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. 2018 Jul 5;12:182. doi: 10.3389/fncel.2018.00182

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Copyright © 2018 Cui, Perez and Venance.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Polarity of spike-timing dependent plasticity (STDP) varies upon number of pairings (N_pairings) and N_pairings = 10 induce spike-timing-dependent long-term potentiation (tLTP) in neocortical layer 2/3. (A) Characteristic voltage responses of a pyramidal cell to a series of 500 ms current pulses from −150 to +90 pA with current steps increasing by 30 pA (black traces) and to +60 pA above spike threshold (gray trace). (B) Cortically-evoked pyramidal cell EPSCs (averages of 15 traces; left panel). Distribution of latency SD, centered on 0.47 ms and fitted by a Gaussian function, indicate a monosynaptic cortical transmission because inferior to 1 ms (right panel). (C) Experimental design of STDP paired stimulations: two spikes evoked in pyramidal cell were paired with a single cortical layer 4 electrical stimulation; this pairing being repeated 10, 50 or 100 times at 1 Hz. Δt_STDP indicates the time between pre- and postsynaptic stimulations. Δt_STDP < 0 and Δt_STDP > 0 refer to post-pre and pre-post pairings, respectively. (D) 100 pairings induced bidirectional Hebbian plasticity at neocortical layer 2/3 synapses. (D1,D2) Summary of timing-dependent long-term depression (tLTD) and tLTP induced by 100 post-pre (n = 11 cells from 8 rats; Δt_STDP = −17 ± 2 ms) (D1) and 100 pre-post pairings (n = 9 cells from 6 rats; Δt_STDP = +17 ± 2 ms) (D2), respectively. (D3) Graph summarizing STDP (bidirectional Hebbian) occurrence for 100 pairings. (E) 50 pairings induced symmetric anti-Hebbian tLTD. (E1,E2) Summary of STDP experiments showing tLTD for both post-pre (n = 8 cells from 5 rats; Δt_STDP = −15 ± 1 ms) (E1) and pre-post (n = 9 cells from 5 rats; Δt_STDP = +14 ± 2 ms) (E2) pairings. (E3) Graph summarizing STDP (symmetric anti-Hebbian tLTD) occurrence for 50 pairings. (F) 10 pairings induced unidirectional Hebbian plasticity. (F1,F2) Summary of STDP experiments showing the absence of STDP and tLTP induced by 10 post-pre (n = 7 cells from 3 rats; Δt_STDP = −16 ± 2 ms) (F1) and 10 pre-post (n = 15 cells from 6 rats; Δt_STDP = +13 ± 1 ms) (F2) pairings, respectively. (F3) Graph summarizing STDP (unidirectional Hebbian) occurrence for 10 pairings. (G) Summary graph illustrating the cortical STDP expression and polarity for 100, 50 and 10 post-pre and pre-post pairings. (H) Relationship between the number of postsynaptic action potentials (per pairing) and the synaptic efficacy changes after 10 pre-post pairings (one way analyses of variance (ANOVA) test, p = 0.0027, F = 8.794. 1 spike vs. 2 spikes, p = 0.0892; 2 spikes vs. 3 spikes, p = 0.1545; 1 spike vs. 3 spikes, p = 0.0020). Representative traces are the average of 15 EPSCs during baseline (black traces) and 50 min after STDP protocol (gray traces). Error bars represent SD. *p < 0.05; **p < 0.01; ^***p < 0.005; ***p < 0.0001. ns, not significant.