Frequency-dependent effects of phenytoin on frog junctional transmission: presynaptic mechanisms

Fosphenytoin induced transient pendular nystagmus

Pendular nystagmus was seen after intravenous infusion of fosphenytoin for the treatment of breakthrough seizure. The oscillations were conjugate, quasi-sinusoidal, and regular. The nystagmus was resolved within 12h. Phenytoin is known to cause conduction slowing. Such delays in the feedback between the cerebellum and the oculomotor neural integrator may cause instability of the integrator. An improvement in the conduction delay following post-ictal recovery may resolve the pendular nystagmus.

Effects of anticonvulsant drugs on axonal conduction in mammalian corpus callosum

The frequency-dependent effect of various anticonvulsant drugs on the conduction in central axons was studied in the corpus callosum of rat and guinea pig brain slices from the parietal region. Extracellularly recorded compound action potentials (CAPs) were evoked by either single stimulus or high frequency stimulation (40-80 Hz). The CAP in rats consisted of an early component (fast axons, 1.2-1.8 m/s) and a late component (slow axons, 0.5-0.7 m/s), while in the guinea pig only the slow phase was observed. Diphenylhydantoin increased the latency of a single response by 10%, and had no effect on the CAP amplitude. In contrast, both phenobarbital and pentobarbital reduced the amplitude of singly evoked CAPs. Stimulation at high frequency alone decreased the CAP amplitude by 10-20%. Identical stimulation in the presence of the drugs further suppressed the CAP amplitude by an additional 31%, with varying degree of drug efficacy. The depressant effect was significant for the slow axons but the fast axons were virtually unaffected by any of the drugs. The results are consistent with the hypothesis that the antiepileptic drugs DPH, Phe and Pnt may block axonal conduction from an epileptic focus into neighbouring areas of the brain.

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

We studied the acute effects of a single dose of phenytoin (250 mg/kg) on peripheral neuromuscular function. The evoked muscle action potentials of the dorsal segmental muscles in the rat tail, and the conduction velocity of the dorsal nerve trunk which innervates them, were measured before and after the intraperitoneal injection of phenytoin. The experiments were performed at different temperatures, 27 (physiological tail temperature), 36 and 37 degrees C (physiological central temperature) in different groups of animals. The amplitudes of the evoked muscle action potentials in the treated groups showed no significant modifications at 27 degrees C, at 36 degrees C a small nonsignificant decrease could be observed, and a complete block occurred at 37 degrees C. The mean blocking time was approximately one hour. No significant variations of conduction velocity were observed at 27 and 36 degrees C, whereas it decreased significantly after 30 minutes at 37 degrees C. The results presented confirm phenytoin toxicity. How far these results, especially the decrease of nerve conduction velocity observed at 37 degrees C, confirm a previous hypothesis which supported that peripheral and central nervous system are affected by phenytoin by similar mechanisms, is discussed.

The effect of diphenylhydantoin on the electroretinogram

Acute administration of diphenylhydantoin (DPH) in rabbits produces a significant increase in the amplitude of the a-wave. A marked increase in the amplitude of the b-wave is also noted but the time course is slower than that for the a-wave. While in controls the oscillatory potential (OP) recordings essentially consist of three major types, recordings taken after DPH injection consist of one major OP (OP2), which appears to be a result of the fusion of the original OP2 with another OP produced by the DPH injection. A similar blend of OPs was also seen in ERGs recorded from three human subjects on DPH therapy.

Effects of phenytoin, ketamine, and atropine methyl nitrate in preventing neuromuscular toxicity of acetylcholinesterase inhibitors soman and diisopropylphosphorofluoridate

Toxic manifestations of acetylcholinesterase inhibitors (AChE-I) include muscle twitching and muscle fiber necrosis, in addition to muscarinic manifestations of acetylcholine excess. The AChE-Is pinacolyl methylphosphonofluoridate (soman) or diisopropylphosphorofluoridate (DFP) were administered to rats to produce spontaneous muscle fiber discharges. Soman produced discharges that arose primarily from the central nervous system (CNS), while those due to DFP were generated from the peripheral nerves as well as the CNS. Three drugs were tested for their potential to reduce muscle fiber discharges: atropine methyl nitrate (AMN), ketamine, and phenytoin. Ketamine caused a significant decrease in discharges of CNS origin, while AMN and phenytoin had no effect. For muscle fiber discharges of peripheral origin, all three drugs produced a significant drop in muscle fiber discharges, but phenytoin showed slightly more efficacy than the others. AChE-I-induced muscle hyperactivity arises from actions on the CNS and on the peripheral nerve in varying proportions for different AChE-Is. Treatment for the toxicity of AChE-Is on muscle may be accomplished by administering drugs with distinctive pharmacological actions at target sites in the CNS and peripheral nervous system (PNS) where AChE-Is exert their effects. By attenuating the effects of AChE-Is at specific CNS or PNS sites, the neuromuscular toxicity can be reduced in a manner specific to the characteristic sites of toxicity of each AChE-I.

Phenytoin: mechanisms of its anticonvulsant action

Phenytoin is a major anticonvulsant drug that is very effective in controlling a wide variety of seizure disorders while impairing neurological function little, if at all. Early work suggested the hypothesis that the drug's effects were due to a selective block of high-frequency neuronal activity. This theory is reevaluated in the light of accumulated observations on the effects of phenytoin in many neuronal and synaptic preparations. Most of these observations can be explained by a use- and frequency-dependent suppression of the sodium action potential by phenytoin, with a consequent filtering out of sustained high-frequency neuronal discharges and synaptic activity. The molecular mechanism for this is a voltage-dependent blockade of membrane sodium channels responsible for the action potential. Through this action, phenytoin obstructs the positive feedback that underlies the development of maximal seizure activity, while normal brain activity, proceeding at lower neuronal firing rates, is spared its depressant action. Other mechanisms of action that may contribute to the drug's efficacy and selectivity are also discussed.

Activity-dependent depression of nerve action potential by phenytoin

The action of the anticonvulsant drug phenytoin was investigated on the responsiveness of isolated amphibian and human nerves to repetitive stimulation. At low frequencies of stimulation (0.5-25 Hz) the drug (at a concentration of 0.1 mM) had no notable effect on the compound nerve action potential. By contrast, at higher rates of stimulation (50-300 Hz), it produced a progressive decrease in amplitude and integral of the compound action potential. This effect was positively correlated with the frequency of nerve activation and was markedly enhanced by elevating the extracellular K+ concentration. Thus, phenytoin induces a use- and frequency-dependent depression of axon conduction, which may contribute to its preferential suppression of the spread of high-frequency seizure discharge in the brain.