Scientists have long believed that the locus coeruleus acts like a megaphone, sending essentially the same noradrenaline signal throughout the brain. However, a new study published in Nature challenges that view.

Using brain imaging, neural recordings, genetic techniques and behavioral experiments in mice, researchers at the Allen Institute discovered that the LC communicates through several distinct pathways. Rather than broadcasting one general message, different groups of locus coeruleus neurons respond to different experiences and send signals to specific destinations.

How the locus coeruleus sends precise brain signals

By tracing the connections of LC neurons, researchers found that different cell groups communicate with specific regions of the nervous system. These pathways help determine where noradrenaline is released and reveal a far more organized communication network than previously understood.

Neurons in the upper, or dorsal, part of the locus coeruleus connect primarily to the cerebral cortex and forebrain. Cells in the lower, or ventral, part mainly send signals to the brainstem and spinal cord. This arrangement allows separate regions of the LC to influence different brain and body functions.

“Our findings suggest that the brain’s noradrenergic system is much more like a targeted postal network than a foghorn,” said study co-author Karel Svoboda, director of neurodynamics at the Allen Institute.

Why the discovery could matter for brain disorders

Understanding how the locus coeruleus is organized could eventually influence the treatment of neurological and mental health conditions.

Noradrenaline plays a central role in attention and mood and is an important target for medications used to treat depression, anxiety and ADHD. LC neurons are also among the brain cells that deteriorate fastest in Alzheimer’s disease, suggesting that the findings could have implications for understanding and treating these conditions.

Current treatments that affect noradrenaline can influence several brain circuits at the same time. This may produce beneficial effects in some areas while causing unwanted effects in others. A clearer map of specialized LC pathways could help scientists develop treatments that act more selectively on the circuits involved in specific symptoms.

This research was supported by the National Institutes of Health Brain Research Through Advancing Innovative Neurotechnologies® Initiative, also known as The BRAIN Initiative®.

Different locus coeruleus neurons support learning and behavior

The researchers also identified important functional differences between groups of LC neurons.

Dorsal LC cells that connect to the cortex were particularly active during learning. When mice performed a decision-making task and changed their choices after an unfavorable outcome, activity in these neurons increased.

Ventral LC neurons behaved differently. Their activity increased shortly before the mice ignored a signal indicating a potential reward. This suggests that these cells may help regulate whether an animal responds to opportunities in its surroundings.

“What emerged was a clear map: neurons in the dorsal LC, which send signals upward to the cortex, are involved in learning, while neurons in the ventral region, which project downward to the brainstem and spinal cord, govern whether the animal engages with its environment,” Svoboda said. “We also showed that these anatomical differences are reflected in different gene expression patterns.”

These findings indicate that the two neuron groups differ not only in their location and connections, but also in their patterns of gene activity and specialized roles.

Researchers identify exceptionally long locus coeruleus neurons

The study involved an extensive examination of mouse brains. Scientists used whole-brain imaging to study about 35,000 neurons, analyzed the genetic profiles of approximately 400,000 cells and reconstructed the complete structure of some of the most complex nerve cells.

During this work, the researchers made another remarkable discovery: locus coeruleus neurons have extremely long axons. These extensions carry signals from nerve cells to other cells, allowing information to travel across the nervous system.

The researchers found that LC axons average about 35 centimeters in length. One unusually long axon had a reconstructed length of 70.32 centimeters, making it one of the longest single neurons ever measured in a mouse.

“This neuron, like many of the other neurons we studied, supplies NE to a very wide area of the cerebral cortex. For the brain, this is very unusual. Most neurons are more specific in their targeting,” said Jeremiah Cohen, an Allen Institute scientist and study co-author. “But this neuron doesn’t release NE everywhere; it ignores the cerebellum, brainstem and spinal cord.”

Despite their exceptional length and broad reach, these neurons do not distribute noradrenaline throughout the entire brain. Their connections remain selective, reinforcing the finding that the locus coeruleus operates through carefully organized signaling pathways.

What the study reveals about learning in the brain

The researchers also identified similarities between the norepinephrine system and the brain’s dopamine signaling system.

Dopamine is another major chemical messenger. It sends signals to the basal ganglia, a brain region involved in habit formation. Scientists believe dopamine and norepinephrine may work together to support learning, helping the brain acquire multiple complex and abstract concepts at the same time.

By revealing the complex connections, specialized functions and unusual structure of locus coeruleus neurons, the study provides a more detailed picture of how the brain communicates and learns.

Because the findings came from experiments in mice, more research is needed to determine how directly they apply to humans. Still, the work could eventually help scientists better understand human brain function and develop treatments that target specific neural circuits instead of affecting the entire noradrenergic system.