This work was supported in part by National Basic Research Program of China (2012CB825504), National Natural Science Foundation of China (91232720 and 9163210042), Chinese Academy of Sciences (CAS) (GJHZ201302 and QYZDY-SSW-SMC015), Bill and Melinda Gates Foundation (OPP1119434), and 100-Talents Program of CAS to Y. Zhu.

1. Preparing the feeding chamber
Note: The feeding chamber for dye-labeling feeding assay consists of two parts: the outside container (as a cover) and the inside container (as food source).
<ol>
	<li>Modify the outside container from a glass vial for culturing Drosophila (with an internal diameter of 31.8 mm and a height of 80 mm) (Figure 1A, 1C). Cover the bottom of the container with a layer of 1% agarose (5 mL) to keep the feeding setup properly humidified and serve as a soft substrate for the flies to walk upon (Figure 1B). The container thereby provides a controlled but naturalistic setting carrying with minimal stress or disturbance to the flies.
	<ol>
		<li>Prepare the outside container. Suspend 0.5 g of agarose in 50 mL of double distilled water, heat with frequent agitation, and boil to dissolve completely on a magnetic stirrer with a heat plate.</li>
		<li>When the 1% agarose cools to about 60 &#176;C, transfer 5 mL of 1% agarose to an empty outside container (Figure 1B), and let the container stay open until cooled down to room temperature to yield an agarose pad.</li>
	</ol>
	</li>
	<li>Prepare dye-labeled food. Composition of the dye-labeled food varies according to different experimental purposes. Here is an example of preparing the dye-labeled food containing only sucrose.
	<ol>
		<li>Add 0.5 g of agarose to 50 mL of double distilled water, heat with frequent agitation, and boil to dissolve completely on a magnetic stirrer with a heat plate.</li>
		<li>When the above 1% agarose cools to about 60 &#176;C, add 1.71 g of sucrose to the agarose to obtain 100 mM sucrose.</li>
		<li>Add 0.25 g of blue dye (erioglaucine disodium) to the agarose solution to obtain 0.5% (w/v) dye-labeled food.</li>
	</ol>
	</li>
	<li>Prepare the inside container. Put the empty inside containers on a flat horizontal surface, then add 750 &#181;L of blue dye-labeled food to individual food container, let it solidify for at least 5 min. 
	Note: The inside container is a small plastic dish with an internal diameter of 13.6 mm and a height of 7.5 mm (Figure 1A, 1C). It is filled with dye-labeled food, and is then positioned at the center of the agarose pad of the outside container.</li>
	<li>Transfer a filled inside container into a prepared outside container and position it at the center of the agarose pad. Let the feeding chambers stay open at room temperature for 40 min until the wall of the outside container is free of condensation. 
	Note: The dry wall is necessary to prevent the flies from being stuck onto the wall by contacting water drops.</li>
	<li>Prepare feeding chambers with dye-free food (same composition but without dye) for background subtraction.</li>
</ol>
2. Preparation of flies
Note: Adult female flies 5 to 10 day after eclosion are normally used for feeding assays. It is recommended to Prepare flies of different genotypes, including genetic controls, and to test in parallel. 20 flies (from one feeding chamber) generate one data point.
<ol>
	<li>Consider the population density for rearing adult flies. Typically, control the population density by varying the number of parental flies according to the strains and the dimensions of culture bottles. As an example, transfer 15 males and 30 females of Canton-S to a culture bottle (with a diameter of 31.8 mm and a height of 80 mm) and let the females to lay eggs for one day then remove the adult flies.</li>
	<li>Prepare flies for thermogenetic activation.
	<ol>
		<li>Use the Gal4/UAS system38 to express dTRPA133 in various neurons by crossing UAS-dTrpA1 flies with different GAL4 driver lines (Figure 2B).</li>
		<li>Keep the adult flies on standard food at 22 &#176;C, 60% relative humidity, and a 12 h/12 h light-dark cycle.</li>
		<li>Two days prior to the feeding assay, sort and collect flies on a CO2 fly pad under a stereo microscope into a fresh food vial and keep a density of 20 flies per vial.</li>
	</ol>
	</li>
	<li>Prepare flies for optogenetic activation.
	<ol>
		<li>Use the Gal4/UAS system38 to express CsChrimson36 in different neurons by crossing UAS-CsChrimson flies with different GAL4 driver lines.</li>
		<li>Collect newly eclosed (within 24 h) flies and transfer them into a vial with standard food containing 400 &#181;M all-trans-retinal.</li>
		<li>To prevent premature activation, wrap the rearing vials with aluminum foil and then put them into a dark box to protect flies from unwanted light stimulation. Keep the flies at 25 &#176;C, 60% humidity in the dark for 4 - 6 days.</li>
		<li>Two days prior to the feeding assay, sort female flies on CO2 fly pad under a stereo microscope. Collect 20 flies into one food vial and keep them in the dark prior to the feeding test.</li>
		<li>Conduct all operations under dim light as quickly as possible to avoid the effects caused by ambient light. Use a LED lamp placed far away from the working area, the intensity at the working area is 5 &#181;W/cm2. 
		Note: Use starved Canton-S flies as a positive control for the feeding assay. Deprive these flies of food and maintain on 1% agarose for 24 h prior to the feeding test. The levels of food consumption of these flies help evaluate the day-to-day fluctuations.</li>
	</ol>
	</li>
</ol>
3. Thermogenetic activation
<ol>
