NMDA RECEPTOR-MEDIATED COMPONENTS OF MINIATURE EXCITATORY SYNAPTIC CURRENTS IN DEVELOPING RAT NEOCORTEX

1. In vitro slices of frontal neocortex were prepared from rat pups at various ages after birth: postnatal days (PN) 3-5, 6-8, and 9-14. Usi ng whole-cell patch-clamp techniques, both spontaneous and evoked exci tatory postsynaptic currents (EPSCs) were recorded from voltage-clampe d layer II-III pyramidal neurons. Developmental changes in EPSCs were examined. 2. Four properties of miniature EPSCs (mEPSCs) were studied: rise time, amplitude, decay time constant (tau), and frequency. These parameters were not tetrodotoxin sensitive and did not exhibit signif icant developmental changes during the first two postnatal weeks. 3. m EPSCs occurred approximately every 2-3 s and had peak amplitudes of 25 -30 pA. Within each age group, certain parameters of mEPSCs were volta ge dependent. mEPSC rise time and decay tau were significantly increas ed at depolarized potentials (-30 to -45 mV) relative to hyperpolarize d (-75 to -90 mV) or resting membrane potential (RMP) (-60 to -70 mV). 4. At threshold stimulation intensity, EPSCs were evoked in an ''all- or-none'' manner. The amplitude and decay tau of evoked unitary EPSCs and mEPSCs were not significantly different. As stimulation intensity was increased, a late EPSC component appeared that was not seen in mEP SCs. At suprathreshold stimulus intensities, EPSC duration was signifi cantly longer in PN 3-5 than in PN 9-14 neurons. 5. The N-methyl-D-asp artate (NMDA) receptor antagonist D(-)2-amino-5-phosphonovaleric acid (APV, 10 muM) significantly decreased mEPSC decay tau and frequency on ly at depolarized membrane potentials. Likewise, EPSCs were depressed by APV to a greater extent at depolarized potentials, and the depressi on was mainly of the late component. mEPSCs recorded at RMP were block ed by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3- dione (5 muM). 6. Removal of extracellular Mg2+ reversibly increased t he decay tau of mEPSCs at RMP but not at depolarized membrane potentia ls. The decay tau and duration of evoked EPSCs were also increased in zero Mg2+. These effects were reversible with application of APV. All effects of zero Mg2+ on mEPSCs and EPSCs were observed as early as PN 3-5. 7. These results indicate that the basic kinetic properties of mE PSCs are present by PN 3-5 and do not change significantly over the fi rst two postnatal weeks. NMDA receptor activation contributes to mEPSC s and sensitivity to Mg2+ appears as early as PN 3-5. Unitary EPSCs re semble mEPSCs, but a late NMDA receptor-mediated component appears in EPSCs as stimulus intensity is increased. The developmental decrease i n the late EPSC component as well as its apparent increase in voltage sensitivity cannot be attributed to changes in characteristics of mini ature synaptic currents.