Two-photon imaging of cerebral hemodynamics and neural activity in awake and anesthetized marmosets

Identification of cortical arteries and veins in awake mice using two-photon microscopy

Distinguishing arteries from veins in the cerebral cortex is critical for studying hemodynamics under pathophysiological conditions, which plays an important role in the diagnosis and treatment of various vessel-related diseases. However, due to the complexity of the cerebral vascular network, it is challenging to identify arteries and veins in vivo. Here, we demonstrate an artery-vein separation method that employs a combination of multiple scanning modes of two-photon microscopy and a custom-designed stereoscopic fixation device for mice. In this process, we propose a novel method for determining the line scanning direction, which allows us to determine the blood flow directions. The vasculature branches have been identified using an optimized z-stack scanning mode, followed by the separation of blood vessel types according to the directions of blood flow and branching patterns. Using this strategy, the penetrating arterioles and penetrating venules in awake mice could be accurately identified and the type of cerebral thrombus has been also successfully isolated without any empirical knowledge or algorithms. Our research presents a new, more accurate, and efficient method for cortical artery-vein separation in awake mice, providing a useful strategy for the application of two-photon microscopy in the study of cerebrovascular pathophysiology.

Altered projection-specific synaptic remodeling and its modification by oxytocin in an idiopathic autism marmoset model

Alterations in the experience-dependent and autonomous elaboration of neural circuits are assumed to underlie autism spectrum disorder (ASD), though it is unclear what synaptic traits are responsible. Here, utilizing a valproic acid-induced ASD marmoset model, which shares common molecular features with idiopathic ASD, we investigate changes in the structural dynamics of tuft dendrites of upper-layer pyramidal neurons and adjacent axons in the dorsomedial prefrontal cortex through two-photon microscopy. In model marmosets, dendritic spine turnover is upregulated, and spines are generated in clusters and survived more often than in control marmosets. Presynaptic boutons in local axons, but not in commissural long-range axons, demonstrate hyperdynamic turnover in model marmosets, suggesting alterations in projection-specific plasticity. Intriguingly, nasal oxytocin administration attenuates clustered spine emergence in model marmosets. Enhanced clustered spine generation, possibly unique to certain presynaptic partners, may be associated with ASD and be a potential therapeutic target.

Mesoscopic calcium imaging in a head-unrestrained male non-human primate using a lensless microscope

Mesoscopic calcium imaging enables studies of cell-type specific neural activity over large areas. A growing body of literature suggests that neural activity can be different when animals are free to move compared to when they are restrained. Unfortunately, existing systems for imaging calcium dynamics over large areas in non-human primates (NHPs) are table-top devices that require restraint of the animal's head. Here, we demonstrate an imaging device capable of imaging mesoscale calcium activity in a head-unrestrained male non-human primate. We successfully miniaturize our system by replacing lenses with an optical mask and computational algorithms. The resulting lensless microscope can fit comfortably on an NHP, allowing its head to move freely while imaging. We are able to measure orientation columns maps over a 20 mm2 field-of-view in a head-unrestrained macaque. Our work establishes mesoscopic imaging using a lensless microscope as a powerful approach for studying neural activity under more naturalistic conditions.

Genetic Approaches for Neural Circuits Dissection in Non-human Primates

Genetic tools, which can be used for the morphology study of specific neurons, pathway-selective connectome mapping, neuronal activity monitoring, and manipulation with a spatiotemporal resolution, have been widely applied to the understanding of complex neural circuit formation, interactions, and functions in rodents. Recently, similar genetic approaches have been tried in non-human primates (NHPs) in neuroscience studies for dissecting the neural circuits involved in sophisticated behaviors and clinical brain disorders, although they are still very preliminary. In this review, we introduce the progress made in the development and application of genetic tools for brain studies on NHPs. We also discuss the advantages and limitations of each approach and provide a perspective for using genetic tools to study the neural circuits of NHPs.

