Nav: Home

Stopping the brain's memory circuits from overheating

May 03, 2017

The highly interconnected zones of the brain's hippocampus mediate spatial and episodic memory, but to keep memories organized they need the right balance of exciting and calming input. A part of the hippocampus called CA2 has been found to be responsible for this regulation, preventing the local brain circuits from becoming hyperactive. In the absence of CA2 activity, mice experience epilepsy-like activity, a sign that this area is essential for regulating the balance of excitation and inhibition in the brain. A silenced CA2 region has broader implications for information processing in hippocampal circuits, according to a new study from the RIKEN Brain Science Institute (BSI) in Japan and the Université Paris Descartes.

RIKEN BSI team leader Thomas McHugh and colleagues studied mice that had either temporary or permanent CA2 impairment. As they reported on May 3rd in the journal Neuron, CA2 is responsible for maintaining inhibition throughout its connected network.

Investigating the connections from CA2 to hippocampal neighbors CA1 and CA3, the researchers found that optogenetic stimulation there translated into a suppression of signaling in the network, especially in CA3. They further probed this inhibition by shutting down signaling from CA2 with a nerve toxin, which resulted in a 'hyperexcitable' network state within CA3.

These observations, which were made in brain slices, were replicated and expanded in behaving mice. During exploration of open areas and tracks, mice with silenced CA2 activity displayed increases in local field potentials, the summed electric current from a larger group of neurons in the hippocampus. McHugh's research group has previously reported how this kind of brain wave activity organizes spatial coding in the hippocampus. This time they found that large increases in the power of the slow-wave activity of 4-12 Hz, dubbed the theta band, along with bursts of high-frequency oscillations, were spatially triggered. "These episodes of hyperexcitability lasted one or two seconds and were tied to specific locations visited by the mice," said McHugh.

In resting or immobile mice, however, the researchers observed something quite different: short, frequent, large-amplitude voltage spikes reminiscent of epileptic brain activity. "Normal ripple waves across the hippocampus appear to be substituted by these epileptiform-like discharges that originate in CA3, which becomes highly excitable without CA2 gating its activity," observed McHugh. These mice were also more susceptible to seizures induced by an injected neurotoxin compared to control mice.

CA2 thus appears to be a vital part of controlling the spread of excitatory neural activity in the hippocampus, potentially preventing it from entering a state of pathological spiking. Further study is needed to determine how this affects navigation and memory in mice, however. "The hippocampus encodes place, and we saw a subtle shift in the spatial organization of pyramidal cells spiking in the face of CA2 inhibition. We still need to explore how timing and strength of inputs in this degraded network manifests in these interesting changes," said McHugh.

-end-

Reference:

Boehringer R, Polygalov D, Huang AJY, Middleton SJ, Robert V, Wintzer ME, Piskorowski RA, Chevaleyre V, McHugh TJ (2017) Chronic loss of CA2 transmission leads to hippocampal hyperexcitability. Neuron, doi: 10.106/j.neuron.2017.04.14.

RIKEN

Related Memory Articles:

Taking photos of experiences boosts visual memory, impairs auditory memory
A quick glance at any social media platform will tell you that people love taking photos of their experiences -- whether they're lying on the beach, touring a museum, or just waiting in line at the grocery store.
Think you know how to improve your memory? Think again
Research from Katherine Duncan at the University of Toronto suggests we may have to rethink how we improve memory.
Improving memory with magnets
The ability to remember sounds, and manipulate them in our minds, is incredibly important to our daily lives -- without it we would not be able to understand a sentence, or do simple arithmetic.
Who has the better memory -- men or women?
In the battle of the sexes, women have long claimed that they can remember things better and longer than men can.
New study of the memory through optogenetics
A collaboration between Universitat Autònoma de Barcelona and Harvard University pioneers the increase of memory using optogenetics in mice in Spain.
Peppermint tea can help improve your memory
Peppermint tea can improve long-term and working memory and in healthy adults.
A new glimpse into working memory
MIT study finds bursts of neural activity as the brain holds information in mind, overturns a long-held model.
Memory ensembles
For over forty years, neuro-scientists have been interested in the biological mechanisms underlying the storage of the information that our brain records every day.
What is your memory style?
Why is it that some people have richly detailed recollection of past experiences (episodic memory), while others tend to remember just the facts without details (semantic memory)?
Watching a memory form
Neuroscientists at Rosalind Franklin University of Medicine and Science have discovered a novel mechanism for memory formation.

Best Science Podcasts 2017

We have hand picked the best science podcasts for 2017. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: Radiolab

Oliver Sipple
One morning, Oliver Sipple went out for a walk. A couple hours later, to his own surprise, he saved the life of the President of the United States. But in the days that followed, Sipple's split-second act of heroism turned into a rationale for making his personal life into political opportunity. What happens next makes us wonder what a moment, or a movement, or a whole society can demand of one person. And how much is too much?
Now Playing: TED Radio Hour

Future Consequences
From data collection to gene editing to AI, what we once considered science fiction is now becoming reality. This hour, TED speakers explore the future consequences of our present actions. Guests include designer Anab Jain, futurist Juan Enriquez, biologist Paul Knoepfler, and neuroscientist and philosopher Sam Harris.