	<li>Conduct all feeding experiments at the same time of the day, around Zeitgeber time 4 - 631,32, to minimize variations due to circadian rhythms.</li>
	<li>Prepare an incubator (or a small room with a temperature-controlled heater) as the heating environment.</li>
	<li>Before behavioral experiments, equilibrate the prepared feeding chambers at the experimental temperature (30 &#176;C) for 2 h to ensure the temperature is relative uniform in the feeding chamber.</li>
	<li>To start the feeding process, carefully transfer 20 flies from their home vial into a preheated feeding chamber by gentle tapping. Continue the feeding process at 30 &#176;C for 60 min ( Figure 3A) . 
	Note: Frequent agitations, such as opening and closing the incubator door, adversely affect the feeding behavior, thus should be minimized.</li>
	<li>Cease feeding at 60 min by freezing the feeding chambers at -80 &#176;C (or -20 &#176;C) for 30 min. 
	Note: Freezing helps to stop feedings in all chambers simultaneously. Freezing flies at -80 &#176;C serves two purposes, for euthanizing the flies and for storage until homogenization for up to one week.</li>
	<li>For the temperature control group, transfer 20 flies into a room temperature feeding chamber with dye-colored food to start the feeding process at 22 &#176;C for 60 min.</li>
	<li>Cease feeding of the temperature control group by freezing the chambers at -80 &#176;C (or -20 &#176;C) for 30 min.</li>
</ol>
4. Optogenetic activation
<ol>
	<li>Conduct all feeding experiments at the same time of the day, around Zeitgeber time 4 - 631,32, to minimize variations due to circadian rhythms.</li>
	<li>Carefully transfer 20 flies from one vial into a feeding chamber by gentle tapping, and then put the chamber sideways on the illumination setup (Figure 4A, 4B).
	<ol>
		<li>For optogenetic manipulation, use an array of orange LEDs (607 nm) as the source of light stimulation, and fix a horizontal plexiglass plate above the LEDs to support the feeding chambers during illumination. Keep the setup in an incubator at 25 &#176;C with 60% humidity (Figure 4A, 4B).</li>
		<li>Stimulate the genetic control flies in parallel with the Taotie&#62;CsChrimson flies, but in different feeding chambers.</li>
	</ol>
	</li>
	<li>Continue the feeding process for 60 min with back illumination by the orange light.
	<ol>
		<li>Determine the optimal illumination intensities for optogenetics experimentally36. For the Taotie&#62;CsChrimson flies, an intensity of 2.2 mW/mm2 induces paralysis (cessation of locomotion and loss of postural control), therefore, use an intermediate intensity of 0.1 mW/mm2 to stimulate the Taotie&#62;CsChrimson flies during the feeding test.</li>
	</ol>
	</li>
	<li>Cease the feeding by freezing feeding chambers at -80 &#176;C (or -20 &#176;C) for 30 min.</li>
</ol>
5. Visual estimation of food consumption
Note: The visual scoring approach provides a semi-quantitative estimation of food consumptions. Although it is not as objective as the optical method, this method allows for going through a large number of fly strains in a timely fashion, making it suitable for the first round of a genetic screen to quickly narrow down the list of candidates for further tests.
<ol>
	<li>After the feeding process, anesthetize and score the flies on a CO2 fly pad under a stereo microscope. Give individual score to each fly based on the volume and intensity of dye-colored food in its abdomen (Figure 2A). 
	NOTE: Criteria for visual estimation: the score system has 4 levels of food intake corresponding to different volumes and intensities of colored food in the abdomen.
	<ol>
		<li>Score flies without detectable dye-colored food a score of 0; give those with faint (barely detectable) blue dye or with dye-colored food occupying less than one third of the volume of the abdomen a score of 1.</li>
		<li>Score flies with intense dye-colored food occupying one-half of the abdomen a score of 2.</li>
		<li>Score those with intense dyed content occupying greater than half of the abdomen a score of 3 (Figure 2A).</li>
	</ol>
	</li>
	<li>Calculate the proportion of flies with different scores in each experimental condition (Figure 2B).</li>
</ol>
6. Colorimetric quantification of food consumption
<ol>
	<li>After cessation of feeding, pour out all 20 flies from a feeding chamber onto a piece of weighing paper, then transfer them into a 1.5 mL tube using a soft brush. 
	Note: Do not take all of the chambers out of -80 &#176;C storage at the same time to avoid flies sticking to the chamber wall because of water condensation on the cold chamber.</li>
	<li>Add 500 &#181;L of PBST to the tube and homogenize the flies using a grinding mill at 60 Hz for 5 s.
	<ol>
		<li>Confirm sufficient homogenization by observation when there are no detectable fragments of body parts.</li>
	</ol>
	</li>
	<li>Spin the tubes with homogenates for 30 min at 13,000 rpm to clear the debris.</li>
	<li>After centrifugation, transfer 100 &#181;L of supernatant to a well of a 96-well plate.</li>
	<li>After all samples are loaded, put the plate into a plate reader to measure the absorbance of the samples at 630 nm.</li>
	<li>To remove the background absorbance, subtract the absorbance of the supernatants from flies fed on food without dye from the absorbance of the supernatant from the blue food-fed flies. 
	Note: This step is optional, depending on the compositions of the food used. When a fly food (without the dye) generates absorbance at 630 nm, it is necessary to perform this step to eliminate the background.</li>
</ol>

