L‐phenylalanine selectively depresses currents at glutamatergic excitatory synapses

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.

Differential Modulation of Glutamatergic Transmission by 3,5-Dibromo-L-phenylalanine

“…The binding sites for glycine and glutamate are located on subunits 1 and 2, respectively (Laube et al, 1997). Previously, we have demonstrated that L-Phe depressed NMDA receptor function by competing for the glycine-binding site of NMDA receptors and depressed non-NMDA receptor activity by competing for the glutamatebinding site of these receptors (Glushakov et al, 2002(Glushakov et al, , 2003. Similar to L-Phe, 3,5-DBr-L-Phe also is a competitive antagonist of AMPA/kainate receptors.…”

“…During investigation of the cellular mechanisms of brain dysfunction seen in phenylketonuria, we found that L-Phe selectively depresses glutamatergic synaptic transmission by a combination of pre-and postsynaptic actions (Glushakov et al, 2002(Glushakov et al, , 2003. Specifically, L-Phe depresses glutamatergic synaptic transmission by 1) competing for the glycine-binding site of NMDA receptors, 2) competing for the glutamate-binding site of AMPA/kainate receptors, and 3) attenuating glutamate release.…”

A late-diagnosed phenylketonuria case presenting with autism spectrum disorder in early childhood

“…This is the suggested mechanism leading to low dopamine levels at prefrontal in PKU 18 . In addition, the negative effect of high phenylalanine levels on glutamate receptor function also contribute to the brain dysfunctioning in PKU 17,19 . Glushakow et al 19 showed that high levels of phenylalanine selectively and significantly depress the function of glutamate receptors in excitatory synapses.…”

“…In addition, the negative effect of high phenylalanine levels on glutamate receptor function also contribute to the brain dysfunctioning in PKU 17,19 . Glushakow et al 19 showed that high levels of phenylalanine selectively and significantly depress the function of glutamate receptors in excitatory synapses. They suggested three different mechanisms mediating this effect of phenylalanine: 1) Attenuation of glutamate release from the presynaptic neuron, 2) competition for glycine binding site of NMDA receptors, 3) competition for the glutamate binding site of non-NMDA receptors.…”

“…They suggested three different mechanisms mediating this effect of phenylalanine: 1) Attenuation of glutamate release from the presynaptic neuron, 2) competition for glycine binding site of NMDA receptors, 3) competition for the glutamate binding site of non-NMDA receptors. In summary, when we consider the important effects of glutamate receptors in activity dependent synaptic plasticity, longterm memory and associative learning, the impairment of glutamate receptor activity seems to be highly contributing to the brain dysfunction in PKU 19 .…”