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

Active dendritic conductances dynamically regulate GABA release from thalamic interneurons

Inhibitory interneurons in the dorsal lateral geniculate nucleus (dLGN) process visual information by precisely controlling spike timing and by refining the receptive fields of thalamocortical (TC) neurons. Previous studies indicate that dLGN interneurons inhibit TC neurons by releasing GABA from both axons and dendrites. However, the mechanisms controlling GABA release are poorly understood. Here, using simultaneous whole-cell recordings from interneurons and TC neurons and two-photon calcium imaging, we find that synchronous activation of multiple retinal ganglion cells (RGCs) triggers sodium spikes that propagate throughout interneuron axons and dendrites, and calcium spikes that invade dendrites but not axons. These distinct modes of interneuron firing can trigger both a rapid and a sustained component of inhibition onto TC neurons. Our studies suggest that active conductances make LGN interneurons flexible circuit-elements that can shift their spatial and temporal properties of GABA release in response to coincident activation of functionally related subsets of RGCs.

Dendritic spine loss and synaptic alterations in Alzheimer's disease

Dendritic spines are tiny protrusions along dendrites, which constitute major postsynaptic sites for excitatory synaptic transmission. These spines are highly motile and can undergo remodeling even in the adult nervous system. Spine remodeling and the formation of new synapses are activity-dependent processes that provide a basis for memory formation. A loss or alteration of these structures has been described in patients with neurodegenerative disorders such as Alzheimer's disease (AD), and in mouse models for these disorders. Such alteration is thought to be responsible for cognitive deficits long before or even in the absence of neuronal loss, but the underlying mechanisms are poorly understood. This review will describe recent findings and discoveries on the loss or alteration of dendritic spines induced by the amyloid beta (Abeta) peptide in the context of AD.

Side illumination fluorescence emission characteristics from a dye doped polymer optical fiber under two-photon excitation

Two-photon excited (TPE) side illumination fluorescence studies in a Rh6G-RhB dye mixture doped polymer optical fiber (POF) and the effect of energy transfer on the attenuation coefficient is reported. The dye doped POF is pumped sideways using 800 nm, 70 fs laser pulses from a Ti:sapphire laser, and the TPE fluorescence emission is collected from the end of the fiber for different propagation distances. The fluorescence intensity of RhB doped POF is enhanced in the presence of Rh6G as a result of energy transfer from Rh6G to RhB. Because of the reabsorption and reemission process in dye molecules, an effective energy transfer is observed from the shorter wavelength part of the fluorescence spectrum to the longer wavelength part as the propagation distance is increased in dye doped POF. An energy transfer coefficient is found to be higher at shorter propagation distances compared to longer distances. A TPE fluorescence signal is used to characterize the optical attenuation coefficient in dye doped POF. The attenuation coefficient decreases at longer propagation distances due to the reabsorption and reemission process taking place within the dye doped fiber as the propagation distance is increased.

Genetic dissection of neural circuits

Understanding the principles of information processing in neural circuits requires systematic characterization of the participating cell types and their connections, and the ability to measure and perturb their activity. Genetic approaches promise to bring experimental access to complex neural systems, including genetic stalwarts such as the fly and mouse, but also to nongenetic systems such as primates. Together with anatomical and physiological methods, cell-type-specific expression of protein markers and sensors and transducers will be critical to construct circuit diagrams and to measure the activity of genetically defined neurons. Inactivation and activation of genetically defined cell types will establish causal relationships between activity in specific groups of neurons, circuit function, and animal behavior. Genetic analysis thus promises to reveal the logic of the neural circuits in complex brains that guide behaviors. Here we review progress in the genetic analysis of neural circuits and discuss directions for future research and development.

Bright ideas for chemical biology

Small-molecule fluorescent probes embody an essential facet of chemical biology. Although numerous compounds are known, the ensemble of fluorescent probes is based on a modest collection of modular "core" dyes. The elaboration of these dyes with diverse chemical moieties is enabling the precise interrogation of biochemical and biological systems. The importance of fluorescence-based technologies in chemical biology elicits a necessity to understand the major classes of small-molecule fluorophores. Here, we examine the chemical and photophysical properties of oft-used fluorophores and highlight classic and contemporary examples in which utility has been built upon these scaffolds.

