Name: optogenetic perturbation of neural activity with laser illumination in semi-intact drosophila larvae in motion
Uploaded: 2013-07-04T13:00:00
Duration: 7 min 7 s
Description: Watch this Scientific Journal Video about Optogenetic Perturbation of Neural Activity with Laser Illumination in Semi-intact Drosophila Larvae in Motion at JoVE.com

This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas "Mesoscopic Neurocircuitry" (No. 22115002) and "Comprehensive Brain Science Network" (No. 221S0003) of The Ministry of Education, Culture, Sports, Science, and Technology, Japan to A.N. and Grant-in-Aid for Young Scientists (B) (No. 21700344) of Japan Society for the Promotion of Science (JSPS) to H.K.

1. Larvae Preparation 
<ol>
 <li>Maintain fly lines of OK6-Gal4, UAS-ChR2 or OK6-Gal4, UAS-NpHR2 in plastic vials containing standard fly food. </li>
 <li>Spread yeast paste containing all-trans retinal (ATR) at appropriate concentrations (1 mM for ChR2, 10 mM for NpHR2 ("NpHR" for short) on an apple juice agar plate. </li>
 <li>Pick up 2nd or 3rd instar larvae from the vials and put them on the ATR-containing plate. </li>
 <li>Rear them at 25 &deg;C in the dark for an appropriate period of time (1 day for ChR2, 2 days for NpHR.) </li>
</ol>
2. Microscope Setup
<ol>
 <li>Attach a CCD camera (XCD-V60, Sony) to a conventional confocal microscope (in our case, FV1000, Olympus) with a C-mount attachment (magnification 0.35x). </li>
 <li>If there is a shutter along the light path from the objective lens to the CCD camera, which closes during laser scanning, remove it carefully. As this step depends on the microscope setup, contact the microscope manufacturer for technical support, if necessary. </li>
</ol>
3. Dissection
<ol>
 <li>Rinse the ATR-fed larvae with water to remove residual food from the body.</li>
 <li>Put the larva on a sylgard coated dish, dorsal side up (the dorsal side has two tracheal tubes running longitudinally, one either side of the dorsal midline). The thickness of the sylgard is about 5mm. </li>
 <li>Insert an insect pin (Austerlitz Insect pins, &Phi;0.10mm, stainless) into the tail between the tracheal tubes with forceps (#5 Inox, FST by Dumont, Switzerland). The pins, about 10mm long, should be bent or cut to be short enough (~2mm long) to avoid hitting and damaging the surface of the objective lens. Then put the second insect pin into the head of the larva near the mouth hook, black claw-like structure at the anterior end. </li>
 <li>Add Ca2+-free normal saline (NaCl 140 mM, KCl 2 mM, MgCl2 6 mM, HEPES-NaOH 5 mM, Sucrose 36 mM (pH7.1)) to keep the larva moist. </li>
 <li>Make a small incision near the tail with micro scissors (MB-50-7, Napox, Japan). </li>
 <li>From the incision, make a longitudinal cut along the dorsal midline toward the head. Be careful not to damage the ventral nerve cord (VNC) and axons. </li>
 <li>Make a small incision at the head laterally. </li>
 <li>Place 4 pins at each corner of the dissected bodywall. The body wall should be stretched enough to visualize a segmental pattern of the body wall, but not too much to hinder peristaltic motion. </li>
 <li>Remove the internal organs except for the brain and the VNC and rinse the sample with Ca2+-free normal saline. </li>
 <li>Adjust the orientation of the ventral nerve cord to be bilaterally symmetric by changing the position and orientation of insect pins on body wall. To fix the position of the ventral nerve cord, insert a pin through tissue between the brain and the mouth hook down to the sylgard.</li>
 <li>Replace the buffer with 2 mM Ca2+ Ringer solution (NaCl 130 mM, KCl 5mM, MgCl2 2mM, CaCl2 2 mM, HEPES-NaOH 5 mM, Sucrose 36 mM (pH7.3)). </li>
</ol>
4. Imaging with Laser Illumination
<ol>
 <li>Attach a 4x dry objective lens (UPlanSApo 4x, NA 0.16, Olympus, Japan) in the confocal microscope and set the larva preparation on the stage.</li>
 <li>Obtain a transmission image of the dissected preparation with a red laser (633nm) to locate the ventral nerve cord by confocal microscope. For this scanning, be sure not to use a 488nm or 559nm laser, which induces unwanted stimulation of ChR2 or NpHR, respectively.</li>
 <li>Define the region of interest (ROI) on the transmission image. Zoomed mode could be useful for detailed determination of the ROI. </li>
 <li>Change the filter set of the microscope to illuminate with 488nm or 559nm light. </li>
 <li>Illuminate the preparation with a halogen lamp to visualize the body wall. The preparation can be monitored with a CCD camera attached to the confocal microscope. </li>
 <li>Record the motion of the body wall and switch the laser beam on and off while monitoring the motion. </li>
</ol>

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No conflicts of interest declared.

