The BG (BG) are a complex network of several central nervous system nuclei, which orchestrate voluntary movements. Among these nuclei, the substantia nigra pars reticulata (SNr) is a GABAergic nucleus of particular interest, as it is the output nucleus of this network. The SNr integrates information received from striatal GABAergic neurons and from excitatory neurons of sub-thalamic nucleus. The SNr then projects to motor subdivisions of the thalamus, the superior colliculus and the pedonculopontine nucleus. Pioneering anatomical studies in rodents and non-human primates have suggested that the SNr is composed of cells with heterogeneous morphology. Heterogeneity of GABAergic neurons has been characterized in the cortex and in the hippocampus. Similar characterization of GABAergic diversity in the SNr will certainly improve our understanding of the BG function. The striatum, the input nucleus of the BG, expresses mainly two types of cells the medium spiny neurons containing dopamine (DA) receptor type 1 (D1R-MSNs) and the DA receptor type 2 (D2R-MSNs). A fine balance between activation of the two MSN pathways allows motor coordination and this balance relies on DA that is released from SNc DA neurons. In Parkinson’s disease (PD), a lack of DA in the BG modifies this equilibrium and strongly impairs motor control. This disequilibrium produces the typical motor symptoms of the disease, specifically difficulty in initiating movements, resting tremor, stiffness, slowing of movement and postural instability. The cause of PD remains elusive, but numerous studies have focused on different aspects of the disease such as the genetic factors, the neurodegeneration of SNc DA neurons itself, or the appearance of protein aggregates called Lewy bodies. However, the effect of DA depletion on synaptic connectivity in the BG, and the implications of this altered connectivity on BG function have not been investigated. Such network rewiring in PD has only been studied in the striatum and was shown to lead to disequilibrium between the two pathways. It is therefore likely that DA depletion causes rewiring also in the SNr, which would have important implications for BG function.
The goal of this research project is to dissect the microcircuitry of the BG circuitry focusing onto the SNr. Preliminary experiments presented in this project proposal show that the SNr is a heterogeneous GABAergic nucleus and identify the majority of cell types. This inevitably raises the issue that our understanding of the function of the SNr and the BG is over-simplified. Based on this new evidence, we hypothesize that each subpopulation of SNr neurons is differentially involved in a specific circuit to produce a given motor behavior and that the modifications of the specific SNr neuronal circuits leads to the pathological motor symptoms characterizing PD. Hence we focus on:
(1) Dissect the SNr circuitry.
a- Investigate the heterogeneity of SNr neurons
b- Reveal local SNr circuitry
c- Characterize the input/output connectivity of SNr subtype neurons
d- Examine the synaptic plasticity rules of the SNr circuitry
(2) Investigate the DA neuromodulation of each SNr subcircuit.
(3) Assess the role of SNr subtype neurons/subcircuits in driving specific motor behaviors.
(4) Characterize SNr circuitry rewiring in mouse models mimicking PD’s motor symptoms.
The main strategy will rely on the use of cell-specific transgenic mouse lines combined to viral-mediated gene delivery. This will allow the identification and optogenetic manipulation of one SNr subpopulation neuron at a time. In vitro and in vivo electrophysiology will be performed to assess the functional connectivity between SNr neurons and also with their inputs and outputs. DA pharmacological agents will be applied in vitro and in vivo to study DA neuromodulation of the SNr neurons and circuits. Confocal imaging and electron microscopy will represent valuable tools to further confirm neuronal identity and connectivity in baseline conditions and following DA depletion. Finally, motor-based behavioral tests will be combined with optogenetic stimulation of specific SNr neuronal subpopulations to assess their specific role in a precise motor task.
This study of the SNr circuitry will offer insights into the still poorly understood physiological mechanisms linking cell-type specificity and synaptic function to BG network activity and behavior. In addition, the study of modifications or rewiring within this system in a model of PD will provide detailed knowledge of the cellular basis of motor disorders, which may lead to novel therapeutic strategies.