A dynamic zone defines interneuron remodeling in the adult neocortex

Multiline orthogonal scanning temporal focusing (mosTF) microscopy for scattering reduction in in vivo brain imaging

Temporal focusing two-photon microscopy has been utilized for high-resolution imaging of neuronal and synaptic structures across volumes spanning hundreds of microns in vivo. However, a limitation of temporal focusing is the rapid degradation of the signal-to-background ratio and resolution with increasing imaging depth. This degradation is due to scattered emission photons being widely distributed, resulting in a strong background. To overcome this challenge, we have developed multiline orthogonal scanning temporal focusing (mosTF) microscopy. mosTF captures a sequence of images at each scan location of the excitation line. A reconstruction algorithm then reassigns scattered photons back to their correct scan positions. We demonstrate the effectiveness of mosTF by acquiring neuronal images of mice in vivo. Our results show remarkable improvements in in vivo brain imaging with mosTF, while maintaining its speed advantage.

Mapping thalamic innervation to individual L2/3 pyramidal neurons and modeling their 'readout' of visual input

The thalamus is the main gateway for sensory information from the periphery to the mammalian cerebral cortex. A major conundrum has been the discrepancy between the thalamus's central role as the primary feedforward projection system into the neocortex and the sparseness of thalamocortical synapses. Here we use new methods, combining genetic tools and scalable tissue expansion microscopy for whole-cell synaptic mapping, revealing the number, density and size of thalamic versus cortical excitatory synapses onto individual layer 2/3 (L2/3) pyramidal cells (PCs) of the mouse primary visual cortex. We find that thalamic inputs are not only sparse, but remarkably heterogeneous in number and density across individual dendrites and neurons. Most surprising, despite their sparseness, thalamic synapses onto L2/3 PCs are smaller than their cortical counterparts. Incorporating these findings into fine-scale, anatomically faithful biophysical models of L2/3 PCs reveals how individual neurons with sparse and weak thalamocortical synapses, embedded in small heterogeneous neuronal ensembles, may reliably 'read out' visually driven thalamic input.

A subpopulation of cortical VIP-expressing interneurons with highly dynamic spines

Structural synaptic plasticity may underlie experience and learning-dependent changes in cortical circuits. In contrast to excitatory pyramidal neurons, insight into the structural plasticity of inhibitory neurons remains limited. Interneurons are divided into various subclasses, each with specialized functions in cortical circuits. Further knowledge of subclass-specific structural plasticity of interneurons is crucial to gaining a complete mechanistic understanding of their contribution to cortical plasticity overall. Here, we describe a subpopulation of superficial cortical multipolar interneurons expressing vasoactive intestinal peptide (VIP) with high spine densities on their dendrites located in layer (L) 1, and with the electrophysiological characteristics of bursting cells. Using longitudinal imaging in vivo, we found that the majority of the spines are highly dynamic, displaying lifetimes considerably shorter than that of spines on pyramidal neurons. Using correlative light and electron microscopy, we confirmed that these VIP spines are sites of excitatory synaptic contacts, and are morphologically distinct from other spines in L1.

Epitope-preserving magnified analysis of proteome (eMAP)

Synthetic tissue-hydrogel methods have enabled superresolution investigation of biological systems using diffraction-limited microscopy. However, chemical modification by fixatives can cause loss of antigenicity, limiting molecular interrogation of the tissue gel. Here, we present epitope-preserving magnified analysis of proteome (eMAP) that uses purely physical tissue-gel hybridization to minimize the loss of antigenicity while allowing permanent anchoring of biomolecules. We achieved success rates of 96% and 94% with synaptic antibodies for mouse and marmoset brains, respectively. Maximal preservation of antigenicity allows imaging of nanoscopic architectures in 1000-fold expanded tissues without additional signal amplification. eMAP-processed tissue gel can endure repeated staining and destaining without epitope loss or structural damage, enabling highly multiplexed proteomic analysis. We demonstrated the utility of eMAP as a nanoscopic proteomic interrogation tool by investigating molecular heterogeneity in inhibitory synapses in the mouse brain neocortex and characterizing the spatial distributions of synaptic proteins within synapses in mouse and marmoset brains.

Cellular correlates of gray matter volume changes in magnetic resonance morphometry identified by two-photon microscopy

Magnetic resonance imaging (MRI) of the brain combined with voxel-based morphometry (VBM) revealed changes in gray matter volume (GMV) in various disorders. However, the cellular basis of GMV changes has remained largely unclear. We correlated changes in GMV with cellular metrics by imaging mice with MRI and two-photon in vivo microscopy at three time points within 12 weeks, taking advantage of age-dependent changes in brain structure. Imaging fluorescent cell nuclei allowed inferences on (i) physical tissue volume as determined from reference spaces outlined by nuclei, (ii) cell density, (iii) the extent of cell clustering, and (iv) the volume of cell nuclei. Our data indicate that physical tissue volume alterations only account for 13.0% of the variance in GMV change. However, when including comprehensive measurements of nucleus volume and cell density, 35.6% of the GMV variance could be explained, highlighting the influence of distinct cellular mechanisms on VBM results.

Neuroplasticity of the visual cortex: in sickness and in health

Brain plasticity refers to the ability of synaptic connections to adapt their function and structure in response to experience, including environmental changes, sensory deprivation and injuries. Plasticity is a distinctive, but not exclusive, property of the developing nervous system. This review introduces the concept of neuroplasticity and describes classic paradigms to illustrate cellular and molecular mechanisms underlying synapse modifiability. Then, we summarize a growing number of studies showing that the adult cerebral cortex retains a significant degree of plasticity highlighting how the identification of strategies to enhance the plastic potential of the adult brain could pave the way for the development of novel therapeutic approaches aimed at treating amblyopia and other neurodevelopmental disorders. Finally, we analyze how the visual system adjusts to neurodegenerative conditions leading to blindness and we discuss the crucial role of spared plasticity in the visual system for sight recovery.

