Time course studies on the effectiveness of tetrodotoxin in reducing consequences of spinal cord contusion

In vivo imaging in mouse spinal cord reveals that microglia prevent degeneration of injured axons

Microglia, the primary immune cells in the central nervous system, play a critical role in regulating neuronal function and fate through their interaction with neurons. Despite extensive research, the specific functions and mechanisms of microglia-neuron interactions remain incompletely understood. In this study, we demonstrate that microglia establish direct contact with myelinated axons at Nodes of Ranvier in the spinal cord of mice. The contact associated with neuronal activity occurs in a random scanning pattern. In response to axonal injury, microglia rapidly transform their contact into a robust wrapping form, preventing acute axonal degeneration from extending beyond the nodes. This wrapping behavior is dependent on the function of microglial P2Y12 receptors, which may be activated by ATP released through axonal volume-activated anion channels at the nodes. Additionally, voltage-gated sodium channels (NaV) and two-pore-domain potassium (K2P) channels contribute to the interaction between nodes and glial cells following injury, and inhibition of NaV delays axonal degeneration. Through in vivo imaging, our findings reveal a neuroprotective role of microglia during the acute phase of single spinal cord axon injury, achieved through neuron-glia interaction.

An Updated Review of Tetrodotoxin and Its Peculiarities

Tetrodotoxin (TTX) is a crystalline, weakly basic, colorless organic substance and is one of the most potent marine toxins known. Although TTX was first isolated from pufferfish, it has been found in numerous other marine organisms and a few terrestrial species. Moreover, tetrodotoxication is still an important health problem today, as TTX has no known antidote. TTX poisonings were most commonly reported from Japan, Thailand, and China, but today the risk of TTX poisoning is spreading around the world. Recent studies have shown that TTX-containing fish are being found in other regions of the Pacific and in the Indian Ocean, as well as the Mediterranean Sea. This review aims to summarize pertinent information available to date on the structure, origin, distribution, mechanism of action of TTX and analytical methods used for the detection of TTX, as well as on TTX-containing organisms, symptoms of TTX poisoning, and incidence worldwide.

From Poison to Promise: The Evolution of Tetrodotoxin and Its Potential as a Therapeutic

Tetrodotoxin (TTX) is a potent neurotoxin that was first identified in pufferfish but has since been isolated from an array of taxa that host TTX-producing bacteria. However, determining its origin, ecosystem roles, and biomedical applications has challenged researchers for decades. Recognized as a poison and for its lethal effects on humans when ingested, TTX is primarily a powerful sodium channel inhibitor that targets voltage-gated sodium channels, including six of the nine mammalian isoforms. Although lethal doses for humans range from 1.5-2.0 mg TTX (blood level 9 ng/mL), when it is administered at levels far below LD₅₀, TTX exhibits therapeutic properties, especially to treat cancer-related pain, neuropathic pain, and visceral pain. Furthermore, TTX can potentially treat a variety of medical ailments, including heroin and cocaine withdrawal symptoms, spinal cord injuries, brain trauma, and some kinds of tumors. Here, we (i) describe the perplexing evolution and ecology of tetrodotoxin, (ii) review its mechanisms and modes of action, and (iii) offer an overview of the numerous ways it may be applied as a therapeutic. There is much to be explored in these three areas, and we offer ideas for future research that combine evolutionary biology with therapeutics. The TTX system holds great promise as a therapeutic and understanding the origin and chemical ecology of TTX as a poison will only improve its general benefit to humanity.

Clinical Trial Design-A Review-With Emphasis on Acute Intervertebral Disc Herniation

There is a clear need for new methods of treatment of acute disc herniation in dogs, most obviously to address the permanent loss of function that can arise because of the associated spinal cord injury. Clinical trials form the optimal method to introduce new therapies into everyday clinical practice because they are a reliable source of unbiased evidence of effectiveness. Although many designs are available, parallel cohort trials are most widely applicable to acute disc herniation in dogs. In this review another key trial design decision—that between pragmatic and explanatory approaches—is highlighted and used as a theme to illustrate the close relationship between trial objective and design. Acute disc herniation, and acute spinal cord injury, is common in dogs and there is a multitude of candidate interventions that could be trialed. Most current obstacles to large-scale clinical trials in dogs can be overcome by collaboration and cooperation amongst interested veterinarians.

