Mouse Study Identifies a Brand New Type of Neuron Involved in Important Memories

With about 90 billion neurons in our brains, it's no surprise that we're still piecing together a lot of information on how these different types of cells function. A group of neuroscientists has discovered a new type of neuron in mice, as well as its role in the intricate circuitry of mammalian brains.

These cells are found in the hippocampus, a region of the brain that is important in memory and learning. Theta-Off Ripple-On (TORO) is the term given to this type of neuron by the researchers because of its activity pattern.

"TORO-neurons propagate the sharp wave ripples information broadly in the brain and signal that a memory event occurred," explains Marco Capogna of Aarhus University.

These intense wave ripples are considered to be how our brain encodes a key event in episodic memory – such as your first kiss – and are formed in the hippocampus. Electrographic recordings show them as high-frequency electrical spikes in brainwaves, as shown below.

With these sorts of surges, TORO activity appears to synchronize.

Stanford University researcher Gergely Szabo and colleagues used electrophysiological methods to trace the circuit of brain cells involved TORO-neuron activity. TOROs are triggered by pyramidal neurons in the CA3 portion of the hippocampus, which is already well recognized for its function in episodic memories, according to the circuit mapping.

TOROs "are inhibited by inputs coming from other brain areas," according to Capogna and are "inhibitory neurons that release the neurotransmitter GABA."

GABA (gamma-aminobutyric acid) has a calming effect due to its inhibitory action, which slows down brain activity. When the brain generates theta waves as a result of a lot of brain activity, such as when we're moving, most GABA-releasing cells release GABA. TOROs, on the other hand, release this substance most actively while they are at rest.

"We have found that this new type of neuron is maximally active during sharp wave ripples when the animal is awake – but quiet – or deeply asleep," Capogna continues. "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." 

Furthermore, TOROs do not function just on a local level. 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." 

It's as if they're broadcasting a message to the rest of the brain about what's going on in the hippocampus.
While mouse brain research may not often apply to humans, a novel type of neuron with such a significant regulatory role is likely to have been preserved across species.

However, additional study is needed to fully comprehend its significance in our own brains and how it may contribute to memory disorders such as Alzheimer's disease.

Taken together, these data imply TORO cells play a significant role in memory processing, possibly controlling episodic memory in particular.