Rapid neurotransmitter uncaging in spatially defined patterns

Nanoparticle-based optical interfaces for retinal neuromodulation: a review

Degeneration of photoreceptors in the retina is a leading cause of blindness, but commonly leaves the retinal ganglion cells (RGCs) and/or bipolar cells extant. Consequently, these cells are an attractive target for the invasive electrical implants colloquially known as "bionic eyes." However, after more than two decades of concerted effort, interfaces based on conventional electrical stimulation approaches have delivered limited efficacy, primarily due to the current spread in retinal tissue, which precludes high-acuity vision. The ideal prosthetic solution would be less invasive, provide single-cell resolution and an ability to differentiate between different cell types. Nanoparticle-mediated approaches can address some of these requirements, with particular attention being directed at light-sensitive nanoparticles that can be accessed via the intrinsic optics of the eye. Here we survey the available known nanoparticle-based optical transduction mechanisms that can be exploited for neuromodulation. We review the rapid progress in the field, together with outstanding challenges that must be addressed to translate these techniques to clinical practice. In particular, successful translation will likely require efficient delivery of nanoparticles to stable and precisely defined locations in the retinal tissues. Therefore, we also emphasize the current literature relating to the pharmacokinetics of nanoparticles in the eye. While considerable challenges remain to be overcome, progress to date shows great potential for nanoparticle-based interfaces to revolutionize the field of visual prostheses.

Layer 1 of somatosensory cortex: an important site for input to a tiny cortical compartment

Neocortical layer 1 has been proposed to be at the center for top-down and bottom-up integration. It is a locus for interactions between long-range inputs, layer 1 interneurons, and apical tuft dendrites of pyramidal neurons. While input to layer 1 has been studied intensively, the level and effect of input to this layer has still not been completely characterized. Here we examined the input to layer 1 of mouse somatosensory cortex with retrograde tracing and optogenetics. Our assays reveal that local input to layer 1 is predominantly from layers 2/3 and 5 pyramidal neurons and interneurons, and that subtypes of local layers 5 and 6b neurons project to layer 1 with different probabilities. Long-range input from sensory-motor cortices to layer 1 of somatosensory cortex arose predominantly from layers 2/3 neurons. Our optogenetic experiments showed that intra-telencephalic layer 5 pyramidal neurons drive layer 1 interneurons but have no effect locally on layer 5 apical tuft dendrites. Dual retrograde tracing revealed that a fraction of local and long-range neurons was both presynaptic to layer 5 neurons and projected to layer 1. Our work highlights the prominent role of local inputs to layer 1 and shows the potential for complex interactions between long-range and local inputs, which are both in position to modify the output of somatosensory cortex.

Tellurium-based chemical sensors

The various tellurium-based chemical sensors are described. This article focuses on four types of Tellurium sensors such as CdTe quantum dots-based sensor, Te thin films-based sensor, Te nanostructures or nanoparticles-based sensor, and TeO2-based sensor.

A Platform for Spatiotemporal "Matrix" Stimulation in Brain Networks Reveals Novel Forms of Circuit Plasticity

Here we demonstrate a facile method by which to deliver complex spatiotemporal stimulation to neural networks in fast patterns, to trigger interesting forms of circuit-level plasticity in cortical areas. We present a complete platform by which patterns of electricity can be arbitrarily defined and distributed across a brain circuit, either simultaneously, asynchronously, or in complex patterns that can be easily designed and orchestrated with precise timing. Interfacing with acute slices of mouse cortex, we show that our system can be used to activate neurons at many locations and drive synaptic transmission in distributed patterns, and that this elicits new forms of plasticity that may not be observable via traditional methods, including interesting measurements of associational and sequence plasticity. Finally, we introduce an automated "network assay" for imaging activation and plasticity across a circuit. Spatiotemporal stimulation opens the door for high-throughput explorations of plasticity at the circuit level, and may provide a basis for new types of adaptive neural prosthetics.

Inverted Ion Current Rectification-Based Chemical Delivery Probes for Stimulation of Neurons

Ion current rectification (ICR), diodelike behavior in surface-charged nanopores, shows promise in the design of delivery probes for manipulation of neural networks as it can solve diffusive leakages that might be critical in clinical and research applications. However, it has not been achieved because ICR has restrictions in nanosized dimension and low electrolyte concentration, and rectification direction is inappropriate for delivery. Herein, we present a polyelectrolyte gel-filled (PGF) micropipette harnessing inverted ICR as a delivery probe, which quantitatively transports glutamate to stimulate primary cultured neurons with high efficiency while minimizing leakages. Since the gel works as an ensemble of numerous surface-charged nanopores, the current is rectified in the micro-opening and physiological environment. By extending the charge-selective region using the gel, inverted ICR is generated, which drives outward deliveries of major charge carriers. This study will help in exploring new aspects of ICR and broaden its applications for advanced chemical delivery.