Longitudinal Awake Imaging of Deep Mouse Brain Microvasculature with Super-resolution Ultrasound Localization Microscopy

Super-resolution ultrasound localization microscopy (ULM) is an emerging imaging modality that resolves capillary-scale microvasculature in deep tissues. However, existing preclinical ULM applications are largely constrained to anesthetized animals, introducing confounding vascular effects such as vasodilation and altered hemodynamics. As such, ULM quantifications (e.g., vessel diameter, density, and flow velocity) may be confounded by the use of anesthesia, undermining the usefulness of ULM in practice. Here we introduce a method to address this limitation and achieve ULM imaging in awake mouse brain. Pupillary monitoring was used to confirm the awake state during ULM imaging. ULM revealed that veins showed a greater degree of vascularity reduction from anesthesia to awake states than did arteries. The reduction was most significant in the midbrain and least significant in the cortex. ULM also revealed a significant reduction in venous blood flow velocity across different brain regions under awake conditions. Serial in vivo imaging of the same animal brain at weekly intervals demonstrated the highly robust longitudinal imaging capability of the proposed technique. This is the first study demonstrating longitudinal ULM imaging in the awake mouse brain, which is essential for many ULM brain applications that require awake and behaving animals.

Single-microvessel occlusion produces lamina-specific microvascular flow vasodynamics and signs of neurodegenerative change

Recent studies have highlighted the importance of understanding the architecture and function of microvasculature, and dysfunction of these microvessels may underlie neurodegenerative disease. Here, we utilize a high-precision ultrafast laser-induced photothrombosis (PLP) method to occlude single capillaries and then quantitatively study the effects on vasodynamics and surrounding neurons. Analysis of the microvascular architecture and hemodynamics after single-capillary occlusion reveals distinct changes upstream vs. downstream branches, which shows rapid regional flow redistribution and local downstream blood-brain barrier (BBB) leakage. Focal ischemia via capillary occlusions surrounding labeled target neurons induces dramatic and rapid lamina-specific changes in neuronal dendritic architecture. Further, we find that micro-occlusion at two different depths within the same vascular arbor results in distinct effects on flow profiles in layers 2/3 vs layer 4. The current results reveal laminar-scale regulation distinctions in microinfarct response and raise the possibility that relatively greater impacts on microvascular function contribute to cognitive decline in neurodegenerative disease.

Large-volume and deep brain imaging in rabbits and monkeys using COMPACT two-photon microscopy

In vivo imaging has been widely used for investigating the structure and function of neurons typically located within ~ 800 μm below the cortical surface. Due to light scattering and absorption, it has been difficult to perform in-vivo imaging of neurons in deep cortical and subcortical regions of large animals with two-photon microscopy. Here, we combined a thin-wall quartz capillary with a GRIN lens attached to a prism for large-volume structural and calcium imaging of neurons located 2 mm below the surface of rabbit and monkey brains. The field of view was greatly expanded by rotating and changing the depth of the imaging probe inside a quartz capillary. Calcium imaging of layer 5/6 neurons in the rabbit motor cortex revealed differential activity of these neurons between quiet wakefulness and slow wave sleep. The method described here provides an important tool for studying the structure and function of neurons located deep in the brains of large animals.

Large-depth three-photon fluorescence microscopy imaging of cortical microvasculature on nonhuman primates with bright AIE probe In vivo

Multiphoton microscopy has been a powerful tool in brain research, three-photon fluorescence microscopy is increasingly becoming an emerging technique for neurological research of the cortex in depth. Nonhuman primates play important roles in the study of brain science because of their neural and vascular similarity to humans. However, there are few research results of three-photon fluorescence microscopy on the brain of nonhuman primates due to the lack of optimized imaging systems and excellent fluorescent probes. Here we introduced a bright aggregation-induced emission (AIE) probe with excellent three-photon fluorescence efficiency as well as facile synthesis process and we validated its biocompatibility in the macaque monkey. We achieved a large-depth vascular imaging of approximately 1 mm in the cerebral cortex of macaque monkey with our lab-modified three-photon fluorescence microscopy system and the AIE probe. Functional measurement of blood velocity in deep cortex capillaries was also performed. Furthermore, the comparison of cortical deep vascular structure parameters across species was presented on the monkey and mouse cortex. This work is the first in vivo three-photon fluorescence microscopic imaging research on the macaque monkey cortex reaching the imaging depth of ∼1 mm with the bright AIE probe. The results demonstrate the potential of three-photon microscopy as primate-compatible method for imaging fine vascular networks and will advance our understanding of vascular function in normal and disease in humans.

