This work was supported by Grants-in-Aid for Scientific Research (15K07147, 17K07419) from JSPS (to Hiroshi Ishimoto, Hiroko Sano), an Inamori Foundation Research Grant (to HI), and the Program of the Joint Usage/Research Center for Developmental Medicine, at the Institute of Molecular Embryology and Genetics, Kumamoto University (to HS). We thank Dr. Azusa Kamikouchi and Mr. Daichi Yamada for their help in using the imaging system.

1. Preparation of Larval Brain Explants
<ol>
	<li>Make an imaging chamber. 
	NOTE: Stable recording requires that the brain explants be tethered. Here, a low-cost, hand-made imaging chamber is used in the Ca2+ imaging system.
	<ol>
		<li>Using forceps, scratch the bottom center of a plastic culture dish (35 mm x 10 mm) to create a dent for the ventral ganglion of the larval brain making mounting easier (step 1.3, Figure 1).</li>
		<li>With a toothpick, place a small drop of superglue on either side of the dent and attach a tungsten rod (0.125 mm diameter, 6 to 7 mm length) to the glue.</li>
		<li>Using a small piece of putty-like reusable adhesive, make a circular wall (10 mm in diameter and 3 mm high) surrounding the dent (Figure 1). 
		NOTE: The space inside the wall will be later filled with phosphate buffered saline (PBS; 0.14 M NaCl, 0.0027 M KCl, 0.01 M PO43-, pH 7.4 &#177; 0.05 at 25 &#176;C).</li>
	</ol>
	</li>
	<li>Animal preparation 
	NOTE: For calcium imaging, a calcium indicator, GCaMP6s, is expressed in the cell type of interest, which is commonly achieved using combinations of tissue-specific GAL4 and UAS-GCaMP6s. To ensure that the signal is mediated through the candidate receptor, GCaMP6s is also expressed in a mutant defective in the receptor.
	<ol>
		<li>When experimental comparisons for different genotypes such as wild type and mutants are required, age match test larvae to avoid GCaMP6 expression level variations and physiological differences due to developmental stages.</li>
		<li>Keep parental flies (30 flies for each sex) in a culture vial containing standard fly food for 8 h to allow egg-laying on the surface of the food. Culture larvae in mass under a 12-h light-dark cycle at 25 &#176;C and 60-70 % relative humidity.</li>
	</ol>
	</li>
	<li>Dissection of the larval brain
	<ol>
		<li>At 90-120 h after egg laying (AEL), collect larvae and wash them at least 3 times with distilled water to remove adherent food debris.</li>
		<li>Place a larva on a square 1.5-inch watch glass filled with ice cold PBS. 
		NOTE: The next two steps are carried out in this watch glass under the dissecting microscope.</li>
		<li>Use forceps to gently grab the middle part of the larva. Use another forceps to grab the mouth hooks and pull the mouth hooks gently to separate the anterior part of the larva containing the brain from the rest of the body. 
		NOTE: Alternatively, surgical scissors can be used.</li>
		<li>Hold the anterior tip by forceps and turn the larva inside out. Remove extraneous tissue attached to the brain, such as imaginal disks, fat bodies, and ring gland. Gently separate the brain from the mouthparts.</li>
		<li>Apply 200 &#181;L of PBS to the inside of the ring made of the putty-like reusable adhesive in the imaging chamber. Gently suck the dissected brain with PBS into a Pasteur pipette, and then transfer it into the imaging chamber.</li>
	</ol>
	</li>
	<li>Setting the brain into the imaging chamber
	<ol>
		<li>Use forceps to grab muscle fibers extending from the ventral ganglia. Insert the brain gently into the dent underneath the tungsten wire.</li>
		<li>Using another forceps, pull the tungsten wire slightly up to place the brain at the correct position for imaging (Figure 1).</li>
	</ol>
	</li>
</ol>
2. Acquisition of Ca2+ Fluorescence Images
