P each increased dorsal horn levels of pGluA1 and pAkt. Intraplantar BoNT-B inhibited NMDA, but not substance P evoked phosphorylation of GluA1 and Akt. These results suggest that intraplantar toxin is transported centrally to block spinal activation and prevent phosphorylation of a glutamate receptor subunit and a kinase, which otherwise contribute to facilitated states.

OnabotulinumtoxinA decreases interictal CGRP plasma levels in patients with chronic migraine

OnabotulinumtoxinA (onabotA) has shown efficacy in chronic migraine (CM). Its mechanism of action, however, remains obscure. We have analysed whether treatment with onabotA is able to induce changes in interictal plasma calcitonin gene-related peptide (CGRP) concentrations, which have been shown to be increased in patients with CM. Calcitonin gene-related peptide levels were determined in samples obtained from the right antecubital vein using ELISA, outside a migraine attack and having taken no symptomatic medication in the previous 24 hours, in 83 patients with CM (average age 44 years; 94% females) before and 1 month after treatment with 155 to 195 U of onabotA. CGRP levels after onabotA treatment (median, 51.89 pg/mL; range, 199.4-10.2) were significantly lower as compared with CGRP levels obtained before onabotA treatment (median, 74.09 pg/mL; range, 241.0-11.4; P = 0.001). Pretreatment CGRP levels in responders (76.85 pg/mL) were significantly higher than those seen in nonresponders (50.45 pg/mL; P = 0.001). One month after treatment, the CGRP levels did not change in nonresponders (51.89 pg/mL; P not significant), but significantly decreased in responders (52.48 pg/mL; P = 0.003). A number of demographic factors, clinical features, and comorbidities were not different in responders as compared with those of nonresponders. These results confirm that interictal CGRP levels can be of help in predicting the response to onabotA and suggest that the mechanism of action of onabotA in CM is the reversal of sensitization as a result of the inhibition of CGRP release.

Current status and future directions of botulinum neurotoxins for targeting pain processing

Current evidence suggests that botulinum neurotoxins (BoNTs) A1 and B1, given locally into peripheral tissues such as skin, muscles, and joints, alter nociceptive processing otherwise initiated by inflammation or nerve injury in animal models and humans. Recent data indicate that such locally delivered BoNTs exert not only local action on sensory afferent terminals but undergo transport to central afferent cell bodies (dorsal root ganglia) and spinal dorsal horn terminals, where they cleave SNAREs and block transmitter release. Increasing evidence supports the possibility of a trans-synaptic movement to alter postsynaptic function in neuronal and possibly non-neuronal (glial) cells. The vast majority of these studies have been conducted on BoNT/A1 and BoNT/B1, the only two pharmaceutically developed variants. However, now over 40 different subtypes of botulinum neurotoxins (BoNTs) have been identified. By combining our existing and rapidly growing understanding of BoNT/A1 and /B1 in altering nociceptive processing with explorations of the specific characteristics of the various toxins from this family, we may be able to discover or design novel, effective, and long-lasting pain therapeutics. This review will focus on our current understanding of the molecular mechanisms whereby BoNTs alter pain processing, and future directions in the development of these agents as pain therapeutics.

Botulinum Toxin Type a as a Therapeutic Agent against Headache and Related Disorders

Botulinum neurotoxin A (BoNT/A) is a toxin produced by the naturally-occurring Clostridium botulinum that causes botulism. The potential of BoNT/A as a useful medical intervention was discovered by scientists developing a vaccine to protect against botulism. They found that, when injected into a muscle, BoNT/A causes a flaccid paralysis. Following this discovery, BoNT/A has been used for many years in the treatment of conditions of pathological muscle hyperactivity, like dystonias and spasticities. In parallel, the toxin has become a “glamour” drug due to its power to ward off facial wrinkles, particularly frontal, due to the activity of the mimic muscles. After the discovery that the drug also appeared to have a preventive effect on headache, scientists spent many efforts to study the potentially-therapeutic action of BoNT/A against pain. BoNT/A is effective at reducing pain in a number of disease states, including cervical dystonia, neuropathic pain, lower back pain, spasticity, myofascial pain and bladder pain. In 2010, regulatory approval for the treatment of chronic migraine with BoNT/A was given, notwithstanding the fact that the mechanism of action is still not completely elucidated. In the present review, we summarize experimental evidence that may help to clarify the mechanisms of action of BoNT/A in relation to the alleviation of headache pain, with particular emphasis on preclinical studies, both in animals and humans. Moreover, we summarize the latest clinical trials that show evidence on headache conditions that may obtain benefits from therapy with BoNT/A.