Experience-Dependent Development of Dendritic Arbors in Mouse Visual Cortex

The dendritic arbor of neurons constrains the pool of available synaptic partners and influences the electrical integration of synaptic currents. Despite these critical functions, our knowledge of the dendritic structure of cortical neurons during early postnatal development and how these dendritic structures are modified by visual experience is incomplete. Here, we present a large-scale dataset of 849 3D reconstructions of the basal arbor of pyramidal neurons collected across early postnatal development in visual cortex of mice of either sex. We found that the basal arbor grew substantially between postnatal day 7 (P7) and P30, undergoing a 45% increase in total length. However, the gross number of primary neurites and dendritic segments was largely determined by P7. Growth from P7 to P30 occurred primarily through extension of dendritic segments. Surprisingly, comparisons of dark-reared and typically reared mice revealed that a net gain of only 15% arbor length could be attributed to visual experience; most growth was independent of experience. To examine molecular contributions, we characterized the role of the activity-regulated small GTPase Rem2 in both arbor development and the maintenance of established basal arbors. We showed that Rem2 is an experience-dependent negative regulator of dendritic segment number during the visual critical period. Acute deletion of Rem2 reduced directionality of dendritic arbors. The data presented here establish a highly detailed, quantitative analysis of basal arbor development that we believe has high utility both in understanding circuit development as well as providing a framework for computationalists wishing to generate anatomically accurate neuronal models.SIGNIFICANCE STATEMENT Dendrites are the sites of the synaptic connections among neurons. Despite their importance for neural circuit function, only a little is known about the postnatal development of dendritic arbors of cortical pyramidal neurons and the influence of experience. Here we show that the number of primary basal dendritic arbors is already established before eye opening, and that these arbors primarily grow through lengthening of dendritic segments and not through addition of dendritic segments. Surprisingly, visual experience has a modest net impact on overall arbor length (15%). Experiments in KO animals revealed that the gene Rem2 is positive regulator of dendritic length and a negative regulator of dendritic segments.

Long-term activity drives dendritic branch elaboration of a C. elegans sensory neuron

Neuronal activity often leads to alterations in gene expression and cellular architecture. The nematode Caenorhabditis elegans, owing to its compact translucent nervous system, is a powerful system in which to study conserved aspects of the development and plasticity of neuronal morphology. Here we focus on one pair of sensory neurons, termed URX, which the worm uses to sense and avoid high levels of environmental oxygen. Previous studies have reported that the URX neuron pair has variable branched endings at its dendritic sensory tip. By controlling oxygen levels and analyzing mutants, we found that these microtubule-rich branched endings grow over time as a consequence of neuronal activity in adulthood. We also find that the growth of these branches correlates with an increase in cellular sensitivity to particular ranges of oxygen that is observable in the behavior of older worms. Given the strengths of C. elegans as a model organism, URX may serve as a potent system for uncovering genes and mechanisms involved in activity-dependent morphological changes in neurons and possible adaptive changes in the aging nervous system.

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The dendritic tip of an oxygen-sensing neuron grows elaborate microtubule-rich processes in adult C. elegans.

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Dendritic tip elaboration depends on the long-term activity of the neuron and calcium.

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The elaboration correlates with increased sensitivity of the neuron to certain ranges of oxygen as well as higher avoidance of oxygen during bordering behavior.

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The dendritic tip changes may reflect adaptive changes in physiology and behavior during adulthood.

Efficient non-degenerate two-photon excitation for fluorescence microscopy

Non-degenerate two-photon excitation (ND-TPE) has been explored in two-photon excitation microscopy. However, a systematic study of the efficiency of ND-TPE to guide the selection of fluorophore excitation wavelengths is missing. We measured the relative non-degenerate two-photon absorption cross-section (ND-TPACS) of several commonly used fluorophores (two fluorescent proteins and three small-molecule dyes) and generated 2-dimensional ND-TPACS spectra. We observed that the shape of a ND-TPACS spectrum follows that of the corresponding degenerate two-photon absorption cross-section (D-TPACS) spectrum, but is higher in magnitude. We found that the observed enhancements are higher than theoretical predictions.

Enhanced near infrared optical access to the brain with a transparent cranial implant and scalp optical clearing

We report on the enhanced optical transmittance in the NIR wavelength range (900 to 2400 nm) offered by a transparent Yttria-stabilized zirconia (YSZ) implant coupled with optical clearing agents (OCAs). The enhancement in optical access to the brain is evaluated upon comparing ex-vivo transmittance measurements of mice native skull and the YSZ cranial implant with scalp and OCAs. An increase in transmittance of up to 50% and attenuation lengths of up to 2.4 mm (i.e., a five-fold increase in light penetration) are obtained with the YSZ implant and the OCAs. The use of this ceramic implant and the biocompatible optical clearing agents offer attractive features for NIR optical techniques for brain theranostics.

Scattering reduction by structured light illumination in line-scanning temporal focusing microscopy

Line-scanning temporal focusing microscopy (LineTFM) is capable of imaging biological samples more than 10 times faster than two-photon laser point-scanning microscopy (TPLSM), while achieving nearly the same lateral and axial spatial resolution. However, the image contrast taken by LineTFM is lower than that by TPLSM because LineTFM is severely influenced by biological tissue scattering. To reject the scattered photons, we implemented LineTFM using both structured illumination and uniform illumination combined with the HiLo post-processing algorithm, called HiLL microscopy (HiLo-Line-scanning temporal focusing microscopy). HiLL microscopy significantly reduces tissue scattering and improves image contrast. We demonstrate HiLL microscopy with in vivo brain imaging. This approach could potentially find applications in monitoring fast dynamic events and in mapping high resolution structures over a large volume.

Relationships in Ongoing Structural Maintenances of the Two Cerebral Cortices of an Individual Brain

A human brain has separate left and right cerebral cortices, each of which must be continuously structurally maintained during adulthood. There is no understanding of how ongoing structural maintenances of separate parts of a mature individual brain, including the 2 cortices, are related. To explore this issue, this study used an unconventional N-of-1 magnetic resonance imaging time-series paradigm to identify relationships between maintenances of structural thicknesses of the 2 cortices in an adult human brain over week intervals for 6 months. The results suggest that maintenances of left and right cortical thicknesses were symmetrically related in some, but asymmetrically related in other, respects. For matched times, thickness magnitudes and variations on the 2 sides were positively correlated and appeared to reflect maintenance symmetry. Maintenance relationships also extended from earlier to later times with temporal continuity and apparent “if-then” contingencies which were reflected in symmetry and asymmetry dynamics spanning 1- to 2-week periods. The findings suggest concepts of individual brain cortical maintenance symmetry, asymmetry, and temporal continuity dynamics that have not been previously recognized. They have implications for defining cortical maintenance traits or states and for development of N-of-1 precision medicine paradigms that can contribute to understanding individual brain health.