NOTE: Brain explants immersed in PBS are imaged using a fluorescence microscope equipped with a 20X water-immersion objective lens (N.A. = 0.5). PBS does not activate cells in the brain. If Z-axis images are needed, the objective lens is mounted with a piezoelectric-activated lens mover. A spinning disc confocal head is used to enhance the time resolution. For low-light imaging, an electron multiplying charge-coupled device camera is mounted on the microscope. The imaging of GCaMP6s fluorescence (excitation/emission: 488/509 nm) requires a 488-nm laser for excitation with a dichroic beam splitter and an emission filter (e.g., 528 &#177; 38 nm band bandpass filter).
<ol>
	<li>Place the imaging chamber containing the brain explant under the microscope.</li>
	<li>Lower the objective lens until it touches the PBS. Under bright-field illumination, position the brain and bring it into focus. Switch to fluorescent light and adjust the focus on the GCaMP6s-labeled cells. 
	NOTE: The basal green fluorescence of the GCaMP6s should be visible.</li>
	<li>Start acquisition at 250 ms/frame at a resolution of 512 &#215; 512 pixels in water-cooled mode using the appropriate acquisition software (e.g., &#181;Manager). Adjust the exposure time to obtain fluorescence values within the dynamic range of the CCD camera, but not lower than 1000 (arbitrary units), with 16-bit images (dynamic range 0-65,535). 
	NOTE: A low exposure time should be used to minimize photo-bleaching of the GCaMP6s. Exposure time should be optimized in each experiment as it depends on the expression levels of GCaMP6 and the sensitivity of the detection system. It was 100 ms in our experiment using dilp2&#62;GCaMP6s. When z-sections are required for a given imaging depth, several z-section images can be acquired depending on the exposure time and frame intervals. If the camera has enough spatial resolution, the binning size (number of registers on the chip that will be binned into one digital pixel) should be increased to reduce the exposure time.</li>
	<li>Once imaging parameters are determined, take images for 1 min before peptide administration to detect baseline signal intensities (F0).</li>
	<li>Apply the test peptide; for this, dissolve 100 &#181;L of peptide solution in PBS and pipette it directly into the larval bath to yield an optimal concentration. 
	NOTE: The peptide solution should be injected slowly (e.g., flow rate 20 &#181;L/s) into the bath solution using a pipette. Unrelated synthetic peptide dissolved in PBS is used as a negative control.</li>
	<li>Record the GCaMP6s emission for a few minutes.</li>
</ol>
3. Data Analysis
Note: Imaging data are analyzed offline with the ImageJ. During the time-lapse imaging, pipetting causes the larval brain to move. Therefore, positional aberrations of the serial images should be corrected before analysis. The correction can be performed using an ImageJ plug-in, TurboReg as follows.
<ol>
	<li>Open the analysis software and use the first frame (before peptide application) as a reference image.</li>
	<li>Select Plugins | Registration | TurboReg.</li>
	<li>Choose the serial image file as the Source and the reference image as the Target.</li>
	<li>Check Rigid Body (parallel displacement and rotation of images) and Accurate (&#34;Fast&#34; is coarse but fast analysis instead) for the processing method and quality, respectively.</li>
	<li>Click Batch to start the image processing.</li>
	<li>Select multiple regions of interest (ROIs) by using Analyze | Tools | ROI Manager in ImageJ.</li>
	<li>Measure the pixel intensity by clicking More | Multi Measure | OK in the ROI Manager window.</li>
	<li>Calculate &#916;F/F0 = (F - F0)/F0, where F is the ROI intensity at the time of the stimulus, and F0 is the intensity at a time window immediately before a particular experimental event (e.g., 30 frames preceding the stimulus onset).</li>
	<li>Repeat the experiment using multiple brain samples, conducting statistical processing to obtain the mean and standard error of the mean (SEM) at each time point.</li>
</ol>

