Regarding the 1 objective several experiments have been done to describe the network between the DCNs, the Mthal and the brainstem. Injection of retrograde and anterograde tracers in the different brain regions have enabled to draw a map of projections between the 3 structures. The major finding is that neuronal populations inside the DCNs send specific projections to subregions of the brainstem and the Mthal. The anatomical analysis for the retrograde experiment consisted in schematic reconstructions of neuronal populations at the level of the DCNs, with count of the total number of neurons in each DCNs regions. Regarding the anterograde experiment, schematic reconstructions of the synaptic population (originating from DCNs projections) at the level of the brainstem and the Mthal have been made.
The following experiment showed that single neurons in DCNs send collaterals to both the Mthal and the brainstem. Two experiments have been realized to come to this conclusion. First, two distinct retrogrades tracers have been injected in the Mthal and the brainstem, and the number of neurons expressing both tracers in the DCNs has been recorded. The second experiment consisted in a first injection of a retrograde virus in the brainstem. At the level of the DCNs, a second virus has been injected, for which his expression depends on the presence of the first injected virus in the same neurons. In that case, the second virus will express a synaptic tag, enable to see if the infected DCN neuron, projecting to the brainstem, send collaterals to the Mthal as well.These previous experiment identified the intDL, a nucleus of the DCNs, as an interesting candidate to focus on. Indeed, it sends projections to both the Mthal and the PCRt, a region of the brainstem. Because of his projection on the PCRt (see Esposito et al., 2014), the intDL-Mthal-PCRt pathway could be involved in forelimb movements. Moreover, the pathway from the intDL to the Mthal could transmit a copy of the motor command send to the PCRt to the motor cortex. To bring first confirmations of this hypothesis, two different viral strategies have been used in the mice, to show that the IntDL send projections to the thalamocortical neurons. Both experiments confirmed that hypothesis.These anatomical experiments showed that the DCNs send projections from specific neuronal subpopulations to distinct regions inside the Mthal and the brainstem. Moreover, these DCNs populations displayed collaterals projecting to both the Mthal and the brainstem. Inside the DCNs, the IntDL represents an interesting candidate to focus on because of his projections on the Mthal and the PCRt. Finally, the IntDL could send motor information to the motor cortex through the Mthal.Concerning the 2 and 3 objectives, only preliminary results have been obtained. Electrophysiological recordings of the intDL and the Mthal have shown a neuronal activity while mice were performing a forearm tasks, the single pellet reaching tasks. However these first results need to be confirmed on a larger population, and carefully analyzed. Optogenetic experiments have also been realized, consisting in activating acutely the different brain structures. Mthal activation didn’t trigger any movement, but the activation of the intDL led to a retraction of the arm and the closing of the fist. Moreover, the activation of IntDL terminals in the PCRt resulted in distinct arm movements, depending on the position of the fibers on the anteroposterior axis of the PCRt.These works have enabled to draw a map of the connexion between the DCNs, the brainstem and the Mthal. Preliminary functional results indicated a possible implication of the intDL-Mthal-brainstem pathway in the forelimb movements.
