Mechanism of calcium entry during axon injury and degeneration

Rate of neurotoxicant exposure determines morphologic manifestations of distal axonopathy

Exposure to a variety of agricultural, industrial, and pharmaceutical chemicals produces nerve damage classified as a central-peripheral distal axonopathy. Morphologically, this axonopathy is characterized by distal axon swellings and secondary degeneration. Over the past 25 years substantial research efforts have been devoted toward deciphering the molecular mechanisms of these presumed hallmark neuropathic features. However, recent studies suggest that axon swelling and degeneration are related to subchronic low-dose neurotoxicant exposure rates (i.e., mg toxicant/kg/day) and not to the development of neurophysiological deficits or behavioral toxicity. This suggests these phenomena are nonspecific and of uncertain pathophysiologic relevance. This possibility has significant implications for research investigating mechanisms of neurotoxicity, development of exposure biomarkers, design of risk assessment models, neurotoxicant classification schemes, and clinical diagnosis and treatment of toxic neuropathies. In this commentary we will review the evidence for the dose-related dependency of distal axonopathies and discuss how this concept might influence our current understanding of chemical-induced neurotoxicities.

gamma-diketone peripheral neuropathy. I. Quality morphometric analyses of axonal atrophy and swelling

Quantitative morphometric analysis was used to characterize expression of myelinated axon swelling and atrophy in rat peripheral nerve during 2,5-hexanedione (HD) intoxication. HD was administered by gavage according to different daily dosing regiments (100, 175, 250, or 400 mg/kg/day) and four proximodistal nerve regions (5th lumbar spinal nerve, proximal and distal sciatic nerve, and tibial nerve) were examined morphometrically. Morphometric determinations were made at four behavioral endpoints (unaffected, slight, moderate, and severe toxicity) and were correlated to electrophysiologic measurements of peripheral nerve function. Results show that, for all HD dose rates, onsets of behavioral neurotoxicity and nerve dysfunction were generally related to development of abundant axon atrophy. The proximodistal manifestation of atrophy was dependent upon the dosing rate; i.e., the atrophy response produced by subacute intoxication with higher daily dosing rates (250 and 400 mg/kg/day) was restricted to distal nerve regions whereas subchronic induction with lower dosing rates (100 and 175 mg/kg/day) produced abundant fiber atrophy in all proximodistal areas. In contrast to atrophy, axonal swellings constituted an inconsistent minor morphologic response, the expression of which was dependent upon subchronic dosing rates (100-250 mg/kg/day). Subacute HD administration (400 mg/kg/day) produced significant changes in neurobehavior and nerve electrophysiologic parameters in the absence of peripheral axon swelling. Thus, conditional expression of swellings suggests they are an epiphenomenon related to low-dose induction rates. Fiber atrophy, however, was numerically dominant, correlated with nerve dysfunction, and occurred at all dosing levels. These characteristics suggest atrophy is a neurotoxicologically significant feature of gamma-diketone peripheral neuropathy.

Redefining toxic distal axonopathies

Nerve damage classified as a central-peripheral distal axonopathy is produced by a variety of chemicals (e.g. acrylamide, n-hexane). Historically, axon swelling and secondary degeneration have been considered the morphologic hallmarks of toxic axonopathies and substantial research has been devoted toward deciphering corresponding molecular mechanisms. However, recent studies from the author's laboratory investigating rate (mg toxicant/kg/day) and route (i.p. vs gavage) of intoxication have shown that swelling and degeneration were related to neurotoxicant dosing conditions (i.e. low-dose, subchronic exposure) and not to development of neurophysiological deficits or classic behavioral toxicity. This suggests the presumed hallmarks of distal axonopathy are epiphenomena of uncertain pathophysiologic significance. Therefore, the current definition of and chemical classification scheme for toxic distal axonopathies requires re-evaluation.

Electron Probe X-Ray Microanalysis: a Tool for Elucidating the Role of Ions in Neuronal Physiology and Pathophysiology

Electron probe x-ray microanalysis (EPMA) is a quantitative electron microscope technique that measures both water content (percentage water) and total (free plus bound) concentrations of biological elements in selected morphological compartments. Unlike other methods for determination of ion/element concentrations, EPMA permits simultaneous quantitation of several elements (Na, P, S, Cl, K, Ca, and Mg) and allows optical differentiation of nervous tissue cell types (i.e, neurons, glia) with subsequent analysis of respective submembrane regions or organelles (e.g, axoplasm, mitochondria, nuclei). EPMA, therefore, represents a powerful tool for extending our current understanding of elements/ions in neurophysiological processes. In addition, it is presumed that neuropathic injury disrupts normal intraneuronal Na+, K+, and Ca2+ distribution and that the structural and functional consequences are mediated by ion translocation. However, little specific information is available regarding how translocated ions distribute among subcellular anatomical compartments after injury. EPMA quantification of ion/element changes associated with various nervous tissue injury models has helped to elucidate corresponding pathophysiological mechanisms. In this review, we will discuss EPMA and the realized, as well as potential, contributions of this technique to deciphering the role of ions in neuronal physiology and pathophysiology. Our recent studies of axon degeneration during acrylamide intoxication will be described to illustrate the utility of EPMA.

