Principles of Two-Photon Excitation Microscopy and Its Applications to Neuroscience

Understanding, engineering, and modulating the growth of neural networks: An interdisciplinary approach

A deeper understanding of the brain and its function remains one of the most significant scientific challenges. It not only is required to find cures for a plethora of brain-related diseases and injuries but also opens up possibilities for achieving technological wonders, such as brain-machine interface and highly energy-efficient computing devices. Central to the brain's function is its basic functioning unit (i.e., the neuron). There has been a tremendous effort to understand the underlying mechanisms of neuronal growth on both biochemical and biophysical levels. In the past decade, this increased understanding has led to the possibility of controlling and modulating neuronal growth in vitro through external chemical and physical methods. We provide a detailed overview of the most fundamental aspects of neuronal growth and discuss how researchers are using interdisciplinary ideas to engineer neuronal networks in vitro. We first discuss the biochemical and biophysical mechanisms of neuronal growth as we stress the fact that the biochemical or biophysical processes during neuronal growth are not independent of each other but, rather, are complementary. Next, we discuss how utilizing these fundamental mechanisms can enable control over neuronal growth for advanced neuroengineering and biomedical applications. At the end of this review, we discuss some of the open questions and our perspectives on the challenges and possibilities related to controlling and engineering the growth of neuronal networks, specifically in relation to the materials, substrates, model systems, modulation techniques, data science, and artificial intelligence.

Three-dimensional microvascular network reconstruction from in vivo images with adaptation of the regional inhomogeneity in the signal-to-noise ratio

OBJECTIVE

Quantification of angiographic images with two-photon laser scanning fluorescence microscopy (2PLSM) relies on proper segmentation of the vascular images. However, the images contain inhomogeneities in the signal-to-noise ratio (SNR) arising from regional effects of light scattering and absorption. The present study developed a semiautomated quantification method for volume images of 2PLSM angiography by adjusting the binarization threshold according to local SNR along the vessel centerlines.

METHODS

A phantom model made with fluorescent microbeads was used to incorporate a region-dependent binarization threshold.

RESULTS

The recommended SNR for imaging was found to be 4.2-10.6 that provide the true size of imaged objects if the binarization threshold was fixed at 50% of SNR. However, angiographic images in the mouse cortex showed variable SNR up to 45 over the depths. To minimize the errors caused by variable SNR and a spatial extent of the imaged objects in an axial direction, the microvascular networks were three-dimensionally reconstructed based on the cross-sectional diameters measured along the vessel centerline from the XY-plane images with adapted binarization threshold. The arterial volume was relatively constant over depths of 0-500 µm, and the capillary volume (1.7% relative to the scanned volume) showed the larger volumes than the artery (0.8%) and vein (0.6%).

CONCLUSIONS

The present methods allow consistent segmentation of microvasculature by adapting the local inhomogeneity in the SNR, which will be useful for quantitative comparison of the microvascular networks, such as under disease conditions where SNR in the 2PLSM images varies over space and time.

Differential modulation of excitatory and inhibitory populations of superficial dorsal horn neurons in lumbar spinal cord by Aβ-fiber electrical stimulation

Activation of Aβ-fibers is fundamental to numerous analgesic therapies, yet its effects on dorsal horn neuronal activity remain unclear. We used multiphoton microscopy of the genetically encoded calcium indicator GCaMP6s to characterize the effects of Aβ-fiber electrical stimulation (Aβ-ES) on neural activity. Specifically, we quantified somatic responses evoked by C-fiber intensity stimulation before and after a 10-minute train of dorsal root Aβ-ES in superficial dorsal horn (SDH) neurons, in mouse lumbar spinal cord. Aβ-ES did not alter C-fiber-evoked activity when GCaMP6s was virally expressed in all neurons, in an intact lumbar spinal cord preparation. However, when we restricted the expression of GCaMP6s to excitatory or inhibitory populations, we observed that Aβ-ES modestly potentiated evoked activity of excitatory neurons and depressed that of inhibitory neurons. Aβ-ES had no significant effects in a slice preparation in either SDH population. A larger proportion of SDH neurons was activated by Aβ-ES when delivered at a root rostral or caudal to the segment where the imaging and C-fiber intensity stimulation occurred. Aβ-ES effects on excitatory and inhibitory populations depended on the root used. Our findings suggest that Aβ-ES differentially modulates lumbar spinal cord SDH populations in a cell type- and input-specific manner. Furthermore, they underscore the importance of the Aβ-ES delivery site, suggesting that Aβ stimulation at a segment adjacent to where the pain is may improve analgesic efficacy.

Heterogeneous associative plasticity in the auditory cortex induced by fear learning - novel insight into the classical conditioning paradigm

We used two-photon calcium imaging with single-cell and cell-type resolution. Fear conditioning induced heterogeneous tuning shifts at single-cell level in the auditory cortex, with shifts both to CS+ frequency and to the control CS- stimulus frequency. We thus extend the view of simple expansion of CS+ tuned regions. Instead of conventional freezing reactions only, we observe selective orienting responses towards the conditioned stimuli. The orienting responses were often followed by escape behavior.

Low noise, self-phase-modulation-enabled femtosecond fiber sources tunable in 740-1236 nm for wide two-photon fluorescence microscopy applications

We have demonstrated widely tunable Yb:fiber-based laser sources, aiming to replace Ti:sapphire lasers for the nJ-level ultrafast applications, especially for the uses of nonlinear light microscopy. We investigated the influence of different input parameters to obtain an expansive spectral broadening, enabled by self-phase modulation and further reshaped by self-steepening, in the normal dispersion regime before the fiber damage. We also discussed the compressibility and intensity fluctuations of the demonstrated pulses, to reach the transform-limited duration with a very low intensity noise. Most importantly, we have demonstrated clear two-photon fluorescence images from UV-absorbing fluorophores to deep red dye stains.

Restoration of Two-Photon Ca2+ Imaging Data Through Model Blind Spatiotemporal Filtering

Two-photon Ca²⁺ imaging is a leading technique for recording neuronal activities in vivo with cellular or subcellular resolution. However, during experiments, the images often suffer from corruption due to complex noises. Therefore, the analysis of Ca²⁺ imaging data requires preprocessing steps, such as denoising, to extract biologically relevant information. We present an approach that facilitates imaging data restoration through image denoising performed by a neural network combining spatiotemporal filtering and model blind learning. Tests with synthetic and real two-photon Ca²⁺ imaging datasets demonstrate that the proposed approach enables efficient restoration of imaging data. In addition, we demonstrate that the proposed approach outperforms the current state-of-the-art methods by evaluating the qualities of the denoising performance of the models quantitatively. Therefore, our method provides an invaluable tool for denoising two-photon Ca²⁺ imaging data by model blind spatiotemporal processing.