Optogenetic Methods to Investigate Brain Alterations in Preclinical Models

Investigating the neuronal dynamics supporting brain functions and understanding how the alterations in these mechanisms result in pathological conditions represents a fundamental challenge. Preclinical research on model organisms allows for a multiscale and multiparametric analysis in vivo of the neuronal mechanisms and holds the potential for better linking the symptoms of a neurological disorder to the underlying cellular and circuit alterations, eventually leading to the identification of therapeutic/rescue strategies. In recent years, brain research in model organisms has taken advantage, along with other techniques, of the development and continuous refinement of methods that use light and optical approaches to reconstruct the activity of brain circuits at the cellular and system levels, and to probe the impact of the different neuronal components in the observed dynamics. These tools, combining low-invasiveness of optical approaches with the power of genetic engineering, are currently revolutionizing the way, the scale and the perspective of investigating brain diseases. The aim of this review is to describe how brain functions can be investigated with optical approaches currently available and to illustrate how these techniques have been adopted to study pathological alterations of brain physiology.

Advanced high resolution three-dimensional imaging to visualize the cerebral neurovascular network in stroke

As an important method to accurately and timely diagnose stroke and study physiological characteristics and pathological mechanism in it, imaging technology has gone through more than a century of iteration. The interaction of cells densely packed in the brain is three-dimensional (3D), but the flat images brought by traditional visualization methods show only a few cells and ignore connections outside the slices. The increased resolution allows for a more microscopic and underlying view. Today's intuitive 3D imagings of micron or even nanometer scale are showing its essentiality in stroke. In recent years, 3D imaging technology has gained rapid development. With the overhaul of imaging mediums and the innovation of imaging mode, the resolution has been significantly improved, endowing researchers with the capability of holistic observation of a large volume, real-time monitoring of tiny voxels, and quantitative measurement of spatial parameters. In this review, we will summarize the current methods of high-resolution 3D imaging applied in stroke.

Miniature microscopes for manipulating and recording in vivo brain activity

Here we describe the development and application of miniature integrated microscopes (miniscopes) paired with microendoscopes that allow for the visualization and manipulation of neural circuits in superficial and subcortical brain regions in freely behaving animals. Over the past decade the miniscope platform has expanded to include simultaneous optogenetic capabilities, electrically-tunable lenses that enable multi-plane imaging, color-corrected optics, and an integrated data acquisition platform that streamlines multimodal experiments. Miniscopes have given researchers an unprecedented ability to monitor hundreds to thousands of genetically-defined neurons from weeks to months in both healthy and diseased animal brains. Sophisticated algorithms that take advantage of constrained matrix factorization allow for background estimation and reliable cell identification, greatly improving the reliability and scalability of source extraction for large imaging datasets. Data generated from miniscopes have empowered researchers to investigate the neural circuit underpinnings of a wide array of behaviors that cannot be studied under head-fixed conditions, such as sleep, reward seeking, learning and memory, social behaviors, and feeding. Importantly, the miniscope has broadened our understanding of how neural circuits can go awry in animal models of progressive neurological disorders, such as Parkinson's disease. Continued miniscope development, including the ability to record from multiple populations of cells simultaneously, along with continued multimodal integration of techniques such as electrophysiology, will allow for deeper understanding into the neural circuits that underlie complex and naturalistic behavior.

Marmosets: a promising model for probing the neural mechanisms underlying complex visual networks such as the frontal-parietal network

