science

Retrograde tracing

Retrograde tracing is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Retrograde tracing rather than just read about it. In short: Retrograde tracing is a research method used in neuroscience to trace neural connections from their point of termination (the synapse) to their source (the cell body). Retrograde tracing techniques allow for detailed assessment of neuronal connections between a target population of neurons and their inputs throughout the nervous system.

Key takeaways

Retrograde tracing belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Retrograde tracing to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Retrograde tracing from memory before moving on to harder problems.

Reference excerpt

Retrograde tracing is a research method used in neuroscience to trace neural connections from their point of termination (the synapse) to their source (the cell body). Retrograde tracing techniques allow for detailed assessment of neuronal connections between a target population of neurons and their inputs throughout the nervous system. These techniques allow the "mapping" of connections between neurons in a particular structure (e.g. the eye) and the target neurons in the brain. The opposite technique is anterograde tracing, which is used to trace neural connections from their source to their point of termination (i.e. from cell body to synapse). Both the anterograde and retrograde tracing techniques are based on the visualization of axonal transport.

Techniques Retrograde tracing can be achieved through various means, including the use of viral strains as markers of a cell's connectivity to the injection site. The pseudorabies virus (PRV; Bartha strain), for example, may be used as a suitable tracer due to the propensity of the infection to spread upstream through a pathway of synaptically linked neurons, thus revealing the nature of their circuitry. Rabies has been shown to be effective for this system of circuit tracing because of its low level of damage to infected cells, specificity of infecting only neurons, and strict limitation of viral spread between neurons to synaptic regions. These factors allow for highly specific traces that can reveal individual neuronal connections in a circuit without inflicting physical damage on the cells. Another technique involves injecting special "beads" into the brain nuclei of anaesthetized animals. The animals are allowed to survive for a few days and then euthanized. The cells in the origin of projection are visualized through an inverted fluorescence microscope. A specialist technique was developed by Wickersham and colleagues, which employed a modified rabies virus. This virus was capable of infecting a single cell and jumping across one synapse; this allowed the researchers to investigate the local connectivity of neurons.

Rabies virus After being taken up at the synaptic terminal or axon of the target neuron, the rabies virus is enveloped in a vesicle which is transported towards the cell body via axonal dynein. In the wildtype rabies virus, the virus will continue to replicate and spread throughout the central nervous system until it has systemically infected the entire brain. Deletion of the gene encoding glycoprotein (G protein) in rabies limits the spread of the virus strictly to cells that were initially infected. Transsynaptic spread of the virus can be limited to monosynaptic transmission to a neuron of origin by pseudotyping the G protein and putting the gene under Cre-control. This viral spread can be visualized through methods including addition of a fluorescence gene such as green fluorescent protein onto the viral cassette or through immunohistochemistry.

Pseudorabies virus A member of the herpesviridae family, the pseudorabies virus spreads through the CNS in both a retrograde and anterograde fashion, moving up the neural axon into the soma and dendrites in the retrograde application. Deletion of three key membrane protein genes in the PRV-Bartha strain of pseudorabies blocks anterograde spread of the virus and allows for additional manipulations to the viral DNA such as fluorescence to be added, allowing for retrograde circuit tracing.

Fluoro-Gold Fluoro-Gold, also known as hydroxystilbamidine, is a non-viral fluorescent retrograde tracer whose movement up the axon and across the dendritic tree can be visualized via fluorescent microscopy or immunohistochemistry.

See also Anterograde tracing Neural pathway Viral neuronal tracing

References

Further reading Retrograde tracing has been extensively used in a broad array of neuroscience studies, including the following examples:

Song, Chenghui; Ehlers, Vanessa L.; Moyer, James R. (2015-09-30). "Trace Fear Conditioning Differentially Modulates Intrinsic Excitability of Medial Prefrontal Cortex–Basolateral Complex of Amygdala Projection Neurons in Infralimbic and Prelimbic Cortices". Journal of Neuroscience. 35 (39): 13511–13524. doi:10.1523/JNEUROSCI.2329-15.2015. ISSN 0270-6474. PMC 4588614. PMID 26424895. Bácskai, Tímea; Rusznák, Zoltán; Paxinos, George; Watson, Charles (2014-01-01). "Musculotopic organization of the motor neurons supplying the mouse hindlimb muscles: a quantitative study using Fluoro-Gold retrograde tracing". Brain Structure and Function. 219 (1): 303–321. doi:10.1007/s00429-012-0501-7. hdl:20.500.11937/26565. ISSN 1863-2653. PMID 23288256. S2CID 17675285. Schwarz, Lindsay A.; Miyamichi, Kazunari; Gao, Xiaojing J.; Beier, Kevin T.; Weissbourd, Brandon; DeLoach, Katherine E.; Ren, Jing; Ibanes, Sandy; Malenka, Robert C. (2015). "Viral-genetic tracing of the input–output organization of a central noradrenaline circuit". Nature. 524 (7563): 88–92. Bibcode:2015Natur.524...88S. doi:10.1038/nature14600. PMC 4587569. PMID 26131933. Ohara, Shinya; Sato, Sho; Tsutsui, Ken-Ichiro; Witter, Menno P.; Iijima, Toshio (2013-11-06). "Organization of Multisynaptic Inputs to the Dorsal and Ventral Dentate Gyrus: Retrograde Trans-Synaptic Tracing with Rabies Virus Vector in the Rat". PLOS ONE. 8 (11) e78928. Bibcode:2013PLoSO...878928O. doi:10.1371/journal.pone.0078928. ISSN 1932-6203. PMC 3819259. PMID 24223172. DeNardo, Laura A; Berns, Dominic S; DeLoach, Katherine; Luo, Liqun (2015). "Connectivity of mouse somatosensory and prefrontal cortex examined with trans-synaptic tracing". Nature Neuroscience. 18 (11): 1687–1697. doi:10.1038/nn.4131. PMC 4624522. PMID 26457553.

Illustrations

Worked examples

Example 1 — a first encounter with Retrograde tracing

Start with the simplest possible case. Write down what Retrograde tracing claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Retrograde tracing before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Retrograde tracing ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Retrograde tracing

In research: Retrograde tracing appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Retrograde tracing in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Retrograde tracing is common in secondary-school and first-year university syllabi. It links to neighbouring topics Neural coding, so understanding it makes those chapters shorter.
In everyday life: Look for Retrograde tracing outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Retrograde tracing in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Retrograde tracing means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Retrograde tracing out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Retrograde tracing in simple terms?

Why does Retrograde tracing matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Retrograde tracing?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Retrograde tracing.