Plasmonic carriers responsive to pulsed laser irradiation: a review of mechanisms, design, and applications

This review focuses on interactions which govern release from plasmonic carrier systems including liposomes, polymersomes, and nanodroplets under pulsed irradiation.

Probing Single Synapses via the Photolytic Release of Neurotransmitters

The development of two-photon microscopy has revolutionized our understanding of how synapses are formed and how they transform synaptic inputs in dendritic spines-tiny protrusions that cover the dendrites of pyramidal neurons that receive most excitatory synaptic information in the brain. These discoveries have led us to better comprehend the neuronal computations that take place at the level of dendritic spines as well as within neuronal circuits with unprecedented resolution. Here, we describe a method that uses a two-photon (2P) microscope and 2P uncaging of caged neurotransmitters for the activation of single and multiple spines in the dendrites of cortical pyramidal neurons. In addition, we propose a cost-effective description of the components necessary for the construction of a one laser source-2P microscope capable of nearly simultaneous 2P uncaging of neurotransmitters and 2P calcium imaging of the activated spines and nearby dendrites. We provide a brief overview on how the use of these techniques have helped researchers in the last 15 years unravel the function of spines in: (a) information processing; (b) storage; and (c) integration of excitatory synaptic inputs.

Photo-triggered Drug Delivery Systems for Neuron-related Applications

The development of materials, chemistry and genetics has created a great number of systems for delivering antibiotics, neuropeptides or other drugs to neurons in neuroscience research, and has also provided important and powerful tools in neuron-related applications. Although these drug delivery systems can facilitate the advancement of neuroscience studies, they still have limited applications due to various drawbacks, such as difficulty in controlling delivery molecules or drugs to the target region, and trouble of releasing them in predictable manners. The combination of optics and drug delivery systems has great potentials to address these issues and deliver molecules or drugs to the nervous system with extraordinary spatiotemporal selectivity triggered by light. In this review, we will introduce the development of photo-triggered drug delivery systems in neuroscience research and their neuron-related applications including regulating neural activities, treating neural diseases and inducing nerve regenerations.

Rationally designed azobenzene photoswitches for efficient two-photon neuronal excitation

Manipulation of neuronal activity using two-photon excitation of azobenzene photoswitches with near-infrared light has been recently demonstrated, but their practical use in neuronal tissue to photostimulate individual neurons with three-dimensional precision has been hampered by firstly, the low efficacy and reliability of NIR-induced azobenzene photoisomerization compared to one-photon excitation, and secondly, the short cis state lifetime of the two-photon responsive azo switches. Here we report the rational design based on theoretical calculations and the synthesis of azobenzene photoswitches endowed with both high two-photon absorption cross section and slow thermal back-isomerization. These compounds provide optimized and sustained two-photon neuronal stimulation both in light-scattering brain tissue and in Caenorhabditis elegans nematodes, displaying photoresponse intensities that are comparable to those achieved under one-photon excitation. This finding opens the way to use both genetically targeted and pharmacologically selective azobenzene photoswitches to dissect intact neuronal circuits in three dimensions.

The use of chemogenetic approaches in alcohol use disorder research and treatment

Several novel techniques were developed recently to explore neural circuit mechanisms of neuropsychiatric disorders. These techniques include the Designer Receptors Exclusively Activated by Designer Drugs (DREADD)-based chemogenetic tools, which represent valuable platforms for selective and non-invasive control of neural activity with a high degree of spatial resolution. Among all variants, Gq- and Gi-DREADDs are widely used by neuroscientists to dissect out the circuitry and cellular signals. This review is focused on strategies to access a specific neuronal population or circuit using the DREADD technique and summarizes the current knowledge of the DREADDs' application in alcohol use disorder research and therapeutics.

Optogenetic stimulation of complex spatio-temporal activity patterns by acousto-optic light steering probes cerebellar granular layer integrative properties

Optogenetics provides tools to control afferent activity in brain microcircuits. However, this requires optical methods that can evoke asynchronous and coordinated activity within neuronal ensembles in a spatio-temporally precise way. Here we describe a light patterning method, which combines MHz acousto-optic beam steering and adjustable low numerical aperture Gaussian beams, to achieve fast 2D targeting in scattering tissue. Using mossy fiber afferents to the cerebellar cortex as a testbed, we demonstrate single fiber optogenetic stimulation with micron-scale lateral resolution, >100 µm depth-penetration and 0.1 ms spiking precision. Protracted spatio-temporal patterns of light delivered by our illumination system evoked sustained asynchronous mossy fiber activity with excellent repeatability. Combining optical and electrical stimulations, we show that the cerebellar granular layer performs nonlinear integration, whereby sustained mossy fiber activity provides a permissive context for the transmission of salient inputs, enriching combinatorial views on mossy fiber pattern separation.