Experimental spinal cord injury: spatiotemporal characterization of elemental concentrations and water contents in axons and neuroglia

To examine the role of axonal ion deregulation in acute spinal cord injury (SCI), white matter strips from guinea pig spinal cord were incubated in vitro and were subjected to graded focal compression injury. At several postinjury times, spinal segments were removed from incubation and rapidly frozen. X-ray microanalysis was used to measure percent water and dry weight elemental concentrations (mmol/kg) of Na, P, Cl, K, Ca, and Mg in selected morphological compartments of myelinated axons and neuroglia from spinal cord cryosections. As an index of axon function, compound action potentials (CAP) were measured before compression and at several times thereafter. Axons and mitochondria in epicenter of severely compressed spinal segments exhibited early (5 min) increases in mean Na and decreases in K and Mg concentrations. These elemental changes were correlated to a significant reduction in CAP amplitude. At later postcompression times (15 and 60 min), elemental changes progressed and were accompanied by alterations in compartmental water content and increases in mean Ca. Swollen axons were evident at all postinjury times and were characterized by marked element and water deregulation. Neuroglia and myelin in severely injured epicenter also exhibited significant disruptions. In shoulder areas (adjacent to epicenter) of severely injured spinal strips, axons and mitochondria exhibited modest increases in mean Na in conjunction with decreases in K, Mg, and water content. Following moderate compression injury to spinal strips, epicenter axons exhibited early (10 min postinjury) element and water deregulation that eventually recovered to near control values (60 min postinjury). Na(+) channel blockade by tetrodotoxin (TTX, 1 microM) perfusion initiated 5 min after severe crush diminished both K loss and the accumulation of Na, Cl, and Ca in epicenter axons and neuroglia, whereas in shoulder regions TTX perfusion completely prevented subcellular elemental deregulation. TTX perfusion also reduced Na entry in swollen axons but did not affect K loss or Ca gain. Thus graded compression injury of spinal cord produced subcellular elemental deregulation in axons and neuroglia that correlated with the onset of impaired electrophysiological function and neuropathological alterations. This suggests that the mechanism of acute SCI-induced structural and functional deficits are mediated by disruption of subcellular ion distribution. The ability of TTX to reduce elemental deregulation in compression-injured axons and neuroglia implicates a significant pathophysiological role for Na(+) influx in SCI and suggests Na(+) channel blockade as a pharmacotherapeutic strategy.

Separation of carrier mediated and vesicular release of GABA from rat brain slices

In this study the temperature dependence of [3H]GABA release from brain slices evoked by electrical field stimulation and the Na+/K+ ATPase inhibitor ouabain was investigated. [3H]GABA has been taken up and released from hippocampal slices at rest and in response to electrical field stimulation (20 V, 10 Hz, 3 msec, 180 pulses) at 37 degrees C. When the bath temperature was cooled to 7 degrees C, during the sample collection period, the tissue uptake and the resting outflow of [3H]GABA were not significantly changed. In contrast, the stimulation-induced tritium outflow increased both in absolute amount (Bq/g) and in fractional release and the S2/S1 ratio was also higher at 7 degrees C. Perfusion of the slices with tetrodotoxin (TTX, 1 microM) inhibited stimulation-induced [3H]GABA efflux indicating that exocytotic release of vesicular origin is maintained under these conditions. 15 min perfusion with ouabain (10-20 microM) induced massive tritium release both in hippocampal and in striatal slices. However, the fraction of [3H]GABA outflow evoked by ouabain was much higher in the hippocampus than in the striatum. Sequential lowering the bath temperature from 37 degrees C to 17 degrees C completely abolished ouabain-induced [3H]GABA release in both brain regions, indicating that it is a temperature-dependent, carrier-mediated process. When the same experiments were repeated under Ca2+ free conditions, cooling the bath temperature to 17 degrees C, although substantially decreased the release but failed to completely abolish the tritium outflow evoked by ouabain, a significant part was maintained. Our results show that vesicular (field stimulation-evoked) and carrier-mediated (ouabain-induced) release of GABA is differentially affected by low temperature: while vesicular release is unaffected, carrier-mediated release is abolished at low bath temperature. Therefore, lowering the temperature offers a reliable tool to separate these two kinds of release and makes possible to study exclusively the pure neuronal release of GABA of vesicular origin.

