Redefining toxic distal axonopathies

Differential effects of lacosamide, phenytoin and topiramate on peripheral nerve excitability: An ex vivo electrophysiological study

BACKGROUND

Antiepileptic drugs (AEDs) are mainly used to control cortical hyperexcitability. Some of them (e.g. phenytoin (PHT) and topiramate (TPM)) have also effects on the peripheral nervous system (PNS). Lacosamide (LCM) is a novel AED that stabilizes hyperexcitable neuronal membranes by selectively enhancing the slow inactivation of voltage-gated sodium channels (VGSCs). Although the mechanism of action of LCM is fairly well understood, there are no in vitro data available regarding any possible PNS effects of LCM.

OBJECTIVE

To investigate, in vitro, the effects of LCM on peripheral nerve excitability in comparison with PHT and TPM, two AEDs that act, in part, by stabilizing the fast inactivation state of VGSCs.

METHODS

Experiments were conducted on the isolated sciatic nerve of the adult rat using standard electrophysiological methods. The effects of LCM on the amplitude and latency of the evoked compound action potential (CAP) during a 48h period of drug exposure were recorded and compared with the effects of PHT and TPM.

RESULTS

LCM produced inhibitory effects on CAP at concentrations significantly higher than the therapeutic levels (>25μg/ml). At these concentrations (62.57-125.15μg/ml), an acute and immediate increment of the latency and decrement of the amplitude of the CAP were observed. In contrast to LCM, PHT caused an acute decrement in the amplitude as well as an increment in the latency of the CAP even at subtherapeutic levels (5μg/ml). With regard to TPM, the amplitude of the CAP was not affected at the supratherapeutic concentrations but at the therapeutic concentration of 33.94μg/ml a reduced decrement of the CAP amplitude compared to the controls was observed.

CONCLUSIONS

LCM, PHT and TPM exert differential effects on peripheral nerve excitability. PHT inhibited the sciatic nerve CAP even at subtherapeutic levels whereas LCM was safe within the therapeutic concentration range. TPM did not affect the CAP amplitude even at high supratherapeutic concentrations whereas in the therapeutic range a neuroprotective effect was observed. Possible underlying mechanisms and the clinical implications of these findings are discussed.

Acrylamide neurotoxicity

Acrylamide, a food contaminant, belongs to a large class of structurally similar toxic chemicals, 'type-2 alkenes', to which humans are widely exposed. Besides, occupational exposure to acrylamide has received wide attention through the last decades. It is classified as a neurotoxin and there are three important hypothesis considering acrylamide neurotoxicity: inhibition of kinesin-based fast axonal transport, alteration of neurotransmitter levels, and direct inhibition of neurotransmission. While many researchers believe that exposure of humans to relatively low levels of acrylamide in the diet will not result in clinical neuropathy, some neurotoxicologists are concerned about the potential for its cumulative neurotoxicity. It has been shown in several studies that the same neurotoxic effects can be observed at low and high doses of acrylamide, with the low doses simply requiring longer exposures. This review is focused on the neurotoxicity of acrylamide and its possible outcomes.

Axonopathy-Inducing 1,2-Diacetylbenzene Forms Adducts with Motor and Cytoskeletal Proteins Required for Axonal Transport

The aromatic hydrocarbon 1,2-diacetylbenzene (1,2-DAB) is a protein-reactive gamma-diketone metabolite of the neurotoxic solvent 1,2-diethylbenzene (1,2-DEB). The effect of neurotoxic 1,2-DAB and its non-neurotoxic isomer 1,3-DAB has been studied on motor proteins and cytoskeletal proteins of rat spinal cord (SC). For in vitro studies, SC slices were incubated with 1, 2, 5, 10 mM of DAB isomers for 30 min at 37 degrees C. For in vivo studies, rats received (i.p.) 20 mg/kg/day of 1,2-DAB or 1,3-DAB, or vehicle (2% acetone in saline), 5 days a week for 2 weeks. Spinal cord and sciatic nerve proteins were subjected to Western blotting using monoclonal mouse antibodies to NF-M, kinesin, dynein, and tau. Proteins were quantified and paired mean comparisons performed to assess concentration-dependent changes in native protein bands. In vitro, 1,2-DAB produced a concentration-dependent decrease of motor and cytoskeletal proteins. While dynein and tau appeared similarly affected by 1,2-DAB, kinesin was most affected by the toxicant. In vivo, 1,2-DAB affected motor and cytoskeletal proteins of sciatic nerves and spinal cord differentially. In general, sciatic nerve proteins were much more affected than spinal cord proteins. The results show that motor proteins that drive axonal transport anterogradely (kinesin) and retrogradely (dynein), cytoskeletal protein NF-M, which is slowly transported in the anterograde direction, and microtubule-associated protein, tau, which is involved in axonal transport, are differentially impacted by 1,2-DAB. By contrast, non-neurotoxic isomer 1,3-diacetylbenzene (1,3-DAB), had no adverse effect on neural proteins either in vitro or in vivo. 2D-Differential in gel electrophoresis (2D-DIGE) of sciatic nerves from neurotoxic 1,2-DAB and non-neurotoxic 1,3-DAB treated rats revealed 197 and 304 protein spots, respectively.

