A C B electrical stimulation Whole-cell V-clamp cell body dendrite 65 ms 50 pA electrical stimulation Whole-cell V-clamp cell body dendrite 150 mM Glut. Iontophoresis “Slow” “Fast” B 65 ms 50 pA

Concentration profile of neurotransmitter delivery determines AMPAR activation: A. Representation of the experimental design for concurrently stimulating presynaptic release and iontophoretically examining the postsynaptic receptors. Iontophoresis and stimulating electrodes are brought to within 1 µm of an isolated synapse. Filled vertical and horizontal bars, used throughout the figures, represent the “Fast” (1 ms, 100 nA) and “Slow” (10 ms, 10 nA) iontophoretic application parameters. B. AMPA receptors are not activated by a “Slow” flux of glutamate. “Slow” pulses elicited NMDAR-only responses; while “Fast” pulses elicited AMPAR and NMDAR responses from the same site. Because AMPAR activation was sensitive to the instantaneous changes in the concentration of neurotransmitter, “AMPA-quiet” responses could be generated at synapses with functional AMPARs. C. “Silent” synapses contain functional AMPARs. EPSCAMPA-quiet responses, resulting from endogenous transmitter release, were evoked by presynaptic electrical stimulation at a 9 DIV synapse (open vertical bars). Evoked synaptic events were interleaved with alternating iontophoretic applications of neurotransmitter. Although EPSC’sAMPA-quiet were evoked presynaptically, AMPAR responses were clearly visible at this synapse during “Fast” iontophoretic pulses, indicating that endogenous “AMPA-quiet” synaptic events occurred at synapses containing functional AMPARs (n = 4).