Dose responses of three 4-aminopyridine derivatives on axonal conduction in spinal cord trauma

4-AP-3-MeOH Promotes Structural and Functional Spontaneous Recovery in the Acute Sciatic Nerve Stretch Injury

Background:

4-AP-3-MeOH, a derivative of 4-aminopyridine, was developed and demonstrated to prevent nerve pulse diffusion due to myelin damage and significantly enhance axonal conduction following nerve injury. Currently, repurposing the existing drug such as 4-AP-3-MeOH to restore motor function is a promising and potential therapy of peripheral nerve injury. However, to evaluate drug effect on sciatic nerve injury is full of challenge.

Methods:

Sciatic functional index was used to determine and measure the walking track in the stretch injury model. Nerve conductivity was performed by electrical stimulation of a nerve and recording the compound muscle action potential. Myelin thickness and regeneration was imaged and measured with transmission electron microscopy (TEM).

Results:

In this study, we developed a sciatic nerve injury model to minimize the spontaneous recovery mechanism and found that 4-AP-3-MeOH not only improved walking ability of the animals but also reduced the sensitivity to thermal stimulus. More interesting, 4-AP-3-MeOH enhanced and recovered electric conductivity of injured nerve; our TEM results indicated that the axon sheath thickness was increased and myelin was regenerated, which was an important evidence to support the recovery of injured nerve conductivity with 4-AP-3-MeOH treatment.

Conclusions:

In summary, our studies suggest that 4-AP-3-MeOH is a viable and promising approach to the therapy of peripheral nerve injury and in support of repurposing the existing drug to restore motor function.

Blockade of fast A-type and TEA-sensitive potassium channels provide an antiparkinsonian effect in a 6-OHDA animal model

Objective:

To evaluate the effect of K⁺ channels inhibitors in treatment of parkinson’s disease (PD).

Methods:

This prospective comparative study was conducted in the Qazvin University of Medical Sciences, Iran, from April 2015 to January 2016. Male rats (n=37) received intraperitoneal doses of TEA (2 and 5 mg/kg) or 4-AP (0.5 and 1 mg/kg) twice-daily, before a stereotactic injection of 6-hydroxydopamine (6-OHDA) for the following 7 days. The 6-OHDA was injected into right medial forebrain bundle (MFB) of the rat brains. Development and severity of PD were assessed using the apomorphine-induced rotational test, the elevated body swing test and rotarod tests. Concentration of malondialdehyde (MDA), a marker of oxidative stress, was measured in rat sera.

Results:

Tetraethylammonium and 4-AP significantly reduced the number of apomorphine-induced rotations and improved motor learning in the rotarod test at both doses. Administration of 4-AP and TEA together was more effective than single administration of either agent. Malondialdehyde measurement showed that pretreatment with TEA could not prevent 6-OHDA-induced oxidative stress.

Conclusion:

Our results showed that pretreatment with TEA and 4-AP has a neuroprotective effect against 6-OHDA in dopaminergic neurons in the substantia nigra.

Mechanisms by which fibroblast growth factor 20 improves motor performance in a mouse model of Parkinson's disease

Genome-wide studies have reported that Parkinson’s disease is associated with abnormal expression of various growth factors. In this study, male C57BL/6 mice aged 10 weeks were used to establish Parkinson’s disease models using an intraperitoneal injection of 60 mg/kg 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. 28 days later, 10 or 100 ng fibroblast growth factor 20 was injected intracerebroventricularly. The electrophysiological changes in the mouse hippocampus were recorded using a full-cell patch clamp. Expression of Kv4.2 in the substantia nigra was analyzed using a western blot assay. Serum malondialdehyde levels were analyzed by enzyme-linked immunosorbent assay. The motor coordination of mice was evaluated using the rotarod test. The results showed that fibroblast growth factor 20 decreased A-type potassium current in neurons of the substantia nigra, increased long-term potentiation amplitude in the hippocampus, and downregulated Kv4.2 expression. A high dose of fibroblast growth factor 20 reduced serum malondialdehyde levels and enhanced the motor coordination of mice. These findings confirm that fibroblast growth factor 20 has a therapeutic effect on the toxicity induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, and its mechanism of action is associated with the inhibition of A-type K⁺ currents and Kv4.2 expression. All animal procedures were approved by the Animal Care and Use Committee of Qilu Hospital of Shandong University, China in 2017 (approval No. KYLL-2017-0012).

