One Neuron Remembering a Memory

Most people prefer sitting in a room with AC than sitting in a hot field. 

Research scientist Josh Hawk and his team study the neural mechanisms behind remembering and acting upon temperature preference memories like these, except they study this in a worm called Caenorhabditis elegans. Working in Professor Daniel Colón-Ramos’s lab, they recently found one neuron in C. elegans that both adapts to sense temperature better in different environments and changes its output based on experience to convert the worm’s temperature preference to behavior.    

C. elegans show they remember their preferred temperature by migrating towards it when placed on a temperature gradient. Past research had found that one of C. elegans’ neurons, called AFD, senses the temperature changes of the surrounding environment. Researchers knew that AFD is active only above a certain temperature set by experience, but they were uncertain about how that temperature threshold contributed to temperature memory. 

Hawk’s work has shown that the temperature threshold for AFD activity is set not by C. elegans’ preferred temperature but by the temperature of its surrounding environment.

Hawk and his team reached this conclusion through an elegant experiment: they trained three groups of worms to prefer different temperatures (15 C, 20 C, and 25 C) and placed them all in a 20 C environment for 30 minutes. They then relocated the worms onto a plate with a temperature gradient and observed their movements. As expected, the worms migrated to their preferred temperature. The researchers found, however, that the temperature threshold for AFD activity in all three groups was the same: about 20C, the same temperature as the worms’ environment prior to testing.  Through additional experiments, Dr. Hawk and his team confirmed that AFD rapidly adapts to its environment to sense temperature changes, a form of neural plasticity known as sensory adaptation.

Since AFD fires independently of C. elegans’ preferred temperature, the research team was left with the question of how different preferred temperatures lead to different migration patterns.  By measuring the activity of a neuron connected to AFD, known as AIY, they determined that, through the activity of an enzyme in AFD, AFD is able to increase the strength of its connection to AIY in worms preferring to move up a temperature gradient.

Based on these findings, if a worm prefers colder temperatures but moves to warmer temperatures, then AFD senses this increase in temperature but does not send a signal to AIY, indicating that the worm should change direction. If a worm prefers warmer temperatures and moves to warmer temperatures, then AFD sends a signal to AIY, indicating the worm should continue. This selective signaling from AFD to AIY relies on a change in neural connections known as presynaptic plasticity.

These experiments indicate that AFD can change in two ways: sensory adaptation to the current temperature to sense temperature changes and presynaptic plasticity to control a signal to AIY depending on temperature preference.  This understanding of AFD’s versatility is one step forward for Hawk. “We want to understand how C. elegans responds to all of its sensory stimuli and integrates it into behavior,” Hawk said. He believes that lessons from C. elegans’ relatively simple neural system will help neuroscientists understand more complicated neural systems with the “same molecules doing the same tricks.”