Potassium channel blockers as an effective treatment to restore impulse conduction in injured axons

Functional improvement in individuals with chronic spinal cord injury treated with 4-aminopyridine: A systematic review

Study design

Systematic review.

Objective

To provide current evidence on the efficacy of 4-aminopyridine (4-AP) to bring about functional improvement in individuals with chronic traumatic spinal cord injury (SCI).

Methods

The Medline (PubMed), Web of Science and SCOPUS databases were systematically searched for relevant articles on the efficacy of 4-AP to treat SCI, from the dates such articles were first published until May 2022. Full-text versions of all the articles selected were examined independently by two reviewers. Methodological quality was rated using the Modified Jadad Scale, and risk of bias was assessed with the RoB-2 test. Data extracted included human models/types, PRISMA assessment protocols, and the results of each study. Descriptive syntheses are provided.

Results

In total, 28 articles were initially identified, 10 of which were included after screening. Most of the studies reviewed reported some degree of patient improvement in one or more of the following parameters: motor, sensitivity and sexual function, sphincter control, spasticity, ability to function independently, quality of life, central motor conduction, pain, and pulmonary function.

Conclusions

This review confirms the efficacy of 4-AP in improving several conditions resulting from SCI but further research on this topic is warranted. Additional randomized clinical trials with 4-AP involving larger sample sizes are needed, as are consistent outcome measures in order to obtain adequate data for analysis with a view to enhance treatment benefits.

Systematic review registration

https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=334835, PROSPERO CRD42022334835.

Level of Potassium Is Associated with Saturated Fatty Acids in Cell Membranes and Influences the Activation of the 9 and 13 HODE and 5 HETE Synthesis Pathways in PCOS

Potassium helps to maintain the water–electrolyte and acid–base balance. There is little research on the relationship between plasma fatty acids (FAs), inflammatory mediators and red blood cell potassium levels in women with polycystic ovary syndrome (PCOS). This study included 38 Caucasian women with PCOS. Potassium in the erythrocytes was determined by inductively coupled atomic plasma emission spectrometry. The FAs were analysed with gas chromatography, and liquid chromatography was used to separate the eicosanoids. The relationships between the potassium content and the amounts of fatty acids, as well as potassium and arachidonic acid (AAs) derivatives, were analysed. Significant negative correlations were found with, among others, pentadecanoic acid, palmitic acid, stearic acid and arachidic acid, whereas a positive correlation was found with neuronic acid. Positive correlations were observed with 9, 13 HODE (derivatives synthetized from linolenic acid) and 5 oxo ETE and 5 HETE (from 5 LOX pathway). Saturated fatty acids reduce the influx of potassium into the cell by destabilizing the pH of the cytosol, and thus exacerbating the inflammatory response through the activation of the AA cascade. Therefore, improving the flow of potassium inside the cell is important in the treatment of patients.

Quantum Mechanical Aspects in the Pathophysiology of Neuropathic Pain

Neuropathic pain is a challenging complaint for patients and clinicians since there are no effective agents available to get satisfactory outcomes even though the pharmacological agents target reasonable pathophysiological mechanisms. This may indicate that other aspects in these mechanisms should be unveiled to comprehend the pathogenesis of neuropathic pain and thus find more effective treatments. Therefore, in the present study, several mechanisms are chosen to be reconsidered in the pathophysiology of neuropathic pain from a quantum mechanical perspective. The mathematical model of the ions quantum tunneling model is used to provide quantum aspects in the pathophysiology of neuropathic pain. Three major pathophysiological mechanisms are revisited in the context of the quantum tunneling model. These include: (1) the depolarized membrane potential of neurons; (2) the cross-talk or the ephaptic coupling between the neurons; and (3) the spontaneous neuronal activity and the emergence of ectopic action potentials. We will show mathematically that the quantum tunneling model can predict the occurrence of neuronal membrane depolarization attributed to the quantum tunneling current of sodium ions. Moreover, the probability of inducing an ectopic action potential in the axons of neurons will be calculated and will be shown to be significant and influential. These ectopic action potentials are generated due to the formation of quantum synapses which are assumed to be the mechanism behind the ephaptic transmission. Furthermore, the spontaneous neuronal activity and the emergence of ectopic action potentials independently from any adjacent stimulated neurons are predicted to occur according to the quantum tunneling model. All these quantum mechanical aspects contribute to the overall hyperexcitability of the neurons and to the pathogenesis of neuropathic pain. Additionally, providing a new perspective in the pathophysiology of neuropathic pain may improve our understanding of how the neuropathic pain is generated and maintained and may offer new effective agents that can improve the overall clinical outcomes of the patients.