GGF2 (Nrg1-β3) treatment enhances NG2+ cell response and improves functional recovery after spinal cord injury

The adult spinal cord contains a pool of endogenous glial precursor cells, which spontaneously respond to spinal cord injury (SCI) with increased proliferation. These include oligodendrocyte precursor cells that express the NG2 proteoglycan and can differentiate into mature oligodendrocytes. Thus, a potential approach for SCI treatment is to enhance the proliferation and differentiation of these cells to yield more functional mature glia and improve remyelination of surviving axons. We previously reported that soluble glial growth factor 2 (GGF2)- and basic fibroblast growth factor 2 (FGF2)-stimulated growth of NG2(+) cells purified from injured spinal cord in primary culture. This study examines the effects of systemic administration of GGF2 and/or FGF2 after standardized contusive SCI in vivo in both rat and mouse models. In Sprague-Dawley rats, 1 week of GGF2 administration, beginning 24 h after injury, enhanced NG2(+) cell proliferation, oligodendrogenesis, chronic white matter at the injury epicenter, and recovery of hind limb function. In 2',3'-cyclic-nucleotide 3'-phosphodiesterase-enhanced green fluorescent protein mice, GGF2 treatment resulted in increased oligodendrogenesis and improved functional recovery, as well as elevated expression of the stem cell transcription factor Sox2 by oligodendrocyte lineage cells. Although oligodendrocyte number was increased chronically after SCI in GGF2-treated mice, no evidence of increased white matter was detected. However, GGF2 treatment significantly increased levels of P0 protein-containing peripheral myelin, produced by Schwann cells that infiltrate the injured spinal cord. Our results suggest that GGF2 may have therapeutic potential for SCI by enhancing endogenous recovery processes in a clinically relevant time frame.

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.

Emerging repair, regeneration, and translational research advances for spinal cord injury

STUDY DESIGN

Literature review of basic scientific and clinical research in spinal cord injury (SCI).

OBJECTIVE

To provide physicians with an overview of the neurobiologic challenges of SCI, the current status of investigation for novel therapies that have been translated to human clinical trials, and the preclinical, scientific basis for each of these therapies.

SUMMARY OF BACKGROUND DATA

An abundance of recent scientific and clinical research activity has revealed numerous insights into the neurobiology of SCI, and has generated an abundance of potential therapies. An increasing number of such therapies are being translated into human SCI trials. Clinicians who attend to SCI patients are increasingly asked about potential treatments and clinical trials.

METHODS

Published data review of novel treatments that are either currently in human clinical trials for acute SCI or about to initiate clinical evaluation.

RESULTS

A number of treatments have bridged the "translational gap" and are currently either in the midst of human SCI trials, or are about to begin such clinical evaluation. These include minocycline, Cethrin, anti-Nogo antibodies, systemic hypothermia, Riluzole, magnesium chloride in polyethylene glycol, and human embryonic stem cell derived oligodendrocyte progenitors. A systematic review of the preclinical literature on these specific therapies reveals promising results in a variety of different SCI injury models.

CONCLUSION

The SCI community is encouraged by the progression of novel therapies from "bench to bedside" and the initiation of clinical trials for a number of different treatments. The task of clinical evaluation, however, is substantial, and many years will be required before the actual efficacy of the treatments currently in evaluation will be determined.