Interneuron Simplification and Loss of Structural Plasticity As Markers of Aging-Related Functional Decline

Changes in excitatory neuron and synapse structure have been recognized as a potential physical source of age-related cognitive decline. Despite the importance of inhibition to brain plasticity, little is known regarding aging-associated changes to inhibitory neurons. Here we test for age-related cellular and circuit changes to inhibitory neurons of mouse visual cortex. We find no substantial difference in inhibitory neuron number, inhibitory neuronal subtypes, or synapse numbers within the cerebral cortex of aged mice compared with younger adults. However, when comparing cortical interneuron morphological parameters, we find differences in complexity, suggesting that arbors are simplified in aged mice. In vivo two-photon microscopy has previously shown that in contrast to pyramidal neurons, inhibitory interneurons retain a capacity for dendritic remodeling in the adult. We find that this capacity diminishes with age and is accompanied by a shift in dynamics from balanced branch additions and retractions to progressive prevalence of retractions, culminating in a dendritic arbor that is both simpler and more stable. Recording of visually evoked potentials shows that aging-related interneuron dendritic arbor simplification and reduced dynamics go hand in hand with loss of induced stimulus-selective response potentiation (SRP), a paradigm for adult visual cortical plasticity. Chronic treatment with the antidepressant fluoxetine reversed deficits in interneuron structural dynamics and restored SRP in aged animals. Our results support a structural basis for age-related impairments in sensory perception, and suggest that declines in inhibitory neuron structural plasticity during aging contribute to reduced functional plasticity.SIGNIFICANCE STATEMENT Structural alterations in neuronal morphology and synaptic connections have been proposed as a potential physical basis for age-related decline in cognitive function. Little is known regarding aging-associated changes to inhibitory neurons, despite the importance of inhibitory circuitry to adult cortical plasticity and the reorganization of cortical maps. Here we show that brain aging goes hand in hand with progressive structural simplification and reduced plasticity of inhibitory neurons, and a parallel decline in sensory map plasticity. Fluoxetine treatment can attenuate the concurrent age-related declines in interneuron structural and functional plasticity, suggesting it could provide an important therapeutic approach for mitigating sensory and cognitive deficits associated with aging.

Neural architecture: from cells to circuits

Circuit operations are determined jointly by the properties of the circuit elements and the properties of the connections among these elements. In the nervous system, neurons exhibit diverse morphologies and branching patterns, allowing rich compartmentalization within individual cells and complex synaptic interactions among groups of cells. In this review, we summarize work detailing how neuronal morphology impacts neural circuit function. In particular, we consider example neurons in the retina, cerebral cortex, and the stomatogastric ganglion of crustaceans. We also explore molecular coregulators of morphology and circuit function to begin bridging the gap between molecular and systems approaches. By identifying motifs in different systems, we move closer to understanding the structure-function relationships that are present in neural circuits.

Experience-Dependent Structural Plasticity in the Visual System

During development, the environment exerts a profound influence on the wiring of brain circuits. Due to the limited resolution of studies in fixed tissue, this experience-dependent structural plasticity was once thought to be restricted to a specific developmental time window. The recent introduction of two-photon microscopy for in vivo imaging has opened the door to repeated monitoring of individual neurons and the study of structural plasticity mechanisms at a very fine scale. In this review, we focus on recent work showing that synaptic structural rearrangements are a key mechanism mediating neural circuit adaptation and behavioral plasticity in the adult brain. We examine this work in the context of classic studies in the visual systems of model organisms, which have laid much of the groundwork for our understanding of activity-dependent synaptic remodeling and its role in brain plasticity.

An Exploration Into Short-Interval Maintenance of Adult Hemispheric Cortical Thickness at an Individual Brain Level

Adult cerebral cortical structure is thought to be statically maintained over short intervals. This view is based on group average findings but has never been studied at the individual level. This issue was examined with an unconventional longitudinal magnetic resonance imaging design which measured hemispheric mean cortical thickness of an adult man repeatedly at week intervals over 6 months. These measures were compared with measurement error estimates to test the current prediction that thickness measures would be statically maintained within measurement error variation. The results did not support this prediction. Thickness underwent incremental and decremental fluctuations which ranged up to 0.12 mm and 5.83% over week and multiweek intervals and which differed from measurement error variation. These exploratory analyses suggest a working hypothesis that short-interval cortical structural maintenance in an individual can involve fluctuations in thickness. If confirmed, this hypothesis has potential implications for cortical maintenance mechanisms and precision medicine approaches.

Activity-dependent axonal plasticity in sensory systems

The rodent whisker-to-barrel cortex pathway is a classic model to study the effects of sensory experience and deprivation on neuronal circuit formation, not only during development but also in the adult. Decades of research have produced a vast body of evidence highlighting the fundamental role of neuronal activity (spontaneous and/or sensory-evoked) for circuit formation and function. In this context, it has become clear that neuronal adaptation and plasticity is not just a function of the neonatal brain, but persists into adulthood, especially after experience-driven modulation of network status. Mechanisms for structural remodeling of the somatodendritic or axonal domain include microscale alterations of neurites or synapses. At the same time, functional alterations at the nanoscale such as expression or activation changes of channels and receptors contribute to the modulation of intrinsic excitability or input-output relationships. However, it remains elusive how these forms of structural and functional plasticity come together to shape neuronal network formation and function. While specifically somatodendritic plasticity has been studied in great detail, the role of axonal plasticity, (e.g. at presynaptic boutons, branches or axonal microdomains), is rather poorly understood. Therefore, this review will only briefly highlight somatodendritic plasticity and instead focus on axonal plasticity. We discuss (i) the role of spontaneous and sensory-evoked plasticity during critical periods, (ii) the assembly of axonal presynaptic sites, (iii) axonal plasticity in the mature brain under baseline and sensory manipulation conditions, and finally (iv) plasticity of electrogenic axonal microdomains, namely the axon initial segment, during development and in the mature CNS.