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M., Yu, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Drosophila+short+neuropeptide+F+regulates+food+intake+and+body+size.">Drosophila short neuropeptide F regulates food intake and body size.</a> J Biol Chem. 279 (49), 50781-50789 (2004).</li><li>Marella, S., Mann, K., Scott, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dopaminergic+Modulation+of+Sucrose+Acceptance+Behavior+in+Drosophila.">Dopaminergic Modulation of Sucrose Acceptance Behavior in Drosophila.</a> Neuron. 73 (5), 941-950 (2012).</li><li>Albin, S. D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Subset+of+Serotonergic+Neurons+Evokes+Hunger+in+Adult+Drosophila.">A Subset of Serotonergic Neurons Evokes Hunger in Adult Drosophila.</a> Current Biology. 25 (18), 2435-2440 (2015).</li><li>Ro, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Serotonin+signaling+mediates+protein+valuation+and+aging.">Serotonin signaling mediates protein valuation and aging.</a> eLife. 5, e16843 (2016).</li></ol>

The authors declare no competing financial interests.

This report focuses on the technical process of dye-labeling feeding assays of food consumption in the context of thermogenetic and optogenetic activation to manipulate neurons controlling feeding. This simple and reliable protocol will help to elucidate the function of candidate neurons in feeding control, to measure the food preference of flies, and to identify novel players in the feeding control circuits via feeding-based genetic screens24.
The dye labeling strategy...