Measurement of glucose metabolism in rat spinal cord slices with dynamic positron autoradiography

We attempted to measure the regional metabolic rate of glucose (MRglc) in sliced spinal cords in vitro. The thoracic spinal cord of a mature Wister rat was cut into 400-mum slices in oxygenated and cooled (1-4 degrees C) Krebs-Ringer solution. After at least 60 min of preincubation, the spinal cord slices were transferred into double polystyrene chambers and incubated in Krebs-Ringer solution at 36 degrees C, bubbled with 5% O(2)/5% CO(2) gas. To measure MRglc, we used the dynamic positron autoradiography technique (dPAT) with F-18-2-fluoro-2-deoxy-d-glucose ([(18)F]FDG) and the net influx constant of [(18)F]FDG as an index. Uptake curves of [(18)F]FDG were well fitted by straight lines for more than 7 h after the slicing of the spinal cord (linear regression coefficient, r=0.99), indicating a constant uptake of glucose by the spinal cord tissue. The slope (K), which denotes MRglc, is affected by tetrodotoxin, and high K(+) (50 mM) or Ca(2+)-free, high Mg(2+) solution. After 10 min of hypoxia, the K value following reoxygenation was similar to the unloaded control value, but after 45 min of hypoxia, the K value was markedly lower than the unloaded control value, and after >90 min of reoxygenation it was nearly 0. Our results indicate that the living spinal cord slices used retained an activity-dependent metabolism to some extent. This technique may provide a new approach for measuring MRglc in sliced living spinal cord tissue in vitro and for quantifying the dynamic changes in MRglc in response to various interventions such as hypoxia.

Pitfalls in treatment of acute cervical spinal cord injury using high-dose methylprednisolone: a retrospect audit of 111 patients

BACKGROUND

Earlier studies suggested that the use of high-dose IV MP was the gold standard of care for the treatment of ASCI, but this has been debated. This study aims to identify the effects of high-dose MP in treatment of cervical SCI and how the treatment might be improved.

METHODS

The medical records of 138 patients with cervical spinal injury secondary to blunt injuries were retrospectively reviewed to determine the steroid administration protocol, effects, and complications. The findings on admission were compared with those at discharge and at the most recent outpatient follow-up visit. Significant neurologic improvement was defined as increase in at least 1 clinical grade according to the Frankel classification system.

RESULTS

Significantly more motor and sensory recovery was noted (complete ASCI, 69% vs 0; incomplete ASCI, 70% vs 50%) in patients treated with surgery and MP than in patients without such treatment. Moreover, 87% (14/16) of patients with complete ASCI (unlike patients with incomplete [8/28, 28.6%] and mild [2/14, 14.3%] ASCI) treated with MP had steroid-related complications, and 1 patient died from sepsis related to a perforated peptic ulcer. Mean hospitalization was significantly shorter for the patients who underwent tracheostomy (49 days, ranged from 22 to 110 days) vs nontracheostomy(94 days, ranged from 28-268 days).

CONCLUSION

Early intervention with surgery and MP is critical. Although treatment with MP for 24 or 48 hours significantly improves motor and sensory function of patients with ASCI, harmful side effects limit its functional efficacy in patients with complete ASCI. Early tracheostomy can shorten hospital stay in patients with complete ASCI.

Contusion, dislocation, and distraction: primary hemorrhage and membrane permeability in distinct mechanisms of spinal cord injury

Object

In experimental models of spinal cord injury (SCI) researchers have typically focused on contusion and transection injuries. Clinically, however, other injury mechanisms such as fracture–dislocation and distraction also frequently occur. The objective of the present study was to compare the primary damage in three clinically relevant animal models of SCI.

Methods

Contusion, fracture–dislocation, and flexion–distraction animal models of SCI were developed. To visualize traumatic increases in cellular membrane permeability, fluorescein–dextran was infused into the cerebrospi-nal fluid prior to injury. High-speed injuries (approaching 100 cm/second) were produced in the cervical spine of deeply anesthetized Sprague–Dawley rats (28 SCI and eight sham treated) with a novel multimechanism SCI test system. The animals were killed immediately thereafter so that the authors could characterize the primary injury in the gray and white matter.

Sections stained with H & E showed that contusion and dislocation injuries resulted in similar central damage to the gray matter vasculature whereas no overt hemorrhage was detected following distraction. Contusion resulted in membrane disruption of neuronal somata and axons localized within 1 mm of the lesion epicenter. In contrast, membrane compromise in the dislocation and distraction models was observed to extend rostrally up to 5 mm, particularly in the ventral and lateral white matter tracts.