The technology, methodology and models used by visual neuroscientists have provided great insights into the structure and function of individual brain areas. However, complex cognitive functions arise in the brain due to networks comprising multiple interacting cortical areas that are wired together with precise anatomical connections. A prime example of this phenomenon is the frontal–parietal network and two key regions within it: the frontal eye fields (FEF) and lateral intraparietal area (area LIP). Activity in these cortical areas has independently been tied to oculomotor control, motor preparation, visual attention and decision-making. Strong, bidirectional anatomical connections have also been traced between FEF and area LIP, suggesting that the aforementioned visual functions depend on these inter-area interactions. However, advancements in our knowledge about the interactions between area LIP and FEF are limited with the main animal model, the rhesus macaque, because these key regions are buried in the sulci of the brain. In this review, we propose that the common marmoset is the ideal model for investigating how anatomical connections give rise to functionally-complex cognitive visual behaviours, such as those modulated by the frontal–parietal network, because of the homology of their cortical networks with humans and macaques, amenability to transgenic technology, and rich behavioural repertoire. Furthermore, the lissencephalic structure of the marmoset brain enables application of powerful techniques, such as array-based electrophysiology and optogenetics, which are critical to bridge the gaps in our knowledge about structure and function in the brain.

Head-mounted microendoscopic calcium imaging in dorsal premotor cortex of behaving rhesus macaque

Microendoscopic calcium imaging with one-photon miniature microscopes enables unprecedented readout of neural circuit dynamics during active behavior in rodents. In this study, we describe successful application of this technology in the rhesus macaque, demonstrating plug-and-play, head-mounted recordings of cellular-resolution calcium dynamics from large populations of neurons simultaneously in bilateral dorsal premotor cortices during performance of a naturalistic motor reach task. Imaging is stable over several months, allowing us to longitudinally track individual neurons and monitor their relationship to motor behavior over time. We observe neuronal calcium dynamics selective for reach direction, which we could use to decode the animal's trial-by-trial motor behavior. This work establishes head-mounted microendoscopic calcium imaging in macaques as a powerful approach for studying the neural circuit mechanisms underlying complex and clinically relevant behaviors, and it promises to greatly advance our understanding of human brain function, as well as its dysfunction in neurological disease.

Using non-invasive neuroimaging to enhance the care, well-being and experimental outcomes of laboratory non-human primates (monkeys)

Over the past 10-20 years, neuroscience witnessed an explosion in the use of non-invasive imaging methods, particularly magnetic resonance imaging (MRI), to study brain structure and function. Simultaneously, with access to MRI in many research institutions, MRI has become an indispensable tool for researchers and veterinarians to guide improvements in surgical procedures and implants and thus, experimental as well as clinical outcomes, given that access to MRI also allows for improved diagnosis and monitoring for brain disease. As part of the PRIMEatE Data Exchange, we gathered expert scientists, veterinarians, and clinicians who treat humans, to provide an overview of the use of non-invasive imaging tools, primarily MRI, to enhance experimental and welfare outcomes for laboratory non-human primates engaged in neuroscientific experiments. We aimed to provide guidance for other researchers, scientists and veterinarians in the use of this powerful imaging technology as well as to foster a larger conversation and community of scientists and veterinarians with a shared goal of improving the well-being and experimental outcomes for laboratory animals.

Differences in cerebral blood vasculature and flow in awake and anesthetized mouse cortex revealed by quantitative optical coherence tomography angiography

BACKGROUND

Most of the in vivo neurovascular imaging studies are performed in anesthetized animals. However, anesthesia significantly affects cerebral hemodynamics.

NEW METHOD

We applied optical coherence tomography (OCT) methods such as optical microangiography (OMAG) and Doppler optical microangiography (DOMAG) to quantitatively evaluate the effect of anesthesia in cerebral vasculature and blood flow in mouse brain.

RESULTS

The OMAG results indicated the increase of large vessel diameter and capillary density induced by ketamine-xylazine and isoflurane, meaning that both anesthetics caused vasodilation. In addition, the preliminary results from DOMAG showed that isoflurane increased the baseline cerebral blood flow.

COMPARISON WITH EXISTING METHODS

In comparison with other in vivo imaging modalities, OCT can provide label-free assessment of cortical tissue including tissue morphology, cerebral blood vessel network and flow information down to capillary level, with a large field of view and high imaging speed.

CONCLUSIONS

OCT angiography methods demonstrated the ability to measure the differences in the baseline morphological and flow parameters of both large and capillary cerebrovascular networks between awake and anesthetized mice.

