Persistence of experience-induced homeostatic synaptic plasticity through adulthood in superficial layers of mouse visual cortex

Abstract

It is well established that sensory cortices of animals can be modified by sensory experience, especially during a brief early critical period in development. Theoretical analyses indicate that there are two synaptic plasticity mechanisms required: input-specific synaptic modifications and global homeostatic mechanisms to provide stability to neural networks. Experience-dependent homeostatic synaptic plasticity mechanisms have subsequently been demonstrated in the visual cortex of juvenile animals. Here, we report that experience-dependent homeostatic synaptic plasticity persists through adulthood in the superficial layers of the mouse visual cortex. We found that 2 d of visual deprivation in the form of dark rearing is necessary and sufficient to cause an increase in AMPA receptor-mediated miniature EPSC amplitude in layer 2/3 neurons. This increase was rapidly reversed by 1 d of light exposure. This reversible change in synaptic strength persisted in adult mice past the critical period for ocular dominance plasticity, which is reported to end at approximately 1 month of age in rodents. Interestingly, the mechanism of homeostatic synaptic modifications in 3-month-old mice differed from that in young mice (3 weeks old) in that the multiplicative nature of synaptic scaling is lost. Our results demonstrate that the superficial layers of adult mouse visual cortex retain the ability to undergo reversible experience-dependent homeostatic synaptic plasticity.

Figure 1.

The minimum duration of dark rearing necessary and sufficient to maximally increase AMPA receptor-mediated mEPSCs in layer 2/3 neurons is 2 d. a, Dark rearing P21 mice for 1 d did not significantly change mEPSC amplitude or frequency in layer 2/3 neurons. Top left, Comparison of average mEPSC amplitude of cells from P22 NR and 1 d DR (from P21 to P22) mice. Top middle, Average mEPSC traces from NR (left trace) and 1 d DR (right trace) mice. “Scaled” indicates the NR mEPSC average trace scaled (red trace) to match in amplitude that of the 1 d DR and superimposed on the 1 d DR average trace (black trace). Note that there was no difference in mEPSC kinetics. Top right, Comparison of average mEPSC frequency from cells from NR and 1 d DR mice. Bottom, Distribution histogram of mEPSC amplitudes recorded from NR (open black symbols) and 1 d DR mice (closed black symbols). There was no change in the distribution of mEPSC amplitudes recorded from 1 d DR and NR mice. The distribution histogram also clearly shows that there was no overlap between signal and noise and that there is no difference in noise levels between the two groups (open red symbols, NR noise; closed red symbols, dark-rearing noise). Far right, Representative traces of mEPSC recording obtained from layer 2/3 pyramidal cells of normally reared P22 mice (top two traces), and P21 mice dark reared for 1 d (bottom two traces). b, Dark rearing P21 mice for 2 d increased the amplitude of mEPSCs without affecting mEPSC frequency. Top left, Comparison of average mEPSC amplitude of cells. Average mEPSC amplitude from P23 NR mice was significantly different from mice dark reared for 2 d from P21 to P23. Top middle, Average mEPSC traces from NR (left trace) and 2 d DR mice (right trace). “Scaled” indicates the NR mEPSC average trace scaled (red trace) and superimposed on the DR average trace (black trace). Top right, Comparison of average mEPSC frequency from cells from NR and 1 d DR mice. Bottom, Distribution histogram of mEPSCs recorded from 2 d DR mice (closed black symbols) showed a higher percentage of mEPSCs with larger amplitudes (i.e., shift in distribution toward larger amplitudes) compared with NR (open black symbols). There was no difference between distribution of NR noise (open red symbols) and dark-rearing noise (closed red symbols). Far right, Representative traces of mEPSC recording obtained from layer 2/3 pyramidal cells of normal reared P23 mice (top two traces) and P21 mice dark reared for 2 d (bottom two traces). c, Dark rearing P21 mice for 4 d increased mEPSC amplitude without changes in frequency. Top left, Comparison of average mEPSC amplitude of cells from P25 NR mice to that from mice dark reared for 4 d from P21 to P25 (DR). Top middle, Average mEPSC traces from NR (left trace) and 4 d DR (right trace) mice. “Scaled” indicates the NR mEPSC average trace scaled (red trace) and superimposed on the 4 d DR average trace (black trace). Top right, No change in average mEPSC frequency from cells from NR and 4 d DR mice. Bottom, Distribution histogram of mEPSC amplitudes recorded from 4 d DR mice (closed black symbols) shows a higher percentage of mEPSCs with larger amplitudes compared with NR animals (open black symbols). There was no difference between distribution of NR noise (open red symbols) and DR noise (closed red symbols). Far right, Representative traces of mEPSC recording obtained from layer 2/3 pyramidal cells of normally reared P25 mice (top two traces) and P21 mice dark reared for 4 d (bottom two traces). *p < 0.04 with t test.

Figure 2.