Imaging in vivo: watching the brain in action

The appeal of in vivo cellular imaging to any neuroscientist is not hard to understand: it is almost impossible to isolate individual neurons while keeping them and their complex interactions with surrounding tissue intact. These interactions lead to the complex network dynamics that underlie neural computation which, in turn, forms the basis of cognition, perception and consciousness. In vivo imaging allows the study of both form and function in reasonably intact preparations, often with subcellular spatial resolution, a time resolution of milliseconds and a purview of months. Recently, the limits of what can be achieved in vivo have been pushed into terrain that was previously only accessible in vitro, due to advances in both physical-imaging technology and the design of molecular contrast agents.

Enhanced background rejection in thick tissue with differential-aberration two-photon microscopy

When a two-photon excited fluorescence (TPEF) microscope is used to image deep inside tissue, out-of-focus background can arise from both ballistic and nonballistic excitation. We propose a solution to largely reject TPEF background in thick tissue. Our technique is based on differential-aberration imaging with a deformable mirror. By introducing extraneous aberrations in the excitation beam path, we preferentially quench in-focus TPEF signal while leaving out-of-focus TPEF background largely unchanged. A simple subtraction of an aberrated, from an unaberrated, TPEF image then removes background while preserving signal. Our differential aberration (DA) technique is simple, robust, and can readily be implemented with standard TPEF microscopes with essentially no loss in temporal resolution when using a line-by-line DA protocol. We analyze the performance of various induced aberration patterns, and demonstrate the effectiveness of DA-TPEF by imaging GFP-labeled sensory neurons in a mouse olfactory bulb and CA1 pyramidal cells in a hippocampus slice.

Preparation of gene gun bullets and biolistic transfection of neurons in slice culture

Biolistic transfection is a physical means of transfecting cells by bombarding tissue with high velocity DNA coated particles. We provide a detailed protocol for biolistic transfection of rat hippocampal slices, from the initial preparation of DNA coated bullets to the final shooting of the organotypic slice cultures using a gene gun. Gene gun transfection is an efficient and easy means of transfecting neurons and is especially useful for fluorescently labeling a small subset of cells in tissue slice. In this video, we first outline the steps required to coat gold particles with DNA. We next demonstrate how to line the inside of plastic tubing with the gold/DNA bullets, and how to cut this tubing to obtain the plastic cartridges for loading into the gene gun. Finally, we perform biolistic transfection of rat hippocampal slice cultures, demonstrating handling of the Bio-Rad Helios gene gun, and offering trouble shooting advice to obtain healthy and optimally transfected tissue slices.

Locally dynamic synaptic learning rules in pyramidal neuron dendrites

Long-term potentiation (LTP) of synaptic transmission underlies aspects of learning and memory. LTP is input-specific at the level of individual synapses, but neural network models predict interactions between plasticity at nearby synapses. Here we show in mouse hippocampal pyramidal cells that LTP at individual synapses reduces the threshold for potentiation at neighbouring synapses. After input-specific LTP induction by two-photon glutamate uncaging or by synaptic stimulation, subthreshold stimuli, which by themselves were too weak to trigger LTP, caused robust LTP and spine enlargement at neighbouring spines. Furthermore, LTP induction broadened the presynaptic-postsynaptic spike interval for spike-timing-dependent LTP within a dendritic neighbourhood. The reduction in the threshold for LTP induction lasted approximately 10 min and spread over approximately 10 microm of dendrite. These local interactions between neighbouring synapses support clustered plasticity models of memory storage and could allow for the binding of behaviourally linked information on the same dendritic branch.

Multi-photon excitation imaging of dynamic processes in living cells and tissues

Over the past decade, two-photon microscopy has successfully made the transition from the laser laboratory into a true biological research setting. This has been due in part to the recent development of turnkey ultrafast laser systems required for two-photon microscopy, allowing ease of use in nonspecialist laboratories. The advantages of two-photon microscopy over conventional optical sectioning techniques are for greater imaging depths and reduced overall phototoxicity, as such enabling noninvasive intra-vital imaging of cellular and subcellular processes. Greater understanding of these advantages has allowed this technique to be more effectively utilized in a biological research setting. This review will cover the recent widespread uses of two-photon microscopy and highlight the wide range of physiological studies enabled in fields such as neurosciences, developmental biology, immunology, cancer biology, and endocrinology.

