Spike timing-dependent synaptic depression in the in vivo barrel cortex of the rat

Abstract

Spike timing-dependent plasticity (STDP) is a computationally powerful form of plasticity in which synapses are strengthened or weakened according to the temporal order and precise millisecond-scale delay between presynaptic and postsynaptic spiking activity. STDP is readily observed in vitro, but evidence for STDP in vivo is scarce. Here, we studied spike timing-dependent synaptic depression in single putative pyramidal neurons of the rat primary somatosensory cortex (S1) in vivo, using two techniques. First, we recorded extracellularly from layer 2/3 (L2/3) and L5 neurons, and paired spontaneous action potentials (postsynaptic spikes) with subsequent subthreshold deflection of one whisker (to drive presynaptic afferents to the recorded neuron) to produce "post-leading-pre" spike pairings at known delays. Short delay pairings (<17 ms) resulted in a significant decrease of the extracellular spiking response specific to the paired whisker, consistent with spike timing-dependent synaptic depression. Second, in whole-cell recordings from neurons in L2/3, we paired postsynaptic spikes elicited by direct-current injection with subthreshold whisker deflection to drive presynaptic afferents to the recorded neuron at precise temporal delays. Post-leading-pre pairing (<33 ms delay) decreased the slope and amplitude of the PSP evoked by the paired whisker, whereas "pre-leading-post" delays failed to produce depression, and sometimes produced potentiation of whisker-evoked PSPs. These results demonstrate that spike timing-dependent synaptic depression occurs in S1 in vivo, and is therefore a plausible plasticity mechanism in the sensory cortex.

Figure 1.

Experimental protocol for backward pairing. A, Peristimulus time histogram of responses to decreasing amplitudes of stimulation. The amplitude of the subthreshold stimulation used during pairing was 17.5 μm in this example. The bottom graph shows the neuronal response integrated over 60 ms (mean spontaneous activity has been subtracted) as a function of the deflection amplitude. B, Average membrane potential and corresponding PSTH in response to three deflection amplitudes. Bottom graph represents, for nine adjacent whiskers, the PSP amplitude as a function of the deflection amplitude at threshold. Whisker deflections >150 μm systematically produced large PSPs. C, During control and test, 16 trains of stimulation of the PW and of the AW were presented in alternation in a pseudorandom sequence. The trains of stimulation contained five deflections presented at 0.5 Hz. The input waveform for each deflection was a 10 ms rostrocaudal movement followed by a 10 ms plateau and a ramp back to the rest position of the whisker. D, During pairing, a spontaneously emitted action potential triggered a subthreshold deflection of the AW with a fixed delay (0, 10, 20, or 30 ms). One pairing period contained 400 associations between an action potential and a stimulation of the AW.

Figure 2.

Differential depression of response by backward pairing. A, PSTHs of the response of a L5–L6 single unit to control stimulations (before pairing) and to test stimulations (after pairing), as well as the normalized difference of responses for the paired whisker (whisker C1, left column) and the unpaired whisker (whisker C2, right column). Histograms show the response level before and after pairing. *p < 0.05. B, Single unit recorded in L2/3. C3 was chosen as the paired whisker (left column) and whisker C4 was chosen as the unpaired whisker (right column). Histograms show a significant (**p < 0.01) differential depression of the response to the paired whisker but not to the unpaired whisker.

Figure 3.

Specific depression for short delays of pairing. The response modification index for the unpaired whisker (ΔR_PW), the paired whisker (ΔR_AW), and the differential effect (ΔR_diff) is plotted against the delay (Δt) of the pairing for L2/3 (circles) and L5–L6 (diamonds) cells with deflection amplitude at an activation threshold larger than 150 μm. The delay has been corrected to take into account the latency of the cortical response. The histogram shows the mean response modification indices (± SEM) for different delay windows. The depression is significant (t test, *p < 0.05; **p < 0.01) only for pairings at a short delay window (−17 ms < Δt ≤ −7 ms). The response to the unpaired whisker is not significantly modified at any of the tested delay time windows.

Figure 4.