Current Status of and Perspectives on the Application of Marmosets in Neurobiology

The common marmoset (Callithrix jacchus), a small New World primate, is receiving substantial attention in the neuroscience and biomedical science fields because its anatomical features, functional and behavioral characteristics, and reproductive features and its amenability to available genetic modification technologies make it an attractive experimental subject. In this review, I outline the progress of marmoset neuroscience research and summarize both the current status (opportunities and limitations) of and the future perspectives on the application of marmosets in neuroscience and disease modeling.

Mannose-Binding Lectin Drives Platelet Inflammatory Phenotype and Vascular Damage After Cerebral Ischemia in Mice via IL (Interleukin)-1α

Supplemental Digital Content is available in the text.

Objective—

Circulating complement factors are activated by tissue damage and contribute to acute brain injury. The deposition of MBL (mannose-binding lectin), one of the initiators of the lectin complement pathway, on the cerebral endothelium activated by ischemia is a major pathogenic event leading to brain injury. The molecular mechanisms through which MBL influences outcome after ischemia are not understood yet.

Approach and Results—

Here we show that MBL-deficient (MBL^−/−) mice subjected to cerebral ischemia display better flow recovery and less plasma extravasation in the brain than wild-type mice, as assessed by in vivo 2-photon microscopy. This results in reduced vascular dysfunction as shown by the shift from a pro- to an anti-inflammatory vascular phenotype associated with MBL deficiency. We also show that platelets directly bind MBL and that platelets from MBL^−/− mice have reduced inflammatory phenotype as indicated by reduced IL-1α (interleukin-1α) content, as early as 6 hours after ischemia. Cultured human brain endothelial cells subjected to oxygen-glucose deprivation and exposed to platelets from MBL^−/− mice present less cell death and lower CXCL1 (chemokine [C-X-C motif] ligand 1) release (downstream to IL-1α) than those exposed to wild-type platelets. In turn, MBL deposition on ischemic vessels significantly decreases after ischemia in mice treated with IL-1 receptor antagonist compared with controls, indicating a reciprocal interplay between MBL and IL-1α facilitating endothelial damage.

Conclusions—

We propose MBL as a hub of pathogenic vascular events. It acts as an early trigger of platelet IL-1α release, which in turn favors MBL deposition on ischemic vessels promoting an endothelial pro-inflammatory phenotype.

Wearable optical resolution photoacoustic microscopy

Optical resolution photoacoustic microscopy (ORPAM) is an emerging imaging technique, which has been extensively used to study various brain activities and disorders of the anesthetized/restricted rodents with a special focus on the morphological and functional visualization of cerebral cortex. However, it is challenging to develop a wearable photoacoustic microscope, which enables the investigation of brain activities/disorders on freely moving rodents. Here, we report a wearable and robust optical resolution photoacoustic microscope (W-ORPAM), which utilizes a small, light, stable and fast optical scanner. This wearable imaging probe features high spatiotemporal resolution, large field of view (FOV) and easy assembly as well as adjustable optical focus during the in vivo experiment, which makes it accessible to image cerebral cortex activities of freely moving rodents. To demonstrate the advantages of this technique, we used W-ORPAM to monitor both morphological and functional variations of vasculature in cerebral cortex during the induction of ischemia and reperfusion of a freely moving rat.

Emerging ideas and tools to study the emergent properties of the cortical neural circuits for voluntary motor control in non-human primates

For years, neurophysiological studies of the cerebral cortical mechanisms of voluntary motor control were limited to single-electrode recordings of the activity of one or a few neurons at a time. This approach was supported by the widely accepted belief that single neurons were the fundamental computational units of the brain (the “neuron doctrine”). Experiments were guided by motor-control models that proposed that the motor system attempted to plan and control specific parameters of a desired action, such as the direction, speed or causal forces of a reaching movement in specific coordinate frameworks, and that assumed that the controlled parameters would be expressed in the task-related activity of single neurons. The advent of chronically implanted multi-electrode arrays about 20 years ago permitted the simultaneous recording of the activity of many neurons. This greatly enhanced the ability to study neural control mechanisms at the population level. It has also shifted the focus of the analysis of neural activity from quantifying single-neuron correlates with different movement parameters to probing the structure of multi-neuron activity patterns to identify the emergent computational properties of cortical neural circuits. In particular, recent advances in “dimension reduction” algorithms have attempted to identify specific covariance patterns in multi-neuron activity which are presumed to reflect the underlying computational processes by which neural circuits convert the intention to perform a particular movement into the required causal descending motor commands. These analyses have led to many new perspectives and insights on how cortical motor circuits covertly plan and prepare to initiate a movement without causing muscle contractions, transition from preparation to overt execution of the desired movement, generate muscle-centered motor output commands, and learn new motor skills. Progress is also being made to import optical-imaging and optogenetic toolboxes from rodents to non-human primates to overcome some technical limitations of multi-electrode recording technology.

