Identification of the Involvement of Potassium Channels in Fibromyalgia

Fibromyalgia is a central sensitivity syndrome that presents with chronic pain, fatigue, cognitive dysfunction, and disordered sleep. The pathophysiology which due to multisensory hypersensitivity of the central nervous system involves neuronal excitability leading to central sensitization. Treatments of the challenges associated with the complexities of fibromyalgia involve combinations of pharmacological and non-pharmacological therapeutic approaches which often offer limited benefit. Potassium (K + ) channels play a fundamental role in establishing and maintaining stability of neuronal activity. The large molecular diversity and distribution of K + channels support involvement in a broad range of physiological functions. In nociceptive pathways, neuronal hyperexcitability leading to pain sensation has been associated with reduced function of K + channels and loss of cellular control. This article reviews the evidence of involvement of K + channels in fibromyalgia. A potential role both in the pathophysiological processes responsible for the symptoms of fibromyalgia and as therapeutic targets for the management of the condition is considered.


Introduction
Fibromyalgia is part of the spectrum of central sensitivity syndromes characterized by persistent widespread idiopathic or "nociplastic" pain which is accompanied by fatigue, cognitive and mood disturbances, and disordered sleep [1][2][3].Diffuse hyperalgesia (heightened responses to painful stimuli) and allodynia (painful responses to nonpainful stimuli) with lowered pressure pain thresholds are characteristics of the pain associated with fibromyalgia.The diagnosis is often complicated by co-morbidities, e.g., chronic fatigue syndrome, with similar symptoms.Fibromyalgia, dependent on the diagnostic criteria and methods of classification, is estimated to affect 0.4-8% of the population worldwide with prevalence greater in females and increasing with age [4].The pathophysiology of fibromyalgia involves amplified responses of the central nervous system (CNS) to peripheral stimuli associated with neuronal excitability leading to central sensitization (CS) [1].The heightened activity of the CNS responsible for the characteristic fibromyalgia symptoms is initiated by a range of peripheral sensory generators.
Neuronal activity within the CNS and peripheral nervous system (PNS) is suppressed and stabilized by potassium (K + ) channel activity regulating the membrane potential, action potential initiation, axonal conduction and neurotransmitter release.K + channels play a fundamental role in nociceptive processing linking disturbances of their expression and function to the generation of pain due to peripheral and central hyperexcitability [5][6][7][8][9].In addition, the pharmacology of certain anti-nociceptive drugs has been associated with K + channel modulation within the PNS and CNS [6][7][8][9].K + channels are a diverse family of ion channels that are widely distributed in excitable and nonexcitable cells and are involved in the regulation of the resting membrane potential and level of excitation of cells.The large molecular diversity that has been identified and characterized supports involvement of K + channels in a broad range of physiological functions.More than 78 genes in humans are responsible for the diversity of K + channels which are organized into four structural types, voltage-gated (Kv), inward rectifier (Kir), Ca 2+ -activated (K Ca ) and tandempore domain K + channels [7,8,10].An α-subunit containing gating and selectivity filter apparatus forms the K + ionconduction pore which can be co-located with auxiliary βsubunits responsible for modulating function and interaction with cellular signalling pathways.The activation of K + channels facilitates rapid transmembrane K + efflux repolarizing or hyperpolarizing the cell membrane and limiting action potential generation.Neuronal hyperexcitability in nociceptive pathways leading to development of pain sensation due to complete or partial loss of cellular control has been associated with reduced function of K + channels whether inherited or acquired.Dominant-negative suppression of function of K + channels has been described due to gene mutations and altered expression contributing to the development of pain conditions [7,8].
Clinical features related to altered K + channel function present in channelopathies, i.e., conditions due to ion channel dysfunction, have demonstrated similarity to the characteristic fibromyalgia symptoms [11].This narrative review will consider evidence of involvement of K + channels in the pathophysiology of fibromyalgia and as therapeutic targets for treatment of this condition.