Solvatochromy and symmetry breaking in two quadrupolar oligophenylenevinylenes

Electrooptical absorption measurements (EOAM), solvatochromic dependences and quantum chemical simulations testify to large dipole moments change of two quadrupolar oligophenylenevinylenes upon transition to Franck-Condon excited state μ_e^FC. The values of the dipole moments μ_g and μ_e^FC are in the range [(4.2 - 4.9)10³⁰] C m and (30.8 - 47.0)10³⁰C m, respectively. The relations of dipole moments in the ground and excited states determined by EOAM correlate well with results obtained via the solvatochromic method. Calculations carried out by density functional theory (DFT) show that optimized configuration of the ground state of these molecules is not planar. The results from all methods applied unequivocally show the structural symmetry breaking in the studied compounds.

Macroscopic and microscopic imaging modalities for diagnosis and monitoring of urogenital schistosomiasis

Urogenital schistosomiasis remains a major global challenge. Optimal management of this infection depends upon imaging-based assessment of sequelae. Although established imaging modalities such as ultrasonography, plain radiography, magnetic resonance imaging (MRI), narrow band imaging, and computerized tomography (CT) have been used to determine tissue involvement by urogenital schistosomiasis, newer refinements in associated technologies may lead to improvements in patient care. Moreover, application of investigational imaging methods such as confocal laser endomicroscopy and two-photon microscopy in animal models of urogenital schistosomiasis are likely to contribute to our understanding of this infection's pathogenesis. This review discusses prior use of imaging in patients with urogenital schistosomiasis and experimentally infected animals, the advantages and limitations of these modalities, the latest radiologic developments relevant to this infection, and a proposed future diagnostic standard of care for management of afflicted patients.

A Substituent Effect on Two-Photon Absorption of Diphenylacetylene Derivatives with an Electron-Donating/Withdrawing Group

Two-photon absorption for diphenylacetylene derivatives with an electron-donating (ED) or electron-withdrawing (EW) group (DPA-Rs) was investigated by high-sensitivity optical-probing photoacoustic spectroscopy. Two-photon absorption spectra and two-photon absorption cross sections σ⁽²⁾ for DPA-Rs were successfully obtained. Two-photon absorption spectra of DPA-Rs with stronger ED or EW groups display more significant red-shifts and larger σ⁽²⁾ values. Simulated two-photon absorption spectra, using time-dependent density functional theory within the Tamm-Dancoff approximation, compared well with the experimental spectra. Based on the three-state model, the substituent effect on the two-photon absorption for DPA-Rs was expected to manifest in the transition dipole moments and detuning energies. Information obtained from investigating the monosubstituent effect on two-photon absorption of DPA is critical for an improved understanding of two-photon absorption.

Functional Dissection of Glutamatergic and GABAergic Neurons in the Bed Nucleus of the Stria Terminalis

The bed nucleus of the stria terminalis (BNST)-a key part of the extended amygdala-has been implicated in the regulation of diverse behavioral states, ranging from anxiety and reward processing to feeding behavior. Among the host of distinct types of neurons within the BNST, recent investigations employing cell type- and projection-specific circuit dissection techniques (such as optogenetics, chemogenetics, deep-brain calcium imaging, and the genetic and viral methods for targeting specific types of cells) have highlighted the key roles of glutamatergic and GABAergic neurons and their axonal projections. As anticipated from their primary roles in excitatory and inhibitory neurotransmission, these studies established that the glutamatergic and GABAergic subpopulations of the BNST oppositely regulate diverse behavioral states. At the same time, these studies have also revealed unexpected functional specificity and heterogeneity within each subpopulation. In this Minireview, we introduce the body of studies that investigated the function of glutamatergic and GABAergic BNST neurons and their circuits. We also discuss unresolved questions and future directions for a more complete understanding of the cellular diversity and functional heterogeneity within the BNST.

In silico exploration of mouse brain dynamics by focal stimulation reflects the organization of functional networks and sensory processing

Resting-state functional networks such as the default mode network (DMN) dominate spontaneous brain dynamics. To date, the mechanisms linking brain structure and brain dynamics and functions in cognition, perception, and action remain unknown, mainly due to the uncontrolled and erratic nature of the resting state. Here we used a stimulation paradigm to probe the brain’s resting behavior, providing insights on state-space stability and multiplicity of network trajectories after stimulation. We performed explorations on a mouse model to map spatiotemporal brain dynamics as a function of the stimulation site. We demonstrated the emergence of known functional networks in brain responses. Several responses heavily relied on the DMN and were suggestive of the DMN playing a mechanistic role between functional networks. We probed the simulated brain responses to the stimulation of regions along the information processing chains of sensory systems from periphery up to primary sensory cortices. Moreover, we compared simulated dynamics against in vivo brain responses to optogenetic stimulation. Our results underwrite the importance of anatomical connectivity in the functional organization of brain networks and demonstrate how functionally differentiated information processing chains arise from the same system.

We demonstrate how functionally differentiated information processing chains arise from the same anatomical network. The main result of the in-silico mouse brain simulations is the emergence of specific functional networks based on structural data from the mouse brain. When the brain is stimulated, for example, by sensory inputs or direct electrical stimulation, the brain initially responds with activities in specific regions. The brain’s anatomical connectivity constrains the subsequent pattern formation. We built a high-resolution mouse brain network model. The model structure originated from experimental data. We systematically explored the mouse model and investigated the simulated brain dynamics after stimulation. Known functional networks emerged in the simulated brain responses. The default mode network occurred in almost all characteristic response patterns. Simulated brain response dynamics and in-vivo response dynamics of the mouse brain to optogenetic stimulation showed similarities even without parameter tuning. Anatomical connectivity and dynamics shape the functional organization of brain networks.

Two-Photon Excited Polarization-Dependent Autofluorescence of Amyloids as a Label-Free Method of Fibril Organization Imaging

Amyloids are broadly investigated protein misfolding products with characteristic β-sheet assemblies that have an important role in neurodegenerative diseases (e.g., Alzheimer's disease). While they are usually visualized by staining with Thioflavin-T, Congo Red, or other fluorescent markers, it still arouses a controversy over possible staining molecule influence on the amyloid structure or aggregation process. In this work we present, for the first time, the polarization analysis of two-photon excited autofluorescence of amyloids and confirm that polarization dependence of the observed emission can be correlated with the orientation of fibrils. We show the potential of two-photon excited autofluorescence for resolution of molecular organization of fibrils within amyloid superstructures. This label-free method is compatible with two-photon imaging already applied in investigation of neurodegeneration model in mice.

