The worms' avoidance of food that has made them sick in the past is analogous to the strongest form of learning known in humans, Bargmann said. "If you eat something and it gives you food poisoning ?especially if it's a flavor that you haven't experienced before, and especially if it's associated with vomiting, you form a very robust memory of that flavor and won't eat food with that flavor in the future. The smell elicits an instant wave of nausea." That aversion is triggered by signals that the olfactory system sends to the brain. Similar forms of learning have been observed in a broad range of animals, including invertebrates such as snails and fish. "We don't know if they feel nauseous," Bargmann said, "but we do know that they will learn to reject the food that's made them ill."
The group carried out further experiments to explore how worms learn to avoid toxic foods. The results, Bargmann said, suggest "what it is to the worm to be nauseous." The group found that when an infection occurs, a particular set of neurons starts producing massive amounts of the neurotransmitter serotonin, and that this response is required for learning. "We think that this is the signal the worm is using to say 'I've gotten sick,'" Bargmann said.
The role of serotonin suggests that this type of learning may be a particularly fundamental aspect of biology, as it has been carried across species even at the molecular level
Source:Howard Hughes Medical Institute