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.

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.

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.

Potassium channel antagonists 4-aminopyridine and the T-butyl carbamate derivative of 4-aminopyridine improve hind limb function in chronically non-ambulatory dogs; a blinded, placebo-controlled trial

4-Aminopyridine (4-AP) blocks voltage gated potassium channels, restoring conduction to demyelinated axons and improving function in demyelinating conditions, but its use is associated with adverse effects and benefit in spinal cord injury is limited. Derivatives of 4-AP have been developed to improve clinical efficacy while reducing toxicity. We compared the therapeutic effects of orally administered 4-AP and its t-butyl carbamate derivative (t-butyl) with placebo in dogs that had suffered an acute spinal cord injury that left them chronically paralyzed. Nineteen dogs were entered into the trial, conducted in two-week treatment blocks starting with placebo, followed by random assignment to 4-AP or t-butyl, a washout and then the opposite medication followed by placebo. Investigators and owners were blinded to treatment group. Primary outcome measures included open field gait score (OFS), and treadmill based stepping score and regularity index, with additional secondary measures also considered. Thirteen of 19 dogs completed the protocol. Two were euthanized due to unrelated heath problems, two developed side effects and two were unable to complete for unrelated reasons. Dogs showed significant improvement in supported stepping score (from 17.39 to 37.24% with 4-AP; 16.85 to 29.18% with t-butyl p<0.0001) and OFS (from 3.63 to 4.73 with 4-AP; 3.78 to 4.45 with t-butyl, p = 0.005). Response was individually variable and most dramatic in three dogs that were able to walk without support with treatment. No significant difference was found between 4-AP and t-butyl. No adverse effects were reported with t-butyl but gastrointestinal upset and seizures were observed in two dogs with 4-AP. In conclusion, both 4-AP and t-butyl significantly improved supported stepping ability in dogs with chronic spinal cord injury with no adverse effects noted with t-butyl. Drug response varied widely between individuals, highlighting the need to understand the factors that influence canine and human patients' response to therapy.

Paranodal myelin damage after acute stretch in Guinea pig spinal cord

Mechanical injury causes myelin disruption and subsequent axonal conduction failure in the mammalian spinal cord. However, the underlying mechanism is not well understood. In mammalian myelinated axons, proper paranodal myelin structure is crucial for the generation and propagation of action potentials. The exposure of potassium channels at the juxtaparanodal region due to myelin disruption is thought to induce outward potassium currents and inhibit the genesis of the action potential, leading to conduction failure. Using multimodal imaging techniques, we provided anatomical evidence demonstrating paranodal myelin disruption and consequent exposure and redistribution of potassium channels following mechanical insult in the guinea pig spinal cord. Decompaction of paranodal myelin was also observed. It was shown that paranodal demyelination can result from both an initial physical impact and secondary biochemical reactions that are calcium dependent. 4-Aminopyridine (4-AP), a known potassium channel blocker, can partially restore axonal conduction, which further implicates the role of potassium channels in conduction failure. We provide important evidence of paranodal myelin damage, the role of potassium channels in conduction loss, and the therapeutic value of potassium blockade as an effective intervention to restore function following spinal cord trauma.

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

Most axons in the vertebral central nervous system are myelinated by oligodendrocytes. Myelin protects and insulates neuronal processes, enabling the fast, saltatory conduction unique to myelinated axons. Myelin disruption resulting from trauma and biochemical reaction is a common pathological event in spinal cord injury and chronic neurodegenerative diseases. Myelin damage-induced axonal conduction block is considered to be a significant contributor to the devastating neurological deficits resulting from trauma and illness. Potassium channels are believed to play an important role in axonal conduction failure in spinal cord injury and multiple sclerosis. Myelin damage has been shown to unmask potassium channels, creating aberrant potassium currents that inhibit conduction. Potassium channel blockade reduces this ionic leakage and improves conduction. The present review was mainly focused on the development of this technique of restoring axonal conduction and neurological function of demyelinated axons. The drug 4-aminopyridine has recently shown clinical success in treating multiple sclerosis symptoms. Further translational research has also identified several novel potassium channel blockers that may prove effective in restoring axonal conduction.