Calretinin and Neuropeptide Y interneurons are differentially altered in the motor cortex of the SOD1G93A mouse model of ALS

Increasing evidence indicates an excitatory/inhibitory imbalance may have a critical role in the pathogenesis of amyotrophic lateral sclerosis (ALS). Impaired inhibitory circuitry is consistently reported in the motor cortex of both familial and sporadic patients, closely associated with cortical hyperexcitability and ALS onset. Inhibitory network dysfunction is presumably mediated by intra-cortical inhibitory interneurons, however, the exact cell types responsible are yet to be identified. In this study we demonstrate dynamic changes in the number of calretinin- (CR) and neuropeptide Y-expressing (NPY) interneurons in the motor cortex of the familial hSOD1^G93A ALS mouse model, suggesting their potential involvement in motor neuron circuitry defects. We show that the density of NPY-populations is significantly decreased by ~17% at symptom onset (8 weeks), and by end-stage disease (20 weeks) is significantly increased by ~30%. Conversely, the density of CR-populations is progressively reduced during later symptomatic stages (~31%) to end-stage (~36%), while CR-expressing interneurons also show alteration of neurite branching patterns at symptom onset. We conclude that a differential capacity for interneurons exists in the ALS motor cortex, which may not be a static phenomenon, but involves early dynamic changes throughout disease, implicating specific inhibitory circuitry.

An Evolutionarily Conserved Mechanism for Activity-Dependent Visual Circuit Development

Neural circuit development is an activity-dependent process. This activity can be spontaneous, such as the retinal waves that course across the mammalian embryonic retina, or it can be sensory-driven, such as the activation of retinal ganglion cells (RGCs) by visual stimuli. Whichever the source, neural activity provides essential instruction to the developing circuit. Indeed, experimentally altering activity has been shown to impact circuit development and function in many different ways and in many different model systems. In this review, we contemplate the idea that retinal waves in amniotes, the animals that develop either in ovo or utero (namely reptiles, birds and mammals) could be an evolutionary adaptation to life on land, and that the anamniotes, animals whose development is entirely external (namely the aquatic amphibians and fish), do not display retinal waves, most likely because they simply don’t need them. We then review what is known about the function of both retinal waves and visual stimuli on their respective downstream targets, and predict that the experience-dependent development of the tadpole visual system is a blueprint of what will be found in future studies of the effects of spontaneous retinal waves on instructing development of retinorecipient targets such as the superior colliculus (SC) and the lateral geniculate nucleus.

Persistent Structural Plasticity Optimizes Sensory Information Processing in the Olfactory Bulb

In the mammalian brain, the anatomical structure of neural circuits changes little during adulthood. As a result, adult learning and memory are thought to result from specific changes in synaptic strength. A possible exception is the olfactory bulb (OB), where activity guides interneuron turnover throughout adulthood. These adult-born granule cell (GC) interneurons form new GABAergic synapses that have little synaptic strength plasticity. In the face of persistent neuronal and synaptic turnover, how does the OB balance flexibility, as is required for adapting to changing sensory environments, with perceptual stability? Here we show that high dendritic spine turnover is a universal feature of GCs, regardless of their developmental origin and age. We find matching dynamics among postsynaptic sites on the principal neurons receiving the new synaptic inputs. We further demonstrate in silico that this coordinated structural plasticity is consistent with stable, yet flexible, decorrelated sensory representations. Together, our study reveals that persistent, coordinated synaptic structural plasticity between interneurons and principal neurons is a major mode of functional plasticity in the OB.

Inhibitory Synapses Are Repeatedly Assembled and Removed at Persistent Sites In Vivo

Older concepts of a hard-wired adult brain have been overturned in recent years by in vivo imaging studies revealing synaptic remodeling, now thought to mediate rearrangements in microcircuit connectivity. Using three-color labeling and spectrally resolved two-photon microscopy, we monitor in parallel the daily structural dynamics (assembly or removal) of excitatory and inhibitory postsynaptic sites on the same neurons in mouse visual cortex in vivo. We find that dynamic inhibitory synapses often disappear and reappear again in the same location. The starkest contrast between excitatory and inhibitory synapse dynamics is on dually innervated spines, where inhibitory synapses frequently recur while excitatory synapses are stable. Monocular deprivation, a model of sensory input-dependent plasticity, shortens inhibitory synapse lifetimes and lengthens intervals to recurrence, resulting in a new dynamic state with reduced inhibitory synaptic presence. Reversible structural dynamics indicate a fundamentally new role for inhibitory synaptic remodeling--flexible, input-specific modulation of stable excitatory connections.

Characteristics of corticospinal projections to the intrinsic hand muscles in skilled harpists

The process of learning to play a musical instrument necessarily alters the functional organisation of the cortical motor areas that are involved in generating the required movements. In the case of the harp, the demands placed on the motor system are quite specific. During performance, all digits with the sole exception of the little finger are used to pluck the strings. With a view to elucidating the impact of having acquired this highly specialised musical skill on the characteristics of corticospinal projections to the intrinsic hand muscles, focal transcranial magnetic stimulation (TMS) was used to elicit motor evoked potentials (MEPs) in three muscles (of the left hand): abductor pollicis brevis (APB); first dorsal interosseous (FDI); and abductor digiti minimi (ADM) in seven harpists. Seven non-musicians served as controls. With respect to the FDI muscle-which moves the index finger, the harpists exhibited reliably larger MEP amplitudes than those in the control group. In contrast, MEPs evoked in the ADM muscle-which activates the little finger, were smaller in the harpists than in the non-musicians. The locations on the scalp over which magnetic stimulation elicited discriminable responses in ADM also differed between the harpists and the non-musicians. This specific pattern of variation in the excitability of corticospinal projections to these intrinsic hand muscles exhibited by harpists is in accordance with the idiosyncratic functional demands that are imposed in playing this instrument.

