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Unformatted text preview: The NMDA-to-AMPA Ratio at Synapses Onto Layer 2/3 Pyramidal Neurons Is Conserved Across Prefrontal and Visual Cortices Chaelon I. O. Myme, Ken Sugino, Gina G. Turrigiano, and Sacha B. Nelson Department of Biology, Volen Center for Complex Systems, Brandeis University, Waltham, Massachusetts 02454-9110 Submitted 27 January 2003; accepted in final form 28 March 2003 Myme, Chaelon I. O., Ken Sugino, Gina G. Turrigiano, and Sacha B. Nelson. The NMDA-to-AMPA ratio at synapses onto layer 2/3 pyramidal neurons is conserved across prefrontal and visual cortices. J Neurophysiol 90: 771779, 2003. First published April 2, 2003; 10.1152/jn.00070.2003. To better understand regulation of N-methyl- D-aspartate (NMDA) and a-amino-3-hydroxy-5-methyl-4-isox- azolepropionic acid (AMPA) receptor complements across the cortex, and to investigate NMDA receptor (NMDAR)-based models of per- sistent activity, we compared NMDA/AMPA ratios in prefrontal (PFC) and visual cortex (VC) in rat. Whole cell voltage-clamp re- sponses were recorded in brain slices from layer 2/3 pyramidal cells of the medial PFC and VC of rats aged p16 p21. Mixed miniature excitatory postsynaptic currents (mEPSCs) having AMPA receptor (AMPAR)- and NMDAR-mediated components were isolated in nominally 0 Mg 2 1 ACSF. Averaged mEPSCs were well-fit by double exponentials. No significant differences in the NMDA/AMPA ratio (PFC: 27 6 1%; VC: 28 6 3%), peak mEPSC amplitude (PFC: 19.1 6 1 pA; VC: 17.5 6 0.7 pA), NMDAR decay kinetics (PFC: 69 6 8 ms; VC: 67 6 6 ms), or degree of correlation between NMDAR- and AMPAR-mediated mEPSC components were found between the areas (PFC: n 5 27; VC: n 5 28). Recordings from older rats (p26 29) also showed no differences. EPSCs were evoked extra- cellularly in 2 mM Mg 2 1 at depolarized potentials; although the average NMDA/AMPA ratio was larger than that observed for mEPSCs, the ratio was similar in the two regions. In nominally 0 Mg 2 1 and in the presence of CNQX, spontaneous activation of NMDAR increased recording noise and produced a small tonic depo- larization which was similar in both areas. We conclude that this basic property of excitatory transmission is conserved across PFC and VC synapses and is therefore unlikely to contribute to differences in firing patterns observed in vivo in the two regions. I N T R O D U C T I O N An important issue in understanding cortical circuits is the regulation of the function and relative number of N-methyl- D- aspartate (NMDA) and a-amino-3-hydroxy-5-methyl-4-isox- azolepropionic acid (AMPA) receptors at excitatory synapses. Because these receptors have distinct kinetics (AMPA, t 5 27 ms; NMDA, t 5 50 100 ms) and because the NMDA receptor is important for Ca 2 1-dependent plasticity and learn- ing (Bliss and Collingridge 1993; Malenka and Nicoll 1999), the ratio of NMDA receptor (NMDAR) mediated to AMPA receptor (AMPAR) mediated currents at excitatory synapses is likely to be a key determinant of both the short-term integration...
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