Sensory experience restructures thalamocortical axons during adulthood

Abstract

The brain's capacity to rewire is thought to diminish with age. It is widely believed that development stabilizes the synapses from thalamus to cortex and that adult experience alters only synaptic connections between cortical neurons. Here we show that thalamocortical (TC) inputs themselves undergo massive plasticity in adults. We combined whole-cell recording from individual thalamocortical neurons in adult rats with a recently developed automatic tracing technique to reconstruct individual axonal trees. Whisker trimming substantially reduced thalamocortical axon length in barrel cortex but not the density of TC synapses along a fiber. Thus, sensory experience alters the total number of TC synapses. After trimming, sensory stimulation evoked more tightly time-locked responses among thalamorecipient layer 4 cortical neurons. These findings indicate that thalamocortical input itself remains plastic in adulthood, raising the possibility that the axons of other subcortical structures might also remain in flux throughout life.

Figure 1. Whisker Trimming during Adulthood Substantially Reduces the Length of Thalamocortical Axons

(A and B) Reconstructions of thalamocortical axons in control (A) and deprived (B) animals, respectively. Axons in deprived animals correspond to trimmed whiskers. Left, tangential view; right, radial view; lines, barrel borders; dots, branch points and end points. Insets: adult rats were sham trimmed (A) or had all whiskers but two trimmed off (B). (C) Columns targeted by control (black) and deprived (gray) axons. The position of symbols within any given column is arbitrary. (D) Distributions of total axon lengths within cortex. Lines represent means ± SD. (E) Distributions of branch points within cortex. Lines represent means ± SD.

Figure 2. Other Morphological Features Appear Unrelated to Differences in Axonal Length

(A) There is no significant relationship of the length of thalamocortical axon to the size of the barrel innervated, regardless of whether control (black) and deprived (gray) groups are analyzed separately or pooled (all p values > 0.5). (B) Control and deprived groups differ even after normalizing the lengths of axons to the areas of their respective barrels (p = 0.03). (C) Axonal length is relatively uniform across arcs (top) and rows (bottom), with deprivation having a relatively consistent effect at all locations. (D) Tangential section through the barrel field of a control (left) and a deprived (right) rat. (E and F) Trimming did not alter mean area occupied by barrels in tangential plane (E) or thickness in the radial dimension (F). Lines represent means ± SD. (G and H) Field span was calculated along a diagonal separately for L3/4 (G) and L5/6 (H) branches. (I and J) Distributions of field spans for L3/4 (I) and L5/6 (J) branches. (K) Axonal varicosities in a control (top) and a deprived (bottom) rat. (L) Distributions of inter-bouton distances along a randomly selected subset of axonal branches. Lines represent means ± SD for (B), (I), (J), and (L).

Figure 3. Axonal Remodeling Is Limited to Specific Branches

(A) Overlays of all axons for control (left) and deprived (right) animals. Red, axon inside the column as defined by borders of the targeted barrel. (B) Length of axon by depth from pia for each group. Dashed line, difference between groups. (C) Tangential views of overlaid axons. An example partial barrel field is overlaid purely to further illustrate scale. As shown in Figure 1C, the axons most densely innervated a variety of different barrels rather than the same barrel. (D and F) Axon lengths, total and by layer, inside (D) and outside (F) the targeted columns. (E and G) Same as (D) and (F) for branch points. Lines represent means ± SD.

Figure 4. Experience Alters the Synchrony, but Not Magnitude, of Sensory-Evoked Activity among Cortical Layer 4 Neurons

(A) Dual cell-attached recordings were made from the same barrel in control and deprived animals (left), and peristimulus time histograms for each population were plotted (middle). Evoked and spontaneous firing rates are plotted for individual cells (right). Box plots show medians and interquartile ranges. (B) Example rasters and cross-correlograms for a control pair (left) and deprived pair (right). (C) Mean firing rate normalized cross-correlograms for each group. The cross-correlogram of each pair was normalized by the geometric mean of the two cells’ firing rates. (D) Coherence functions averaged over all pairs. Inset: individual coherence values for each pair (mean over 4–20 Hz). (E) Examples of pairs with (left) and without (right) signs of shared synaptic inputs. Both pairs were recorded in trimmed rats. The raw cross-correlogram (CCG) measures total correlated activity of a pair of neurons (top). The shift corrector (middle), the recalculation of the correlogram after shifting one of the spike trains by a trial, measures stimulus-induced correlation. The corrected cross-correlogram (bottom) is their difference. A peak exceeding 3.3 standard deviations (α = 0.001) of the shift corrector (red lines) is taken as evidence of significant shared synaptic input (arrow). (F) Distributions of the strength of significant shared synaptic inputs for individual pairs (circles). Box plots show medians and interquartile ranges.