Simulation of morphologically structured photo-thermal neural stimulation

OBJECTIVE

Rational design of next-generation techniques for photo-thermal excitation requires the development of tools capable of modeling the effects of spatially- and temporally-dependent temperature distribution on cellular neuronal structures.

APPROACH

We present a new computer simulation tool for predicting the effects of arbitrary spatiotemporally-structured photo-thermal stimulation on 3D, morphologically realistic neurons. The new simulation tool is based on interfacing two generic platforms, NEURON and MATLAB and is therefore suited for capturing different kinds of stimuli and neural models.

MAIN RESULTS

Simulation results are validated using photo-absorber induced neuro-thermal stimulation (PAINTS) empirical results, and advanced features are explored.

SIGNIFICANCE

The new simulation tool could have an important role in understanding and investigating complex optical stimulation at the single-cell and network levels.

TRPA1 expression and its functional activation in rodent cortex

TRPA1 is a non-selective cation channel involved in pain sensation and neurogenic inflammation. Although TRPA1 is well established in a number of organs including the nervous system, its presence and function in the mammalian cortex remains unclear. Here, we demonstrate the expression of TRPA1 in rodent somatosensory cortex through immunostaining and investigate its functional activation by whole-cell electrophysiology, Ca²⁺ imaging and two-photon photoswitching. Application of TRPA1 agonist (AITC) and antagonist (HC-030031) produced significant modulation of activity in layer 5 (L5) pyramidal neurons in both rats and mice; AITC increased intracellular Ca²⁺ concentrations and depolarized neurons, and both effects were blocked by HC-030031. These modulations were absent in the TRPA1 knockout mice. Next, we used optovin, a reversible photoactive molecule, to activate TRPA1 in individual L5 neurons of rat cortex. Optical control of activity was established by applying a tightly focused femtosecond-pulsed laser to optovin-loaded neurons. Light application depolarized neurons (n = 17) with the maximal effect observed at λ = 720 nm. Involvement of TRPA1 was further confirmed by repeating the experiment in the presence of HC-030031, which diminished the light modulation. These results demonstrate the presence of TRPA1 in L5 pyramidal neurons and introduce a highly specific approach to further understand its functional significance.

A distinct entorhinal cortex to hippocampal CA1 direct circuit for olfactory associative learning

Lateral and medial parts of entorhinal cortex (EC) convey nonspatial 'what' and spatial 'where' information, respectively, into hippocampal CA1, via both the indirect EC layer 2→ hippocampal dentate gyrus→CA3→CA1 and the direct EC layer 3→CA1 paths. However, it remains elusive how the direct path transfers distinct information and contributes to hippocampal learning functions. Here we report that lateral EC projection neurons selectively form direct excitatory synapses onto a subpopulation of morphologically complex, calbindin-expressing pyramidal cells (PCs) in the dorsal CA1 (dCA1), while medial EC neurons uniformly innervate all dCA1 PCs. Optogenetically inactivating the distinct lateral EC-dCA1 connections or the postsynaptic dCA1 calbindin-expressing PC activity slows olfactory associative learning. Moreover, optetrode recordings reveal that dCA1 calbindin-expressing PCs develop more selective spiking responses to odor cues during learning. Thus, our results identify a direct lateral EC→dCA1 circuit that is required for olfactory associative learning.

Optogenetic Stimulation and Recording of Primary Cultured Neurons with Spatiotemporal Control

We studied a network of cortical neurons in culture and developed an innovative optical device to stimulate optogenetically a large neuronal population with both spatial and temporal precision. We first describe how to culture primary neurons expressing channelrhodopsin. We then detail the optogenetic setup based on the workings of a fast Digital Light Processing (DLP) projector. The setup is able to stimulate tens to hundreds neurons with independent trains of light pulses that evoked action potentials with high temporal resolution. During photostimulation, network activity was monitored using patch-clamp recordings of up to 4 neurons. The experiment is ideally suited to study recurrent network dynamics or biological processes such as plasticity or homeostasis in a network of neurons when a sub-population is activated by distinct stimuli whose characteristics (correlation, rate, and, size) were finely controlled.