The effects of intraplantar and intrathecal botulinum toxin type B on tactile allodynia in mono and polyneuropathy in the mouse

BACKGROUND

Mononeuropathies (MNs: nerve ligation) and polyneuropathies (PNs: cisplatin) produce unilateral and bilateral tactile allodynia, respectively. We examined the effects of intraplantar (IPLT) and intrathecal (IT) botulinum toxin B (BoNT-B) on this allodynia.

METHODS

Mice (male c57Bl/6) were prepared with an L5 nerve ligation. Others received cisplatin (IP 2.3 mg/kg/d, every other day for 6 injections). Saline and BoNT-B were administered through the IPLT or IT route. We examined mechanical allodynia (von Frey hairs) before and at intervals after BoNT. As a control, we injected IPLT BoNT-B treated with dithiothreitol to cleave heavy chain from light chain. We measured motor function using acute thermal escape and sensorimotor tests.

RESULTS

MN and PN mice showed a persistent ipsilateral and bilateral allodynia, respectively. IPLT BoNT-B resulted in an ipsilateral dorsal horn reduction in the synaptic protein target of BoNT-B (vesicle-associated membrane protein) and a long-lasting (up to approximately 17 days) reversal of allodynia in PN and MN models. The predominant effect after IPLT delivery was ipsilateral to IPLT BoNT. The effects of IPLT BoNT-B in MN mice were blocked by prior reduction of BoNT-B with dithiothreitol. IT BoNT-B in mice with PN resulted in a bilateral reversal of allodynia. With these dosing parameters, hind paw placing and stepping reflexes were unaltered, and there were no changes in thermal escape latencies. After cisplatin, dorsal root ganglions displayed increases in activation transcription factor 3, which were reduced by IT, but not IPLT BoNT-B.

CONCLUSIONS

BoNT-B given IPLT and IT yields a long-lasting attenuation of the allodynia in mice displaying MN and PN allodynia.

Botulinum Toxin for Neuropathic Pain: A Review of the Literature

Botulinum neurotoxin (BoNT), derived from Clostridium botulinum, has been used therapeutically for focal dystonia, spasticity, and chronic migraine. Its spectrum as a potential treatment for neuropathic pain has grown. Recent opinions on the mechanism behind the antinociceptive effects of BoNT suggest that it inhibits the release of peripheral neurotransmitters and inflammatory mediators from sensory nerves. There is some evidence showing the axonal transport of BoNT, but it remains controversial. The aim of this review is to summarize the experimental and clinical evidence of the antinociceptive effects, mechanisms, and therapeutic applications of BoNT for neuropathic pain conditions, including postherpetic neuralgia, complex regional pain syndrome, and trigeminal neuralgia. The PubMed and OvidSP databases were searched from 1966 to May 2015. We assessed levels of evidence according to the American Academy of Neurology guidelines. Recent studies have suggested that BoNT injection is an effective treatment for postherpetic neuralgia and is likely efficient for trigeminal neuralgia and post-traumatic neuralgia. BoNT could also be effective as a treatment for diabetic neuropathy. It has not been proven to be an effective treatment for occipital neuralgia or complex regional pain syndrome.

Therapeutic use of botulinum toxin in migraine: mechanisms of action

Migraine pain represents sensations arising from the activation of trigeminal afferents, which innervate the meningeal vasculature and project to the trigeminal nucleus caudalis (TNC). Pain secondary to meningeal input is referred to extracranial regions innervated by somatic afferents that project to homologous regions in the TNC. Such viscerosomatic convergence accounts for referral of migraine pain arising from meningeal afferents to particular extracranial dermatomes. Botulinum toxins (BoNTs) delivered into extracranial dermatomes are effective in and approved for treating chronic migraine pain. Aside from their well-described effect upon motor endplates, BoNTs are also taken up in local afferent nerve terminals where they cleave soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, and prevent local terminal release. However, a local extracranial effect of BoNT cannot account for allthe effects of BoNT upon migraine. We now know that peripherally delivered BoNTs are taken up in sensory afferents and transported to cleave SNARE proteins in the ganglion and TNC, prevent evoked afferent release and downstream activation. Such effects upon somatic input (as from the face) likewise would not alone account for block of input from converging meningeal afferents. This current work suggests that BoNTs may undergo transcytosis to cleave SNAREs in second-order neurons or in adjacent afferent terminals. Finally, while SNAREs mediate exocytotic release, they are also involved in transport of channels and receptors involved in facilitated pain states. The role of such post-synaptic effects of BoNT action in migraine remains to be determined.

