Developmental switch in spike timing-dependent plasticity at layers 4-2/3 in the rodent barrel cortex

Abstract

Sensory deprivation during the critical period induces long-lasting changes in cortical maps. In the rodent somatosensory cortex (S1), its precise initiation mechanism is not known, yet spike timing-dependent plasticity (STDP) at layer 4 (L4)-L2/3 synapses are thought to be crucial. Whisker stimulation causes "L4 followed by L2/3" cell firings, while acute single whisker deprivation suddenly reverses the sequential order in L4 and L2/3 neurons in the deprived column (Celikel et al., 2004). Reversed spike sequence then leads to long-term depression through an STDP mechanism (timing-dependent long-term depression), known as deprivation-induced suppression at L4-L2/3 synapses (Bender et al., 2006a), an important first step in the map reorganization. Here we show that STDP properties change dramatically on postnatal day 13-15 (P13-P15) in mice S1. Before P13, timing-dependent long-term potentiation (t-LTP) was predominantly induced regardless of spiking order. The induction of t-LTP required postsynaptic influx of Ca(2+), an activation of protein kinase A, but not calcium/calmodulin-dependent protein kinase II. Consistent with the strong bias toward t-LTP, whisker deprivation (all whiskers in Row "D") from P7-P12 failed to induce synaptic depression at L4-L2/3 synapses in the deprived column, but clear depression was seen if deprivation occurred after P14. Random activation of L4, L2/3 cells, as may occur in response to whisker stimulation before P13 during network formation, led to potentiation under the immature STDP rule, as predicted from the bias toward t-LTP regardless of spiking order. These findings describe a developmental switch in the STDP rule that may underlie the transition from synapse formation to circuit reorganization at L4-L2/3 synapses, both in distinct activity-dependent manners.

Figure 1.

t-LTP and t-LTD during the second and third postnatal weeks. A–D, Examples of pre-post (+8 ms) and post-pre (−8 ms) timing stimulation at the indicated ages. During the third postnatal week (P16, B), post-pre stimulation caused LTD (67.8 ± 16.4% of control), while the same stimulation caused LTP (D, 125.1 ± 30.2) during the second postnatal week. Sample recordings are displayed above. Input resistance (in MΩ) and membrane potentials (in mV) are displayed below. E, F, Mean experiment time course across all cells for pre-post (+8 ms, E) and post-pre (−8 ms, F) stimulation for ages > P15 (squares) and < P13 (circles) are shown. G, Summary of the effect of pre-post (+8 ms, filled bar) and post-pre stimulation (−8, open bar) during the second (<P13) and third (P15<) postnatal weeks. *p < 0.05 and **p < 0.01.

Figure 2.

Developmental switch in STDP occurring at the end of the second postnatal week. A, B, The graphs show the relationship between applied timing delay and the resultant changes in EPSP amplitude for the indicated age groups. In the bottom graphs, connected lines indicate changes in EPSP amplitudes in 10 different pairing delay groups (mean ± SEM). Ranges for delays were as follows: A, 0 to +12 ms, +25 ms, +26 to +50 ms, +51 to +100 ms, and 0 to −10 ms, −11 to −25 ms, −26 to −50 ms, −51 to −100 ms, −150 ms, and −200 to −250 ms. B, 0 to +12 ms, +25 ms, +26 to +50 ms, +51 to +100 ms, +150 to 200 ms, and 0 to −10 ms, −11 to −25 ms, −26 to −50 ms, −51 to −150 ms, and −200 to −250 ms. C, Histograms showing the fractions of cells exhibiting LTP (gray), LTD (black), and no change (white) elicited by pre-before-post (+timing) or post-before-pre (−timing) stimulation during the second (P8–P13) and third (P14–P20) postnatal weeks. LTP was defined as those EPSPs changed ≥ 5% (i.e., ≥ 105% of control), and LTD was defined as those that changed ≥ 5% (i.e., ≤95% of control). Histograms based on all data points in graphs A and B tested for various timing delays ranging from +100 ms to −250 ms. Numbers in parentheses within the bars indicate the number of observed EPSPs. There was a significant difference between the second and third postnatal weeks for −timing stimulation (χ² test, p < 0.001). D, Neither presynaptic nor postsynaptic activity alone induced LTP. Plasticity was not observed when only EPSPs (1 Hz, 90 s) or only APs (1 Hz, 90 s, by current injection) were elicited without pairing with postsynaptic APs or presynaptic stimulations, respectively. When EPSPs alone were elicited, EPSP magnitude was 98.1 ± 1.5% of control (p = 0.23, n = 15), and when APs alone were elicited, EPSP magnitude was 102.0 ± 4.2% of control (p = 0.64, n = 10), based on a two-tailed, one sample t test.