The Quantum Tunneling of Ions Model Can Explain the Pathophysiology of Tinnitus

Tinnitus is a well-known pathological entity in clinical practice. However, the pathophysiological mechanisms behind tinnitus seem to be elusive and cannot provide a comprehensive understanding of its pathogenesis and clinical manifestations. Hence, in the present study, we explore the mathematical model of ions’ quantum tunneling to propose an original pathophysiological mechanism for the sensation of tinnitus. The present model focuses on two major aspects: The first aspect is the ability of ions, including sodium, potassium, and calcium, to depolarize the membrane potential of inner hair cells and the neurons of the auditory pathway. This membrane depolarization is induced via the quantum tunneling of ions through closed voltage-gated channels. The state of membrane depolarization can be a state of hyper-excitability or hypo-excitability, depending on the degree of depolarization. Both of these states aid in understanding the pathophysiology of tinnitus. The second aspect is the quantum tunneling signals between the demyelinated neurons of the auditory pathway. These signals are mediated via the quantum tunneling of potassium ions, which exit to the extracellular fluid during an action potential event. These quantum signals can be viewed as a “quantum synapse” between neurons. The formation of quantum synapses results in hyper-excitability among the demyelinated neurons of the auditory pathway. Both of these aspects augment and amplify the electrical signals in the auditory pathway and result in a loss of the spatiotemporal fidelity of sound signals going to the brain centers. The brain interprets this hyper-excitability and loss of spatiotemporal fidelity as tinnitus. Herein, we show mathematically that the quantum tunneling of ions can depolarize the membrane potential of the inner hair cells and neurons of the auditory pathway. Moreover, we calculate the probability of action potential induction in the neurons of the auditory pathway generated by the quantum tunneling signals of potassium ions.

A cross-sectional comparison of performance, neurophysiological and MRI outcomes of responders and non-responders to fampridine treatment in multiple sclerosis - An explorative study

OBJECTIVE

To compare baseline physical and cognitive performance, neurophysiological, and magnetic resonance imaging (MRI) outcomes and examinetheir interrelationship inparticipants with Multiple Sclerosis (MS), already established aseither responder or non-responder to Fampridine treatment, andto examine associationswiththe expanded disability status scale (EDSS) and 12-item MS walking scale (MSWS-12).

METHODS

Baseline data from an explorative longitudinal observational study were analyzed. Participants underwent the Timed 25-Foot Walk Test (T25FW), Six Spot Step Test (SSST), Nine-Hole Peg Test, Five Times Sit-to-Stand Test, Symbol Digit Modalities Test (SDMT), neurophysiological testing, including central motor conduction time (CMCT), peripheral motor conduction time (PMCT), motor evoked potential (MEP) amplitudesand electroneuronographyof the lower extremities, and brain MRI (brain volume, number and volume of T2-weighted lesions and lesion load normalized to brain volume).

RESULTS

41 responders and 8 non-responders were examined. There were no intergroup differences inphysical performance, cognitive, neurophysiological, andMRI outcomes (p > 0.05).CMCT was associated withT25FW, SSST, EDSS, and MSWS-12,(p < 0.05). SDMT was associated with the number and volume of T2-weighted lesions, and lesion load normalized to brain volume (p < 0.05).

CONCLUSION

No differences were identified between responders and non-responders to Fampridine treatment regarding physical and cognitive performance, neurophysiological or MRI outcomes. The results call for cautious interpretation and further large-scale studies are needed to expand ourunderstanding of underlying mechanisms discriminating Fampridine responders and non-responders.CMCT may be used as a marker of disability and walking impairment, while SDMT was associated with white matter lesions estimated by MRI. ClinicalTrials.gov identifier: NCT03401307.

