Animals respond to whole-field visual motion with compensatory eye and body movements in order to stabilize both their gaze and position with respect to their surroundings. In zebrafish, rotational stimuli need to be distinguished from translational stimuli to drive the optokinetic and the optomotor responses, respectively. Here, we systematically characterize the neural circuits responsible for these operations using a combination of optogenetic manipulation and in vivo calcium imaging during optic flow stimulation. By recording the activity of thousands of neurons within the area pretectalis (APT), we find four bilateral pairs of clusters that process horizontal whole-field motion and functionally classify eleven prominent neuron types with highly selective response profiles. APT neurons are prevalently direction selective, either monocularly or binocularly driven, and hierarchically organized to distinguish between rotational and translational optic flow. Our data predict a wiring diagram of a neural circuit tailored to drive behavior that compensates for self-motion.
Genetic approaches to control DNA expression in different brain areas have provided an excellent system to characterize gene function in health and disease of animal models. With respect to others, in utero electroporation of exogenous DNA into progenitor cells committed to specific brain areas is the optimal solution in terms of simplicity and velocity. Indeed, this method entails one quick and easy surgical procedure aimed at DNA injection in the embryonic brain followed by brief exposure to a strong electric field by a bipolar electrode. Nevertheless, the technique is still lacking the necessary control and reliability in addressing the field. Moving from a theoretical model that accounts for the morphology and the dielectric properties of the embryonic brain, we developed here a set of novel and reliable experimental configurations based on the use of three electrodes for electroporation in mouse. Indeed, by means of a full 3D model of the embryonic brain and the surrounding environment, we showed that the distribution of the electric field can be finely tuned in order to target specific brain regions at a desired temporal window by proper placement of the three electrodes. In the light of this theoretical background, we manufactured a three-electrode device and performed model-guided experimental sessions. The result was an increased spatial control, extended time frames and unprecedented reliability of the genetic manipulation, with respect to the current state of the art. In particular, the outcomes of this method applied into the mouse model are reported here for the first time.
In the absence of external stimuli, the mammalian neocortex shows intrinsic network oscillations. These dynamics are characterized by translaminar assemblies of neurons whose activity synchronizes rhythmically in space and time. How different cortical layers influence the formation of these spontaneous cellular assemblies is poorly understood. We found that excitatory neurons in supragranular and infragranular layers have distinct roles in the regulation of intrinsic low-frequency oscillations in mice in vivo. Optogenetic activation of infragranular neurons generated network activity that resembled spontaneous events, whereas photoinhibition of these same neurons substantially attenuated slow ongoing dynamics. In contrast, light activation and inhibition of supragranular cells had modest effects on spontaneous slow activity. This study represents, to the best of our knowledge, the first causal demonstration that excitatory circuits located in distinct cortical layers differentially control spontaneous low-frequency dynamics.
The ?2 auxiliary subunit of voltage-gated sodium channels (VGSCs) appears at early stages of brain development. It is abundantly expressed in the mammalian central nervous system where it forms complexes with different channel isoforms, including Na(v)1.2. From the structural point of view, ?2 is a transmembrane protein: at its extracellular N-terminus an Ig-like type C2 domain mediates the binding to the pore-forming alpha subunit with disulfide bonds and the interactions with the extracellular matrix. Given this structural versatility, ?2 has been suggested to play multiple functions ranging from channel targeting to the plasma membrane and gating modulation to control of cell adhesion. We report that, when expressed in Chinese Hamster Ovary cells CHO-K1, the subunit accumulates at the perimetral region of adhesion and particularly in large lamellipodia-like membrane processes where it induces formation of filopodia-like structures. When overexpressed in developing embryonic rat hippocampal neurons in vitro, ?2 specifically promotes formation of filopodia-like processes in dendrites leading to expansion of the arborization tree, while axonal branching remains unaltered. In contrast to this striking and highly specific effect on dendritic morphology, the targeting of functional sodium channels to the plasma membrane, including the preferential localization of Na(v)1.2 at the axon, and their gating properties are only minimally affected. From these and previously reported observations it is suggested that ?2, among its multiple functions, may contribute to promote dendritic outgrowth and to regulate neuronal wiring at specific stages of neuronal development.
The acquisition of high-resolution images in three dimensions is of utmost importance for the morphological and functional investigation of biological tissues. Here, we present a laser scanning two-photon microscope with remote and motionless control of the focus position. The movement of the excitation spot along the propagation direction is achieved by shaping the laser wavefront with a spatial light modulator. Depending on the optical properties of the objective in use, this approach allows z movements in a range of tens to hundreds of micrometers with small changes of the point spread function. We applied this technique for the three-dimensional (3D) imaging of fluorescent cells in the mouse neocortex in vivo. The presented system bypasses the limitations of microscopes based on moving objectives, enabling high-resolution inertia-free 3D imaging.
Regeneration of functional connectivity within a neural network after different degrees of lesion is of utmost clinical importance. To test pharmacological approaches aimed at recovering from a total or partial damage of neuronal connections within a circuit, it is necessary to develop a precise method for controlled ablation of neuronal processes. We combined a UV laser microdissector to ablate neural processes in vitro at single neuron and neural network level with infrared holographic optical tweezers to carry out force spectroscopy measurements. Simultaneous force spectroscopy, down to the sub-pico-Newton range, was performed during laser dissection to quantify the tension release in a partially ablated neurite. Therefore, we could control and measure the damage inflicted to an individual neuronal process. To characterize the effect of the inflicted injury on network level, changes in activity of neural subpopulations were monitored with subcellular resolution and overall network activity with high temporal resolution by concurrent calcium imaging and microelectrode array recording. Neuronal connections have been sequentially ablated and the correlated changes in network activity traced and mapped. With this unique combination of electrophysiological and optical tools, neural activity can be studied and quantified in response to controlled injury at the subcellular, cellular, and network level.
