How memories become permanent decoded

Scientists have for the first time identified the mechanism that regulates rhythmic brain waves important for making memory permanent.
Every time we learn something new, the memory does not only need to be acquired, it also needs to be stabilised in a process called memory consolidation.
Brain waves are considered to play an important role in this process, but the underlying mechanism that dictates their shape and rhythm was still unknown.
The new study shows that one of the brain waves important for consolidating memory is dominated by synaptic inhibition.
Sharp wave ripples (SWRs) are one of three major brain waves coming from the hippocampus.

The study by researchers at the Institute of Science and Technology Austria (IST Austria), found the mechanism that generates this oscillation of neuronal activity in mice.

"Our results shed light on the mechanisms underlying this high-frequency network oscillation," said Peter Jonas, professor at IST Austria.
"As our experiments provide information both about the phase and the location of the underlying conductance, we were able to show that precisely timed synaptic inhibition is the current generator for sharp wave ripples," said Jonas.
When neurons oscillate in synchrony, their electrical activity adds together so that measurements of field potential can pick them up.

SWRs are one of the most synchronous oscillations in the brain nd have been suggested to play a key role in making memories permanent.
Researchers wanted to identify whether ripples are caused by a temporal modulation of excitation or of inhibition at the synapse, the connection between neurons.
They found that the frequency of both excitatory and inhibitory events at the synapse increased during SWRs. However, quantitatively synaptic inhibition dominated over excitation during the generation of SWRs.
Furthermore, the magnitude of inhibitory events positively correlated with SWR amplitude, indicating that the inhibitory events are the driver of the oscillation.
Inhibitory events were phase locked to individual cycles of ripple oscillations. Finally, the researchers showed that so-called PV+ interneurons - neurons that provide inhibitory output onto other neurons - are mainly responsible for generating SWRs.

Researchers proposed a model involving two specific regions in the hippocampus, CA1 and CA3.
In their model SWRs are generated by a combination of tonic excitation from the CA3 region and phasic inhibition within the CA1 region.
"In our ripple model, inhibition ensures the precise timing of neuronal firing," said Jian Gan, researcher in the group of Peter Jonas.
"This could be critically important for preplay or replay of neuronal activity sequences, and the consolidation of memory. Inhibition may be the crucial player to make memories permanent," Gan said.
The study was published in the journal Neuron.