The action of phenytoin on a composite electrical‐chemical synapse in the lamprey spinal cord

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Abstract
The effect of phenytoin (PTN), 20 μg per milliliter, was tested on the composite electrical‐chemical synapse between pairs of giant interneurons in the isolated spinal cords of river lampreys (Ichthiomyzon unicuspis). The main observations were that: (1) PTN reversibly reduced the chemical component of the excitatory postsynaptic potential by up to 70%; (2) PTN greatly reduced or eliminated posttetanic potentiation (PTP) of the chemical component; (3) PTN produced little or no decrease in the size of the electrical component; and (4) PTN did not cause blockage of the electrical component during high‐frequency stimulation of the presynaptic neuron. Since the electrical component reflects the presynaptic spike, the suppression of PTP cannot be explained by blockage of this spike during the tetanus. These four observations are consistent with a current hypothesis concerning the anticonvulsant effect of PTN—that the drug inhibits calcium entry into stimulated presynaptic terminals. This would block PTP by reducing the accumulation of calcium in the terminal during repetitive stimulation.