An intrathecal bolus of cyclosporin A before injury preserves mitochondrial integrity and attenuates axonal disruption in traumatic brain injury

Traumatic brain injury evokes multiple axonal pathologies that contribute to the ultimate disconnection of injured axons. In severe traumatic brain injury, the axolemma is perturbed focally, presumably allowing for the influx of Ca2+ and initiation of Ca2+ -sensitive, proaxotomy processes. Mitochondria in foci of axolemmal failure may act as Ca2+ sinks that sequester Ca2+ to preserve low cytoplasmic calcium concentrations. This Ca2+ load within mitochondria, however, may cause colloid osmotic swelling and loss of function by a Ca2+ -induced opening of the permeability transition pore. Local failure of mitochondria, in turn, can decrease production of high-energy phosphates necessary to maintain membrane pumps and restore ionic balance in foci of axolemmal permeability change. The authors evaluated the ability of the permeability transition pore inhibitor cyclosporin A (CsA) to prevent mitochondrial swelling in injured axonal segments demonstrating altered axolemmal permeability after impact acceleration injury in rat. At the electron microscopic level, statistically fewer abnormal mitochondria were seen in traumatically injured axons from CsA-pretreated injured animals. Further, this mitochondrial protection translated into axonal protection in a second group of injured rats, whose brains were reacted with antibodies against amyloid precursor protein, a known marker of injured axons. Pretreatment with CsA significantly reduced the number of axons undergoing delayed axotomy, as evidenced by a decrease in the density of amyloid precursor protein-immunoreactive axons. Collectively, these studies demonstrate that CsA protects both mitochondria and the related axonal shaft, suggesting that this agent may be of therapeutic use in traumatic brain injury.

Oxygen/glucose deprivation in hippocampal slices: altered intraneuronal elemental composition predicts structural and functional damage

Effects of oxygen/glucose deprivation (OGD) on subcellular elemental composition and water content were determined in nerve cell bodies from CA1 areas of rat hippocampal slices. Electron probe x-ray microanalysis was used to measure percentage water and concentrations of Na, P, K, Cl, Mg, and Ca in cytoplasm, nucleus, and mitochondria of cells exposed to normal and oxygen/glucose deficient medium. As an early (2 min) consequence of OGD, evoked synaptic potentials were lost, and K, Cl, P, and Mg concentrations decreased significantly in all morphological compartments. As exposure to in vitro OGD continued, a negative DC shift in interstitial voltage occurred ( approximately 5 min), whereas general elemental disruption worsened in cytoplasm and nucleus (5-42 min). Similar elemental changes were noted in mitochondria, except that Ca levels increased during the first 5 min of OGD and then decreased over the remaining experimental period (12-42 min). Compartmental water content decreased early (2 min), returned to control after 12 min of OGD, and then exceeded control levels at 42 min. After OGD (12 min), perfusion of hippocampal slices with control oxygenated solutions (reoxygenation) for 30 min did not restore synaptic function or improve disrupted elemental composition. Notably, reoxygenated CA1 cell compartments exhibited significantly elevated Ca levels relative to those associated with 42 min of OGD. When slices were incubated at 31 degreesC (hypothermia) during OGD/reoxygenation, neuronal dysfunction and elemental deregulation were minimal. Results show that in vitro OGD causes loss of transmembrane Na, K, and Ca gradients in CA1 neurons of hippocampal slices and that hypothermia can obtund this damaging process and preserve neuronal function.

Excessive release of [3H]noradrenaline and glutamate in response to simulation of ischemic conditions in rat spinal cord slice preparation: effect of NMDA and AMPA receptor antagonists