Temporal and local perturbation of neural activity is an invaluable technique for analyzing the network dynamics of neural circuits. In this protocol, we present a method to manipulate neural activity by optogenetics using a laser. The laser's high directionality allows more localized optogenetic stimulation than wide field stimulation using mercury or Xenon lamp. Though laser illuminations have already been applied to optogenetics in previous studies, special setups such as glass fiber, micromanipulator, and laser source, are required in most previous studies. In this protocol, we use a conventional confocal microscopy for local illumination. Since the confocal microscope system is widely used, this method will open the opportunity to apply higher resolution optogenetics in many labs.
The protocol has two critical points: larval dissection and laser power. Firstly, if the brain, the ventral nerve cord or a motor nerve is damaged during dissection, the larvae will exhibit less or none spontaneous peristaltic motion. Precision is therefore essential, especially when making an incision on the dorsal side with spring scissors and removing internal organs with forceps. Details about the dissection have been reported previously21. Secondly, if sufficient stimulation is to be achieved, a laser power of about 0.1~1 mW/ mm2 for ChR2 or 1~10 mW/mm2 for NpHR is required. We evaluated the laser power as total light power below an objective lens divided by an estimated area of illumination. We measured the total light power using a power meter (mobiken, Sanwa M.I. Technos, Japan). We estimated the area of illumination as circular constant times the square of the wave length of the laser, which roughly gives illuminated area in diffraction limit. Effective illumination power on the sample can be adjusted not only by the power of the laser output, but also by changing the scanning speed and number of repetitions. We scanned laser with 20-100 &mu;sec/pixel for 63 times. In addition, the expression level of optogenetics protein and the amount of ATR are also critical for stimulation efficiency. Accordingly, if larvae showed no optogenetic response, the following points should be checked and corrected: 1) laser power (by optimizing microscope system), 2) expression level of optogenetics protein (by checking genotype and rearing temperature), and 3) amount of ATR (by adjusting concentration of ATR and feeding period). NpHR requires higher concentration of ATR than ChR2 does by unknown reasons 13. ChR2 works well in larvae reared in food containing lesser ATR (e.g. 0.1 mM) than described here (1 mM). As feeding larvae to 1 mM ATR is sufficient for making ChR2 photo-reactive, we recommend this concentration for the first trial in ChR2 experiments. The concentration of ATR can subsequently be titrated down from 1 mM. As to exposure duration for ATR, we recommend the duration described here for robust optogenetic control. We often failed to observe optogenetic response when feeding larvae to ATR for shorter duration.
In this protocol, laser illumination and image acquisition are operated by a separate computer. So, clear detection of spatiotemporal pattern of laser illumination by CCD camera is critical for data analysis. If laser spot or line is dim on CCD image, you should optimize the power of halogen lamp and gain value of the CCD camera to visualize the laser illumination while keeping body wall visible.
Spatiotemporal illumination shown in this protocol provides information on larval motor circuits; however, the method has some limitations. As to the "spatial" aspect, location of the illumination is recorded by low magnification CCD image used for videotaping body wall movements. Accordingly, illumination area can be assigned at segment level, but not at cellular level. As to the "temporal" aspect, time lag between switching on the laser scan by confocal microscope and illumination by laser depends on the confocal microscope system and scanning condition. Consequently, some testing by trial and error is required to adjust illumination timing. Since the timing of illumination can be determined in CCD image movies using adequate software like ImageJ, the critical factor in temporal resolution is frame rate of CCD camera imaging. We typically set the frame rate for 7.5-15 frames per second.
Advances in optogenetic tools provide us a chance to modify this protocol. We used 3rd instar larvae possessing OK6-Gal4 and UAS-ChR2[H134R] or UAS-NpHR2. Other optogenetic tools instead of ChR2[H134R] or NpHR2 such as ChR2[T159C/E123T], NpHR3 or Arch can boost the efficiency of the activity control22. In addition, using other gal4 lines or analyzing in other developmental stages can provide more information about the motor circuits.
In this protocol, motor activity was monitored by transmission images of the bodywall contraction with a CCD camera. The activity of motor neurons with calcium-sensitive fluorescence molecules (e.g. GCaMP) can also be directly monitored while manipulating neural activity with ChR2 or NpHR. In this case, blue light illumination in a broad area and a highly sensitive CCD camera (e.g. EMCCD camera) are used instead of a halogen lamp and the normal CCD camera previously described herein. To improve spatial resolution of stimulation while monitoring bodywall movement, using an additional objective lens may be a more advanced method: illumination of the ventral nerve cord by a high magnification objective lens (e.g. 40x) from above the sample and monitoring bodywall by low magnification lens (e.g. 4x) below the sample. This dual lens system may allow us to stimulate nervous system in higher spatial resolution. 
The protocol shown here can be applied to other neural circuits, provided optogenetic tools can be expressed in the nervous system and preparation in which sufficient light can be delivered into the neural tissue can be established.

