Peri-Infarct Hot-Zones Have Higher Susceptibility to Optogenetic Functional Activation-Induced Spreading Depolarizations

Optogenetic Spreading Depolarizations Do Not Worsen Acute Ischemic Stroke Outcome

BACKGROUND

Spreading depolarizations (SDs) are believed to contribute to injury progression and worsen outcomes in focal cerebral ischemia because exogenously induced SDs have been associated with enlarged infarct volumes. However, previous studies used highly invasive methods to trigger SDs that can directly cause tissue injury (eg, topical KCl) and confound the interpretation. Here, we tested whether SDs indeed enlarge infarcts when induced via a novel, noninjurious method using optogenetics.

METHODS

Using transgenic mice expressing channelrhodopsin-2 in neurons (Thy1-ChR2-YFP), we induced 8 optogenetic SDs to trigger SDs noninvasively at a remote cortical location in a noninjurious manner during 1-hour distal microvascular clip or proximal an endovascular filament occlusion of the middle cerebral artery. Laser speckle imaging was used to monitor cerebral blood flow. Infarct volumes were then quantified at 24 or 48 hours.

RESULTS

Infarct volumes in the optogenetic SD arm did not differ from the control arm in either distal or proximal middle cerebral artery occlusion, despite a 6-fold and 4-fold higher number of SDs, respectively. Identical optogenetic illumination in wild-type mice did not affect the infarct volume. Full-field laser speckle imaging showed that optogenetic stimulation did not affect the perfusion in the peri-infarct cortex.

CONCLUSIONS

Altogether, these data show that SDs induced noninvasively using optogenetics do not worsen tissue outcomes. Our findings compel a careful reexamination of the notion that SDs are causally linked to infarct expansion.

Real-time whole-brain imaging of hemodynamics and oxygenation at micro-vessel resolution with ultrafast wide-field photoacoustic microscopy

High-speed high-resolution imaging of the whole-brain hemodynamics is critically important to facilitating neurovascular research. High imaging speed and image quality are crucial to visualizing real-time hemodynamics in complex brain vascular networks, and tracking fast pathophysiological activities at the microvessel level, which will enable advances in current queries in neurovascular and brain metabolism research, including stroke, dementia, and acute brain injury. Further, real-time imaging of oxygen saturation of hemoglobin (sO₂) can capture fast-paced oxygen delivery dynamics, which is needed to solve pertinent questions in these fields and beyond. Here, we present a novel ultrafast functional photoacoustic microscopy (UFF-PAM) to image the whole-brain hemodynamics and oxygenation. UFF-PAM takes advantage of several key engineering innovations, including stimulated Raman scattering (SRS) based dual-wavelength laser excitation, water-immersible 12-facet-polygon scanner, high-sensitivity ultrasound transducer, and deep-learning-based image upsampling. A volumetric imaging rate of 2 Hz has been achieved over a field of view (FOV) of 11 × 7.5 × 1.5 mm³ with a high spatial resolution of ~10 μm. Using the UFF-PAM system, we have demonstrated proof-of-concept studies on the mouse brains in response to systemic hypoxia, sodium nitroprusside, and stroke. We observed the mouse brain's fast morphological and functional changes over the entire cortex, including vasoconstriction, vasodilation, and deoxygenation. More interestingly, for the first time, with the whole-brain FOV and micro-vessel resolution, we captured the vasoconstriction and hypoxia simultaneously in the spreading depolarization (SD) wave. We expect the new imaging technology will provide a great potential for fundamental brain research under various pathological and physiological conditions.

Loss of Kir6.1 facilitates peri-infarct depolarizations in focal cerebral ischemia

OBJECTIVE

Peri-infarct depolarizations (PIDs) are spontaneous waves that propagate slowly across the penumbra region following stroke, contributing to secondary infarct growth and negatively affecting stroke outcomes. K_ATP channels are generally spread in the brain. Under conditions of ischemia and/or hypoxia, K_ATP channels play a cytoprotective role in neurons. However, it is still unknown whether K_ATP channels are involved in the initiation and propagation of PIDs.

METHODS

The Kir6.1 knockout (Kir6.1^-/-) mice, Kir6.2 knockout (Kir6.2^-/-) mice, and wild-type C57Bl6 mice (n = 8) were used. The middle cerebral artery occlusion (MCAO) stroke model was made and PIDs were detected by an optical intrinsic signal (OIS) imaging system.

