Ian R. Winship

In Vivo Voltage-Sensitive Dye Imaging in Adult Mice Reveals That Somatosensory Maps Lost to Stroke Are Replaced over Weeks by New Structural and Functional Circuits with Prolonged Modes of Activation within Both the Peri-Infarct Zone and Distant Sites

After brain damage such as stroke, topographically organized sensory and motor cortical representations remap onto adjacent surviving tissues. It is conceivable that cortical remapping is accomplished by changes in the temporal precision of sensory processing and regional connectivity in the cortex. To understand how the adult cortex remaps and processes sensory signals during stroke recovery, we performed in vivo imaging of sensory-evoked changes in membrane potential, as well as multiphoton imaging of dendrite structure and tract tracing. In control mice, forelimb stimulation evoked a brief depolarization in forelimb cortex that quickly propagated to, and dissipated within, adjacent motor/hindlimb areas (<100 ms). One week after forelimb cortex stroke, the cortex was virtually unresponsive to tactile forelimb stimulation. After 8 weeks recovery, forelimb-evoked depolarizations reemerged with a characteristic pattern in which responses began within surviving portions of forelimb cortex (<20 ms after stimulation) and then spread horizontally into neighboring peri-infarct motor/hindlimb areas in which depolarization persisted 300–400% longer than controls. These uncharacteristically prolonged responses were not limited to the remapped peri-infarct zone and included distant posteromedial retrosplenial cortex, millimeters from the stroke. Structurally, the remapped peri-infarct area selectively exhibited high levels of dendritic spine turnover, shared more connections with retrosplenial cortex and striatum, and lost inputs from lateral somatosensory cortical regions. Our findings demonstrate that sensory remapping during stroke recovery is accompanied by the development of prolonged sensory responses and new structural circuits in both the peri-infarct zone as well as more distant sites.

Rapid astrocyte calcium signals correlate with neuronal activity and onset of the hemodynamic response in vivo

Elevation of intracellular Ca2+ in astrocytes can influence cerebral microcirculation and modulate synaptic transmission. Recently, in vivo imaging studies identified delayed, sensory-driven Ca2+ oscillations in cortical astrocytes; however, the long latencies of these Ca2+ signals raises questions in regards to their suitability for a role in short-latency modulation of cerebral microcirculation or rapid astrocyte-to-neuron communication. Here, using in vivo two-photon Ca2+ imaging, we demonstrate that ∼5% of sulforhodamine 101-labeled astrocytes in the hindlimb area of the mouse primary somatosensory cortex exhibit short-latency (peak amplitude ∼0.5 s after stimulus onset), contralateral hindlimb-selective sensory-evoked Ca2+ signals that operate on a time scale similar to neuronal activity and correlate with the onset of the hemodynamic response as measured by intrinsic signal imaging. The kinetics of astrocyte Ca2+ transients were similar in rise and decay times to postsynaptic neuronal transients, but decayed more slowly than neuropil Ca2+ transients that presumably reflect presynaptic transients. These in vivo findings suggest that astrocytes can respond to sensory activity in a selective manner and process information on a subsecond time scale, enabling them to potentially form an active partnership with neurons for rapid regulation of microvascular tone and neuron–astrocyte network properties.

In Vivo Calcium Imaging Reveals Functional Rewiring of Single Somatosensory Neurons after Stroke

Functional mapping and microstimulation studies suggest that recovery after stroke damage can be attributed to surviving brain regions taking on the functional roles of lost tissues. Although this model is well supported by data, it is not clear how activity in single neurons is altered in relation to cortical functional maps. It is conceivable that individual surviving neurons could adopt new roles at the expense of their usual function. Alternatively, neurons that contribute to recovery may take on multiple functions and exhibit a wider repertoire of neuronal processing. In vivo two-photon calcium imaging was used in adult mice within reorganized forelimb and hindlimb somatosensory functional maps to determine how the response properties of individual neurons and glia were altered during recovery from ischemic damage over a period of 2–8 weeks. Single-cell calcium imaging revealed that the limb selectivity of individual neurons was altered during recovery from ischemia, such that neurons normally selective for a single contralateral limb processed information from multiple limbs. Altered limb selectivity was most prominent in border regions between stroke-altered forelimb and hindlimb macroscopic map representations, and peaked 1 month after the targeted insult. Two months after stroke, individual neurons near the center of reorganized functional areas became more selective for a preferred limb. These previously unreported forms of plasticity indicate that in adult animals, seemingly hardwired cortical neurons first adopt wider functional roles as they develop strategies to compensate for loss of specific sensory modalities after forms of brain damage such as stroke.

