Reconstruction of axonal collaterals from sparse locus coeruleus noradrenergic neurons reveals their non-uniform projections in the cerebral cortex.

The locus coeruleus (LC) is composed primarily of norepinephrine (NE) neurons that project widely throughout the brain. The LC plays a critical role in controlling behavior, particularly arousal. Previously, it was thought that the LC-NE system functioned by releasing NE uniformly throughout the brain. However, recent evidence suggests that its cortical projections are organized into modules that coordinate diverse-and sometimes opposing-functions, such as the formation and extinction of fear memories. Nevertheless, many details remain unclear and require data from the axon collaterals of sparse neurons. We modified a viral tracing protocol using a dual-recombinase system to trace the axonal collaterals of sparse LC-NE neurons projecting to the cingulate cortex (CgC). Our results show that a small number of LC-NE neurons have broad cortical projections, though the pattern is not uniform. Centered-log ratio transformation of NE fiber distribution across the cortex and hippocampus reveals a few preferential targets (PTs) of the labeled LC-NE neurons' axonal projections. The summed NE fiber length in these defined PTs is enriched relative to the geometric mean of all other cortical and hippocampal regions where NE fibers were detected. Notably, these PTs, including the CgC (where the retrograde viral infusion was made), the retrosplenial cortex, the dorsal hippocampus, the somatosensory cortex, and the motor cortices, seem to be functionally related to navigation. In summary, our results suggest that the cortical projections of LC-NE neurons are organized into PTs, which are functionally correlated, and non-PTs, which are more divergent and lack functional correlation.