Glushakov A V, Dennis D M, Sumners C, Seubert C N, Martynyuk A E
Department of Anesthesiology, University of Florida, Gainesville, Florida 32610-0254, USA.
J Neurosci Res. 2003 Apr 1;72(1):116-24. doi: 10.1002/jnr.10569.
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.
为探究L-苯丙氨酸（L-Phe）抑制谷氨酸能突触传递并因此导致苯丙酮尿症（PKU）脑功能障碍这一假说，我们采用膜片钳技术研究了L-Phe对大鼠和小鼠海马及大脑皮质培养神经元中自发性和微小兴奋性突触后电流（s/mEPSCs）的影响。L-Phe降低了谷氨酸受体（GluR）s/mEPSCs中N-甲基-D-天冬氨酸（NMDA）和非NMDA成分的幅度和频率。L-Phe抑制非NMDA受体mEPSC频率的半数抑制浓度（IC50）为0.98±0.13 mM，这是经典PKU患者脑中可见的浓度。相比之下，D-Phe的作用明显较小，而L-亮氨酸（一种与L-Phe竞争脑转运体的氨基酸）对mEPSCs无影响。与GluR s/mEPSCs不同，GABA受体mIPSCs未被L-Phe减弱。细胞外高浓度甘氨酸可防止L-Phe对外源性激动剂激活的NMDAR电流以及NMDAR s/mEPSC幅度的减弱，但不能防止NMDAR s/mEPSC频率的减弱。另一方面，L-Phe显著降低了低浓度而非高浓度外源性激动剂激活的非NMDAR电流。NMDAR s/mEPSC频率的甘氨酸非依赖性减弱表明L-Phe导致突触前谷氨酸释放减少，而NMDAR和非NMDAR s/mEPSCs幅度的降低分别与L-Phe竞争NMDARs和非NMDARs的甘氨酸结合位点和谷氨酸结合位点一致。在经典PKU特征性条件下GluR活性显著降低这一发现表明，GluR功能受损对PKU相关智力发育迟缓有潜在重要影响，并为GluR功能受损的潜在生理后果提供了重要见解。
