The Journal of Neurophysiology Vol. 77 No. 5 May 1997, pp. 2416-2426
Copyright ©1997 The American Physiological Society

Rapid Kinetics and Inward Rectification of Miniature EPSCs in Layer I Neurons of Rat Neocortex

Fu-Ming Zhou and John J. Hablitz

Neurobiology Research Center and Department of Physiology and Biophysics, University of Alabama at Birmingham, Birmingham, Alabama 35294

Zhou, Fu-Ming and John J. Hablitz. Rapid kinetics and inward rectification of miniature EPSCs in layer I neurons of rat neocortex. J. Neurophysiol. 77: 2416-2426, 1997. With the use of the whole cell patch-clamp technique combined with visualization of neurons in brain slices, we studied the properties of miniature excitatory postsynaptic currents (mEPSCs) in rat neocortical layer I neurons. At holding potentials (50 to 70 mV) near the resting membrane potential (RMP), mEPSCs had amplitudes of 5-100 pA and were mediated mostly by -amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptors. Amplitude histograms were skewed toward large events. An N-methyl-D-aspartate (NMDA) component was revealed by depolarization to 30 mV or by the use of a Mg²⁺-free bathing solution. At RMP, averaged AMPA mEPSCs had a 10-90% rise time of ~0.3 ms (uncorrected for instrument filtering). The decay of averaged mEPSCs was best fit by double-exponential functions in most cases. The fast, dominating component had a decay time constant of ~1.2 ms and comprised ~80% of the total amplitude. A small slow component had a decay time constant of ~4 ms. Positive correlations were found between rise and decay times of both individual and averaged mEPSCs, indicative of dendritic filtering. Some large-amplitude mEPSCs and spontaneous EPSCs (recorded in the absence of tetrodotoxin) had slower kinetics, suggesting a role of asynchronous transmitter release in shaping EPSCs. The amplitudes of mEPSCs were much smaller at +60 mV than at 60 mV, indicating that synaptic AMPA-receptor-mediated currents were inwardly rectifying. These results suggest that neocortical layer I neurons receive both NMDA- and AMPA-receptor-mediated synaptic inputs. The rapid decay of EPSCs appears to be largely determined by AMPA receptor deactivation. The observed rectification of synaptic responses suggests that synaptic AMPA receptors in layer I neurons may lack GluR-2 subunits and may be Ca²⁺ permeable.

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