A tiny region deep in the brain sends targeted signals instead of one broadcast, a mouse study found

A small cluster of cells deep in the mouse brainstem does not shout one message to the whole brain, as long assumed, but sends different signals down different routes. The finding, published in Nature by researchers at the Allen Institute, concerns the locus coeruleus, the source of the brain’s norepinephrine, and was repackaged by ScienceDaily on Oct. 22 and 23.

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Karel Svoboda, director of Neural Dynamics at the Allen Institute and a co-author, summed up the result with an analogy. In his words, the findings suggest the brain’s norepinephrine system is “far more like a targeted postal network than a foghorn.” The work was done in mice, and extending it to people remains a separate question.

Locus coeruleus neurons in the mouse brainstem

The locus coeruleus is a small group of nerve cells in the brainstem that makes norepinephrine, a chemical involved in learning and attention. The structure was long considered fairly uniform, in effect a single broadcaster sending the same signal to the cortex at once. The ScienceDaily summary of the study says the new data point the other way. The paper, titled “Topographic structure and function of locus coeruleus noradrenaline neurons,” lists Zhixiao Su as first author and Jeremiah Cohen as final author, and its DOI record is the stable citation for the work.

The Allen Institute’s own news story, dated Sept. 16, places the original publication a month before the ScienceDaily reprint, which is why the date on the aggregator page is later than the research itself. The study was funded by the National Institutes of Health’s BRAIN Initiative.

35,000 neurons and 400,000 cells

The scale of the mapping is what makes the claim hard to dismiss. The team imaged almost 35,000 neurons across the whole mouse brain and profiled the genes of nearly 400,000 cells, and it reconstructed several unusually elaborate neurons in full. The axons of locus coeruleus cells averaged about 35 centimeters in length, and the longest reconstructed axon measured 70.32 centimeters, which the Allen Institute describes as the longest single neuron ever measured in a mouse.

Cohen, a principal scientist at the Allen Institute for Neural Dynamics, said that this neuron, like many others the team studied, supplies norepinephrine to a very large volume of the cerebral cortex. That detail cuts both ways: individual cells reach widely, yet which cells reach where is organized. Cohen suggested in The Transmitter’s coverage that norepinephrine from dorsal neurons could act as a learning signal, and his team plans to study how cortical circuits use it and how it interacts with dopamine and serotonin. The preprint reported some axons up to 73 centimeters, while the published Nature figure for the longest is 70.32 centimeters, so the later number is the one used here.

Dorsal and ventral cells do different jobs

The organization follows position. Neurons in the upper, or dorsal, part of the locus coeruleus project mainly to the cortex and forebrain, and they became active when mice were learning, including when animals switched choices after a negative outcome. Neurons in the lower, ventral, part project to the brainstem and spinal cord, and their activity rose just before mice ignored cues that could have signaled a reward. Gene expression patterns mirrored the anatomical split.

The result had a public preview. The Transmitter reported in May on an April preprint of the work, which had not yet been peer reviewed at that point. Nelson Totah of the University of Helsinki, who was not involved, said the study was “the bread-and-butter work that the locus coeruleus field needed,” and Rebecca Jordan of the University of Edinburgh said it went to the next level in showing the spatial organization of heterogeneity. The preprint described gene expression shifting gradually from the dorsal to the ventral end of the structure, tracking differences in cell shape, electrical activity and projection targets, with central neurons that target the cerebellum showing an intermediate pattern. The April preprint listing carries the same title family, “Topographic structure and function of locus coeruleus norepinephrine neurons.”

What a targeted norepinephrine system could change

Norepinephrine is a target of common medications for depression, anxiety and ADHD, and locus coeruleus neurons are among the first to degenerate in Alzheimer’s disease, according to the Allen Institute. If the system works as a set of routes rather than a single broadcast, treatments might one day aim at particular regions instead of the whole network. That is a hope rather than a result, since the work describes mouse circuits and has not tested any therapy. The authors also suggest that norepinephrine and dopamine may work together as a learning system, a question Cohen’s team plans to pursue. Because the paper describes where the signals go and when they fire, rather than proving what each route does to behavior, the next experiments, which would switch particular groups of cells on or off, are what would show whether the postal-network picture holds up. Gene expression profiling of the cell types, an approach outside commentators such as Robert Froemke of NYU Grossman School of Medicine have said could help researchers record and manipulate specific cell types, is the likely tool for that work.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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