A new study from the University of Toronto has shown that fear memories created closely in time are stored by the same group of brain cells, suggesting a new way that memories can be connected.
Review of Rashid AJ, Yan C, Mercaldo V, Hinsang HL, Park S, Cole CJ, De Cristofaro A, Yu J, Ramakrishnan C, Lee SY, Deisseroth K, Frankland PW, Josselyn SA (2016) Competition between engrams influences fear memory formation and recall. Science, 353.6297: 1095-9203, doi:10.1126/science.aaf0594
In order to understand how memories are connected, we must first understand how memories are stored. The brain is made up of billions of individual specialized cells called neurons, which transmit electrical impulses like wires. When we create a memory, several specific neurons are activated at the same time, forming a unique pattern called an engram. When that same group of neurons is activated again later, we recognize the memory.
Our memories are not isolated, however; they are organized into complex structures based on cause and effect. Engrams interact with each other and link together to form these structures, but we know very little about how this takes place. If we could predict which neurons will become part of an engram, we could better understand how memories are connected. “If we can understand how new memories talk to old memories, that opens up lots of new directions for research on memory problems like dementia,” says Madeleine Slater, an undergraduate neuroscience student at Brown University who spoke with me about the study.
A team of researchers at the University of Toronto set out to do just that. They first created two fear memories in the amygdala of rats (a part of the brain which processes fear) by shocking them when they heard two specific sounds to see if they were connected (Rashid et al., 2016). They introduced light molecules into the amygdala that only turn on when a neuron is activated, which showed that if the two memories were made 6 hours apart, the engrams overlapped (most of the neurons in the first one were also in the second one) (Rashid et al., 2016). If the memories were made 24 hours apart, however, the engrams were completely separate (Rashid et al., 2016).
They also discovered that if the two memories were made within 6 hours apart, the rats had a much stronger memory of the second sound (which they measured by the length of time the rats were frozen after hearing that sound again the next day) (Rashid et al., 2016). If these fear memories were created 18 or 24 hours apart, though, the rats’ memory was not as strong (Rashid et al., 2016).
Something, then, must happen when a cell becomes part of an engram that makes it easier to activate. We know from previous research that a molecule called CREB, which turns on genes that make the neuron activate more easily, is increased in neurons that become part of an engram (Rashid et al., 2016). In the next experiment, the researchers attached a colored molecule to CREB and took pictures of the amygdala so they could see where it was (Rashid et al., 2016). This time, they only created one fear memory and checked how much CREB stayed in the amygdala over time (Rashid et al., 2016). They found that CREB was increased for about 6 hours, and then it dropped back down to normal (Rashid et al., 2016). This matches the data from before, which means means that CREB could cause engram overlap by making the neurons that become part of the first engram easier to activate and therefore more likely to become part of the second one (Rashid et al., 2016).
Next, the researchers wanted to choose which neurons would and would not become part of an engram. They accomplished this through two different techniques. In the first method, they introduced special molecules into a few neurons that would turn these neurons on when they were exposed to blue light (making them easier to activate) and turn them off when they were exposed to red light (making them harder to activate) (Rashid et al., 2016). The second method is similar; the researchers introduced extra CREB into a few neurons so that they would be easier to activate, but at the same time they introduced special lock molecules they had made that would turn the neuron off when the researchers introduced special key molecules that they had also made (Rashid et al., 2016). Through both methods, the researchers were able to successfully choose certain neurons to become part of each memory by making them easier to activate (Rashid et al., 2016). They could force memories made 6 hours apart to form separate engrams when they should use the same one, and they could force memories made 24 hours apart to use the same engram when they should be separate (Rashid et al., 2016). This was strong evidence supporting their hypothesis that neurons which are easier to activate are more likely to become part of a memory allowing those memories to link together and strengthen each other (Rashid et al., 2016).
There is one final piece of the puzzle, however, which is critically important for this system to work correctly. Just as these experiments used the red light and the lock molecules to turn neurons off, the brain similarly must turn the surrounding neurons off in order to form an engram. In fact, we can observe this in the last experiment. When the researchers forced close memories to use separate engrams that should use the same engram, the second memory was extremely weak (Rashid et al., 2016). This means that when a memory is being made, the neurons surrounding the engram are even harder to activate than normal (Rashid et al., 2016). We can remove this effect, however, by using the lock and key method to turn off the helper neurons which we know have the job of turning off those surrounding neurons (Rashid et al., 2016). This makes the neurons that surround the first engram easier to turn on while the second memory is being made, which makes that second memory much stronger (Rashid et al., 2016). This shows that in order for us to make strong memories, the functions of turning on neurons and turning off neurons must be balanced and working together.
“These results are convincing because the researchers were able to change which neurons entered the engram by changing how easy they were to activate,” Slater emphasized. “This means that a neuron’s level of excitability actually determines how likely it is to become part of an engram.
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