A little nervous system with big learning potential

From the Singhvi Lab, Basic Sciences Division

The human brain is an amazing thing. Maybe the most amazing thing. Tens of billions of neurons and glia, intricately organized, working in harmony to allow you to think, to create, to learn and remember and understand, not to mention the mundane tasks like breathing that keep us alive. It’s so complex, no one really understands how it does what it does. In fact, it’s so complex that we may never fully understand it. On the other end of the complexity spectrum is the brain of the nematode worm C. elegans. The human nervous system, by our best estimates, contains 86 billion neurons. The C. elegans nervous system has 302 neurons. Now that’s a nervous system simple enough to really wrap our heads around. But what does a worm do, really? They wriggle forward, they wriggle backward. What are they going to teach us about the complex workings of the human brain? As it turns out, packed into those 302 neurons is an astounding set of functions. They coordinate complex behaviors like feeding and mating. They regulate diverse behavioral responses to environmental stimuli. They even possess the rudimentary ability to do something we consider much more human-like: to learn from experience, remember, and adjust their future behavior in response. With this model, we have the ability to dissect the neurological basis of learning and memory in a simple nervous system. Maybe this little worm has a lot to teach us after all.

Thermotaxis – directed movement along a temperature gradient – provides a powerful model of learning in C. elegans. “Thermotaxis [within the physiological temperature range] is not an innately hard-wired behavior…rather, the animal’s thermotaxis memory and preference exhibits experience-dependent plasticity on a timescale of hours, based on the animal’s cultivation conditions,” write worm neuroscientists Dr. Aakanksha Singhvi, Assistant Professor in Fred Hutch’s Basic Sciences Division, Dr. Mason Klein, Assistant Professor at the University of Miami, and Singhvi lab member Dr. Stephan Raiders in a new article published in bio-protocol. In layman’s terms, C. elegans will remember the temperature at which they lived in the past, and, if placed on a temperature gradient, will return to that temperature, presumably following the logic that “if that temperature proved a good feeding ground for me before – and the fact that I’m still alive suggests that it did – then it probably will again.”

Some of the neurons and glia that regulate thermotaxis have been identified, but how they encode temperature memory and use that information to regulate behavior is less clear. The challenge with studying behavior as a phenotype is that it can be quite variable, and even subtle changes can be quite important. Thus, a sensitive and reliable behavioral assay is needed to understand how experimental manipulations of these neurons affect thermotaxis behavior.

Measuring thermotaxis behavior is a particular challenge, explains Dr. Singhvi. “C. elegans has one of the most sensitive temperature sensors in the world…[this] means that measuring temperature behaviors in the animal are tricky, because the slightest changes in how we raise the animals can impact their behavior responses.” Indeed, the field has a history of conflicting results that may be due to such slight differences. In the current work, the group set out to design a more consistent and reproducible thermotaxis assay.

First, says Dr. Singhvi, “we built a state-of-the-art ThermMax environmental room which allows us to perform our assays under tightly controlled humidity and temperature. This is key for our animal behavior assays. If not, the exquisite sensitivity of the neuron means that the animal behaviors would otherwise fluctuate drastically with time of day, or seasons.” They then designed an assay device to generate a stable, 1-dimensional temperature gradient. This consists of a flat metal plate, onto which the worms can be placed, attached on each side to independently controlled heating/cooling units used to create, monitor, and maintain the gradient, and a port for an overhead camera to record and automatically track worm behavior. Beyond its ability to generate a precise temperature gradient, a major advantage of this device over previous assay devices is that it was designed to simultaneously measure the behavior of up to three groups of worms. This is particularly important, the authors note, because even if you can’t ensure a perfectly stable gradient, by placing control worms and experimental worms on the plate at the same time and measuring their behavior in tandem you can ensure that they are experiencing the exact same conditions. Ultimately, says Dr. Singhvi, this “allows us to be more confident in our conclusions, and also allows higher throughput.” With a reliable assay as their foundation, it’s now time for the group to continue the hard work of dissecting the basis of learning and memory in these remarkable little worms.

The nematode worm C. elegans. Image from Maria Gallegos

This work was supported by the National Institutes of Health, The Glenn Foundation for Medical Research, the American Federation for Aging Research, The Simons Foundation, the Anderson Foundation, and the Marco J. Heidner Foundation.

Raiders S, Klein M, Singhvi A. 2022. Multiplexing Thermotaxis Behavior Measurement in Caenorhabditis elegans. Bio Protoc. 12(7):e4370.