Structural Components of Synaptic Plasticity and Memory Consolidation

Consolidation of implicit memory in the invertebrate Aplysia and explicit memory in the mammalian hippocampus are associated with remodeling and growth of preexisting synapses and the formation of new synapses. Here, we compare and contrast structural components of the synaptic plasticity that underlies these two distinct forms of memory. In both cases, the structural changes involve time-dependent processes. Thus, some modifications are transient and may contribute to early formative stages of long-term memory, whereas others are more stable, longer lasting, and likely to confer persistence to memory storage. In addition, we explore the possibility that trans-synaptic signaling mechanisms governing de novo synapse formation during development can be reused in the adult for the purposes of structural synaptic plasticity and memory storage. Finally, we discuss how these mechanisms set in motion structural rearrangements that prepare a synapse to strengthen the same memory and, perhaps, to allow it to take part in other memories as a basis for understanding how their anatomical representation results in the enhanced expression and storage of memories in the brain.

Longitudinal in vivo two-photon fluorescence imaging

Fluorescence microscopy is an essential technique for the basic sciences, especially biomedical research. Since the invention of laser scanning confocal microscopy in the 1980s, which enabled imaging both fixed and living biological tissue with 3D precision, high-resolution fluorescence imaging has revolutionized biological research. Confocal microscopy, by its very nature, has one fundamental limitation. Due to the confocal pinhole, deep tissue fluorescence imaging is not practical. In contrast (no pun intended), two-photon fluorescence microscopy allows, in principle, the collection of all emitted photons from fluorophores in the imaged voxel, dramatically extending our ability to see deep into living tissue. Since the development of transgenic mice with genetically encoded fluorescent protein in neocortical cells in 2000, two-photon imaging has enabled the dynamics of individual synapses to be followed for up to 2 years. Since the initial landmark contributions to this field in 2002, the technique has been used to understand how neuronal structure are changed by experience, learning, and memory and various diseases. Here we provide a basic summary of the crucial elements that are required for such studies, and discuss many applications of longitudinal two-photon fluorescence microscopy that have appeared since 2002.

Spectral-resolved multifocal multiphoton microscopy with multianode photomultiplier tubes

Multiphoton excitation fluorescence microscopy is the preferred method for in vivo deep tissue imaging. Many biological applications demand both high imaging speed and the ability to resolve multiple fluorophores. One of the successful methods to improve imaging speed in a highly turbid specimen is multifocal multiphoton microscopy (MMM) based on use of multi-anode photomultiplier tubes (MAPMT). This approach improves imaging speed by using multiple foci for parallelized excitation without sacrificing signal to noise ratio (SNR) due to the scattering of emission photons. In this work, we demonstrate that the MAPMT based MMM can be extended with spectral resolved imaging capability. Instead of generating multiple excitation foci in a 2D grid pattern, a linear array of foci is generated. This leaves one axis of the 2D MAPMT available for spectral dispersion and detection. The spectral-resolved MMM can detect several emission signals simultaneously with high imaging speed optimized for high-throughput, high-contents applications. The new procedure is illustrated using imaging data from the kidney, peripheral nerve regeneration and dendritic morphological data from the brain.

Reassignment of scattered emission photons in multifocal multiphoton microscopy

Multifocal multiphoton microscopy (MMM) achieves fast imaging by simultaneously scanning multiple foci across different regions of specimen. The use of imaging detectors in MMM, such as CCD or CMOS, results in degradation of image signal-to-noise-ratio (SNR) due to the scattering of emitted photons. SNR can be partly recovered using multianode photomultiplier tubes (MAPMT). In this design, however, emission photons scattered to neighbor anodes are encoded by the foci scan location resulting in ghost images. The crosstalk between different anodes is currently measured a priori, which is cumbersome as it depends specimen properties. Here, we present the photon reassignment method for MMM, established based on the maximum likelihood (ML) estimation, for quantification of crosstalk between the anodes of MAPMT without a priori measurement. The method provides the reassignment of the photons generated by the ghost images to the original spatial location thus increases the SNR of the final reconstructed image.

Comparison of intravital thinned skull and cranial window approaches to study CNS immunobiology in the mouse cortex

Fluorescent imaging coupled with high-resolution femto-second pulsed infrared lasers allows for interrogation of cellular interactions deeper in living tissues than ever imagined. Intra-vital imaging of the central nervous system (CNS) has provided insights into neuronal development, synaptic transmission, and even immune interactions. In this review we will discuss the two most common intravital approaches for studying the cerebral cortex in the live mouse brain for pre-clinical studies, the thinned skull and cranial window techniques, and focus on the advantages and drawbacks of each approach. In addition, we will discuss the use of neuronal physiologic parameters as determinants of successful surgical and imaging preparation.

An evolutionarily conserved protein CHORD regulates scaling of dendritic arbors with body size

Most organs scale proportionally with body size through regulation of individual cell size and/or cell number. Here we addressed how postmitotic and morphologically complex cells such as neurons scale with the body size by using the dendritic arbor of one Drosophila sensory neuron as an assay system. In small adults eclosed under a limited-nutrition condition, the wild-type neuron preserved the branching complexity of the arbor, but scaled down the entire arbor, making a “miniature”. In contrast, mutant neurons for the Insulin/IGF signaling (IIS) or TORC1 pathway exhibited “undergrowth”, which was characterized by decreases in both the branching complexity and the arbor size, despite a normal diet. These contrasting phenotypes hinted that a novel regulatory mechanism contributes to the dendritic scaling in wild-type neurons. Indeed, we isolated a mutation in the gene CHORD/morgana that uncoupled the neuron size and the body size: CHORD mutant neurons generated miniature dendritic arbors regardless of the body size. CHORD encodes an evolutionarily conserved co-chaperone of HSP90. Our results support the notion that dendritic growth and branching are controlled by partly separate mechanisms. The IIS/TORC1 pathways control both growth and branching to avert underdevelopment, whereas CHORD together with TORC2 realizes proportional scaling of the entire arbor.

A30P α-Synuclein interferes with the stable integration of adult-born neurons into the olfactory network

Impaired olfaction is an early symptom in Parkinson disease (PD), although the exact cause is as yet unknown. Here, we investigated the link between PD-related mutant α-Synuclein (α-SYN) pathology and olfactory deficit, by examining the integration of adult-born neurons in the olfactory bulb (OB) of A30P α-SYN overexpressing mice. To this end, we chose to label one well-known vulnerable subpopulation of adult-born cells, the dopaminergic neurons. Using in vivo two-photon imaging, we followed the dynamic process of neuronal turnover in transgenic A30P α-SYN and wild-type mice over a period of 2.5 months. Our results reveal no difference in the number of cells that reach, and possibly integrate into, the glomerular layer in the OB. However, in mutant transgenic mice these new neurons have a significantly shortened survival, resulting in an overall reduction in the addition of neurons to the glomerular layer over time. We therefore propose unstable integration and impaired homeostasis of functional new neurons as a likely contributor to odour discrimination deficits in mutant α-SYN mice.