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The authors have nothing to disclose.

The Ca2+ imaging method described here is a useful system for testing the function of hormones in the brain. In vivo, the brain receives hormones from various endocrine organs. In addition, the brain constantly perceives ascending sensory information, which causes spontaneous neuronal activation. This ex vivo system eliminates such noise and yields a better signal/noise ratio than in vivo imaging. In this system, molecules can be tested singly or in combination. In addition to peptid...

Organ-to-organ communication is an evolutionarily conserved strategy for maintaining homeostasis to cope with environmental changes. In humans, a variety of hormones arereleased from the endocrine glands into the circulation. Many of these hormones target the hypothalamus of the brain, which regulates metabolic processes and fundamental behaviors such as feeding1,2. Many hormones have been discovered using mammalian models. However, the mechanisms of their action, especially the interorgan networks in which they participate, remain largely unclear.
Drosophila melanogaster has emerged as a useful model for studying organ-to-organ communication. In insects, many physiological processes are controlled by hormones. Early studies focusing on growth and metamorphosis used large insects. In these studies, the removal or transplantation of specific organs predicted the existence of inter-organ signaling molecules; later, Juvenile hormone (JH), Prothoracicotropic hormone (PTTH), and Ecdysone were biochemically purified3,4,5. A large family of intercellular signaling peptides is thought to be involved in various physiological events during the insect life cycle6,7. Most of these peptides act on G-protein-coupled receptors (GPCRs), although the specific GPCRs were initially difficult to identify using conventional approaches. The publication of the Drosophila whole genome sequence8&#160;was the breakthrough that enabled the identification of Drosophila bioactive peptides based on their homology to those found in other insects. In addition, the receptors for several peptides were identified from GPCRs predicted in the genome using cell-culture-based GPCR-ligand binding assays. Next, expression analyses predicted the organ-to-organ pathways elicited by these peptides and receptors. Notably, many of the putative peptide receptors are expressed in the brain, suggesting that the brain is a major target of peptide hormones9. Moreover, the advanced genetic tools in Drosophila have contributed to the identification of physiological roles of peptide-GPCR combinations. For instance, the GAL4 and LexA-based binary transcriptional systems enable gene knockdown or overexpression in a spatially and temporally controlled manner. GAL4, a transcription factor identified in yeast, binds to a specific cis-regulatory sequence called Upstream Activation Sequence (UAS). In the GAL4/UAS system, the driver line provides tissue-specific or stage-specific GAL4 expression and the responder line carries UAS upstream of the gene of interest or the construct to drive shRNA expression. LexA/LexAop system is based on a similar mechanism. Phenotypic analyses of tissue-specific peptide-knockdown and tissue-specific GPCR-knockdown animals can reveal information about the peptide-GPCR signaling&#39;s mode and site of action. However, the conclusions that can be reached solely by genetic data are limited. On the other hand, once the putative target of a particular peptide hormone is narrowed down to a tissue or cell type, ex vivo calcium imaging in organ explants can be used to elucidate the organ-to-organ communication mediated by peptide-GPCR signaling. Upon the activation of a Gq-coupled GPCR, the intracellular Ca2+ concentration is increased due to the release of Ca2+ from the ER10. In the brain, neural activation also elevates the intracellular Ca2+ levels. Such Ca2+ increases can be detected by a calcium sensor, GCaMP, which undergoes conformational changes in the presence of Ca2+ resulting in fluorescence emission11.
In this article, a calcium imaging method using Drosophila brain explants is described. To test the ability of a peptide to activate specific neurons, a test peptide is applied to the GCaMP-expressing brain explant, and fluorescence changes are monitored by confocal microscopy. The involvement of the GPCR is then confirmed by performing the same assay using a mutant brain lacking the GPCR. This combination of imaging and genetics provides precise information about organ-to-organ communication by peptides-GPCRs. Possible modifications and applications of this protocol are also discussed.

<table border="0" cellpadding="0" cellspacing="0" style="width:1033px;" width="1032">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="40">
			<td height="40" style="height:40px;width:295px;">Tungsten rod</td>
			<td style="width:327px;">A-M systems, Sequim, WA, USA</td>
			<td style="width:165px;">717000</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">Blu Tack</td>
			<td>Bostik, Paris, France</td>
			<td>3049100</td>
			<td>putty-like reusable adhesives</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">Watch glass, square, 1 5/8 in</td>
			<td>Carolina Biological Supply Company, Burlington, NC, USA</td>
			<td>742300</td>
			<td></td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">PBS</td>
			<td>TAKARA Bio Inc., Kusatsu, Shiga, Japan</td>
			<td>T900</td>
			<td>Phosphated buffered salts</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">CCHa2</td>
			<td>SCRUM Inc., Tokyo, Japan</td>
			<td></td>
			<td>Custum-synthesized peptide</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">Ghrerin</td>
			<td>Peptide institute Inc., Osaka, Japan</td>
			<td>4372-s</td>
			<td></td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">Nociceptin</td>
			<td>Peptide institute Inc., Osaka, Japan</td>
			<td>4313-v</td>
			<td></td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">Axio Imager A2</td>
			<td>Carl Zeiss, Oberkochen, Germany</td>
			<td>Axio Imager A2</td>
			<td>fluorescence microscope</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">Objective W N-Achroplan 20x/0.5 M27</td>
			<td>Carl Zeiss, Oberkochen, Germany</td>
			<td>420957-9900-000</td>
			<td>water-immersion objective lens</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">P-725 PIFOC objective scanner with long travel range</td>
			<td>Physik Instrumente GmbH &#38; Co. KG, Karlsruhe, Germany</td>
			<td>N/A</td>
			<td>piezoelectric-activated lens mover</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">Confocal Scanner Unit CSU-W1</td>
			<td>Yokogawa Electric Corporation, Tokyo, Japan</td>
			<td>CSU-W1</td>
			<td>spinning disc confocal head</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">ImagEM C9100-13</td>
			<td>Hamamatsu Photonics, Sizuoka, Japan</td>
			<td>C9100-13</td>
			<td>EM-CCD camera</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">OBIS 488 nm LS 60 mW</td>
			<td>Coherent, Santa Clara, CA, USA</td>
			<td>1178770</td>
			<td>488-nm laser</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">488/568/647 nm Yokogawa dichroic beamsplitter</td>
			<td>Semrock, Rochester, NY, USA</td>
			<td>Di01-T405/488/568/647-13x15x0.5</td>
			<td>dichroic beam splitter</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">528/38 nm BrightLine single-band bandpass filter</td>
			<td>Semrock, Rochester, NY, USA</td>
			<td>FF01-528/38-25</td>
			<td>emission filter</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">&#181;Manager</td>
			<td>Open Imaging, Inc.</td>
			<td>N/A</td>
			<td>https://micro-manager.org</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">ImageJ</td>
			<td>U. S. National Institutes of Health</td>
			<td>N/A</td>
			<td>https://imagej.nih.gov/ij/</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;">TurboReg</td>
			<td>Philippe Th&#233;venaz, Biomediccal Imaging Group, Swiss Federal Institute of Technology Lausanne</td>
			<td>N/A</td>
			<td>http://bigwww.epfl.ch/thevenaz/turboreg/</td>
		</tr>
	</tbody>
</table>

