Brain circuits endow behavioral flexibility. Here, we study circuits encoding flexible chemotaxis in C. elegans, where the animal navigates up or down NaCl gradients (positive or negative chemotaxis) to reach the salt concentration of previous growth (the setpoint). The ASER sensory neuron mediates positive and negative chemotaxis by regulating the frequency and direction of reorientation movements in response to salt gradients. Both salt gradients and setpoint memory are encoded in ASER temporal activity patterns. Distinct temporal activity patterns in interneurons immediately downstream of ASER encode chemotactic movement decisions. Different interneuron combinations regulate positive vs. negative chemotaxis. We conclude that sensorimotor pathways are segregated immediately after the primary sensory neuron in the chemotaxis circuit, and sensory representation is rapidly transformed to motor representation at the first interneuron layer. Our study reveals compact encoding of perception, memory, and locomotion in an experience-dependent navigational behavior in C. elegans.

Nitric oxide (NO) is released into the air by NO-producing organisms; however, it is unclear if animals utilize NO as a sensory cue. We show that C. elegans avoids Pseudomonas aeruginosa (PA14) in part by detecting PA14-produced NO. PA14 mutants deficient for NO production fail to elicit avoidance and NO donors repel worms. PA14 and NO avoidance are mediated by a chemosensory neuron (ASJ) and these responses require receptor guanylate cyclases and cyclic nucleotide gated ion channels. ASJ exhibits calcium increases at both the onset and removal of NO. These NO-evoked ON and OFF calcium transients are affected by a redox sensing protein, TRX-1/thioredoxin. TRX-1’s trans-nitrosylation activity inhibits the ON transient whereas TRX-1’s de-nitrosylation activity promotes the OFF transient. Thus, C. elegans exploits bacterially produced NO as a cue to mediate avoidance and TRX-1 endows ASJ with a bi-phasic response to NO exposure.

This review discusses the potential usefulness of several selected polypeptide growth factors as treatments for stroke. Distinctions between global vs. focal cerebral ischemia, permanent vs. temporary focal ischemia, and acute stroke vs. stroke recovery are first discussed. Potential routes of administration of growth factors are also considered. The growth factors basic fibroblast growth factor (bFGF), osteogenic protein-1 (OP-1), vascular endothelial growth factor (Veg-f), erythropoietin (EPO), and granulocyte colony stimulating factor (G-CSF) all show potential usefulness in animal models of acute stroke and stroke recovery. Two of these factors, bFGF and EPO, have reached human clinical trials for acute stroke, and the data are discussed. Future directions in this field are also discussed.

Summary An animal’s survival strongly depends on a nervous system that can rapidly process and integrate the changing quality of its environment and promote the most appropriate physiological responses. This is amply demonstrated in the nematode worm Caenorhabditis elegans, where its sensory system has been shown to impact multiple physiological traits that range from behavior and developmental plasticity to longevity. Because of the accessibility of its nervous system and the number of tools available to study and manipulate its neural circuitry, C. elegans has thus become an important model organism in dissecting the mechanisms through which the nervous system promotes survival. Here we review our current understanding of how the C. elegans sensory system affects diverse physiological traits, whose coordination would be essential for survival under fluctuating environments. The knowledge we derive from the C. elegans studies should provide testable hypotheses in discovering similar mechanisms in higher animals.