Metaplasticity cannot be induced in juvenile <i>NesCreIrs2KO</i> mice.

<p><b>A</b>: A priming stimulus (10 Hz; small arrow) applied 20–30 min prior to TBS (large arrow) significantly inhibited subsequent LTP induction in juvenile control mice (+/+) for at least 50 min (average EPSP slope change: 10 Hz primed: 104±4%, N = 7, n = 7), when compared to un-primed LTP recorded 50 min following induction (average EPSP slope change: Control: 158±9%, N = 7, n = 7, p<0.0005). <b>B</b>: In juvenile <i>NesCreIrs2KO</i> mice (−/−), the same priming stimulus (10 Hz; small arrow) did not affect TBS-induced LTP (average EPSP slope change: 10 Hz primed; 135±5%, N = 7, n = 10), compared with the un-primed LTP measured 50 min post-TBS (average EPSP slope change: Control; 126±11%, N = 5, n = 7; p = 0.47). Insets in <b>A</b> and <b>B</b> show representative EPSP traces (average of 4 consecutive sweeps) taken prior to and 50 min following TBS, from individual experiments in control (<b>A</b>) and <i>NesCreIrs2KO</i> (<b>B</b>) slices to which 10 Hz priming stimuli had previously been applied. Scale bars: 0.4 mV; 10 ms. <b>C</b>: Enhancing NMDA receptor activity by lowering the concentration of extracellular Mg²⁺ to 1 mM and increasing the Ca²⁺ concentration to 3 mM, caused an attenuation of TBS-induced LTP in juvenile control mice (+/+) (average EPSP slope change: Low Mg²⁺/High Ca²⁺: 135±3%, N = 5, n = 6) compared with LTP recorded for 60 min under standard ionic conditions (average EPSP slope change: Control: 160±9%, N = 7, n = 7, p<0.05). <b>D</b>: The same ionic conditions to enhance NMDA receptor activity did not significantly alter the level of LTP obtained in <i>NesCreIrs2KO</i> mice (−/−) (average EPSP slope change: Low Mg²⁺/High Ca²⁺: 127±4, N = 4, n = 5) compared with that recorded in control conditions 60 min following induction (average EPSP slope change: Control: 128±11%, N = 5, n = 7). <b>E</b>: Metaplasticity induced in control mice (+/+) by altering extracellular divalent cation concentrations to enhance NMDA receptor activity yielded an LTP of similar magnitude to that observed in <i>NesCreIrs2KO</i> mice (−/−) under the same conditions. <b>F</b>: Representative EPSP traces (average of 4 consecutive sweeps) from individual experiments performed in low Mg²⁺/high Ca²⁺ conditions, taken prior to application of TBS and following 60 min of LTP in both control (+/+) and <i>NesCreIrs2KO</i> (−/−) mice.