The phasmids are bilateral sensory organs located in the tail of Caenorhabditis elegans and other nematodes. The similar structures of the phasmids and the amphid chemosensory organs in the head have long suggested a chemosensory function for the phasmids. However, the PHA and PHB phasmid neurons are not required for chemotaxis or for dauer formation, and no direct proof of a chemosensory function of the phasmids has been obtained. C. elegans avoids toxic chemicals by reversing its movement, and this behavior is mediated by sensory neurons of the amphid, particularly, the ASH neurons. Here we show that the PHA and PHB phasmid neurons function as chemosensory cells that negatively modulate reversals to repellents. The antagonistic activity of head and tail sensory neurons is integrated to generate appropriate escape behaviors: detection of a repellent by head neurons mediates reversals, which are suppressed by antagonistic inputs from tail neurons. Our results suggest that C. elegans senses repellents by defining a head-to-tail spatial map of the chemical environment.

ASH sensory neurons are required in Caenorhabditis elegans for a wide range of avoidance behaviors in response to chemical repellents, high osmotic solutions and nose touch. The ASH neurons are therefore hypothesized to be polymodal nociceptive neurons. To understand the nature of polymodal sensory response and adaptation at the cellular level, we expressed the calcium indicator protein cameleon in ASH and analyzed intracellular Ca 2 þ responses following stimulation with chemical repellents, osmotic shock and nose touch. We found that a variety of noxious stimuli evoked strong responses in ASH including quinine, denatonium, detergents, heavy metals, both hyper-and hypo-osmotic shock and nose touch. We observed that repeated chemical stimulation led to a reversible reduction in the magnitude of the sensory response, indicating that adaptation occurs within the ASH sensory neuron. A key component of ASH adaptation is GPC-1, a G-protein c-subunit expressed specifically in chemosensory neurons. We hypothesize that G-protein c-subunit heterogeneity provides a mechanism for repellent-specific adaptation, which could facilitate discrimination of a variety of repellents by these polymodal sensory neurons.

An animal's ability to detect and avoid toxic compounds in the environment is crucial for survival. We show that the nematode Caenorhabditis elegans avoids many water-soluble substances that are toxic and that taste bitter to humans. We have used laser ablation and a genetic cell rescue strategy to identify sensory neurons involved in the avoidance of the bitter substance quinine, and found that ASH, a polymodal nociceptive neuron that senses many aversive stimuli, is the principal player in this response. Two G protein alpha subunits GPA-3 and ODR-3, expressed in ASH and in different, nonoverlapping sets of sensory neurons, are necessary for the response to quinine, although the effect of odr-3 can only be appreciated in the absence of gpa-3. We identified and cloned a new gene, qui-1, necessary for quinine and SDS avoidance. qui-1 codes for a novel protein with WD-40 domains and which is expressed in the avoidance sensory neurons ASH and ADL.

A thorough understanding of nerve regeneration in Caenorhabditis elegans requires performing femtosecond laser nanoaxotomy while minimally affecting the worm. We present a microfluidic device that fulfills such criteria and can easily be automated to enable high-throughput genetic and pharmacological screenings. Using the 'nanoaxotomy' chip, we discovered that axonal regeneration occurs much faster than previously described and surprisingly the distal fragment of the severed axon regrows in the absence of anesthetics.The understanding of the biological mechanisms of nerve regeneration and degeneration after injury holds the key to developing novel therapies for human neurodegenerative diseases. These processes can be studied in model organisms by severing axons in a controlled manner, and then observing their regrowth and functional recovery. The ideal predisposition of the nematode Caenorhabditis elegans for such studies recently became accessible by the demonstration of precise nanoaxotomy using ultrafast laser pulses 1 . However, the side effects that the chemicals used to immobilize the worms for laser nanoaxotomy might have on nerve regeneration are difficult to evaluate, unless nanosurgery can be performed in vivo without anesthetics. The environment in which both surgery and monitoring are performed should have a minimal impact on the studied organism and its biological processes. To achieve this goal, we designed a microfluidic device that allows us to sever axons in C. elegans using ultrashort laser pulses with the same high precision as we demonstrated previously 1,2 while monitoring the subsequent axonal regeneration activity.Several microfluidic devices and microelectromechanical systems (MEMS) have recently been developed for C. elegans including, Petri dish-based microfluidics 3 , microfluidic traps 4,5,6 , the "CD-worm" 7 , a shadow imaging platform 8 , microfluidic maze structures 6,9 , a cantilever force MEMS sensor 10 and a platform to capture and sort worms 11 . However no demonstration of nanosurgery on-a-chip has been reported so far.Correspondence should be addressed to A.B. (ben-yakar@mail.utexas.edu The integrated microfluidic device we designed presents several unique features that are critical for the success of in vivo nerve regeneration studies: the worms are held directly against the glass cover for ideal focusing and precise nanosurgery; the trap is adjustable to the size of the worms allowing immobilization of worms at various developmental stages (L4 to adult size); and the system integrates feeding modules and thus allows long term follow-up studies of the axotomized worms as well as their sorting and screening.The high-throughput microfluidic system integrates two separate modules (Fig. 1a), a trapping module for nanosurgery and time-lapse imaging ( Fig. 1c-e) and a feeding module for recovery of the operated worms (Fig. 1b). Follow-up imaging of injured axons and their regrowth is performed using the same trapping module. Depending on the outcome of the imaging session,...

A set of conserved molecules guides axons along the metazoan dorsal-ventral axis. Recently, Wnt glycoproteins have been shown to guide axons along the anterior-posterior (A/P) axis of the mammalian spinal cord. Here, we show that, in the nematode Caenorhabditis elegans, multiple Wnts and Frizzled receptors regulate the anterior migrations of neurons and growth cones. Three Wnts are expressed in the tail, and at least one of these, EGL-20, functions as a repellent. We show that the MIG-1 Frizzled receptor acts in the neurons and growth cones to promote their migrations and provide genetic evidence that the Frizzleds MIG-1 and MOM-5 mediate the repulsive effects of EGL-20. While these receptors mediate the effects of EGL-20, we find that the Frizzled receptor LIN-17 can antagonize MIG-1 signaling. Our results indicate that Wnts play a key role in A/P guidance in C. elegans and employ distinct mechanisms to regulate different migrations.