Pathophysiology of Fibromyalgia
Evidence in patients with FM predicts responses to augmented sensory input evokes disturbed neurotransmitter functioning and neuroplasticity leading to neuronal hyperexcitability and hypersensitivity in the CNS due to enhanced excitatory and reduced inhibitory processes [1,2,12].These findings are consistent with the development of central sensitization (Fig. 1) [1,12].Although heightened activity of the CNS can be initiated by peripheral sensory generators, such as nerve pathologies, neuroinflammation, skeletal muscle abnormalities and ischaemia, constant nociceptor activation due to a comorbid condition can also evoke secondary fibromyalgia [2,3,12].Pain associated with fibromyalgia is characterized by neuropathic components in that it is chronic and intractable and unlike nociceptive pain, which serves to protect from potential or actual injury, persists after tissue healing has taken place.Even in the apparent absence of sensory stimulus symptoms of pain can present indicating that it is maladaptive and has no obvious biological purpose [2,12].
In brain spectroscopy studies of patients with fibromyalgia severity of pain correlated with raised levels of neurotransmitters glutamate, glutamine, and glycine in regions of the brain associated with pain processing [1,2,13].Consequently, in the spinal cord and brain the glutamate N-methyl-D-aspartate (NMDA) receptors are exposed to elevated glutamate levels.In addition, the cerebrospinal fluid (CSF) of patients with fibromyalgia has elevated levels of substance P, endogenous opioids, nerve growth factor, brain-derived neurotrophic factor and attenuated levels of 5-hydroxyindoleacetic acid, a metabolite of serotonin, and 3-methoxy-4-hydroxyphenylglycol, a metabolite of noradrenaline [1,2,13].Serum serotonin and L-tryptophan are also suppressed relative to healthy subjects in people with fibromyalgia [1,2,13].Consistent with these findings, the activity of descending serotonergic-noradrenergic efferent pathways in people with fibromyalgia has been observed to be reduced resulting in altered diffuse noxious inhibitory control (DNIC) of pain pathways [14].DNIC is the descending, from the CNS to the spinal cord, control of nociception which relies on painful conditioning stimulation of one part of the body to inhibit pain in another part and inefficiency has been implicated in the development of chronic pain [14].
A characteristic of fibromyalgia is non-restorative sleep associated with poor sleep quality [15][16][17].Superficial, fragmented and non-restorative sleep with restless leg syndrome and morning fatigue are common complaints amongst people with fibromyalgia.Diminished slow-wave sleep, which is essential to feeling refreshed upon awakening, during non-rapid eye movement sleep (NREM) is often reported [15][16][17].Persistent pain related to heightened states of arousal has been linked to lower sleep efficiency and patterns of sleep discontinuity [18].Altered production of melatonin has also been reported in patients with fibromyalgia with lower secretion in dark hours related to sleep disturbance and raised secretion in daytime contributing to pain, fatigue and mood symptoms [19].
Dysautonomia, neuroinflammation, increased proinflammatory with reduced anti-inflammatory cytokine profiles and reduced reactivity to stress of the hypothalamic pituitary adrenal axis have also been proposed to be involved in the genesis and enhancement of symptoms of fibromyalgia [2,12,13].

Sensory Axonal Excitability
Investigation of sensory axonal excitability in patients with fibromyalgia revealed enhanced superexcitability relative to healthy controls [20].The technique used measured membrane polarization, ion channel function and paranodal/internodal condition of peripheral nerves.Superexcitability is influenced by the conductance of paranodal fast K + channels with increased conductance reducing intensity and period of superexcitability, and reduced conductance contributing to increased superexcitability [21,22].The symptoms of fibromyalgia assessed by the Fibromyalgia Impact Questionnaire (FIQ), an evaluation instrument of fibromyalgia patient status, progress, and outcomes, negatively correlated with the increased subexcitability suggesting compensatory K + channel activity attenuating the severity of the condition.Neuronal cell excitability can be monitored by creating subthreshold depolarizing or hyperpolarizing currents within the membrane, i.e., threshold electrotonus (TE).In patients with fibromyalgia a slight increase in sensory hyperpolarizing TE was observed which would also be consistent with instability of K + channels [20].The dysregulated K + channel conductance observed in, at least, the nociceptive pathway would be consistent with the hypersensitivity underlying symptoms in patients with fibromyalgia.

Thalamic Cells
Although findings have been inconsistent, some studies have suggested an abnormal intrusion of alpha activity (8-13 Hz oscillations) into the delta activity (1-4 Hz oscillations) that occurs during slow-wave sleep in people with fibromyalgia [15][16][17]23].The pain experienced by patients with fibromyalgia may be exacerbated by these sleep abnormalities.Depolarization of cells within the thalamus due to alterations in conductance of K + currents has been suggested to underlie alpha-delta sleep [24].Increasing thalamic K + currents was suggested to lead to the restoration of delta sleep.The drug sodium oxybate, which has been shown to improve quality of sleep and levels of pain in patients with fibromyalgia [25], restores delta sleep following modulation of molecular targets, including K + channels, in the thalamocortical cells [24].