Acrylamide: review of toxicity data and dose-response analyses for cancer and noncancer effects

Acrylamide (ACR) is used in the manufacture of polyacrylamides and has recently been shown to form when foods, typically containing certain nutrients, are cooked at normal cooking temperatures (e.g., frying, grilling or baking). The toxicity of ACR has been extensively investigated. The major findings of these studies indicate that ACR is neurotoxic in animals and humans, and it has been shown to be a reproductive toxicant in animal models and a rodent carcinogen. Several reviews of ACR toxicity have been conducted and ACR has been categorized as to its potential to be a human carcinogen in these reviews. Allowable levels based on the toxicity data concurrently available had been developed by the U.S. EPA. New data have been published since the U.S. EPA review in 1991. The purpose of this investigation was to review the toxicity data, identify any new relevant data, and select those data to be used in dose-response modeling. Proposed revised cancer and noncancer toxicity values were estimated using the newest U.S. EPA guidelines for cancer risk assessment and noncancer hazard assessment. Assessment of noncancer endpoints using benchmark models resulted in a reference dose (RfD) of 0.83 microg/kg/day based on reproductive effects, and 1.2 microg/kg/day based on neurotoxicity. Thyroid tumors in male and female rats were the only endpoint relevant to human health and were selected to estimate the point of departure (POD) using the multistage model. Because the mode of action of acrylamide in thyroid tumor formation is not known with certainty, both linear and nonlinear low-dose extrapolations were conducted under the assumption that glycidamide or ACR, respectively, were the active agent. Under the U.S. EPA guidelines (2005), when a chemical produces rodent tumors by a nonlinear or threshold mode of action, an RfD is calculated using the most relevant POD and application of uncertainty factors. The RfD was estimated to be 1.5 microg/kg/day based on the use of the area under the curve (AUC) for ACR hemoglobin adducts under the assumption that the parent, ACR, is the proximate carcinogen in rodents by a nonlinear mode of action. When the mode of action in assumed to be linear in the low-dose region, a risk-specific dose corresponding to a specified level of risk (e.g., 1 x 10-5) is estimated, and, in the case of ACR, was 9.5 x 10-2 microg ACR/kg/day based on the use of the AUC for glycidamide adduct data. However, it should be noted that although this review was intended to be comprehensive, it is not exhaustive, as new data are being published continuously.

Assessment of nociception in acrylamide-induced neuropathy in rats

Acrylamide was intraperitoneally administered to male Sprague-Dawley rats at four different doses (5, 10, 20 and 30 mg/kg) three times a week for 5 consecutive weeks. Because of motor dysfunction, the 30 mg/kg dose was not used for behavioral pain tests. Clinical status remained good throughout the experiment and no motor deficit was observed at the other doses. We showed that acrylamide administration at low doses and cumulative dose (CD) range of 35-140 mg/kg produced mechanical allodynia and rapid, marked heat (42 degrees C) and cold (10 degrees C) allodynia after tail immersion test. Mechanical and thermal hyperalgesia appeared after higher cumulative doses (70-280 mg/kg), except for cold (4 degrees C) hyperalgesia (20-80 mg/kg). All the modifications persisted throughout all study, except the mechanical hyperalgia. All the cumulative doses tested were lower than those generally reported to induce motor dysfunction (CD>250 mg/kg), confirming that CD may be considered to be a suitable index in assessing neurological signs and suggesting that early detection of acrylamide neurotoxicity would be possible using the sensory tests, especially those for detecting allodynia thresholds.

A novel endogenous erythropoietin mediated pathway prevents axonal degeneration

Clinically relevant peripheral neuropathies (such as diabetic and human immunodeficiency virus sensory neuropathies) are characterized by distal axonal degeneration, rather than neuronal death. Here, we describe a novel, endogenous pathway that prevents axonal degeneration. We show that in response to axonal injury, periaxonal Schwann cells release erythropoietin (EPO), which via EPO receptor binding on neurons, prevents axonal degeneration. We demonstrate that the relevant axonal injury signal that stimulates EPO production from surrounding glial cells is nitric oxide. In addition, we show that this endogenous pathway can be therapeutically exploited by administering exogenous EPO. In an animal model of distal axonopathy, systemic EPO administration prevents axonal degeneration, and this is associated with a reduction in limb weakness and neuropathic pain behavior. Our in vivo and in vitro data suggest that EPO prevents axonal degeneration and therefore may be therapeutically useful in a wide variety of human neurological diseases characterized by axonopathy.

