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To regain sensorimotor functions after stroke, surviving neural circuits must reorganize and form new connections. Although the thalamus is critical for processing and relaying sensory information to the cortex, little is known about how stroke affects the structure and function of these connections, or whether a therapeutic approach targeting these circuits can improve recovery. Here we reveal with in vivo calcium imaging that stroke in somatosensory cortex dampens the excitability of surviving thalamocortical circuits. Given this deficit, we hypothesized that chronic transcranial window optogenetic stimulation of thalamocortical axons could facilitate recovery. Using two-photon imaging, we show that optogenetic stimulation promotes the formation of new and stable thalamocortical synaptic boutons, without impacting axon branch dynamics. Stimulation also enhances the recovery of somatosensory cortical circuit function and forepaw sensorimotor abilities. These results demonstrate that an optogenetic approach can rewire thalamocortical circuits and restore function in the damaged brain. Stroke recovery requires circuit reorganization and therapeutic efforts have focused on rewiring cortical circuits after stroke, but what about thalamic inputs? Here, the authors examine how thalamocortical axons are affected by stroke and use optogenetic stimulation to promote recovery.

Cortical capillaries are prone to obstruction, which over time, could have a major impact on brain angioarchitecture and function. The mechanisms that govern the removal of these obstructions and what long-term fate awaits obstructed capillaries, remains a mystery. We estimate that ~0.12% of mouse cortical capillaries are obstructed each day (lasting >20 min), preferentially in superficial layers and lower order branches. Tracking natural or microsphere-induced obstructions revealed that 75–80% of capillaries recanalized within 24 hr. Remarkably, 30% of all obstructed capillaries were pruned by 21 days, including some that had regained flow. Pruning involved regression of endothelial cells, which was not compensated for by sprouting. Using this information, we predicted capillary loss with aging that closely matched experimental estimates. Genetic knockdown or inhibition of VEGF-R2 signaling was a critical factor in promoting capillary recanalization and minimizing subsequent pruning. Our studies reveal the incidence, mechanism and long-term outcome of capillary obstructions which can also explain age-related capillary rarefaction.

Passage of molecules across the central nervous system is tightly regulated by the blood-brain barrier (BBB) and blood-spinal cord barrier (BSCB), which restrict entry of many substances, including opioid medications. Here, we examined the effects of opioid withdrawal on BBB and BSCB integrity by measuring extravascular levels of peripherally injected dyes – Evans Blue (high molecular weight) and sodium fluorescein (NaFl, low molecular weight) – in the brain and spinal cord. In morphine-dependent male and female mice, repeated naloxone challenge induced robust withdrawal behaviors concomitant with region specific dye extravasation. In a fixed dose morphine paradigm, Evans Blue extravasation was highest within the cortex, hippocampus, cerebellum, and brainstem (pons and medulla) in male mice, and in the hypothalamus in female mice. By contrast, NaFl extravasation remained unchanged in both sexes. In an escalating dose morphine paradigm, Evans Blue extravasation was most prominent in the brainstem (pons and medulla) of both sexes, as well as in the lumbar of male mice and cervical spinal cord of female mice. NaFl extravasation in these regions was unchanged in male but reduced in female mice. These findings suggest that repeated opioid withdrawal alters permeability of the BBB and BSCB in discrete regions of the brain and spinal cord.

Stroke profoundly disrupts cortical excitability which impedes recovery, but how it affects the function of specific inhibitory interneurons, or subpopulations therein, is poorly understood. Interneurons expressing vasoactive intestinal peptide (VIP) represent an intriguing stroke target because they can regulate cortical excitability through disinhibition. Here we chemogenetically augmented VIP interneuron excitability in a murine model of photothrombotic stroke and show that it enhances somatosensory responses and improves recovery of paw function. Using longitudinal calcium imaging, we discovered that stroke primarily disrupts the fidelity (fraction of responsive trials) and predictability of sensory responses within a subset of highly active VIP neurons. Partial recovery of responses occurred largely within these active neurons and was not accompanied by the recruitment of minimally active neurons. Importantly, chemogenetic stimulation preserved sensory response fidelity and predictability in highly active neurons. These findings provide a new depth of understanding into how stroke and prospective therapies (chemogenetics), can influence subpopulations of inhibitory interneurons. Stroke profoundly disrupts cortical excitability which impedes recovery, but how stroke affects inhibitory interneurons is poorly understood. Here, the authors show that functional impairments after stroke are associated with the disruption of a highly active subpopulation of interneurons expressing vasoactive intestinal peptide (VIP), which could be ameliorated by chemogenetic stimulation.

The manner in which fully mature peri-infarct cortical dendritic arbors remodel after stroke, and thus may possibly contribute to stroke-induced changes in cortical receptive fields, is unknown. In this study, we used longitudinal in vivo two-photon imaging to investigate the extent to which brain ischemia can trigger dendritic remodeling of pyramidal neurons in the adult mouse somatosensory cortex, and to determine the nature by which remodeling proceeds over time and space. Before the induction of stroke, dendritic arbors were relatively stable over several weeks. However, after stroke, apical dendritic arbor remodeling increased significantly (dendritic tip growth and retraction), particularly within the first 2 weeks after stroke. Despite a threefold increase in structural remodeling, the net length of arbors did not change significantly over time because dendrite extensions away from the stroke were balanced by the shortening of tips near the infarct. Therefore, fully mature cortical pyramidal neurons retain the capacity for extensive structural plasticity and remodel in a balanced and branch-specific manner.