Study of laser uncaging induced morphological alteration of rat cortical neurites using atomic force microscopy

Activity-dependent structural remodeling is an important aspect of neuronal plasticity. In the previous researches, neuronal structure variations resulting from external interventions were detected by the imaging instruments such as the fluorescence microscopy, the scanning/transmission electron microscopy (SEM/TEM) and the laser confocal microscopy. In this article, a new platform which combined the photochemical stimulation with atomic force microscopy (AFM) was set up to detect the activity-dependent structural remodeling. In the experiments, the cortical neurites on the glass coverslips were stimulated by locally uncaged glutamate under the ultraviolet (UV) laser pulses, and a calcium-related structural collapse of neurites (about 250 nm height decrease) was observed by an AFM. This was the first attempt to combine the laser uncaging with AFM in living cell researches. With the advantages of highly localized stimulation (<5 μm), super resolution imaging (<3.8 nm), and convenient platform building, this system was suitable for the quantitative observation of the neuron mechanical property variations and morphological alterations modified by neural activities under different photochemical stimulations, which would be helpful for studying physiological and pathological mechanisms of structural and functional changes induced by the biomolecule acting.

All-optical bidirectional neural interfacing using hybrid multiphoton holographic optogenetic stimulation

Our understanding of neural information processing could potentially be advanced by combining flexible three-dimensional (3-D) neuroimaging and stimulation. Recent developments in optogenetics suggest that neurophotonic approaches are in principle highly suited for noncontact stimulation of network activity patterns. In particular, two-photon holographic optical neural stimulation (2P-HONS) has emerged as a leading approach for multisite 3-D excitation, and combining it with temporal focusing (TF) further enables axially confined yet spatially extended light patterns. Here, we study key steps toward bidirectional cell-targeted 3-D interfacing by introducing and testing a hybrid new 2P-TF-HONS stimulation path for accurate parallel optogenetic excitation into a recently developed hybrid multiphoton 3-D imaging system. The system is shown to allow targeted all-optical probing of in vitro cortical networks expressing channelrhodopsin-2 using a regeneratively amplified femtosecond laser source tuned to 905 nm. These developments further advance a prospective new tool for studying and achieving distributed control over 3-D neuronal circuits both in vitro and in vivo.

Optogenetic approaches to retinal prosthesis

The concept of visual restoration via retinal prosthesis arguably started in 1992 with the discovery that some of the retinal cells were still intact in those with the retinitis pigmentosa disease. Two decades later, the first commercially available devices have the capability to allow users to identify basic shapes. Such devices are still very far from returning vision beyond the legal blindness. Thus, there is considerable continued development of electrode materials, and structures and electronic control mechanisms to increase both resolution and contrast. In parallel, the field of optogenetics--the genetic photosensitization of neural tissue holds particular promise for new approaches. Given that the eye is transparent, photosensitizing remaining neural layers of the eye and illuminating from the outside could prove to be less invasive, cheaper, and more effective than present approaches. As we move toward human trials in the coming years, this review explores the core technological and biological challenges related to the gene therapy and the high radiance optical stimulation requirement.

Synthesis and biological evaluation of bis-CNB-GABA, a photoactivatable neurotransmitter with low receptor interference and chemical two-photon uncaging properties

Photoactivatable "caged" neurotransmitters allow optical control of neural tissue with high spatial and temporal precision. However, the development of caged versions of the chief vertebrate inhibitory neurotransmitter, γ-amino butyric acid (GABA), has been limited by the propensity of caged GABAs to interact with GABA receptors. We describe herein the synthesis and application of a practically useful doubly caged GABA analog, termed bis-α-carboxy-2-nitrobenzyl-GABA (bis-CNB-GABA). Uncaging of bis-CNB-GABA evokes inward GABAergic currents in cerebellar molecular layer interneurons with rise times of 2 ms, comparable to flash duration. Response amplitudes depend on the square of flash intensity, as expected for a chemical two-photon uncaging effect. Importantly, prior to uncaging, bis-CNB-GABA is inactive at the GABAA receptor, evoking no changes in holding current in voltage-clamped neurons and showing an IC50 of at least 2.5 mM as measured using spontaneous GABAergic synaptic currents. Bis-CNB-GABA is stable in solution, with an estimated half-life of 98 days in the light. We expect that bis-CNB-GABA will prove to be an effective tool for high-resolution chemical control of brain circuits.

Optical developments for optogenetics

Brain intricacies and the difficulty that scientists encounter in revealing its function with standard approaches such as electrical stimulation of neurons have led to the exploration of new tools that enable the study of neural circuits in a remote and non-invasive way. To this end, optogenetics has initialised a revolution for neuroscience in the last decade by enabling simultaneous monitoring and stimulation of specific neuronal populations in intact brain preparations through genetically targeted expression of light sensitive proteins and molecular photoswitches. In addition to ongoing molecular probe development and optimisation, novel optical techniques hold immense potential to amplify and diversify the utility of optogenetic methods. Importantly, by improving the spatio-temporal resolution of light stimulation, neural circuits can be photoactivated in patterns mimicking endogenous physiological processes. The following synopsis addresses the possibilities and limitations of optical stimulation methods applied to and developed for activation of neuronal optogenetic tools.