Forward peristaltic locomotion in Drosophila larvae occurs by the propagation of muscular contraction from posterior to anterior segments. This motion is realized by sequential activation of motor neurons within the ventral nerve cord along the longitudinal body axis. To examine the circuitry behind this patterned propagating activity, local perturbation of neural activity could be an informative approach. Although pharmacological assay can control specific substance, such as neurotransmitters, in neural tissue, the effect of pharmacological drugs can be similar among the almost all segments because the larval ventral nerve cord is repetitive structure composed of similar neuromeres along the longitudinal axis. Alternatively, one could express optogenetic or temperature-sensitive probe molecules in a small number of segments. However, such specific gal4 lines are very difficult to generate. Here we present a method for manipulating neurons in a few segments using laser illumination. Taking advantage of the spatially confined illumination in confocal microscopy, one can perturb the activity of neurons in a local area. Combined with the expression pattern of the probe by subsets of neuron-specific gal4 lines, this laser illumination method greatly improves spatial resolution for perturbation. And because this method requires only a conventional confocal microscope, the purchase of additional laboratory equipment is typically not necessary. 
Using this technique, we examined the role of motor neuronal activity during the propagation of motor output13. By blocking the activities of motor neurons within a few segments during peristaltic motion, we were able to test whether the activity of motor neurons themselves is required for the propagation of the motor output signal. Local and transient blockade of motor neurons arrested the propagation of peristaltic motion. After the blockade was removed, propagation appeared at the segment where the wave had been arrested. This phenomenon suggests that without motor neuronal activation, the propagative wave cannot progress along the ventral nerve cord any further, and thus the activation of motor neurons is necessary for peristaltic motion. We have previously reported detailed data and discussions about this phenomenon in Inada et al. (2011) 13. Here, we describe the method to perturb neural activity interactively while monitoring the motion of the dissected larvae. Using various gal4 lines and series of spatiotemporal patterns for activity manipulation, one can investigate circuitry logic through perturbation response properties of the motor circuit.

<table border="1">
<tbody>
 <tr>
 <td>Name</td>
 <td>Company</td>
 <td >Catalog Number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>BX61WI Microscope</td>
 <td>Olympus Corporation</td>
 <td>BX61WI</td>
 <td></td>
 </tr>
 <tr>
 <td>Fluoview FV1000 confocal system</td>
 <td>Olympus Corporation</td>
 <td>FV1000</td>
 <td></td>
 </tr>
 <tr>
 <td>CCD camera</td>
 <td>Sony</td>
 <td>XCD-V60</td>
 <td></td>
 </tr>
 <tr>
 <td>all-trans retinal</td>
 <td>Sigma</td>
 <td>R2500-500MG</td>
 <td>200 mM stock in 95% EtOH</td>
 </tr>
 <tr>
 <td>Silpot184</td>
 <td>Dow Corning Toray</td>
 <td>3255981</td>
 <td></td>
 </tr>
 </tbody>
</table>

optogenetic perturbation of neural activity with laser illumination in semi-intact drosophila larvae in motion

Drosophila larval locomotion is a splendid model system in developmental and physiological neuroscience, by virtue of the genetic accessibility of the underlying neuronal components in the circuits1-6. Application of optogenetics7,8 in the larval neural circuit allows us to manipulate neuronal activity in spatially and temporally patterned ways9-13. Typically, specimens are broadly illuminated with a mercury lamp or LED, so specificity of the target neurons is controlled by binary gene expression systems such as the Gal4-UAS system14,15. In this work, to improve the spatial resolution to "sub-genetic resolution", we locally illuminated a subset of neurons in the ventral nerve cord using lasers implemented in a conventional confocal microscope. While monitoring the motion of the body wall of the semi-intact larvae, we interactively activated or inhibited neural activity with channelrhodopsin16,17 or halorhodopsin18-20, respectively. By spatially and temporally restricted illumination of the neural tissue, we can manipulate the activity of specific neurons in the circuit at a specific phase of behavior. This method is useful for studying the relationship between the activities of a local neural assembly in the ventral nerve cord and the spatiotemporal pattern of motor output.