Quantifying the amount of food ingested is important for evaluating multiple aspects of feeding controls by the brain in responding to the internal needs (such as hunger states) and external factors (such as food quality and palatability)1,2,3,4,5,6,7,8,9. In recent years, the efforts of deciphering the neural substrates of feeding control in Drosophila lead to the development of multiple assays to directly quantify the amount of food ingested or serve as an indicator of feeding motivation10,11,12,13,14,15,16.
The CApillary FEeder (CAFE) assay12,13 was developed to measure the amount of consumption of liquid food in a glass microcapillary. The CAFE assay is highly sensitive and reproducible17 and simplifies the measurement of food consumption, especially for quantifying long-term feeding18. However, this assay requires the flies to climb to the tip of the microcapillary and feed upside-down, which is not suitable for all strains. Additionally, because the flies to be tested using the CAFE assay have to be reared on liquid food, the effect of these rearing conditions on metabolism status or the potential malnutrition remains to be determined.
The Proboscis Extension Response (PER) assay11,14 counts the frequency of proboscis extension responses toward gentle touches of food drops. PER assay proved as an excellent way to evaluate feeding motivation of individual fly and asses the influence of palatability and content of food18,19. However, it is not a direct quantification of intake amount.
Recently, a semi-automatic method, the manual feeding assay (MAFE)15, was developed. In MAFE, a single immobilized fly is fed manually with a microcapillary containing food. Given that proboscis extension responses and food consumption can be monitored simultaneously, MAFE is suitable for assessing nutrient values and the effects of pharmacological manipulation. However, immobilizing a fly might negatively impact its behavioral performance, including feeding.
Additionally, fly Proboscis and Activity Detector (FlyPAD)10 was developed to automatically quantify feeding behavior. Using machine vision methods, FlyPAD records physical interactions between a fly and food to quantify the frequency and duration of proboscis extensions as an indicator of feeding motivation. FlyPAD provides a high-throughput approach to monitor the feeding behaviors of a free-moving fly, although the sensitivity and robustness of this system remains to be further confirmed by more studies12.
Labeling strategies are frequently used to estimate food ingestion in flies. It is common to label food with chemical tracers and, after feeding, measure the amount of ingested tracer to calculate the quantity of food intake. Radioactive tracers16,17,20,21,22,23,24,25 allow for the detection through the cuticle without homogenization of the flies. This method provides remarkably low variability and high sensitivity18, and is feasible for long-term study of food intake. However, the availability of usable radioisotopes and different rates of absorbance and excretion should be taken into consideration when working with this assay.
Labeling and tracing food intake with non-toxic food colors is a safer and simpler alternative2,3,26,27,28. Flies are homogenized after feeding with food containing soluble and non-absorbable dyes, and the amount of the ingested dye is later quantified using a spectrophotometer3,24,28,29. The labeling strategy is easy to perform and provides high efficiency, but with a caveat. The volume of food intake estimated from the ingested dye is smaller than the actual volume because excretion begins as early as 15 min after flies start feeding17. Additionally, the assay assesses food ingestion typically within a 60-min period, which is only suitable for investigation of short-term feeding behavior24,28. Moreover, multiple internal and external factors, such as genotype17, gender17, mated state17, rearing density30, circadian rhythm31,32, and food quality3,8,16, influence food intake. Therefore, the feeding duration might need to be adjusted according to specific experimental conditions. Besides facilitating the quantification of food intake, food colors are also used to assess food choices2,19,27, and to visualize the meniscus in a microcapillary in CAFE assay12.
Here, we introduce a protocol combined manipulation of neuronal activity with dye-labeling approach. This strategy has been proved useful in our neurogenetic study on feeding control in adult fruit flies24. The visual scoring method allows for a quick estimation of food consumption; thus, it is useful for screening through a large number of strains in a timely fashion. The candidates from the screen are then analyzed in detail using a colorimetric method to provide objective and precise quantification in additional study.
Besides the feeding assays, we also describe the thermogenetic27,33,34,35 and optogenetic36 methods of forcibly activating target neurons in Drosophila. To activate neurons by thermogenetic operation is simple and convenient with Drosophila Transient Receptor Potential Ankyrin 1 (dTRPA1), which is a temperature- and voltage-gated cation channel that increases neuronal excitability when the ambient temperature rises above 23 &#176;C33,37; however, testing animals at high temperatures might produce adverse effects on behavior. Another effective approach to activate neurons in Drosophila is using optogenetics with CsChrimson36, which is a red-shifted variant of channelrhodopsin that increases the excitability of neurons when exposed to light. Optogenetics offers higher temporal resolution and lesser disturbance to behaviors than thermogenetics. Combining the quantitative measurement of food intake with the manipulation of neuronal activity represents an effective approach for studying the neural mechanisms of feeding.
We describe in detail the preparation of the feeding chamber and the flies to be tested. Using Taotie-Gal4 flies as a model24, we describe activating neurons by thermogenetics and optogenetics. Two assays of quantification of food consumption with dye-labeled food are also described in the protocol.