RESULTS

Much more PIDs appeared in Kir6.1^-/-mice than that in Kir6.2^-/- and WT mice in both the first hour and 4 hours following MCAO (3.9 ± 0.7 vs. 1.5 ± 0.3, p < 0,05; 3.9 ± 0.7 vs. 1.9 ± 0.3, p < 0.05; 20.0 ± 2.5 vs. 10.4 ± 2.4, p < 0.05; 20.0 ± 2.5 vs. 11.3 ± 1.4, p < 0.05). Furthermore, the first PID occurred much earlier in Kir6.1^-/- mice than that in Kir6.2^-/- mice and WT mice (21.3 ± 2.1 min vs. 34.1 ± 4.8 min, p < 0.05; 21.3 ± 2.1 min vs. 38.8 ± 3.4 min, p < 0.01). No significant differences in other characteristics of PIDs including originating sites, duration time, propagation patterns, and velocity were detected. Additionally, the migration of originating sites was observed.

CONCLUSION

This study shows that loss of Kir6.1, not Kir6.2 facilitates the induction of PIDs in focal cerebral ischemia, indicating that Kir6.1-forming channels in the brain may provide protection against PIDs.

Frequency selective neuronal modulation triggers spreading depolarizations in the rat endothelin-1 model of stroke

Ischemia is one of the most common causes of acquired brain injury. Central to its noxious sequelae are spreading depolarizations (SDs), waves of persistent depolarizations which start at the location of the flow obstruction and expand outwards leading to excitotoxic damage. The majority of acute stage of stroke studies to date have focused on the phenomenology of SDs and their association with brain damage. In the current work, we investigated the role of peri-injection zone pyramidal neurons in triggering SDs by optogenetic stimulation in an endothelin-1 rat model of focal ischemia. Our concurrent two photon fluorescence microscopy data and local field potential recordings indicated that a ≥ 60% drop in cortical arteriolar red blood cell velocity was associated with SDs at the ET-1 injection site. SDs were also observed in the peri-injection zone, which subsequently exhibited elevated neuronal activity in the low-frequency bands. Critically, SDs were triggered by low- but not high-frequency optogenetic stimulation of peri-injection zone pyramidal neurons. Our findings depict a complex etiology of SDs post focal ischemia and reveal that effects of neuronal modulation exhibit spectral and spatial selectivity.

Pericytes: Intrinsic Transportation Engineers of the CNS Microcirculation

Pericytes in the brain are candidate regulators of microcirculatory blood flow because they are strategically positioned along the microvasculature, contain contractile proteins, respond rapidly to neuronal activation, and synchronize microvascular dynamics and neurovascular coupling within the capillary network. Analyses of mice with defects in pericyte generation demonstrate that pericytes are necessary for the formation of the blood-brain barrier, development of the glymphatic system, immune homeostasis, and white matter function. The development, identity, specialization, and progeny of different subtypes of pericytes, however, remain unclear. Pericytes perform brain-wide 'transportation engineering' functions in the capillary network, instructing, integrating, and coordinating signals within the cellular communicome in the neurovascular unit to efficiently distribute oxygen and nutrients ('goods and services') throughout the microvasculature ('transportation grid'). In this review, we identify emerging challenges in pericyte biology and shed light on potential pericyte-targeted therapeutic strategies.

Characterization of optogenetically-induced cortical spreading depression in awake mice using graphene micro-transistor arrays

Objective.The development of experimental methodology utilizing graphene micro-transistor arrays to facilitate and advance translational research into cortical spreading depression (CSD) in the awake brain.Approach.CSDs were reliably induced in awake nontransgenic mice using optogenetic methods. High-fidelity DC-coupled electrophysiological mapping of propagating CSDs was obtained using flexible arrays of graphene soultion-gated field-effect transistors (gSGFETs).Main results.Viral vectors targetted channelrhopsin expression in neurons of the motor cortex resulting in a transduction volume ⩾1 mm³. 5-10 s of continous blue light stimulation induced CSD that propagated across the cortex at a velocity of 3.0 ± 0.1 mm min^-1. Graphene micro-transistor arrays enabled high-density mapping of infraslow activity correlated with neuronal activity suppression across multiple frequency bands during both CSD initiation and propagation. Localized differences in the CSD waveform could be detected and categorized into distinct clusters demonstrating the spatial resolution advantages of DC-coupled recordings. We exploited the reliable and repeatable induction of CSDs using this preparation to perform proof-of-principle pharmacological interrogation studies using NMDA antagonists. MK801 (3 mg kg^-1) suppressed CSD induction and propagation, an effect mirrored, albeit transiently, by ketamine (15 mg kg^-1), thus demonstrating this models' applicability as a preclinical drug screening platform. Finally, we report that CSDs could be detected through the skull using graphene micro-transistors, highlighting additional advantages and future applications of this technology.Significance.CSD is thought to contribute to the pathophysiology of several neurological diseases. CSD research will benefit from technological advances that permit high density electrophysiological mapping of the CSD waveform and propagation across the cortex. We report anin vivoassay that permits minimally invasive optogenetic induction, combined with multichannel DC-coupled recordings enabled by gSGFETs in the awake brain. Adoption of this technological approach could facilitate and transform preclinical investigations of CSD in disease relevant models.