Holographic optogenetic stimulation of patterned neuronal activity for vision restoration

When natural photoreception is disrupted, as in outer-retinal degenerative diseases, artificial stimulation of surviving nerve cells offers a potential strategy for bypassing compromised neural circuits. Recently, light-sensitive proteins that photosensitize quiescent neurons have generated unprecedented opportunities for optogenetic neuronal control, inspiring early development of optical retinal prostheses. Selectively exciting large neural populations are essential for eliciting meaningful perceptions in the brain. Here we provide the first demonstration of holographic photo-stimulation strategies for bionic vision restoration. In blind retinas, we demonstrate reliable holographically patterned optogenetic stimulation of retinal ganglion cells with millisecond temporal precision and cellular resolution. Holographic excitation strategies could enable flexible control over distributed neuronal circuits, potentially paving the way towards high-acuity vision restoration devices and additional medical and scientific neuro-photonics applications.

Two-way communication with neural networks in vivo using focused light

Neuronal networks process information in a distributed, spatially heterogeneous manner that transcends the layout of electrodes. In contrast, directed and steerable light offers the potential to engage specific cells on demand. We present a unified framework for adapting microscopes to use light for simultaneous in vivo stimulation and recording of cells at fine spatiotemporal resolutions. We use straightforward optics to lock onto networks in vivo, to steer light to activate circuit elements and to simultaneously record from other cells. We then actualize this 'free' augmentation on both an 'open' two-photon microscope and a leading commercial one. By following this protocol, setup of the system takes a few days, and the result is a noninvasive interface to brain dynamics based on directed light, at a network resolution that was not previously possible and which will further improve with the rapid advance in development of optical reporters and effectors. This protocol is for physiologists who are competent with computers and wish to extend hardware and software to interface more fluidly with neuronal networks.

Microfluidic construction of minimalistic neuronal co-cultures

In this paper we present compartmentalized neuron arraying (CNA) microfluidic circuits for the preparation of neuronal networks using minimal cellular inputs (10-100-fold less than existing systems). The approach combines the benefits of microfluidics for precision single cell handling with biomaterial patterning for the long term maintenance of neuronal arrangements. A differential flow principle was used for cell metering and loading along linear arrays. An innovative water masking technique was developed for the inclusion of aligned biomaterial patterns within the microfluidic environment. For patterning primary neurons the technique involved the use of meniscus-pinning micropillars to align a water mask for plasma stencilling a poly-amine coating. The approach was extended for patterning the human SH-SY5Y neuroblastoma cell line using a poly(ethylene glycol) (PEG) back-fill and for dopaminergic LUHMES neuronal precursors by the further addition of a fibronectin coating. The patterning efficiency Epatt was >75% during lengthy in chip culture, with ∼85% of the outgrowth channels occupied by neurites. Neurons were also cultured in next generation circuits which enable neurite guidance into all outgrowth channels for the formation of extensive inter-compartment networks. Fluidic isolation protocols were developed for the rapid and sustained treatment of the different cellular and sub-cellular compartments. In summary, this research demonstrates widely applicable microfluidic methods for the construction of compartmentalized brain models with single cell precision. These minimalistic ex vivo tissue constructs pave the way for high throughput experimentation to gain deeper insights into pathological processes such as Alzheimer and Parkinson Diseases, as well as neuronal development and function in health.

Optogenetic manipulation of neural and non-neural functions

Optogenetic manipulation of the neuronal activity enables one to analyze the neuronal network both in vivo and in vitro with precise spatio-temporal resolution. Channelrhodopsins (ChRs) are light-sensitive cation channels that depolarize the cell membrane, whereas halorhodopsins and archaerhodopsins are light-sensitive Cl(-) and H(+) transporters, respectively, that hyperpolarize it when exogenously expressed. The cause-effect relationship between a neuron and its function in the brain is thus bi-directionally investigated with evidence of necessity and sufficiency. In this review we discuss the potential of optogenetics with a focus on three major requirements for its application: (i) selection of the light-sensitive proteins optimal for optogenetic investigation, (ii) targeted expression of these selected proteins in a specific group of neurons, and (iii) targeted irradiation with high spatiotemporal resolution. We also discuss recent progress in the application of optogenetics to studies of non-neural cells such as glial cells, cardiac and skeletal myocytes. In combination with stem cell technology, optogenetics may be key to successful research using embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) derived from human patients through optical regulation of differentiation-maturation, through optical manipulation of tissue transplants and, furthermore, through facilitating survival and integration of transplants.

Molecular catch and release: controlled delivery using optical trapping with light-responsive liposomes

Gold-coated liposomes are maneuvered using an optical trap to achieve precise delivery of encapsulated molecular cargo. Movement and payload release from these plasmon resonant nanocapsules are independently controlled using a pulsed trapping beam. This technology enables in vitro delivery of a payload to a selected cell and may be applied to the interrogation of individual cells within their biological microenvironment.

