Monosynaptic Cerebellar Modulation of the Substantia Nigra

Abstract

The survival of an organism depends on its ability to find and consume energy and reproduce in a constantly changing environment. Doing so necessitates a range of movements, but exactly how the brain plans, selects, and executes precise motor sequences remains unclear. Two subcortical structures, the cerebellum and basal ganglia, communicate with the cerebral cortex and thalamus to generate motor output. It has long been thought that the basal ganglia and cerebellum serve separate but complementary roles in motor control, and that information flows between the cortex and the basal ganglia or cerebellum through separate, parallel "loops." Thus integration of basal ganglia and cerebellar outputs occurs only at the level of the cortex. A growing body of evidence, however, suggests that these circuits are not as separate as originally thought. It has been shown across several species that there are a number of pathways in the brain capable of mediating communication directly from the cerebellum to the basal ganglia, and vice versa. In contrast to the long, multisynaptic loops through cortex, these pathways enable the cerebellum and basal ganglia to coordinate their outputs on faster timescales. In this thesis, I examine and characterize a pathway from the cerebellum to the substantia nigra and examine how the cerebellum might be involved in a movement disorder often attributed to basal ganglia dysfunction.

The substantia nigra is a structure within the basal ganglia that consists of the pars compacta (SNc), a nucleus made of modulatory, dopaminergic neurons that project to and modulate the input structure of the basal ganglia, and the pars reticulata (SNr), which is a major output nucleus of the basal ganglia. There is anatomical evidence dating almost fifty years of a projection from the cerebellar nuclei to the dopaminergic neurons of the SNc, but whether neurons in the cerebellum can drive activity of these dopamine neurons on a fast timescale is unknown. In contrast, cerebellar projections to SNr have not previously been reported. Here, we test the hypothesis that the cerebellum can modulate the SNc and SNr through a direct, monosynaptic connection, using a combination of optogenetics and in vivo and in vitro electrophysiology. We found, in vivo, that neurons in SNc and SNr rapidly increased the number of spikes fired immediately following optogenetic stimulation of cerebellar fibers. Experiments in vitro determined that stimulation of cerebellar fibers evoked excitatory currents in both structures and confirmed that this modulation occurred through a monosynaptic pathway. Taken together, these data provide support for a cortex-independent pathway capable of relaying information directly from the cerebellum to the SNc and SNr and have important implications for understanding how the cerebellum and basal ganglia interact to generate movements under normal and pathological conditions.

Previous work from our laboratory has shown that cerebellar-basal ganglia interactions are relevant in a number of movement disorders, including dystonia. The second part of this thesis examines how the cerebellum is implicated in a movement disorder typically associated with basal ganglia dysfunction. Myoclonus dystonia (DYT11) is an inherited movement disorder caused by loss-of-function mutations in SGCE and characterized by involuntary jerking of the upper body (myoclonus) and sustained contraction of agonist and antagonist muscles that result in painful, twisted postures (dystonia). A striking feature of this disorder is that patients frequently report improvement of motor symptoms after consumption of alcohol. Unfortunately, the neural basis of DYT11 is unclear, although the basal ganglia and cerebellum have been implicated. Here, we test the hypothesis that the cerebellum contributes to motor symptoms in DYT11. To that end, we generated a mouse model of DYT11 using short hairpin RNA (shRNA) to knock down sgce in the adult mouse. We found that knockdown of sgce in the cerebellum, but not the basal ganglia, produced dystonia and repetitive jerk-like movements in mice and showed that these symptoms improved after administration of ethanol, consistent with what is seen in patients. Further, we performed extracellular recordings from dystonic mice and found that cerebellar neurons fire aberrantly in dystonic mice.

The work in this thesis demonstrates that the cerebellum and basal ganglia are more interconnected than previously thought. The cerebellum is capable of affecting the activity of substantia nigra pars compacta, a modulatory nucleus within the basal ganglia, and the pars reticulata, a major output nucleus of the basal ganglia. Further, this work illustrates how interactions between the basal ganglia and cerebellum may have significant implications for movement disorders, as disorders like dystonia frequently associated with the basal ganglia may also involve the cerebellum.