Amazing stuff!
"In their mapping of LC [locus coeruleus] 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,”...
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,” ... “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. ..."
From the abstract:
"Noradrenaline (also known as norepinephrine) is released throughout most of the central nervous system by neurons in the locus coeruleus.
Here we identified a relationship between the morphologies, gene expression and activity of noradrenergic neurons in locus coeruleus in mice.
Axonal projections of individual neurons were extensive but largely confined to subsets of brain regions. Axonal projections and graded gene expression correlated with locations of cell bodies in locus coeruleus.
In a behavioural task requiring continuous learning from actions, dorsal locus coeruleus neurons projecting to isocortex were activated when mice switched choices and by reward-prediction-error signals that drive learning.
Background activity of neurons in ventral locus coeruleus was higher when mice ignored stimuli indicating potential reward availability.
These observations reveal a topographically organized structure and function of a neurotransmitter system and show that it contains learning signals for flexible behaviour."
Topography of locus coeruleus norepinephrine neurons in the mouse brain. Approximately 100 fully reconstructed neurons are superimposed. Neurons are colored by their locations. Dorsal neurons (purple) project to the front of the brain, ventral (yellow) neurons project to the back.
Reconstruction of 70 cm LC neuron in mouse, the longest ever measured.
Fig. 1: Organization of LC-NE neurons and their brain-wide projections.
Fig. 4: LC-NE neurons are active during action-outcome learning.
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