Vascular pathology is associated with cognitive impairment in diseases such as type 1 diabetes; however, how capillary flow is affected and the underlying mechanisms remain elusive. Here we show that capillaries in the diabetic mouse brain in both sexes are prone to stalling, with blocks consisting primarily of erythrocytes in branches off ascending venules. Screening for circulating inflammatory cytokines revealed persistently high levels of interleukin-10 (IL-10) in diabetic mice. Contrary to expectation, stimulating IL-10 signalling increased capillary obstruction, whereas inhibiting IL-10 receptors with neutralizing antibodies or endothelial specific knockdown in diabetic mice reversed these impairments. Chronic treatment of diabetic mice with IL-10 receptor neutralizing antibodies improved cerebral blood flow, increased capillary flux and diameter, downregulated haemostasis and cell adhesion-related gene expression, and reversed cognitive deficits. These data suggest that IL-10 signalling has an unexpected pathogenic role in cerebral microcirculatory defects and cognitive impairment associated with type 1 diabetes. Interleukin-10 promotes the formation of microcirculatory defects in the brain associated with cognitive impairment in a mouse model of type 1 diabetes.

Recovery from stroke is rarely complete as humans and experimental animals typically show lingering deficits in sensory function. One explanation for limited recovery could be that rewired cortical networks do not process sensory stimuli with the same temporal precision as they normally would. To examine how well peri-infarct and more distant cortical networks process successive vibrotactile stimulations of the affected forepaw (a measure of temporal fidelity), we imaged cortical depolarizations with millisecond temporal resolution using voltage-sensitive dyes. In control mice, paired forepaw stimulations (ranging from 50 to 200 milliseconds apart) induced temporally distinct depolarizations in primary forelimb somatosensory (FLS1) cortex, and to a lesser extent in secondary FLS (FLS2) cortex. For mice imaged 3 months after stroke, the first forepaw stimulus reliably evoked a strong depolarization in the surviving region of FLS1 and FLS2 cortex. However, depolarizations to subsequent forepaw stimuli were significantly reduced or completely absent (for stimuli ≤100 milliseconds apart) in the FLS1 cortex, whereas FLS2 responses were relatively unaffected. Our data reveal that stroke induces long-lasting impairments in how well the rewired FLS1 cortex processes temporal aspects of sensory stimuli. Future therapies directed at enhancing the temporal fidelity of cortical circuits may be necessary for achieving full recovery of sensory functions.

Although microglia possess the unique ability to migrate, whether mobility is evident in all microglia, is sex dependent, and what molecular mechanisms drive this, is poorly understood in the adult brain. Using longitudinal in vivo imaging of sparsely labelled microglia, we find a relatively small population of microglia are mobile under normal conditions. Following injury (microbleed), the population of mobile microglia increased in a sex-dependent manner, with male microglia migrating significantly greater distances towards the microbleed relative to their female counterparts. To understand the signaling pathways involved, we interrogated the role of interferon gamma (IFNγ). Our data show that in male mice, stimulating microglia with IFNγ or inducible microglial specific knockdown of IFNγ receptor 1 stimulates or inhibits migration, respectively, whereas female microglia were generally unaffected. These findings highlight the diversity of microglia migratory responses to injury, its dependence on sex and the signaling mechanisms that modulate this behavior.

Cerebral angiogenesis is well appreciated in development and after injury, but the extent to which it occurs across cortical regions in normal adult mice and underlying mechanisms, is incompletely understood. Using in vivo imaging, we show that angiogenesis in anterior-medial cortical regions (retrosplenial and sensorimotor cortex), was exceptionally rare. By contrast, angiogenesis was significantly elevated in posterior-lateral regions such as visual cortex, primarily within 200µm of the cortical surface. There were no regional differences in vessel pruning or sex effects except for the length and depth of new capillaries. To understand mechanisms, we surveyed gene expression and found Notch related genes were enriched in ultra-stable retrosplenial versus visual cortex. Using endothelial specific knockdown of Notch1, cerebral angiogenesis was significantly increased along with genes implicated in angiogenesis (Apln, Angpt2, Cdkn1a). Our study shows that angiogenesis is regionally dependent and manipulations of Notch1 signaling could unlock the angiogenic potential of the mature vasculature.

Adeno-associated viruses (AAVs) are used in a wide array of experimental situations for driving expression of biosensors, recombinases and opto/chemo-genetic actuators in the brain. However, conventional approaches for minimally invasive, spatially precise and ultra-sparse AAV mediated transduction of cells during imaging experiments, has remained a significant challenge. Here we show that intravenous injection of commercially available AAVs at different doses, combined with laser based perforation of single cortical capillaries through a cranial widow, allows for ultra-sparse, titrate-able, and micron level precision for delivery of viral vectors with relatively little inflammation or tissue damage. Further, we show the utility of this approach for eliciting sparse expression of GCaMP6, channel-rhodopsin or fluorescent reporters in neurons and astrocytes within specific functional domains in normal and stroke damaged cortex. This technique represents a facile approach for targeted delivery of viral vectors that should assist in the study of cell types and circuits in the cerebral cortex.