KCHN2 Protein
Lower plasma levels of KCHN2 protein, which forms the Kv11.1 subtype of the hERG (Ether-a-go-go-related gene) Kv channel were observed in patients with fibromyalgia compared to healthy controls [26].In contrast, the levels of the proteins KCHN6 and KCHN7 that form the Kv11.2 and Kv11.3 subtypes of the hERG channel were not different in patients with fibromyalgia and healthy controls.Kv11 channels are expressed in the CNS and are thought to contribute to adaptive firing of neurons [27,28].At the spinal cord level Kv11.1 channels have been demonstrated to be involved in nociceptive modulation [29].Hyperexcitable states due to a lack of accommodation of nerve cells to repetitive stimuli may occur due to altered hERG expression.A relationship between the plasma levels of KCHN2 and the symptoms of fibromyalgia has not yet been established.

Kv Channel Autoantibodies
Autoantibodies targeting Kv channels associated with contactin-associated protein-2 (CASPR2)-IgG-positivity have been observed to contribute to hyperexcitability which could be responsible for persistent pain experienced by people with fibromyalgia [30].CASPR2 is a member of the neurexin family that is expressed in the PNS and CNS and colocalizes with Kv1 subtype channels.In people with persistent pain conditions, such as fibromyalgia, the prevalence of symptoms correlated with a positive Kv channelcomplex immunoglobulin status.Disruption of Kv localization at paranodal axons may be involved in the link between CASPR2 autoimmunity and pain.Immune modulation therapy, prednisone, methylprednisolone, intravenous immune globulin, methotrexate or hydroxychloroquine, reduced the state of pain in Kv channel-complex seropositive patients allowing the discontinuation of opioid analgesics in some cases [30].
3.1.5Substance P Substance P (SP) levels are elevated in the CSF of people with fibromyalgia [1,2].SP is a mediator of pain involved in the generation and transmission of pain signalling which can also exhibit antinociceptive properties [31].CS results due to SP activating excitatory post-synaptic potential in the CNS, whilst peripheral neurogenic inflammation is caused by the release of SP from nociceptive nerve fibres [32,33].SP decreases the activity of Kir channels in locus coeruleus and nucleus basalis neurons, and IK Ca subtype channels in stellate ganglion neurons leading to neuronal excitation [34][35][36].Within the PNS SP has been suggested to sensitize nociceptors due to a decrease of low-threshold K + channel (Kv4 subtype) activity in addition to activating excitatory mechanisms [37].Thus, raised levels of SP observed in people with fibromyalgia will lead to suppression of the inhibitory K + channel activity within peripheral sensory small neurons (nociceptors) resulting in neuronal excitation and increased nociceptive responses.In contrast, SP enhances the current of Kv7 subtype channels in the PNS and the afferent dorsal root ganglion neurons whilst inhibiting T-type calcium currents which dampen neuronal excitability and evokes an anti-nociceptive effect [38,39].The balance between pro-and anti-nociception in people with fibromyalgia will be dependent on the distribution and concentration of the elevated SP levels.

Pregabalin
The gabapentinoid pregabalin has demonstrated clinically significant improvements of core symptoms in patients with fibromyalgia and is a recommended treatment of the condition [40].The benefits reported have been dose-related pain alleviation with improvement in quality of life and sleep [40].The efficacy exhibited by pregabalin as a treatment of pain is a consequence of preventing sensory propagation with reduction of neuronal excitability.Alpha-2-delta (α2δ) subunits of the voltage-activated calcium channels, which are expressed in pain processing areas of the spinal dorsal horn, periaqueductal gray, ante-rior cingulate cortex, insula and amygdala, have been the focus of the mechanism of action of pregabalin leading to inhibition of neurotransmitter, such as glutamate and glutamine, release, and pain control [41].Evidence, however, supports the involvement of K + channels in the action of pregabalin on pain processing and analgesia.Pregabalin activated ATP-dependent K + channels (K ATP ) in isolated hippocampal neuron-derived cells and an anti-nociceptive effect in a formalin-induced pain rat model involved the activation of K ATP channels by pregabalin [42,43].Suppression of the α2δ subunit function by the extracellular N terminus region of the BK Ca channel in dorsal root ganglion neurons reducing calcium ion influx and neuronal excita-tion could be enhanced by pregabalin binding to the α2δ subunit [44].Pregabalin has also been reported to enhance apamin-sensitive SK Ca currents in rat dorsal root ganglion neurons [45].