Visualizing mRNA localization and local protein translation in neurons

Fluorescent proteins (FPs) have been successfully used to study the localization and interactions of proteins in living cells. They have also been instrumental in analyzing the proteins involved in the localization of RNAs in different cell types, including neurons. With the development of methods that also tag RNAs via fluorescent proteins, researchers now have a powerful tool to covisualize RNAs and associated proteins in living neurons. Here, we review the current status of the use of FPs in the study of transport and localization of ribonucleoprotein particles (RNPs) in neurons and provide key protocols used to introduce transgenes into cultured neurons, including calcium-phosphate-based transfection and nucleofection. These methods allow the fast and efficient expression of fluorescently tagged fusion proteins in neurons at different stages of differentiation and form the basis for fluorescent protein-based live cell imaging in neuronal cultures. Additional protocols are given that allow the simultaneous visualization of RNP proteins and cargo RNAs in living neurons and aspects of the visualization of fluorescently tagged proteins in neurons, such as colocalization studies, are discussed. Finally, we review approaches to visualize the local synthesis of proteins in distal dendrites and axons.

In Vivo Calcium Imaging of Neural Network Function

Spatiotemporal activity patterns in local neural networks are fundamental to brain function. Network activity can now be measured in vivo using two-photon imaging of cell populations that are labeled with fluorescent calcium indicators. In this review, we discuss basic aspects of in vivo calcium imaging and highlight recent developments that will help to uncover operating principles of neural circuits.

Stable in vivo imaging of densely populated glia, axons and blood vessels in the mouse spinal cord using two-photon microscopy

In vivo imaging has revolutionized our understanding of biological processes in brain physiology and pathology. However, breathing-induced movement artifacts have impeded the application of this powerful tool in studies of the living spinal cord. Here we describe in detail a method to image stably and repetitively, using two-photon microscopy, the living spinal tissue in mice with dense fluorescent cells or axons, without the need for animal intubation or image post-processing. This simplified technique can greatly expand the application of in vivo imaging to study spinal cord injury, regeneration, physiology and disease.

Albumin therapy improves local vascular dynamics in a rat model of primary microvascular thrombosis: a two-photon laser-scanning microscopy study

BACKGROUND AND PURPOSE

High-dose human albumin is robustly neuroprotective in preclinical ischemia models and is currently in phase III clinical trial for acute ischemic stroke. To explore the hypothesis that albumin's protective effect is mediated in part by salutary intravascular mechanisms, we assessed microvascular hemodynamics in a model of laser-induced cortical arteriolar thrombosis.

METHODS

The cortical microcirculation of anesthetized, physiologically monitored Sprague-Dawley rats was studied in vivo via a frontoparietal cranial window (intact dura) by two-photon laser-scanning microscopy after plasma-labeling with fluorescein-dextran. Focal thrombosis was produced in 30- to 50-mum cortical arterioles by laser irradiation. Arteriolar flow velocity was measured repeatedly by line-scanning. At 30 minutes post-thrombosis, animals were treated with either human albumin, 2 g/kg, or with saline control.

RESULTS

Baseline arteriolar flow velocity averaged 3.5+/-1.8 mm/s and was reduced to 10% to 13% of control values by laser-induced thrombosis, which also led to focal vasodilatation (mean, 49% above baseline diameter). Saline treatment at 30 minutes post-thrombosis failed to influence arteriolar flow velocity, which remained depressed at 10% to 22% of control throughout the subsequent 60- to 90-minute observation period. By contrast, albumin treatment induced a prompt rise in median flow velocity to 38% of control by 10 minutes post-treatment, and to 61% to 67% of control by 50 to 60 minutes.

CONCLUSIONS

High-dose albumin therapy induces a prompt, sustained improvement in microvascular hemodynamics distal to a cortical arteriolar thrombosis; these data support an important intravascular component to albumin's protective effect in acute cerebral ischemia.

Estimating firing rates from calcium signals in locust projection neurons in vivo

Combining intracellular electrophysiology and multi-photon calcium imaging in vivo, we studied the relationship between calcium signals (sampled at 500-750 Hz) and spike output in principal neurons in the locust antennal lobe. Our goal was to determine whether the firing rate of individual neurons can be estimated in vivo with calcium imaging and, if so, to measure directly the accuracy and resolution of our estimates. Using the calcium indicator Oregon Green BAPTA-1, we describe a simple method to reconstruct firing rates from dendritic calcium signals with 80-90% accuracy and 50 ms temporal resolution.

