Researchers at the Swedish medical university Karolinska Institutet have discovered a mechanism that controls the brain's ability to create lasting memories. In experiments on genetically manipulated mice, they were able to switch on and off the animals' ability to form lasting memories by adding a substance to their drinking water. The findings, which are published in the scientific journal PNAS, are of potential significance to the future treatment of Alzheimer's and stroke.
"We are constantly being swamped with sensory impression," says Professor Lars Olson, who led the study. "After a while, the brain must decide what's to be stored long term. It's this mechanism for how the connections between nerve fibers are altered so as to store selected memories that we've been able to describe."
The ability to convert new sensory impressions into lasting memories in the brain is the basis for all learning. Much is known about the first steps of this process, those that lead to memories lasting a few hours, whereby altered signalling between neurons causes a series of chemical changes in the connections between nerve fibers, called synapses. However, less is understood about how the chemical changes in the synapses are converted into lasting memories stored in the cerebral cortex.
A research group team at Karolinska Institutet has now discovered that signalling via a receptor molecule called nogo receptor 1 (NgR1) in the nerve membrane plays a key part in this process. When nerve cells are activated, the gene for NgR1 is switched off, and the team suspected that this inactivation might be important in the creation of long-term memories. To test this hypothesis they created mice with an extra NgR1 gene that could remain active even when the normal NgR1 was switched off.
"Doing this, we found that the ability to retain something in the memory for the first 24 hours was normal in the genetically modified mice," says Professor Olson. "However, two different memory tests showed that the mice had serious difficulties converting their normal short-term memories to long-term ones, the kind that last for months."