Worms are capable of forming memories that associate a certain sound, smell or tone of voice with a particular outcome, according to a new report by Hebrew University researchers.
The most famous story of associative learning dates back to the 1890s, when Russian physiologist Ivan Pavlov was researching salivation in dogs in response to being fed. Pavlov predicted that the dogs would salivate in response to the food placed in front of them. However, he learned that any object or event that the dogs learned to associate with food (such as the lab assistant or eventually a bell) would trigger the same response.
But little is known about the physiology behind this phenomenon, such as how do living things leverage these memories to prepare for future adversities?
Dr. Alon Zaslaver and research associate Dr. Yifat Eliezer at Hebrew University of Jerusalem’s Genetics Department discovered that even a very basic animal life form like the C. elegans worm has the ability to learn from past experiences. They also pinpointed the exact neurons that store these memories and the physiological changes the worms undergo when they retrieve memories to cope with future hardships. The C. elegans worm only contains 302 neuron cells.
“We trained the worms to form associative memories,” explained Zaslaver. “However, instead of feeding the worms like with Pavlov’s dogs, we starved them for a day – and instead of ringing a bell, we sprayed a scent that the worms like.
“We hoped that by linking this odor with hunger, the worms would learn that from now on, this pleasant odor signals a distressed situation,” he continued.
A naive worm, that did not go through the training, likes the scent and lingers nearby. (credit: Current Biology)
The next day, the team fed the worms and sprayed the odor again, which led the worms to enter a defensive mode and turn on their stress-protective genes. In other words, the worms now associated what used to be a pleasant odor with hardship.
“However, when we again subjected the worms to starvation, they were better able to survive the hardship than before their associative-learning training,” Zaslaver said.
The response is similar to humans with post-traumatic stress disorder (PTSD) – a mental health condition that’s triggered by a terrifying event – either experiencing it or witnessing it. Those who experience PTSD can have anxiety attacks, flashbacks, nightmares or other symptoms – even after the smallest exposure to a similar scene or scent.
“In a way, their PTSD had helped them,” Zaslaver said of the worms’ response to the scent.
Because the worms have so few memories, the researchers could pinpoint the specific neurons that hold associative memories.
“It’s very rare that you can look at a neuron and say ‘Here, here is the memory,’” he said, explaining that once the neurons were located, the researchers were able to then genetically engineer new worms whose “fight or flight” neurons could be activated simply by shining a light on them – without spraying the scent.
Now, Zaslaver and his team are using their findings to better understand the molecular changes that take place within human memory neurons and the privation-mode responses they provoke – to alleviate and possibly cure PTSD triggers.