Organic bioelectronics for electronic-to-chemical translation in modulation of neuronal signaling and machine-to-brain interfacing

BACKGROUND

A major challenge when creating interfaces for the nervous system is to translate between the signal carriers of the nervous system (ions and neurotransmitters) and those of conventional electronics (electrons).

SCOPE OF REVIEW

Organic conjugated polymers represent a unique class of materials that utilizes both electrons and ions as charge carriers. Based on these materials, we have established a series of novel communication interfaces between electronic components and biological systems. The organic electronic ion pump (OEIP) presented in this review is made of the polymer-polyelectrolyte system poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The OEIP translates electronic signals into electrophoretic migration of ions and neurotransmitters.

MAJOR CONCLUSIONS

We demonstrate how spatio-temporally controlled delivery of ions and neurotransmitters can be used to modulate intracellular Ca(2+) signaling in neuronal cells in the absence of convective disturbances. The electronic control of delivery enables strict control of dynamic parameters, such as amplitude and frequency of Ca(2+) responses, and can be used to generate temporal patterns mimicking naturally occurring Ca(2+) oscillations. To enable further control of the ionic signals we developed the electrophoretic chemical transistor, an analog of the traditional transistor used to amplify and/or switch electronic signals. Finally, we demonstrate the use of the OEIP in a new "machine-to-brain" interface by modulating brainstem responses in vivo.

GENERAL SIGNIFICANCE

This review highlights the potential of communication interfaces based on conjugated polymers in generating complex, high-resolution, signal patterns to control cell physiology. We foresee widespread applications for these devices in biomedical research and in future medical devices within multiple therapeutic areas. This article is part of a Special Issue entitled Organic Bioelectronics-Novel Applications in Biomedicine.

NIR-activated content release from plasmon resonant liposomes for probing single-cell responses

Technological limitations have prevented the interrogation and manipulation of cellular activity in response to bioactive molecules within model and living systems that is required for the development of diagnostic and treatment modalities for diseases, such as cancer. In this work, we demonstrate that gold-coated liposomes are capable of encapsulation and on-demand release of signaling molecules with a spatial and temporal resolution leading to activation of individual cells. As a model system, we used cells modified to overexpress a certain G-protein coupled receptor, the CCK2 receptor, and achieved its activation in a single cell via the localized release of its agonist. This content release was triggered by illumination of the liposomes at wavelengths corresponding to the plasmon resonance of the gold coating. The use of plasmon resonant liposomes may enable on-demand release of a broad range of molecules using biologically safe near-infrared light and without molecule chemical modification. In combination with the spectral tunability of plasmon resonant coating, this technology may allow for multiplexed interrogation of complex and diverse signaling pathways in model or living tissues with unprecedented spatial and temporal control.

The spatial pattern of light determines the kinetics and modulates backpropagation of optogenetic action potentials

Optogenetics offers an unprecedented ability to spatially target neuronal stimulations. This study investigated via simulation, for the first time, how the spatial pattern of excitation affects the response of channelrhodopsin-2 (ChR2) expressing neurons. First we described a methodology for modeling ChR2 in the NEURON simulation platform. Then, we compared four most commonly considered illumination strategies (somatic, dendritic, axonal and whole cell) in a paradigmatic model of a cortical layer V pyramidal cell. We show that the spatial pattern of illumination has an important impact on the efficiency of stimulation and the kinetics of the spiking output. Whole cell illumination synchronizes the depolarization of the dendritic tree and the soma and evokes spiking characteristics with a distinct pattern including an increased bursting rate and enhanced back propagation of action potentials (bAPs). This type of illumination is the most efficient as a given irradiance threshold was achievable with only 6 % of ChR2 density needed in the case of somatic illumination. Targeting only the axon initial segment requires a high ChR2 density to achieve a given threshold irradiance and a prolonged illumination does not yield sustained spiking. We also show that patterned illumination can be used to modulate the bAPs and hence spatially modulate the direction and amplitude of spike time dependent plasticity protocols. We further found the irradiance threshold to increase in proportion to the demyelination level of an axon, suggesting that measurements of the irradiance threshold (for example relative to the soma) could be used to remotely probe a loss of neural myelin sheath, which is a hallmark of several neurodegenerative diseases.

Simultaneous multi-site two-photon photostimulation in three dimensions

We demonstrate simultaneous multi-site two-photon photolysis of caged neurotransmitters with close to diffraction-limited resolution in all three dimensions (3D). We use holographic projection of multiple focal spots, which allows full control over the 3D positions of uncaging sites with a high degree of localized excitation. Our system incorporates a two-photon imaging setup to visualize the 3D morphology of the neurons in order to accurately determine the photostimulation sites. We show its application to studies of synaptic integration by performing simultaneous and controlled glutamate delivery at multiple locations on dendritic trees.