Potent block of potassium channels by MEK inhibitor U0126 in primary cultures and brain slices

U0126 (1,4-diamino-2,3-dicyano-1,4-bis (2-aminophenylthio) butadiene), a widely used mitogen-activated protein kinase kinase (MEK) inhibitor, was found to accelerate voltage-gated K⁺ channel (K_V) inactivation in heterologous cells expressing several types of K_V. The goal of this study was to examine whether U0126 at a concentration thought to specifically inhibit MEK signaling also inhibits K_V in native neurons of primary cultures or brain slices. U0126 caused a dose-dependent inhibition of both the transient (I_A) and sustained (I_DR) components of K⁺ currents in hippocampal neurons. U0126 also exhibited much higher potency on the I_A and I_DR than the classical K_V blockers 4-aminopyridine (4-AP) and tetraethylammonium (TEA). Consistent with its inhibitory effect on K_V, U0126 broadened action potential duration, profoundly affected the repolarizing phase, and dramatically reduced firing frequency in response to current pulse injections. Despite the potent and reversible action of U0126 on K_v channels, PD98059, a structurally-unrelated MEK inhibitor, did not induce such an effect, suggesting U0126 may act independently of MEK inhibition. Together, these results raise cautions for using U0126 as a specific inhibitor for studying MEK signaling in neurons; on the other hand, further studies on the blocking mechanisms of U0126 as a potent inhibitor of K_V may provide useful insights into the structure-function relationship of K_V in general.

Effectiveness of 4-Aminopyridine for the Management of Spasticity in Spinal Cord Injury: A Systematic Review

Background: Spasticity is a common secondary complication of spinal cord injury (SCI), which can severely impact functional independence and quality of life. 4-Aminopyridine (4-AP) is a potassium channel blocker that has been studied as an intervention for spasticity in individuals with SCI. Objective: To conduct a systematic review of the available evidence regarding the effectiveness of 4-AP for the management of spasticity in individuals with SCI. Methods: A comprehensive literature search was conducted on five electronic databases for articles published in English up to January 2017. Studies were included if (1) the sample size was three or more subjects, (2) the population was ≥50% SCI, (3) the subjects were ≥18 years old, (4) the treatment was 4-AP via any route, and (5) spasticity was assessed before and after the intervention. Subject characteristics, study design, intervention protocol, assessment methods, side effects, adverse events, and outcomes were extracted from selected studies. Randomized controlled trials (RCTs) were evaluated for methodological quality using the Physiotherapy Evidence Database (PEDro) tool. Levels of evidence were assigned using a modified Sackett scale. Results: Nine studies met inclusion criteria with a pooled sample size of 591 subjects. Six studies were RCTs (PEDro = 6-10, Level 1 evidence) and three studies were pre-post tests (Level 4 evidence). There was a wide range in duration, severity, and level of SCI across subjects. Oral 4-AP was investigated in five studies; one study reported significant improvements on the Ashworth Scale (AS), while the remaining four studies found no improvement. Three studies found no significant improvements on the Spasm Frequency Scale. Intravenous 4-AP was investigated in three studies; no significant improvements were found on the AS or in the Reflex Score. Intrathecal 4-AP was investigated in one study, which did not find significant improvements on the AS. Conclusion: There is weak evidence supporting the effectiveness of 4-AP in reducing spasticity post SCI. Future research should utilize contemporary measures of spasticity and address methodological limitations such as small sample sizes.

