The nematode (worm) C. elegans is one of the widely studied animal model organisms in biology. It develops through 4 larval stages (L1-L4) in 2 to 3 days before becoming a young adult. Biological assays involving C. elegans frequently require a large number of animals that are appropriately staged and exhibit a similar behaviour. We have developed a new method to synchronize animals that relies on the electrotactic response (electric field-induced motion) of C. elegans to sort them in parallel based on their age, size and phenotype. By using local electric field traps in a microfluidic device, we can efficiently sort worms from a mixed culture in a semi-continuous flow manner (with a minimum throughput of 78 worms per minute per load-run) and obtain synchronized populations of animals. In addition to sorting larvae, our device can also distinguish between young and old adults efficiently. Unlike fluorescent based sorting systems that use active imaging based feedback, this method is passive and automatic and uses the innate behaviour of the worm. Considering that the entire procedure takes only a few minutes to run and is cost-effective, it promises to simplify and accelerate experiments requiring homogeneous cultures of worms as well as to facilitate isolation of mutants that have abnormal electrotaxis. More importantly, our method of isolating and separating worms using locomotion as a defining characteristic promises development of advanced microfluidics-based systems to study the neuronal basis of movement-related defects in worms and facilitate high-throughput chemical screening and drug discovery.

The nematode (worm) C. elegans is a leading multicellular animal model to study neuronal-basis of behavior. Worms respond to a wide range of stimuli and exhibit characteristic movement patterns. Here we describe the use of a microfluidics setup to probe neuronal activity that relies on the innate response of C. elegans to swim toward the cathode in the presence of a DC electric field (termed “electrotaxis”). Using this setup, we examined mutants affecting sensory and dopaminergic neurons and found that their electrotactic responses were defective. Such animals moved with reduced speed (35–80% slower than controls) with intermittent pauses, abnormal turning and slower body bends. A similar phenotype was observed in worms treated with neurotoxins 6-OHDA (6- hydroxy dopamine), MPTP (1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine) and rotenone (20–60% slower). We also found that neurotoxin effects could be suppressed by pre-exposing worms to a known neuroprotective compound acetaminophen. Collectively, these results show that microfluidic electrotaxis can identify alterations in dopamine and amphid neuronal signaling based on swimming responses of C. elegans. Further characterization has revealed that the electrotactic swimming response is highly sensitive and reliable in detecting neuronal abnormalities. Thus, our microfluidics setup could be used to dissect neuronal function and toxin-induced neurodegeneration. Among other applications, the setup promises to facilitate genetic and chemical screenings to identify factors that mediate neuronal signaling and neuroprotection.

The nematodes (worms) Caenorhabditiselegans and Caenorhabditisbriggsae are well-known model organisms to study the basis of animal development and behaviour. Their sinusoidal pattern of movement is highly stereotypic and serves as a tool to monitor defects in neurons and muscles that control movement. Until recently, a simple yet robust method to initiate movement response on-demand did not exist. We have found that the electrical stimulation in a microfluidic channel, using constant DC electric field, induces movement (termed electrotaxis) that is instantaneous, precise, sensitive, and fully penetrant. We have further characterized this behaviour and, in this paper, demonstrate that electrotaxis can also be induced using a pulse DC electric signal. Worms responded to pulse DC signals with as low as 30% duty cycle by moving towards the negative electrode at the same speed as constant DC fields (average speed of C. elegans = 296 ± 43 μm/s and C. briggsae = 356 ± 20 μm/s, for both constant and pulse DC electric fields with various frequencies). C. briggsae was found to be more sensitive to electric signals compared to C. elegans. We also investigated the turning response of worms to a change in the direction of constant and pulse DC signals. The response for constant DC signal was found to be instantaneous and similar for most worms. However, in the case of pulse DC signal, alterations in duty cycle affected the turning response time as well as the number of responding worms. Our findings show that pulse DC method allows quantitative measurement of response behaviour of worms and suggest that it could be used as a tool to study the neuronal basis of such a behaviour that is not observed under constant DC conditions.

The nematode Caenorhabditis elegans is a versatile model organism for biomedical research because of its conservation of disease-related genes and pathways as well as its ease of cultivation. Several C. elegans disease models have been reported, including neurodegenerative disorders such as Parkinson's disease (PD), which involves the degeneration of dopaminergic (DA) neurons 1 . Both transgenes and neurotoxic chemicals have been used to induce DA neurodegeneration and consequent movement defects in worms, allowing for investigations into the basis of neurodegeneration and screens for neuroprotective genes and compounds 2,3 . Screens in lower eukaryotes like C. elegans provide an efficient and economical means to identify compounds and genes affecting neuronal signaling. Conventional screens are typically performed manually and scored by visual inspection; consequently, they are time-consuming and prone to human errors. Additionally, most focus on cellular level analysis while ignoring locomotion, which is an especially important parameter for movement disorders.We have developed a novel microfluidic screening system (Figure 1) that controls and quantifies C. elegans' locomotion using electric field stimuli inside microchannels. We have shown that a Direct Current (DC) field can robustly induce on-demand locomotion towards the cathode ("electrotaxis") 4 . Reversing the field's polarity causes the worm to quickly reverse its direction as well. We have also shown that defects in dopaminergic and other sensory neurons alter the swimming response 5 . Therefore, abnormalities in neuronal signaling can be determined using locomotion as a read-out. The movement response can be accurately quantified using a range of parameters such as swimming speed, body bending frequency and reversal time.Our work has revealed that the electrotactic response varies with age. Specifically, young adults respond to a lower range of electric fields and move faster compared to larvae 4 . These findings led us to design a new microfluidic device to passively sort worms by age and phenotype 6 .We have also tested the response of worms to pulsed DC and Alternating Current (AC) electric fields. Pulsed DC fields of various duty cycles effectively generated electrotaxis in both C. elegans and its cousin C. briggsae Implementation of the electric field in a microfluidic environment enables rapid and automated execution of the electrotaxis assay. This approach promises to facilitate high-throughput genetic and chemical screens for factors affecting neuronal function and viability. Video LinkThe video component of this article can be found at

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