Figure 3.

Developmental transition from potentiation to depression after post-pre (−25 ms) timing stimulation at approximately the end of the second postnatal week. A, Resultant potentiation or depression versus postnatal day. B, Linear regression analysis obtained from the same dataset shown in A. Regression equation: Amplitude = 248.6 −11.1 × Postnatal Day, R² = 0.27 (p < 0.0001). The regression line crosses Amplitude = 100% at P13.4. The gray area around the regression line indicates the 95% confidence interval.

Figure 4.

Changes in the STDP curve every 2 d during the second and third postnatal week. Various timings of pre-post and post-pre stimulations were applied and the effect breakdown is illustrated for each age group. The graphs indicate that before P13, bilateral STDPs (potentiation for both positive and negative timing delays) were consistently induced, whereas after P14 classical bidirectional STDP (potentiation by positive timing delay and depression by negative timing delay) were induced. Connected lines represent mean ± SEM.

Figure 5.

After P15, t-LTP was CaMKII dependent, but was PKA dependent before P13. A, BAPTA, a Ca²⁺ chelator, in the recording pipettes (1 mm) blocked the induction of LTP by both pre-post (+8 ms) and post-pre (−25 ms) stimulation, as also summarized in B and C. B, C, d-AP5 (50 μm) applied to the bath blocked LTP induction by pre-post (B) and post-pre (C) stimulation. D, E, Rp-cAMP-S, a PKA inhibitor, as well as PKI 6–22, another PKA inhibitor, applied to postsynaptic cells through the recording pipette blocked the induction of LTP by pre-post stimulation only during the second postnatal week (D), which is summarized in the right columns of E. LTP by negative timing (−25 ms) at < P13 was also inhibited by PK I6–22 (C). In contrast, KN-93 and AIP, both CaMKII inhibitors, blocked LTP induction only during the third postnatal week (E). In all graphs, points represent mean ± SEM.

Figure 6.

Whisker deprivation fails to induce suppression of L4–L2/3 transmission until P12–P14. A, Left, Photograph illustrating the right-sided whiskers of a mouse. All whiskers in Row D were plucked, with the other whiskers left intact. Middle, Whisker plucking and electrophysiological recording schedule. Right, Photograph of a brain slice from the barrel cortex in the recording chamber, showing the barrels corresponding to whiskers A–E. A stimulating electrode was placed on L4 of column E and the tip of a glass micropipette was placed at L2/3 of column E for recordings. B1, B2, Two examples of cytochrome oxidase staining (left), FP recordings (middle), and stimulus–response curves constructed from the FPs (right). Slices were prepared from P13 (B1) and P18 (B2) mice. C1–C3, Summary of mean stimulus–response curves from all slices from (C1) P12–P14 (n = 14), (C2) P17–P20 (n = 24), and (C3) P18–P20 (n = 28).

Figure 7.

Random-delay timing stimulation causes potentiation before P13, while depression after P15. A, Example of random-delay stimulation. EPSPs were elicited by presynaptic stimulation (shown by red dots) and APs were elicited by postsynaptic current injection such that the delay between the AP and EPSP (AP–EPSP delay) was randomly distributed as also illustrated in B and C. B, AP–EPSP delays (+50 to −50 ms) were plotted for the 90 stimuli. C, Histogram of AP–EPSP delays (+50 to −50 ms) showing that delays were evenly distributed in this range. D, AP–EPSP delays (+50 to −50 ms) plotted for the 90 stimuli, in which AP–EPSP delays were randomly distributed in a Gaussian manner. E, Histogram of AP–EPSP delays (+50 to −50 ms) showing that the distribution is random in a Gaussian manner. F, The mean time course across all cells following random delay stimulation are shown for the second (red circle) and third (blue square) postnatal week, causing LTP and LTD, respectively. G, Developmental changes in the effect of random-delay stimulation. Random-delay stimulation led to LTP during the second postnatal week, but LTD during the third postnatal week, and the transition occurred at P13–P15. Asterisks represent significance, **p < 0.01.