Parallel Evaluation of Two Potassium Channel Blockers in Restoring Conduction in Mechanical Spinal Cord Injury in Rat

Myelin damage is a hallmark of spinal cord injury (SCI), and potassium channel blocker (PCB) is proven effective to restore axonal conduction and regain neurological function. Aiming to improve this therapy beyond the U.S. Food and Drug Administration-approved 4-aminopyridine (4-AP), we have developed multiple new PCBs, with 4-aminopyridine-3-methanol (4-AP-3-MeOH) being the most potent and effective. The current study evaluated two PCBs, 4-AP-3-MeOH and 4-AP, in parallel in both ex vivo and in vivo rat mechanical SCI models. Specifically, 4-AP-3-MeOH induced significantly greater augmentation of axonal conduction than 4-AP in both acute and chronic injury. 4-AP-3-MeOH had no negative influence on the electrical responsiveness of rescued axons whereas 4-AP-recruited axons displayed a reduced ability to follow multiple stimuli. In addition, 4-AP-3-MeOH can be applied intraperitoneally at a dose that is at least 5 times higher (5 mg/kg) than that of 4-AP (1 mg/kg) in vivo. Further, 5 mg/kg of 4-AP-3-MeOH significantly improved motor function whereas both 4-AP-3-MeOH (1 and 5 mg/kg) and, to a lesser degree, 4-AP (1 mg/kg) alleviated neuropathic pain-like behavior when applied in rats 2 weeks post-SCI. Based on these and other findings, we conclude that 4-AP-3-MeOH appears to be more advantageous over 4-AP in restoring axonal conduction because of the combination of its higher efficacy in enhancing the amplitude of compound action potential, lesser negative effect on axonal responsiveness to multiple stimuli, and wider therapeutic range in both ex vivo and in vivo application. As a result, 4-AP-3-MeOH has emerged as a strong alternative to 4-AP that can complement the effectiveness, and even partially overcome the shortcomings, of 4-AP in the treatment of neurotrauma and degenerative diseases where myelin damage is implicated.

Optic nerve regeneration in mammals: Regenerated or spared axons?

Intraorbital optic nerve crush in rodents is widely used as a model to study axon regeneration in the adult mammalian central nervous system. Recent studies using appropriate genetic manipulations have revealed remarkable abilities of mature retinal ganglion cell (RGC) axons to regenerate after optic nerve injury, with some studies demonstrating that axons can then go on to re-innervate a number of central visual targets with partial functional restoration. However, one confounding factor inherent to optic nerve crush injury is the potential incompleteness of the initial lesion, leaving spared axons that later on could erroneously be interpreted as regenerating distal to the injury site. Careful examination of axonal projection pattern and morphology may facilitate separating spared from regenerating RGC axons. Here we discuss morphological criteria and strategies that may be used to differentiate spared versus regenerated axons in the injured mammalian optic nerve.

Elevated axonal membrane permeability and its correlation with motor deficits in an animal model of multiple sclerosis

Background

It is increasingly clear that in addition to myelin disruption, axonal degeneration may also represent a key pathology in multiple sclerosis (MS). Hence, elucidating the mechanisms of axonal degeneration may not only enhance our understanding of the overall MS pathology, but also elucidate additional therapeutic targets. The objective of this study is assess the degree of axonal membrane disruption and its significance in motor deficits in EAE mice.

Methods

Experimental Autoimmune Encephalomyelitis was induced in mice by subcutaneous injection of myelin oligodendrocyte glycoprotein/complete Freud’s adjuvant emulsion, followed by two intraperitoneal injections of pertussis toxin. Behavioral assessment was performed using a 5-point scale. Horseradish Peroxidase Exclusion test was used to quantify the disruption of axonal membrane. Polyethylene glycol was prepared as a 30% (w/v) solution in phosphate buffered saline and injected intraperitoneally.

Results

We have found evidence of axonal membrane disruption in EAE mice when symptoms peak and to a lesser degree, in the pre-symptomatic stage of EAE mice. Furthermore, polyethylene glycol (PEG), a known membrane fusogen, significantly reduces axonal membrane disruption in EAE mice. Such PEG-mediated membrane repair was accompanied by significant amelioration of behavioral deficits, including a delay in the emergence of motor deficits, a delay of the emergence of peak symptom, and a reduction in the severity of peak symptom.

Conclusions

The current study is the first indication that axonal membrane disruption may be an important part of the pathology in EAE mice and may underlies behavioral deficits. Our study also presents the initial observation that PEG may be a therapeutic agent that can repair axolemma, arrest axonal degeneration and reduce motor deficits in EAE mice.

