Summary: Researchers have identified a previously undescribed neuron in the hippocampus that becomes active during the formation of new memories.
Source: Aarhus University
The hippocampus has long been known as a key brain structure where memories are encoded and stored through coordinated changes in neuronal activity.
A well-known hallmark of memory-related activity in the hippocampus is the occurrence of sharp wave ripples (SWRs). These are brief, high-frequency electrical events generated within hippocampal circuits and are widely associated with episodic memory—the recall of specific events and experiences from an individual’s life, such as childhood moments, a first date, or an old phone number.
Despite extensive study of SWRs, the exact cellular players and circuit mechanisms that are recruited when these fast oscillations occur have not been fully described. New research now reports the discovery of a distinct type of hippocampal neuron in mice that is tightly linked to SWR events and may play a central role in how episodic memories are signaled and propagated across the brain.
Professor Marco Capogna and Assistant Professor Wen-Hsien Hou from the Department of Biomedicine at Aarhus University contributed to the identification and characterization of this novel neuron type. The collaborative study, which also involved Professor Ivan Soltesz’s group at Stanford University, appears in the journal Neuron.
A neuron active during ripples but silent during theta
The newly described cell shows a striking pattern of activity: it is strongly active during sharp wave ripples when the animal is either quietly awake or in deep sleep, yet it is essentially silent during theta states—slower, synchronized activity that occurs when an animal moves or during certain dreaming sleep phases. Because of this clear on–off relationship with theta and ripple states, the researchers named the cell type “theta off–ripples on,” abbreviated TORO.
The study provides a detailed account of TORO neurons’ functional role, their local and long-range connections, and their influence on fast hippocampal oscillations linked to memory. By mapping circuits and monitoring activity, the team showed how TOROs fit into the broader network dynamics of the hippocampus and beyond.
“Why are TORO neurons so selectively engaged by sharp wave ripples? To address this, we mapped inputs and outputs,” says Professor Marco Capogna. “We found that TOROs receive excitatory drive from CA3 pyramidal neurons within the hippocampus while receiving inhibitory control from external sources such as the septum. That combination makes them particularly sensitive to the specific pattern of activity that defines SWRs.”
The researchers further show that TORO neurons are GABAergic inhibitory cells. Like many inhibitory interneurons, their axons send output locally within the hippocampus, shaping activity there, but they also extend projections to other brain regions such as the septum and parts of the cortex. Through these outputs, TORO neurons can broadcast the occurrence of sharp wave ripples beyond the hippocampus and help coordinate distributed brain networks when a memory trace is being consolidated or retrieved.
To observe TORO activity the team used complementary approaches. Electrophysiology recorded the cells’ electrical signals over time, while calcium imaging captured intracellular calcium transients as a proxy for neuronal activation. These methods together allowed precise correlation of TORO firing with SWR events and behavioral states.
While the findings clarify an important element of hippocampal circuitry, the authors emphasize that further work is needed. A key next step is to establish a causal link between TORO activity and memory performance—demonstrating that manipulating these neurons alters memory formation or recall. It will also be important to investigate whether dysfunction of TORO neurons or disruption of sharp wave ripples contributes to memory impairments observed in conditions such as dementia and Alzheimer’s disease.
Overall, the discovery of TORO neurons advances our understanding of how fast hippocampal oscillations are generated and distributed, and it opens new avenues for research into the cellular basis of episodic memory and its disorders.
About this memory research news
Author: Line Rønn
Source: Aarhus University
Contact: Line Rønn – Aarhus University
Image: The image is in the public domain
Original Research: The findings are published in Neuron