Tools and tutorial on practical ray tracing for microscopy

Significance: An advanced understanding of optical design is necessary to create optimal systems but this is rarely taught as part of general curriculum. Compounded by the fact that professional optical design software tools have a prohibitive learning curve, this means that neither knowledge nor tools are easily accessible. Aim: In this tutorial, we introduce a raytracing module for Python, originally developed for teaching optics with ray matrices, to simplify the design and optimization of optical systems. Approach: This module is developed for ray matrix calculations in Python. Many important concepts of optical design that are often poorly understood such as apertures, aperture stops, and field stops are illustrated. Results: The module is explained with examples in real systems with collection efficiency, vignetting, and intensity profiles. Also, the optical invariant, an important benchmark property for optical systems, is used to characterize an optical system. Conclusions: This raytracing Python module will help improve the reader's understanding of optics and also help them design optimal systems.

Neutrophils infiltrate into the spiral ligament but not the stria vascularis in the cochlea during lipopolysaccharide-induced inflammation

It has been challenging to apply intravital imaging for monitoring the inner ear, as the anatomical location and intricate structure hamper the access of imaging instruments to the inner ear of live mice. By employing intravital imaging of the cochlea in live mice with two-photon microscopy, we investigated neutrophil infiltration into the cochlea tissue and its characteristics under a lipopolysaccharide (LPS)-induced inflammatory state. Methods: Cochlea inflammation was induced by LPS injection to the middle ear. Using two-photon intravital microscopy with specifically designed surgical exteriorization of the cochlea in live mice, we investigated the dynamic features of neutrophils in the lateral wall of the cochlea. The molecular expression pattern of the cochlea lateral wall was also investigated during the LPS-induce inflammation. Results: Despite the contention of whether neutrophils are recruited to the spiral ligament (SL) during inflammation, we observed that LPS-induced inflammation of the middle ear, which mimics acute otitis media, triggered neutrophil migration to the SL in the lateral wall. Notably, massive neutrophil infiltration to the SL occurred 2 days after LPS inoculation, but there was no neutrophil infiltration into the stria vascularis (SV) region. At 1 day after LPS-induced cochlear inflammation, increased mRNA expression of interleukin-1β, interleukin-6 were identified in both the SL and SV, while the ICAM-1 mRNA expression increased only in the SL. The differential reactivity of ICAM-1 is likely responsible for the different neutrophil recruitment pattern in the cochlea. Conclusion: Intravital imaging of the cochlea revealed that neutrophil recruitment and infiltration during inflammation are spatially controlled and exclusively observed in the SL but not in the SV and organ of Corti.

Axially resolved volumetric two-photon microscopy with an extended field of view using depth localization under mirrored Airy beams

It is a great challenge in two-photon microscopy (2PM) to have a high volumetric imaging speed without sacrificing the spatial and temporal resolution in three dimensions (3D). The structure in 2PM images could be reconstructed with better spatial and temporal resolution by the proper choice of the data processing algorithm. Here, we propose a method to reconstruct 3D volume from 2D projections imaged by mirrored Airy beams. We verified that our approach can achieve high accuracy in 3D localization over a large axial range and is applicable to continuous and dense sample. The effective field of view after reconstruction is expanded. It is a promising technique for rapid volumetric 2PM with axial localization at high resolution.

Prolonged exposure to stressors suppresses exploratory behavior in zebrafish larvae

Environmental stressors induce rapid physiological and behavioral shifts in vertebrate animals. However, the neurobiological mechanisms responsible for stress-induced changes in behavior are complex and not well understood. Similar to mammalian vertebrates, zebrafish adults display a preference for dark environments that is associated with predator avoidance, enhanced by stressors, and broadly used in assays for anxiety-like behavior. Although the larvae of zebrafish are a prominent model organism for understanding neural circuits, few studies have examined the effects of stressors on their behavior. This study examines the effects of noxious chemical and electric shock stressors on locomotion and light preference in zebrafish larvae. We found that both stressors elicited similar changes in behavior. Acute exposure induced increased swimming activity, while prolonged exposure depressed activity. Neither stressor produced a consistent shift in light-dark preference, but prolonged exposure to these stressors resulted in a pronounced decrease in exploration of different visual environments. We also examined the effects of exposure to a noxious chemical cue using whole-brain calcium imaging, and identified neural correlates in the area postrema, an area of the hindbrain containing noradrenergic and dopaminergic neurons. Pharmaceutical blockade experiments showed that α-adrenergic receptors contribute to the behavioral response to an acute stressor but are not necessary for the response to a prolonged stressor. These results indicate that zebrafish larvae have complex behavioral responses to stressors comparable to those of adult animals, and also suggest that these responses are mediated by similar neural pathways.

Infrared Excitation Induces Heating and Calcium Microdomain Hyperactivity in Cortical Astrocytes

Unraveling how neural networks process and represent sensory information and how these cellular signals instruct behavioral output is a main goal in neuroscience. Two-photon activation of optogenetic actuators and calcium (Ca²⁺) imaging with genetically encoded indicators allow, respectively, the all-optical stimulation and readout of activity from genetically identified cell populations. However, these techniques locally expose the brain to high near-infrared light doses, raising the concern of light-induced adverse effects on the biology under study. Combining 2P imaging of Ca²⁺ transients in GCaMP6f-expressing cortical astrocytes and unbiased machine-based event detection, we demonstrate the subtle build-up of aberrant microdomain Ca²⁺ transients in the fine astroglial processes that depended on the average rather than peak laser power. Illumination conditions routinely being used in biological 2P microscopy (920-nm excitation, ∼100-fs, and ∼10 mW average power) increased the frequency of microdomain Ca²⁺ events but left their amplitude, area, and duration largely unchanged. Ca²⁺ transients in the otherwise silent soma were secondary to this peripheral hyperactivity that occurred without overt morphological damage. Continuous-wave (nonpulsed) 920-nm illumination at the same average power was as damaging as femtosecond pulses, unraveling the dominance of a heating-mediated damage mechanism. In an astrocyte-specific inositol 3-phosphate receptor type-2 knockout mouse, near-infrared light-induced Ca²⁺ microdomains persisted in the small processes, underpinning their resemblance to physiological inositol 3-phosphate receptor type-2-independent Ca²⁺ signals, whereas somatic hyperactivity was abolished. We conclude that, contrary to what has generally been believed in the field, shorter pulses and lower average power can help to alleviate damage and allow for longer recording windows at 920 nm.