Structural plasticity of axon terminals in the adult

There is now conclusive evidence for widespread ongoing structural plasticity of presynaptic boutons and axon side-branches in the adult brain. The plasticity complements that of postsynaptic spines, but axonal plasticity samples larger volumes of neuropil, and has a larger impact on circuit remodeling. Axons from distinct neurons exhibit unique ratios of stable (t1/2>9 months) and dynamic (t1/2 5-20 days) boutons, which persist as spatially intermingled subgroups along terminal arbors. In addition, phases of side-branch dynamics mediate larger scale remodeling guided by synaptogenesis. The plasticity is most pronounced during critical periods; its patterns and outcome are controlled by Hebbian mechanisms and intrinsic neuronal factors. Novel experience, skill learning, life-style, and age can persistently modify local circuit structure through axonal structural plasticity.

Diolistics: incorporating fluorescent dyes into biological samples using a gene gun

The hand-held gene gun provides a rapid and efficient method of incorporating fluorescent dyes into cells, a technique that is becoming known as diolistics. Transporting fluorescent dyes into cells has, in the past, used predominantly injection or chemical methods. The use of the gene gun, combined with the new generation of fluorescent dyes, circumvents some of the problems of using these methods and also enables the study of cells that have proved difficult traditionally to transfect (e.g. those deep in tissues and/or terminally differentiated); in addition, the use of ion- or metabolite-sensitive dyes provides a route to study cellular mechanisms. Diolistics is also ideal for loading cells with optical nanosensors – nanometre-sized sensors linked to fluorescent probes. Here, we discuss the theoretical considerations of using diolistics, the advantages compared with other methods of inserting dyes into cells and the current uses of the technique, with particular consideration of nanosensors.

Optical brain imaging in vivo: techniques and applications from animal to man

Optical brain imaging has seen 30 years of intense development, and has grown into a rich and diverse field. In-vivo imaging using light provides unprecedented sensitivity to functional changes through intrinsic contrast, and is rapidly exploiting the growing availability of exogenous optical contrast agents. Light can be used to image microscopic structure and function in vivo in exposed animal brain, while also allowing noninvasive imaging of hemodynamics and metabolism in a clinical setting. This work presents an overview of the wide range of approaches currently being applied to in-vivo optical brain imaging, from animal to man. Techniques include multispectral optical imaging, voltage sensitive dye imaging and speckle-flow imaging of exposed cortex, in-vivo two-photon microscopy of the living brain, and the broad range of noninvasive topography and tomography approaches to near-infrared imaging of the human brain. The basic principles of each technique are described, followed by examples of current applications to cutting-edge neuroscience research. In summary, it is shown that optical brain imaging continues to grow and evolve, embracing new technologies and advancing to address ever more complex and important neuroscience questions.

Two-photon microscopy: shedding light on the chemistry of vision

Two-photon microscopy (TPM) has come to occupy a prominent place in modern biological research with its ability to resolve the three-dimensional distribution of molecules deep inside living tissue. TPM can employ two different types of signals, fluorescence and second harmonic generation, to image biological structures with subcellular resolution. Two-photon excited fluorescence imaging is a powerful technique with which to monitor the dynamic behavior of the chemical components of tissues, whereas second harmonic imaging provides novel ways to study their spatial organization. Using TPM, great strides have been made toward understanding the metabolism, structure, signal transduction, and signal transmission in the eye. These include the characterization of the spatial distribution, transport, and metabolism of the endogenous retinoids, molecules essential for the detection of light, as well as the elucidation of the architecture of the living cornea. In this review, we present and discuss the current applications of TPM for the chemical and structural imaging of the eye. In addition, we address what we see as the future potential of TPM for eye research. This relatively new method of microscopy has been the subject of numerous technical improvements in terms of the optics and indicators used, improvements that should lead to more detailed biochemical characterizations of the eyes of live animals and even to imaging of the human eye in vivo.

