Imaging rapid redistribution of sensory-evoked depolarization through existing cortical pathways after targeted stroke in mice

Abstract

Evidence suggests that recovery from stroke damage results from the production of new synaptic pathways within surviving brain regions over weeks. To address whether brain function might redistribute more rapidly through preexisting pathways, we examined patterns of sensory-evoked depolarization in mouse somatosensory cortex within hours after targeted stroke to a subset of the forelimb sensory map. Brain activity was mapped with voltage-sensitive dye imaging allowing millisecond time resolution over 9 mm(2) of brain. Before targeted stroke, we report rapid activation of the forelimb area within 10 ms of contralateral forelimb stimulation and more delayed activation of related areas of cortex such as the hindlimb sensory and motor cortices. After stroke to a subset of the forelimb somatosensory cortex map, function was lost in ischemic areas within the forelimb map center, but maintained in regions 200-500 microm blood flow deficits indicating the size of a perfused, but nonfunctional, penumbra. In many cases, stroke led to only partial loss of the forelimb map, indicating that a subset of a somatosensory domain can function on its own. Within the forelimb map spared by stroke, forelimb-stimulated responses became delayed in kinetics, and their center of activity shifted into adjacent hindlimb and posterior-lateral sensory areas. We conclude that the focus of forelimb-specific somatosensory cortex activity can be rapidly redistributed after ischemic damage. Given that redistribution occurs within an hour, the effect is likely to involve surviving accessory pathways and could potentially contribute to rapid behavioral compensation or direct future circuit rewiring.

Fig. 1.

Mapping sensory-evoked depolarization with VSD after stroke targeted to a subset of the somatosensory cortex. (A) Schematic showing C57 Bl6 mouse after craniotomy. The black lines at the left of the scheme represent moving shafts that stimulate the fore- and hindlimb. (B) Maps of VSD fluorescence changes indicating depolarization in the sensorimotor cortex in response to stimulation of forelimb (FL, green), hindlimb (HL, red), and the overlap of forelimb and hindlimb map (yellow). (C) Maps of VSD fluorescence as in B after photothrombotic stroke targeted to arterioles indicated by black arrows. Loss of blood flow (determined from the difference of speckle signals as shown in D and E) overlaid as blue shade. (D) Laser speckle image to determine blood flow (area in B). Darker tones indicate higher velocity blood flow. (E) Laser speckle image showing that blood flow was blocked at positions indicated by arrows in D. (Calibration bar indicates speckle contrast; Sp, standard deviation/mean.)

Fig. 2.

The delayed component of the response to forelimb stimulation is centered in the hindlimb sensory map after forelimb area-targeted stroke. (A) VSD fluorescence signal response in the sensorimotor cortex to tactile stimulation of the forelimb (Upper) or hindlimb (Lower) 1 h before stroke induction. (B) VSD fluorescence signal responses as in A, 80 min after targeted photothrombotic focal stroke in the cortical area that represents the anterior forelimb map. (Upper, second from left) The stroke focus, determined by speckle imaging, is outlined. The delayed component of the response to forelimb stimulation (≥100 ms) is centered in areas that respond with short latency to hindlimb stimulation. Shown are averaged results of 20–40 trials.

Fig. 3.

Relationship between the loss of forelimb area blood flow and the redistribution of forelimb-stimulated activity within the sensorimotor cortex. (A–D) profiles of the VSD fluorescence responses to forelimb (FL, green) and hindlimb stimulation (HL, red) and the difference of speckle images of blood flow before and after stroke (ΔSp, blue). Profiles are aligned across animals to the right, lateral border, of the response to hindlimb stimulation. The profiles were then normalized separately for fore- and hindlimb stimulation to the peak response amplitudes before stroke. (A) Profile of the immediate response (0–100 ms after stimulation) before, and (B) 80 min after, focal stroke in the forelimb area. (C) Profile of the delayed response (100–200 ms after stimulation) before stroke and (D) 80 min after stroke. (E) Demonstration of the position used to obtain profiles of the VSD signal and (F) of the laser speckle difference images. To quantify the map shift, blue vertical lines (connected to each other at the top) indicate the lateral (Right) point of the half-maximum VSD response to forelimb stimulation and the medial (Left) point of the half-maximum of the speckle difference profile in A and D. We used these half-maximum points to define the border of the maps. (G) Quantitative comparison of overlap and distance between the borders of the limb-stimulated VSD signal and speckle maps. See Table S1 for further description and values plotted in G. Average data from 8 mice is shown.

Fig. 4.

Stroke targeted to the center of the forelimb area spares delayed forelimb responses that spread to the hindlimb area. (A–D) Temporal plots of the VSD response to fore or hindlimb stimulation, each with an image of 1 example showing the areas from which we determined the time courses. (A and B) Plot of VSD response to forelimb stimulation before (A) and after (B) stroke. (C and D) Plots of VSD response to hindlimb stimulation before (C) and after (D) stroke. In the image in B and D, loss of blood flow as determined by change of laser speckle signal is indicated in blue to display the ischemic area (blue shaded). VSD signals were averaged for 0.25-mm² regions of interest and normalized, separately for fore- and hindlimb stimulation, to the peak maximum of response amplitudes before stroke. Averaged data from 9 mice is shown.

Fig. 5.

Schematic illustration of rapid sensory response redistribution mechanisms for somatosensory cortex function. (A) Control (before stroke). Sensory signals coming from the contralateral (left) front paw are first processed within the contralateral thalamus, routed to the primary somatosensory cortex (FL, green), and then to nearby cortical regions such as the sensory HL area. (B) After stroke in the FL area, thalamocortical connections carrying FL-derived signals are lost, which could enhance the importance of intracortical and thalamocortical connections that bring FL signals to areas that represent the HL in the sensorimotor cortex. This scheme may explain the stroke-related spatial redistribution of cortical activity that we observed (Fig. 3). The smaller number of intact thalamocortical connections, or the circuitous route the signals take, may explain why the responses to forelimb stimulation observed in off-target areas such as hindlimb cortex are slower after stroke (Fig. 4).