<table><tbody><tr><td>UAS-CsChrimson</td><td>Bloomintoon</td><td>55135</td><td></td></tr><tr><td>UAS-dTrpA1</td><td>Bloomintoon</td><td>26263</td><td></td></tr><tr><td>TDC1-GAL4&amp;nbsp;</td><td>Bloomintoon</td><td>9312</td><td></td></tr><tr><td>TDC2-GAL4</td><td>Bloomintoon</td><td>9313</td><td></td></tr><tr><td>sNPF-GAL4</td><td></td><td></td><td>Provided by Z. Zhao</td></tr><tr><td>NPF-GAL4</td><td></td><td></td><td>Provided by Y. Rao</td></tr><tr><td>TH-GAL4</td><td></td><td></td><td>Provided by Y. Rao</td></tr><tr><td>5-HT-GAL4</td><td></td><td></td><td>Provided by Y. Rao</td></tr><tr><td>AKH-GAL4</td><td></td><td></td><td>Provided by Y. Rao</td></tr><tr><td>dip2-GAL4</td><td></td><td></td><td>Provided by Y. Rao</td></tr><tr><td>Taotie-GAL4</td><td></td><td></td><td>Provided by J. Carlson</td></tr><tr><td>Agarose</td><td>Biowest</td><td>G-10</td><td></td></tr><tr><td>Sucrose</td><td>Sigma</td><td>S7903</td><td></td></tr><tr><td>Erioglaucine disodium salt</td><td>Sigma</td><td>861146</td><td></td></tr><tr><td>all-trans-retinal&amp;nbsp;</td><td>Sigma&amp;nbsp;</td><td>R2500</td><td>stored in darkness</td></tr><tr><td>Triton X-100</td><td>Amresco</td><td>9002-93-01</td><td></td></tr><tr><td>Fly food</td><td></td><td>1 L food contains: 77.7 g corn meal, 32.19 g yeast, 5 g agar, 0.726 g CaCl&lt;sub&gt;2&lt;/sub&gt;, 31.62 g sucrose, 63.2 g glucose, 2 g potassium sorbate, pH&amp;nbsp;&amp;nbsp;&amp;nbsp;</td><td></td></tr><tr><td>&amp;nbsp;1x PBS buffer&amp;nbsp;</td><td></td><td>1 L 1X PBS contains: 8 g Nacl, 0.2 g Kcl, 1.44 g Na&lt;sub&gt;2&lt;/sub&gt;HPO&lt;sub&gt;4&lt;/sub&gt;, 0.24 g KH&lt;sub&gt;2&lt;/sub&gt;PO&lt;sub&gt;4&lt;/sub&gt;, pH 7.4</td><td></td></tr><tr><td>PBST buffer</td><td></td><td>1X PBS with 1% Triton X-100</td><td></td></tr><tr><td>&amp;nbsp;Grinding mill</td><td>Shang Hai Jing Xin</td><td>Tissuelyser-24</td><td></td></tr><tr><td>Incubator</td><td>Ning Bo Jiang Nan</td><td>HWS-80</td><td></td></tr><tr><td>Magnetic stirrer with a heat plate</td><td>Chang Zhou Bo Yuan</td><td>CJJ 78-1</td><td></td></tr><tr><td>Spectrometer</td><td>Thorlabs</td><td>CCS200/M</td><td></td></tr><tr><td>Microplate Spectrophotometer</td><td>Thermo Scientific&amp;nbsp;</td><td>Multiskan GO Type: 1510, REF 51119200</td><td></td></tr><tr><td>Fluorescence stereo microscope&amp;nbsp;</td><td>Leica</td><td>&amp;nbsp;M205FA</td><td></td></tr><tr><td>Stereo microscope</td><td>Leica&amp;nbsp;</td><td>S6E</td><td></td></tr><tr><td>Outside container</td><td>Jiang Su Hai Men</td><td>glass vial with a diameter of 31.8 mm and a height of 80 mm (inside dimension)</td><td></td></tr><tr><td>Inside container&amp;nbsp;</td><td>Beijing Yi Ran machinery factory</td><td>plastic dish with a diameter of 13.6 mm and a height of 7.5 mm (inside dimension)</td><td></td></tr><tr><td>1.5 mL Eppendorf tubes</td><td>Hai Men Ning Mong</td><td></td><td></td></tr><tr><td>&amp;nbsp;96 well plate</td><td>Corning Incorporated</td><td>&amp;nbsp;Costar 3599</td><td></td></tr><tr><td>LEDs</td><td>Xin Xing Yuan Guangdian</td><td>607 nm, 3W&amp;nbsp;</td><td>https://item.taobao.com/item.htm?id=20158878058</td></tr></tbody></table>

combining quantitative food-intake assays and forcibly activating neurons to study appetite in drosophila

Food consumption is under the tight control of the brain, which integrates the physiological status, palatability, and nutritional contents of the food, and issues commands to start or stop feeding. Deciphering the processes underlying the decision-making of timely and moderate feeding carries major implications in our understanding of physiological and psychological disorders related to feeding control. Simple, quantitative, and robust methods are required to measure the food ingestion of animals after experimental manipulation, such as forcibly increasing the activities of certain target neurons. Here, we introduced dye-labeling-based feeding assays to facilitate the neurogenetic study of feeding control in adult fruit flies. We review available feeding assays, and then describe our methods step-by-step from setup to analysis, which combine thermogenetic and optogenetic manipulation of neurons controlling feeding motivation with dye-labeled food intake assay. We also discuss the advantages and limitations of our methods, compared with other feeding assays, to help readers choose an appropriate assay.

Thermogenetic screen.
Abnormally increased appetite causes elevated food intake, regardless of physiological needs. We utilized this scheme to design a high-throughput behavioral screen to obtain genetic handles of neurons related to hunger and satiated states (Figure 1). The screen yielded Taotie-Gal424. When the Taotie-Gal4 neurons were forcibly activated ...

Watch this Scientific Journal Video about Combining Quantitative Food-intake Assays and Forcibly Activating Neurons to Study Appetite in Drosophila at JoVE.com

Combining Quantitative Food-intake Assays and Forcibly Activating Neurons to Study Appetite in Drosophila

Quantitative food-intake assays with dyed food provide a robust and high-throughput means to evaluate feeding motivation. Combining the food consumption assay with thermogenetic and optogenetic screens is a powerful approach to investigate the neural circuits underlying appetite in adult Drosophila melanogaster.

Combining Quantitative Food-intake Assays and Forcibly Activating Neurons to Study Appetite in Drosophila

Food consumption is under the tight control of the brain, which integrates the physiological status, palatability, and nutritional contents of the food, ...

combining-quantitative-food-intake-assays-forcibly-activating-neurons

Research

JoVE Journal

Behavior

8.2K Views.  University of Science and Technology of China. Kwantitatieve voedselinname testen met geverfd voedsel bieden een robuuste en high-throughput betekent om voeding motivatie. De voedsel consumptie-bepaling te combineren met thermogenetic en optogenetic schermen is een krachtige aanpak van de onderzoeken van de neurale circuits die ten grondslag liggen aan de eetlust in volwassen Drosophila melanogaster.