The present study is aimed to design a prototype of hybrid silicon-muscle cell junction, analog to an artificial neuromuscular junction prototype and relevant to the development of advanced neuro-prostheses and bionic systems. The device achieves focal Electric Capacitive Stimulation (ECS) by coupling of single cells and semiconductors, without electrochemical reaction with the substrate. A voltage change applied to a stimulation spot beneath an electrogenic cell leads to a capacitive current (charge accumulation) that opens voltage-gated ion channels in the membrane and generates an action potential. The myo-electronic junction was employed to chronically stimulate muscle cells via ECS and to induce cytosolic calcium transients in myotubes, fibers isolated from mouse FDB (fast [Ca(2+)](i) transients) and surprisingly also in undifferentiated myoblasts (slow [Ca(2+)](i) waves). The hybrid junction elicited, via chronic ECS, a differential reprogramming of single muscle cells by inducing early muscle contraction maturation and plasticity effects, such as NFAT-C3 nuclear translocation. In addition, in the presence of agrin, chronic ECS induced a modulation of AchR clustering which simulates in vitro synaptogenesis. This methodology can coordinate the myogenic differentiation, thus offering direct but non-invasive single cell/wiring, providing a platform for regenerative medicine strategies.
A key issue in the realization of retinal prosthetic devices is reliable transduction of information carried by light into specific patterns of electrical activity in visual information processing networks. Soft organic materials can be used to couple artificial sensors with neuronal tissues. Here, we interface a network of primary neurons with an organic blend. We show that primary neurons can be successfully grown onto the polymer layer without affecting the optoelectronic properties of the active material or the biological functionality of neuronal network. Moreover, action potentials can be triggered in a temporally reliable and spatially selective manner with short pulses of visible light. Our results may lead to new neuronal communication and photo manipulation techniques, thus paving way to the development of artificial retinas and other neuroprosthetic interfaces based on organic photodetectors.
Holographic microscopy is increasingly recognized as a promising tool for the study of the central nervous system. Here we present a "holographic module", a simple optical path that can be combined with commercial scanheads for simultaneous imaging and uncaging with structured two-photon light. The present microscope is coupled to two independently tunable lasers and has two principal configurations: holographic imaging combined with galvo-steered uncaging and holographic uncaging combined with conventional scanning imaging. We applied this flexible system for simultaneous two-photon imaging and photostimulation of neuronal cells with complex light patterns, opening new perspectives for the study of brain function in situ and in vivo.
Although astrocytes are increasingly recognized as important modulators of neuronal excitability and information transfer at the synapse, whether these cells regulate neuronal network activity has only recently started to be investigated. In this article, we highlight the role of astrocytes in the modulation of circuit function with particular focus on sleep-related rhythmogenesis. We discuss recent data showing that these glial cells regulate slow oscillations, a specific thalamocortical activity that characterizes non-REM sleep, and sleep-associated behaviors. Based on these findings, we predict that our understanding of the genesis and tuning of thalamocortical rhythms will necessarily go through an integrated view of brain circuits in which non-neuronal cells can play important neuromodulatory roles.
The generation of 3D networks of primary neurons is a big challenge in neuroscience. Here, a novel method is presented for a 3D neuronal culture on superhydrophobic (SH) substrates. How nano-patterned SH devices stimulate neurons to build 3D networks is investigated. Scanning electron microscopy and confocal imaging show that soon after plating neurites adhere to the nanopatterned pillar sidewalls and they are subsequently pulled between pillars in a suspended position. These neurons display an enhanced survival rate compared to standard cultures and develop mature networks with physiological excitability. These findings underline the importance of using nanostructured SH surfaces for directing 3D neuronal growth, as well as for the design of biomaterials for neuronal regeneration.
In utero electroporation is a powerful tool to transfect and manipulate neural-precursor cells of the rodent parietal cortex and their progeny in vivo. Although this technique can potentially target numerous brain areas, reliability of transfection in some brain regions is low or physical access is limited. Here we present a new in utero electroporation configuration based on the use of three electrodes, the relative position and polarities of which can be adjusted. The technique allows easy access and exceedingly reliable monolateral or bilateral transfection at brain locations that could only be sporadically targeted before. By improvement in the efficiency of the electrical field distribution, demonstrated here by a mathematical simulation, the multi-electrode configuration also extends the developmental timeframe for reliable in utero electroporation, allowing for the first time specific transfection of Purkinje cells in the rat cerebellum.
The calcium ion is a fundamental second messenger that plays crucial roles in the pathophysiology of brain cells. In this chapter, we will focus on the measurement of calcium fluctuations as a reporter of cellular excitability of both neurons and glial cells in the intact central nervous system. We will first describe the methodological aspects of in vivo two-photon fluorescence calcium imaging and then review recent data highlighting the ways in which this technique is revolutionizing our understanding of brain circuits at the cellular level. Finally, we will discuss recent technical advancements that promise to open new horizons in the optical investigation of brain function in awake, behaving animals.
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