ex vivo calcium imaging for visualizing brain responses to endocrine signaling in drosophila

Organ-to-organ communication by endocrine signaling, for example, from the periphery to the brain, is essential for maintaining homeostasis. As a model animal for endocrine research, Drosophila melanogaster, which has sophisticated genetic tools and genome information, is being increasingly used. This article describes a method for the calcium imaging of Drosophila brain explants. This method enables the detection of the direct signaling of a hormone to the brain. It is well known that many peptide hormones act through G-protein-coupled receptors (GPCRs), whose activation causes an increase in the intracellular Ca2+concentration. Neural activation also elevates intracellular Ca2+ levels, from both Ca2+ influx and the release of Ca2+ stored in the endoplasmic reticulum (ER). A calcium sensor, GCaMP, can monitor these Ca2+ changes. In this method, GCaMP is expressed in the neurons of interest, and the GCaMP-expressing larval brain is dissected and cultured ex vivo. The test peptide is then applied to the brain explant, and the fluorescent changes in GCaMP are detected using a spinning disc confocal microscope equipped with a CCD camera. Using this method, any water-soluble molecule can be tested, and various cellular events associated with neural activation can be imaged using the appropriate fluorescent indicators. Moreover, by modifying the imaging chamber, this method can be used to image other Drosophila organs or the organs of other animals.

Using this protocol, the activation of brain insulin-producing cells (IPCs) by the peptide hormone, CCHa2, was examined. IPCs of the wild-type brain were labeled with GCaMP6s using a combination of dilp2-GAL412 (a gift from Dr. Rulifson) and UAS-GCaMP6s13 (a gift from Dr. Kim). Larval brain explants were treated with a synthetic CCHa2 peptide, and the fluorescence from GCaMP6s was recorded in real time. In this experiment, ...

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Ex Vivo Calcium Imaging for Visualizing Brain Responses to Endocrine Signaling in Drosophila

This paper describes a protocol for ex vivo calcium imaging of the Drosophila brain. In this method, natural or synthetic compounds can be applied to the buffer to test their ability to activate particular neurons in the brain.

Ex Vivo Calcium Imaging for Visualizing Brain Responses to Endocrine Signaling in Drosophila

Organ-to-organ communication by endocrine signaling, for example, from the periphery to the brain, is essential for maintaining homeostasis. As a model ...

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9.1K Views.  Nagoya University. This paper describes a protocol for ex vivo calcium imaging of the Drosophila brain. In this method, natural or synthetic compounds can be applied to the buffer to test their ability to activate particular neurons in the brain.

Video: Ex Vivo Calcium Imaging for Visualizing Brain Responses to Endocrine Signaling in Drosophila