In the present study we investigated the effects of NMDA and non-NMDA glutamate receptor antagonists on the ischemia-evoked release of [3H]noradrenaline from rat spinal cord slices. An in vitro ischemia model (oxygen and glucose deprivation) was used to simulate the ischemic conditions known to cause neuronal injury. Spinal cord slices were loaded with [3H]noradrenaline and superfused with Krebs solution in a micro-organ bath. Both axonal stimulation and ischemia increased the release of [3H]noradrenaline, but the release in response to glucose and oxygen deprivation was [Ca2+]o independent. Dizocilpine (MK-801), an NMDA receptor antagonist, suppressed the release of [3H]noradrenaline produced by ischemia, while it enhanced the release of [3H]noradrenaline evoked by electrical field stimulation. In contrast, LY300168 (GYKI-53655) [(+/-)-3-N-methylcarbamyde-1-(4-aminophenyl)-4-methyl-1.8-methylen e-dioxy-5H-2.3-benzodiazepine] and its (-)isomer LY303070 (GYKI-53784) [(-)-3-N-methylcarbamyde-1-(4-aminophenyl)-4-methyl-1.8-methylene- dioxy-5H-2.3-benzodiazepine] AMPA receptor antagonists, had no effect on the release of [3H]noradrenaline evoked by either electrical stimulation or ischemia. Desipramine, a noradrenaline uptake inhibitor, potentiated the release of [3H]noradrenaline evoked by ischemia, while in the absence of [Ca2+]o but under conditions when [3H]noradrenaline release was further increased, it reduced the release. Dizocilpine also decreased glutamate and aspartate release, measured by high performance liquid chromatography, during ischemia. It is concluded that glutamate release and NMDA receptors, but not AMPA receptors, are involved in the acute effect of oxygen and glucose deprivation on the excessive release of noradrenaline and that this release is not related to physiological axonal conduction.

Cyclosporin A limits calcium-induced axonal damage following traumatic brain injury

In traumatic axonal injury, Ca2+ influx across a focally damaged axolemma precipitates local mitochondrial failure, degradation of the subaxolemmal spectrin network and compaction of neurofilaments, which collectively contribute to axonal failure. In previous studies, cyclosporin A pretreatment preserved mitochondrial integrity and attenuated axonal failure following trauma. Here we investigate whether this CsA-linked protection was related to the concomitant blunting of intra-axonal, Ca2+-induced cytoskeletal changes in traumatic axonal injury, assessed with antibodies targeting spectrin proteolysis and neurofilament compaction. CsA pretreatment dramatically reduced Ca2+-induced cytoskeletal damage following injury; CsA-treated rats, compared with vehicle-treated rats, displayed a 70% decrease in immunoreactive/damaged profiles. We suggest that CsA-mediated preservation of mitochondrial integrity enables the restoration of ionic and metabolic homeostasis thereby short-circuiting Ca2+-induced proteolysis in injured axons.

In vitro central nervous system models of mechanically induced trauma: a review

Injury is one of the leading causes of death among all people below the age of 45 years. In the United States, traumatic brain injury (TBI) and spinal cord injury (SCI) together are responsible for an estimated 90,000 disabled persons annually. To improve treatment of the patient and thereby decrease the associated mortality, morbidity, and cost, several in vivo models of central nervous system (CNS) injury have been developed and characterized over the past two decades. To complement the ability of these in vivo models to reproduce the sequelae of human CNS injury, in vitro models of neuronal injury have also been developed. Despite the inherent simplifications of these in vitro systems, many aspects of the posttraumatic sequelae are faithfully reproduced in cultured cells, including ultrastructural changes, ionic derangements, alterations in electrophysiology, and free radical generation. This review presents a number of these in vitro systems, detailing the mechanical stimuli, the types of tissue injured, and the in vivo injury conditions most closely reproduced by the models. The data generated with these systems is then compared and contrasted with data from in vivo models of CNS injury. We believe that in vitro models of mechanical injury will continue to be a valuable tool to study the cellular consequences and evaluate the potential therapeutic strategies for the treatment of traumatic injury of the CNS.

Biochemical and morphologic characterization of acrylamide peripheral neuropathy

To determine whether reduced Na+/K+-ATPase activity might be involved in acrylamide (ACR)-induced peripheral axon swelling and degeneration, rubidium (Rb+) transport was measured as an index of enzyme function. x-ray microanalysis was used to quantify elemental Rb uptake and accumulation in internodal myelinated axons, mitochondria, Schwann cells, and myelin of rat tibial nerve cryosections. Results demonstrated impairment of Rb uptake in tibial axons from orally intoxicated (2.8 mM ACR for 34 days), moderately affected rats. In severely affected oral rats (49 days), complete inhibition of Rb transport and frank axon degeneration were evident. However, in moderate-to-severely affected rats exposed to ACR via ip injection (50 mg/kg/day for 11 days), neither structural nor enzymatic changes were present in tibial fibers. These findings in nerve cryosections suggested inhibition of axolemmal Na+ pump activity and degeneration were dependent upon route of ACR administration. This possibility was substantiated by a quantitative longitudinal morphometric study of conventionally fixed tibial nerve. Oral ACR treatment (2.8 mM ACR for 15-49 days) was associated with progressive axon degeneration, which was preceded by atrophy. Axonal swellings were rarely (<1%) observed. In contrast, ip ACR injection (50 mg/kg/day for 5-11 days) produced classic behavioral neurotoxicity but did not alter axon morphology in tibial nerve. Thus, fiber degeneration and decreased Na+ pump activity were consequences of subchronic oral ACR administration. This parallel expression suggests a mechanistic relationship. However, the corresponding general neurotoxicological significance is unclear since, behavioral toxicity induced by ip ACR develops without structural and enzymatic changes in tibial nerve.

