Arai A, Silberg J, Kessler M, Lynch G
Center for the Neurobiology of Learning and Memory, University of California, Irvine 92717-3800, USA.
Neuroscience. 1995 Jun;66(4):815-27. doi: 10.1016/0306-4522(94)00616-d.
The binding affinity of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors for [3H]AMPA is increased 10-30-fold by the chaotropic anion thiocyanate. The present experiments tested if thiocyanate alters AMPA receptor mediated current fluxes and if any such effects are reflected in the waveform of synaptic responses. Currents were measured after a step application of glutamate or AMPA to patches excised from pyramidal cells of hippocampal slice cultures. Application of 1 mM AMPA produced responses with an average peak amplitude of 86 pA at -50 mV and a 10-90% rise time of 1.7 +/- 0.1 ms; the responses desensitized to a steady-state level below 10% of the peak current with a time constant of 11.1 +/- 0.7 ms. Glutamate in presence of D-amino-phosphonopentanoate produced similar responses which were inhibited by 6-cyano-7-nitro-quinoxaline-dione and enhanced by aniracetam or cyclothiazide and thus are characteristic for AMPA receptors. Thiocyanate accelerated the decay of AMPA responses two-fold and reduced the peak current by 30-50% with an EC50 of 3.2 mM which is comparable to its EC50 for enhancing binding. Effects on the desensitization of glutamate induced responses were much smaller and only evident at the highest thiocyanate concentration; no effect was seen on response amplitude. Binding and physiological effects can be adequately explained by assuming that thiocyanate enhances conversion from the sensitive to the desensitized state of the receptor and reduces ligand dissociation from the desensitized state. Synaptic responses were measured in disinhibited hippocampal slices. Perfusion with 20 mM sodium thiocyanate increased the slope of the field excitatory postsynaptic potential by 44.9 +/- 4.2% and reduced its decay time by 10.4 +/- 4.3%. The former effect appears to result at least in part from an increase in transmitter release since it was accompanied by a decrease in paired-pulse facilitation and was reduced in magnitude after enhancing transmitter release. The decrease in the decay time constant points to an effect of thiocyanate on AMPA receptors in situ which is similar to that seen in excised patches. These results demonstrate that an increase in binding affinity may be indicative of reduced rather than enhanced current flow through AMPA receptors. In addition, the results provide further evidence that the kinetics of the AMPA receptor channel contribute significantly to at least the decay phase of fast excitatory synaptic responses.
离液序列高的阴离子硫氰酸盐可使α-氨基-3-羟基-5-甲基-4-异恶唑丙酸（AMPA）受体对[3H]AMPA的结合亲和力提高10至30倍。本实验检测硫氰酸盐是否会改变AMPA受体介导的电流通量，以及此类效应是否会在突触反应波形中体现。在对海马切片培养的锥体细胞切除的膜片施加谷氨酸或AMPA阶跃后测量电流。施加1 mM AMPA在-50 mV时产生的反应平均峰值幅度为86 pA，10-90%上升时间为1.7±0.1 ms；反应以11.1±0.7 ms的时间常数脱敏至峰值电流的10%以下的稳态水平。在D-氨基膦酰戊酸存在下的谷氨酸产生类似反应，这些反应被6-氰基-7-硝基喹喔啉-二酮抑制，并被茴拉西坦或环噻嗪增强，因此是AMPA受体的特征。硫氰酸盐使AMPA反应的衰减加快两倍，并使峰值电流降低30-50%，EC50为3.2 mM，这与其增强结合的EC50相当。对谷氨酸诱导反应脱敏的影响要小得多，且仅在最高硫氰酸盐浓度下明显；对反应幅度无影响。通过假设硫氰酸盐增强受体从敏感状态到脱敏状态的转变并减少配体从脱敏状态的解离，可以充分解释结合和生理效应。在去抑制的海马切片中测量突触反应。用20 mM硫氰酸钠灌注使场兴奋性突触后电位的斜率增加44.9±4.2%，并使其衰减时间减少10.4±4.3%。前一种效应似乎至少部分是由于递质释放增加所致，因为它伴随着双脉冲易化的减少，并且在增强递质释放后幅度减小。衰减时间常数的降低表明硫氰酸盐对原位AMPA受体有影响，这与在切除的膜片中观察到的类似。这些结果表明，结合亲和力的增加可能表明通过AMPA受体的电流减少而非增强。此外，结果提供了进一步的证据，即AMPA受体通道的动力学至少对快速兴奋性突触反应的衰减阶段有显著贡献。
