SYDNEY: Neuroscientists have looked to rats to help solve the mystery of how memories are stored in the brain, finding that certain cortical neurons – nerve cells that make up the cortex of the brain – must be ‘tagged’ and set aside at the exact time a memory is formed.
The results of the study, which involved observing how rats communicate food safety messages to each other, signify an important step toward understanding how memories are formed, stored and ultimately lost. These findings could lay the groundwork for future medical developments in the fight against diseases such as Alzheimer’s.
“If we could identify the tags, we could find a connection between the impairment of these tags and memory loss – or the inability to effectively store remote memories,” said co-author and CNRS researcher Bruno Bontempi of the Institute of Neurodegenerative Diseases in Bordeaux, France. “It’s a long way away, but this research is paving the way for the potential development of treatments.”
From formation to storage
The formation of short-term memories occurs in a region of the brain called the hippocampus, however, the long-term storage of these memories – so that they can be retrieved months and even years later – happens in a region of the brain called the cortex.
Prior to this study, it was known that the consolidation of remote memories required some kind of interaction between these systems, but scientists were unsure as to when in the communication process this tagging took place.
“In order for remote associative memories to be stored there needs to be an active dialogue between the hippocampus and the cortex,” Bontempi explained. “The hippocampus is a fast learner, which is constantly processing information, but is unable to store that information. The cortex meanwhile is a slow learner, but has the ability to store memories.”
The Ratatouille study
During the study, which was published in Science, the research team investigated the way rats passed information about food safety amongst each other by exploiting a special inherent behavior called ‘social transmission of food preference’ (STFP).
STFP in rats became well documented with the early development of rat poisons. Chemicals that carried a distinct smell or taste and rapidly lethal effects became ineffective because the rats would carry this smell back to their colonies.
Upon dying, the rats would socially transmit to the others that the poison was unsafe to eat, causing researchers to develop a more effective tasteless, odourless and slow-acting poison.
With this fact in mind, the research team fed a demonstrator rat cumin powder and put it in a cage with an observer rat for 30 minutes. Through this single interaction, the observer rat learned that cumin was a safe food by smelling the odour on the breath of the demonstrator rat – thus forming a memory about this smell.
Early tagging ensures memory storage
The researchers used this method to train several rats. They then tested their memory retrieval one day after the interaction, and then again, 30 days after – a considerably long time in the life of a rat. In both instances, when the rats were given a choice between cumin powder and another food substance, they were drawn to the cumin.
By isolating the hippocampus and orbitofrontal cortex with drugs at different points in the memory consolidation process, researchers were able to pinpoint when each region of the brain was required for recollection: the orbitofrontal cortex – part of the cortical network – was needed after 30 days, but played no part in retrieving the memory immediately after it was formed.
“We have shown that cortical neurons need to be tagged early on so that this dialogue can take place and so memories can be consolidated in the cortex. You need a simultaneous activation,” said Bontempi, whose team is yet to identify the proteins involved in the tagging process.
Tags likely exist in human brains
Bontempi said the findings surrounding ‘tagging’ and memory storage could have potential applications in the medical field for humans. “We think this process should be present across different species and there is no reason to believe these tags don’t exist in human brains,” he said. “If we can figure out how to act on these tags then we might be on our way to improving remote memory.”
Jürgen Götz, director of the Alzheimer’s and Parkinson’s laboratories at the University of Sydney’s Brain and Mind Research Institute, agreed that the research could have potential implications for Alzheimer’s.
“By understanding how memory is formed one can also understand how memory is lost. Alzheimer’s patients often memorise what they learned a long time ago but forget the memories they have acquired recently,” said Götz, who was not involved with the study.
“The researchers looked into both recent and remote memories and determined the molecular and network changes underlying their formation. As the hippocampus is affected early on in the degenerative process in Alzheimer’s disease, understanding its role in enduring memory formation may be critical in understanding the steps leading to dementia.”
Institute of Neurodegenerative Diseases, CNRS UMR 5293, University of Bordeaux
J. David Sweatt at the University of Alabama– author of a perspectives piece in Science
University of Sydney, Brain and Mind Research Institute
Science issue of February 18