Laser Speckle Contrast Imaging of Collateral Blood Flow during Acute Ischemic Stroke

Collateral vasculature may provide an alternative route for blood flow to reach the ischemic tissue and partially maintain oxygen and nutrient support during ischemic stroke. However, much about the dynamics of stroke-induced collateralization remains unknown. In this study, we used laser speckle contrast imaging to map dynamic changes in collateral blood flow after middle cerebral artery occlusion in rats. We identified extensive anastomatic connections between the anterior and middle cerebral arteries that develop after vessel occlusion and persist for 24 hours. Augmenting blood flow through these persistent yet dynamic anastomatic connections may be an important but relatively unexplored avenue in stroke therapy.

A Dissociation of Motion and Spatial-Pattern Vision in the Avian Telencephalon: Implications for the Evolution of “Visual Streams”

The ectostriatum is a large visual structure in the avian telencephalon. Part of the tectofugal pathway, the ectostriatum receives a large ascending thalamic input from the nucleus rotundus, the homolog of the mammalian pulvinar complex. We investigated the effects of bilateral lesions of the ectostriatum in pigeons on visual motion and spatial-pattern perception tasks. To test motion perception, we measured performance on a task requiring detection of coherently moving random dots embedded in dynamic noise. To test spatial-pattern perception, we measured performance on the detection of a square wave grating embedded in static noise. A double dissociation was revealed. Pigeons with lesions to the caudal ectostriatum showed a performance deficit on the motion task but not the grating task. In contrast, pigeons with lesions to the rostral ectostriatum showed a performance deficit on the grating task but not the motion task. Thus, in the avian telencephalon, there is a separation of visual motion and spatial-pattern perception as there is in the mammalian telencephalon. However, this separation of function is in the targets of the tectofugal pathway in pigeons rather than in the thalamofugal pathway as described in mammals. The implications of these findings with respect to the evolution of the visual system are discussed. Specifically, we suggest that the principle of parallel visual streams originated in the tectofugal pathway rather than the thalamofugal pathway.

Metabotropic NMDA receptor signaling couples Src family kinases to pannexin-1 during excitotoxicity

Overactivation of neuronal N-methyl-D-aspartate receptors (NMDARs) causes excitotoxicity and is necessary for neuronal death. In the classical view, these ligand-gated Ca2+-permeable ionotropic receptors require co-agonists and membrane depolarization for activation. We report that NMDARs signal during ligand binding without activation of their ion conduction pore. Pharmacological pore block with MK-801, physiological pore block with Mg2+ or a Ca2+-impermeable NMDAR variant prevented NMDAR currents, but did not block excitotoxic dendritic blebbing and secondary currents induced by exogenous NMDA. NMDARs, Src kinase and Panx1 form a signaling complex, and activation of Panx1 required phosphorylation at Y308. Disruption of this NMDAR-Src-Panx1 signaling complex in vitro or in vivo by administration of an interfering peptide either before or 2 h after ischemia or stroke was neuroprotective. Our observations provide insights into a new signaling modality of NMDARs that has broad-reaching implications for brain physiology and pathology.

Remapping the Somatosensory Cortex after Stroke: Insight from Imaging the Synapse to Network:

Together, thousands of neurons with similar function make up topographically oriented sensory cortex maps that represent contralateral body parts. Although this is an accepted model for the adult cortex, whether these same rules hold after stroke-induced damage is unclear. After stroke, sensory representations damaged by stroke remap onto nearby surviving neurons. Here, we review the process of sensory remapping after stroke at multiple levels ranging from the initial damage to synapses, to their rewiring and function in intact sensory circuits. We introduce a new approach using in vivo 2-photon calcium imaging to determine how the response properties of individual somatosensory cortex neurons are altered during remapping. One month after forelimb-area stroke, normally highly limb-selective neurons in surviving peri-infarct areas exhibit remarkable flexibility and begin to process sensory stimuli from multiple limbs as remapping proceeds. Two months after stroke, neurons within remapped regions develop a stronger response preference. Thus, remapping is initiated by surviving neurons adopting new roles in addition to their usual function. Later in recovery, these remapped forelimb-responsive neurons become more selective, but their new topographical representation may encroach on map territories of neurons that process sensory stimuli from other body parts. Neurons responding to multiple limbs may reflect a transitory phase in the progression from their involvement in one sensorimotor function to a new function that replaces processing lost due to stroke.