Tonic regulation of middle meningeal artery diameter by ATP-sensitive potassium channels

Activation of ATP-sensitive potassium (KATP) channels in arterial smooth muscle (ASM) contributes to vasodilation evoked by a variety of endogenous and exogenous compounds. Although controversial, activation of KATP channels by neuropeptides such as calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase activating peptide (PACAP) in the trigeminovascular system, including the middle meningeal artery (MMA), has been linked to migraine headache. The objective of the current study was to determine if ongoing KATP channel activity also influences MMA diameter. In the absence of other exogenous compounds, the KATP channel inhibitors glibenclamide and PNU37883A induced constriction of isolated and pressurized MMAs. In contrast, KATP channel inhibition did not alter cerebral artery diameter. Consistent with tonic KATP activity in MMA, glibenclamide also induced ASM membrane potential depolarization and increased cytosolic Ca2+. Inhibitors of cAMP-dependent protein kinase (PKA) abolished basal KATP activation in MMA and caused a marked decrease in sensitivity to the synthetic KATP channel opener, cromakalim. In vivo MMA constriction in response to gibenclamide was observed using two-photon imaging of arterial diameter. Together these results indicate that PKA-mediated tonic KATP channel activity contributes to the regulation of MMA diameter.

Comparative Principles for Next-Generation Neuroscience

Neuroscience is enjoying a renaissance of discovery due in large part to the implementation of next-generation molecular technologies. The advent of genetically encoded tools has complemented existing methods and provided researchers the opportunity to examine the nervous system with unprecedented precision and to reveal facets of neural function at multiple scales. The weight of these discoveries, however, has been technique-driven from a small number of species amenable to the most advanced gene-editing technologies. To deepen interpretation and build on these breakthroughs, an understanding of nervous system evolution and diversity are critical. Evolutionary change integrates advantageous variants of features into lineages, but is also constrained by pre-existing organization and function. Ultimately, each species’ neural architecture comprises both properties that are species-specific and those that are retained and shared. Understanding the evolutionary history of a nervous system provides interpretive power when examining relationships between brain structure and function. The exceptional diversity of nervous systems and their unique or unusual features can also be leveraged to advance research by providing opportunities to ask new questions and interpret findings that are not accessible in individual species. As new genetic and molecular technologies are added to the experimental toolkits utilized in diverse taxa, the field is at a key juncture to revisit the significance of evolutionary and comparative approaches for next-generation neuroscience as a foundational framework for understanding fundamental principles of neural function.

Generation of genetically engineered non-human primate models of brain function and neurological disorders

Research with non-human primates (NHP) has been essential and effective in increasing our ability to find cures for a large number of diseases that cause human suffering and death. Extending the availability and use of genetic engineering techniques to NHP will allow the creation and study of NHP models of human disease, as well as broaden our understanding of neural circuits in the primate brain. With the recent development of efficient genetic engineering techniques that can be used for NHP, there's increased hope that NHP will significantly accelerate our understanding of the etiology of human neurological and neuropsychiatric disorders. In this article, we review the present state of genetic engineering tools used in NHP, from the early efforts to induce exogeneous gene expression in macaques and marmosets, to the latest results in producing germline transmission of different transgenes and the establishment of knockout lines of specific genes. We conclude with future perspectives on the further development and employment of these tools to generate genetically engineered NHP.