Changes in [Ca2+]0 during anoxia in CNS white matter

Irreversible anoxic injury of axons in the rat optic nerve requires the presence of extracellular Ca2+. To test the hypothesis that Ca2+ enters an intracellular compartment during anoxia we monitored [Ca2+]0 in this CNS white matter tract using ion-sensitive microelectrodes. Periods of anoxia lasting 15 min resulted in a rapid, reversible increase in [Ca2+]0 accompanied by transient loss of nerve conduction. This increase in [Ca2+]0 was apparently the result of extracellular space shrinkage. Anoxic periods lasting 60 min resulted in an initial rise followed by a sustained fall in [Ca2+]0, indicative of net influx of Ca2+ into an intracellular compartment. Following reoxygenation after 60 min of anoxia, [Ca2+]0 slowly returned toward control levels but nerve conduction recovered incompletely, indicating irreversible loss of function. Removal of bath Ca2+ lowered [Ca2+]0 to about 100 microM, prevented the anoxia-induced fall in [Ca2+]0, and protected against irreversible loss of the compound action potential.

Na+ channel block prevents the ischemia-induced release of norepinephrine from spinal cord slices

The principal finding of the present study with rat spinal cord slices was the novel demonstration of the [Ca2+]o-independent effect of ischemia on norepinephrine release and its antagonism by tetrodotoxin and low temperature (10 degrees C). Our finding that tetrodotoxin antagonized the effects of glucose deprivation on norepinephrine release in a [Ca2+]o-independent way suggests that Na+ channel block alone, i.e., the prevention of Na+ accumulation, may account for the protective action. Low temperature completely prevented the effect of ischemia on norepinephrine release but did not change the release associated with axonal activity. This finding is in good agreement with the observation that small changes in brain temperature critically determine the extent of neuronal injury from ischemia and suggests that both [Ca2+]o-independent release and cell injury are associated with the norepinephrine membrane carrier. It is suggested, therefore, that drugs able to attenuate the increase in [Na+]i during ischemia may be useful agents to protect against ischemic damage if given before the insult.

Mechanisms of calcium and sodium fluxes in anoxic myelinated central nervous system axons

Electron probe X-ray microanalysis was used to measure water content and concentrations of elements (i.e. Na, K, Cl and Ca) in selected morphological compartments of rat optic nerve myelinated axons. Transaxolemmal movements of Na+ and Ca2+ were modified experimentally and corresponding effects on axon element and water compositions were determined under control conditions and following in vitro anoxic challenge. Also characterized were effects of modified ion transport on axon responses to postanoxia reoxygenation. Blockade of Na+ entry by tetrodotoxin (1 microM) or zero Na+/Li(+)-substituted perfusion reduced anoxic increases in axonal Na and Ca concentrations. Incubation with zero-Ca2+/EGTA perfusate prevented axoplasmic and mitochondrial Ca accumulation during anoxia but did not affect Na increases or K losses in these compartments. Inhibition of Na(+)-Ca2+ exchange with bepridil (30 microM) selectively prevented increases in intra-axonal Ca, whereas neither nifedipine (5 microM) nor nimodipine (5 microM) influenced the effects of anoxia on axonal Na, K or Ca. X-ray microanalysis also showed that prevention of Na and Ca influx during anoxia obtunded severe elemental deregulation normally associated with reoxygenation. Results of the present study suggest that during anoxia, Na+ enters axons mainly through voltage-gated Na+ channels and that subsequent increases in axoplasmic Na+ are functionally coupled to extra-axonal Ca2+ import. Na+i-dependent, Ca2+o entry is consistent with reverse operation of the axolemmal Na(+)-Ca2+ exchanger and we suggest this route represents a primary mechanism of Ca2+ influx. Our findings also implicate a minor route of Ca2+ entry directly through Na+ channels.