Reversible modification of AMPA receptor-mediated mEPSC amplitude in juvenile mice. Increase in mEPSC amplitude by 2 d of dark rearing was reversed by re-exposing the DR mice to light for 1 d (D+L). Top, Comparison of average mEPSC amplitudes from NR, 2 d DR, and 2 d DR followed by 1 d re-exposure to light (D+L). Middle, Average mEPSC traces from NR (left trace), DR (right trace), and D+L (right trace) mice. “Scaled” indicates the average mEPSC traces of NR (red solid line trace) and D+L (red dotted line trace) scaled and superimposed on the DR average trace (black trace). Note no difference in mEPSC kinetics across the three groups. Below the average traces, representative traces of mEPSC recording obtained from layer 2/3 pyramidal cells of normally reared P23 mice (top two traces), P21 mice dark reared for 2 d (middle two traces), and 2 d dark-reared P23 mice re-exposed to light for 1 d (bottom two traces) are shown. Bottom, No change in average mEPSC frequency across NR, DR, and D+L groups. *p < 0.0001 with Fisher's PLSD post hoc test after one-way ANOVA.

Figure 3.

Reversible homeostatic synaptic modification in adult mice. a, Dark rearing P36 mice for 2 d until P38 (DR) significantly increased mEPSC amplitude compared with NR, whereas 1 d of re-exposure to light (D+L) reversed this increase. Top, Comparison of average mEPSC amplitude in NR, 2 d DR, and 2 d DR followed by 1 d re-exposure to (D+L) in P38 mice. Middle, Average mEPSC trace from NR (left trace), DR (middle trace), and D+L (right trace) groups. “Scaled” indicates that average mEPSC traces of NR (red solid trace) and D+L (red dotted trace) were scaled and superimposed on the average trace of DR (black trace). Below the average traces are the representative traces of mEPSC recording obtained from layer 2/3 pyramidal cell of normally reared P38 mice (top two traces), P36 mice dark reared for 2 d (middle two traces), and 2 d dark-reared P38 mice re-exposed to light for 1 d (bottom two traces). Bottom, No change in average mEPSC frequency across NR, DR, and D+L groups at P38. b, Dark rearing for 2 d from P95 to P97 significantly increased mEPSC amplitude compared with NR, and this was reversed by 1 d of re-exposure to light (D+L). Top, Comparison of average mEPSC amplitude between NR, 2 d DR, and 2 d DR followed by 1 d of re-exposure to light (D+L). Middle, Average mEPSC trace from NR (black trace), DR (black middle trace), and D+L (black right trace) mice. “Scaled” indicates the NR mEPSC average trace (red solid trace) and D+L mEPSC average trace (red dotted trace) scaled and superimposed on the DR average trace (black trace). Below the average mEPSC traces are representative traces of mEPSC recording obtained from layer 2/3 pyramidal cells of normally reared P97 mice (top two traces), P95 mice dark reared for 2 d (middle two traces), and 2 d dark-reared P97 mice re-exposed to light for 1 d (bottom two traces). Bottom, No change in average mEPSC frequency across NR, DR, and D+L groups. *p < 0.003 with Fisher's PLSD post hoc test after one-way ANOVA.

Figure 4.

