Bilateral activity-dependent interactions in the developing corticospinal system

Abstract

Activity-dependent competition between the corticospinal (CS) systems in each hemisphere drives postnatal development of motor skills and stable CS tract connections with contralateral spinal motor circuits. Unilateral restriction of motor cortex (M1) activity during an early postnatal critical period impairs contralateral visually guided movements later in development and in maturity. Silenced M1 develops aberrant connections with the contralateral spinal cord whereas the initially active M1, in the other hemisphere, develops bilateral connections. In this study, we determined whether the aberrant pattern of CS tract terminations and motor impairments produced by early postnatal M1 activity restriction could be abrogated by reducing activity-dependent synaptic competition from the initially active M1 later in development. We first inactivated M1 unilaterally between postnatal weeks 5-7. We next inactivated M1 on the other side from weeks 7-11 (alternate inactivation), to reduce the competitive advantage that this side may have over the initially inactivated side. Alternate inactivation redirected aberrant contralateral CS tract terminations from the initially silenced M1 to their normal spinal territories and reduced the density of aberrant ipsilateral terminations from the initially active side. Normal movement endpoint control during visually guided locomotion was fully restored. This reorganization of CS terminals reveals an unsuspected late plasticity after the critical period for establishing the pattern of CS terminations in the spinal cord. Our findings show that robust bilateral interactions between the developing CS systems on each side are important for achieving balance between contralateral and ipsilateral CS tract connections and visuomotor control.

Figure 1.

Alternate inactivation promotes redistribution of CS terminations to intermediate and ventral motor laminas after previous M1 inactivation. Color-coded average density maps are shown for age-matched controls (left column; n = 4), animals with unilateral inactivation (center; n = 4), and animals with alternate inactivation (right; n = 6). Axon density (terminals and preterminal axons) maps for are shown in A and bouton density maps in B. The contours in A graph the densest regions (yellow; maximal 40%) from the individual animals comprising the averages. Scale bar, 500 μm; color scale: A, 0–216 μm axon/μm² area; B, 0–3.5 × 10⁻² boutons/μm.

Figure 2.

Unilateral inactivation (inact.) shifts the distribution of CS terminations dorsally in the gray matter and alternate inactivation restores terminations to the intermediate and ventral laminas. Dorsoventral distributions of axon (left) and bouton (right) density from the dorsal to ventral margins of the contralateral gray mater are shown. Solid lines plot the mean pixel values at each normalized depth; light shading plots ±SEM. Data from controls (ctrl.) are shown in green, unilateral inactivation in red, and alternate inactivation in blue. To normalize for differences in the size of the gray matter across animals, all graphs were interpolated to 1000 points between the dorsal and ventral margins of the gray matter. Scale bars: axons, 500 μm axon/μm² area; varicosities, 5 × 10⁻³ boutons/μm.

Figure 3.

A, B, The proportion of axonal labeling (light gray) and boutons (dark gray) within laminas 2–4 (A) and laminas 5–7 (B) after alternate inactivation are similar to controls. Data from controls, unilateral inactivation, and alternate inactivation are shown. Asterisks show significant differences for axon labeling and bouton density between the unilateral inactivation group and either the controls (p < 0.01) or the alternate inactivation (p < 0.01) groups. Error bars indicate SEM.

Figure 4.

CS terminations have similar distribution immediately after the alternate inactivation (PW 11) and 4 weeks later (PW 15). Dorsoventral distributions of axon (left) and bouton (right) density from the dorsal to ventral margins of the contralateral gray mater are shown for animals examined at PW 11 (n = 2; black) and PW 15/16 (n = 4; red). Solid lines and shading are similar to Figure 2. Scale bars: axons, 500 μm axon/μm² area; varicosities, 5 × 10⁻³ boutons/μm.

Figure 5.

Alternate inactivation reduces ipsilateral CS tract terminations. A–C, Averaged regional distribution of ipsilateral CS axon terminations for controls (n = 4; A, replotted from Fig. 1A), animals receiving unilateral inactivation only (B; n = 3) and alternate inactivation (C; n = 3). Conventions are similar to those in Figure 1A. We used either LY (n = 2) or BDA (n = 1) to anterogradely label projections from the initially active (right) side, depending on which tracer we did not use for the initially inactive side. The contours in B and C plot the region of maximal contralateral label (inner; 40%; yellow) and an estimate of total contralateral label (outer; 80%; light blue). All maps are plotted using the same color scale. Scale bar, 500 μm.

Figure 6.

Alternate inactivation restores end point control during visually guided locomotion. A, Forward distance of the forepaw contralateral to the first inactivation by week of representative cats after unilateral (filled circles) or alternate inactivation (open circles). Forward distance remains higher than controls (dotted line, average, n = 4) even 2 months after unilateral inactivation. The dashed line represents the average forward distance for unilaterally inactivated cats (n = 5). After alternate inactivation, forward distance returns to control levels 1 month after the second inactivation. Average control forward distance is plotted as a dotted line and unilateral inactivation as a dashed line. B, Plot showing the lack of recovery in animals with unilateral inactivation (inact.) and recovery after alternate inactivation. Unilateral inactivation resulted in persistent overstepping (dotted lines; n = 4; data from a previous study for animals tested longer than 16 weeks) (Friel et al., 2007). After alternate inactivation (solid lines), all animals recovered in <15 weeks.