Intrinsic excitability in the pathophysiology of the olivo-cerebellar circuit

The structure and function of neuronal circuits can be modified by experience during the encoding of memories, or under pathological conditions. The overall goal of my thesis is to investigate the role of intrinsic excitability (I) in neuroplasticity and (II) in the pathophysiology of the cerebellum. As a first aim, we investigated activity-dependent structural plasticity of cerebellar climbing fibers (CFs). These convey a teaching signal to Purkinje cells (PCs) that is crucial for learning. These fibers originate in the inferior olive (IO), located in the brainstem, and can undergo dramatic structural rearrangements following injuries. Nonetheless, it is still needed to be clarified whether their structure may depend also on their activity. To answer this question, we investigated how modifications of CF intrinsic excitability affect their structure and the physiology of the olivo-cerebellar circuit. To do this, we chronically altered CF intrinsic excitability by in vivo knocking-down voltage-gated sodium channels in the IO of adult mice. We analyzed the 3D morphology and presynaptic terminals of treated CFs, as well as the postsynaptic spines on PCs that synapse with treated CFs and we looked at functional consequences in CF synaptic transmission. We compared the effects with those obtained by knocking-down the growth-associated protein GAP-43, a potential candidate for mediating physiological structural changes of CFs. We show that knocking-down voltage-gated sodium channels cause a modification of CFs consisting in shorter and fewer branches, with a compensatory increase of axonal varicosities and dendritic spines, showing that activity-dependent structural plasticity can occur in CFs. By patch-clamp recording we show that these modifications are associated to slower kinetics of synaptic currents, suggesting that they can potentially affect cerebellar function. As a second aim, we investigated the possible role of alterations of intrinsic excitability in the cerebellar pathophysiology of a mouse model of multiple sclerosis (MS). Neuronal function can be modified under pathological conditions. In MS, a chronic demyelinating disease, in addition to demyelination, synaptic impairments in the grey matter contribute to the pathogenesis of the experimental autoimmune encephalomyelitis EAE, the most common mouse model for MS. It is however unclear whether other neuronal impairments occur, affecting the function of neuronal circuits before demyelination becomes the prominent pathogenic event. Some lines of evidence suggest that neuronal intrinsic excitability may be affected by neuroinflammation early during the disease, thus affecting synaptic integration and signal processing. We therefore focused on cerebellar PCs intrinsic excitability during the presymptomatic and acute phase of EAE. We observed, for the first time to our knowledge, an increase in PCs intrinsic excitability in EAE mice compared to their control, both at the presymptomatic and symptomatic phase of the disease, partially caused by a dysregulation of SK-type calcium-dependent potassium channels. Our results suggest that SK channels may be a good candidate to be further investigated as a possible therapeutic target to treat MS. These results are included in two manuscripts currently in preparation that will be listed at the end of the conclusions.