Calcium Imaging of Odor-evoked Responses in the Drosophila Antennal Lobe

The antennal lobe is the primary olfactory center in the insect brain and represents the anatomical and functional equivalent of the vertebrate olfactory bulb1-5. Olfactory information in the external world is transmitted to the antennal lobe by olfactory sensory neurons (OSNs), which segregate to distinct regions of neuropil called glomeruli according to the specific olfactory receptor they express. Here, OSN axons synapse with both local interneurons (LNs), whose processes can innervate many different glomeruli, and projection neurons (PNs), which convey olfactory information to higher olfactory brain regions.
Optical imaging of the activity of OSNs, LNs and PNs in the antennal lobe - traditionally using synthetic calcium indicators (e.g. calcium green, FURA-2) or voltage-sensitive dyes (e.g. RH414) - has long been an important technique to understand how olfactory stimuli are represented as spatial and temporal patterns of glomerular activity in many species of insects6-10. Development of genetically-encoded neural activity reporters, such as the fluorescent calcium indicators G-CaMP11,12 and Cameleon13, the bioluminescent calcium indicator GFP-aequorin14,15, or a reporter of synaptic transmission, synapto-pHluorin16 has made the olfactory system of the fruitfly, Drosophila melanogaster, particularly accessible to neurophysiological imaging, complementing its comprehensively-described molecular, electrophysiological and neuroanatomical properties2,4,17. These reporters can be selectively expressed via binary transcriptional control systems (e.g. GAL4/UAS18, LexA/LexAop19,20, Q system21) in defined populations of neurons within the olfactory circuitry to dissect with high spatial and temporal resolution how odor-evoked neural activity is represented, modulated and transformed22-24.
Here we describe the preparation and analysis methods to measure odor-evoked responses in the Drosophila antennal lobe using G-CaMP25-27. The animal preparation is minimally invasive and can be adapted to imaging using wide-field fluorescence, confocal and two-photon microscopes.

Calcium Imaging of Odor-evoked Responses in the Drosophila Antennal Lobe

We describe an established technique to measure and analyze odor-evoked calcium responses in the antennal lobe of living Drosophila melanogaster.

The antennal lobe is the primary olfactory center in the insect brain and represents the anatomical and functional equivalent of the vertebrate olfactory ...

Imaging Calcium Responses in GFP-tagged Neurons of Hypothalamic Mouse Brain Slices

Despite an enormous increase in our knowledge about the mechanisms underlying the encoding of information in the brain, a central question concerning the precise molecular steps as well as the activity of specific neurons in multi-functional nuclei of brain areas such as the hypothalamus remain. This problem includes identification of the molecular components involved in the regulation of various neurohormone signal transduction cascades. Elevations of intracellular Ca2+ play an important role in regulating the sensitivity of neurons, both at the level of signal transduction and at synaptic sites.
New tools have emerged to help identify neurons in the myriad of brain neurons by expressing green fluorescent protein (GFP) under the control of a particular promoter. To monitor both spatially and temporally stimulus-induced Ca2+ responses in GFP-tagged neurons, a non-green fluorescent Ca2+ indicator dye needs to be used. In addition, confocal microscopy is a favorite method of imaging individual neurons in tissue slices due to its ability to visualize neurons in distinct planes of depth within the tissue and to limit out-of-focus fluorescence. The ratiometric Ca2+ indicator fura-2 has been used in combination with GFP-tagged neurons1. However, the dye is excited by ultraviolet (UV) light. The cost of the laser and the limited optical penetration depth of UV light hindered its use in many laboratories. Moreover, GFP fluorescence may interfere with the fura-2 signals2. Therefore, we decided to use a red fluorescent Ca2+ indicator dye. The huge Stokes shift of fura-red permits multicolor analysis of the red fluorescence in combination with GFP using a single excitation wavelength. We had previously good results using fura-red in combination with GFP-tagged olfactory neurons3. The protocols for olfactory tissue slices seemed to work equally well in hypothalamic neurons4. Fura-red based Ca2+ imaging was also successfully combined with GFP-tagged pancreatic &beta;-cells and GFP-tagged receptors expressed in HEK cells5,6. A little quirk of fura-red is that its fluorescence intensity at 650 nm decreases once the indicator binds calcium7. Therefore, the fluorescence of resting neurons with low Ca2+ concentration has relatively high intensity. It should be noted, that other red Ca2+-indicator dyes exist or are currently being developed, that might give better or improved results in different neurons and brain areas.

In this protocol, we update recent progress in imaging Ca2+ signals of GFP-tagged neurons in brain tissue slices using a red fluorescent Ca2+ indicator dye.

Despite an enormous increase in our knowledge about the mechanisms underlying the encoding of information in the brain, a central question concerning the ...