Energy and electron transfer in enhanced two-photon-absorbing systems with triplet cores

Enhanced two-photon-absorbing (2PA) systems with triplet cores are currently under scrutiny for several biomedical applications, including photodynamic therapy (PDT) and two-photon microscopy of oxygen. The performance of so far developed molecules, however, is substantially below expected. In this study we take a detailed look at the processes occurring in these systems and propose ways to improve their performance. We focus on the interchromophore distance tuning as a means for optimization of two-photon sensors for oxygen. In these constructs, energy transfer from several 2PA chromophores is used to enhance the effective 2PA cross section of phosphorescent metalloporphyrins. Previous studies have indicated that intramolecular electron transfer (ET) can act as an effective quencher of phosphorescence, decreasing the overall sensor efficiency. We studied the interplay between 2PA, energy transfer, electron transfer, and phosphorescence emission using Rhodamine B-Pt tetrabenzoporphyrin (RhB-PtTBP) adducts as model compounds. 2PA cross sections (sigma2) of tetrabenzoporphyrins (TBPs) are in the range of several tens of GM units (near 800 nm), making TBPs superior 2PA chromophores compared to regular porphyrins (sigma2 values typically 1-2 GM). Relatively large 2PA cross sections of rhodamines (about 200 GM in 800-850 nm range) and their high photostabilities make them good candidates as 2PA antennae. Fluorescence of Rhodamine B (lambda(fl) = 590 nm, phi(fl) = 0.5 in EtOH) overlaps with the Q-band of phosphorescent PtTBP (lambda(abs) = 615 nm, epsilon = 98 000 M(-1) cm(-1), phi(p) approximately 0.1), suggesting that a significant amplification of the 2PA-induced phosphorescence via fluorescence resonance energy transfer (FRET) might occur. However, most of the excitation energy in RhB-PtTBP assemblies is consumed in several intramolecular ET processes. By installing rigid nonconducting decaproline spacers (Pro10) between RhB and PtTBP, the intramolecular ETs were suppressed, while the chromophores were kept within the Förster r0 distance in order to maintain high FRET efficiency. The resulting assemblies exhibit linear amplification of their 2PA-induced phosphorescence upon increase in the number of 2PA antenna chromophores and show high oxygen sensitivity. We also have found that PtTBPs possess unexpectedly strong forbidden S0 --> T1 bands (lambda(max) = 762 nm, epsilon = 120 M-1 cm-1). The latter may overlap with the laser spectrum and lead to unwanted linear excitation.

Progression of bacterial infections studied in real time--novel perspectives provided by multiphoton microscopy

The holy grail of infection biology is to study a pathogen within its natural infectious environment, the living host. Advances in in vivo imaging techniques have begun to introduce the possibility to visualize, in real time, infection progression within a living model. In this review we detail the current advancements and knowledge in multiphoton microscopy and how it can be related to the field of microbial infections. This technology is a new and very valuable tool for in vivo imaging, and using this technique it is possible to begin to study various microbes within their natural infectious environment - the living host.

Why Is Wallerian Degeneration in the CNS So Slow?

Wallerian degeneration (WD) is the set of molecular and cellular events by which degenerating axons and myelin are cleared after injury. Why WD is rapid and robust in the PNS but slow and incomplete in the CNS is a longstanding mystery. Here we review current work on the mechanisms of WD with an emphasis on deciphering this mystery and on understanding whether slow WD in the CNS could account for the failure of CNS axons to regenerate.

An optical neural interface:in vivocontrol of rodent motor cortex with integrated fiberoptic and optogenetic technology

Neural interface technology has made enormous strides in recent years but stimulating electrodes remain incapable of reliably targeting specific cell types (e.g. excitatory or inhibitory neurons) within neural tissue. This obstacle has major scientific and clinical implications. For example, there is intense debate among physicians, neuroengineers and neuroscientists regarding the relevant cell types recruited during deep brain stimulation (DBS); moreover, many debilitating side effects of DBS likely result from lack of cell-type specificity. We describe here a novel optical neural interface technology that will allow neuroengineers to optically address specific cell types in vivo with millisecond temporal precision. Channelrhodopsin-2 (ChR2), an algal light-activated ion channel we developed for use in mammals, can give rise to safe, light-driven stimulation of CNS neurons on a timescale of milliseconds. Because ChR2 is genetically targetable, specific populations of neurons even sparsely embedded within intact circuitry can be stimulated with high temporal precision. Here we report the first in vivo behavioral demonstration of a functional optical neural interface (ONI) in intact animals, involving integrated fiberoptic and optogenetic technology. We developed a solid-state laser diode system that can be pulsed with millisecond precision, outputs 20 mW of power at 473 nm, and is coupled to a lightweight, flexible multimode optical fiber, approximately 200 microm in diameter. To capitalize on the unique advantages of this system, we specifically targeted ChR2 to excitatory cells in vivo with the CaMKIIalpha promoter. Under these conditions, the intensity of light exiting the fiber ( approximately 380 mW mm(-2)) was sufficient to drive excitatory neurons in vivo and control motor cortex function with behavioral output in intact rodents. No exogenous chemical cofactor was needed at any point, a crucial finding for in vivo work in large mammals. Achieving modulation of behavior with optical control of neuronal subtypes may give rise to fundamental network-level insights complementary to what electrode methodologies have taught us, and the emerging optogenetic toolkit may find application across a broad range of neuroscience, neuroengineering and clinical questions.