Ion channel blockers and spinal cord injury

The activation of a delayed secondary cascade of unsatisfactory cellular and molecular responses after a primary mechanical insult to the spinal cord causes the progressive degeneration of this structure. Disturbance of ionic homeostasis is part of the secondary injury process and plays an integral role in the early stage of spinal cord injury (SCI). The secondary pathology of SCI is complex and involves disturbance of the homeostasis of K(+) , Na(+) , and Ca(2+) . The effect of ion channel blockers on chronic SCI has also been proved. In this Mini-Review, we provide a comprehensive summary of the effects of ion channel blockers on the natural responses after SCI. Combination therapy is based on the roles of ions and disturbance of their homeostasis in SCI. The effects of ion channel blockers suggest that they have potential in the treatment of SCI, although the complexity of their effects shows that further knowledge is needed before they can be applied clinically.

Novel Potassium Channel Blocker, 4-AP-3-MeOH, Inhibits Fast Potassium Channels and Restores Axonal Conduction in Injured Guinea Pig Spinal Cord White Matter

We have demonstrated that 4-aminopyridine-3-methanol (4-AP-3-MeOH), a 4-aminopyridine derivative, significantly restores axonal conduction in stretched spinal cord white-matter strips and shows no preference in restoring large and small axons. This compound is 10 times more potent when compared with 4-AP and other derivatives in restoring axonal conduction. Unlike 4-AP, 4-AP-3-MeOH can restore axonal conduction without changing axonal electrophysiological properties. In addition, we also have confirmed that 4-AP-3-MeOH is indeed an effective blocker of IA based on patch-clamp studies using guinea pig dorsal root ganglia cells. Furthermore, we have also provided the critical evidence to confirm the unmasking of potassium channels following mechanical injury. Taken together, our data further supports and implicates the role of potassium channels in conduction loss and its therapeutic value as an effective target for intervention to restore function in spinal cord trauma. Furthermore, due to its high potency and possible low side effect of impacting electrophysiological properties, 4-AP-3-MeOH is perhaps the optimal choice in reversing conduction block in spinal cord injury compared with other derivatives previously reported from this group.

In-vitro approaches for studying blast-induced traumatic brain injury

Traumatic brain injury caused by explosive or blast events is currently divided into four phases: primary, secondary, tertiary, and quaternary blast injury. These phases of blast-induced traumatic brain injury (bTBI) are biomechanically distinct, and can be modeled in both in-vivo and in-vitro systems. The purpose of this review is to consider the mechanical phases of bTBI, how these phases are reproduced with in-vitro models, and to review findings from these models to assess how each phase of bTBI can be examined in more detail. Highlighted are some important gaps in the literature that may be addressed in the future to better identify the exact contributing mechanisms for bTBI. These in-vitro models, viewed in combination with in-vivo models and clinical studies, can be used to assess both the mechanisms and possible treatments for this type of trauma.

N-(4-pyridyl) methyl carbamate inhibits fast potassium currents in guinea pig dorsal root ganglion cells

Axonal demyelination is a critical pathological phenomenon associated with spinal cord injury and multiple sclerosis (MS). Previous studies demonstrated that 4-Aminopyridine, a fast potassium channel blocker, enhances impulse conduction on damaged and/or demyelinated axons, allowing for functional recovery in spinal cord injuries and MS, but with severe therapeutic limitations. To continue to explore the therapeutic value of blocking fast potassium channels while circumventing the side effects of 4-AP, we have developed three novel 4-AP derivatives that enhance impulse conduction in spinal cord trauma. In the current study, we have shown that one of these three derivatives, N-(4-pyridyl) methyl carbamates (MC), significantly inhibits a fast, I(A) like potassium current in guinea pig dorsal root ganglion cells in a whole cell patch clamp configuration. This inhibition of I(A) likely plays a critical role in MC's ability to restore conduction in mechanically injured spinal cord axons and may present a viable alternative to 4-AP for individuals with spinal cord injury or MS. From this, compounds with greater efficacy and perhaps less side effects will likely emerge in the near future, which will greatly enhance the functional restoration and lessen the suffering of SCI and MS patients.

