Sodium channels and pain therapy

Cutaneous pain in disorders affecting peripheral nerves

Our ability to quickly detect and respond to harmful environmental stimuli is vital for our safety and survival. This inherent acute pain detection is a "gift" because it both protects our body from harm and allows healing of damaged tissues [1]. Damage to tissues from trauma or disease can result in distorted or amplified nociceptor signaling and sensitization of the spinal cord and brain (Central Nervous System; CNS) pathways to normal input from light touch mechanoreceptors. Together, these processes can result in nagging to unbearable chronic pain and extreme sensitivity to light skin touch (allodynia). Unlike acute protective pain, chronic pain and allodynia serve no useful purpose and can severely reduce the quality of life of an affected person. Chronic pain can arise from impairment to peripheral neurons, a phenomenon called "peripheral neuropathic pain." Peripheral neuropathic pain can be caused by many insults that directly affect peripheral sensory neurons, including mechanical trauma, metabolic imbalance (e.g., diabetes), autoimmune diseases, chemotherapeutic agents, viral infections (e.g., shingles). These insults cause "acquired" neuropathies such as small-fiber neuropathies, diabetic neuropathy, chemotherapy-induced peripheral neuropathy, and post herpetic neuralgia. Peripheral neuropathic pain can also be caused by genetic factors and result in hereditary neuropathies that include Charcot-Marie-Tooth disease, rare channelopathies and Fabry disease. Many acquired and hereditary neuropathies affect the skin, our largest organ and protector of nearly our entire body. Here we review how cutaneous nociception (pain perceived from the skin) is altered following diseases that affect peripheral nerves that innervate the skin. We provide an overview of how noxious stimuli are detected and encoded by molecular transducers on subtypes of cutaneous afferent endings and conveyed to the CNS. Next, we discuss several acquired and hereditary diseases and disorders that cause painful or insensate (lack of sensation) cutaneous peripheral neuropathies, the symptoms and percepts patients experience, and how cutaneous afferents and other peripheral cell types are altered in function in these disorders. We highlight exciting new research areas that implicate non-neuronal skin cells, particularly keratinocytes, in cutaneous nociception and peripheral neuropathies. Finally, we conclude with ideas for innovative new directions, areas of unmet need, and potential opportunities for novel cutaneous therapeutics that may avoid CNS side effects, as well as ideas for improved translation of mechanisms identified in preclinical models to patients.

Molecular mechanisms underlying the actions of arachidonic acid-derived prostaglandins on peripheral nociception

Arachidonic acid-derived prostaglandins not only contribute to the development of inflammation as intercellular pro-inflammatory mediators, but also promote the excitability of the peripheral somatosensory system, contributing to pain exacerbation. Peripheral tissues undergo many forms of diseases that are frequently accompanied by inflammation. The somatosensory nerves innervating the inflamed areas experience heightened excitability and generate and transmit pain signals. Extensive studies have been carried out to elucidate how prostaglandins play their roles for such signaling at the cellular and molecular levels. Here, we briefly summarize the roles of arachidonic acid-derived prostaglandins, focusing on four prostaglandins and one thromboxane, particularly in terms of their actions on afferent nociceptors. We discuss the biosynthesis of the prostaglandins, their specific action sites, the pathological alteration of the expression levels of related proteins, the neuronal outcomes of receptor stimulation, their correlation with behavioral nociception, and the pharmacological efficacy of their regulators. This overview will help to a better understanding of the pathological roles that prostaglandins play in the somatosensory system and to a finding of critical molecular contributors to normalizing pain.

An open-label pilot study evaluating the effectiveness of the heated lidocaine/tetracaine patch for the treatment of pain associated with carpal tunnel syndrome

OBJECTIVES

Carpal tunnel syndrome (CTS) is a common entrapment neuropathy of the median nerve at the wrist that is characterized by pain, paresthesias, weakness, and loss of dexterity. This pilot study was conducted to evaluate the heated lidocaine/tetracaine patch (HLT patch) as a conservative treatment for pain of CTS.