Video: Combining Quantitative Food-intake Assays and Forcibly Activating Neurons to Study Appetite in Drosophila

Simultaneous Recording of Calcium Signals from Identified Neurons and Feeding Behavior of Drosophila melanogaster

To study neuronal networks in terms of their function in behavior, we must analyze how neurons operate when each behavioral pattern is generated. Thus, simultaneous recordings of neuronal activity and behavior are essential to correlate brain activity to behavior. For such behavioral analyses, the fruit fly, Drosophila melanogaster, allows us to incorporate genetically encoded calcium indicators such as GCaMP1, to monitor neuronal activity, and to use sophisticated genetic manipulations for optogenetic or thermogenetic techniques to specifically activate identified neurons2-5. Use of a thermogenetic technique has led us to find critical neurons for feeding behavior (Flood et al., under revision). As a main part of feeding behavior, a Drosophila adult extends its proboscis for feeding6 (proboscis extension response; PER), responding to a sweet stimulus from sensory cells on its proboscis or tarsi. Combining the protocol for PER7 with a calcium imaging technique8 using GCaMP3.01, 9, I have established an experimental system, where we can monitor activity of neurons in the feeding center &#x2013; the suboesophageal ganglion (SOG), simultaneously with behavioral observation of the proboscis. I have designed an apparatus ("Fly brain Live Imaging and Electrophysiology Stage": "FLIES") to accommodate a Drosophila adult, allowing its proboscis to freely move while its brain is exposed to the bath for Ca2+ imaging through a water immersion lens. The FLIES is also appropriate for many types of live experiments on fly brains such as electrophysiological recording or time lapse imaging of synaptic morphology. Because the results from live imaging can be directly correlated with the simultaneous PER behavior, this methodology can provide an excellent experimental system to study information processing of neuronal networks, and how this cellular activity is coupled to plastic processes and memory.

Simultaneous Recording of Calcium Signals from Identified Neurons and Feeding Behavior of Drosophila melanogaster

The fruit fly, Drosophila melanogaster, extends its proboscis for feeding, responding to a sugar stimulus from its proboscis or tarsus. I have combined observations of the proboscis extension response (PER) with a calcium imaging technique, allowing us to monitor the activity of neurons in the brain, simultaneously with behavioral observation.

To study neuronal networks in terms of their function in behavior, we must analyze how neurons operate when each behavioral pattern is generated.  Thus, ...

Neuroscience

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 ...

A Single-fly Assay for Foraging Behavior in Drosophila

For many animals, hunger promotes changes in the olfactory system in a manner that facilitates the search for appropriate food sources. In this video article, we describe an automated assay to measure the effect of hunger or satiety on olfactory dependent food search behavior in the adult fruit fly Drosophila melanogaster. In a light-tight box illuminated by red light that is invisible to fruit flies, a camera linked to custom data acquisition software monitors the position of six flies simultaneously. Each fly is confined to walk in individual arenas containing a food odor at the center. The testing arenas rest on a porous floor that functions to prevent odor accumulation. Latency to locate the odor source, a metric that reflects olfactory sensitivity under different physiological states, is determined by software analysis. Here, we discuss the critical mechanics of running this behavioral paradigm and cover specific issues regarding fly loading, odor contamination, assay temperature, data quality, and statistical analysis.

A Single-fly Assay for Foraging Behavior in Drosophila

In this video article, we describe an automated assay to measure the effect of hunger or satiety on olfactory dependent food search behavior in the adult fruit fly Drosophila melanogaster.

For many animals, hunger promotes changes in the olfactory system in a manner that facilitates the search for appropriate food sources. In this video ...

A High Throughput Microplate Feeder Assay for Quantification of Consumption in Drosophila

Quantifying food intake in Drosophila is used to study the genetic and physiological underpinnings of consumption-associated traits, their environmental factors, and the toxicological and pharmacological effects of numerous substances. Few methods currently implemented are amenable to high throughput measurement. The Microplate Feeder Assay (MFA) was developed for quantifying the consumption of liquid food for individual flies using absorbance. In this assay, flies consume liquid food medium from select wells of a 1536-well microplate. By incorporating a dilute tracer dye into the liquid food medium and loading a known volume into each well, absorbance measurements of the well acquired before and after consumption reflect the resulting change in volume (i.e., volume consumed). To enable high throughput analysis with this method, a 3D-printed coupler was designed that allows flies to be sorted individually into 96-well microplates. This device precisely orients 96- and 1536-well microplates to give each fly access to up to 4 wells for consumption, thus enabling food preference quantification in addition to regular consumption. Furthermore, the device has barrier strips that toggle between open and closed positions to allow for controlled containment and release of a column of samples at a time. This method enables high throughput measurements of consumption of aqueous solutions by many flies simultaneously. It also has the potential to be adapted to other insects and to screen consumption of nutrients, toxins, or pharmaceuticals.