Lidocaine
Intravenous lidocaine has been demonstrated to reduce hyperalgesia, pain intensity and FIQ scores in people with fibromyalgia [46].Although inhibition of sodium channels is considered the primary mechanism of action, lidocaine suppresses neuronal activity within the dorsal horn due to hyperpolarization following blockade of Kv channels [47,48].Hyperpolarization-activated cyclic nucleotidegated (HCN) channels, which are related to Kv channels and generate inward K + current producing rhythmic electrical activity in specialized peripheral sensory and CNS neurons, are inhibited by lidocaine at a concentration range for systemic use [49,50].Thus, the activity of HCN channel activity evoking repetitive firing of nociceptive neurons leading to pain is suppressed by lidocaine [50].

Flupirtine
Patients with fibromyalgia reported an improvement in their symptoms, particularly pain and physical activity, following flupirtine treatment [51].Flupirtine evokes hyperpolarization and stabilization of neuronal membranes due to activation of Kv7 subtype channels, with subsequent NMDA receptor activity reduction [52].Kv7 channels are present in central terminals of primary afferents and spinal cord dorsal horn neurons and involved in the regulation of peripheral and central nociceptive pathway activity [52,53].Activation of neuronal Kv7 channels evokes stabilization of membrane excitability producing inhibition of action potential initiation and thereby these channels are a therapeutic target for the treatment of diseases involving neuronal hyperexcitability [54,55].The location of Kv7 channels in other neuronal areas responsible for the pharmacological properties of flupirtine are relevant to the management of the spectrum of symptoms characteristic of fibromyalgia [52,53].Thus, K + channel activation, at least in nociceptive pathways, by flupirtine appears to be responsible for the benefit to patients with fibromyalgia.

Melatonin
Melatonin regulates the sleep-awake cycle due to synchronization of circadian rhythm which impacts on fatigue and mood levels and enhances pain inhibition [56,57].Some studies in patients with fibromyalgia have reported altered levels of melatonin with secretion during dark hours being lowered contributing to disordered sleep leading to daytime pain and fatigue and raised daytime secretion enhancing the pain, fatigue, and mood symptoms further disturbing sleep quality [19].Although such findings could suggest abnormal secretion of melatonin being involved in the pathophysiology of fibromyalgia, inconsistencies have been reported where studies have observed no difference in the melatonin levels in patients with fibromyalgia and healthy controls [19].Administration of melatonin as a treatment of fibromyalgia reduced pain and fatigue, and improved sleep quality and quality of life measurements [19].The anti-nociceptive effects of melatonin following stimulation of melatonergic receptors have been associated with the activation of SK Ca , BK Ca , K ATP and G-protein coupled Kir3 channels [58,59].Cell hyperpolarization due to efflux of K + ions in regions of the nervous system such as the suprachiasmatic nucleus and cerebellar cells contribute to the anti-nociceptive effect of melatonin [59,60].

ASP0819
In rat reserpine-induced myalgia and vagotomyinduced myalgia models that mimic pain features of fibromyalgia, such as hyperalgesic symptoms, the activator of IK Ca (also known as K Ca 3.1) channels ASP0819 reduced responses of neuronal excitation in peripheral nerve fibres at the dorsal root responsible for pain [61].Increases in cytosolic calcium evokes opening of K Ca channels regulating calcium-signalling and membrane potentials in excitable and non-excitable cells.The IK Ca subtype is a voltageinsensitive channel widely expressed in numerous cell types and tissues, including dorsal root ganglion [62,63].Activation of IK Ca channels leads to a period after each action potential of reduced excitability and thereby evokes cellular stability [63].ASP0819 improved sleep, pain and FIQ scores in people with fibromyalgia consistent with IK Ca channels being a therapeutic target in such a clinical condition that involves unregulated nerve excitation [64].

Palmitoylethanolamide
Palmitoylethanolamide (PEA) is a peroxisome proliferator-activated receptor (PPARα) agonist that improved pain and FIQ quality of life scores in people with fibromyalgia [65,66].Interestingly the analgesic effects of PEA in a formalin-induced pain mouse model have been demonstrated to involve the activation of IK Ca , BK Ca and K ATP channels [67].In a prostaglandin E 2 -induced hyperalgesia rat paw model peripheral antinociception due to PEA treatment involved the activation of K ATP channels, but not Kv, IK Ca or BK Ca channels [68].In addition to exhibiting PPARα agonist properties PEA, which is an endocannabinoid-like structurally related N-acylethanolamine, has been shown to stimulate cannabinoid receptors which may be responsible for the K + channel activation.Plasma levels of PEA were reported to be elevated in patients with fibromyalgia and thereby may be part of a compensatory mechanism involving K + channel activation within the complex pathophysiology of this condition [69].

