Table 2. Summary of pros and cons of each described method.
| Method | Pros | Cons | Outcome | Reference |
|---|---|---|---|---|
| Muscle injection of AAV-G (serotype 2.6)+RabV (Figure 1A) | Avoids the possibility of retrograde disynaptic transfer from second order motor neurons due to restriction of G expression to targeted motor neurons | The labelled premotor population could be contaminated by anterogradely labelled neurons from primary sensory neurons. | Flexor- extensor segregation No flexor- extensor segregation |
(Tripodi et al., 2011) Present study |
| Avoids the possibility of retrograde disynaptic transfer from premotor spinal interneurons. | ||||
| Muscle injection of AAV-flex-G (serotype 2.6)+RabV in ChatCre/+ mice (Figure 1B) | Avoids the possibility of retrograde disynaptic transfer from second order motor neurons due to restriction of G expression to targeted motor neurons | Conditional expression of G may be inefficient | Flexor- extensor segregation No flexor- extensor segregation |
(Wang et al., 2017) Present study |
| Avoids the possibility of retrograde disynaptic transfer from premotor spinal interneurons. | ||||
| Avoids potential anterograde sensory contamination. | ||||
| Central injection of AAV-flex-G (serotype 2.9) in ChatCre/+ mice followed by muscle injection of RabV, in adults (Figure 1C) | Limits the issue of potential disynaptic transfer from cholinergic interneurons | Potential for disynaptic transfer from cholinergic premotor interneurons, transsynaptically labelled motor neurons and mis-targeted primary motor neurons | Flexor- extensor segregation | Takeoka and Arber, 2019 |
| Avoids potential anterograde tracing from sensory neurons | ||||
| Genetically driven expression of G in ChatCre/+ or Olig2Cre/+ mice + muscle RabV injection in neonates (Figure 1D and E) | Avoids potential anterograde tracing from sensory neurons | Potential for disynaptic transfer from premotor spinal interneurons, transsynaptically labelled motor neurons and mis-targeted primary motor neurons. | No flexor- extensor segregation | Present study |
| Ensures homogenous expression of G in all motor neurons | ||||
| Muscle injection of PRV-Bartha with strictly timed fixation of tissue (Figure 1F) | High efficiency in transsynaptic transmission. Not reliant on viral recombination. | Timed fixation does not guarantee that transsynaptic jumps occur only up to the second order | No flexor-extensor segregation | Present study |