Acrolein-mediated conduction loss is partially restored by K⁺ channel blockers

Acrolein-mediated myelin damage is thought to be a critical mechanism leading to conduction failure following neurotrauma and neurodegenerative diseases. The exposure and activation of juxtaparanodal voltage-gated K(+) channels due to myelin damage leads to conduction block, and K(+) channel blockers have long been studied as a means for restoring axonal conduction in spinal cord injury (SCI) and multiple sclerosis (MS). In this study, we have found that 100 μM K(+) channel blockers 4-aminopyridine-3-methanol (4-AP-3-MeOH), and to a lesser degree 4-aminopyridine (4-AP), can significantly restore compound action potential (CAP) conduction in spinal cord tissue following acrolein-mediated myelin damage using a well-established ex vivo SCI model. In addition, 4-AP-3-MeOH can effectively restore CAP conduction in acrolein-damaged axons with a range of concentrations from 0.1 to 100 μM. We have also shown that while both compounds at 100 μM showed no preference of small- and large-caliber axons when restoring CAP conduction, 4-AP-3-MeOH, unlike 4-AP, is able to augment CAP amplitude while causing little change in axonal responsiveness measured in refractory periods and response to repetitive stimuli. In a prior study, we show that 4-AP-3-MeOH was able to functionally rescue mechanically injured axons. In this investigation, we conclude that 4-AP-3-MeOH is an effective K(+) channel blocker in restoring axonal conduction following both primary (physical) and secondary (chemical) insults. These findings also suggest that 4-AP-3-MeOH is a viable alternative of 4-AP for treating myelin damage and improving function following central nervous system trauma and neurodegenerative diseases.

Molecular mechanisms of acrolein-mediated myelin destruction in CNS trauma and disease

Myelin is a critical component of the nervous system facilitating efficient propagation of electrical signals and thus communication between the central and peripheral nervous systems and the organ systems that they innervate throughout the body. In instances of neurotrauma and neurodegenerative disease, injury to myelin is a prominent pathological feature responsible for conduction deficits, and leaves axons vulnerable to damage from noxious compounds. Although the pathological mechanisms underlying myelin loss have yet to be fully characterized, oxidative stress (OS) appears to play a prominent role. Specifically, acrolein, a neurotoxic aldehyde that is both a product and an instigator of OS, has been observed in studies to elicit demyelination through calcium-independent and -dependent mechanisms and also by affecting glutamate uptake and promoting excitotoxicity. Furthermore, pharmacological scavenging of acrolein has demonstrated a neuroprotective effect in animal disease models, by conserving myelin's structural integrity and alleviating functional deficits. This evidence indicates that acrolein may be a key culprit of myelin damage while acrolein scavenging could potentially be a promising therapeutic approach for patients suffering from nervous system trauma and disease.

From demyelination to remyelination: the road toward therapies for spinal cord injury

Myelin integrity is crucial for central nervous system (CNS) physiology while its preservation and regeneration after spinal cord injury (SCI) is key to functional restoration. Disturbance of nodal organization acutely after SCI exposes the axon and triggers conduction block in the absence of overt demyelination. Oligodendrocyte (OL) loss and myelin degradation follow as a consequence of secondary damage. Here, we provide an overview of the major biological events and underlying mechanisms leading to OL death and demyelination and discuss strategies to restrain these processes. Another aspect which is critical for SCI repair is the enhancement of endogenously occurring spontaneous remyelination. Recent findings have unveiled the complex roles of innate and adaptive immune responses in remyelination and the immunoregulatory potential of the glial scar. Moreover, the intimate crosstalk between neuronal activity, oligodendrogenesis and myelination emphasizes the contribution of rehabilitation to functional recovery. With a view toward clinical applications, several therapeutic strategies have been devised to target SCI pathology, including genetic manipulation, administration of small therapeutic molecules, immunomodulation, manipulation of the glial scar and cell transplantation. The implementation of new tools such as cellular reprogramming for conversion of one somatic cell type to another or the use of nanotechnology and tissue engineering products provides additional opportunities for SCI repair. Given the complexity of the spinal cord tissue after injury, it is becoming apparent that combinatorial strategies are needed to rescue OLs and myelin at early stages after SCI and support remyelination, paving the way toward clinical translation.