Botulinum toxin in migraine: Role of transport in trigemino-somatic and trigemino-vascular afferents

Migraine secondary to meningeal input is referred to extracranial regions innervated by somatic afferents that project to homologous regions in the trigeminal nucleus caudalis (TNC). Reported efficacy of extracranial botulinum toxin (BoNT) in treating migraine is surprising since a local extracranial effect of BoNT cannot account for its effect upon meningeal input. We hypothesize that intradermal BoNT acts through central transport in somatic afferents. Anesthetized C57Bl/6 mice (male) received unilateral supraorbital (SO) injections of BoNT-B (1.5 U/40 μl) or saline. 3 days later, mice received ipsilateral (ipsi)-SO capsaicin (20 μl of 0.5mM solution) or meningeal capsaicin (4 μl of 0.35 μM). Pre-treatment with ipsi-SO BoNT-B i) decreased nocicsponsive ipsilateral wiping behavior following ipsi-SO capsaicin; ii) produced cleavage of VAMP in the V1 region of ipsi-TG and in TG neurons showing WGA after SO injection; iii) reduced expression of c-fos in ipsi-TNC following ipsi-SO capsaicin; iv) reduced c-fos activation and NK-1 internalization in ipsi-TNC secondary to ipsi-meningeal capsaicin; and vi) SO WGA did not label dural afferents. We conclude that BoNT-B is taken up by peripheral afferents and transported to central terminals where it inhibits transmitter release resulting in decreased activation of second order neurons. Further, this study supports the hypothesis that SO BoNT exerts a trans-synaptic action on either the second order neuron (which receives convergent input from the meningeal afferent) or the terminal/TG of the converging meningeal afferent.

Blockade of glutamate release by botulinum neurotoxin type A in humans: a dermal microdialysis study

BACKGROUND

The analgesic action of botulinum neurotoxin type A (BoNTA) has been linked to the blockade of peripheral release of neuropeptides and neurotransmitters in animal models; however, there is no direct evidence of this in humans.

OBJECTIVES

To investigate the effect of BoNTA on glutamate release in humans, using an experimental model of pain and sensitization provoked by capsaicin plus mild heat.

METHODS

Twelve healthy volunteers (six men, six women) were pretreated with BoNTA (10 U) on the volar forearm and with a saline control on the contralateral side. Dermal microdialysis was applied one week later to collect interstitial samples before and after the application of a capsaicin patch (8%) plus mild heat (40°C⁄60 min) to provoke glutamate release, pain and vasodilation. Samples were collected every hour for 3 h using linear microdialysis probes (10 mm, 100 kD). Dialysate was analyzed for glutamate concentration. Pain intensity and skin vasomotor reactions (temperature and blood flow changes) were also recorded.

RESULTS

BoNTA significantly reduced glutamate release compared with saline (P<0.05). The provoked pain intensity was lower in the BoNTA-pretreated arm (P<0.01). The reduction in pain scores was not correlated with glutamate level. Cutaneous blood flow (P<0.05), but not cutaneous temperature (P≥0.05), was significantly reduced by BoNTA. There was a correlation between glutamate level and skin blood flow (r=0.58⁄P<0.05) but not skin temperature (P≥0.05). No differences according to sex were observed in any response.

CONCLUSIONS

The present study provided the first direct evidence supporting the inhibitory effect of BoNTA on glutamate release in human skin, which is potentially responsible for some of the analgesic action of BoNTA.