Ex Vivo Optogenetic Dissection of Fear Circuits in Brain Slices

Optogenetic approaches are now widely used to study the function of neural populations and circuits by combining targeted expression of light-activated proteins and subsequent manipulation of neural activity by light. Channelrhodopsins (ChRs) are light-gated cation-channels and when fused to a fluorescent protein their expression allows for visualization and concurrent activation of specific cell types and their axonal projections in defined areas of the brain. Via stereotactic injection of viral vectors, ChR fusion proteins can be constitutively or conditionally expressed in specific cells of a defined brain region, and their axonal projections can subsequently be studied anatomically and functionally via ex vivo optogenetic activation in brain slices. This is of particular importance when aiming to understand synaptic properties of connections that could not be addressed with conventional electrical stimulation approaches, or in identifying novel afferent and efferent connectivity that was previously poorly understood. Here, a few examples illustrate how this technique can be applied to investigate these questions to elucidating fear-related circuits in the amygdala. The amygdala is a key region for acquisition and expression of fear, and storage of fear and emotional memories. Many lines of evidence suggest that the medial prefrontal cortex (mPFC) participates in different aspects of fear acquisition and extinction, but its precise connectivity with the amygdala is just starting to be understood. First, it is shown how ex vivo optogenetic activation can be used to study aspects of synaptic communication between mPFC afferents and target cells in the basolateral amygdala (BLA). Furthermore, it is illustrated how this ex vivo optogenetic approach can be applied to assess novel connectivity patterns using a group of GABAergic neurons in the amygdala, the paracapsular intercalated cell cluster (mpITC), as an example.

Ex Vivo Optogenetic Dissection of Fear Circuits in Brain Slices

Optogenetic approaches are widely used to manipulate neural activity and assess the consequences for brain function. Here, a technique is outlined that&#160;upon in vivo expression of the optical activator Channelrhodopsin, allows for ex vivo analysis of synaptic properties of specific long range and local neural connections in fear-related circuits.

Optogenetic approaches are now widely used to study the function of neural populations and circuits by combining targeted expression of light-activated ...

In Vivo Functional Brain Imaging Approach Based on Bioluminescent Calcium Indicator GFP-aequorin

Functional in vivo imaging has become a powerful approach to study the function and physiology of brain cells and structures of interest. Recently a new method of Ca2+-imaging using the bioluminescent reporter GFP-aequorin (GA) has been developed. This new technique relies on the fusion of the GFP and aequorin genes, producing a molecule capable of binding calcium and&#160;&#8212; with the addition of its cofactor coelenterazine &#8212; emitting bright light that can be monitored through a photon collector. Transgenic lines carrying the GFP-aequorin gene have been generated for both mice and Drosophila. In Drosophila, the GFP-aequorin gene has been placed under the control of the GAL4/UAS binary expression system allowing for targeted expression and imaging within the brain. This method has subsequently been shown to be capable of detecting both inward Ca2+-transients and Ca2+-released from inner stores. Most importantly it allows for a greater duration in continuous recording, imaging at greater depths within the brain, and recording at high temporal resolutions (up to 8.3 msec). Here we present the basic method for using bioluminescent imaging to record and analyze Ca2+-activity within the mushroom bodies, a structure central to learning and memory in the fly brain.

In Vivo Functional Brain Imaging Approach Based on Bioluminescent Calcium Indicator GFP-aequorin

Here we present a novel Ca2+-imaging approach using a bioluminescent reporter. This approach uses a fused construct GFP-aequorin which binds to Ca2+ and emits light, eliminating the need for light excitation. Significantly this method permits long continuous imaging, access to deep brain structures and high temporal resolution.

Functional in vivo imaging has become a powerful approach to study the function and physiology of brain cells and structures of interest. Recently a new ...

Ex Vivo Imaging of Cell-specific Calcium Signaling at the Tripartite Synapse of the Mouse Diaphragm

The electrical activity of cells in tissues can be monitored by electrophysiological techniques, but these are usually limited to the analysis of individual cells. Since an increase of intracellular calcium (Ca2+) in the cytosol often occurs because of the electrical activity, or in response to a myriad of other stimuli, this process can be monitored by the imaging of cells loaded with fluorescent calcium-sensitive dyes.&#160; However, it is difficult to image this response in an individual cell type within whole tissue because these dyes are taken up by all cell types within the tissue. In contrast, genetically encoded calcium indicators (GECIs) can be expressed by an individual cell type and fluoresce in response to an increase of intracellular Ca2+, thus permitting the imaging of Ca2+ signaling in entire populations of individual cell types. Here, we apply the use of the GECIs GCaMP3/6 to the mouse neuromuscular junction, a tripartite synapse between motor neurons, skeletal muscle, and terminal/perisynaptic Schwann cells. We demonstrate the utility of this technique in classic ex vivo tissue preparations. Using an optical splitter, we perform dual-wavelength imaging of dynamic Ca2+ signals and a static label of the neuromuscular junction (NMJ) in an approach that could be easily adapted to monitor two cell-specific GECI or genetically encoded voltage indicators (GEVI) simultaneously. Finally, we discuss the routines used to capture spatial maps of fluorescence intensity. Together, these optical, transgenic, and analytic techniques can be employed to study the biological activity of distinct cell subpopulations at the NMJ in a wide variety of contexts.