Demonstration of electron diffraction from membrane protein crystals grown in a lipidic mesophase after lamella preparation by focused ion beam milling at cryogenic temperatures

Electron diffraction experiments on crystals of membrane proteins grown in lipidic mesophases have not been possible owing to a thick layer of viscous crystallization medium around the crystals. Here it is shown that focused ion beam milling at cryogenic temperatures (cryo-FIB milling) can remove the viscous layer, and high-quality electron diffraction on a FIB-milled lamella of a bacteriorhodopsin 3D crystal is demonstrated.

Electron crystallography of sub-micrometre-sized 3D protein crystals has emerged recently as a valuable field of structural biology. In meso crystallization methods, utilizing lipidic mesophases, particularly lipidic cubic phases (LCPs), can produce high-quality 3D crystals of membrane proteins (MPs). A major step towards realizing 3D electron crystallography of MP crystals, grown in meso, is to demonstrate electron diffraction from such crystals. The first task is to remove the viscous and sticky lipidic matrix that surrounds the crystals without damaging the crystals. Additionally, the crystals have to be thin enough to let electrons traverse them without significant multiple scattering. In the present work, the concept that focused ion beam milling at cryogenic temperatures (cryo-FIB milling) can be used to remove excess host lipidic mesophase matrix is experimentally verified, and then the crystals are thinned to a thickness suitable for electron diffraction. In this study, bacteriorhodopsin (BR) crystals grown in a lipidic cubic mesophase of monoolein were used as a model system. LCP from a part of a hexagon-shaped plate-like BR crystal (∼10 µm in thickness and ∼70 µm in the longest dimension), which was flash-frozen in liquid nitro­gen, was milled away with a gallium FIB under cryogenic conditions, and a part of the crystal itself was thinned into a ∼210 nm-thick lamella with the ion beam. The frozen sample was then transferred into an electron cryo-microscope, and a nanovolume of ∼1400 × 1400 × 210 nm of the BR lamella was exposed to 200 kV electrons at a fluence of ∼0.06 e Å⁻². The resulting electron diffraction peaks were detected beyond 2.7 Å resolution (with an average peak height to background ratio of >2) by a CMOS-based Ceta 16M camera. The results demonstrate that cryo-FIB milling produces high-quality lamellae from crystals grown in lipidic mesophases and pave the way for 3D electron crystallography on crystals grown or embedded in highly viscous media.

Nuclear crowding and nonlinear diffusion during interkinetic nuclear migration in the zebrafish retina

An important question in early neural development is the origin of stochastic nuclear movement between apical and basal surfaces of neuroepithelia during interkinetic nuclear migration. Tracking of nuclear subpopulations has shown evidence of diffusion - mean squared displacements growing linearly in time - and suggested crowding from cell division at the apical surface drives basalward motion. Yet, this hypothesis has not yet been tested, and the forces involved not quantified. We employ long-term, rapid light-sheet and two-photon imaging of early zebrafish retinogenesis to track entire populations of nuclei within the tissue. The time-varying concentration profiles show clear evidence of crowding as nuclei reach close-packing and are quantitatively described by a nonlinear diffusion model. Considerations of nuclear motion constrained inside the enveloping cell membrane show that concentration-dependent stochastic forces inside cells, compatible in magnitude to those found in cytoskeletal transport, can explain the observed magnitude of the diffusion constant.

Three-photon imaging of synthetic dyes in deep layers of the neocortex

Multiphoton microscopy has emerged as the primary imaging tool for studying the structural and functional dynamics of neural circuits in brain tissue, which is highly scattering to light. Recently, three-photon microscopy has enabled high-resolution fluorescence imaging of neurons in deeper brain areas that lie beyond the reach of conventional two-photon microscopy, which is typically limited to ~ 450 µm. Three-photon imaging of neuronal calcium signals, through the genetically-encoded calcium indicator GCaMP6, has been used to successfully record neuronal activity in deeper neocortical layers and parts of the hippocampus in rodents. Bulk-loading cells in deeper cortical layers with synthetic calcium indicators could provide an alternative strategy for labelling that obviates dependence on viral tropism and promoter penetration, particularly in non-rodent species. Here we report a strategy for visualized injection of a calcium dye, Oregon Green BAPTA-1 AM (OGB-1 AM), at 500-600 µm below the surface of the mouse visual cortex in vivo. We demonstrate successful OGB-1 AM loading of cells in cortical layers 5-6 and subsequent three-photon imaging of orientation- and direction- selective visual responses from these cells.

LED Zappelin': An open source LED controller for arbitrary spectrum visual stimulation and optogenetics during 2-photon imaging

Two-photon (2P) microscopy is a cornerstone technique in neuroscience research. However, combining 2P imaging with spectrally arbitrary light stimulation can be challenging due to crosstalk between stimulation light and fluorescence detection. To overcome this limitation, we present a simple and low-cost electronic solution based on an ESP32 microcontroller and a TLC5947 LED driver to rapidly time-interleave stimulation and detection epochs during scans. Implemented for less than $100, our design can independently drive up to 24 arbitrary spectrum LEDs to meet user requirements. We demonstrate the utility of our stimulator for colour vision experiments on the in vivo tetrachromatic zebrafish retina and for optogenetic circuit mapping in Drosophila.

Imaging the Neuroimmune Dynamics Across Space and Time

The immune system is essential for maintaining homeostasis, as well as promoting growth and healing throughout the brain and body. Considering that immune cells respond rapidly to changes in their microenvironment, they are very difficult to study without affecting their structure and function. The advancement of non-invasive imaging methods greatly contributed to elucidating the physiological roles performed by immune cells in the brain across stages of the lifespan and contexts of health and disease. For instance, techniques like two-photon in vivo microscopy were pivotal for studying microglial functional dynamics in the healthy brain. Through these observations, their interactions with neurons, astrocytes, blood vessels and synapses were uncovered. High-resolution electron microscopy with immunostaining and 3D-reconstruction, as well as super-resolution fluorescence microscopy, provided complementary insights by revealing microglial interventions at synapses (phagocytosis, trogocytosis, synaptic stripping, etc.). In addition, serial block-face scanning electron microscopy has provided the first 3D reconstruction of a microglial cell at nanoscale resolution. This review will discuss the technical toolbox that currently allows to study microglia and other immune cells in the brain, as well as introduce emerging methods that were developed and could be used to increase the spatial and temporal resolution of neuroimmune imaging. A special attention will also be placed on positron emission tomography and the development of selective functional radiotracers for microglia and peripheral macrophages, considering their strong potential for research translation between animals and humans, notably when paired with other imaging modalities such as magnetic resonance imaging.