Structure, molecular modeling, and function of the novel potassium channel blocker urotoxin isolated from the venom of the Australian scorpion Urodacus yaschenkoi

This communication reports the structural and functional characterization of urotoxin, the first K(+) channel toxin isolated from the venom of the Australian scorpion Urodacus yaschenkoi. It is a basic peptide consisting of 37 amino acids with an amidated C-terminal residue. Urotoxin contains eight cysteines forming four disulfide bridges with sequence similarities resembling the α-potassium channel toxin 6 (α-KTx-6) subfamily of peptides; it was assigned the systematic number of α-KTx-6.21. Urotoxin is a potent blocker of human voltage-gated potassium channel (Kv)1.2 channels, with an IC50 of 160 pM, whereas its affinity for other channels tested was in the nanomolar range (hKv1.1, IC50 = 253 nM; hKv1.3, IC50 = 91 nM; and hKCa3.1, IC50 = 70 nM). The toxin had no effect on hKv1.4, hKv1.5, human ether-à-go-go-related gene type 1 (hERG1), or human ether-à-go-go-like (hELK2) channels. Multiple sequence alignments from the venom gland transcriptome showed the existence of four other new peptides similar to urotoxin. Computer modeling of urotoxin's three-dimensional structure suggests the presence of the α/β-scaffold characteristic of other scorpion toxins, although very likely forming an uncommon disulfide pairing pattern. Using molecular dynamics, a model for the binding of this peptide to human Kv1.2 and hKv1.1 channels is presented, along with the binding of an in silico mutant urotoxin (Lys25Ala) to both channels. Urotoxin enriches our knowledge of K(+) channel toxins and, due to its high affinity for hKv1.2 channels, it may be a good candidate for the development of pharmacologic tools to study the physiologic functions of K(+) channels or related channelopathies and for restoring axonal conduction in demyelinated axons.

Subtle paranodal injury slows impulse conduction in a mathematical model of myelinated axons

This study explores in detail the functional consequences of subtle retraction and detachment of myelin around the nodes of Ranvier following mild-to-moderate crush or stretch mediated injury. An equivalent electrical circuit model for a series of equally spaced nodes of Ranvier was created incorporating extracellular and axonal resistances, paranodal resistances, nodal capacitances, time varying sodium and potassium currents, and realistic resting and threshold membrane potentials in a myelinated axon segment of 21 successive nodes. Differential equations describing membrane potentials at each nodal region were solved numerically. Subtle injury was simulated by increasing the width of exposed nodal membrane in nodes 8 through 20 of the model. Such injury diminishes action potential amplitude and slows conduction velocity from 19.1 m/sec in the normal region to 7.8 m/sec in the crushed region. Detachment of paranodal myelin, exposing juxtaparanodal potassium channels, decreases conduction velocity further to 6.6 m/sec, an effect that is partially reversible with potassium ion channel blockade. Conduction velocity decreases as node width increases or as paranodal resistance falls. The calculated changes in conduction velocity with subtle paranodal injury agree with experimental observations. Nodes of Ranvier are highly effective but somewhat fragile devices for increasing nerve conduction velocity and decreasing reaction time in vertebrate animals. Their fundamental design limitation is that even small mechanical retractions of myelin from very narrow nodes or slight loosening of paranodal myelin, which are difficult to notice at the light microscopic level of observation, can cause large changes in myelinated nerve conduction velocity.

Therapeutic approaches for spinal cord injury

This study reviews the literature concerning possible therapeutic approaches for spinal cord injury. Spinal cord injury is a disabling and irreversible condition that has high economic and social costs. There are both primary and secondary mechanisms of damage to the spinal cord. The primary lesion is the mechanical injury itself. The secondary lesion results from one or more biochemical and cellular processes that are triggered by the primary lesion. The frustration of health professionals in treating a severe spinal cord injury was described in 1700 BC in an Egyptian surgical papyrus that was translated by Edwin Smith; the papyrus reported spinal fractures as a "disease that should not be treated." Over the last biological or pharmacological treatment method. Science is unraveling the mechanisms of cell protection and neuroregeneration, but clinically, we only provide supportive care for patients with spinal cord injuries. By combining these treatments, researchers attempt to enhance the functional recovery of patients with spinal cord injuries. Advances in the last decade have allowed us to encourage the development of experimental studies in the field of spinal cord regeneration. The combination of several therapeutic strategies should, at minimum, allow for partial functional recoveries for these patients, which could improve their quality of life.

