Figure 7. Recovery of synaptic AMPARs from desensitization is substantially accelerated in more mature synapses.

A, eEPSCs at V_h=−70 mV were evoked in a P7 synapse using a single stimulus (A1) and a 100 Hz train (A2) under control conditions (grey trace) and in the presence of 250 μm puff-applied kainate (black trace). KA responses without synaptic stimulation (A3, grey trace) were subtracted from raw traces (A3, black trace) to measure the blocked current fraction of I_KA attributable to desensitization of AMPARs by synaptically released glutamate (black traces in A1 and A2). B, similar experiment as illustrated in A in a P13 synapse using 100 Hz (B1) and 300 Hz (B2) trains. Control traces plotted in grey. Black traces were obtained after subtracting kainate responses without synaptic stimulation (B3). Note the faster time scale in B compared with A. Peak amplitudes of eEPSC recorded in the absence of KA are indicated by the arrowheads in (A1 and 2) and (B1 and 2). C, superposition of the difference currents for the cells shown in A and B to facilitate comparison. D, recovery of synaptic AMPARs from desensitization after short 100 Hz trains was significantly slower in immature synapses. The time course was well approximated with single exponentials (C, superimposed smooth curves) having mean time constants of 484 ± 90 ms (P5–7, n = 8) versus 21 ± 6 ms (P12–14, n = 4). E, in immature synapses, the recovery from desensitization is independent of the amount of released glutamate. The blocked fraction of I_KA increased from 159 ± 45 to 517 ± 119 pA when the number of stimuli was increased from 1 to 30 (100 Hz). This was not accompanied by a significant slow down of the recovery from desensitization (p_r= 0.071, n = 5). F, ratio of eEPSC amplitudes in the presences and absence of puff-applied KA. Note that in immature synapses, the first two eEPSCs were significantly smaller when evoked on top of I_KA.