Local and transient activation of motor neurons with ChR2.
We expressed ChR2 in motor neurons. The axons of motor neurons emerging from each VNC segment project to a corresponding segment of the body wall. When we stimulated one or two segments of the VNC with a blue laser, muscles in the corresponding segments exhibited contractions (Figure 1). 
Local and transient inhibition of motor neurons with NpHR.
We expressed NpHR in motor neurons. When we stimulated a few (1~2) segments of the VNC with a yellow laser during the peristaltic muscle contractions, the propagating wave was arrested at the corresponding segments of the body wall (Figure 2). Then the wave restarted from the arrested segments when we switched off the laser illumination13. 
<img src="/files/ftp_upload/50513/50513fig1.jpg" alt="Figure 1" fo:content-width="5.5in" fo:src="/files/ftp_upload/50513/50513fig1highres.jpg"/> 
Figure 1. Local activation of motor neurons with ChR2. A. Scheme of the local activation of filleted preparation. By blue-light illumination of a few segments of the ventral nerve cord expressing ChR2 in motor neurons, muscles in the corresponding segments are contracted (arrow head). B. Transmission images of a dissected larva. Local illumination of the ventral nerve cord (arrow heads) induces segmental contraction of body wall muscles (arrows).
<img src="/files/ftp_upload/50513/50513fig2.jpg" alt="Figure 1" fo:content-width="5.5in" fo:src="/files/ftp_upload/50513/50513fig2highres.jpg"/> 
Figure 2. Local inhibition of motor neurons with NpHR. A. Scheme of the local inhibition of filleted preparation. By yellow-light illumination of a few segments of the ventral nerve cord expressing NpHR in motor neurons, muscles contraction during spontaneously-occurred peristaltic motion (arrow head in the top) are relaxed (arrow head in the bottom.) B. Transmission images of a dissected larva. Local illumination of the ventral nerve cord (arrow heads) relaxed the spontaneously-contracted body wall segments (arrows).

Watch this Scientific Journal Video about Optogenetic Perturbation of Neural Activity with Laser Illumination in Semi-intact Drosophila Larvae in Motion at JoVE.com

Optogenetic Perturbation of Neural Activity with Laser Illumination in Semi-intact Drosophila Larvae in Motion

Here we describe a protocol for optogenetic manipulation of motoneuronal activity while monitoring changes in motor output (muscle contraction) in semi-intact Drosophila larvae using lasers within a conventional confocal microscope. This technique enables researchers to achieve local perturbation of neural activity in a few neuromeres to elucidate the dynamics of motor circuits.

Optogenetic Perturbation of Neural Activity with Laser Illumination in Semi-intact Drosophila Larvae in Motion

Drosophila larval locomotion is a splendid model system in developmental and physiological neuroscience, by virtue of the genetic accessibility of the ...

optogenetic-perturbation-neural-activity-with-laser-illumination-semi

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Neuroscience

In vivo Laser Axotomy in C. elegans

Neurons communicate with other cells via axons and dendrites, slender membrane extensions that contain pre- or post-synaptic specializations. If a neuron is damaged by injury or disease, it may regenerate. Cell-intrinsic and extrinsic factors influence the ability of a neuron to regenerate and restore function. Recently, the nematode C. elegans has emerged as an excellent model organism to identify genes and signaling pathways that influence the regeneration of neurons1-6. The main way to initiate neuronal regeneration in C. elegans is laser-mediated cutting, or axotomy. During axotomy, a fluorescently-labeled neuronal process is severed using high-energy pulses. Initially, neuronal regeneration in C. elegans was examined using an amplified femtosecond laser5. However, subsequent regeneration studies have shown that a conventional pulsed laser can be used to accurately sever neurons in vivo and elicit a similar regenerative response1,3,7.
We present a protocol for performing in vivo laser axotomy in the worm using a MicroPoint pulsed laser, a turnkey system that is readily available and that has been widely used for targeted cell ablation. We describe aligning the laser, mounting the worms, cutting specific neurons, and assessing subsequent regeneration. The system provides the ability to cut large numbers of neurons in multiple worms during one experiment. Thus, laser axotomy as described herein is an efficient system for initiating and analyzing the process of regeneration.