Reversal of BoNT/A-mediated inhibition of muscle paralysis by 3,4-diaminopyridine and roscovitine in mouse phrenic nerve-hemidiaphragm preparations

Botulinum neurotoxins (BoNTs) comprise a family of neurotoxic proteins synthesized by anaerobic bacteria of the genus Clostridium. Each neurotoxin consists of two polypeptide chains: a 100kDa heavy chain, responsible for binding and internalization into the nerve terminal of cholinergic motoneurons and a 50kDa light chain that mediates cleavage of specific synaptic proteins in the host nerve terminal. Exposure to BoNT leads to cessation of voltage- and Ca(2+)-dependent acetylcholine (ACh) release, resulting in flaccid paralysis which may be protracted and potentially fatal. There are no approved therapies for BoNT intoxication once symptoms appear, and specific inhibitors of the light chain developed to date have not been able to reverse the consequences of BoNT intoxication. An alternative approach for treatment of botulism is to focus on compounds that act by enhancing ACh release. To this end, we examined the action of the K(+) channel blocker 3,4-diaminopyridine (3,4-DAP) in isolated mouse hemidiaphragm muscles intoxicated with 5pM BoNT/A. 3,4-DAP restored tension within 1-3min of application, and was effective even in totally paralyzed muscle. The Ca(2+) channel activator (R)-roscovitine (Ros) potentiated the action of 3,4-DAP, allowing for use of lower concentrations of the K(+) channel blocker. In the absence of 3,4-DAP, Ros was unable to augment tension in BoNT/A-intoxicated muscle. This is the first report demonstrating the efficacy of the combination of 3,4-DAP and Ros for the potential treatment of BoNT/A-mediated muscle paralysis.

Coupled left-shift of Nav channels: modeling the Na⁺-loading and dysfunctional excitability of damaged axons

Injury to neural tissue renders voltage-gated Na⁺ (Nav) channels leaky. Even mild axonal trauma initiates Na⁺-loading, leading to secondary Ca²⁺-loading and white matter degeneration. The nodal isoform is Nav1.6 and for Nav1.6-expressing HEK-cells, traumatic whole cell stretch causes an immediate tetrodotoxin-sensitive Na⁺-leak. In stretch-damaged oocyte patches, Nav1.6 current undergoes damage-intensity dependent hyperpolarizing- (left-) shifts, but whether left-shift underlies injured-axon Nav-leak is uncertain. Nav1.6 inactivation (availability) is kinetically limited by (coupled to) Nav activation, yielding coupled left-shift (CLS) of the two processes: CLS should move the steady-state Nav1.6 "window conductance" closer to typical firing thresholds. Here we simulated excitability and ion homeostasis in free-running nodes of Ranvier to assess if hallmark injured-axon behaviors--Na⁺-loading, ectopic excitation, propagation block--would occur with Nav-CLS. Intact/traumatized axolemma ratios were varied, and for some simulations Na/K pumps were included, with varied in/outside volumes. We simulated saltatory propagation with one mid-axon node variously traumatized. While dissipating the [Na⁺] gradient and hyperactivating the Na/K pump, Nav-CLS generated neuropathic pain-like ectopic bursts. Depending on CLS magnitude, fraction of Nav channels affected, and pump intensity, tonic or burst firing or nodal inexcitability occurred, with [Na⁺] and [K⁺] fluctuating. Severe CLS-induced inexcitability did not preclude Na⁺-loading; in fact, the steady-state Na⁺-leaks elicited large pump currents. At a mid-axon node, mild CLS perturbed normal anterograde propagation, and severe CLS blocked saltatory propagation. These results suggest that in damaged excitable cells, Nav-CLS could initiate cellular deterioration with attendant hyper- or hypo-excitability. Healthy-cell versions of Nav-CLS, however, could contribute to physiological rhythmic firing.