METHODS

Twenty adult patients (mean age = 44 ± 12 years) with pain secondary to unilateral CTS and electrodiagnostic evidence of mild-to-moderate CTS enrolled in this open-label study. Patients were treated with a single HLT patch placed over the junction of forearm and wrist on the palmar aspect of the wrist twice daily (morning and evening at 12-hour intervals) for 2 hours. At baseline and during the 2-week study, patients graded their pain intensity with an 11-point numerical rating scale (0 = no pain, 10 = worst imaginable pain). Pain interference with general activity, work, and sleep was evaluated with a similar 0-to-10-point scale.

RESULTS

Fifteen patients completed the 14-day treatment period. Mean average pain intensity score decreased from 5.1 ± 1.5 at baseline to 2.5 ± 1.6 at end of study in the per-protocol population (P < 0.001). Two-thirds of the patients demonstrated clinically meaningful pain relief (≥ 30% reduction in average pain score), with 40% of the patients reaching this threshold by the third treatment day. Similar improvements were observed for pain interference scores. The HLT patch was generally well tolerated.

CONCLUSION

The HLT patch resulted in clinically meaningful reduction in pain intensity in the majority of patients with mild-to-moderate CTS and may represent a targeted nonsurgical treatment for pain associated with CTS.

Sodium channel Na v 1.7 immunoreactivity in painful human dental pulp and burning mouth syndrome

BACKGROUND

Voltage gated sodium channels Na v 1.7 are involved in nociceptor nerve action potentials and are known to affect pain sensitivity in clinical genetic disorders.

AIMS AND OBJECTIVES

To study Na v 1.7 levels in dental pulpitis pain, an inflammatory condition, and burning mouth syndrome (BMS), considered a neuropathic orofacial pain disorder.

METHODS

Two groups of patients were recruited for this study. One group consisted of patients with dental pulpitis pain (n = 5) and controls (n = 12), and the other patients with BMS (n = 7) and controls (n = 10). BMS patients were diagnosed according to the International Association for the Study of Pain criteria; a pain history was collected, including the visual analogue scale (VAS). Immunohistochemistry with visual intensity and computer image analysis were used to evaluate levels of Na v 1.7 in dental pulp tissue samples from the dental pulpitis group, and tongue biopsies from the BMS group.

RESULTS

There was a significantly increased visual intensity score for Na v 1.7 in nerve fibres in the painful dental pulp specimens, compared to controls. Image analysis showed a trend for an increase of the Na v 1.7 immunoreactive % area in the painful pulp group, but this was not statistically significant. When expressed as a ratio of the neurofilament % area, there was a strong trend for an increase of Na v 1.7 in the painful pulp group. Na v 1.7 immunoreactive fibres were seen in abundance in the sub-mucosal layer of tongue biopsies, with no significant difference between BMS and controls.

CONCLUSION

Na v 1.7 sodium channel may play a significant role in inflammatory dental pain. Clinical trials with selective Na v 1.7 channel blockers should prioritize dental pulp pain rather than BMS.

Behavioral models of pain states evoked by physical injury to the peripheral nerve

Physical injury or compression of the root, dorsal root ganglion, or peripheral sensory axon leads to well-defined changes in biology and function. Behaviorally, humans report ongoing painful dysesthesias and aberrations in function, such that an otherwise innocuous stimulus will yield a pain report. These behavioral reports are believed to reflect the underlying changes in nerve function after injury, wherein increased spontaneous activity arises from the neuroma and dorsal root ganglion and spinal changes increase the response of spinal projection neurons. These pain states are distinct from those associated with tissue injury and pose particular problems in management. To provide for developing an understanding of the underlying mechanisms of these pain states and to promote development of therapeutic agents, preclinical models involving section, compression, and constriction of the peripheral nerve or compression of the dorsal root ganglion have been developed. These models give rise to behaviors, which parallel those observed in the human after nerve injury. The present review considers these models and their application.

Quantification of neural protein in extirpated tooth pulp

Because the pulp tissue extirpated during root canal procedures might serve as a valuable resource with which to assess underlying mechanisms of persistent pain, we sought to determine whether standard Western blotting techniques could be used to quantify neural proteins in pulp extirpated from teeth with irreversible pulpitis. Pulp harvested from healthy intact teeth extracted for orthodontic reasons was used for comparison. The neural marker PGP9.5 was detectable in all samples tested. A membrane enrichment protocol enabled detection of even low abundance, high molecular weight proteins such as the sodium channel alpha-subunit NaV1.8. Importantly, it was possible to quantify a approximately 6-fold increase in the relative density of NaV1.8 in inflamed pulp compared with control pulp. Our results suggest that it should be possible to use extirpated tooth pulp to validate mechanisms of persistent pain implicated in preclinical studies as well as evaluate the therapeutic efficacy of novel antinociceptive interventions.