Ex Vivo Imaging of Cell-specific Calcium Signaling at the Tripartite Synapse of the Mouse Diaphragm

Here we present a protocol to image calcium signaling in populations of individual cell types at the murine neuromuscular junction.

The electrical activity of cells in tissues can be monitored by electrophysiological techniques, but these are usually limited to the analysis of ...

In Vivo Optical Calcium Imaging of Learning-Induced Synaptic Plasticity in Drosophila melanogaster

Decades of research in many model organisms have led to the current concept of synaptic plasticity underlying learning and memory formation. Learning-induced changes in synaptic transmission are often distributed across many neurons and levels of processing in the brain. Therefore, methods to visualize learning-dependent synaptic plasticity across neurons are needed. The fruit fly Drosophila melanogaster represents a particularly favorable model organism to study neuronal circuits underlying learning. The protocol presented here demonstrates a way in which the processes underlying the formation of associative olfactory memories, i.e., synaptic activity and their changes, can be monitored in vivo. Using the broad array of genetic tools available in Drosophila, it is possible to specifically express genetically encoded calcium indicators in determined cell populations and even single cells. By fixing a fly in place, and opening the head capsule, it is possible to visualize calcium dynamics in these cells whilst delivering olfactory stimuli. Additionally, we demonstrate a set-up in which the fly can be subjected, simultaneously, to electric shocks to the body. This provides a system in which flies can undergo classical olfactory conditioning - whereby a previously na&#239;ve odor is learned to be associated with electric shock punishment - at the same time as the representation of this odor (and other untrained odors) is observed in the brain via two-photon microscopy. Our lab has previously reported the generation of synaptically localized calcium sensors, which enables one to confine the fluorescent calcium signals to pre- or postsynaptic compartments. Two-photon microscopy provides a way to spatially resolve fine structures. We exemplify this by focusing on neurons integrating information from the mushroom body, a higher-order center of the insect brain. Overall, this protocol provides a method to examine the synaptic connections between neurons whose activity is modulated as a result of olfactory learning.

In Vivo Optical Calcium Imaging of Learning-Induced Synaptic Plasticity in Drosophila melanogaster

Here we present a protocol with which pre- and/or postsynaptic calcium can be visualized in the context of Drosophila learning and memory. In vivo calcium imaging using synaptically localized calcium sensors is combined with a classical olfactory conditioning paradigm such that the synaptic plasticity underlying this type of associative learning may be determined.

Decades of research in many model organisms have led to the current concept of synaptic plasticity underlying learning and memory formation. ...

Preparing Adult Drosophila melanogaster for Whole Brain Imaging during Behavior and Stimuli Responses

We present a method developed specifically to image the whole Drosophila brain during ongoing behavior such as walking. Head fixation and dissection are optimized to minimize their impact on behavior. This is first achieved by using a holder that minimizes movement hindrances. The back of the fly&#8217;s head is glued to this holder at an angle that allows optical access to the whole brain while retaining the fly&#8217;s ability to walk, groom, smell, taste and see. The back of the head is dissected to remove tissues in the optical path and muscles responsible for head movement artefacts. The fly brain can subsequently be imaged to record brain activity, for instance using calcium or voltage indicators, during specific behaviors such as walking or grooming, and in response to different stimuli. Once the challenging dissection, which requires considerable practice, has been mastered, this technique allows to record rich data sets relating whole brain activity to behavior and stimulus responses.

Preparing Adult Drosophila melanogaster for Whole Brain Imaging during Behavior and Stimuli Responses

We present a method specifically tailored to image the whole brain of adult Drosophila during behavior and in response to stimuli. The head is positioned to allow optical access to the whole brain, while the fly can move its legs and the antennae, the tip of the proboscis, and the eyes can receive sensory stimuli.

We present a method developed specifically to image the whole Drosophila brain during ongoing behavior such as walking. Head fixation and dissection are ...

