Ablation of a Neuronal Population Using a Two-photon Laser and Its Assessment Using Calcium Imaging and Behavioral Recording in Zebrafish Larvae

The overall goal of this experiment is to test the effect of laser ablation of certain neurons on circuit activity and behavior. With this experimental technique we can examine the necessity of neurons in a circuit underlying behavior. This method can help answer our key questions in the behavior and neuroscience field such as how prey is perceived and how prey captured behavior is generated by neural activity.

The main advantage of this technique is that neurons can be specifically ablated because of the genetic labeling of these neurons and also high spatial resolution of the laser. After raising larvae and mounting them in 2%low melting agarose according to the text protocol place the agarose embedded larvae under a two photon laser scanning microscope and start the microscopy system. Open the image acquisition software and under the laser tab click the on button to turn on the laser.

Wait for the status of the laser to change from busy to mode locked. On the channels tab set the laser's wavelength to 880 nanometers for EGFP ablation or 800 nanometers for GCaMP ablation. Select the 20X subjective lens by manually moving the lens revolver.

Then, click the locate tab, click GFP to change the optical pathways and directly view the fluorescence by eye. Center the zebra fish larval brain in the field of view, then click the acquisition tab to go back to two photon microscopy and set the laser power and gain. As a record of the before ablation condition click Z stack to select the Z stack option.

Then, after focusing on the ventral end of the larval brain click the set first button to set the lower limit and after focusing on the dorsal most surface of the brain click the set last button to set the upper limit. Finally, click the start experiment button to run the Z stack image acquisition. Next, select the 63X subjective lens by manually moving the lens revolver.

Then, click the locate tab to switch to epifluorescence microscopy. Center the cells to be ablated by eye then click the acquisition tab to go back to laser scanning microscopy. In the acquisition mode tab set the frame size to 256 by 256.

Click live and observe the neurons of interest. Then, starting from the dorsal most side find a focal plane in which the cells to be ablated are in focus. Using the regions function mark a small circular area about 1/3 the cell's size in diameter on each neuron as an ROI.

Set the scan speed to 13.9 seconds per 256 by 256 pixels which corresponds to a laser dwell time of approximately 200 microseconds per pixel or 200 microseconds per half micron. Also, set the iteration cycle to four repetitions. Click start experiment to execute the time series and bleaching functions.

Compare the image before ablation to the image after ablation in the time series cycle to ensure that the fluorescence in the targeted cells is gone. Next, move the focal plane slightly deeper and choose the next focal plane where unablated cells appear. Carryout the bleaching function as just demonstrated and repeat the process until all the cells in the neural structure of interest have been ablated.

Following ablation check the cells for abolished fluorescence and repeat the laser irradiation if necessary. Following irradiation the larvae needs to be recovered. Carefully make tiny perpendicular cuts in the agarose around the larvae.

Then, wait for the fish to swim out of the agarose on its own when the anesthetic wears off. After culturing paramecia and anesthetizing a zebra fish larva according to the text protocol place the larvae into 2%low melting point agarose and immediately remove any excess agarose so that the surface of the agarose looks slightly convex. Using a dissecting needle quickly orient the larvae in an upright position.

Then, using a surgical knife make a few small cuts in the agarose to remove the agarose around the zebra fish head which makes space for the paramecium to swim. Carefully remove the residual agarose by gently pouring system water on the chamber. Next, put one paramecium in the recording chamber to serve as a visual stimulus.

Then place the recording chamber under an epifluorescence microscope equipped with a scientific CMOS camera and select a 2.5X objective lens. Start the image acquisition application for the equipped camera. Click sequence pane and select hard disk record on the pane.

In the scan setting section set the frame count to 900 or any other total frame number desired. In the capture pane set the exposure time at 30 milliseconds and binning to two by two. Click live on the capture pane to locate the larvae in the camera view and focus and then click stop live and click the start button.

Save the movie in cxd format and analyze the data according to the text protocol. After setting up the behavioral recording system and placing a larva into a recording chamber according to the text protocol use a micro pipette to collect 50 paramecia and place them in the recording chamber. To place a cover slip on top of the recording chamber put a small drop of water on the cover slip before placing it on the water surface of the recording chamber to avoid introducing air in the chamber.

Place the recording chamber in the lighting system. Then, start the time lapse recording at 10 frames per second for 11 minutes. Finally, carryout analysis according to the text protocol.

In this experiment with the Gal4 line that labels nuclei in the pretectal area and a subpopulation of olfactory bulb neurons. The pretectal neurons were laser ablated bilaterally. Olfactory neurons were ablated as a control.

The results show that the two photon laser can ablate targeted cells while leaving adjacent neurites unaffected. In this experiment the functional connectivity of pretectal neurons that project their axons toward the ILH ipsilaterally were investigated. Both the pretectum and the ILH showed neuronal activity in the proximal presence of prey suggesting that both neuronal activities are visually driven.

Calcium imaging was also used to observe neuronal activity in the pretectum and the ILH in pretectal neuron ablated larvae. The left pretectum that was laser ablated showed residual to no neuronal activity. In addition, the ipsilateral ILH displayed dramatically displaced neuronal activity suggesting that the major input to the ILH comes from the ipsilateral pretectum.

In contrast the ILH on the other side of the laser ablated pretectum showed neuronal activity comparable to the neuronal activity in the ipsilateral pretectum which suggests that the right ILH is receiving inputs from the right pretectum. Once mastered laser ablation can be done in about one hour for each zebra fish larva. While attempting this procedure it's important to remember to ablate only the targeted neurons with proper laser irradiation avoiding heat damage by the laser.

Following this procedure ablation of neurons in other brain areas like forebrain or hindbrain can be performed in order to answer additions questions like how these neurons are involved in cognitive functions or motor functions. After its development this technique paved the way for researchers in the field of behavioral neuroscience to explore functional neural circuits in zebra fish models. After watching this video you should have a good understanding of how to specifically laser ablate neurons in the zebra fish larvae brain and assess the functional and behavioral consequences.

Don't forget that working with a laser can be extremely hazardous and always comply with the manufacturer's instructions while preforming this procedure.