Scientists publishing in Neuron have described how a newly discovered neuron type may be involved with the formation of memory in the hippocampus, which is marked by high-frequency electrical events.
It is known that memory is represented by changes in the hippocampus. One of the well-established changes in the hippocampus that has been associated with memory is the presence of so-called sharp wave ripples (SWR). These are brief, high-frequency electrical events generated in the hippocampus, and they are believed to represent a major event occurring in the brain in the so-called episodic memory, such as recalling a life event or a friend’s phone number.
However, what happens in the hippocampus when SRWs are generated has not been well understood.
Now a new study sheds light on the existence of a neuron type in the mouse hippocampus that might be a key to better understanding of episodic memory.
Professor Marco Capogna and Assistant professor Wen-Hsien Hou have contributed to the discovery of the novel neuron that is associated with sharp wave ripples and memory.
Possible disruption in dementia and Alzheimer’s
The study describes the novel neuron type in the hippocampus.
“We have found that this new type of neuron is maximally active during SWRs when the animal is awake – but quiet – or deeply asleep. In contrast, the neuron is not active at all when there is a slow, synchronized neuronal population activity called “theta” that can occur when an animal is awake and moves or in a particular type of sleep when we usually dream,” Prof Capogna said.
Because of this dichotomic activity, this novel type of neuron is named theta off-ripples on (TORO).
“How come, TORO-neurons are so sensitive to SWRs? The paper tries to answer this question by describing the functional connectivity of TORO-neurons with other neurons and brain areas, an approach called circuit mapping. We find that TOROs are activated by other types of neurons in the hippocampus, namely CA3 pyramidal-neurons and are inhibited by inputs coming from other brain areas, such as the septum,” Prof Capogna explained.
“Furthermore, the study finds that TOROs are inhibitory neurons that release the neurotransmitter GABA. They send their output locally – as most GABAergic neurons do – within the hippocampus, but also project and inhibit other brain areas outside the hippocampus, such as the septum and the cortex. In this way, TORO-neurons propagate the SWR information broadly in the brain and signal that a memory event occurred,” he concluded.
The team has monitored the activity of the neuron by using electrophysiology – a technique that detects activity of the neurons by measuring voltage versus time, and by using imaging that detects activity by measuring changes in calcium signalling inside the neurons.
Demonstrating a causal link between the activity of TORO-nerve cells and memory will be the next step, and exploring whether inhibition of TORO-neurons and sharp wave ripples occurs in dementia and Alzheimer’s diseases.
Source: Aarhus University