Adaptive optics two-photon endomicroscopy enables deep-brain imaging at synaptic resolution over large volumes

Optical deep-brain imaging in vivo at high resolution has remained a great challenge over the decades. Two-photon endomicroscopy provides a minimally invasive approach to image buried brain structures, once it is integrated with a gradient refractive index (GRIN) lens embedded in the brain. However, its imaging resolution and field of view are compromised by the intrinsic aberrations of the GRIN lens. Here, we develop a two-photon endomicroscopy by adding adaptive optics based on direct wavefront sensing, which enables recovery of diffraction-limited resolution in deep-brain imaging. A new precompensation strategy plays a critical role to correct aberrations over large volumes and achieve rapid random-access multiplane imaging. We investigate the neuronal plasticity in the hippocampus, a critical deep brain structure, and reveal the relationship between the somatic and dendritic activity of pyramidal neurons.

Two-Photon Voltage Imaging of Spontaneous Activity from Multiple Neurons Reveals Network Activity in Brain Tissue

Recording the electrical activity of multiple neurons simultaneously would greatly facilitate studies on the function of neuronal circuits. The combination of the fast scanning by random-access multiphoton microscopy (RAMP) and the latest two-photon-compatible high-performance fluorescent genetically encoded voltage indicators (GEVIs) has enabled action potential detection in deep layers in in vivo brain. However, neuron connectivity analysis on optically recorded action potentials from multiple neurons in brain tissue has yet to be achieved. With high expression of a two-photon-compatible GEVI, ASAP3, via in utero electroporation and RAMP, we achieved voltage recording of spontaneous activities from multiple neurons in brain slice. We provide evidence for the developmental changes in intralaminar horizontal connections in somatosensory cortex layer 2/3 with a greater sensitivity than calcium imaging. This method thus enables investigation of neuronal network connectivity at the cellular resolution in brain tissue.

Bidirectional Optogenetic Control of Inhibitory Neurons in Freely-Moving Mice

OBJECTIVE

Optogenetic manipulations of excitable cells enable activating or silencing specific types of neurons. By expressing two types of exogenous proteins, a single neuron can be depolarized using light of one wavelength and hyperpolarized with another. However, routing two distinct wavelengths into the same brain locality typically requires bulky optics that cannot be implanted on the head of a freely-moving animal.

METHODS

We developed a lens-free approach for constructing dual-color head-mounted, fiber-based optical units: any two wavelengths can be combined.

RESULTS

Here, each unit was comprised of one 450 nm and one 638 nm laser diode, yielding light power of 0.4 mW and 8 mW at the end of a 50 micrometer multimode fiber. To create a multi-color/multi-site optoelectronic device, a four-shank silicon probe mounted on a microdrive was equipped with two dual-color and two single-color units, for a total weight under 3 g. Devices were implanted in mice expressing the blue-light sensitive cation channel ChR2 and the red-light sensitive chloride pump Jaws in parvalbumin-immunoreactive (PV) inhibitory neurons. The combination of dual-color units with recording electrodes was free from electromagnetic interference, and device heating was under 7°C even after prolonged operation.

CONCLUSION

Using these devices, the same cortical PV cell could be activated and silenced. This was achieved for multiple cells both in neocortex and hippocampus of freely-moving mice.

SIGNIFICANCE

This technology can be used for controlling spatially intermingled neurons that have distinct genetic profiles, and for controlling spike timing of cortical neurons during cognitive tasks.

Three-Dimensional Distribution of Cochlear Macrophages in the Lateral Wall of Cleared Cochlea

Objectives

Resident macrophages are well known to be present in the cochlea, but the exact patterns thereof in spiral ligaments have not been discussed in previous studies. We sought to document the distribution of macrophages in intact cochleae using three-dimensional imaging.

Methods

Cochleae were obtained from C-X3-C motif chemokine receptor 1^+/GFP mice, and organ clearing was performed. Three-dimensional images of cleared intact cochleae were reconstructed using two-photon microscopy. The locations of individual macrophages were investigated using 100-μm stacked images to reduce bias. Cochlear inflammation was then induced by lipopolysaccharide (LPS) inoculation into the middle ear through the tympanic membrane. Four days after inoculation, three-dimensional images were obtained.

Results

Macrophages were scarce in areas adjacent to the stria vascularis, particularly the area just beneath it even though many have suspected macrophages to be abundant in this area. This finding remained consistent upon LPS-induced cochlear inflammation, despite a significant increase in the number of macrophages, compared to non-treated cochlea.

Conclusion

Resident macrophages in spiral ligaments are scarce in areas adjacent to the stria vascularis.

Automated Curation of CNMF-E-Extracted ROI Spatial Footprints and Calcium Traces Using Open-Source AutoML Tools

In vivo 1-photon (1p) calcium imaging is an increasingly prevalent method in behavioral neuroscience. Numerous analysis pipelines have been developed to improve the reliability and scalability of pre-processing and ROI extraction for these large calcium imaging datasets. Despite these advancements in pre-processing methods, manual curation of the extracted spatial footprints and calcium traces of neurons remains important for quality control. Here, we propose an additional semi-automated curation step for sorting spatial footprints and calcium traces from putative neurons extracted using the popular constrained non-negative matrixfactorization for microendoscopic data (CNMF-E) algorithm. We used the automated machine learning (AutoML) tools TPOT and AutoSklearn to generate classifiers to curate the extracted ROIs trained on a subset of human-labeled data. AutoSklearn produced the best performing classifier, achieving an F1 score >92% on the ground truth test dataset. This automated approach is a useful strategy for filtering ROIs with relatively few labeled data points and can be easily added to pre-existing pipelines currently using CNMF-E for ROI extraction.

Second Harmonic Generation Imaging of Collagen in Chronically Implantable Electrodes in Brain Tissue

Advances in neural engineering have brought about a number of implantable devices for improved brain stimulation and recording. Unfortunately, many of these micro-implants have not been adopted due to issues of signal loss, deterioration, and host response to the device. While glial scar characterization is critical to better understand the mechanisms that affect device functionality or tissue viability, analysis is frequently hindered by immunohistochemical tissue processing methods that result in device shattering and tissue tearing artifacts. Devices are commonly removed prior to sectioning, which can itself disturb the quality of the study. In this methods implementation study, we use the label free, optical sectioning method of second harmonic generation (SHG) to examine brain slices of various implanted intracortical electrodes and demonstrate collagen fiber distribution not found in normal brain tissue. SHG can easily be used in conjunction with multiphoton microscopy to allow direct intrinsic visualization of collagen-containing glial scars on the surface of cortically implanted electrode probes without imposing the physical strain of tissue sectioning methods required for other high resolution light microscopy modalities. Identification and future measurements of these collagen fibers may be useful in predicting host immune response and device signal fidelity.