A High Throughput Microplate Feeder Assay for Quantification of Consumption in Drosophila

The microplate feeder assay offers an economical, high throughput method for quantifying liquid food consumption in Drosophila. A 3D-printed device connects a 96-well microplate in which flies are housed to a 1536-well microplate from which flies consume a feeding solution with a tracer dye. The solution volume decline is measured spectrophotometrically.

Quantifying food intake in Drosophila is used to study the genetic and physiological underpinnings of consumption-associated traits, their environmental ...

A Simple Way to Measure Alterations in Reward-seeking Behavior Using Drosophila melanogaster

We describe a protocol for measuring ethanol self-administration in fruit flies (Drosophila melanogaster) as a proxy for changes in reward states. We demonstrate a simple way to tap into the fly reward system, modify experiences related to natural reward, and use voluntary ethanol consumption as a measure for changes in reward states. The approach serves as a relevant tool to study the neurons and genes that play a role in experience-mediated changes of internal state. The protocol is composed of two discrete parts: exposing the flies to rewarding and nonrewarding experiences, and assaying voluntary ethanol consumption as a measure of the motivation to obtain a drug reward. The two parts can be used independently to induce the modulation of experience as an initial step for further downstream assays or as an independent two-choice feeding assay, respectively. The protocol does not require a complicated setup and can therefore be applied in any laboratory with basic fly culture tools.

A Simple Way to Measure Alterations in Reward-seeking Behavior Using Drosophila melanogaster

We describe a protocol for inducing rewarding and nonrewarding experiences in fruit flies (Drosophila melanogaster) using voluntary ethanol consumption as a measure for changes in reward states.

We describe a protocol for measuring ethanol self-administration in fruit flies (Drosophila melanogaster) as a proxy for changes in reward states. We ...

The CApillary FEeder Assay Measures Food Intake in Drosophila melanogaster

For most animals, feeding is an essential behavior for securing survival, and it influences development, locomotion, health and reproduction. Ingestion of the right type and quantity of food therefore has a major influence on quality of life. Research on feeding behavior focuses on the underlying processes that ensure actual feeding and unravels the role of factors regulating internal energy homeostasis and the neuronal bases of decision-making. The model organism Drosophila melanogaster, with its great variety of genetically traceable tools for labeling and manipulating single neurons, allows mapping of neuronal networks and identification of molecular signaling cascades involved in the regulation of food intake. This report demonstrates the CApillary FEeder assay (CAFE) and shows how to measure food intake in a group of flies for time spans ranging from hours to days. This easy-to-use assay consists of glass capillaries filled with liquid food that flies can freely access and feed on. Food consumption in the assay is accurately determined using simple measurement tools. Herein we describe step-by-step the method from setup to successful execution of the CAFE assay, and provide practical examples to analyze the food intake of a group of flies under controlled conditions. The reader is guided through possible limitations of the assay, and advantages and disadvantages of the method compared to other feeding assays in D. melanogaster are evaluated.

The CApillary FEeder Assay Measures Food Intake in Drosophila melanogaster

The CApillary FEeder (CAFE) assay is a simple, budget-friendly, highly reliable method for investigating mechanisms underlying food intake. Used with the highly versatile genetic model organism Drosophila melanogaster, it provides a powerful means of gaining new insights into regulatory mechanisms of food intake.

For most animals, feeding is an essential behavior for securing survival, and it influences development, locomotion, health and reproduction. Ingestion of ...

Quantification of Macronutrients Intake in a Thermogenetic Neuronal Screen using Drosophila Larvae

Foraging and feeding behaviors allow animals to access sources of energy and nutrients essential for their development, health, and fitness. Investigating the neuronal regulation of these behaviors is essential for the understanding of the physiological and molecular mechanisms underlying nutritional homeostasis. The use of genetically tractable animal models such as worms, flies, and fish greatly facilitates these types of studies. In the last decade, the fruit fly Drosophila melanogaster has been used as a powerful animal model by neurobiologists investigating the neuronal control of feeding and foraging behaviors. While undoubtedly valuable, most studies examine adult flies. Here, we describe a protocol that takes advantage of the simpler larval nervous system to investigate neuronal substrates controlling feeding behaviors when larvae are exposed to diets differing in their protein and carbohydrates content. Our methods are based on a quantitative colorimetric no-choice feeding assay, performed in the context of a neuronal thermogenetic-activation screen. As a read-out, the amount of food eaten by larvae over a 1 h interval was used when exposed to one of the three dye-labeled diets that differ in their protein to carbohydrates (P:C) ratios. The efficacy of this protocol is demonstrated in the context of a neurogenetic screen in larval Drosophila, by identifying candidate neuronal populations regulating the amount of food eaten in diets of different macronutrient quality. We were also able to classify and group the genotypes tested into phenotypic classes. Besides a brief review of the currently available methods in the literature, the advantages and limitations of these methods are discussed and, also, some suggestions are provided about how this protocol might be adapted to other specific experiments.

