Temperature dependence of the toxic effects of phenytoin on peripheral neuromuscular function of the rat tail

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.

Antioxidants in peripheral nerve

Oxidative stress and antioxidants have been related in a wide variety of ways with nervous tissue. This review attempts to gather the most relevant information related to a) the antioxidant status in non pathologic nervous tissue; b) the hypothesis and evidence for oxidative stress (considered as the disequilibrium between prooxidants and antioxidants in the cell) as the responsible mechanism of diverse neurological diseases; and c) the correlation between antioxidant alterations and neural function, in different experimental neuropathies. Decreased antioxidant availability has been observed in different neurological disorders in the central nervous system, for example, Parkinson's disease, Alzheimer's disease, epilepsy, amyotrophic lateral sclerosis, cerebral ischaemia, etc. Moreover, the experimental manipulation of the antioxidant defense has led in some cases to interesting experimental models in which electrophysiological alterations are associated with the metabolic modifications induced. In view of the electrophysiological and biochemical effects of some protein kinase C inhibitors on different neural experimental models, special attention is dedicated to the role of this kinase in peripheral nervous tissue. The nervous tissue, central as well as peripheral, has two main special features that are certainly related to its antioxidant metabolism: the lipid-enriched membrane and myelin sheaths, and cellular excitability. The former explains the importance of the glutathione (GSH)-conjugating activity towards 4-hydroxy-nonenal, a biologically active product of lipid peroxidation, present in nervous tissue and in charge of its inactivation. The impairment of the latter by oxidative damage or experimental manipulation of antioxidant metabolism is discussed. Work on different experimental neuropathies from author's laboratory has been primarily used to provide information about the involvement of free radical damage and antioxidants in peripheral nerve metabolic and functional impairment.

Phenytoin-induced glutathione depletion in rat peripheral nerve

Administration of high doses (150-250 mg/kg body weight) of phenytoin (DPH) promote a 40% decrease in glutathione (GSH) content of rat sciatic nerve. This DPH-induced GSH depletion is accompanied with an electrophysiological impairment of peripheral neuromuscular function. H7 (20 mg/kg body weight IP, 30 min prior to DPH), a protein kinase C inhibitor, was able to prevent the DPH-induced GSH depletion only at the lower DPH dose used. This same inhibitor completely prevented the electrophysiological impairment at the lower DPH dose, and only partially at the higher DPH dose used. These results confirm the hypothesis of a DPH-dependent activation of PKC (that might be triggered by, or be the consequence of, the reduction of the intracellular antioxidant GSH), as one of the pathophysiological mechanisms involved in DPH-induced neurotoxicity.

Nerve conduction velocity decrease and synaptic transmission alterations in caffeine-treated rats

The action of caffeine on peripheral neuromuscular function was studied by means of in vivo determinations of electrophysiological parameters, i.e., amplitude of extracellularly recorded muscle action potentials and nerve conduction velocity in the dorsal skeletal muscle and caudal nerve of the rat tail, respectively. Repeated exposure of the rats was carried out by adding caffeine to the drinking water for 10 days. Here we report the novel finding that motor nerve conduction velocity showed a significant decrease in caffeine-treated animals, whereas no change was observed in the amplitude of indirectly evoked extracellular muscle action potentials. The physiological recovery of the amplitude of the compound muscle action potential observed in nonintoxicated rats after high-frequency stimulation (10 Hz) was not observed in intoxicated animals and is also discussed.

Prevention of the acute neurotoxic effects of phenytoin on rat peripheral nerve by H7, an inhibitor of protein kinase C

The neurotoxic effects of a single dose of phenytoin (150 mg/kg body weight) alone or 30 min after H7 (a protein kinase C inhibitor) injection (20 mg/kg body weight) were investigated in terms of peripheral neuromuscular function and Na+,K(+)-ATPase activity of the sciatic nerve. This intraperitoneal injection of phenytoin induced complete blockade of muscle action potentials in the dorsal segmental muscles of the rat tail evoked by electric stimulation of the caudal nerve and a 40% decrease in the Na+,K(+)-ATPase activity of the rat sciatic nerve when compared with control values, measured as the difference between total and ouabain-insensitive ATPase activity. Prior administration of H7 resulted in the complete prevention of both effects. Implications of protein kinase C inhibition in phenytoin neurotoxicity are discussed.