Separate cell types encode memory’s time, place

Distinct groups of brain cells stamp memories with place and time information, new research suggests. The results, published in the Sept. 23 Neuron, provide clues to how the brain weaves together different aspects of a scene to form cohesive memories.

Cells in a brain region called the entorhinal cortex of a mouse can be divided into two groups: island cells, which cluster into small groups, and surrounding ocean cells. The new work “shows very cleanly that those two cell populations likely mediate different functions,” says functional neuroanatomist Menno Witter of the Norwegian University of Science and Technology in Trondheim. Ocean cells help detect where a memory is formed, while island cells handle aspects of time, experiments on mice reveal.

That finding comes from mice altered so that their ocean cells or island cells glowed when active. Researchers led by Susumu Tonegawa at MIT moved these mice between two distinct environments—one room had a steel rod floor, while another had a white plastic floor, for instance. Some ocean cells fired off signals in the rod-floored room, while others became active in the plastic-floored room. Island cells, in contrast, didn’t discriminate between the rooms, the researchers found, suggesting that ocean cells, not island cells, store information about a mouse’s context.

In separate experiments, researchers shut down ocean cells by genetically engineering them to respond to light. After light shone on the cells, these mice didn’t seem as fearful in a room in which they had previously received a foot shock. That finding suggests that the mice had trouble remembering the context in which they had been zapped.

Ocean cells seem to be important for remembering distinct contexts, but only when the two locations are very different, Tonegawa says. When the two rooms were similar, ocean cells no longer showed strong preferences for one room over the other. Instead, cells in the hippocampus, a brain area important for memory known to collaborate with the entorhinal cortex, may take over the job, Tonegawa says.

Island cells don’t care which room they’re in, but they are important for storing time information, the team found. In this experiment, island cells helped the mice remember that a tone came 20 seconds before a shock. When researchers shut down island cells, the mice showed differences in when they froze in response to the tone.

Messages that travel from the entorhinal cortex to the hippocampus are known to be involved in memory, but the details of that pathway aren’t clear. By finding different job descriptions for ocean and island cells, the researchers show that the entorhinal cortex isn’t just a passive conduit that feeds raw information into the hippocampus, as some researchers thought. Instead, entorhinal cortex cells are capable of sorting information by time and place, Tonegawa says.

The entorhinal cortex “is a very complex area with lots of different cell types,” says Witter. This region is known to house cells that help animals find their way around, including cells that orient an animal in space (SN Online: 10/6/14), cells that detect borders and recently discovered cells that act as speedometers (SN: 8/8/15, p. 8). More experiments will reveal whether ocean or island cells have multiple identities, helping with both memories and navigation, Witter says.