Impact of the Kinetics of PP1 Activity and CaMKII Phosphorylation on the Number of Spike-Pair Presentations Leading to Transitions

<div><p>(A,B) Time course of PP1 activity (<i>k</i>₁₂ · <i>D</i>) (A) and phosphorylated CaMKII subunit concentration (B) during and after the STDP stimulation protocol for different values of <i>R</i> and <i>Q</i> (deterministic stimulation protocol, 60 spike pairs with Δ<i>t</i> = 15 ms). The buildup of PP1 activity (<i>k</i>₁₂ · <i>D</i>) is shown in (A) for <i>R</i> = 0.002 (blue line), <i>R</i> = 0.078 (green line) and <i>R</i> = 1 (red line; same curve as in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.0030221#pcbi-0030221-g005" target="_blank">Figure 5</a>C). <i>Q</i> has no impact on PP1 dynamics. The time course of phosphorylated CaMKII subunit concentration is depicted in (B) for five sets of values of <i>R</i> and <i>Q</i> (indicated close to the corresponding curves).</p><p>(C,D) The impact of the number of spike-pair presentations and of <i>R</i> and <i>Q</i> on the Δ<i>t</i> ranges for which transitions occur is depicted for STDP stimulation protocols evoking deterministic calcium transients. White region: no change (relative change in fraction of synapses in the UP state is 0); diagonally striped regions: down-to-up transitions (relative change in fraction of synapses in the UP state is +1); shaded regions: up-to-down transitions (relative change in fraction of synapses in the UP state is −1). In each case, down-to-up and up-to-down transition regions in the same color correspond to the same choice of <i>R</i> (C) or <i>Q</i> (D). All the cases in (C) use <i>Q</i> = 1 paired with: <i>R</i> = 0.078: red regions; <i>R</i> = 1: green regions; and <i>R</i> = 2: blue region. In (D): blue regions: <i>Q</i> = 0.83 (no down-to-up transitions); green regions: <i>Q</i> = 1; and red regions: <i>Q</i> = 1.67. <i>R</i> = 1 is utilized in all cases in (D), i.e., the green regions in (C) and (D) are identical (see <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.0030221#pcbi-0030221-t001" target="_blank">Table 1</a> for parameters, <i>k</i>_CaN = 18 1/s, Δ<i>Ca</i>_pre = 0.17 μM).</p>