Electrotaxis Assay: A Method to Observe Locomotion in C. elegans

- Without sensory stimuli, C. elegans movement is generally forward and is interrupted by reversals and turns-- random reorienting maneuvers that result in a change of direction. C. elegans are electroreceptive. That is, they can sense electric fields. When exposed to a mild direct current or DC, worms move in a straight uninterrupted path in the direction of the negative electrode or cathode, a behavior termed electrotaxis.

To more accurately observe the worm's locomotion, place the animal in a microfluidic device and apply a DC electric field. Reverse the direction of the field, and the worm will turn the opposite way-- towards the direction of the cathode. Apply an alternating current, AC, and the worm will stop. The alternating change in the field's direction overwhelms the worm's neuromotor systems, resulting in the worm's immobilization. Reapply the DC field, and the worm will resume forward motion towards the cathode.

In the following example protocol, we will see a setup demonstrating an electrotaxis assay in a microfluidic device.

- Begin this step by placing the just assembled micro-channel onto an xy- movable stage of a microscope with a mounted camera connected to a monitor. Connect the power supply to the micro-channel's electrodes.

After confirming that the resistance of the micro-channel is around 0.6 mega-ohms, attach the output tube of the micro-channel to a disposable syringe. Then submerge the mouth of the inlet tube into a solution of nematodes suspended in M9 physiological buffer. Apply negative pressure inside the syringe to aspirate liquid into the channel.

When the inlet and outlet tubes are both filled, disconnect the syringe and hydro-statically manipulate the flow by adjusting the tube's relative height to place a worm in the center of the channel. Then lay both tubes flat at the same elevation. To maintain zero flow inside the micro-channel when switching worm samples during an electrotaxis experiment, after leveling both the inlet tubes to the same elevation, if there is still flow make small adjustments to the tube's height relative to each other.

- If we are ever unsure of whether the flow is really zero, we can induce a reversal in the worm's movement by changing the polarity of the electric field and then evaluating the worm's linear velocity as it turns around.

- Now, set the power supply to the appropriate voltage. Activate the electric signal, and allow one minute of pre-exposure for the worm to acclimatize to the field, during which time the worm should begin moving towards the cathode. When the minute has passed, begin recording.

When the experiment is finished, remove all the liquid and worms from the channel. Rinse the chamber with de-ionized water and leave the device on a hot plate at 125 degrees Celsius to dry.