Caggiano et al

Question 1

Background

Answer 1

Gait selection is ubiquitous yet mysterious
• Different speeds of locomotion are associated with different
gaits
• How are these gaits initiated, selected, and maintained? (gait
control)
• The mesencephalic locomotor region (MLR) is one locus for gait
control
• MLR consists of portions of both the cuneiform nucleas (CnF) and
pedunculopontine nucleus (PPN)
• Do these two nuclei play different roles in gait control?

Question 2

Methods

Answer 2

CnF/PPN optogenetic perturbations
• Use Vglut2cremice to target excitatory neurons
• Vglut = glutamate transporter = excitatory cells (maybe)
• Inject a virus (AAV) containing a cre-dependent channelrhodopsin (ChR2) + fluorescent tag for vis.
• End up with CnF- or PPN-specific population control

Chemogenetic manipulations
• DREADD’s can be used to inactivate a region chronically
• Can also be virally-targeted to specific regions like optotools

Question 3

Figure 1

Answer 3

CnF and PPN excitation uniquely drive gait induction.
• CnF seems to stochastically drive faster gaits, while PPN drives slower gaits with higher reliability
• With higher power, it is possible to activate faster speeds for CnF, but not PPN.

Question 4

Figure 2

Answer 4

CnF and PPN inhibition is sufficient to decrease slow locomation rates.
• Inhibiting CnF or PPN is sufficient to reduce locomotion speed
• Both regions are additive; inhibiting both creates a stronger change in behavioral response than inhibiting just one
• This also means that each region is able to ‘independently’ generate
slower gaits

Question 5

Figure 3

Answer 5

CnF is necessary for the production of fast-gait
locomotion.

 DREADD inhibition of CnF
leads to a decrease in fast 
gaits associated with escape
• Inhibiting PPN leads to a 
decrease in locomotion 
velocity, but fast gaits are still  
driven via CnF opto activation
• Thus, PPN is not required for 
fast gaits, but can regulate 
CnF driven fast gaits

Question 6

Figure 4

Answer 6

CnF and PPN show unique and nuanced
activity-speed relationships.

• CnF firing rates closely 
track speed
• PPN firing rates can both 
track speed and signal the 
onset of locomotion
• CnF neurons are more 
selective for higher speeds, 
while PPN neurons are 
more selective for lower 
speeds

Question 7

Figure 5

Answer 7

PPN exhibits selective control of exploratory
behaviors.

• Slow gaits are often associated with exploratory behaviors
• Inhibiting PPN, but not CnF, leads to a decrease in exploratory head-dips
• Activating PPN, but notCnF, is sufficient to induce head-dips
- PPN likely drives exploratory behaviors

Question 8

Figure 6

Answer 8

PPN and CnF have distinct inputs.

• Rabies retrograde tracing
• PPN has wider inputs than the CfN
• However, CfN hasmore relativeinputs from themidbrain than PPN
• Reciprocal connections

Question 9

Summary of results

Answer 9

• PPN and CnF both contribute to slower gaits
• Only CnF can elicit high-speed gaits
• MLR is not homogenous
• PPN supports slow, explorative behavior
• CnF supports fast, escape behavior

Question 10

Critiques

Answer 10

• The authors do a good job of characterizing the roles of CnF and
PPN, using a variety of tools
• However, their analysis of neural activity is somewhat limited
• Furthermore, N is quite low (2 at times)
• What sort of computational/processing role do CnF and PPN
play? Do they simply encode a gait or do they generate some
sort of rhythm?
• Effect sizes are not large across many experiments