Neurites containing the neurofilament-triplet proteins are selectively vulnerable to cytoskeletal pathology in Alzheimer's disease and transgenic mouse models

Amyloid-β plaque accumulation in Alzheimer’s disease (AD) is associated with dystrophic neurite (DN) formation and synapse loss in principal neurons, but interneuron pathology is less clearly characterized. We compared the responses of neuronal processes immunoreactive for either neurofilament triplet (NF⁺) or calretinin (CR⁺) to fibrillar amyloid (Aβ) plaques in human end-stage and preclinical AD, as well as in APP/PS1 and Tg2576 transgenic mouse AD models. Neurites traversing the Aβ plaque core, edge, or periphery, defined as 50, 100, and 150% of the plaque diameter, respectively, in human AD and transgenic mouse tissue were compared to age-matched human and wild-type mouse controls. The proportion of NF⁺ neurites exhibiting dystrophic morphology (DN) was significantly larger than the proportion of dystrophic CR⁺ neurites in both human AD and transgenic mice (p < 0.01). Additionally, the number of NF⁺, but not CR⁺, DNs, correlated with Aβ plaque size. We conclude that CR⁺ interneurons appear to be more resistant than NF⁺ neurons to AD-mediated cytoskeletal pathology.

Recent progress in tissue optical clearing

Tissue optical clearing technique provides a prospective solution for the application of advanced optical methods in life sciences. This paper gives a review of recent developments in tissue optical clearing techniques. The physical, molecular and physiological mechanisms of tissue optical clearing are overviewed and discussed. Various methods for enhancing penetration of optical-clearing agents into tissue, such as physical methods, chemical-penetration enhancers and combination of physical and chemical methods are introduced. Combining the tissue optical clearing technique with advanced microscopy image or labeling technique, applications for 3D microstructure of whole tissues such as brain and central nervous system with unprecedented resolution are demonstrated. Moreover, the difference in diffusion and/or clearing ability of selected agents in healthy versus pathological tissues can provide a highly sensitive indicator of the tissue health/pathology condition. Finally, recent advances in optical clearing of soft or hard tissue for in vivo imaging and phototherapy are introduced. [Formula: see text].

Recent progress in tissue optical clearing

Tissue optical clearing technique provides a prospective solution for the application of advanced optical methods in life sciences. This paper gives a review of recent developments in tissue optical clearing techniques. The physical, molecular and physiological mechanisms of tissue optical clearing are overviewed and discussed. Various methods for enhancing penetration of optical-clearing agents into tissue, such as physical methods, chemical-penetration enhancers and combination of physical and chemical methods are introduced. Combining the tissue optical clearing technique with advanced microscopy image or labeling technique, applications for 3D microstructure of whole tissues such as brain and central nervous system with unprecedented resolution are demonstrated. Moreover, the difference in diffusion and/or clearing ability of selected agents in healthy versus pathological tissues can provide a highly sensitive indicator of the tissue health/pathology condition. Finally, recent advances in optical clearing of soft or hard tissue for in vivo imaging and phototherapy are introduced. [Formula: see text].

The dendritic spines of interneurons are dynamic structures influenced by PSA-NCAM expression

Excitatory neurons undergo dendritic spine remodeling in response to different stimuli. However, there is scarce information about this type of plasticity in interneurons. The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) is a good candidate to mediate this plasticity as it participates in neuronal remodeling and is expressed by some mature cortical interneurons, which have reduced dendritic arborization, spine density, and synaptic input. To study the connectivity of the dendritic spines of interneurons and the influence of PSA-NCAM on their dynamics, we have analyzed these structures in a subpopulation of fluorescent spiny interneurons in the hippocampus of glutamic acid decarboxylase-enhanced green fluorescent protein transgenic mice. Our results show that these spines receive excitatory synapses. The depletion of PSA in vivo using the enzyme Endo-Neuraminidase-N (Endo-N) increases spine density when analyzed 2 days after, but decreases it 7 days after. The dendritic spine turnover was also analyzed in real time using organotypic hippocampal cultures: 24 h after the addition of EndoN, we observed an increase in the apparition rate of spines. These results indicate that dendritic spines are important structures in the control of the synaptic input of hippocampal interneurons and suggest that PSA-NCAM is relevant in the regulation of their morphology and connectivity.

Highly specific structural plasticity of inhibitory circuits in the adult neocortex

Inhibitory neurons are known to play a vital role in defining the window for critical period plasticity during development, and it is increasingly apparent that they continue to exert powerful control over experience-dependent cortical plasticity in adulthood. Recent in vivo imaging studies demonstrate that long-term plasticity of inhibitory circuits is manifested at an anatomical level. Changes in sensory experience drive structural remodeling in inhibitory interneurons in a cell-type and circuit-specific manner. Inhibitory synapse formation and elimination can occur with a great deal of spatial and temporal precision and are locally coordinated with excitatory synaptic changes on the same neuron. We suggest that the specificity of inhibitory synapse dynamics may serve to differentially modulate activity across the dendritic arbor, to selectively tune parts of a local circuit, or potentially discriminate between activities in distinct local circuits. We further review evidence suggesting that inhibitory circuit structural changes instruct excitatory/inhibitory balance while enabling functional reorganization to occur through Hebbian forms of plasticity.

Structural plasticity of interneurons in the adult brain: role of PSA-NCAM and implications for psychiatric disorders

Neuronal structural plasticity is known to have a major role in cognitive processes and in the response of the CNS to aversive experiences. This type of plasticity involves processes ranging from neurite outgrowth/retraction or dendritic spine remodeling, to the incorporation of new neurons to the established circuitry. However, the study of how these structural changes take place has been focused mainly on excitatory neurons, while little attention has been paid to interneurons. The exploration of these plastic phenomena in interneurons is very important, not only for our knowledge of CNS physiology, but also for understanding better the etiology of different psychiatric and neurological disorders in which alterations in the structure and connectivity of inhibitory networks have been described. Here we review recent work on the structural remodeling of interneurons in the adult brain, both in basal conditions and after chronic stress or sensory deprivation. We also describe studies from our laboratory and others on the putative mediators of this interneuronal structural plasticity, focusing on the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). This molecule is expressed by some interneurons in the adult CNS and, through its anti-adhesive and insulating properties, may participate in the remodeling of their structure. Finally, we review recent findings on the possible implication of PSA-NCAM on the remodeling of inhibitory neurons in certain psychiatric disorders and their treatments.