Inotropic effects of the K+ channel blocker 3,4-diaminopyridine on fatigued diaphragm muscle

K(+) channels play important roles in skeletal muscle contraction by regulating action potential duration. Blocking these channels, for example with 3,4-diaminopyridine (DAP), augments muscle force considerably, and these force increases are maintained well during fatigue-inducing contractions. The present study tested the hypothesis that K(+) channel blockade also improves force of previously fatigued muscle. Rat diaphragm underwent fatigue-inducing stimulation in vitro with four different stimulation protocols consisting of 20 Hz vs. 50 Hz trains and 1 min vs. 4 min stimulation durations. DAP administered at the onset of the recovery period produced significant force increases irrespective of the amount of antecedent force loss. These force gains considerably exceeded those resulting from normal force recovery in untreated muscle. Furthermore contraction time was prolonged by DAP in all cases, and half-relaxation time was prolonged by DAP in most cases. Several differences were found compared with previous studies of DAP in fresh muscle, including smaller magnitude and slower time course of force increases. Intracellular electrophysiological recordings found smaller effects of DAP on action potential overshoot and time-depolarization integral in previously stimulated compared with fresh muscle. These data indicate that K(+) channel blockade does indeed increase force of fatigued diaphragm, but to an attenuated extent relative to its effects on non-fatigued muscle, which can be explained on the basis of electrophysiological findings. Nonetheless DAP-induced force increases were usually sufficient to restore force to values present prior to the onset of fatigue-inducing stimulation.

4-Aminopyridine derivatives enhance impulse conduction in guinea-pig spinal cord following traumatic injury

4-Aminopyridine (4-AP), a potassium channel blocker, is capable of restoring conduction in the injured spinal cord. However, the maximal tolerated level of 4-AP in humans is 100 times lower than the optimal dose in in vitro animal studies due to its substantially negative side effects. As an initial step toward the goal of identifying alternative potassium channel blockers with a similar ability of enhancing conduction and with fewer side effects, we have synthesized structurally distinct pyridine-based blockers. Using isolated guinea-pig spinal cord white matter and a double sucrose gap recording device, we have found three pyridine derivatives, N-(4-pyridyl)-methyl carbamate (100 microM), N-(4-pyridyl)-ethyl carbamate (100 microM), and N-(4-pyridyl)-tertbutyl (10 microM) can significantly enhance conduction in spinal cord white matter following stretch. Similar to 4-AP, the derivatives did not preferentially enhance conduction based on axonal caliber. Unlike 4-AP, the derivatives did not change the overall electrical responsiveness of axons to multiple stimuli, indicating the axons recruited by the derivatives conducted in a manner similar to healthy axons. These results demonstrate the ability of novel constructs to serve as an alternative to 4-AP for the purpose of reversing conduction deficits.

Fampridine-SR for multiple sclerosis and spinal cord injury

Fampridine-SR is a sustained-release tablet form of the K(+) channel-blocking compound 4-aminopyridine that has been shown to restore conduction in focally demyelinated axons, to enhance synaptic transmission in many types of neurons and to potentiate muscle contraction. The present review describes the mechanism of action and chemistry of Fampridine-SR, its pharmacokinetics and safety, and the outcomes of clinical trials of its safety and efficacy for enhancing neuromuscular function in patients with multiple sclerosis or spinal cord injury. Randomized clinical trials completed to date indicate that this form of K(+) channel blockade may be useful for the improvement of walking ability in patients with multiple sclerosis.

Inotropic effects of the K+ channel blocker 3,4-diaminopyridine: differential responses of rat soleus and extensor digitorum longus

The K+ channel blocker 3,4-diaminopyrindine (DAP) increases diaphragm force, use of which could potentially improve muscle performance during functional neuromuscular stimulation. To determine the extent of hindlimb muscle force augmentation, and delineate whether DAP effects vary in muscles comprised of mainly slow versus fast fibers, rat soleus, extensor digitorum longus (EDL) and diaphragm muscle samples were studied in vitro. DAP increased force of all three muscles, but at high concentrations the force increases were transient and were followed by declines in force below baseline. The maximum DAP-induced twitch force increase was smaller for soleus (38 +/-7%) than both EDL (94+/-12%) (P < 0.05) and diaphragm (93+/-13%) (P < 0.01). During fatigue-inducing 20 Hz stimulation (tested at an intermediate DAP concentration), force of soleus muscle remained significantly elevated by DAP for the entire testing period, force of DAP-treated EDL muscle rapidly declined to values in untreated muscle, and force of DAP-treated diaphragm had an intermediate force-time profile. Muscles varied in extent to which isometric contractile kinetics were altered by DAP. Thus, the K+ channel blocker DAP improves contractile performance of limb muscles, but the profile of improvement is distinct between the soleus and EDL muscles.