Caenorhabditis elegans as a model to study bitter taste: identification of neurons and molecules involved in detection and avoidance of repellent stimuli

Detection of water-soluble substances present in the environment is commonly defined as taste and is crucial for animal's survival. In the nematode Caenorhabditis elegans repellents trigger an avoidance reaction, consisting in a reversal of the forward movement. We developed a quantitative single worm assay for this behavior and showed that many repellents, which induce an avoidance reaction in the nematode, are toxic to animals, are recognized as bitter by humans and are discarded in double choice assays by other vertebrates. We thus believe that avoidance of repellents by C. elegans is a useful model to study the molecules and the cellular mechanisms underlying bitter taste.
Laser ablations of specific neurons and a genetic cell rescue strategy were used to identify in C. elegans the sensory neurons involved in the detection of repellent stimuli. ASH is the main sensory neuron for the detection of quinine and of a variety of other aversive stimuli. Other sensory neurons, in the head and in the tail, are also involved with a modulating role. Our results show that chemical avoidance and positive chemotaxis use different strategies. Avoidance is based on spatial integration of signals from the head and the tail, is extremely rapid but is poorly accurate in localizing the source of stimuli. Positive chemotaxis is based instead on temporal integration of stimuli (only from the head) and, albeit slower, can be extremely accurate, Hilliard MA, Bargmann CI, Bazzicalupo P., Curr Biol., 12:730-4. (2002).
Behavioral responses are mediated by the combined function of several neurons. To understand the functioning of a single neuron we expressed Camaleon, a genetically encoded calcium indicator, in ASH and analyzed the Ca++ fluxes specifically within ASH, after stimulation of live worms with various repellents. We found that ASH is able to maintain high levels of Ca++ when a persistent repellent stimulus is applied, suggesting that it responds to the presence of the repellent rather than to a change in its concentration. We also showed that the adaptation we observed in the avoidance responses to certain repellents, is due at least in part to adaptation occurring within the sensory neuron ASH.
Using our behavioral assay and the calcium imaging approach we identified some of the molecules that, inside the sensory cells, are necessary for sensing quinine (and/or other repellents). Two G protein alpha subunits expressed in ASH, GPA-3 and ODR-3, are necessary for the response to quinine. If both are missing there is no response to quinine and to some other repellent stimuli. In addtion we identified and cloned a new gene, qui-1, necessary for quinine and SDS avoidance. qui-1 codes for a novel WD-40-repeats-containing-protein expressed in the avoidance sensory neurons ASH and ADL. QUI-1 orthologs are present in vertebrates including mammals. Their function is at present unknown but understanding the function of QUI-1 in C. elegans may reveal a new, conserved player in the signaling pathway of G protein coupled receptors, Hilliard MA, Bergamasco C, Arbucci S, Plasterk RH, Bazzicalupo P., EMBO J., 23:1101-1111(2004).