Distinct neuronal populations in the mouse reticular formation regulate locomotion, grasping and breathing

Animals constantly interact with their environment including other living
creatures. The nervous system constantly integrates this multitude of sensory
information with its internal state and with previous experience to produce a
coherent behavioral output. Interestingly, many outputs of neural computations
generate motor behaviours. Motor neurons (MNs) are the final command lines
regulating muscular contractions and they are located in different regions of the
nervous system. In my PhD Thesis, I focused on the motor neuron pools residing
in the spinal cord (limb-innervating and phrenic motor neurons), work that allowed
us to get insight into neuronal circuits responsible for the execution of three
fundamental behaviours: (1). grasping, (2). locomotion and (3). breathing.
As an entry point into this study, we needed to reveal the channels the brain uses
to communicate with MNs innervating limb muscles. We took advantage of rabies
tracing technology and found that the reticular formation (RF) contains a number
of neuronal populations with direct access to MNs and other spinal cord circuits.
Retrograde tracing experiments revealed many identifiable clusters of neurons in
the RF, providing us with first insight how to study their role in motor program
execution. We first focused on a nucleus located in the caudal RF with
preferential connectivity to forelimb-innervating MNs known as MdV (medullary
reticular formation ventral part). MdV glutamatergic neurons exhibit connectivity
to some motor pools innervating a precise set of forelimb muscles and the same
MdV-vGlut2 neurons are essential for the grasping phase during a pellet reaching
task in mice. We speculate that MdV-vGlut2 neurons explicit their command
through circuits in the spinal cord but they also have a special channel to bypass
spinal cord circuits and directly interact with MNs.
Moving rostral into the brainstem I then focused on a different area which turned
out to be key in motor control, it is called magnocellular nucleus. It shows a
completely different connectivity pattern to spinal motor neurons; it has access
to both the cervical and lumbar spinal cord. We found that different
subpopulations in the magnocellular nucleus have access to the spinal cord but
only optogenetic activation of vGlut2 positive neurons located in the lateral
paragigantocellular nucleus (LPGi) produced stereotyped full body locomotion.
These same neurons are needed for high speed locomotion as revealed by loss
of function experiments. LPGi-vGlut2 neurons make preferential synaptic
contacts with interneurons (INs) located in the ventral part of the spinal cord (SC)
which are believed to be part of the central pattern generators known as essential
elements for locomotion rhythm and pattern generation. Other subpopulations
within the medulla show a very distinct role in behaviour when optogenetically
activated: Gi-vGlut2 neurons induce very reliable head turning and distinct
subpopulations of inhibitory neurons mediate behavioural arrest. Interestingly,
distinct inhibitory populations induce a phenotypically different behavioural arrest
when activated in vivo, which could provide a first insight into their physiological
role. We found that the two key populations regulating the speed of the animal
are intermingled within the LPGi, likely leading to cancellation of the behaviour
when simultaneously activated in vivo as we could demonstrate by stimulating all
2
LPGi neurons irrespective of neurotransmitter fate. We hypothesize that this
might be one of the reasons why they were not characterized previously.
A very different and extremely critical motor output is breathing. In collaboration
with the Fortin lab in Paris, we shed light on the neuronal populations in the RF
with direct access to phrenic motor neurons which innervate the diaphragm. We
revealed how genetically identified populations are assembled in a region laterocaudal to LPGi to generate a functional respiratory rhythm complex composed of
distinct cell types.
Together, these three projects allowed me to explore and study different aspects
of motor control and to understand how circuits in the RF are wired up and how
the nervous system takes advantage of specific connectivity profiles in the adult
to produce motor programs that everybody uses on a daily basis.