Size-Dependent Axonal Bouton Dynamics following Visual Deprivation In Vivo

Abstract

Persistent synapses are thought to underpin the storage of sensory experience, yet little is known about their structural plasticity in vivo. We investigated how persistent presynaptic structures respond to the loss of primary sensory input. Using in vivo two-photon (2P) imaging, we measured fluctuations in the size of excitatory axonal boutons in L2/3 of adult mouse visual cortex after monocular enucleation. The average size of boutons did not change after deprivation, but the range of bouton sizes was reduced. Large boutons decreased, and small boutons increased. Reduced bouton variance was accompanied by a reduced range of correlated calcium-mediated neural activity in L2/3 of awake animals. Network simulations predicted that size-dependent plasticity may promote conditions of greater bidirectional plasticity. These predictions were supported by electrophysiological measures of short- and long-term plasticity. We propose size-dependent dynamics facilitate cortical reorganization by maximizing the potential for bidirectional plasticity.

Keywords: GCaMP; LTP; axonal bouton; homeostasis; network; plasticity; population coupling; presynaptic; sensory deprivation; visual cortex.

Graphical abstract

Figure 1

Size-Dependent Bouton Dynamics in Deprived Cortex (A) Time course of 2P imaging paradigm and enucleation. Axon from L2/3 in V1m of Thy1-GFP mouse is shown. Arrows indicate lost (red), new (cyan), and persistent boutons (gray). Scale bar, 2 μm. (B) Intensity profiles from three persistent boutons above. Red, average taken at 3 depths. Scale bar, 2 μm. (C) Comparison of analysis program versus summed volume. (Inset) Bouton size over short (0.5 hr) versus longer periods (6 hr) is shown. (D) Turnover ratios for deprived and control cortex. (Inset) Percentage of disappearing (D) and new (N) boutons at 3 days is shown. (E) Histogram of bouton size in axonal backbone units (0 day). (F) Histogram of Log₁₀ bouton sizes from (E). (G) Average persistent bouton size per region in control and deprived cortices. (H) Variance of persistent axonal boutons per region in control and deprived cortices. (I and K) Log₁₀ bouton sizes in deprived (I, 16 days) and control (K, 16 days) cortices. (J and L) Difference in deprived (J) and control (L) Log₁₀ bouton sizes between 16 days and 0 day. Gray lines, 25^th and 75^th percentiles. (L, inset) Absolute difference in bouton size from Log₁₀ mean at baseline (0 day, black, open), in control cortex (16 days, black, filled) and deprived cortex (16 days, red, filled), is shown. (M) Normalized change in size of small, medium, and large boutons relative to baseline in control and deprived cortices. (N–P) Time course of size-dependent bouton dynamics in deprived (red) and control (black) cortices for large (N), small (O), and all boutons (P). Scale bar, 1 μm. (Q) Rate of change per day between imaging sessions for boutons at different sizes, relative to the mean size, in control and deprived cortices. (R) Absolute rate of change per day for boutons in deprived cortex between 0 and 3 days or 9 and 16 days. (S) Persistent bouton size (i) and the variance (ii) of bouton sizes in axonal backbone units in control and deprived cortices. For statistical comparisons and n values, see Table S1. NS, not significant; ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001. Error bars, mean and SEM.

Figure 2

Reduced Range of Correlated Network Activity (A) Awake behaving imaging of GCaMP5 at 0 day or 3 days with example calcium trace. Gray dashed line is activity threshold. Scale bars, 100%ΔF/F₀ and 50 s. (B and C) Mean (B) and variance (C) of activity in imaged region normalized to the baseline (0 day) for control and deprived cortices. Gray points show each region. (D) Example neurons highlighted by regions of interest (ROIs) with color corresponding to population coupling scale bar. GCaMP5 traces show population rate (gray trace), and color-coded traces are according to coupling scores (scores next to traces). Scale bars, individual cells: 50%ΔF/F₀ and 50 s; average population rate trace: 10%ΔF/F₀ and 50 s; region: 20 μm. (E and F) Mean (E) and variance (F) of population coupling scores per region normalized to the baseline (0 day) for control and deprived cortices. Gray points show each imaged region. (G and H) Normalized population coupling scores from deprived mice at 0 day (G) and 3 days (H). (I) Difference between deprived 3 days (from H) and deprived 0 day (from G) normalized population coupling distributions. Gray dashed lines are 25^th and 75^th percentiles. (J and K) Absolute normalized population coupling distributions in deprived (J) and control (K) cortices at 0 day and 3 days. (L) Normalized population coupling scores at 0 day and 3 days for neurons in control and deprived cortices. (M) Normalized population coupling scores for neurons in control and deprived cortices at 3 days that were either (left) weakly coupled or (right) strongly coupled to population activity when measured at 0 day. For statistical comparisons and n values, see Table S2. ^∗p < 0.05 and ^∗∗∗p < 0.001. Error bars, mean and SEM.

Figure 3

Network Simulation of Strength-Dependent Dynamics (A) Control simulation. Addition and subtraction symbols show weight strengths between neurons. (B) (i) Feedforward input weights into the neurons in the recurrent control network. Neurons 2 and 3 receive similar inputs, as do neurons 4–6, neurons 7 and 8, and neurons 9 and 10. Strong weights are yellow and weak weights are blue. (ii) Connection strengths from the presynaptic (x axis) to the postsynaptic neurons (y axis) within the control network. Neurons with correlated inputs are (yellow) bidirectionally connected, other neurons are (light blue) weakly connected, but not (dark blue) self-connected. (C) Deprived simulation with reduced range of correlated input. (D and E) The strength of weak weights (D) increases and the strength of strong weights (E) decreases (average of every 20^th time point). (Inset) SD of all weights measured at time points 1 and 700 is shown. (F) Schematic of plasticity simulation. Deprived model is presented with bursts of correlated input and the change in weights measured. (G and H) Deprived networks show greater plasticity when given bursts of positively (G) or negatively (H) correlated input. (I and J) Control network where weights (1–4) are established with highly correlated input (I) and then tested using novel input (J, i.e., a different set of neurons receives correlated inputs); weights 1 and 4 strengthen and weights 2 and 3 weaken. (K and L) Schematic of deprived network (K) and testing with novel input patterns (L). In the cartoon, synaptic weights 1 and 4 strengthen and synaptic weights 2 and 3 weaken after new input. (M) Deprived model learns novel input faster than control. For statistical comparisons and n values, see Table S3. ^∗p < 0.05 and ^∗∗∗p < 0.001. Error bars, mean and SEM.

Figure 4

Enhanced Bidirectional Plasticity in Deprived Cortex (A) Schematic of deprived mouse and slice electrophysiology in V1m L2/3. Timeline for the characterization of PPR, LTP/LTD induction, and PPR re-testing is shown. (B and C) PPR in control (B) and deprived (C) cortices. (D) Absolute difference of PPRs from population mean and (inset) average PPR in control and deprived cortices. (E and F) LTP (E) and LTD (F) in deprived and control cortices. (Insets) Examples of excitatory postsynaptic potentials (EPSPs) from deprived cortex during baseline (E and F, black) and following either (red) LTP (E) or LTD (F) induction. Scale bars, 25 ms, 5 mV. (G and H) Normalized change in PPR relative to baseline in control (G) and deprived (H) cortices after (left) LTP or (right) LTD. (I) Change in PPR after induction of (left) LTP or (right) LTD in deprived and control cortex. Values are normalized to baseline. For statistical comparisons and n values, see Table S4. NS, not significant; ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001. Error bars, mean and SEM.