A newly discovered insulin receptor gene was found to be necessary for worms to learn a taste avoidance task. In a recently reported study, worms learned to expect food when placed in an unpleasant taste situation while other worms learned they would not get food and would starve under this same unpleasant condition. Those that learned they would get food gravitated to the unpleasant taste condition at a later time but the worms that starved avoided the unpleasant testing condition. However, when worms with a defect in this newly discovered insulin receptor gene were put to the task, they could not perform taste avoidance learning.
The study was recently reported in the journal Science. A group of Japanese scientists carried out the study and they were associated with the Graduate School of Science at the University of Tokyo in Japan.
The newly discovered insulin receptor gene was named DAF-2c. The authors suggested that this form of the insulin receptor gene is important for taste aversion learning. The study showed that DAF-2c is translocated from the cell body of chemosensory neurons to the synaptic region when a conditioning stimulus induces taste avoidance learning. They also showed that P13K pathway activity was required downstream of the receptor. The authors stated that synaptic P13K is necessary for the behavioral switch during the learning task.
In the experiment, roundworms were placed in a high salt concentration condition and some worms were given food. Other worms did not receive food when in the high salt condition and they learned that being in the high salt condition meant they would starve. This can be viewed as a typical case of associative learning. Associative learning is what happened when Pavlov rang a bell before feeding his dogs. After the associative learning took place in Pavlov’s experiment, the dogs would salivate at just the sound of the bell. In the reported experiment, the salt water condition was associated with food for some of the worms, however, for other worms, the salt water condition was associated with starvation.
The expectation for the reported study is that those worms that did not get a reward (food) under the salt condition would avoid this condition at a later time while the worms that learned to expect food would gravitate to the high salt condition. In worms that did not carry the mutated insulin receptor gene, this is exactly what happened. They behaved as expected with the worms expecting food gravitating to the high salt concentration and the worms that starved avoiding the high salt concentration.
When the tests were carried out with worms that had the defective DAF-2c version of the insulin receptor gene, the expected did not happen. The worms with this defective gene could not learn this task and failed to avoid the salt condition that was associated with starvation.
Of course, worms might be interesting creatures in their own right, but what the researchers are actually interested in is how might this DAF-2c insulin receptor gene relate to learning in humans. The authors of the study pointed out that previous studies have linked insulin levels with Alzheimer’s disease in humans. The hope is the information learned in this study can be applied to humans in future studies. Learning that an insulin receptor gene is necessary for worms to learn a taste avoidance task may mean that this insulin receptor gene has some relevance to learning in humans.
By Margaret Lutze