Chronic stress alters inhibitory networks in the medial prefrontal cortex of adult mice

Chronic stress in experimental animals induces dendritic atrophy and decreases spine density in principal neurons of the medial prefrontal cortex (mPFC). This structural plasticity may play a neuroprotective role and underlie stress-induced behavioral changes. Different evidences indicate that the prefrontocortical GABA system is also altered by stress and in major depression patients. In the amygdala, chronic stress induces dendritic remodeling both in principal neurons and in interneurons. However, it is not known whether similar structural changes occur in mPFC interneurons. The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) may mediate these changes, because it is known to influence the dendritic organization of adult cortical interneurons. We have analyzed the dendritic arborization and spine density of mPFC interneurons in adult mice after 21 days of restraint stress and have found dendritic hypertrophy in a subpopulation of interneurons identified mainly as Martinotti cells. This aversive experience also decreases the number of glutamate decarboxylase enzyme, 67 kDa isoform (GAD67) expressing somata, without affecting different parameters related to apoptosis, but does not alter the number of interneurons expressing PSA-NCAM. Quantitative retrotranscription-polymerase chain reaction (qRT-PCR) analysis of genes related to general and inhibitory neurotransmission and of PSA synthesizing enzymes reveals increases in the expression of NCAM, synaptophysin and GABA(A)α1. Together these results show that mPFC inhibitory networks are affected by chronic stress and suggest that structural plasticity may be an important feature of stress-related psychiatric disorders where this cortical region, specially their GABAergic system, is altered.

Development and plasticity of the primary visual cortex

Hubel and Wiesel began the modern study of development and plasticity of primary visual cortex (V1), discovering response properties of cortical neurons that distinguished them from their inputs and that were arranged in a functional architecture. Their findings revealed an early innate period of development and a later critical period of dramatic experience-dependent plasticity. Recent studies have used rodents to benefit from biochemistry and genetics. The roles of spontaneous neural activity and molecular signaling in innate, experience-independent development have been clarified, as have the later roles of visual experience. Plasticity produced by monocular visual deprivation (MD) has been dissected into stages governed by distinct signaling mechanisms, some of whose molecular players are known. Many crucial questions remain, but new tools for perturbing cortical cells and measuring plasticity at the level of changes in connections among identified neurons now exist. The future for the study of V1 to illuminate cortical development and plasticity is bright.

Clustered dynamics of inhibitory synapses and dendritic spines in the adult neocortex

A key feature of the mammalian brain is its capacity to adapt in response to experience, in part by remodeling of synaptic connections between neurons. Excitatory synapse rearrangements have been monitored in vivo by observation of dendritic spine dynamics, but lack of a vital marker for inhibitory synapses has precluded their observation. Here, we simultaneously monitor in vivo inhibitory synapse and dendritic spine dynamics across the entire dendritic arbor of pyramidal neurons in the adult mammalian cortex using large-volume, high-resolution dual-color two-photon microscopy. We find that inhibitory synapses on dendritic shafts and spines differ in their distribution across the arbor and in their remodeling kinetics during normal and altered sensory experience. Further, we find inhibitory synapse and dendritic spine remodeling to be spatially clustered and that clustering is influenced by sensory input. Our findings provide in vivo evidence for local coordination of inhibitory and excitatory synaptic rearrangements.

Imaging neocortical neurons through a chronic cranial window

The rich structural dynamics of axonal arbors and neuronal circuitry can only be revealed through direct and repeated observations of the same neuron(s) over time, preferably in vivo. This protocol describes a long-term, high-resolution method for imaging neocortical neurons in vivo, using a combination of two-photon laser scanning microscopy (2PLSM) and a surgically implanted chronic cranial window. The window is used because the skull of most mammals is too opaque to allow high-resolution imaging of cortical neurons. Using this method, it is feasible to image the smallest neuronal structures in the superficial layers of the neocortex, such as dendritic spines and axonal boutons. Because the surface area of the craniotomy is relatively large, this technique is even suitable for use when labeled neurons are relatively uncommon. The surgery and imaging procedures are illustrated with examples from our studies of structural plasticity in the developing or adult mouse brain. The protocol is optimized for adult mice; we have used mice up to postnatal day 511 (P511). With minor modifications, it is possible to image neurons in rats and mice from P2. Most of our studies have used the Thy1 promoter to drive expression of fluorophores in subsets of cortical neurons.

Dendritic structural plasticity

Dendrites represent the compartment of neurons primarily devoted to collecting and computating input. Far from being static structures, dendrites are highly dynamic during development and appear to be capable of plastic changes during the adult life of animals. During development, it is a combination of intrinsic programs and external signals that shapes dendrite morphology; input activity is a conserved extrinsic factor involved in this process. In adult life, dendrites respond with more modest modifications of their structure to various types of extrinsic information, including alterations of input activity. Here, the author reviews classical and recent evidence of dendrite plasticity in invertebrates and vertebrates and current progress in the understanding of the molecular mechanisms that underlie this plasticity. Importantly, some fundamental questions such as the functional role of dendrite remodeling and the causal link between structural modifications of neurons and plastic processes, including learning, are still open.