Quantification of Macronutrients Intake in a Thermogenetic Neuronal Screen using Drosophila Larvae

Described here is a protocol that enables the colorimetric quantification of the amount of food eaten within a defined interval of time by Drosophila melanogaster larvae exposed to diets of different macronutrient quality. These assays are conducted in the context of a neuronal thermogenetic screen.

Foraging and feeding behaviors allow animals to access sources of energy and nutrients essential for their development, health, and fitness. Investigating ...

A Rapid Food-Preference Assay in Drosophila

To select food with nutritional value while avoiding the consumption of harmful agents, animals need a sophisticated and robust taste system to evaluate their food environment. The fruit fly, Drosophila melanogaster, is a genetically tractable model organism that is widely used to decipher the molecular, cellular, and neural underpinnings of food preference. To analyze fly food preference, a robust feeding method is needed. Described here is a two-choice feeding assay, which is rigorous, cost-saving, and fast. The assay is Petri-dish-based and involves the addition of two different foods supplemented with blue or red dye to the two halves of the dish. Then, ~70 prestarved, 2-4-day-old flies are placed in the dish and&#160;allowed to choose between blue and red foods in the dark for about 90 min. Examination of the abdomen of each fly is followed by the calculation of the preference index. In contrast to multiwell plates, each Petri dish takes only ~20 s to fill and saves time and effort. This feeding assay can be employed to quickly determine whether flies like or dislike a particular food.

A Rapid Food-Preference Assay in Drosophila

We present a protocol for a two-choice feeding assay for flies. This feeding assay is fast and easy to run and is suitable not only for small-scale laboratory research, but also for high-throughput behavioral screens in flies.

To select food with nutritional value while avoiding the consumption of harmful agents, animals need a sophisticated and robust taste system to evaluate ...

In Vivo Calcium Imaging of Taste-Induced Neural Responses in Adult Drosophila

For nearly two decades, in vivo calcium imaging has been an effective method for measuring cellular responses to taste stimuli in the fruit fly model organism, Drosophila melanogaster. A key strength of this methodology is its ability to record taste-induced neural responses in awake animals without the need for anesthesia. This approach employs binary expression systems (e.g., Gal4-UAS) to express the calcium indicator GCaMP in specific neurons of interest. This protocol describes a procedure in which flies expressing GCaMP are mounted with the labellum securely positioned, enabling fluorescence in the brain to be recorded at millisecond resolution under a confocal microscope while a solution is applied to the labellum, stimulating all labellar taste sensilla. The examples provided focus on calcium responses in primary gustatory receptor neurons of D. melanogaster. However, this approach can be adapted to record from other neurons of interest within the brain of Drosophilids or other insect species. This imaging method enables researchers to simultaneously record collective calcium responses from groups of gustatory neurons across the labellum, complementing electrophysiological tip recordings that quantify action potentials from individual neurons. The in vivo calcium imaging technique outlined here has been instrumental in uncovering molecular and cellular mechanisms of chemosensation, identifying unique temporal response patterns in primary taste neurons, investigating mechanisms of gustatory modulation, and exploring taste processing in downstream circuits.

A calcium imaging approach enables the recording of taste-induced responses in the brains of awake flies while a solution is applied to the labellum. Primary gustatory responses in Drosophila melanogaster are used as an example, but this protocol can be adapted to study downstream neurons or other species.

For nearly two decades, in vivo calcium imaging has been an effective method for measuring cellular responses to taste stimuli in the fruit fly model ...

Taste Preference Assay: A Method for Measuring Feeding Behavior in Drosophila

Source: Bantel, A. P. and Tessier, C. R. <a href="https://www.jove.com/video/54403/" target="_blank">Taste Preference Assay for Adult Drosophila</a>. J. Vis. Exp. (2016).
This video describes the taste preference assay, a behavioral method used to measure attraction or avoidance towards colored solutions that taste differently by assessing the fly&#39;s abdominal coloration after ingestion of the preferred substance. The featured protocol demonstrates the procedure used to measure flies&#39; preference towards solutions of varying sucrose concentrations.

Source: Bantel, A. P. and Tessier, C. R. Taste Preference Assay for Adult Drosophila. J. Vis. Exp. (2016).
This video describes the taste preference ...

Combining Quantitative Food-intake Assays and Forcibly Activating Neurons to Study Appetite in Drosophila

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