Wired together, change together: Spike timing modifies transmission in converging assemblies

The precise timing of neuronal spikes may lead to changes in synaptic connectivity and is thought to be crucial for learning and memory. However, the effect of spike timing on neuronal connectivity in the intact brain remains unknown. Using closed-loop optogenetic stimulation in CA1 of freely moving mice, we generated unique spike patterns between presynaptic pyramidal cells (PYRs) and postsynaptic parvalbumin (PV)–immunoreactive cells. The stimulation led to spike transmission changes that occurred together across all presynaptic PYRs connected to the same postsynaptic PV cell. The precise timing of all presynaptic and postsynaptic cell spikes affected transmission changes. These findings reveal an unexpected plasticity mechanism, in which the spike timing of an entire cell assembly has a more substantial impact on effective connectivity than that of individual cell pairs.


This PDF file includes:
Figs. S1 to S8 Tables S1 to S3

Fig. S1 .
Fig. S1.PYR spikes trigger closed-loop light stimulation with millisecond timescale resolution.(A) Photos of optical fibers attached to the four silicon probes which were implanted in PV::ChR2 mice and employed for closed-loop stimulation experiments (table S2).(B) An example of a PSTH from a trigger PYR with high CLE.The CLE is calculated as the area under the peak (green) in the -5≤t<0 ms ROI, and the baseline activity is determined by hollow median filtering (5 ms halfwidth) of the PSTH (black line).***: p<0.001,Poisson test.(C) Distribution of CLE values for all PYRs recorded during Stimulation sessions (black) and for the PYRs participating in Stimulation pairs as shown in table S2.Here, in D, and in F, ***: p<0.001,U-test.(D) Distributions of CLE values for the Stimulation PYRs.PYRs which exhibit a consistent PSTH peak within the closed-loop ROI (p<0.001Poisson test) are defined as "trigger" PYRs.(E) The number of trigger PYRs in every session.The color represents the CLE of each PYR.Sessions are sorted chronologically.(F) Distribution of PSTH peak time lags for trigger (n=198) and non-trigger PYRs (n=491).SDs are 8 ms for trigger PYRs and larger (14 ms) for non-trigger PYRs (p<0.001,permutation test).

Fig. S2 .
Fig. S2.Rate changes during closed-loop stimulation.(A) Gain PSTHs of 689 PYRs during closedloop stimulation, with zero time lag indicating light onset (as in Fig. 3A).Every gain PSTH is scaled by the baseline firing rate, estimated for every unit during a 15 ms period starting 30 ms before light onset and ending 15 ms before light onset.(B) Same as A, for the 122 PV cells.(C) Mean light-induced firing rate gain of all 689 PYRs (purple) and 122 PV cells (green).Error bands, SEM.(D) Surprise, defined as the base-10 logarithm of the p-value estimating the probability that the median gain of PYRs and PVs exceeds unity for each 1 ms PSTH bin (Wilcoxon's signed-rank test comparing to unity gain null).The dashed line corresponds to the Bonferroni-corrected chance level (0.05/101).(E) Distribution of the light-induced firing rate gain for the n=689/122 PYR/PV cells during light stimulation.Here and in F, n.s./***: p>0.05/p<0.001,Wilcoxon's signed-rank test compared to a unity gain null.(F) Distribution of the post-light gain, computed as the ratio between the mean firing rates during the 10 ms post-light and the baseline firing rates.

Fig. S3 .
Fig. S3.STGs are similar for pairs in the same converging assembly.(A) STG of every CA1 PYRinterneurons pair vs. mean STG of all peers.Peers, PYR-interneuron pairs that share the same postsynaptic interneuron, forming a CA.n=1261 pairs in 139 CAs recorded from hippocampal region CA1 of three mice(34).Here and in B-H, n.s./*/**/***: p>0.05/p<0.05/p<0.01/p<0.001,permutation test.(B) STGs of neocortical PYR-interneuron pairs vs. mean peers STG.n=1099 pairs in 100 CAs recorded from six mice(20).(C) The STGBefore of every pair vs. the mean STGBefore of all non-peer pairs.Here and in D, data are shown for the stimulated 1014/1026 PYR-PV pairs as in Fig. 2C.(D) Same as C for the STGAfter.(E) The STGBefore of every pair vs. the mean STGBefore of all peers.Here and in F-H, data are shown for the 374/388 PYR-PV Control pairs which were part of a CA with more than one presynaptic PYR, and did not undergo light stimulation during the "Experience" epoch.(F) Same as E for the STGAfter.(G) STG change vs. the mean STG change of all peers.(H) Same as G for non-peer pairs.

Fig. S4 .
Fig. S4.PYRs which belong to the same converging assembly are synchronized.(A) Mean synchronous firing between a single PYR and either peer PYRs or non-peer PYRs.n=623/689 PYRs which have both peers and non-peers PYR recorded on a different shank.Here and in B, ***: p<0.001,Wilcoxon's paired test.(B) Same as A for the fraction of synchronized peers and synchronized non-peers of each PYR.(C) Distribution of the fraction of pairs that exhibit an STG increase in every CA.(D) Distribution of the firing rate change for PYRs (n=689) and PV cells (n=122).The firing rate change is defined as the base-2 logarithm of the ratio between the firing rate during the After epoch and the firing rate during the Before epoch.Median [IQR] values are -0.06 [-0.61 0.47] for the PYRs, and -0.06 [-0.32 0.18] for the PV cells.n.s./*: p>0.05/p<0.05,Wilcoxon's test comparing to a zero-change null.(E-F) STG changes vs. firing rate changes of the presynaptic or the postsynaptic cell.

Fig. S5 .
Fig. S5.Changes in PV synchronous firing are smaller than changes in PYR-PV transmission.(A-B) CCHs between two simultaneously recorded PV cells during the Before (A) and the After (B) epochs.(C) Overlaid conditional rate histograms for the no-light epochs, obtained by removing the baseline activity.(D) Synchrony of PV pairs during the After epoch vs. the Before epoch.Inset, Distribution of the number of simultaneously recorded of PV pairs.PV pairs recorded on the same shank are not included.(E) Distribution of the PYR-PV STG change (blue) as in Fig. 1L, and the PV-PV Sync change computed from the data shown in D.Here and in F, n.s./***: p>0.05/p<0.001,U-test.(F) Magnitude (absolute value) of STG changes (blue) as in Fig. 1M, and magnitude of PV-PV Sync.