Loss of “multiplicative synaptic scaling” in adult mice. a, Increase in mEPSC amplitude by 2 d of DR in P23 mice follows the rule of multiplicative scaling. Cumulative probability of mEPSC amplitudes (generated by compiling 100 consecutive mEPSCs from each neuron) in visual cortex neurons from P23 NR (gray solid line) and 2 d DR mice (black solid line). Superimposed on the graph is a cumulative probability of mEPSC amplitudes from NR that are multiplied by a factor (1.2) to match the average mEPSC amplitude of that from DR (NR_scaled, red dotted line). There was no statistically significant difference between cumulative probability of DR and NR_scaled (Kolmogorov–Smirnov test, p > 0.3). Inset, Distribution histogram plotting the percentage of mEPSCs against mEPSC amplitude of NR_scaled (red bars) and DR (black bars). Superimposed on the graph is a subtraction of the two distributions (blue solid line). Note that there was no observable shift in distribution. b, Decrease in mEPSC amplitude by re-exposing 2 d DR mice to 1 d of light (D+L) in P23 mice also occurs in a multiplicative manner. Cumulative probability of mEPSC amplitudes from 2 d dark-reared P23 mice (DR, black solid line) and 2 d of DR followed by 1 d of light exposure (D+L, gray solid line). Superimposed on the graph is a cumulative probability of mEPSC amplitudes from DR that are multiplied by a factor (0.7) to match the average mEPSC amplitude of D+L (DR_scaled, red dotted line). There was no statistically significant difference between the cumulative probability of D+L and that of DR_scaled (Kolmogorov–Smirnov test, p > 0.4). Inset, Distribution histogram plotting the percentage of mEPSCs against the mEPSC amplitude of DR_scaled (red bars) and D+L (black bars). Superimposed on the graph is a subtraction of the two distributions (blue solid line). Note no noticeable shift in distribution. c, Multiplicative scaling of mEPSC amplitude by 2 d of DR is preserved in P38 mice. Cumulative probability of mEPSC amplitudes in visual cortex neurons from P38 NR (gray solid line) and 2 d DR (black solid line) is shown. Superimposed on the graph is a cumulative probability of mEPSC amplitudes from NR that are multiplied by a factor (1.6) to match the average mEPSC amplitude of DR (NR_scaled, red dotted line). There was no statistically significant difference between the cumulative probability of DR and NR_scaled (Kolmogorov–Smirnov test, p > 0.1). Inset, Distribution histogram plotting the percentage of mEPSCs against mEPSC amplitude of NR_scaled (red bars) and DR (black bars). Superimposed on the graph is a subtraction of the two distributions (blue solid line). Note that there were a bit more fluctuations in the subtraction graph when compared with that seen at P23 (inset in a). d, Reversal of DR-induced increase in mEPSC amplitude by re-exposure to light (D+L) does not conform to the multiplicative scaling mechanism. Cumulative probability of mEPSC amplitudes in visual cortex neurons from P38 DR (black solid line) and dark-reared followed by 1 d of light re-exposure (D+L, gray solid line) is shown. Superimposed is a cumulative probability of mEPSC amplitudes from DR that are multiplied by a factor (0.6) to match the average mEPSC amplitude of D+L (DR_scaled, red dotted line). There was a statistically significant difference between the cumulative probability of D+L and DR_scaled (Kolmogorov–Smirnov test, p < 0.01), suggesting that the reduction in mEPSC amplitude by re-exposure to light does not follow the rules of multiplicative synaptic scaling. Inset, Distribution histogram plotting the percentage of mEPSCs against mEPSC amplitude of DR_scaled (red bars) and D+L (black bars). Superimposed on the graph is a subtraction of the two distributions (blue solid line). Note that there was a more noticeable difference in distribution as seen in the subtraction graph than in P38 NR_scaled to DR comparison (inset in c). e, Increase in mEPSC amplitude by 2 d of DR in P97 mice does not occur in a multiplicative manner. Cumulative probability of mEPSC amplitudes in visual cortex neurons from P97 NR (gray solid line) and 2 d DR mice (black solid line) are plotted. Superimposed on the graph is a cumulative probability of mEPSC amplitudes from NR mice that are multiplied by a factor (1.2) to match the average mEPSC amplitude of DR mice (NR_scaled, red dotted line). There was a statistically significant difference between the cumulative probability of DR and NR_scaled (Kolmogorov–Smirnov test, p < 0.001). This suggests that global multiplicative scaling mechanism does not operate at this age. Inset, Distribution histogram plotting the percentage of mEPSCs against mEPSC amplitude of NR_scaled (red bars) and DR (black bars). Note that there was an observable shift in distribution between NR_scaled and DR, which is shown as a large positive peak followed by a depression in the superimposed subtraction graph (blue solid line). f, Decrease in mEPSC amplitude by re-exposing DR mice to 1 d of light (D+L) at P97 also does not occur via a global multiplicative scaling mechanism. Cumulative probability of mEPSC amplitudes in visual cortex neurons from P97 DR mice (black solid line) and DR mice followed by 1 d of light exposure (D+L, gray solid line) are shown. Superimposed on the graph is a cumulative probability of mEPSC amplitudes from DR mice that are multiplied by a factor (0.7) to match the average mEPSC amplitude of D+L mice (DR_scaled, red dotted line). There was a statistically significant difference between the cumulative probability of DR and D+L_scaled (Kolmogorov–Smirnov test, p < 0.001). This suggests that a global multiplicative scaling down does not occur at this age. Inset, Distribution histogram plotting the percentage of mEPSCs against the mEPSC amplitude of DR_scaled (red bars) and D+L (black bars). Note that there was an observable shift in the distribution between NR_scaled and DR, which is shown as a large positive peak followed by a depression in the superimposed subtraction graph (blue solid line).

Figure 5.

Persistence of experience-induced reversible changes in mEPSC amplitude in the superficial layers of visual cortex. a, Comparison of average mEPSC amplitudes during development in NR (open squares), 2 d DR (black squares), and 2 d DR re-exposed to 1 d light (D+L: gray squares) mice. Black dashed line indicates eye opening, which occurs around P14 in mice. There was a developmental decrease in average mEPSC amplitude that occurred after eye opening (after P13), which plateaus around P21 (one-way ANOVA, F_(3,39) = 3.249, p < 0.04). There was no change in mEPSC amplitude with 2 d of DR before eye opening, whereas in all other ages recorded, we saw significant increases in mEPSC amplitude. This increase in amplitude was reversed by 1 d of light exposure. b, Comparison of average mEPSC frequency across age in NR (open squares), 2 d DR (black squares), and DR mice re-exposed to 1 d light (D+L; gray squares). There was no significant difference in mEPSC frequency across normal development or between NR, DR, and D+L mice regardless of age.