The overall goal of this novel procedure is to inflict closed traumatic brain injuries in the fruit fly, Drosophila, to establish a new chronic traumatic encephalopathy model. This method can help answer key questions in the chronic traumatic encephalopathy field, such as how the repetitive trauma induces neuron degeneration. The main advantage of this technique is that it delivers the repetitive impacts of adjustable strengths to the fly head directly without causing damage to other parts of the animal body.
Real world demonstration of this method is critical as assembly of the device and fly loading steps are difficult to learn, because the Drosophila head is so small and it is important to apply the impact only to the head. To make the apparatus used to concuss or strike the fruit flies, first, make the impactor portion of the apparatus. Remove the plunger from a one milliliter polycarbonate tuberculin syringe and then cut the barrel at the one milliliter mark.
For an impactor, use the aerosol barrier found on a 200 microliter pipette tip. This impactor should be three millimeters long and four millimeters in diameter. Then, pack the impactor into the syringe barrel with the flat side towards the nozzle.
To provide force, attach the syringe barrel to a carbon dioxide flow regulator. The regulator must have an On Off toggle switch and good control of the gas flow rate. Then, clamp the barrel vertically to an iron stand, such that the impactor will fall to the bottom of the barrel.
Next, make the fly holder. Start with a 200 microliter pipette tip. Cut the pipette's tip five millimeters from the small end to make a 8 millimeter diameter opening.
A fly head will fit through this hole, but the thorax will not. Lastly, make a for the fly holder and impactor device. First, cut a one milliliter pipette tip 44 millimeters from the small opening.
Second, cut a six millimeter length from the needle cover for a one milliliter syringe. Then, push the two together. Anesthetize a single, two-day old adult female fly using carbon dioxide.
From the carbon dioxide pad, use a fine brush to gently transfer the fly into the holder. Then, gently tap the holder until the head protrudes from the tip. Of the proboscis is exposed, use a blunted, one milliliter syringe needle to gently tuck it back inside the tip.
It is critical to keep the fly body, especially the mouth part, inside the holder. Otherwise, the fly may die from damage to the internal organs or from not being able to ingest food. Next, set the gas pressure to 100 kilopascals and adjust the flow as needed.
Once the fly is loaded, tighten the holder to the syringe barrel using the connector, such that the fly head faces downward. Then, using the toggle switch, send a burst of gas that moves the impactor to strike the fly once and only once. Now, detach the fly holder and dump the fly back onto the carbon dioxide pad.
Brush the fly into an empty vial until it recovers. Keep one fly per vial. The recovery takes only a few minutes.
Repeat the process to test four flies per experimental group. Processing two groups, a test and a sham, should only take 20 to 30 minutes. Treat shams just like test animals, except do not include an aerosol barrier in the impactor tube.
To track fly movements in a Petri dish, first, fill a six centimeter dish with transparent silicon elastomer to make the tracking arena. Leave a three millimeter gap between the silicon and the lid so that flies can walk freely in the gap, but not take flight. Beneath the tracking arena, have a white background.
Next, anesthetize four flies from the same treatment group and place them in the arena. Let the flies acclimate for an hour to the arena, which should be fully lit and at about 22 degrees Celsius. Not hotter than room temperature.
After an hour, use a charge-coupled device camera positioned above the arena to record the fly's activity for five minutes. After the five minute recording, anesthetize the flies in the arena and return them to a new home vial. Then, dispose of the arena.
Later, analyze the recordings using the freely available Ctrax software. This software generates tracking data, which can be exported in a programmable language compatible format, such as the MATLAB format. From the data, calculate the distance traveled per frame, the mean walking distance for each fly, and the average distance traveled per fly.
For statistical significance, expect to test about 100 flies per treatment group. To establish a chronic traumatic encephalopathy model, the effectiveness of a single, closed-head injury made by the impacting apparatus at different speeds was analyzed using two-day old Canton-S females. The gas pressure was held at 100 kilopascals.
Flies exposed to a single strike at the highest tested flow rate, exhibited minimal external defects. Despite no clear evidence for external damage, injuries at a flow rate of 15 liters per minute were acutely lethal, resulting in less than a 10%survival within 24 hours. Survival increased with lower flow rates.
One hundred percent survival was achieved at five liter per minute or less, so five liters per minute was selected as the standard for the model. Just after being struck, flies gradually recovered their mobility within four minutes. Recovery for the shams was about a minute quicker.
Post-impact, video-assisted movement tracking was used to measure locomotive function. During the two days after the impact, locomotion slowly returned to normal. This evidence for gradual recovery is in line with CTE recovery in humans.
To evaluate long term effects, a repetitive traumatic brain injury protocol of five strikes over five days was applied. Twenty days post-treatment, the locomotion of surviving members in each group was compared and the treated group clearly walked less. The lifespan of the treated group was also substantially reduced, with a median at 26 days compared to 37 days for controls.
After its development, this technique paves the way for researchers in the field of mild repetitive traumatic brain injury to explore pathogenic mechanisms in the classic animal model Drosophila melanogaster. While attempting this procedure, it is important to remember to shield the proboscis, thorax and abdomen from direct impact by the aerosol barrier.