Circuit mapping by ultraviolet uncaging of glutamate

In laser photostimulation, small clusters of neurons in brain slices are induced to fire action potentials by focal glutamate uncaging, and synaptic connectivity between photoexcited presynaptic neurons and individual postsynaptic neurons is assessed by intracellular recording of synaptic events. With a scanner, this process can be repeated sequentially across a patterned array of stimulus locations, generating maps of neurons' local sources of synaptic inputs. Laser scanning photostimulation (LSPS) based on patterned glutamate uncaging offers an efficient, quantitative, optical-electrophysiological way to map synaptic circuits in brain slices. The efficacy of glutamate-based photostimulation for circuit mapping (in contrast to electrical stimulation) derives from the ability to stimulate neurons with high precision and speed, and without stimulating axons of passage. This protocol describes the components, assembly, and operation of a laser scanning microscope for ultraviolet (UV) uncaging, along with experimental methods for circuit mapping in brain slices. It presents a general approach and a set of guidelines for quantitative circuit mapping using "standard" LSPS methods based on single-photon glutamate uncaging using a UV laser, a pair of scanning mirror galvanometers, a patch-clamp setup, and open-source data acquisition software.

Acousto-optical deflector-based patterned ultraviolet uncaging of neurotransmitter for the study of neuronal integration

The method of patterned photoactivation is a natural fit for the study of neuronal dendritic integration. Photoactivatable molecules that influence a wide range of extracellular and intracellular neurophysiological functions are available. The choice of photosensitive molecules depends on the research question and will influence the design of the experimental apparatus. This article describes an acousto-optical deflector (AOD)-based system for rapid ultraviolet (UV) photolysis in arbitrary spatial and temporal patterns. Some basics of caged neurotransmitters and the theory of operation of AODs are covered, as are descriptions for implementing the system.

Construction, alignment, and implementation of an acousto-optical deflector-based system for patterned uncaging with ultraviolet light

The method of patterned photoactivation is a natural fit for the study of neuronal dendritic integration. Photoactivatable molecules that influence a wide range of extracellular and intracellular neurophysiological functions are available. The choice of photosensitive molecules depends on the research question and will influence the design of the experimental apparatus. An acousto-optical deflector (AOD)-based system can be used for rapid ultraviolet (UV) photolysis in arbitrary spatial and temporal patterns. Photolysis-activated "caged" diffusible molecules or newer light-sensitive membrane proteins can be used in this system. This protocol describes the addition of a UV beam for uncaging to a homebuilt two-photon microscope. The goal is to get UV light from the light source (laser) to the approximate center of the objective's back aperture, passing through a pair of perpendicularly oriented AODs along the way. The protocol also describes the fine alignment of the UV beam and the implementation of AOD-based beam steering. Performing the final alignment with the beam passing through the AODs will ensure that the system is optimized for the idiosyncrasies of the crystals.

An interdisciplinary learning experience in neuro-optics

How can a Ph.D. student initially trained as a biologist take part in the development of a multineuronal recording method that requires cross interaction between physics, neurobiology and mathematics? Beyond student training in the laboratory, interdisciplinary research calls for a new style of academic training of young researchers. Here we present an innovative approach to graduate student academic training that fills the need for multidisciplinary knowledge and provides students, in addition, with a deeper understanding of the interdisciplinary approach to scientific research.

Instrumentation of a compact random-access photostimulator based on acousto-optic deflectors

Recently developed optogenetics provides a fast, non-invasive, and efficient method for cell activation. However, it is difficult for the optical stimulators used for optogenetics to realize selective multi-site fast activation. In this paper, we developed a random-access photostimulator based on a pair of perpendicularly oriented acousto-optic deflectors. Precise laser targeting in the x-y plane was verified, and the lateral spatial resolution of laser intensity after the objective was measured as ∼1.38 μm. Photostimulaton of ChETA-expressing astrocytes induced reliable inward currents only if the laser beam was directed onto the targeted cell. In the ChR2-expresing neuron, multiple locations along two dendrites were stimulated, and spatiotemporal integration was observed in the soma with fast multi-site activation. These results demonstrated that this random-access photostimulator would be a powerful tool for selective multi-site fast activation. The compact and modular design of this photostimulator makes it easily integrated with different commercial microscopes, and thus widely popularized in many laboratories.

The age of enlightenment: evolving opportunities in brain research through optical manipulation of neuronal activity

Optical manipulation of neuronal activity has rapidly developed into the most powerful and widely used approach to study mechanisms related to neuronal connectivity over a range of scales. Since the early use of single site uncaging to map network connectivity, rapid technological development of light modulation techniques has added important new options, such as fast scanning photostimulation, massively parallel control of light stimuli, holographic uncaging, and two-photon stimulation techniques. Exciting new developments in optogenetics complement neurotransmitter uncaging techniques by providing cell-type specificity and in vivo usability, providing optical access to the neural substrates of behavior. Here we review the rapid evolution of methods for the optical manipulation of neuronal activity, emphasizing crucial recent developments.