Analgesic effects of botulinum neurotoxin type A in a model of allyl isothiocyanate- and capsaicin-induced pain in mice

We evaluate analgesic effects of BoNT/A in relation to the two main transient receptor potentials (TRP), the vanilloid 1 (TRPV1) and the ankyrin 1 (TRPA1), having a role in migraine pain. BoNT/A (15 pg/mouse) was injected in the inner side of the medial part of hindlimb thigh of mice, where the superficial branch of femoral artery is located. We chosen this vascular structure because it is similar to other vascular structures, such as the temporal superficial artery, whose perivascular nociceptive fibres probably contributes to migraine pain. After an interval, ranging from 7 to 30 days, capsaicin (agonist of TRPV1) or allyl isothiocyanate (AITC; agonist of TRPA1) were injected in the same region previously treated with BoNT/A and nocifensive response to chemicals-induced pain was recorded. In absence of BoNT/A, capsaicin and AITC induced extensive nocifensive response, with a markedly different temporal profile: capsaicin induced maximal pain during the first 5 min, while AITC induced maximal pain at 15-30 min after injection. Pretreatment with BoNT/A markedly reduced both the capsaicin- and AITC-induced pain for at least 21 days. These data suggest a long lasting analgesic effect of BoNT/A exerted via prevention of responsiveness of TRPV1 and TRPA1 toward their respective agonists.

Reporting of sample size calculations in analgesic clinical trials: ACTTION systematic review

Sample size calculations determine the number of participants required to have sufficiently high power to detect a given treatment effect. In this review, we examined the reporting quality of sample size calculations in 172 publications of double-blind randomized controlled trials of noninvasive pharmacologic or interventional (ie, invasive) pain treatments published in European Journal of Pain, Journal of Pain, and Pain from January 2006 through June 2013. Sixty-five percent of publications reported a sample size calculation but only 38% provided all elements required to replicate the calculated sample size. In publications reporting at least 1 element, 54% provided a justification for the treatment effect used to calculate sample size, and 24% of studies with continuous outcome variables justified the variability estimate. Publications of clinical pain condition trials reported a sample size calculation more frequently than experimental pain model trials (77% vs 33%, P < .001) but did not differ in the frequency of reporting all required elements. No significant differences in reporting of any or all elements were detected between publications of trials with industry and nonindustry sponsorship. Twenty-eight percent included a discrepancy between the reported number of planned and randomized participants. This study suggests that sample size calculation reporting in analgesic trial publications is usually incomplete. Investigators should provide detailed accounts of sample size calculations in publications of clinical trials of pain treatments, which is necessary for reporting transparency and communication of pre-trial design decisions.

PERSPECTIVE

In this systematic review of analgesic clinical trials, sample size calculations and the required elements (eg, treatment effect to be detected; power level) were incompletely reported. A lack of transparency regarding sample size calculations may raise questions about the appropriateness of the calculated sample size.

Botulinum toxin A, brain and pain

Botulinum neurotoxin type A (BoNT/A) is one of the most potent toxins known and a potential biological threat. At the same time, it is among the most widely used therapeutic proteins used yearly by millions of people, especially for cosmetic purposes. Currently, its clinical use in certain types of pain is increasing, and its long-term duration of effects represents a special clinical value. Efficacy of BoNT/A in different types of pain has been found in numerous clinical trials and case reports, as well as in animal pain models. However, sites and mechanisms of BoNT/A actions involved in nociception are a matter of controversy. In analogy with well known neuroparalytic effects in peripheral cholinergic synapses, presently dominant opinion is that BoNT/A exerts pain reduction by inhibiting peripheral neurotransmitter/inflammatory mediator release from sensory nerves. On the other hand, growing number of behavioral and immunohistochemical studies demonstrated the requirement of axonal transport for BoNT/A's antinociceptive action. In addition, toxin's enzymatic activity in central sensory regions was clearly identified after its peripheral application. Apart from general pharmacology, this review summarizes the clinical and experimental evidence for BoNT/A antinociceptive activity and compares the data in favor of peripheral vs. central site and mechanism of action. Based on literature review and published results from our laboratory we propose that the hypothesis of peripheral site of BoNT/A action is not sufficient to explain the experimental data collected up to now.

Selective inhibition of meningeal nociceptors by botulinum neurotoxin type A: therapeutic implications for migraine and other pains

Background

Meningeal and other trigeminal nociceptors are thought to play important roles in the initiation of migraine headache. Currently, the only approved peripherally administered chronic migraine prophylactic drug is onabotulinumtoxinA. The purpose of this study was to determine how botulinum neurotoxin type A (BoNT-A) affects naïve and sensitized meningeal nociceptors.

