A series of experiments has shown that neurons are not the only brain cells involved in memory recall.
The study, published in Nature, could inform research into diseases of memory loss, like Alzheimer’s, as well as diseases where memories cannot be suppressed, like post-traumatic stress disorder.
The brain’s remarkable adaptability, or plasticity, allows us to learn new skills, create new memories and respond to injury. But the physical basis of memory formation and recall remains an active area of neuroscience.
The prevailing theory is that groups of neurons called “ensembles” or “engrams” activate when a memory is made and again when that memory is retrieved. Importantly, these neuron engrams lie near to star-shaped support cells called astrocytes.
The team of researchers from Baylor College of Medicine in the US used mice models to show that astrocytes trigger memory recall in the hippocampus. The hippocampus is a brain structure deep within the temporal lobe that plays a critical role in learning and memory.
Roger Marek from the Queensland Brain Institute at The University of Queensland, who was not involved with the study, tells Cosmos that the new research builds upon previous work showing that astrocytes in the hippocampus facilitate fear memories.
“However, the current paper neatly maps and identifies the precise role of hippocampal astrocytes in contextual fear conditioning, as well as their interaction with the neuronal ensembles,” says Marek.
A typical experiment consisted of conditioning mice to freeze in response to a fearful situation so that they would remember to freeze in response to the same situation later. The researchers identified, or “tagged”, the engram neurons and nearby astrocytes associated with the freeze behaviour.
When mice were in non-fearful environments, they did not freeze. “However, when the astrocyte ensemble in these mice in the non-fearful environment was activated, the animals froze,” says co-first author Wookbong Kwon. “[This shows] that astrocyte activation stimulates memory recall.”
The research team also tested a potential mechanism underlying astrocyte activation. They had previously identified a gene, called NFIA, as involved in regulating memory circuits. “When we deleted the NFIA gene in astrocytes that were active during a learning event, the animals were not able to recall the specific memory associated with the learning event, but they could recall other memories,” says Kwon.
“These findings speak to the nature of the role of astrocytes in memory,” says corresponding author Benjamin Deneen.
“The paper emphasises the importance of non-neuronal cells in the brain to regulate learning and memory,” agrees Marek.
Marek, whose research employs a similar engram tagging technique, states that, “these findings will definitely be beneficial for our research by considering the function of these astrocytes in the formation of a new memory, and to better understand the pathology of anxiety disorders and memory-related disorders such as Alzheimer’s.”
