After an hour of neurons, patterns and rhythms, Piali Sengupta talked about behavior in probably the most accessible nuroethological model system, the nematode worm Caenorhabditis elegans. She started out by detailing the kind of temperature-sensitive ion channels. Of course one of these channels is the famous capsaicin receptor which reacts both to heat and to the main component of chili peppers. Even though many such channels are known covering various temperature ranges, it is not known how they are expressed in sensory neurons and how temperature is coded in the brain. Detailing a series of very elegant experiments, Piali showed us some of the neurobiological mechanisms underlying thermosensory behavior in the worm.
In the soil nematode C. elegans, thermotactic behavior depends on a memory of the previous cultivation temperature and perception of the ambient temperature. Worms crawl around by undulating forward motion interspersed by sharp turns rather than crawling in smooth curves. These two components were used to quantify the worm's thermotactic behavior. In what is surprisingly similar to the biased random walk movements of E. coli, the worms increase turning frequency under unfavorable conditions with apparently a random turn direction, leading to just as effective a thermotactic behavior as the tumbling behavior in E. coli leads to chemotaxis in the bacterium.
One of the many amazing features of C. elegans is that every single neuron is identified and its connectivity completely mapped out. Starting at sensory neurons, one can thus start to identify the circuitry mediating thermotaxis and manipulate it to understand how it works. In order to study the circuitry, Piali's lab uses optophysiology, i.e., imaging of neuronal activity (in this case a FRET-based method involving calcium sensors). The first result she showed was that the temperature-sensitive AFD sensory neurons in the animal show a sharp activity threshold which lies exactly at the cultivation temperature. Thermosensation in the AFD neurons occurs in the sensory endings of the neurons which project to the very anterior tip of the animal. The AFD neurons synapse onto downstream AIY neurons which (maybe not entirely surprisingly) exhibit similar activity patterns as the AFD sensory neurons.
The next behavior she analyzed was thermal tracking. Thermal tracking refers to a behavior where the animals move within an isotherm in a temperature gradient. This entails actively suppressing turning behavior and only move straight. The AFD sensory neurons are sensitive enough to sense the tiny temperature differences necessary for tracking. Piali generated expression profiles of the AFD neurons to be able to specifically knock out certain genes in the AFD neurons to understand their role in tracking behavior. Diacyl glycerol kinase dgk-3 converts DAG (diacyl glycerol) into PA (phosphatidic acid) and when you knock the gene out the mutant animals have only a very subtle phenotype. When shifting cultivation temperature, the worms temperature preference changes more slowly than in wildtype animals. This phenotype is rescued by expressing the dgk-3 gene only in the AFD neurons. Gain of function mutations of dgk-3 exhibit the opposite phenotype. As could be expected, the behavioral phenotypes are paralleled in the dynamics of the activity threshold in the AFD neurons.
It is currently not known which molecular processes lead to the AFD neurons resetting their activity threshold to changes in cultivation temperature.
How is turning suppressed in thermal tracking? Apparently the AFD neurons inhibit turning via the downstream AIY neurons. Turning frequency then is regulated by antagonistic interactions on the AIY neuron level with another set of temperature sensitive neurons, the olfactory AWC neurons which inhibit the AIY neurons. Activity in the AWC neurons stops the AIY neurons from inhibiting turning and turning frequency goes up. Besides inhibiting the AIY neurons, the AWC neurons also activate AIB neurons which promote turning behavior. The AIB neurons are also downstream of the AFD neurons but by reciprocal inhibitory synapses.