Acrylamide disturbs the subcellular distribution of GABAA receptor in brain neurons

Mechanisms underlying the action of acrylamide on neurons were studied by monitoring the expression of GABA(A) receptor (R) in cultured brain neurons derived from chicken embryos. In situ trypsinization of the neurons and 3H-flunitrazepam binding assay were employed to examine the subcellular distribution of GABA(A)R. A 3-h exposure of the cultured neurons to 10 mM of acrylamide raised reversibly the proportion of intracellular (trypsin-resistant) 3H-flunitrazepam binding sites by about 48% and decreased cell surface binding 24% from respective control values, without altering total cellular binding and the affinity of the ligand. Moreover, the acrylamide treatment induced more intense perikaryal immunostaining of GABA(A)R alpha subunit proteins than that in control neurons but did not change the total level of cellular alpha immunostain, in accordance with the binding data. In the cell bodies of acrylamide-treated neurons, the level of neurofilament-200 kDa proteins was similar to control, whereas the tubulin protein content was significantly lowered approximately 51% from control, as revealed by quantifying the immunostained cytoskeletal elements. In addition, electron microscopic observations found reductions in the numbers of microtubules and neurofilaments in the perikarya of acrylamide-treated neurons. As exhibited by the 3H-leucine and 3H-monosaccharide incorporation experiments, the exposure to acrylamide inhibited the rate of general protein synthesis in the culture by 21%, while the rate of glycosylation remained unaltered. Furthermore, in situ hybridization analysis showed that acrylamide did not modify the expression of GABA(A)R alpha subunit mRNAs. Taken together, these data suggest that acrylamide may downregulate the microtubular system and disintegrate neurofilaments, and thereby block the intracellular transport of GABA(A)R, resulting in the accumulation of intracellular receptors.

Amino acid and protein targets of 1,2-diacetylbenzene, a potent aromatic gamma-diketone that induces proximal neurofilamentous axonopathy

The gamma-diketone analogs 1,2-diacetylbenzene (1,2-DAB) and 2,5-hexanedione (2,5-HD), but not the delta-diketone 1,3-diacetylbenzene (1,3-DAB) or the beta-diketone 2,4-hexanedione, induce neuropathological changes in the rodent central and peripheral nervous systems. The molecular targets of these neurotoxic aromatic and aliphatic gamma-diketones, and of their nonneurotoxic structural analogs and ninhydrin, are examined by assessing their differential reactivity with neural and nonneural amino acids and proteins in vitro and in vivo. Whereas 1,2-DAB is chromogenic and forms polymers with amino acids (notably lysine) and proteins (especially lysine-rich proteins), 1,3-DAB lacks these properties. Ninhydrin forms a chromophore without evidence of protein polymerization. 1,2-DAB preferentially targets neurofilament over microtubule protein in vitro and in situ. Based on protein reactivity, 1,2-DAB is three orders of magnitude more reactive than 2,5-HD. Lysine-rich neurofilament protein subunits NF-H and NF-M are more susceptible than lysine-poor NF-L and beta-tubulin to 1,2-DAB. These observations correlate with the development of proximal (1,2-DAB) and distal (2,5-HD) neurofilament-filled axonal swellings and segregated intact microtubules observed during systemic treatment with aromatic and aliphatic gamma-diketones.

Early spatial memory deficit induced by 2,5-hexanedione in the rat

2,5-Hexanedione (2,5-HD), the major common neurotoxic metabolite of n-hexane and methyl n-butyl ketone, causes a delayed neuropathy with associated sensorimotor impairments. The question arises as to whether specific cognitive deficits occur even prior to changes in sensorimotor ability. The present experiments examined the effects of 2,5-HD on spatial navigation of rats in a water maze at levels/times that did not affect spontaneous exploratory motor activity in an open field holeboard apparatus. Exposure to 1% 2,5-HD in the drinking water for 2 weeks did not significantly affect escape learning, as measured by latency to find a hidden platform. However, 2,5-HD treated animals were impaired in the use of a spatial strategy during a recall test. A similar impairment in spatial memory was observed after i.p. injection of 500 mg/kg/day 2,5-HD for 4 days, in the absence of significant changes in sensorimotor ability or weight loss. Thus 2,5-HD may mediate some of the cognitive effects of hexacarbons and these changes can occur prior to the development of motor symptoms.