Two-photon probes for in vivo multicolor microscopy of the structure and signals of brain cells

Imaging the brain of living laboratory animals at a microscopic scale can be achieved by two-photon microscopy thanks to the high penetrability and low phototoxicity of the excitation wavelengths used. However, knowledge of the two-photon spectral properties of the myriad fluorescent probes is generally scarce and, for many, non-existent. In addition, the use of different measurement units in published reports further hinders the design of a comprehensive imaging experiment. In this review, we compile and homogenize the two-photon spectral properties of 280 fluorescent probes. We provide practical data, including the wavelengths for optimal two-photon excitation, the peak values of two-photon action cross section or molecular brightness, and the emission ranges. Beyond the spectroscopic description of these fluorophores, we discuss their binding to biological targets. This specificity allows in vivo imaging of cells, their processes, and even organelles and other subcellular structures in the brain. In addition to probes that monitor endogenous cell metabolism, studies of healthy and diseased brain benefit from the specific binding of certain probes to pathology-specific features, ranging from amyloid-β plaques to the autofluorescence of certain antibiotics. A special focus is placed on functional in vivo imaging using two-photon probes that sense specific ions or membrane potential, and that may be combined with optogenetic actuators. Being closely linked to their use, we examine the different routes of intravital delivery of these fluorescent probes according to the target. Finally, we discuss different approaches, strategies, and prerequisites for two-photon multicolor experiments in the brains of living laboratory animals.

Two-photon imaging of neuronal activity in motor cortex of marmosets during upper-limb movement tasks

Two-photon imaging in behaving animals has revealed neuronal activities related to behavioral and cognitive function at single-cell resolution. However, marmosets have posed a challenge due to limited success in training on motor tasks. Here we report the development of protocols to train head-fixed common marmosets to perform upper-limb movement tasks and simultaneously perform two-photon imaging. After 2–5 months of training sessions, head-fixed marmosets can control a manipulandum to move a cursor to a target on a screen. We conduct two-photon calcium imaging of layer 2/3 neurons in the motor cortex during this motor task performance, and detect task-relevant activity from multiple neurons at cellular and subcellular resolutions. In a two-target reaching task, some neurons show direction-selective activity over the training days. In a short-term force-field adaptation task, some neurons change their activity when the force field is on. Two-photon calcium imaging in behaving marmosets may become a fundamental technique for determining the spatial organization of the cortical dynamics underlying action and cognition.

Marmosets are an important model organism in neuroscience but there has only been limited success in training them on behavioral tasks. Here the authors report their ability to train marmosets in various motor tasks and simultaneously image neural dynamics in motor cortex with 2-photon imaging.

VEGF signaling regulates the fate of obstructed capillaries in mouse cortex

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.

Development of stereotaxic recording system for awake marmosets (Callithrix jacchus)

The common marmoset has been proposed as a potential alternative to macaque monkey as a primate model for neuroscience and medical research. Here, we have newly developed a stereotaxic neuronal recording system for awake marmosets under the head-fixed condition by modifying that for macaque monkeys. Using this system, we recorded neuronal activity in the cerebral cortex of awake marmosets and successfully identified the primary motor cortex by intracortical microstimulation. Neuronal activities of deep brain structures, such as the basal ganglia, thalamus, and cerebellum, in awake marmosets were also successfully recorded referring to magnetic resonance images. Our system is suitable for functional mapping of the brain, since the large recording chamber allows access to arbitrary regions over almost the entire brain, and the recording electrode can be easily moved stereotaxically from one site to another. In addition, our system is desirable for neuronal recording during task performance to assess motor skills and cognitive function, as the marmoset sits in the marmoset chair and can freely use its hands. Moreover, our system can be used in combination with cutting-edge techniques, such as two-photon imaging and optogenetic manipulation. This recording system will contribute to boosting neuroscience and medical research using marmosets.