Material and methods

Using electrophysiological techniques, we identified 43 C- and 36 Aδ-meningeal nociceptors, and measured their spontaneous and evoked firing before and after BoNT-A administration to intracranial dura and extracranial suture-receptive fields.

Results

As a rule, BoNT-A inhibited C- but not Aδ-meningeal nociceptors. When applied to nonsensitized C-units, BoNT-A inhibited responses to mechanical stimulation of the dura with suprathreshold forces. When applied to sensitized units, BoNT-A reversed mechanical hypersensitivity. When applied before sensitization, BoNT-A prevented development of mechanical hypersensitivity. When applied extracranially to suture branches of intracranial meningeal nociceptors, BoNT-A inhibited the mechanical responsiveness of the suture branch but not dural axon. In contrast, BoNT-A did not inhibit C-unit responses to mechanical stimulation of the dura with threshold forces, or their spontaneous activity.

Discussion

The study provides evidence for the ability of BoNT-A to inhibit mechanical nociception in peripheral trigeminovascular neurons. These findings suggest that BoNT-A interferes with neuronal surface expression of high-threshold mechanosensitive ion channels linked preferentially to mechanical pain by preventing their fusion into the nerve terminal membrane.

Time course analysis of the effects of botulinum neurotoxin type A on pain and vasomotor responses evoked by glutamate injection into human temporalis muscles

The effect of botulinum neurotoxin type A (BoNTA) on glutamate-evoked temporalis muscle pain and vasomotor responses was investigated in healthy men and women over a 60 day time course. Subjects participated in a pre-BoNTA session where their responses to injection of glutamate (1 M, 0.2 mL) and saline (0.2 mL) into the temporalis muscles were assessed. On Day 1, BoNTA (5 U) was injected into one temporalis muscle and saline into the contralateral temporalis muscle, in a randomized order. Subjects then received intramuscular injections of glutamate (1 M, 0.2 mL) into the left and right temporalis muscles at 3 h and subsequently 7, 30 and 60 days post-injection of BoNTA. Pain intensity, pain area, and neurogenic inflammation (skin temperature and skin blood perfusion) were recorded. Prior to BoNTA treatment, glutamate evoked significantly greater pain and vasomotor reactions (P < 0.001) than saline. BoNTA significantly reduced glutamate-evoked pain intensity (P < 0.05), pain area (P < 0.01), skin blood perfusion (P < 0.05), and skin temperature (P < 0.001). The inhibitory effect of BoNTA was present at 3 h after injection, peaked after 7 days and returned to baseline by 60 days. Findings from the present study demonstrated a rapid action of BoNTA on glutamate-evoked pain and neurogenic inflammation, which is in line with animal studies.

Botulinum toxin B in the sensory afferent: transmitter release, spinal activation, and pain behavior

We addressed the hypothesis that intraplantar botulinum toxin B (rimabotulinumtoxin B: BoNT-B) has an early local effect upon peripheral afferent terminal releasing function and, over time, will be transported to the central terminals of the primary afferent. Once in the terminals it will cleave synaptic protein, block spinal afferent transmitter release, and thereby prevent spinal nociceptive excitation and behavior. In mice, C57Bl/6 males, intraplantar BoNT-B (1 U) given unilaterally into the hind paw had no effect upon survival or motor function, but ipsilaterally decreased: (1) intraplantar formalin-evoked flinching; (2) intraplantar capsaicin-evoked plasma extravasation in the hind paw measured by Evans blue in the paw; (3) intraplantar formalin-evoked dorsal horn substance P (SP) release (neurokinin 1 [NK1] receptor internalization); (4) intraplantar formalin-evoked dorsal horn neuronal activation (c-fos); (5) ipsilateral dorsal root ganglion (DRG) vesicle-associated membrane protein (VAMP); (6) ipsilateral SP release otherwise evoked bilaterally by intrathecal capsaicin; (7) ipsilateral activation of c-fos otherwise evoked bilaterally by intrathecal SP. These results indicate that BoNT-B, after unilateral intraplantar delivery, is taken up by the peripheral terminal, is locally active (blocking plasma extravasation), is transported to the ipsilateral DRG to cleave VAMP, and is acting presynaptically to block release from the spinal peptidergic terminal. The observations following intrathecal SP offer evidence for a possible transsynaptic effect of intraplantar BoNT. These results provide robust evidence that peripheral BoNT-B can alter peripheral and central terminal release from a nociceptor and attenuate downstream nociceptive processing via a presynaptic effect, with further evidence suggesting a possible postsynaptic effect.