Analysis of response depression as a function of the delay from stimulus onset. A, To assess the effect of pairing at different delays from stimulus onset, the responses to whisker stimulation were integrated on 20-ms-long time windows, the delay of which varied from 0 to 50 ms from stimulus onset. B, The figure presents the mean response modifications for the paired (ΔR_AW, close triangles) and the unpaired (ΔR_PW, open triangles) whiskers at short-delay pairings (−17 ms < Δt ≤ −7 ms) as a function of the delay of the integration windows from stimulus onset, as defined in A. *p < 0.05 between ΔR_AW and ΔR_PW. Because of the phasic response of the neurons, the number of neurons taken into account for each integration window varies.

Figure 5.

Experimental protocol for whole-cell induction of STDP in vivo. A, Examples of regular spiking (putative pyramidal neuron, top), intrinsic bursting (putative pyramidal neuron, middle), and fast spiking (putative inhibitory interneuron, bottom) firing patterns. B, Subthreshold receptive field for one neuron. Each trace shows a wPSP elicited by the indicated whisker. The PW for this neuron was D2. C, Distribution of PSP latencies for the population of recorded neurons (n = 36) in response to stimulation of the PW (left) and the AW (right). D, STDP protocol for whole-cell experiments. During the control and the test periods, the PW and the AW were deflected alternately. During pairing, PW deflection was paired with current injection to elicit postsynaptic spikes.

Figure 6.

Induction of spike timing-dependent synaptic depression in a representative neuron. A, Firing pattern in response to 500 ms current injection. B, Pairing protocol in which current-evoked spikes preceded the PW-evoked PSP. Histogram, Distribution of times of all evoked spikes, relative to PSP onset (black arrowhead), during pairing. Mean Δt for this cell was −26 ms (white arrowhead). C, Results of pairing. The amplitude of each wPSP during the experiment (small points) and the average of each 10 trials (thick line) for the paired PW (top) and unpaired AW (bottom) whiskers are shown. Right, wPSP averaged over 50 trials during baseline (thin line) and after pairing (thick line).

Figure 7.

Induction of spike timing-dependent synaptic potentiation in a representative neuron. A, Regular spiking pattern in response to 500 ms current injection. B, Pairing protocol in which current-evoked postsynaptic spikes followed the PW-evoked PSP. Mean Δt for this cell was +15 ms (white arrowhead). C, Results of pairing on PW (paired) and AW (unpaired) wPSPs. Conventions are as in Figure 6.

Figure 8.

Learning rule for spike timing-dependent synaptic depression in L2/3 in vivo. Pairing-induced changes in initial slope (left) and amplitude (right) of the wPSP as a function of delay between postsynaptic spikes and wPSP onset. Magnitude of plasticity is quantified using ΔPSPAmplitude and ΔPSPSlope indices (see Materials and Methods). Each point is one pairing. Circles, RS cells. Squares, IB and unclassified cells. Filled symbols show statistically significant pairings (Kolmogorov–Smirnoff test, p < 0.01). Gray region, Mean ± SD for pseudopairing experiments in which no spikes were evoked during the pairing period. Pairings with Δt <0 were fitted with an exponential function. Bottom plots show results for RS cells only. The exponential fit computed on all pairings is reproduced on the RS plots (dashed line).

Figure 9.

Mean time course of pairing-induced plasticity. The top plot shows the number of experiments contributing to each time point in the mean time course (data from a given pairing protocol was limited to the sweep range over which input resistance, series resistance, and Vm remained stable) (see Materials and Methods). A1, Pairing-evoked changes in wPSP slope, for pairings with Δt between −7 and −33 ms. Open gray symbols, All experiments (n = 23, each pairing was counted as one experiment). Filled symbols, Experiments that resulted in significant plasticity (i.e., the same experiments denoted by filled symbols in Fig. 8; n = 12). Bars show SEM across experiments. A2, Pairing-evoked changes in wPSP amplitude for pairings with Δt between −7 and −33 ms. Symbols as in A1. B1, Mean time course for wPSP slope for pairings with Δt >0 ms. Open symbols, All experiments (n = 7). Filled symbols, Experiments that resulted in significant plasticity (n = 4). B2, Mean time course for wPSP amplitude for pairings with Δt >0 ms. Symbols are as in B1.