Ca(2+) signaling in dendritic spines

Recent studies have revealed that Ca(2+) signals evoked by action potentials or by synaptic activity within individual dendritic spines are regulated at multiple levels. Ca(2+) influx through glutamate receptors and voltage-sensitive Ca(2+) channels located on spines depends on the channel subunit composition, the activity of kinases and phosphatases, the local membrane potential and past patterns of activity. Furthermore, sources of spine Ca(2+) interact nonlinearly such that activation of one Ca(2+) channel can enhance or depress the activity of others. These studies have revealed that each spine is a complex and partitioned Ca(2+) signaling domain capable of autonomously regulating the electrical and biochemical consequences of synaptic activity.

Two-Photon Calcium Imaging of Network Activity in XFP-Expressing Neurons in the Mouse

Fluorescent protein (XFP) expression in postnatal neurons allows the anatomical and physiological investigation of identified subpopulations of interneurons with established techniques. However, the spatiotemporal pattern of activity of these XFP neurons within a network and their role in the functional output of the network are more challenging issues to investigate. Here we apply two-photon excitation laser scanning microscopy to mouse spinal cord locomotor networks and present the methodology by which calcium activity can be recorded in XFP-expressing neurons. Such activity can be studied both in relation to neighboring non-XFP neurons in a spinal cord slice preparation and in relation to functional locomotor output monitored by ventral root activity in the intact in vitro spinal cord. Thus the network properties and functional correlates with locomotion of identified populations of interneurons can be studied simultaneously.

Multiple-color optical activation, silencing, and desynchronization of neural activity, with single-spike temporal resolution

The quest to determine how precise neural activity patterns mediate computation, behavior, and pathology would be greatly aided by a set of tools for reliably activating and inactivating genetically targeted neurons, in a temporally precise and rapidly reversible fashion. Having earlier adapted a light-activated cation channel, channelrhodopsin-2 (ChR2), for allowing neurons to be stimulated by blue light, we searched for a complementary tool that would enable optical neuronal inhibition, driven by light of a second color. Here we report that targeting the codon-optimized form of the light-driven chloride pump halorhodopsin from the archaebacterium Natronomas pharaonis (hereafter abbreviated Halo) to genetically-specified neurons enables them to be silenced reliably, and reversibly, by millisecond-timescale pulses of yellow light. We show that trains of yellow and blue light pulses can drive high-fidelity sequences of hyperpolarizations and depolarizations in neurons simultaneously expressing yellow light-driven Halo and blue light-driven ChR2, allowing for the first time manipulations of neural synchrony without perturbation of other parameters such as spiking rates. The Halo/ChR2 system thus constitutes a powerful toolbox for multichannel photoinhibition and photostimulation of virally or transgenically targeted neural circuits without need for exogenous chemicals, enabling systematic analysis and engineering of the brain, and quantitative bioengineering of excitable cells.