Conclusions

Given the pivotal nature of hemorrhagic necrosis and plasma membrane compromise in the initiation of downstream SCI pathomechanisms, the aforementioned differences suggest the presence of mechanism-specific injury regions, which may alter future clinical treatment paradigms.

Systematic analysis of axonal damage and inflammatory response in different white matter tracts of acutely injured rat spinal cord

The mechanisms of white matter (WM) damage during secondary degeneration are a fundamental issue in the pathophysiology of central nervous system (CNS) diseases. Our main goal was to describe the pattern of an acute inflammatory response and secondary damage to axons in different WM tracts of acutely injured rat spinal cord. Adult rats were deeply anesthetized and injected with 20 nmol of NMDA into the spinal cord ventral horn on T7. Animals were perfused after survival times of 1 day, 3 days and 7 days. Ten micrometer sections were submitted to immunocytochemical analysis for activated macrophages/microglia, neutrophils and damaged axons. There were inflammatory response and progressive tissue destruction of ventral WM (VWM) with formation of microcysts in both VWM and lateral WM (LWM). In the VWM, the number of beta-amyloid precursor protein (beta-APP) end-bulbs increased from 1 day with a peak at 3 days, decreasing by 7 days following the injection. APP end-bulbs were present in the dorsal WM (DWM) at 3 days survival time but were not in the LWM. Electron microscopic analysis revealed different degrees of myelin disruption and axonal pathology in the vacuolated WM up to 14 mm along the rostrocaudal axis. Quantitative analysis revealed a significant loss of medium and large axons (P < 0.05), but not of small axons (P > 0.05). Our results suggest that bystander axonal damage and myelin vacuolation are important secondary component of the pathology of WM tracts following rat SCI. Further studies are needed to understand the mechanisms of these pathological events.

Chronic alterations in the cellular composition of spinal cord white matter following contusion injury

Spinal cord injury (SCI) involves the loss of neurons and glia due to initial mechanical and secondary biochemical mechanisms. Treatment with the sodium channel blocker tetrodotoxin (TTX) reduces acute white matter pathology and increases both axon density and hindlimb function chronically at 6 weeks after injury. We investigated the cellular composition of residual white matter chronically to determine whether TTX also has a significant effect on the numbers and types of cells present. Rats received an incomplete thoracic contusion injury, in the presence or absence of TTX (0.15 nmole) injected focally, beginning at 15 min prior to injury. Six weeks later, cell density was significantly increased in the residual white matter of the dorsal, lateral, and ventral funiculi, both rostral and caudal to the injury site in both TTX-treated and injury control groups. Oligodendrocyte and astrocyte density was similar to normal but large numbers of cells expressing microglia/macrophage markers were present. Labeling with the progenitor markers nestin and NG2 showed that precursor cell density had also doubled or tripled as compared with uninjured controls. Some of these cells were also labeled for antigens that indicate their possible progression along an oligodendrocyte or astrocyte lineage. Our results support the hypothesis that the beneficial effect of TTX in SCI is related to its preservation of axons per se; no effect on chronic white matter cell composition was detected. They highlight the profound changes in cellular composition in preserved white matter chronically at 6 weeks after injury, including the accumulation of endogenous progenitor cells and the persistence of activated macrophages/microglia. The manipulation of these endogenous cells may be used in the future to enhance recovery after SCI.

Potential effects of tetrodotoxin exposure to human glial cells postulated using microarray approach