The neurology of itch

Research over the past 15 years has helped to clarify the anatomy and physiology of itch, the clinical features of neuropathic itch syndromes and the scientific underpinning of effective treatments. Two itch-sensitive pathways exist: a histamine-stimulated pathway that uses mechanically insensitive C-fibres, and a cowhage-stimulated pathway primarily involving polymodal C-fibres. Interactions with pain continue to be central to explaining various aspects of itch. Certain spinal interneurons (Bhlhb5) inhibit itch pathways within the dorsal horn; they may represent mediators between noxious and pruritic pathways, and allow scratch to inhibit itch. In the brain, functional imaging studies reveal diffuse activation maps for itch that overlap, but not identically, with pain maps. Neuropathic itch syndromes are chronic itch states due to dysfunction of peripheral or central nervous system structures. The most recognized are postherpetic itch, brachioradial pruritus, trigeminal trophic syndrome, and ischaemic stroke-related itch. These disorders affect a patient's quality of life to a similar extent as neuropathic pain. Treatment of neuropathic itch focuses on behavioural interventions (e.g., skin protection) followed by stepwise trials of topical agents (e.g., capsaicin), antiepileptic drugs (e.g., gabapentin), injection of other agents (e.g., botulinum A toxin), and neurostimulation techniques (e.g., cutaneous field stimulation). The involved mechanisms of action include desensitization of nerve fibres (in the case of capsaicin) and postsynaptic blockade of calcium channels (for gabapentin). In the future, particular histamine receptors, protease pathway molecules, and vanilloids may serve as targets for novel antipruritic agents.

Subcutaneous botulinum toxin for chronic post-thoracotomy pain

OBJECTIVE

Botulinum toxin is a neurotoxin that has been widely used in chronic pain for the treatment of multiple conditions with a component of localized muscle spasm. Recent studies suggest that botulinum toxin is effective in the treatment of neuropathic pain syndromes such as post-herpetic neuralgia.

CASE REPORT

We report the case of a 67-year-old man who underwent atypical segmentectomy of a right lower lobe lung nodule. The patient was referred to our pain management department with a of 2-year history persistent pain along the thoracotomy scar having a predominantly neuropathic component, refractory to standard treatments. He was successfully treated with subcutaneous botulinum toxin type A.

DISCUSSION

On the basics of our own experience and on the analysis of the reports published in the literature, fractioned subcutaneous injections of botulinum toxin may be useful for the treatment of various chronic localized pain conditions including chronic post-thoracotomy pain.

Antidromic vasodilatation and the migraine mechanism

Despite the fact that an unprecedented series of new discoveries in neurochemistry, neuroimaging, genetics and clinical pharmacology accumulated over the last 20 years has significantly increased our current knowledge, the underlying mechanism of the migraine headache remains elusive. The present review article addresses, from early evidence that emerged at the end of the nineteenth century, the role of ‘antidromic vasodilatation’ as part of the more general phenomenon, currently defined as neurogenic inflammation, in the unique type of pain reported by patients suffering from migraine headaches. The present paper describes distinctive orthodromic and antidromic properties of a subset of somatosensory neurons, the vascular- and neurobiology of peptides contained in these neurons, and the clinical–pharmacological data obtained in recent investigations using provocation tests in experimental animals and human beings. Altogether, previous and recent data underscore that antidromic vasodilatation, originating from the activation of peptidergic somatosensory neurons, cannot yet be discarded as a major contributing mechanism of the throbbing head pain and hyperalgesia of migraine.

Botulinum toxin type-A effect as a preemptive treatment in a model of acute trigeminal pain: a pre-clinical double-blind and placebo-controlled study

The purpose of this study was to investigate if botulinum neurotoxin type-A (BoNT/A) had a preemptive antinociceptive effect in a formalin-induced orofacial pain model (FT). To test this hypothesis, male Rattus norvegicus were injected with isotonic saline solution 0.9% or BoNT/A administered as a 40 μl bolus, lateral to their nose, at 24 hours, 8, 15, 22, 29 or 36 days pre-FT. The procedures were repeated 42 days later. Influence on motor activity was assessed through the open-field test. Pain scores corresponded to the time spent rubbing and flicking the injected area. Animals pre-treated with BoNT/A at the first protocol (8 days subgroup) showed reduced inflammatory scores (p=0.011). For the other groups no significant results were observed at any phase. Motor activity was similar in both groups. BoNT/A showed to be effective preventing inflammatory pain up to eight days after the first treatment, an effect not reproduced on the second dose administration.