High-speed fiber-optic scanning nonlinear endomicroscopy for imaging neuron dynamicsin vivo

Fiber-optic-based two-photon fluorescence endomicroscopy is emerging as an enabling technology for in vivo histological imaging of internal organs and functional neuronal imaging on freely-behaving animals. However, high-speed imaging remains challenging due to the expense of miniaturization and lack of suited fast beam scanners. For many applications, a higher imaging speed is highly desired, especially for monitoring functional dynamics such as transient dendritic responses in neuroscience. This Letter reports the development of a fast fiber-optic scanning endo-microscope with an imaging speed higher than 26 frames/s. In vivo neural dynamics imaging with the high-speed endomicroscope was performed on a freely-behaving mouse over the primary motor cortex that expressed GCaMP6m. The results demonstrate its capability of real-time monitoring of transient neuronal dynamics with very fine temporal resolution.

AI in Medical Imaging Informatics: Current Challenges and Future Directions

This paper reviews state-of-the-art research solutions across the spectrum of medical imaging informatics, discusses clinical translation, and provides future directions for advancing clinical practice. More specifically, it summarizes advances in medical imaging acquisition technologies for different modalities, highlighting the necessity for efficient medical data management strategies in the context of AI in big healthcare data analytics. It then provides a synopsis of contemporary and emerging algorithmic methods for disease classification and organ/ tissue segmentation, focusing on AI and deep learning architectures that have already become the de facto approach. The clinical benefits of in-silico modelling advances linked with evolving 3D reconstruction and visualization applications are further documented. Concluding, integrative analytics approaches driven by associate research branches highlighted in this study promise to revolutionize imaging informatics as known today across the healthcare continuum for both radiology and digital pathology applications. The latter, is projected to enable informed, more accurate diagnosis, timely prognosis, and effective treatment planning, underpinning precision medicine.

Real-time 3D movement correction for two-photon imaging in behaving animals

Two-photon microscopy is widely used to investigate brain function across multiple spatial scales. However, measurements of neural activity are compromised by brain movement in behaving animals. Brain motion-induced artifacts are typically corrected using post hoc processing of two-dimensional images, but this approach is slow and does not correct for axial movements. Moreover, the deleterious effects of brain movement on high-speed imaging of small regions of interest and photostimulation cannot be corrected post hoc. To address this problem, we combined random-access three-dimensional (3D) laser scanning using an acousto-optic lens and rapid closed-loop field programmable gate array processing to track 3D brain movement and correct motion artifacts in real time at up to 1 kHz. Our recordings from synapses, dendrites and large neuronal populations in behaving mice and zebrafish demonstrate real-time movement-corrected 3D two-photon imaging with submicrometer precision.

Mapping astrocyte activity domains by light sheet imaging and spatio-temporal correlation screening

Astrocytes are a major type of glial cell in the mammalian brain, essentially regulating neuronal development and function. Quantitative imaging represents an important approach to study astrocytic signaling in neural circuits. Focusing on astrocytic Ca²⁺ activity, a key pathway implicated in astrocye-neuron interaction, we here report a strategy combining fast light sheet fluorescence microscopy (LSFM) and correlative screening-based time series analysis, to map activity domains in astrocytes in living mammalian nerve tissue. Light sheet of micron-scale thickness enables wide-field optical sectioning to image astrocytes in acute mouse brain slices. Using both chemical and genetically encoded Ca²⁺ indicators, we demonstrate the complementary advantages of LSFM in mapping Ca²⁺ domains in astrocyte populations as compared to epifluorescence and two-photon microscopy. Our approach then revealed distinct kinetics of Ca²⁺ signals between cortical and hypothalamic astrocytes in resting conditions and following the activation of adrenergic G protein coupled receptor (GPCR). This observation highlights the activity heterogeneity across regionally distinct astrocyte populations, and indicates the potential of our method for investigating dynamic signals in astrocytes.

Investigation of intact mouse cochleae using two-photon laser scanning microscopy

OBJECTIVES

The investigation of cochlear hair cells and lateral wall is a time-consuming and labor-intensive process. However, it is a mandatory experiment in audiology research. Here we suggest a novel method for investigating the inner ear microstructures from intact cochleae using two-photon laser scanning microscopy (TPLSM). This technique guarantees fewer artifacts and technical simplicity.

METHODS

Using TPLSM, we investigated the whole mount cochleae, decalcified cochleae, and cleared cochleae of wild type C57BL/6 mice. CX3CR1^+/GFP mice were used to investigate the feasibility of visualizing cellular structures in the cochlear spiral ligament. All samples were investigated without staining.

RESULTS

Endogenous fluorescence emission from the outer hair cells was strong enough to be distinguished from the other structures in all samples. From the single apical view, 50 and 90% of the whole hair cells of the decalcified cochleae and cleared cochleae, respectively, could be visualized without staining using TPLSM. Capillary structure of stria vascularis and spiral ligament could be visualized by endogenous fluorescence without staining.

CONCLUSION

We successfully investigated the hair cells and lateral wall of mouse cochleae using TPLSM without using staining or any destructive procedures. This method is easier, faster, and more reliable than conventional methods.

Ultrafast Two-Photon Imaging of a High-Gain Voltage Indicator in Awake Behaving Mice

Optical interrogation of voltage in deep brain locations with cellular resolution would be immensely useful for understanding how neuronal circuits process information. Here, we report ASAP3, a genetically encoded voltage indicator with 51% fluorescence modulation by physiological voltages, submillisecond activation kinetics, and full responsivity under two-photon excitation. We also introduce an ultrafast local volume excitation (ULoVE) method for kilohertz-rate two-photon sampling in vivo with increased stability and sensitivity. Combining a soma-targeted ASAP3 variant and ULoVE, we show single-trial tracking of spikes and subthreshold events for minutes in deep locations, with subcellular resolution and with repeated sampling over days. In the visual cortex, we use soma-targeted ASAP3 to illustrate cell-type-dependent subthreshold modulation by locomotion. Thus, ASAP3 and ULoVE enable high-speed optical recording of electrical activity in genetically defined neurons at deep locations during awake behavior.

Comparing the fundamental imaging depth limit of two-photon, three-photon, and non-degenerate two-photon microscopy

We have systematically characterized the degradation of imaging quality with depth in deep brain multi-photon microscopy, utilizing our recently developed numerical model that computes wave propagation in scattering media. The signal-to-background ratio (SBR) and the resolution determined by the width of the point spread function are obtained as functions of depth. We compare the imaging quality of two-photon (2PM), three-photon (3PM), and non-degenerate two-photon microscopy (ND-2PM) for mouse brain imaging. We show that the imaging depth of 2PM and ND-2PM are fundamentally limited by the SBR, while the SBR remains approximately invariant with imaging depth for 3PM. Instead, the imaging depth of 3PM is limited by the degradation of the resolution, if there is sufficient laser power to maintain the signal level at large depth. The roles of the concentration of dye molecules, the numerical aperture of the input light, the anisotropy factor g, noise level, input laser power, and the effect of temporal broadening are also discussed.