In vivo Laser Axotomy in C. elegans

A protocol to cut neurons in C. elegans with a MicroPoint pulsed laser is presented. We describe setting up the system, immobilizing worms, and severing labeled neurons. Advantages include a relatively low-cost system and the ability to sever neuronal processes or ablate cells in vivo.

Neurons communicate with other cells via axons and dendrites, slender membrane extensions that contain pre- or post-synaptic specializations. If a neuron ...

Local and Global Methods of Assessing Thermal Nociception in Drosophila Larvae

In this article, we demonstrate assays to study thermal nociception in Drosophila larvae. One assay involves spatially-restricted (local) stimulation of thermal nociceptors1,2 while the second involves a wholesale (global) activation of most or all such neurons3. Together, these techniques allow visualization and quantification of the behavioral functions of Drosophila nociceptive sensory neurons.
The Drosophila larva is an established model system to study thermal nociception, a sensory response to potentially harmful temperatures that is evolutionarily conserved across species1,2. The advantages of Drosophila for such studies are the relative simplicity of its nervous system and the sophistication of the genetic techniques that can be used to dissect the molecular basis of the underlying biology4-6 In Drosophila, as in all metazoans, the response to noxious thermal stimuli generally involves a "nocifensive" aversive withdrawal to the presented stimulus7. Such stimuli are detected through free nerve endings or nociceptors and the amplitude of the organismal response depends on the number of nociceptors receiving the noxious stimulus8. In Drosophila, it is the class IV dendritic arborization sensory neurons that detect noxious thermal and mechanical stimuli9 in addition to their recently discovered role as photoreceptors10. These neurons, which have been very well studied at the developmental level, arborize over the barrier epidermal sheet and make contacts with nearly all epidermal cells11,12. The single axon of each class IV neuron projects into the ventral nerve cord of the central nervous system11 where they may connect to second-order neurons that project to the brain.
Under baseline conditions, nociceptive sensory neurons will not fire until a relatively high threshold is reached. The assays described here allow the investigator to quantify baseline behavioral responses or, presumably, the sensitization that ensues following tissue damage. Each assay provokes distinct but related locomotory behavioral responses to noxious thermal stimuli and permits the researcher to visualize and quantify various aspects of thermal nociception in Drosophila larvae. The assays can be applied to larvae of desired genotypes or to larvae raised under different environmental conditions that might impact nociception. Since thermal nociception is conserved across species, the findings gleaned from genetic dissection in Drosophila will likely inform our understanding of thermal nociception in other species, including vertebrates.

Local and Global Methods of Assessing Thermal Nociception in Drosophila Larvae

In this article, we demonstrate assays to study thermal nociception in Drosophila larvae. One assay involves spatially-restricted (local) stimulation of thermal nociceptors1,2 while the second involves a wholesale (global) activation of most or all such neurons3. Together, these techniques allow visualization and quantification of the behavioral functions of Drosophila nociceptive sensory neurons.

In this article, we demonstrate assays to study thermal nociception in Drosophila larvae. One assay involves spatially-restricted (local) stimulation of ...

Visualization of Proprioceptors in Drosophila Larvae and Pupae

Proprioception is the ability to sense the motion, or position, of body parts by responding to stimuli arising within the body. In fruitflies and other insects proprioception is provided by specialized sensory organs termed chordotonal organs (ChOs) 2. Like many other organs in Drosophila, ChOs develop twice during the life cycle of the fly. First, the larval ChOs develop during embryogenesis. Then, the adult ChOs start to develop in the larval imaginal discs and continue to differentiate during metamorphosis.
The development of larval ChOs during embryogenesis has been studied extensively 10,11,13,15,16. The centerpiece of each ChO is a sensory unit composed of a neuron and a scolopale cell. The sensory unit is stretched between two types of accessory cells that attach to the cuticle via specialized epidermal attachment cells 1,9,14. When a fly larva moves, the relative displacement of the epidermal attachment cells leads to stretching of the sensory unit and consequent opening of specific transient receptor potential vanilloid (TRPV) channels at the outer segment of the dendrite 8,12. The elicited signal is then transferred to the locomotor central pattern generator circuit in the central nervous system.
Multiple ChOs have been described in the adult fly 7. These are located near the joints of the adult fly appendages (legs, wings and halters) and in the thorax and abdomen. In addition, several hundreds of ChOs collectively form the Johnston's organ in the adult antenna that transduce acoustic to mechanical energy 3,5,17,4.
In contrast to the extensive knowledge about the development of ChOs in embryonic stages, very little is known about the morphology of these organs during larval stages. Moreover, with the exception of femoral ChOs 18 and Johnston's organ, our knowledge about the development and structure of ChOs in the adult fly is very fragmentary.
Here we describe a method for staining and visualizing ChOs in third instar larvae and pupae. This method can be applied together with genetic tools to better characterize the morphology and understand the development of the various ChOs in the fly.