Targeted treatment of pruritus: a look into the future

Recent advances in pruritus research have elucidated mediators and neuronal pathways involved in itch transmission, and this fast emerging knowledge may possibly be translated into new therapies in the near future. In the skin and peripheral nerves, potential mediator and receptor therapeutic targets include the H4 histamine receptor, protease-activated receptor 2, serine proteases, cathepsin S, peripheral mu- and kappa-opioid receptors, interleukin-31, transient receptor potential vanilloid 1 and 3, fatty acid amide hydrolase, nerve growth factor and its receptor, acetylcholine, and the Mas-related G protein-coupled receptors. In the spinal cord, gastrin-related peptide and its receptor, as well as substance P and its receptor neurokinin receptor-1 serve as potential therapeutic targets. In the brain, reduction of itch perception and modulation of emotions may possibly be achieved through drugs acting on the anterior cingulate cortex. Clinically, management of pruritus should be instituted early and should address the skin pathology, peripheral neuropathy, central sensitization, and the cognito-affective aspects of the disease.

Targeted treatment of pruritus: a look into the future

Recent advances in pruritus research have elucidated mediators and neuronal pathways involved in itch transmission, and this fast emerging knowledge may possibly be translated into new therapies in the near future. In the skin and peripheral nerves, potential mediator and receptor therapeutic targets include the H4 histamine receptor, protease-activated receptor 2, serine proteases, cathepsin S, peripheral mu- and kappa-opioid receptors, interleukin-31, transient receptor potential vanilloid 1 and 3, fatty acid amide hydrolase, nerve growth factor and its receptor, acetylcholine, and the Mas-related G protein-coupled receptors. In the spinal cord, gastrin-related peptide and its receptor, as well as substance P and its receptor neurokinin receptor-1 serve as potential therapeutic targets. In the brain, reduction of itch perception and modulation of emotions may possibly be achieved through drugs acting on the anterior cingulate cortex. Clinically, management of pruritus should be instituted early and should address the skin pathology, peripheral neuropathy, central sensitization, and the cognito-affective aspects of the disease.

[Botulinum toxin and painful peripheral neuropathies: what should be expected?]

Botulinum toxin type A (BTX-A) is a potent neurotoxin that blocks acetylcholine release from presynaptic nerve terminals by cleaving the SNARE complex. BTX-A has been reported to have analgesic effects independent of its action on muscle tone. The most robust results have been observed in patients with neuropathic pain. Neuropathic pain due to peripheral lesions has been the most widely studied. BTX-A has shown its efficacy on pain and allodynia in various animal models of inflammatory neuropathic pain. The only randomized, double-blind, placebo-controlled trial in patients with focal painful neuropathies due to nerve trauma or postherpetic neuralgia demonstrated significant effects on average pain intensity from 2 weeks after the injections to 14 weeks. Most patients reported pain during the injections, but there were no further local or systemic side effects. The efficacy of BTX-A in painful peripheral neuropathies has been more recently studied. Results were positive in the only study in an animal model of peripheral neuropathy. One study in patients with diabetic painful peripheral neuropathy demonstrated a significant decrease in Visual Analog Scale. In conclusion, one session of multiple intradermal injection of BTX-A produces long-lasting analgesic effects in patients with focal painful neuropathies and diabetic neuropathic pain, and is particularly well tolerated. The findings are consistent with a reduction of peripheral sensitisation, the place of a possible central effect remaining to define. Further studies are needed to assess some important issues, i.e. BTX-A efficacy in patients with small fiber neuropathies and the relevance of early and repeated injections. Future studies could also provide valuable insights into pathophysiology of neuropathic pain.