EZcalcium: Open-Source Toolbox for Analysis of Calcium Imaging Data

Fluorescence calcium imaging using a range of microscopy approaches, such as two-photon excitation or head-mounted “miniscopes,” is one of the preferred methods to record neuronal activity and glial signals in various experimental settings, including acute brain slices, brain organoids, and behaving animals. Because changes in the fluorescence intensity of genetically encoded or chemical calcium indicators correlate with action potential firing in neurons, data analysis is based on inferring such spiking from changes in pixel intensity values across time within different regions of interest. However, the algorithms necessary to extract biologically relevant information from these fluorescent signals are complex and require significant expertise in programming to develop robust analysis pipelines. For decades, the only way to perform these analyses was for individual laboratories to write their custom code. These routines were typically not well annotated and lacked intuitive graphical user interfaces (GUIs), which made it difficult for scientists in other laboratories to adopt them. Although the panorama is changing with recent tools like CaImAn, Suite2P, and others, there is still a barrier for many laboratories to adopt these packages, especially for potential users without sophisticated programming skills. As two-photon microscopes are becoming increasingly affordable, the bottleneck is no longer the hardware, but the software used to analyze the calcium data optimally and consistently across different groups. We addressed this unmet need by incorporating recent software solutions, namely NoRMCorre and CaImAn, for motion correction, segmentation, signal extraction, and deconvolution of calcium imaging data into an open-source, easy to use, GUI-based, intuitive and automated data analysis software package, which we named EZcalcium.

Aggregation-Induced Plasmon Coupling-Enhanced One- and Two-Photon Excitation Fluorescence by Silver Nanoparticles

Plasmon coupling-induced intense local electrical field in the gap of closely packed metal nanoparticles (NPs) has been known capable of significantly enhancing optical properties of chromophores. Here, we have investigated aggregation-induced plasmon coupling-enhanced one-photon excitation (1PE) and two-photon excitation (2PE) fluorescence of dyes using Ag NPs of three different sizes (20, 36, and 48 nm). The fluorescence of a model dye, Rhodamine B isothiocyanate (RiTC), was prequenched by attaching to Ag NPs and subsequently enhanced upon forming aggregates of Ag NPs. It was found that aggregates of larger sized Ag NPs gave larger 1PE and 2PE fluorescence enhancement on the basis of free dyes, while aggregates of smaller counterparts displayed larger enhancement on the basis of the corresponding prequenched ones. 1PE and 2PE fluorescence were enhanced by 2.5- and 10.2-fold by aggregated 48 nm Ag NPs compared to free dyes and by 8.0- and 22.5-fold by aggregated 20 nm Ag NPs compared to the quenched ones, respectively. This scheme achieved fluorescence enhancement significantly beyond the level of fluorescence recovery, much larger than conventional turn-on fluorescence probes, which is attractive for developing sensitive fluorescence turn-on-based detection with reduced background.

Aberration-free 3D imaging via DMD-based two-photon microscopy and sensorless adaptive optics

In this Letter, we present a new, to our knowledge, aberration-free 3D imaging technique based on digital micromirror device (DMD)-based two-photon microscopy and sensorless adaptive optics (AO), where 3D random-access scanning and modal wavefront correction are realized using a single DMD chip at 22.7 kHz. Specifically, the DMD is simultaneously used as a deformable mirror to modulate a distorted wavefront and a fast scanner to maneuver the laser focus in a 3D space by designed binary holograms. As such, aberration-free 3D imaging is realized by superposing the wavefront correction and 3D scanning holograms. Compared with conventional AO devices and methods, the DMD system can apply optimal wavefront correction information to different imaging regions or even individual pixels without compromising the scanning speed and device resolution. In the experiments, we first focus the laser through a diffuser and apply sensorless AO to retrieve a corrected focus. After that, the DMD performs 3D scanning on a Drosophila brain labeled with green fluorescent protein. The two-photon imaging results, where optimal wavefront correction information is applied to 3×3 separate regions, demonstrate significantly improved resolution and image quality. The new DMD-based imaging solution presents a compact, low-cost, and effective solution for aberration-free two-photon deep tissue imaging, which may find important applications in the field of biophotonics.

Recent advances in two-photon absorbing probes based on a functionalized dipolar naphthalene platform

Two-photon microscopy (TPM) techniques have been highlighted over the past two decades throughout various fields, including physics, chemistry, biology, and medicine. In particular, the two-photon near-infrared excitation of fluorophores or molecular probes emitting fluorescence have ushered in a new biomedical era, specifically in the deep-tissue imaging of biologically relevant species. Non-linear two-photon optics enables the development of 3D fluorescence images via focal point excitation of biological samples with low photo-damage and photo-bleaching. Many studies have disclosed the relationship between the chemical structure of fluorophores and their two-photon absorbing properties. In this review, we have summarized the recent advances in two-photon absorbing probes based on a functionalized electron donor (D)-acceptor (A) type dipolar naphthalene platform (FDNP) that was previously reported between 2015 and 2019. Our systematic outline of the synthesis, photophysical properties, and examples of two-photon imaging applications will provide useful context for the future development of new naphthalene backbone-based two-photon probes.

Dynamic Tracking Algorithm for Time-Varying Neuronal Network Connectivity using Wide-Field Optical Image Video Sequences

Propagation of signals between neurons and brain regions provides information about the functional properties of neural networks, and thus information transfer. Advances in optical imaging and statistical analyses of acquired optical signals have yielded various metrics for inferring neural connectivity, and hence for mapping signal intercorrelation. However, a single coefficient is traditionally derived to classify the connection strength between two cells, ignoring the fact that neural systems are inherently time-variant systems. To overcome these limitations, we utilized a time-varying Pearson's correlation coefficient, spike-sorting, wavelet transform, and wavelet coherence of calcium transients from DIV 12-15 hippocampal neurons from GCaMP6s mice after applying various concentrations of glutamate. Results provide a comprehensive overview of resulting firing patterns, network connectivity, signal directionality, and network properties. Together, these metrics provide a more comprehensive and robust method of analyzing transient neural signals, and enable future investigations for tracking the effects of different stimuli on network properties.