Visualization of Proprioceptors in Drosophila Larvae and Pupae

A method to immunostain and visualize chordotonal organs in larvae and pupae of Drosophila melanogaster is described.

Proprioception is the ability to sense the motion, or position, of body parts by responding to stimuli arising within the body. In fruitflies and other ...

Appetitive Associative Olfactory Learning in Drosophila Larvae

In the following we describe the methodological details of appetitive associative olfactory learning in Drosophila larvae. The setup, in combination with genetic interference, provides a handle to analyze the neuronal and molecular fundamentals of specifically associative learning in a simple larval brain.
Organisms can use past experience to adjust present behavior. Such acquisition of behavioral potential can be defined as learning, and the physical bases of these potentials as memory traces1-4. Neuroscientists try to understand how these processes are organized in terms of molecular and neuronal changes in the brain by using a variety of methods in model organisms ranging from insects to vertebrates5,6. For such endeavors it is helpful to use model systems that are simple and experimentally accessible. The Drosophila larva has turned out to satisfy these demands based on the availability of robust behavioral assays, the existence of a variety of transgenic techniques and the elementary organization of the nervous system comprising only about 10,000 neurons (albeit with some concessions: cognitive limitations, few behavioral options, and richness of experience questionable)7-10.
Drosophila larvae can form associations between odors and appetitive gustatory reinforcement like sugar11-14. In a standard assay, established in the lab of B. Gerber, animals receive a two-odor reciprocal training: A first group of larvae is exposed to an odor A together with a gustatory reinforcer (sugar reward) and is subsequently exposed to an odor B without reinforcement 9. Meanwhile a second group of larvae receives reciprocal training while experiencing odor A without reinforcement and subsequently being exposed to odor B with reinforcement (sugar reward). In the following both groups are tested for their preference between the two odors. Relatively higher preferences for the rewarded odor reflect associative learning - presented as a performance index (PI). The conclusion regarding the associative nature of the performance index is compelling, because apart from the contingency between odors and tastants, other parameters, such as odor and reward exposure, passage of time and handling do not differ between the two groups9.

Appetitive Associative Olfactory Learning in Drosophila Larvae

Drosophila larvae are able to associate odor stimuli with gustatory reward. Here we describe a simple behavioral paradigm that allows the analysis of appetitive associative olfactory learning.

In the following we describe the methodological  details of appetitive associative olfactory learning in Drosophila larvae. The setup, in combination with ...

Multi-unit Recording Methods to Characterize Neural Activity in the Locust (Schistocerca Americana) Olfactory Circuits

Detection and interpretation of olfactory cues are critical for the survival of many organisms. Remarkably, species across phyla have strikingly similar olfactory systems suggesting that the biological approach to chemical sensing has been optimized over evolutionary time1. In the insect olfactory system, odorants are transduced by olfactory receptor neurons (ORN) in the antenna, which convert chemical stimuli into trains of action potentials. Sensory input from the ORNs is then relayed to the antennal lobe (AL; a structure analogous to the vertebrate olfactory bulb). In the AL, neural representations for odors take the form of spatiotemporal firing patterns distributed across ensembles of principal neurons (PNs; also referred to as projection neurons)2,3. The AL output is subsequently processed by Kenyon cells (KCs) in the downstream mushroom body (MB), a structure associated with olfactory memory and learning4,5. Here, we present electrophysiological recording techniques to monitor odor-evoked neural responses in these olfactory circuits.
First, we present a single sensillum recording method to study odor-evoked responses at the level of populations of ORNs6,7. We discuss the use of saline filled sharpened glass pipettes as electrodes to extracellularly monitor ORN responses. Next, we present a method to extracellularly monitor PN responses using a commercial 16-channel electrode3. A similar approach using a custom-made 8-channel twisted wire tetrode is demonstrated for Kenyon cell recordings8. We provide details of our experimental setup and present representative recording traces for each of these techniques.

Multi-unit Recording Methods to Characterize Neural Activity in the Locust Schistocerca Americana Olfactory Circuits

We demonstrate variations of the extracellular multi-unit recording technique to characterize odor-evoked responses in the first three stages of the invertebrate olfactory pathway. These techniques can easily be adapted to examine ensemble activity in other neural systems as well.