Reduced conduction failure of the main axon of polymodal nociceptive C-fibres contributes to painful diabetic neuropathy in rats

Painful diabetic neuropathy is a common complication of diabetes mellitus and can affect many aspects of life and severely limit patients' daily functions. Signals of painful diabetic neuropathy are believed to originate in the peripheral nervous system. However, its peripheral mechanism of hyperalgesia has remained elusive. Numerous studies have accumulated that polymodal nociceptive C-fibres play a crucial role in the generation and conduction of pain signals and sensitization of which following injury or inflammation leads to marked hyperalgesia. Traditionally, the number of nociceptive primary afferent firings is believed to be determined at the free nerve endings, while the extended main axon of unmyelinated C-fibres only involves the reliable and faithful propagation of firing series to the central terminals. We challenged this classic view by showing that conduction of action potential can fail to occur in response to repetitive activity when they travel down the main axon of polymodal nociceptive C-fibres. Quantitative analysis of conduction failure revealed that the degree of conduction failure displays a frequency-dependent manner. Local administration of low threshold, rapidly activating potassium current blocker, α-dendrotoxin (0.5 nM) and persistent sodium current blocker, low doses of tetrodotoxin (<100 nM) on the main axon of C-fibres can reciprocally regulate the degree of conduction failure, confirming that conduction failure did occur along the main axon of polymodal nociceptive C-fibres. Following streptozotocin-induced diabetes, a subset of polymodal nociceptive C-fibres exhibited high-firing-frequency to suprathreshold mechanical stimulation, which account for about one-third of the whole population of polymodal nociceptive C-fibres tested. These high-firing-frequency polymodal nociceptive C-fibres in rats with diabetes displayed a marked reduction of conduction failure. Delivery of low concentrations of tetrodotoxin and Nav1.8 selective blocker, A-803467 on the main axon of C-fibres was found to markedly enhance the conduction failure in a dose-dependent manner in diabetic rats. Upregulated expression of sodium channel subunits Nav1.7 and Nav1.8 in both small dorsal root ganglion neurons and peripheral C-fibres as well as enhanced transient and persistent sodium current and increased excitability in small dorsal root ganglion neurons from diabetic rats might underlie the reduced conduction failure in the diabetic high-firing-frequency polymodal nociceptive C-fibres. This study shed new light on the functional capability in the pain signals processing for the main axon of polymodal nociceptive C-fibres and revealed a novel mechanism underlying diabetic hyperalgesia.

Involvement of Kv1.3 and p38 MAPK signaling in HIV-1 glycoprotein 120-induced microglia neurotoxicity

Inflammatory responses mediated by activated microglia play a pivotal role in the pathogenesis of human immunodeficiency virus type 1 (HIV-1)-associated neurocognitive disorders. Studies on identification of specific targets to control microglia activation and resultant neurotoxic activity are imperative. Increasing evidence indicate that voltage-gated K⁺ (K_v) channels are involved in the regulation of microglia functionality. In this study, we investigated K_v1.3 channels in the regulation of neurotoxic activity mediated by HIV-1 glycoprotein 120 (gp120)-stimulated rat microglia. Our results showed treatment of microglia with gp120 increased the expression levels of K_v1.3 mRNA and protein. In parallel, whole-cell patch-clamp studies revealed that gp120 enhanced microglia K_v1.3 current, which was blocked by margatoxin, a K_v1.3 blocker. The association of gp120 enhancement of K_v1.3 current with microglia neurotoxicity was demonstrated by experimental results that blocking microglia K_v1.3 attenuated gp120-associated microglia production of neurotoxins and neurotoxicity. Knockdown of K_v1.3 gene by transfection of microglia with K_v1.3-siRNA abrogated gp120-associated microglia neurotoxic activity. Further investigation unraveled an involvement of p38 MAPK in gp120 enhancement of microglia K_v1.3 expression and resultant neurotoxic activity. These results suggest not only a role K_v1.3 may have in gp120-associated microglia neurotoxic activity, but also a potential target for the development of therapeutic strategies.