In vivo Calcium Imaging of Mouse Geniculate Ganglion Neuron Responses to Taste Stimuli

Within the last ten years, advances in genetically encoded calcium indicators (GECIs) have promoted a revolution in in vivo functional imaging. Using calcium as a proxy for neuronal activity, these techniques provide a way to monitor the responses of individual cells within large neuronal ensembles to a variety of stimuli in real time. We, and others, have applied these techniques to image the responses of individual geniculate ganglion neurons to taste stimuli applied to the tongues of live anesthetized mice. The geniculate ganglion is comprised of the cell bodies of gustatory neurons innervating the anterior tongue and palate as well as some somatosensory neurons innervating the pinna of the ear. Imaging the taste-evoked responses of individual geniculate ganglion neurons with GCaMP has provided important information about the tuning profiles of these neurons in wild-type mice as well as a way to detect peripheral taste miswiring phenotypes in genetically manipulated mice. Here we demonstrate the surgical procedure to expose the geniculate ganglion, GCaMP fluorescence image acquisition, initial steps for data analysis, and troubleshooting. This technique can be used with transgenically encoded GCaMP, or with AAV-mediated GCaMP expression, and can be modified to image particular genetic subsets of interest (i.e., Cre-mediated GCaMP expression). Overall, in vivo calcium imaging of geniculate ganglion neurons is a powerful technique for monitoring the activity of peripheral gustatory neurons and provides complementary information to more traditional whole-nerve chorda tympani recordings or taste behavior assays.

Here we present how to expose the geniculate ganglion of a live, anesthetized laboratory mouse and how to use calcium imaging to measure the responses of ensembles of these neurons to taste stimuli, allowing for multiple trials with different stimulants. This allows for in depth comparisons of which neurons respond to which tastants.

Within the last ten years, advances in genetically encoded calcium indicators (GECIs) have promoted a revolution in in vivo functional imaging. Using ...

In vivo Calcium Imaging in Mouse Inferior Olive

Inferior olive (IO), a nucleus in the ventral medulla, is the only source of climbing fibers that form one of the two input pathways entering the cerebellum. IO has long been proposed to be crucial for motor control and its activity is currently considered to be at the center of many hypotheses of both motor and cognitive functions of the cerebellum. While its physiology and function have been relatively well studied on single-cell level in vitro, presently there are no reports on the organization of the IO network activity in living animals. This is largely due to the extremely challenging anatomical location of the IO, making it difficult to subject to conventional fluorescent imaging methods, where an optic path must be created through the entire brain located dorsally to the region of interest.
Here we describe an alternative method for obtaining state-of-the-art -level calcium imaging data from the IO network. The method takes advantage of the extreme ventral location of the IO and involves a surgical procedure for inserting a gradient-refractive index (GRIN) lens through the neck viscera to come into contact with the ventral surface of the calcium sensor GCaMP6s-expressing IO in anesthetized mice. A representative calcium imaging recording is shown to demonstrate the feasibility to record IO neuron activity after the surgery. While this is a non-survival surgery and the recordings must be conducted under anesthesia, it avoids damage to life-critical brainstem nuclei and allows conducting large variety of experiments investigating spatiotemporal activity patterns and input integration in the IO. This procedure with modifications could be used for recordings in other, adjacent regions of the ventral brainstem.

In vivo Calcium Imaging in Mouse Inferior Olive

We present a protocol to expose the brainstem of adult mouse from the ventral side. By using a gradient-refractive index lens with a miniature microscope, calcium imaging can be used to examine the activity of inferior olive neural somata in vivo.

Inferior olive (IO), a nucleus in the ventral medulla, is the only source of climbing fibers that form one of the two input pathways entering the ...

Brain Pericyte Calcium and Hemodynamic Imaging in Transgenic Mice In Vivo

Recent advances in protein biology and mouse genetics have made it possible to measure intracellular calcium fluctuations of brain cells in vivo and to correlate this with local hemodynamics. This protocol uses transgenic mice that have been prepared with a chronic cranial window and express the genetically encoded calcium indicator, RCaMP1.07, under the &#945;-smooth muscle actin promoter to specifically label mural cells, such as vascular smooth muscle cells and ensheathing pericytes. Steps are outlined on how to prepare a tail vein catheter for intravenous injection of fluorescent dyes to trace blood flow, as well as how to measure brain pericyte calcium and local blood vessel hemodynamics (diameter, red blood cell velocity, etc.) by two photon microscopy in vivo through the cranial window in ketamine/xylazine anesthetized mice. Finally, details are provided for the analysis of calcium fluctuations and blood flow movies via the image processing algorithms developed by Barrett et al. 2018, with an emphasis on how these processes can be adapted to other cellular imaging data.

Brain Pericyte Calcium and Hemodynamic Imaging in Transgenic Mice In Vivo

This protocol presents steps to acquire and analyze fluorescent calcium images from brain ensheathing pericytes and blood flow data from nearby blood vessels in anesthetized mice. These techniques are useful for studies of mural cell physiology and can be adapted to investigate calcium transients in any cell type.

Recent advances in protein biology and mouse genetics have made it possible to measure intracellular calcium fluctuations of brain cells in vivo and to ...