Prevention of the collapse of pial collaterals by remote ischemic perconditioning during acute ischemic stroke

Collateral circulation is a key variable determining prognosis and response to recanalization therapy during acute ischemic stroke. Remote ischemic perconditioning (RIPerC) involves inducing peripheral ischemia (typically in the limbs) during stroke and may reduce perfusion deficits and brain damage due to cerebral ischemia. In this study, we directly investigated pial collateral flow augmentation due to RIPerC during distal middle cerebral artery occlusion (MCAo) in rats. Blood flow through pial collaterals between the anterior cerebral artery (ACA) and the MCA was assessed in male Sprague Dawley rats using in vivo laser speckle contrast imaging (LSCI) and two photon laser scanning microscopy (TPLSM) during distal MCAo. LSCI and TPLSM revealed that RIPerC augmented collateral flow into distal MCA segments. Notably, while control rats exhibited an initial dilation followed by a progressive narrowing of pial arterioles 60 to 150-min post-MCAo (constricting to 80-90% of post-MCAo peak diameter), this constriction was prevented or reversed by RIPerC (such that vessel diameters increased to 105-110% of post-MCAo, pre-RIPerC diameter). RIPerC significantly reduced early ischemic damage measured 6 h after stroke onset. Thus, prevention of collateral collapse via RIPerC is neuroprotective and may facilitate other protective or recanalization therapies by improving blood flow in penumbral tissue.

Genetically Encoded Calcium Indicators as Probes to Assess the Role of Calcium Channels in Disease and for High-Throughput Drug Discovery

The calcium ion (Ca²⁺) is an important signaling molecule implicated in many cellular processes, and the remodeling of Ca²⁺ homeostasis is a feature of a variety of pathologies. Typical methods to assess Ca²⁺ signaling in cells often employ small molecule fluorescent dyes, which are sometimes poorly suited to certain applications such as assessment of cellular processes, which occur over long periods (hours or days) or in vivo experiments. Genetically encoded calcium indicators are a set of tools available for the measurement of Ca²⁺ changes in the cytosol and subcellular compartments, which circumvent some of the inherent limitations of small molecule Ca²⁺ probes. Recent advances in genetically encoded calcium sensors have greatly increased their ability to provide reliable monitoring of Ca²⁺ changes in mammalian cells. New genetically encoded calcium indicators have diverse options in terms of targeting, Ca²⁺ affinity and fluorescence spectra, and this will further enhance their potential use in high-throughput drug discovery and other assays. This review will outline the methods available for Ca²⁺ measurement in cells, with a focus on genetically encoded calcium sensors. How these sensors will improve our understanding of the deregulation of Ca²⁺ handling in disease and their application to high-throughput identification of drug leads will also be discussed.

Long-Term Two-Photon Imaging in Awake Macaque Monkey

Successful application of two-photon imaging with genetic tools in awake macaque monkeys will enable fundamental advances in our understanding of higher cognitive function at the level of molecular and neuronal circuits. Here we report techniques for long-term two-photon imaging in awake macaque monkeys. Using genetically encoded indicators including GCaMP5 and GCaMP6s delivered by AAV2/1 into the visual cortex, we demonstrate that high-quality two-photon imaging of large neuronal populations can be achieved and maintained in awake monkeys for months. Simultaneous intracellular recording and two-photon calcium imaging confirm that fluorescence activity is linearly proportional to neuronal spiking activity across a wide range of firing rates (10 Hz to 150 Hz). By providing two-photon imaging access to cortical neuronal populations at single-cell or single dendritic spine resolution in awake monkeys, the techniques reported can help bridge the use of modern genetic and molecular tools and the study of higher cognitive function.

Anatomical and functional neuroimaging in awake, behaving marmosets

The common marmoset (Callithrix jacchus) is a small New World monkey that has gained significant recent interest in neuroscience research, not only because of its compatibility with gene editing techniques, but also due to its tremendous versatility as an experimental animal model. Neuroimaging modalities, including anatomical (MRI) and functional magnetic resonance imaging (fMRI), complemented by two-photon laser scanning microscopy and electrophysiology, have been at the forefront of unraveling the anatomical and functional organization of the marmoset brain. High-resolution anatomical MRI of the marmoset brain can be obtained with remarkable cytoarchitectonic detail. Functional MRI of the marmoset brain has been used to study various sensory systems, including somatosensory, auditory, and visual pathways, while resting-state fMRI studies have unraveled functional brain networks that bear great correspondence to those previously described in humans. Two-photon laser scanning microscopy of the marmoset brain has enabled the simultaneous recording of neuronal activity from thousands of neurons with single cell spatial resolution. In this article, we aim to review the main results obtained by our group and by our colleagues in applying neuroimaging techniques to study the marmoset brain. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 373-389, 2017.