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Researchers erase, then restore memories in rats

Eternal Sunshine of the Spotless Mouse? Researchers in California have successfully meddled with rats' memories by stimulating nerves.

Jim Carrey elects to forget Kate Winslet in "Eternal Sunshine of the Spotless Mind". Focus Features

If you could forget a painful memory, would you? It's a hypothetical question, but maybe it could really happen one day -- and, more interestingly, maybe lost memories could be restored. It's this last idea that was of particular interest to researchers at the University of California, San Diego School of Medicine, who have managed to accomplish both in lab rats.

Roberto Malinow, MD, PhD, professor of neurosciences and senior author of the study published in the journal Nature, firstly conditioned a fear response in the rats. They stimulated nerves in the brains of rats genetically engineered to be more sensitive to light. At the same time as stimulating the nerve, the researchers administered an electric shock to the rat's foot -- leading the rat to associate the nerve stimulation with pain, and react with fear.

Then, the team set about erasing the memory of that fear. Science has long hypothesised that strengthened connections between neurons -- called long-term potentiation (LTP) -- is the basis of what forms memories. After forming these memories, the researchers observed that the optically stimulated synapses had strengthened.

The next step was to re-weaken these connections with a low-frequency train of optical pulses. After this treatment, the rats no longer gave a fear response to the nerve stimulation.

But the next part is where it gets really interesting: the researchers then strengthened the neuronal connections up again using a high-frequency train of optical pulses -- and, once again, the rats responded to the nerve stimulation with fear, even though they had not been shocked again.

"We can form a memory, erase that memory and we can reactivate it, at will, by applying a stimulus that selectively strengthens or weakens synaptic connections," Malinow explained.

The potential uses for this research have implications beyond forgetting a lover -- the basis of Michel Gondry's 2004 movie -- offering possible treatments for mental illness such as PTSD, and neurodegenerative disorders such as dementia.

"We have shown that the damaging products that build up in the brains of Alzheimer's disease patients can weaken synapses in the same way that we weakened synapses to remove a memory," Malinow said. "So this line of research could suggest ways to intervene in the process."