Zonal Organization of the Vestibulocerebellum in Pigeons (Columba livia): I. Climbing Fiber Input to the Flocculus

Previous studies in pigeons have shown that the neurons in the medial column of the inferior olive respond best to patterns of optic flow resulting from self‐rotation. With respect to the axis of rotation, there are two functional groups: rVA neurons prefer rotation about the vertical axis, whereas rH45 neurons respond best to rotation about an horizontal axis oriented at 45 degrees ipsilateral azimuth. The rVA and rH45 neurons are located in the caudal and rostral margins of the medial column, respectively. These olivary neurons project as climbing fibers to the contralateral flocculus. In this study, injections of anterograde tracers into the medial column were used to investigate the zonal organization of the climbing fiber input to the flocculus of pigeons. Iontophoretic injections of either cholera toxin subunit‐B or biotinylated dextrin amine were made into the medial column of the inferior olive at locations responsive to rVA or rH45 rotational optic flow. Anterogradely labeled climbing fibers in the flocculus showed a clear zonal organization. There were four parasagittal bands spanning both folia IXcd and X consisting of two rVA zones interdigitated with two rH45 zones. These findings are compared with the zonal organization of the flocculus in mammalian species. J. Comp. Neurol. 456:127–139, 2003.

Topographic Organization of Inferior Olive Cells Projecting to Translational Zones in the Vestibulocerebellum of Pigeons

In the nodulus and ventral uvula of pigeons, there are four parasagittal zones containing Purkinje cells responsive to patterns of optic flow that results from self‐translation along a particular axis in three‐dimensional space. By using a three‐axis system to describe the preferred direction of translational optic flow, where +X, +Y, and +Z represent rightward, upward, and forward self‐motion, respectively, the four cell types are: +Y, −Y, −X−Z, and −X+Z (assuming recording from the left side of the head). The −X−Z zone is the most medial, followed in sequence by the −X+Z, −Y zone, and the +Y zones. In this study, we injected the retrograde tracer cholera toxin subunit B into each of the four translational zones to determine the origin of the climbing fiber inputs in the inferior olive. Retrograde labeling in the inferior olive was found in the ventrolateral margin of the medial column from injections into all four translational zones; however, there was a clear functional topography. Retrograde labeling from −Y zone injections was found most rostrally in the medial column, whereas retrogradely labeled cells from −X−Z zone injections were found most caudally in the medial column. Labeling from +Y and −X+Z zone injections were found between the labeling from −Y zones and −X−Z zones, with +Y labeling located slightly caudal to −X+Z labeling. J. Comp. Neurol. 419:87–95, 2000.

Augmenting collateral blood flow during ischemic stroke via transient aortic occlusion

Collateral circulation provides an alternative route for blood flow to reach ischemic tissue during a stroke. Blood flow through the cerebral collaterals is a critical predictor of clinical prognosis after stroke and response to recanalization, but data on collateral dynamics and collateral therapeutics are lacking. Here, we investigate the efficacy of a novel approach to collateral blood flow augmentation to increase collateral circulation by optically recording blood flow in leptomeningeal collaterals in a clinically relevant model of ischemic stroke. Using high-resolution laser speckle contrast imaging (LSCI) during thromboembolic middle cerebral artery occlusion (MCAo), we demonstrate that transiently diverting blood flow from peripheral circulation towards the brain via intra-aortic catheter and balloon induces persistent increases in blood flow through anastomoses between the anterior and middle cerebral arteries. Increased collateral flow restores blood flow in the distal middle cerebral artery segments to baseline levels during aortic occlusion and persists for over 1 hour after removal of the aortic balloon. Given the importance of collateral circulation in predicting stroke outcome and response to treatment, and the potential of collateral flow augmentation as an adjuvant or stand-alone therapy for acute ischemic stroke, this data provide support for further development and translation of collateral therapeutics including transient aortic occlusion.