Sodium channels play an important role in many neurological disorders and also in prostate cancer. Tetrodotoxin (TTX), a blocker of voltage-gated sodium channels has been chiefly used as a molecular probe for the study and characterization of these channels. The regulation of gene expression in response for the exposure of TTX to glial cells which are reported to be involved in neurodegenerative process is poorly understood. Therefore, the present study aims to develop a repository of genes and map it on a few pivotal neurodegenerative pathways to speculate the effect of TTX. Using Affymetrix GeneChip (HG-U133A), we have selected a subset of 692 differentially expressed genes, several of which are-cullin 4A (CUL4A), ubiquitin carrier protein (E2-EPF), proteasome (prosome, macropain) subunit, beta type, 8 (large multifunctional protease 7) (PSMB8), protein tyrosine phosphatase type IVA (PTP4A1), intercellular adhesion molecule 1 (ICAM1), prostaglandin-endoperoxide synthase 2 (PTGS2), and caspase 1 (CASP1). These genes, which facilitate some of the neurodegenerative pathways, such as ubiquitin, proteasome, inflammation and kinases, were identified to be up- or down-regulated for the TTX treatment. Thus, the selected genes were further examined on ubiquitin-proteasome mediated inflammatory responses pathway as ample evidence for the role of glial cell-mediated inflammation in the neurodegenerative process are available. In summary, our result provides a basic understanding of the differentially expressed genes along with one of the possible pathway which may have been modulated by the exposure of TTX.

Acute physiological response of mammalian central neurons to axotomy: ionic regulation and electrical activity

The transection of the axon of central neurons has dramatic consequences on the damaged cells and nerves. Injury activates molecular programs leading to a complex repertoire of responses that, depending on the cellular context, include activation of sprouting, axonal degeneration, and cell death. Although the cellular mechanisms started at the time of lesion are likely to shape the changes affecting injured cells, the acute physiological reaction to trauma of mammalian central neurons is not completely understood yet. To characterize the physiology of the acute response to axonal transection, we have developed a model of in vitro axotomy of neurons cultured from the rodent cortex. Imaging showed that axotomy caused an increase of calcium in the soma and axon. Propagation of the response to the soma required the activation of voltage-dependent sodium channels, since it was blocked by tetrodotoxin. The electrophysiological response to axotomy was recorded in patched neurons kept in the current clamp configuration: injury was followed by vigorous spiking activity that caused a sodium load and the activation of transient calcium currents that were opened by each action potential. The decrease of the electrochemical gradient of sodium caused inversion of the Na-Ca exchanger that provided an additional mean of entry for calcium. Finally, we determined that inhibition of the physiological response to axotomy hindered the regeneration of a new neurite. These data provide elements of the framework required to link the axotomy itself to the downstream molecular machinery that contributes to the determination of the long-term fate of injured neurons and axons.

Estrogen as a neuroprotective agent in the treatment of spinal cord injury

The following review is a brief discussion about spinal cord injury and the possibility of using estrogen as a neuroprotective agent. There are several pathways by which secondary cell death can occur following spinal cord injury, including infiltration of inflammatory cells, generation of reactive oxygen species, decreases in spinal cord blood flow, and increases in intracellular Ca(2+) levels. This secondary damage leads to apoptotic cell death, and the neuroprotective effects of pharmacologic agents have been investigated using experimentally induced spinal cord injury in animals. Currently, only high-dose methylprednisolone is advocated for the treatment of patients following spinal cord injury. Estrogen has been shown to be neuroprotective in both in vitro and in vivo studies. There are several possible mechanisms of action by which estrogen may attenuate damage following spinal cord injury and improve functional outcome. Estrogen has been shown to have anti-inflammatory properties. Estrogen levels are correlated with an increase in post-traumatic blood flow to injured tissue. Estrogen may also upregulate protein levels of anti-apoptotic Bcl-2 and may attenuate the post-traumatic influx of Ca(2+).

Spinal cord contusion models

Most human spinal cord injuries involve contusions of the spinal cord. Many investigators have long used weight-drop contusion animal models to study the pathophysiology and genetic responses of spinal cord injury. All spinal cord injury therapies tested to date in clinical trial were validated in such models. In recent years, the trend has been towards use of rats for spinal cord injury studies. The MASCIS Impactor is a well-standardized rat spinal cord contusion model that produces very consistent graded spinal cord damage that linearly predicts 24-h lesion volumes, 6-week white matter sparing, and locomotor recovery in rats. All aspects of the model, including anesthesia for male and female rats, age rather than body weight criteria, and arterial blood gases were empirically selected to enhance the consistency of injury.