New clues about memory function
When something piques our interest or we are expecting something, unique wave patterns called gamma oscillations pass through our brains. These oscillations are believed to play a role in higher-level functions (although their precise role is not clear), and their disturbance has been seen in epilepsy, schizophrenia, Alzheimer’s disease and autism.
Within the brain are billions of cells known as astrocytes, whose purpose is to support and protect neurons. In the past this was believed to be their only function, but scientists now know that they play a part in seizure generation. Researchers at the Salk Institute in California have now shown that astrocytes have an important role in the generation of gamma oscillations, and that these waves are crucial for a particular type of memory function.
In earlier work, the group found that astrocytes became active immediately before gamma oscillations in rodent brains. This suggested that the astrocytes could be influencing the oscillations – a theory that they were keen to test.
During their latest study the researchers disabled astrocyte function in the brains of rodents using viruses carrying a toxin (neurons were unaffected). They then added a chemical to trigger gamma waves and found that brain tissue with disabled astrocytes produced weaker gamma waves than tissue containing healthy cells.
In the next stage of the experiment the scientists incorporated three genes into the animals’ brains, which would allow them to selectively ‘turn on and off’ the toxin in astrocytes at will. They consistently found that gamma waves were dampened in animals with blocked astrocytes, and that turning off the toxin reversed this effect.
The rodents with disabled astrocytes seemed perfectly healthy, but when they underwent cognitive tests and were compared to ‘normal’ animals, they failed in one major area: new object recognition. As expected, the healthy animals spent more time with new items placed in their environment than they did with more familiar objects. However, the modified animals treated all objects the same – their new object recognition memory was completely gone.
These results are surprising. Astrocytes operate a lot slower than neurons, and no one suspected that they were involved in such high-speed brain activity. However, this evidence suggests that astrocytes actively supply the right environment for gamma waves to occur, and that these in turn make the brain more likely to learn and change the strength of its neuronal connections.
The recognition system is hugely important and includes recognising people, places, facts and past events. These findings will help scientists to better understand how gamma oscillations are generated, and their role in recognition memory. Future research will hopefully look into exactly how gamma waves can become disrupted in epilepsy and other conditions, leading to memory difficulties. Ultimately this could lead to the development of new treatments for memory loss.
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