Deep in the brainstem sits a small but important cluster of neurons known as the locus coeruleus (LC), Latin for “blue place.” It makes up a tiny fraction of the brain’s cells but plays an important role by releasing a crucial chemical messenger known as norepinephrine (NE). NE activates the body’s response to stress or excitement, impacts attention and learning, and affects heart rate.

For a long time, scientists believed the LC (called the blue place because it contains neuromelanin, a dark bluish pigment formed by dopamine and NE) worked like a single loudspeaker, indiscriminately broadcasting a uniform NE signal throughout the brain, triggering it to release this important neurotransmitter. But in a new study published in Nature, Allen Institute scientists used a combination of brain imaging, neural recordings, genetic tools, and behavioral experiments in mice to illustrate that it actually transmits multiple signals through distinct routes with great precision depending on what the mouse experiences.

The ExA-SPIM microscope was used in this study to image brain regions with extreme precision.

‍

tailored messages instead of one loudspeaker/

In their mapping of LC neurons, researchers discovered that they sent their connections—and therefore also the signals that activate the release of norepinephrine—to specific “addresses” elsewhere in the brain. Neurons in the upper (dorsal) part of the LC sent their connections mainly to the cerebral cortex and forebrain, while neurons in the lower (ventral) area communicated with the brainstem and spinal cord.

“The findings suggest the brain's norepinephrine system is far more like a targeted postal network than a foghorn,” said Karel Svoboda, director of Neural Dynamics at the Allen Institute and study co-author.

why this matters/

NE is crucial for regulating mood and attention; therefore, it is the target of common medications for treating depression, anxiety, and ADHD. LC neurons are also among the first to degenerate in Alzheimer’s disease, so the research could have important implications for treating these disorders.

With new information about the precise organization of LC neurons, future therapies could also target specific regions, rather than flood the entire system and affect some circuits positively and others negatively.

The research was supported by the National Institutes of Health’s Brain Research Through Advancing Innovative Neurotechnologies® Initiative, or The BRAIN Initiative®.

the learning cells/

Researchers also found the LC neurons leading to the cortex carried learning signals. When the mice performed a decision-making task, such as switching choices after a negative outcome, dorsal LC neurons became active. In contrast, ventral LC neurons showed elevated firing just before mice would ignore cues that might offer potential rewards.

“What emerged was a clear map: neurons in the dorsal LC that send signals upward to the cortex are involved in learning, whereas neurons in the ventral region projecting downward to the brainstem and spinal cord govern whether animals engage with their environment at all,” said Svoboda. “We also showed that these anatomical differences are mirrored by distinct gene expression patterns.”

longest axon ever discovered/

In the study, researchers precisely mapped the mouse brain, including whole-brain imaging of almost 35,000 neurons and genetic profiling of nearly 400,000 cells, plus complete reconstruction of some of the brain’s most complex cells. Scientists also discovered incredible physical characteristics of LC neurons. They found that the neurons have axons—the long projections of a nerve cell that carry signals to other cells—averaging about 35 centimeters in length. One axon measured 70.32 centimeters, leading to the longest single neuron ever measured in a mouse.  

“This neuron, like many others that we studied, supplies NE to a very large volume ofthe cerebral cortex. For the brain, this is highly unusual. Most neurons aremore specific in their targets,” said Jeremiah Cohen, scientist at the Allen Institute and study co-author. “But this neuron doesn't release NE everywhere. It ignores the cerebellum, brainstem, and spinal cord.”

Additionally, researchers drew parallels between NE and dopamine, the brain’s other major chemical messenger, which carries signals to the basal ganglia, a region involved in habit formation. They believe that these two systems work together as a learning platform, allowing the brain to learn multiple complex and abstract concepts simultaneously.

By mapping the contours and architecture of the brain’s mysterious “blue place,” this new research deepens our understanding of neuronal communications and learning in the animal brain and may one day help scientists develop more targeted and precise treatments for neurological conditions in humans.