Precise spatiotemporal control of optogenetic activation using an acousto-optic device

Light activation and inactivation of neurons by optogenetic techniques has emerged as an important tool for studying neural circuit function. To achieve a high resolution, new methods are being developed to selectively manipulate the activity of individual neurons. Here, we report that the combination of an acousto-optic device (AOD) and single-photon laser was used to achieve rapid and precise spatiotemporal control of light stimulation at multiple points in a neural circuit with millisecond time resolution. The performance of this system in activating ChIEF expressed on HEK 293 cells as well as cultured neurons was first evaluated, and the laser stimulation patterns were optimized. Next, the spatiotemporally selective manipulation of multiple neurons was achieved in a precise manner. Finally, we demonstrated the versatility of this high-resolution method in dissecting neural circuits both in the mouse cortical slice and the Drosophila brain in vivo. Taken together, our results show that the combination of AOD-assisted laser stimulation and optogenetic tools provides a flexible solution for manipulating neuronal activity at high efficiency and with high temporal precision.

Scanless two-photon excitation of channelrhodopsin-2

Light-gated ion channels and pumps have made it possible to probe intact neural circuits by manipulating the activity of groups of genetically similar neurons. What is needed now is a method for precisely aiming the stimulating light at single neuronal processes, neurons or groups of neurons. We developed a method that combines generalized phase contrast with temporal focusing (TF-GPC) to shape two-photon excitation for this purpose. The illumination patterns are generated automatically from fluorescence images of neurons and shaped to cover the cell body or dendrites, or distributed groups of cells. The TF-GPC two-photon excitation patterns generated large photocurrents in Channelrhodopsin-2-expressing cultured cells and neurons and in mouse acute cortical slices. The amplitudes of the photocurrents can be precisely modulated by controlling the size and shape of the excitation volume and, thereby, be used to trigger single action potentials or trains of action potentials.

Using light to control signaling cascades in live neurons

Understanding the complexity of neuronal biology requires the manipulation of cellular processes with high specificity and spatio-temporal precision. The recent development of synthetic photo-activatable proteins designed using the light-oxygen-voltage and phytochrome domains provides a new set of tools for genetically targeted optical control of cell signaling. Their modular design, functional diversity, precisely controlled activity and in vivo applicability offer many advantages for investigating neuronal function. Although designing these proteins is still a considerable challenge, future advances in rational protein design and a deeper understanding of their photoactivation mechanisms will allow the development of the next generation of optogenetic techniques.

Detection of input sites in scanning photostimulation data based on spatial correlations

Scanning photostimulation is a well-established method for studying the functional microcircuitry in brain slices. Light-evoked responses are thereby taken as an indicator for a connected presynaptic partner. Such an approach thus requires a clear distinction between the photo-evoked and the spontaneous responses. Here we show that, for a data set from entorhinal cortex layer II with high spontaneous synaptic rates of up to 10Hz, it is possible to identify presynaptic sites. The underlying detection algorithm is based on the finding that a presynaptic cell has several neighboring activation sites, resulting in the clustered appearance of specific photo-evoked inputs. The main idea behind this approach is to identify "hit" locations at which the number of intracellularly recorded synaptic events is significantly larger as expected from the hypothesis of statistical independence. The algorithm works without making use of EPSC amplitude information and for single trials, i.e., each site is stimulated only once. The hit maps are tested upon reliability by repeated stimulations and by blocking synaptically mediated responses via TTX. Furthermore, based on the hit density of surrogate data, we devise a Bayesian formalism to estimate the number of presynaptic partners. In these simulations we find good agreement between estimated and real number of input cells, which shows that the hit density can be used as a reliable measure for afferent connectivity.

A compact Acousto-Optic Lens for 2D and 3D femtosecond based 2-photon microscopy

We describe a high speed 3D Acousto-Optic Lens Microscope (AOLM) for femtosecond 2-photon imaging. By optimizing the design of the 4 AO Deflectors (AODs) and by deriving new control algorithms, we have developed a compact spherical AOL with a low temporal dispersion that enables 2-photon imaging at 10-fold lower power than previously reported. We show that the AOLM can perform high speed 2D raster-scan imaging (>150 Hz) without scan rate dependent astigmatism. It can deflect and focus a laser beam in a 3D random access sequence at 30 kHz and has an extended focusing range (>137 mum; 40X 0.8NA objective). These features are likely to make the AOLM a useful tool for studying fast physiological processes distributed in 3D space.