Phenytoin blocks N-methyl-D-aspartate responses of mouse central neurons

Intracellularly recorded depolarizing responses of mouse spinal cord neurons in cell culture to N-methyl-D-aspartate (NMDA) applied by pressure ejection at 37 degrees C had a reversal potential of about -13 mV. Amplitude increased when [Mg++]o was less than 1.0 mM or glycine was added to the buffer. Desensitization was complete within 30 pressure applications of NMDA (P30) at 2-s inter-response intervals (IRI; timed from return of one response to resting potential until next application) in bicarbonate buffer and was glycine-sensitive. Desensitization was insignificant in phosphate buffer. In both buffers, 8 x 10(-6) M phenytoin (PT) blocked responses reversibly by P10 of 10(-5) M NMDA at 0.2 Hz (overlapping responses) and at short 2-s IRI (responses not overlapping). At frequencies < or = 0.1 Hz or IRI > or = 5 s, desensitization and block were less prominent or inapparent. Block by PT was observed 1) in single isolated neurons; 2) in 7 mM [Mg++]o-, 150 mM [K+]o-, or tetrodotoxin (TTX)-containing buffer to suppress spontaneous synaptic activity and action potentials and 3) when voltage-dependent Mg++ block was removed by depolarization or in 0.1 mM Mg++, with or without glycine supplementation. The block was not competitive. The PT metabolite, 5-(4-hydroxyphenyl)-5-phenylhydantoin (80 microM), did not block responses to NMDA. Use- and frequency-dependent block of NMDA responses may contribute to clinical effects of PT, e.g., during sustained rapid activity along pathways excited by NMDA-preferring glutamate receptors.