How does the brain know how fast the body is moving? Previous research in animals suggested that voluntarily-generated locomotion—rather than passive movement, such as being moved on a cart—is crucial for updating the brain’s understanding of location. Studies also found that cells in the hippocampus, a key brain area for spatial navigation, know where the body will be in the near future. However, the sources of a speed input to brain areas that track the body’s position in space are poorly understood.

In a new study, a team of researchers in the lab of Ivan Soltesz, PhD, hypothesized that an incoming speed signal to the hippocampus comes from a brain area that simultaneously controls locomotion while broadcasting a copy of this locomotor drive as a future speed signal to other parts of the brain.

The team identified a small brain area deep in the rodent hypothalamus, called the supramammillary nucleus, as an ideal candidate, based on its unique brain connections to the midbrain where locomotion is controlled, and the hippocampus, as well as other brain areas involved in spatial navigation.

Through a combination of recording techniques and experiments that manipulate the activity of select groups of supramammillary cells, the research team found a population of cells that regulate future locomotor speed and relay this signal to the hippocampus. Importantly, cells in the hippocampus that keep track of the animal’s speed were found to be selectively controlled by supramammillary activation.

In effect, at the same time that the supramammillary nucleus controls movement of the body, it also sends a ‘signal’ to tell other areas of the brain involved in spatial navigation how fast the body will move in the near future, according to the researchers.

Jordan Farrell, PhD, is the lead author of the study and a postdoctoral scholar in the laboratory of Ivan Soltesz, PhD, the principal investigator. Their findings were published in the December 2021 issue of Science.

Clinical relevance

The Soltesz laboratory has a longstanding interest in understanding how different groups of neurons in the hippocampus contribute to its physiological functions, such as memory and spatial navigation, but also to its pathological role in epilepsy. 

Farrell described the study's results as having potential clinical relevance to seizure-related disorders. "In the healthy brain, we observed that supramammillary neurons exerted strong control over hippocampal network activity," he said. "We wonder whether this control of hippocampal activity could be applied to temporal lobe epilepsy, where seizures most often involve the hippocampus and are highly resistant to medications."

The paper's authors included Jordan S. Farrell, Matthew Lovett-Barron, Peter M. Klein, Fraser T. Sparks, Tilo Gschwind, Anna L. Ortiz, Biafra Ahanonu, Susanna Bradbury, Satoshi Terada, Mikko Oijala, Ernie Hwaun, Barna Dudok, Gergely Szabo, Mark J. Schnitzer, Karl Deisseroth, Attila Losonczy, and Ivan Soltesz.

Read more about the paper: https://www.science.org/doi/10.1126/science.abh427