Study shows fruit flies avoid bad experiences too
The brain is without a doubt an amazing and complex organ. And we use it to help us remember positive events and avoid negative ones. For example, sharing a bowl of ice cream with a best friend is a good experience while taking a bite of a too-hot slice of pizza is a bad one. But humans are not the only ones with the ability to do this. The fruit fly Drosophila melanogaster can also learn to seek good events and dodge the bad ones.
An international team of scientists led by the Max Planck Institute of Neurobiology in Germany have discovered that dopamine-containing neurons linked to the mushroom body of the fly brain contribute to the insect's ability to form positive and negative memories. They localised and identified four types of nerve cells that use dopamine to communicate with other neurons; three assume various functions in mediating negative stimuli and the fourth allows the fly to establish positive memories. The finding is presented in the journal Nature.
The mushroom body, which is a prominent brain structure in the fruit fly's brain, receives the dopamine signals that are released by the neurons. 'It is really surprising that similar dopamine-releasing nerve cells can play such different roles,' says co-author Hiromu Tanimoto of the Max Planck Institute of Neurobiology.
The researchers from China, France, Germany and the United States evaluated how individual neurons function in the fruit fly. The insects were presented with an odour that was linked to a negative stimulus, a foot shock. The team observed that the flies learned to avoid this odour in future.
In another test, they replaced the shock with artificial activation of defined sets of neurons during an odour presentation. According to the researchers, the transient activation of only the neurons was enough to signal a negative stimulus, which compelled the fly to form a negative odour memory. This was the result regardless if a negative stimulus was present or not.
The team also found that the main difference between the three types of neurons is the stability of the induced memories. One cell type is associated with long-lasting memory, while the others are responsible for short-lived memories. 'Punishing events induce aversive memories with different stabilities by combining distinct dopamine cells in the fly brain,' says Dr Tanimoto.
They also used the same method to show that other dopamine cells signal reward to establish positive odour memory. Artificial activation of these cells resulted in the flies remembering the smell and working to find the source of the award. They did this even if the sugar reward was missing. The researchers' work shows that this process is influenced by specific dopamine neurons.
According to the researchers, the messenger substance dopamine is important for fruit flies and other insects. Results generated in such studies help us understand that functional diversity of dopamine is a highly conserved mechanism in brains.
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Data Source Provider: Nature; Max Planck Institute of Neurobiology
Document Reference: Liu, C., et al. 'A subset of dopamine neurons signals reward for odour memory in Drosophila', Nature, 2012. doi:10.1038/nature11304
Subject Index: Coordination, Cooperation; Life Sciences; Scientific Research