Download Genetic tracing shows noradrenergic neuronal circuitries of locus

Genetic tracing shows noradrenergic neuronal circuitries of locus coeruleus
Youngbuhm Huh
The technique to visualize complicated and extended neural networks in the brain is critical to
understanding the functional organization of complex motor and sensory systems and is particularly valuable
when the object is to visualize the neuronal plasticity of these systems in injury and disease. During the past
several decades, studies using neuronal tracers have provided much information about the neuronal circuitry
of the brain. After application or injection, tracers such as horseradish peroxidase or wheat germ agglutinin
(WGA) are transported through the axons or dendrites of neurons, where they can be visualized by
histochemical or immunohistochemical techniques.
Although transneuronal transport of tracers has been used in several circuitry mapping studies,
conventional tracing methods have limitations, due to the difficulty in delivering tracer molecules to neurons
of a given phenotype. To circumvent this limitation for the study of transsynaptic noradrenergic circuitry, we
generated a recombinant adenoviral construct that can direct the expression of the transsynaptic tracer, WGA
under the synthetic noradrenergic-specific promoter. When these PRS-WGA adenoviral constructs were
stereotactically injected into the locus coeruleus containing major noradrenergic cell bodies, WGA was
robustly expressed in noradrenergic neurons of the locus coeruleus. Then, we established that WGA is
transported to specific target areas in a time-dependent manner. One day after adenoviral injection, WGAexpressing cell bodies were detected in the locus coeruleus, the mesencephalic trigeminal nucleus, the medial
parabrachial nucleus, the lateral parabrachial nucleus, the motor trigeminal nucleus, the principal sensory
trigeminal nucleus, A5 noradrenergic cells and the vestibular nucleus. Six days after adenoviral injection,
WGA positive cells were found in the lateral periaqueductal gray, the dorsolateral periaqueductal gray,
dorsomedial hypothalamic nucleus, the lateral hypothalamic area and the paraventricular nucleus of the brain.
However, we could not differentiate the WGA positive neurons between primary target neurons and
secondary WGA transported neurons. We tested several possible tracers using the cell type specific promoter
system and finally we found the TRX in the presynaptic terminal of noradrenergic neurons. Therefore, we can
visualize the primary target neurons and transsynaptic connections of LC noradrenergic neurons.
Because WGA-TRX expression in PRS-tracer adenovirus injected rats is under the control of
noradrenergic specific promoter, the circuits that are revealed are system specific. This approach will be
useful for transsynaptic mapping and functional characterization of noradrenergic neuronal pathways in
normal and diseased brains.