Inflammatory Mediators Enhance the Excitability of Chronically Compressed Dorsal Root Ganglion Neurons

A laterally herniated disk, spinal stenosis, and various degenerative or traumatic diseases of the spine can sometimes lead to a chronic compression and inflammation of the dorsal root ganglion and chronic abnormal sensations including pain. After a chronic compression of the dorsal root ganglion (CCD) in rats, the somata in the dorsal root ganglion (DRG) become hyperexcitable, and some exhibit ectopic, spontaneous activity (SA). Inflammatory mediators have a potential role in modulating the excitability of DRG neurons and therefore may contribute to the neuronal hyperexcitability after CCD. In this study, an inflammatory soup (IS) consisting of bradykinin, serotonin, prostaglandin E2, and histamine (each 10−6M) was applied topically to the DRG. The responses of DRG neurons were electrophysiologically recorded extracellularly from teased dorsal root fibers or intracellularly from the somata in the intact DRG or from dissociated neurons within 30 h of culture. In all three preparations, IS remarkably increased the discharge rates of SA CCD neurons and evoked discharges in more silent-CCD than control neurons. IS slightly depolarized the resting membrane potential and decreased the current and voltage thresholds of action potential in both intact and dissociated neurons, although the magnitude of depolarization or decrease in action potential threshold was not significantly different between CCD and control. IS-evoked responses were found in a proportion of neurons in each size category including those with and without nociceptive properties. Inflammatory mediators, by increasing the excitability of DRG somata, may contribute to CCD-induced neuronal hyperexcitability and to hyperalgesia and tactile allodynia.

[Neuropathic pain due to malignancy: mechanisms, clinical manifestations and therapy]

INTRODUCTION

Neuropathic pain in cancer patients requires a focused clinical evaluation based on knowledge of common neuropathic pain syndromes.

DEFINITION

Neuropathic pain is a non-nociceptive pain or "deafferentation" pain, which suggests abnormal production of impulses by neural tissue that is separated from afferent input. Impulses arise from the peripheral nervous system or central nervous system. CAUSES OF NEUROPATHIC PAIN DUE TO MALIGNANCY: Neuropathic pain is caused directly by cancer-related pathology (compression/infiltration of nerve tissue, combination of compression/infiltration) or by diagnostic and therapeutic procedures (surgical procedures, chemotherapy, radiotherapy).

MECHANISMS

Pathophysiological mechanisms are very complex and still not clear enough. Neuropathic pain is generated by electrical hyperactivity of neurons along the pain pathways. Peripheral mechanisms (primary sensitization of nerve endings, ectopically generated action potentials within damaged nerves, abnormal electrogenesis within sensory ganglia) and central mechanisms (loss of input from peripheral nociceptors into dorsal horn, aberrant sprouting within dorsal horn, central sensitization, loss of inhibitory interneurons, mechanisms at higher centers) are involved.

DIAGNOSIS

The quality of pain presents as spontaneous pain (continuous and paroxysmal), abnormal pain (allodynia, hyperalgesia, hyperpathia), paroxysmal pain.

CLINICAL MANIFESTATIONS

Clinically, neuropathic pain is described as the pain in the peripheral nerve (cranial nerves, other mononeuropathies, radiculopathy, plexopathy, paraneoplastic peripheral neuropathy) and relatively infrequent, central pain syndrome.

THERAPY

Treatment of neuropathic pain remains a challenge for clinicians, because there is no accepted algorithm for analgesic treatment of neuropathic pain. Pharmacotherapy is considered to be the first line therapy. Opioids combined with non-steroidal antiinflammatory drugs are warranted. If patient is relatively unresponsive to an opioid, a trial with adjuvant analgesics might be considered. Tricyclic antidepressants might be selected for patients with continuous dysesthesia, and anticonvulsants might be used if the pain is predominanty lancinating or paroxysmal. The complexity of neuropathic syndromes and underlying etiologic mechanisms warrant clinical trials to determine appropriate treatment.