Rapid redistribution of synaptic PSD-95 in the neocortex in vivo

Most excitatory synapses terminate on dendritic spines. Spines vary in size, and their volumes are proportional to the area of the postsynaptic density (PSD) and synaptic strength. PSD-95 is an abundant multi-domain postsynaptic scaffolding protein that clusters glutamate receptors and organizes the associated signaling complexes. PSD-95 is thought to determine the size and strength of synapses. Although spines and their synapses can persist for months in vivo, PSD-95 and other PSD proteins have shorter half-lives in vitro, on the order of hours. To probe the mechanisms underlying synapse stability, we measured the dynamics of synaptic PSD-95 clusters in vivo. Using two-photon microscopy, we imaged PSD-95 tagged with GFP in layer 2/3 dendrites in the developing (postnatal day 10–21) barrel cortex. A subset of PSD-95 clusters was stable for days. Using two-photon photoactivation of PSD-95 tagged with photoactivatable GFP (paGFP), we measured the time over which PSD-95 molecules were retained in individual spines. Synaptic PSD-95 turned over rapidly (median retention times τ_r ~ 22–63 min from P10–P21) and exchanged with PSD-95 in neighboring spines by diffusion. PSDs therefore share a dynamic pool of PSD-95. Large PSDs in large spines captured more diffusing PSD-95 and also retained PSD-95 longer than small PSDs. Changes in the sizes of individual PSDs over days were associated with concomitant changes in PSD-95 retention times. Furthermore, retention times increased with developmental age (τ_r ~ 100 min at postnatal day 70) and decreased dramatically following sensory deprivation. Our data suggest that individual PSDs compete for PSD-95 and that the kinetic interactions between PSD molecules and PSDs are tuned to regulate PSD size.

Using two-photon microscopy and photoactivation of a fluorescently tagged synaptic protein (PSD-95), the authors demonstrated rapid turnover of these molecules in dendritic spines of the mouse sensory cortex in vivo.

Acousto-optic modulator system for femtosecond laser pulses

Femtosecond laser pulses have made a revolution in multiphoton excitation microscopy, micromachining, and optical storage for their unprecedented high peak power. However, modulation of their intensity with acousto-optic modulator (AOM) is frustrated by dispersion which results in a significant stretch in pulse width. Here we report a scheme composed of two acousto-optic deflectors (AODs) to modulate the intensity of the femtosecond laser pulses with simultaneous compensation for the temporal dispersion. With commercial AODs, we demonstrated such an AOM system for the femtosecond laser pulses with overall transmission efficiency of around 80%. The pulse width of the exit beam is 115-177 fs for an input pulse of 110 fs, across the wavelength range of 720-920 nm when the temporal dispersion compensation is optimally tuned at 800 nm. The fluorescence intensity in a two-photon microscopy experiment performed using this system increased 5.5-fold over that of the uncompensated AOM.

Form and function in systems neuroscience

'Form follows function' is an architectural philosophy attributed to the great American architect Louis Sullivan, and later taken up by the Bauhaus movement. It stresses that the form of a building should reflect its function. Neuroscientists have used the converse of this dictum to learn the functions of neural circuits, believing that if we study neural architecture, it will lead us to an understanding of how neural systems function. New tools for studying the structure of neural circuits are being developed, so it is important to discuss what the old techniques have taught us about how to derive function from the form of a neural circuit.

Glutamate and GABA receptors and transporters in the basal ganglia: what does their subsynaptic localization reveal about their function?

GABA and glutamate, the main transmitters in the basal ganglia, exert their effects through ionotropic and metabotropic receptors. The dynamic activation of these receptors in response to released neurotransmitter depends, among other factors, on their precise localization in relation to corresponding synapses. The use of high resolution quantitative electron microscope immunocytochemical techniques has provided in-depth description of the subcellular and subsynaptic localization of these receptors in the CNS. In this article, we review recent findings on the ultrastructural localization of GABA and glutamate receptors and transporters in monkey and rat basal ganglia, at synaptic, extrasynaptic and presynaptic sites. The anatomical evidence supports numerous potential locations for receptor-neurotransmitter interactions, and raises important questions regarding mechanisms of activation and function of synaptic versus extrasynaptic receptors in the basal ganglia.

Imaging spatiotemporal dynamics of neuronal signaling using fluorescence resonance energy transfer and fluorescence lifetime imaging microscopy

The spatiotemporal localization of neuronal signaling is important for triggering neuronal responses in specific locations at precise times. Fluorescence resonance energy transfer imaging enables measurement of spatiotemporal dynamics of signaling activity in live neurons. Although the usefulness of fluorescence resonance energy transfer is well recognized, there are many difficulties in applying it, particularly when imaging in neuronal micro-compartments in light-scattering brain tissue. Fluorescence resonance energy transfer has been imaged using several techniques including intensity-based methods, fluorescence lifetime imaging and fluorescence anisotropy imaging. These methods have different advantages and disadvantages, and thus are suitable in different applications.