Fast, 3D Isotropic Imaging of Whole Mouse Brain Using Multiangle-Resolved Subvoxel SPIM

The recent integration of light-sheet microscopy and tissue-clearing has facilitated an important alternative to conventional histological imaging approaches. However, the in toto cellular mapping of neural circuits throughout an intact mouse brain remains highly challenging, requiring complicated mechanical stitching, and suffering from anisotropic resolution insufficient for high-quality reconstruction in 3D. Here, the use of a multiangle-resolved subvoxel selective plane illumination microscope (Mars-SPIM) is proposed to achieve high-throughput imaging of whole mouse brain at isotropic cellular resolution. This light-sheet imaging technique can computationally improve the spatial resolution over six times under a large field of view, eliminating the use of slow tile stitching. Furthermore, it can recover complete structural information of the sample from images subject to thick-tissue scattering/attenuation. With Mars-SPIM, a digital atlas of a cleared whole mouse brain (≈7 mm × 9.5 mm × 5 mm) can readily be obtained with an isotropic resolution of ≈2 µm (1 µm voxel) and a short acquisition time of 30 min. It provides an efficient way to implement system-level cellular analysis, such as the mapping of different neuron populations and tracing of long-distance neural projections over the entire brain. Mars-SPIM is thus well suited for high-throughput cell-profiling phenotyping of brain and other mammalian organs.

In vivo deep-tissue microscopy with UCNP/Janus-dendrimers as imaging probes: resolution at depth and feasibility of ratiometric sensing

Lanthanide-based upconverting nanoparticles (UCNPs) are known for their remarkable ability to convert near-infrared energy into higher energy light, offering an attractive platform for construction of biological imaging probes. Here we focus on in vivo high-resolution microscopy - an application for which the opportunity to carry out excitation at low photon fluxes in non-linear regime makes UCNPs stand out among all multiphoton probes. To create biocompatible nanoparticles we employed Janus-type dendrimers as surface ligands, featuring multiple carboxylates on one 'face' of the molecule, polyethylene glycol (PEG) residues on another and Eriochrome Cyanine R dye as the core. The UCNP/Janus-dendrimers showed outstanding performance as vascular markers, allowing for depth-resolved mapping of individual capillaries in the mouse brain down to a remarkable depth of ∼1000 μm under continuous wave (CW) excitation with powers not exceeding 20 mW. Using a posteriori deconvolution, high-resolution images could be obtained even at high scanning speeds in spite of the blurring caused by the long luminescence lifetimes of the lanthanide ions. Secondly, the new UCNP/dendrimers allowed us to evaluate the feasibility of quantitative analyte imaging in vivo using a popular ratiometric UCNP-to-ligand excitation energy transfer (EET) scheme. Our results show that the ratio of UCNP emission bands, which for quantitative sensing should respond selectively to the analyte of interest, is also strongly affected by optical heterogeneities of the medium. On the other hand, the luminescence decay times of UCNPs, which are independent of the medium properties, are modulated via EET only insignificantly. As such, quantitative analyte sensing in biological tissues with UCNP-based probes still remains a challenge.

Reversible Loss of Hippocampal Function in a Mouse Model of Demyelination/Remyelination

Demyelination of axons in the central nervous system (CNS) is a hallmark of multiple sclerosis (MS) and other demyelinating diseases. Cycles of demyelination, followed by remyelination, appear in the majority of MS patients and are associated with the onset and quiescence of disease-related symptoms, respectively. Previous studies in human patients and animal models have shown that vast demyelination is accompanied by wide-scale changes to brain activity, but details of this process are poorly understood. We used electrophysiological recordings and non-linear fluorescence imaging from genetically encoded calcium indicators to monitor the activity of hippocampal neurons during demyelination and remyelination over a period of 100 days. We found that synaptic transmission in CA1 neurons was diminished in vitro, and that neuronal firing rates in CA1 and the dentate gyrus (DG) were substantially reduced during demyelination in vivo, which partially recovered after a short remyelination period. This new approach allows monitoring how changes in synaptic transmission induced by cuprizone diet affect neuronal activity, and it can potentially be used to study the effects of therapeutic interventions in protecting the functionality of CNS neurons.

Gating a Single Cell: A Label-Free and Real-Time Measurement Method for Cellular Progression

There is an ever-growing need for more advanced methods to study the response of cancer cells to new therapies. To determine cancer cells' response from a cell-mortality perspective to various cancer therapies, we report a label-free and real time method to monitor the in situ response of individual HeLa cells using a single cell gated transistor (SCGT). As a cell undergoes apoptotic cell death, it experiences changes in morphology and ion concentrations. This change is well in line with the threshold voltage of the SCGT, which has been verified by correlating the data with the cell morphologies by scanning electron microscopy and the ion-concentration analysis by inductively-coupled plasma mass spectrometry (ICPMS). This SCGT could replace patch clamps to study single cell activity via direct measurement in real time. Importantly, this SCGT can be used to study the electrical response of a single cell to stimuli that leaves the membrane intact.

Red-light-sensitive BODIPY photoprotecting groups for amines and their biological application in controlling heart rhythm

Protection of amine functionality with a BODIPY-derived photocleavable protecting group enables the control of heart beat frequency with red light.

Operant conditioning paradigm for juxtacellular recordings in functionally identified cortical neurons during motor execution in head-fixed rats

BACKGROUND

Understanding the configuration of neural circuits and the specific role of distinct cortical neuron types involved in behavior, requires the study of structure-function and connectivity relationships with single cell resolution in awake behaving animals. Despite head-fixed behaving rats have been used for in vivo measuring of neuronal activity, it is a concern that head fixation could change the performance of behavioral task.

NEW METHOD

We describe the procedures for efficiently training Wistar rats to develop a behavioral task, involving planning and execution of a qualified movement in response to a visual cue under head-fixed conditions. The behavioral and movement performance in freely moving vs head-fixed conditions was analyzed.

RESULTS

The best behavioral performance was obtained in the rats that were trained first in freely moving conditions and then placed in a head-restrained condition compared with the animals which first were habituated to head-restriction and then learned the task. Moreover, head restriction did not alter the movement performance. Stable juxtacellular recordings from sensorimotor cortex neurons were obtained while the rats were performing forelimb movements. Biocytin electroporation and retrograde tracer injections, permits identify the hodology of individual long-range projecting neurons.

COMPARISON WITH EXISTING METHODS

Our method shows no difference in the behavioral performance of head fixed and freely moving conditions. Also includes a computer aided design of a discrete and ergonomic head-post allowing enough stability to perform juxtacellular recording and labeling of cortical neurons.

CONCLUSIONS

Our method is suitable for the in vivo characterization of neuronal circuits and their long-range connectivity.

Smart diagnostic nano-agents for cerebral ischemia

A summary of the latest developments on imaging techniques and smart nano-diagnostics used for ischemic stroke.