Redistribution of Na(V)1.8 in uninjured axons enables neuropathic pain

The underlying mechanisms of neuropathic pain are poorly understood, and existing treatments are mostly ineffective. We recently demonstrated that antisense mediated "knock-down" of the sodium channel isoform, Na(V)1.8, reverses neuropathic pain behavior after L5/L6 spinal nerve ligation (SNL), implicating a critical functional role of Na(V)1.8 in the neuropathic state. Here we have investigated mechanisms through which Na(V)1.8 contributes to the expression of experimental neuropathic pain. Na(V)1.8 does not appear to contribute to neuropathic pain through an action in injured afferents because the channel is functionally downregulated in the cell bodies of injured neurons and does not redistribute to injured terminals. Although there was little change in Na(V)1.8 protein or functional channels in the cell bodies of uninjured neurons in L4 ganglia, there was a striking increase in Na(V)1.8 immunoreactivity along the sciatic nerve. The distribution of Na(V)1.8 reflected predominantly the presence of functional channels in unmyelinated axons. The C-fiber component of the sciatic nerve compound action potential (CAP) was resistant (>40%) to 100 microm TTX after SNL, whereas both A- and C-fiber components of sciatic nerve CAP were blocked (>90%) by 100 microm TTX in sham-operated rats or the contralateral sciatic nerve of SNL rats. Attenuating expression of Na(V)1.8 with antisense oligodeoxynucleotides prevented the redistribution of Na(V)1.8 in the sciatic nerve and reversed neuropathic pain. These observations suggest that aberrant activity in uninjured C-fibers is a necessary component of pain associated with partial nerve injury. They also suggest that blocking Na(V)1.8 would be an effective treatment of neuropathic pain.

Prostaglandin E(2) modulates TTX-R I(Na) in rat colonic sensory neurons

This study was performed to determine the impact of the inflammatory mediator prostaglandin E(2) (PGE(2)) on the biophysical properties of tetrodotoxin resistant voltage-gated Na(+) currents (TTX-R I(Na)) in colonic dorsal root ganglion (DRG) neurons. TTX-R I(Na) was studied in DRG neurons from thoracolumbar (TL: T(13)-L(2)) and lumbosacral (LS: L(6)-S(2)) DRG retrogradely labeled following the injection of DiIC(18) (DiI) into the wall of the descending colon of adult male rats. TTX-R I(Na) in colonic DRG neurons had a high threshold for activation [V(0.5) of conductance-voltage (G-V) curve = -3.1 +/- 1.0 (SE) mV] and steady-state availability (V(0.5) for H-infinity curve = -18.4 +/- 1.4 mV), was slowly inactivating (10.6 +/- 1.4 ms at 0 mV), and recovered rapidly from inactivation (83.5 +/- 5.0% of the current recovered with a time constant of 1.3 +/- 0.1 ms at -80 mV). TTX-R I(Na) was present in every colonic DRG neuron studied (n = 62). PGE(2) induced a rapid (<15 s) increase in TTX-R I(Na) that was associated with a hyperpolarizing shift in the G-V curve (3.4 +/- 0.7 mV), an increase in the rate of inactivation (4.21 +/- 0.7 ms at 0 mV), and no change in steady-state availability. There was no statistically significant difference (P > 0.05) between TL and LS colonic DRG neurons with respect to the biophysical properties of TTX-R I(Na), the current density or the magnitude of PGE(2)-induced changes in the current. However, both the proportion of TL and LS neurons in which TTX-R I(Na) was modulated by PGE(2) (16 of 16 TL neurons and 12 of 14 LS neurons) as well as the magnitude of PGE(2)-induced changes in the current were significantly larger in colonic DRG neurons than in the total population of DRG neurons. These results suggest that changes in nociceptive processing associated with inflammation of the colon does not reflect differences between TL and LS neurons with respect to the properties of TTX-R I(Na), distribution of current, or magnitude of inflammatory mediator-induced changes in the current. However, these results do suggest modulation of TTX-R I(Na) in colonic afferents is an underlying mechanism of hyperalgesia and pain associated with inflammation of the colon and that this current constitutes a novel target for therapeutic relief of visceral inflammatory pain.