Re-engineering clostridial neurotoxins for the treatment of chronic pain: current status and future prospects

Clostridial neurotoxins from the botulinum neurotoxin (BoNT) family are protein complexes, derived from the bacterium Clostridium botulinum, which potently inhibit acetylcholine release and result in a reversible blockade of the neuromuscular junction. This feature led to the clinical development of BoNT-A for a number of neuromuscular disorders. BoNT-A toxins are commercially available as three different preparations: Dysport/Azzalure, Botox/Vistabel, and Xeomin/Bocouture. Although BoNT-A preparations have not yet been approved for the treatment of pain, a substantial body of preclinical and clinical evidence shows that BoNT-A is effective in treating a number of different types of pain. It is thought to exert an analgesic effect both via muscle-relaxant properties and also directly, via inhibition of nociceptive neuropeptides. This review explores the mechanistic basis of this analgesic effect, summarizing current knowledge of the structure-function relationship of BoNT and discussing effects on both motor and pain neurons. For a complete picture of the analgesic properties of BoNT-A, clinical evidence of efficacy in myofascial pain and neuropathic pain is considered in tandem with a mechanistic rationale for activity. Patients experiencing chronic pain are clear candidates for treatment with a modified clostridial endopeptidase that would provide enduring inhibition of neurotransmitter release. A strong preclinical evidence base underpins the concept that re-engineering of BoNT could be used to enhance the analgesic potential of this neurotoxin, and it is hoped that the first clinical studies examining re-engineered BoNT-A will confirm this potential.

Activation of transient receptor potential vanilloid subtype 1 increases secretion of the hypofunctional, transplanted submandibular gland

Hyposecretion occurs in most patients early after submandibular gland autotransplantation for severe keratoconjunctivitis sicca. Endogenous transient receptor potential vanilloid subtype 1 (TRPV1) has been recently demonstrated in rabbit submandibular glands, and activation of TRPV1 by capsaicin increases secretion in isolated glands, but the TRPV1-mediated secretory mechanism remains to be elucidated. The purpose of this study was to verify whether activation of TRPV1 by capsaicin could improve the secretion of transplanted gland and its underlying mechanism. The salivary flow of the transplanted glands was significantly decreased, and the mRNA and protein levels of TRPV1 and aquaporin 5 (AQP5) were downregulated in the transplanted glands. Topical capsaicin cream increased secretion and upregulated levels of TRPV1 and AQP5 in transplanted glands. Moreover, in cultured submandibular gland cells, capsaicin increased the mRNA expression of AQP5 and led to redistribution of AQP5 from the cytoplasm to the plasma membrane via TRPV1 activation. Capsaicin enhanced the phosphorylation of extracellular signal-regulated kinase (ERK). Preincubation of cells with PD98059, an inhibitor of ERK kinase, suppressed the capsaicin-induced mRNA expression of AQP5. In summary, the capsaicin-induced secretory mechanism involved activation of TRPV1 and upregulation of AQP5 in an ERK-dependent manner and promoted the redistribution of AQP5 in submandibular gland cells. Activation of TRPV1 may provide a new therapeutic strategy to improve submandibular gland hypofunction.

Peptide-mediated transdermal delivery of botulinum neurotoxin type A reduces neurogenic inflammation in the skin

Release of inflammatory pain mediators from peripheral sensory afferent endings contributes to the development of a positive feedback cycle resulting in chronic inflammation and pain. Botulinum neurotoxin type A (BoNT-A) blocks exocytosis of neurotransmitters and may therefore block the release of pain modulators in the periphery. Subcutaneous administration of BoNT-A (2.5, 5 and 10U) reduced plasma extravasation (PE) caused by electrical stimulation of the saphenous nerve or capsaicin in the rat hindpaw skin (ANOVA, Post hoc Tukey, p<0.05, n=6). Subcutaneous BoNT-A also reduced blood flow changes evoked by saphenous nerve stimulation (ANOVA, Post hoc Tukey, p<0.05, n=6). Subcutaneous BoNT-A had no effect on PE induced by local injection of substance P (SP) or vasodilation induced by local CGRP injection. Although BoNT-A is an effective treatment for a wide range of painful conditions, the toxin's large size necessitates that it be injected at numerous sites. We found that a short synthetic peptide (TD-1) can facilitate effective transdermal delivery of BoNT-A through intact skin. Coadministration of TD-1 and BoNT-A to the hindpaw skin resulted in a significant reduction in PE evoked by electrical stimulation. The findings show that BoNT-A can be administered subcutaneously or topically with a novel transdermal delivery peptide to reduce inflammation produced by activating nociceptors in the skin. Peptide-mediated delivery of BoNT-A is an easy and non-invasive way of administering the toxin that may prove to be useful in clinical practice.