Suvorexant
In patients with fibromyalgia treatment with the orexin receptor antagonist suvorexant, which would allow recovery of Kv7 channel activity, resulted in an improved sleep time and subsequent reduced pain sensitivity [70].The hypocretin/orexin neurons within the lateral hypothalamus are involved in sleep-wake control where stimulation of neuronal activity leads to sleep-to-wake transition, whilst suppression induces NREM sleep [71].Hyperexcitability in hypocretin/orexin neurons in a mouse model leading to destabilized sleep involved a decreased density and impaired functioning of Kv7 channels [72].When the Kv7 channel activity was augmented with flupirtine the resulting hyperpolarization suppressed the activity of the hypocretin/orexin neurons, increased NREM sleep and restoration of sleep stability.Interestingly, the flupirtine enhanced sleep quality was related to an improvement in cognitive function in the mice, a characteristic of fibromyalgia that was not assessed during the clinical trial of suvorexant.

Conclusions
Evidence is presented demonstrating involvement of K + channels in the chronic pain condition fibromyalgia, which is summarized in Table 1.Hyperexcitability and increased sensitivity of cells, particularly neurons, is proposed to be the basis of the pathophysiology of fibromyalgia, however a full understanding of the underlying processes is currently not available.The limited knowledge regarding the pathophysiology has often hindered the development of specific diagnostic tools, resulting in a reliance on symptom profiling, and the identification of effective therapeutic approaches.
Modulation of K + channel expression and function has been identified in patients with fibromyalgia which would be consistent with an altered inhibitory mechanism to regulate cell activity.The absence of damping mechanisms related to K + channel activity leads to enhanced cell excitation that would be responsible for the characteristics of fibromyalgia, such as pain initiation and processing, sleep disturbance and cognitive dysfunction.Activation of K + channels is a component of the mechanism of action of several pharmacological treatments that have demonstrated efficacy in the management of fibromyalgia (Fig. 2).
Identification of the role K + channels play within the pathophysiology and/or as a therapeutic target for drugs will provide clues to an understanding of the cause(s) of and processes within the altered biology responsible for fibromyalgia.The diversity and abundance of K + channels offer a breadth of functions that are consistent with the characteristic complexity of fibromyalgia.

Fig. 1 .
Fig. 1.Model of the pathophysiology of fibromyalgia.Postulated relationship between hypersensitivity due to stressors activating generators of neuronal hyperexcitability resulting in the development of central sensitization a characteristic of fibromyalgia.

Fig. 2 .
Fig. 2. Schematic diagram of potassium (K + ) channel involvement in pharmacological treatments of fibromyalgia.ASP0819, flupirtine, melatonin and pregabalin have direct K + channel activating properties leading to increased K + ion efflux.Melatonin and PEA exhibit indirect channel activating properties through involvement of the NO/cGMP pathway and PPARα.Pregabalin inhibitory action on α2δ subunits of the Ca 2+ channel evokes an interaction with BKCa channels leading to suppression of Ca 2+ channel expression and Ca 2+ ion influx.Suvorexant evokes antagonistic activity at the OX receptor reducing receptor-induced suppression of Kv7 channel expression.Lidocaine inhibits influx of K + ions through HCN channels.The altered transmembrane ion movement will lead to suppression of neuronal hyperexcitability associated with central sensitization characteristic of fibromyalgia.Blue lines indicate activating properties, red lines indicate inhibitory properties, black line indicates antagonistic properties and green line indicates channel-channel interaction.Abbreviations: α2δ, alpha2delta subunit; AP, action potential; HCN, hyperpolarization-activated cyclic nucleotide-gated channel; KATP, ATP-dependent potassium channel; BKCa, large conductance calcium-activated potassium channel; IKCa, intermediate conductance calcium-activated potassium channel; SKCa, small conductance calcium-activated potassium channel; Kv, voltage-gated potassium channel; NO/cGMP, nitric oxide/cyclic guanosine monophosphate; OX, orexin; PEA, palmitoylethanolamide; PPARα, peroxisome proliferator-activated receptor; RMP, resting membrane potential.