CPG15 regulates synapse stability in the developing and adult brain

Use-dependent selection of optimal connections is a key feature of neural circuit development and, in the mature brain, underlies functional adaptation, such as is required for learning and memory. Activity patterns guide circuit refinement through selective stabilization or elimination of specific neuronal branches and synapses. The molecular signals that mediate activity-dependent synapse and arbor stabilization and maintenance remain elusive. We report that knockout of the activity-regulated gene cpg15 in mice delays developmental maturation of axonal and dendritic arbors visualized by anterograde tracing and diolistic labeling, respectively. Electrophysiology shows that synaptic maturation is also delayed, and electron microscopy confirms that many dendritic spines initially lack functional synaptic contacts. While circuits eventually develop, in vivo imaging reveals that spine maintenance is compromised in the adult, leading to a gradual attrition in spine numbers. Loss of cpg15 also results in poor learning. cpg15 knockout mice require more trails to learn, but once they learn, memories are retained. Our findings suggest that CPG15 acts to stabilize active synapses on dendritic spines, resulting in selective spine and arbor stabilization and synaptic maturation, and that synapse stabilization mediated by CPG15 is critical for efficient learning.

Inhibitory dendrite dynamics as a general feature of the adult cortical microcircuit

The mammalian neocortex is functionally subdivided into architectonically distinct regions that process various types of information based on their source of afferent input. Yet, the modularity of neocortical organization in terms of cell type and intrinsic circuitry allows afferent drive to continuously reassign cortical map space. New aspects of cortical map plasticity include dynamic turnover of dendritic spines on pyramidal neurons and remodeling of interneuron dendritic arbors. While spine remodeling occurs in multiple cortical regions, it is not yet known whether interneuron dendrite remodeling is common across primary sensory and higher-level cortices. It is also unknown whether, like pyramidal dendrites, inhibitory dendrites respect functional domain boundaries. Given the importance of the inhibitory circuitry to adult cortical plasticity and the reorganization of cortical maps, we sought to address these questions by using two-photon microscopy to monitor interneuron dendritic arbors of thy1-GFP-S transgenic mice expressing GFP in neurons sparsely distributed across the superficial layers of the neocortex. We find that interneuron dendritic branch tip remodeling is a general feature of the adult cortical microcircuit, and that remodeling rates are similar across primary sensory regions of different modalities, but may differ in magnitude between primary sensory versus higher cortical areas. We also show that branch tip remodeling occurs in bursts and respects functional domain boundaries.

Loss of sensory input causes rapid structural changes of inhibitory neurons in adult mouse visual cortex

A fundamental property of neuronal circuits is the ability to adapt to altered sensory inputs. It is well established that the functional synaptic changes underlying this adaptation are reflected by structural modifications in excitatory neurons. In contrast, the degree to which structural plasticity in inhibitory neurons accompanies functional changes is less clear. Here, we use two-photon imaging to monitor the fine structure of inhibitory neurons in mouse visual cortex after deprivation induced by retinal lesions. We find that a subset of inhibitory neurons carry dendritic spines, which form glutamatergic synapses. Removal of visual input correlates with a rapid and lasting reduction in the number of inhibitory cell spines. Similar to the effects seen for dendritic spines, the number of inhibitory neuron boutons dropped sharply after retinal lesions. Together, these data suggest that structural changes in inhibitory neurons may precede structural changes in excitatory circuitry, which ultimately result in functional adaptation following sensory deprivation.

Long-term imaging reveals dynamic changes in the neuronal composition of the glomerular layer

The mammalian olfactory bulb (OB) contains a rich and highly heterogeneous network of local interneurons (INs). These INs undergo continuous turnover in the adult OB in a process known as "adult neurogenesis." Although the overall magnitude of adult neurogenesis has been estimated, the detailed dynamics of the different subpopulations remains largely unknown. Here we present a novel preparation that enables long-term in vivo time-lapse imaging in the mouse OB through a chronic cranial window in a virtually unlimited number of sessions. Using this preparation, we followed the turnover of a specific neuronal population in the OB, the dopaminergic (DA) neurons, for as long as 9 months. By following the same population over long periods of time, we found clear addition and loss of DA neurons in the glomerular layer. Both cell addition and loss increased over time. The numbers of new DA cells were consistently and significantly higher than lost DA cells, suggesting a net increase in the size of this particular population with age. Over a 9 month period of adult life, the net addition of DA neurons reached ∼ 13%. Our data argue that the fine composition of the bulbar IN network changes throughout adulthood rather than simply being replenished.

Chronic stress induces changes in the structure of interneurons and in the expression of molecules related to neuronal structural plasticity and inhibitory neurotransmission in the amygdala of adult mice

Chronic stress in experimental animals, one of the most accepted models of chronic anxiety and depression, induces structural remodeling of principal neurons in the amygdala and increases its excitation by reducing inhibitory tone. These changes may be mediated by the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), a molecule related to neuronal structural plasticity and expressed by interneurons in the adult CNS, which is downregulated in the amygdala after chronic stress. We have analyzed the amygdala of adult mice after 21 days of restraint stress, studying with qRT-PCR the expression of genes related to general and inhibitory neurotransmission, and of PSA synthesizing enzymes. The expression of GAD67, synaptophysin and PSA-NCAM was also studied in specific amygdaloid nuclei using immunohistochemistry. We also analyzed dendritic arborization and spine density, and cell activity, monitoring c-Fos expression, in amygdaloid interneurons. At the mRNA level, the expression of GAD67 and of St8SiaII was significantly reduced. At the protein level there was an overall reduction in the expression of GAD67, synaptophysin and PSA-NCAM, but significant changes were only detected in specific amygdaloid regions. Chronic stress did not affect dendritic spine density, but reduced dendritic arborization in interneurons of the lateral and basolateral amygdala. These results indicate that chronic stress modulates inhibitory neurotransmission in the amygdala by regulating the expression of molecules involved in this process and by promoting the structural remodeling of interneurons. The addition of PSA to NCAM by St8SiaII may be involved in these changes.

Experience-dependent structural plasticity in the cortex

Synapses are the fundamental units of neuronal circuits. Synaptic plasticity can occur through changes in synaptic strength, as well as through the addition/removal of synapses. Two-photon microscopy in combination with fluorescence labeling offers a powerful tool to peek into the living brain and follow structural reorganization at individual synapses. Time-lapse imaging depicts a dynamic picture in which experience-dependent plasticity of synaptic structures varies between different cortical regions and layers, as well as between neuronal subtypes. Recent studies have demonstrated that the formation and elimination of synaptic structures happens rapidly in a subpopulation of cortical neurons during various sensorimotor learning experiences, and that stabilized synaptic structures are associated with long lasting memories for the task. Therefore, circuit plasticity, mediated by structural remodeling, provides an underlying mechanism for learning and memory.