Detection and interpretation of olfactory cues are critical for the survival  of many organisms. Remarkably, species across phyla have strikingly similar ...

Optogenetic Stimulation of Escape Behavior in Drosophila melanogaster

A growing number of genetically encoded tools are becoming available that allow non-invasive manipulation of the neural activity of specific neurons in Drosophila melanogaster1. Chief among these are optogenetic tools, which enable the activation or silencing of specific neurons in the intact and freely moving animal using bright light. Channelrhodopsin (ChR2) is a light-activated cation channel that, when activated by blue light, causes depolarization of neurons that express it. ChR2 has been effective for identifying neurons critical for specific behaviors, such as CO2 avoidance, proboscis extension and giant-fiber mediated startle response2-4. However, as the intense light sources used to stimulate ChR2 also stimulate photoreceptors, these optogenetic techniques have not previously been used in the visual system. Here, we combine an optogenetic approach with a mutation that impairs phototransduction to demonstrate that activation of a cluster of loom-sensitive neurons in the fly's optic lobe, Foma-1 neurons, can drive an escape behavior used to avoid collision. We used a null allele of a critical component of the phototransduction cascade, phospholipase C-&beta;, encoded by the norpA gene, to render the flies blind and also use the Gal4-UAS transcriptional activator system to drive expression of ChR2 in the Foma-1 neurons. Individual flies are placed on a small platform surrounded by blue LEDs. When the LEDs are illuminated, the flies quickly take-off into flight, in a manner similar to visually driven loom-escape behavior. We believe that this technique can be easily adapted to examine other behaviors in freely moving flies.

Optogenetic Stimulation of Escape Behavior in Drosophila melanogaster

Genetically encoded optogenetic tools enable noninvasive manipulation of specific neurons in the Drosophila brain. Such tools can identify neurons whose activation is sufficient to elicit or suppress particular behaviors. Here we present a method for activating Channelrhodopsin2 that is expressed in targeted neurons in freely walking flies.

A  growing number of genetically encoded tools are becoming available that allow non-invasive  manipulation of the neural activity of specific neurons in ...

Neural Activity Propagation in an Unfolded Hippocampal Preparation with a Penetrating Micro-electrode Array

This protocol describes a method for preparing a new in vitro flat hippocampus preparation combined with a micro-machined array to map neural activity in the hippocampus. The transverse hippocampal slice preparation is the most common tissue preparation to study hippocampus electrophysiology. A longitudinal hippocampal slice was also developed in order to investigate longitudinal connections in the hippocampus. The intact mouse hippocampus can also be maintained in vitro because its thickness allows adequate oxygen diffusion. However, these three preparations do not provide direct access to neural propagation since some of the tissue is either missing or folded. The unfolded intact hippocampus provides both transverse and longitudinal connections in a flat configuration for direct access to the tissue to analyze the full extent of signal propagation in the hippocampus in vitro. In order to effectively monitor the neural activity from the cell layer, a custom made penetrating micro-electrode array (PMEA) was fabricated and applied to the unfolded hippocampus. The PMEA with 64 electrodes of 200 &#181;m in height could record neural activity deep inside the mouse hippocampus. The unique combination of an unfolded hippocampal preparation and the PMEA provides a new in-vitro tool to study the speed and direction of propagation of neural activity in the two-dimensional CA1-CA3 regions of the hippocampus with a high signal to noise ratio.

We have developed an in vitro unfolded hippocampus which preserves CA1-CA3 array of neurons. Combined with the penetrating micro-electrode array, neural activity can be monitored in both the longitudinal and transverse orientations. This method provides advantages over hippocampal slice preparations as the propagation in the entire hippocampus can be recorded simultaneously.

This protocol describes a method for preparing a new in vitro flat hippocampus preparation combined with a micro-machined array to map neural activity in ...

Extracellular Recording of Neuronal Activity Combined with Microiontophoretic Application of Neuroactive Substances in Awake Mice

