As sufferers of post-traumatic stress syndrome know all too well, frightening experiences can be strong, long-lasting and notoriously difficult to erase. Now, we’re starting to understand why. Far from trying to purge these memories, the brain actively protects them by hiring a group of molecular bodyguards called CSPGs (or chondroitin sulphate proteoglycans in full).
By studying the brains of rats, Nadine Gogolla from Harvard University found that CSPGs – large chains of sugars and proteins – accumulate in the space around nerve cells and form defensive nets around a select few. Dissolve these nets, and the rats’ fearful memories were more easily erased.
The nets start to form round about the time when rats reach adulthood and their fearful memories become harder to erase. As adults, rats can learn to be scared of an inoffensive sensation, like the sound of a buzzer, for the rest of their lives, if it’s paired with an unpleasant one, like an electric shock.
However, the strength of this terror starts to wane if the rats repeatedly hear the ominous buzz without any nasty consequences. This process is called extinction, but it’s nowhere near as permanent or robust as the creation of the original fear. The minute the shock returns, the fear response recovers. This happens so quickly that the memory clearly hadn’t been erased or overwritten – the rat had merely learned to block it out.
Things are different in puphood. Before their third week of life, a rat’s fear memories can be easily erased; only afterwards do they become indelible. Gogolla thinks that the nets are the reason why. She looked at the brains of baby rats in their first month of life, and focused on their amygdala, a pair of almond-shaped structures that have roles in processing emotions. The number of CSPG nets in these regions shot up as the days went past, but particularly in the third week, when the switch from erasable fear memories to permanent ones takes place. That’s no coincidence.
The wiping of unwanted memories is a common staple of science-fiction and if you believe this weekend’s headlines, you might think that the prospect has just become a reality. The Press Association said that a “drug helps erase fearful memories“, while the ever-hyperbolic Daily Mail talked about a “pill to erase bad memories“. The comparisons to The Eternal Sunshine of the Spotless Mind were inevitable, but the actual study, while fascinating and important, isn’t quite the mind-wiper these headlines might have you believe.
The drug in question is propranolol, commonly used to treat high blood pressure and prevent migraines in children. But Merel Kindt and colleagues from the University of Amsterdam have found that it can do much more. By giving it to people before they recalled a scary memory about a spider, they could erase the fearful response it triggered.
The critical thing about the study is that the entire memory hadn’t been erased in a typical sci-fi way. Kindt had trained the volunteers to be fearful of spidery images by pairing them with electric shocks. Even after they’d been given propranolol, they still expected to receive a shock when they saw a picture of a spider – they just weren’t afraid of the prospect. The drug hadn’t so much erased their memories, as dulled their emotional sting. It’s more like removing all the formatting from a Word document than deleting the entire file. Congatulations to Forbes and Science News who actually got it right.
Kindt’s work hinges on the fact that memories of past fears aren’t as fixed as previously thought. When they are brought back to mind, proteins at the synapses – the junctions between two nerve cells – are broken down and have to be created from scratch. This process is called “reconsolidation” and scientists believe that it helps to incorporate new information into existing memories. The upshot is that when we recall old memories, they have to be rebuilt on some level, which creates an opportunity for changing them.
A few years ago, two American scientists managed to use propranolol to banish fearful responses in rats. They injected the animals in their amygdalae, a part of their brains involved in processing emotional memories. The drug didn’t stop a fearful memory from forming in the first place, but it did impair the memory when the rats tried to retrieve it. Now, Kindt has shown that the chemical has the same effect in humans.