The ability to learn associations between events is critical for survival, but it has not been clear how different pieces of information stored in memory may be linked together by populations of neurons. In an opensource study published in Cell Reports, synchronous activation of distinct neuronal ensembles caused mice to artificially associate the memory of a foot shock with the unrelated memory of exploring a safe environment, triggering an increase in fear-related behaviour when the mice were re-exposed to the non-threatening environment. The findings from the University of Toyama suggest that co-activated cell ensembles become wired together to link two distinct memories that were previously stored independently in the brain.
The team explain that memory is the basis of all higher brain functions, including consciousness, and it also plays an important role in psychiatric diseases such as post-traumatic stress disorder. By showing how the brain associates different types of information to generate a qualitatively new memory that leads to enduring changes in behaviour, the data findings could have important implications for the treatment of these debilitating conditions.
Previous studies have shown that subpopulations of neurons activated during learning are reactivated during subsequent memory retrieval, and reactivation of a cell ensemble triggers the retrieval of the corresponding memory. Moreover, artificial reactivation of a specific neuronal ensemble corresponding to a pre-stored memory can modify the acquisition of a new memory, thereby generating false or synthetic memories. However, these studies employed a combination of sensory input and artificial stimulation of cell ensembles. Until now, researchers had not linked two distinct memories using completely artificial means.
The current study used a fear-learning paradigm in mice followed by a technique called optogenetics, which involves genetically modifying specific populations of neurons to express light-sensitive proteins that control neuronal excitability, and then delivering blue light through an optic fiber to activate those cells.
In the behavioural paradigm, one group of mice spent six minutes in a cylindrical enclosure while another group explored a cube-shaped enclosure, and 30 minutes later, both groups of mice were placed in the cube-shaped enclosure, where a foot shock was immediately delivered. Two days later, mice that were re-exposed to the cube-shaped enclosure spent more time frozen in fear than mice that were placed back in the cylindrical enclosure.
The researchers then used optogenetics to reactivate the unrelated memories of the safe cylinder-shaped environment and the foot shock. Stimulation of neuronal populations in memory-related brain regions called the hippocampus and amygdala, which were activated during the learning phase, caused mice to spend more time frozen in fear when they were later placed back in the cylindrical enclosure, as compared with stimulation of neurons in either the hippocampus or amygdala, or no stimulation at all.
The data findings show that synchronous activation of distinct cell ensembles can generate artificial links between unrelated pieces of information stored in memory, resulting in long-lasting changes in behaviour. By modifying this technique, the team will next attempt to artificially dissociate memories that are physiologically connected. The team surmise that this may contribute to the development of new treatments for psychiatric disorders such as post-traumatic stress disorder, whose main symptoms arise from unnecessary associations between unrelated memories.
Source: University of Toyama
