L-phenylalanine selectively depresses currents at glutamatergic excitatory synapses

J Neurosci Res. 2003 Apr 1;72(1):116-24. doi: 10.1002/jnr.10569.

Abstract

To explore the hypothesis that L-phenylalanine (L-Phe) depresses glutamatergic synaptic transmission and thus contributes to brain dysfunction in phenylketonuria (PKU), the effects of L-Phe on spontaneous and miniature excitatory postsynaptic currents (s/mEPSCs) in rat and mouse hippocampal and cerebrocortical cultured neurons were studied using the patch-clamp technique. L-Phe depressed the amplitude and frequency of both N-methyl-D-aspartate (NMDA) and non-NMDA components of glutamate receptor (GluR) s/mEPSCs. The IC(50) of L-Phe to inhibit non-NMDAR mEPSC frequency was 0.98 +/- 0.13 mM, a brain concentration seen in classical PKU. In contrast, D-Phe had a significantly smaller effect, whereas L-leucine, an amino acid that competes with L-Phe for brain transporter, had no effect on mEPSCs. Unlike GluR s/mEPSCs, GABA receptor mIPSCs were not attenuated by L-Phe. A high extracellular concentration of glycine prevented the attenuation by L-Phe of NMDAR current, activated by exogenous agonist, and of NMDAR s/mEPSC amplitude, but not of NMDAR s/mEPSC frequency. On the other hand, L-Phe significantly depressed non-NMDAR current activated by low but not high concentrations of exogenous agonists. Glycine-independent attenuation of NMDAR s/mEPSC frequency suggests decreased presynaptic glutamate release caused by L-Phe, whereas decreased amplitudes of NMDAR and non-NMDAR s/mEPSCs are consistent with competition of L-Phe for the glycine- and glutamate-binding sites of NMDARs and non-NMDARs, respectively. The finding that GluR activity is significantly depressed at conditions characteristic of classical PKU indicates a potentially important contribution of impaired GluR function to PKU-related mental retardation and provides important insights into the potential physiological consequences of impaired GluR function.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cells, Cultured
  • Cerebral Cortex / cytology
  • Cerebral Cortex / drug effects
  • Cerebral Cortex / physiology
  • Dose-Response Relationship, Drug
  • Excitatory Postsynaptic Potentials / drug effects*
  • Excitatory Postsynaptic Potentials / physiology
  • Hippocampus / cytology
  • Hippocampus / drug effects
  • Hippocampus / physiology
  • Mice
  • Neurons / drug effects*
  • Neurons / physiology
  • Phenylalanine / pharmacology*
  • Rats
  • Receptors, Glutamate / physiology*
  • Synapses / drug effects*
  • Synapses / physiology

Substances

  • Receptors, Glutamate
  • Phenylalanine