Differences in the activity of neurotransmitters and neuromodulators, and consequently different neural responses, can be found between anesthetized and awake animals. Therefore, methods allowing the manipulation of synaptic systems in awake animals are required in order to determine the contribution of synaptic inputs to neuronal processing unaffected by anesthetics. Here, we present methodology for the construction of electrodes to simultaneously record extracellular neural activity and release multiple neuroactive substances at the vicinity of the recording sites in awake mice. By combining these procedures, we performed microiontophoretic injections of gabazine to selectively block GABAA receptors in neurons of the inferior colliculus of head-restrained mice. Gabazine successfully modified neural response properties such as the frequency response area and stimulus-specific adaptation. Thus, we demonstrate that our methods are suitable for recording single-unit activity and for dissecting the role of specific neurotransmitter receptors in auditory processing.
The main limitation of the described procedure is the relatively short recording time (~3 hr), which is determined by the level of habituation of the animal to the recording sessions. On the other hand, multiple recording sessions can be performed in the same animal. The advantage of this technique over other experimental procedures used to manipulate the level of neurotransmission or neuromodulation (such as systemic injections or the use of optogenetic models), is that the drug effect is confined to the local synaptic inputs to the target neuron. In addition, the custom-manufacture of electrodes allows adjustment of specific parameters according to the neural structure and type of neuron of interest (such as the tip resistance for improving the signal-to-noise ratio of the recordings).

We present methods for the construction of electrodes to simultaneously record extracellular neural activity and release multiple neuroactive substances at the vicinity of the recording sites in awake mice. This technique allows the detailed analysis of putative local synaptic inputs to the neuron of interest.

Differences in the activity of neurotransmitters and neuromodulators, and consequently different neural responses, can be found between anesthetized and ...

Novel Assay for Cold Nociception in Drosophila Larvae

How organisms sense and respond to noxious temperatures is still poorly understood. Further, the mechanisms underlying sensitization of the sensory machinery, such as in patients experiencing peripheral neuropathy or injury-induced sensitization, are not well characterized. The genetically tractable Drosophila model has been used to study the cells and genes required for noxious heat detection, which has yielded multiple conserved genes of interest. Little is known however about the cells and receptors important for noxious cold sensing. Although, Drosophila does not survive prolonged exposure to cold temperatures (&#8804;10 &#186;C), and will avoid cool, preferring warmer temperatures in behavioral preference assays, how they sense and possibly avoid noxious cold stimuli has only recently been investigated.
Here we describe and characterize the first noxious cold (&#8804;10 &#186;C) behavioral assay in Drosophila. Using this tool and assay, we show an investigator how to qualitatively and quantitatively assess cold nociceptive behaviors. This can be done under normal/healthy culture conditions, or presumably in the context of disease, injury or sensitization. Further, this assay can be applied to larvae selected for desired genotypes, which might impact thermosensation, pain, or nociceptive sensitization. Given that pain is a highly conserved process, using this assay to further study&#160;thermal nociception will likely glean important understanding of pain processes in other species, including vertebrates.

Novel Assay for Cold Nociception in Drosophila Larvae

Here we demonstrate a novel assay to study cold nociception in Drosophila larvae. This assay utilizes a custom-built Peltier probe capable of applying a focal noxious cold stimulus and results in quantifiable cold-specific behaviors. This technique will allow further cellular and molecular dissection of cold nociception.

How organisms sense and respond to noxious temperatures is still poorly understood. Further, the mechanisms underlying sensitization of the sensory ...

Tracking Drosophila Larval Behavior in Response to Optogenetic Stimulation of Olfactory Neurons

The ability of insects to navigate toward odor sources is based on the activities of their first-order olfactory receptor neurons (ORNs). While a considerable amount of information has been generated regarding ORN responses to odorants, the role of specific ORNs in driving behavioral responses remains poorly understood. Complications in behavior analyses arise due to different volatilities of odorants that activate individual ORNs, multiple ORNs activated by single odorants, and the difficulty in replicating naturally observed temporal variations in olfactory stimuli using conventional odor-delivery methods in the laboratory. Here, we describe a protocol that analyzes Drosophila larval behavior in response to simultaneous optogenetic stimulation of its ORNs. The optogenetic technology used here allows for specificity of ORN activation and precise control of temporal patterns of ORN activation. Corresponding larval movement is tracked, digitally recorded, and analyzed using custom written software. By replacing odor stimuli with light stimuli, this method allows for a more precise control of individual ORN activation in order to study its impact on larval behavior. Our method could be further extended to study the impact of second-order projection neurons (PNs) as well as local neurons (LNs) on larval behavior. This method will thus enable a comprehensive dissection of olfactory circuit function and complement studies on how olfactory neuron activities translate in to behavior responses.

Tracking Drosophila Larval Behavior in Response to Optogenetic Stimulation of Olfactory Neurons

This protocol analyzes navigational behavior of Drosophila larva in response to simultaneous optogenetic stimulation of its olfactory neurons. Light of 630 nm wavelength is used to activate individual olfactory neurons expressing a red-shifted channel rhodopsin. Larval movement is simultaneously tracked, digitally recorded, and analyzed using custom-written software.