The directional flow of information between different parts of the brain is mediated by a single neuron. Neurons are composed of cell bodies, dendrites that receive incoming information, and prominent axons that send information to other neuronal cells. The synapse at the end of the axon forms a connection with the dendrites of the proximal neuron cell.
In neuron tracking experiments, compounds are injected into brain tissue. These compounds will diffuse from the application site within the cell and help to observe the morphology of individual neurons, including dendrites and axon extensions. Most importantly, Any connection area between neurons and other distant brains. It is advisable to track compounds to outline neurons in the direction of information flow in order to understand which brain areas communicate with each other and how they communicate (that is, where the signals come from and what their signals may mean). Anterograde tracing outlines neurons from the cell body to the end of the axon; retrograde tracing outlines the neuron in the opposite direction, from their axon end to their cell body.
The anterograde and retrograde tracking takes advantage of the existing transport pathways in neurons. Anterograde transport is usually used to transport organelles, such as mitochondria, and macromolecules such as actin and myosin, and enzymes for the synthesis of transmitters. Retrograde transport is used to transport endocytosed substances or target molecules for degradation. These two pathways also use different cytoskeletal mechanisms to promote transport: retrograde transport depends on dynein, while anterograde transport depends on kinesin. The different speeds of the various forms of retrograde and anterograde transport indicate the existence of several parallel mechanisms.
With the advent of viral vector technology, new methods for studying neuronal connections have been developed. These methods are less neurotoxic and are better compatible with other neuroscience methods (such as electrophysiological recording). The origin of these methods are naturally occurring viruses that infect, persist and migrate within neurons, and are also able to spread across synaptic connections. The most famous is the rabies virus (RABV), which moves retrogradely from the peripheral infection site to the central nervous system, where it replicates, spreads, and causes neurotoxicity, it will be fatal if not treated in time. The herpes simplex virus (HSV) also migrates through neurons, where it can replicate and in doing so spreads to multiple synaptic connections. When these viruses are used to mark neuronal connections, most of the neuronal cells will be marked overtime due to the continuous replication of the virus. Although this is useful, if you have too many labeled neurons, it may be difficult to accurately map network connections.
AAV vectors have been widely used by neuroscientists for some time due to their superior characteristics in brain research and the ability to introduce genetic material into neuronal cells. The AAV genome is a single-stranded DNA molecule, and AAV vectors are defective in replication (that is, they cannot spread between cells as easily as natural RABV or HSV). AAV does not have a membrane envelope like lentivirus, but is contained in a capsid, which is composed of viral proteins VP1, VP2, and VP3. To date, hundreds of variants or serotypes have been discovered naturally or genetically engineered to make their capsid proteins differ.
The ideal comprehensive tool set for neuron tracking and network recognition requires forward and retrograde labeling, synaptic restriction, and controlled single and multiple synaptic propagation. AAV vectors cover some of these properties, and their use allows visualization of a single connected network within the rodent brain. A better understanding of the pathways and mechanisms of AAV intracellular transport can facilitate the development of more powerful AAV serotypes and may lead to controlled transsynaptic transmission of AAV vectors. It is also possible to improve neuron tracking by combining optogenetic-controlled transgenic and conditional transgenic mice, which can not only reveal the anatomical structure of neuronal connections, but also display functionally related neuronal connections, that is, activity-dependent neuronal tracing . In addition to mapping neuronal networks, if improved AAV vectors can reach the affected parts of the brain in a less invasive way, they can also become powerful tools in the field of gene therapy.