Fear conditioning is a form of associative learning that is known to involve different brain areas, notably the amygdala, the prefrontal cortex and the periaqueductal grey (PAG). Here, we describe the functional role of pathways that link the cerebellum with the fear network. We found that the cerebellar fastigial nucleus (FN) sends glutamatergic projections to vlPAG that synapse onto glutamatergic and GABAergic vlPAG neurons. Chemogenetic and optogenetic manipulations revealed that the FN-vlPAG pathway controls bi-directionally the strength of the fear memories, indicating an important role in the association of the conditioned and unconditioned stimuli, a function consistent with vlPAG encoding of fear prediction error. Moreover, FN-vlPAG projections also modulate extinction learning. We also found a FN-parafascicular thalamus pathway, which may relay cerebellar influence to the amygdala and modulates anxiety behaviors. Overall, our results reveal multiple contributions of the cerebellum to the emotional system.

SummaryThe medial habenula (MHb) is an epithalamic hub contributing to expression and extinction of aversive states by bridging forebrain areas and midbrain monoaminergic centers. Although contradictory information exists regarding their synaptic properties, the physiology of the excitatory inputs to the MHb from the posterior septum remains elusive. Here, combining optogenetics-based mapping with ex vivo and in vivo physiology, we examine the synaptic properties of posterior septal afferents to the MHb and how they influence behavior. We demonstrate that MHb cells receive sparse inputs producing purely glutamatergic responses via calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), heterotrimeric GluN2A-GluN2B-GluN1 N-methyl-D-aspartate (NMDA) receptors, and inhibitory group II metabotropic glutamate receptors. We describe the complex integration dynamics of these components by MHb cells. Finally, we combine ex vivo data with realistic afferent firing patterns recorded in vivo to demonstrate that efficient optogenetic septal stimulation in the MHb induces anxiolysis and promotes locomotion, contributing long-awaited evidence in favor of the importance of this septo-habenular pathway.

Tau assemblies have prion‐like properties: they propagate from one neuron to another and amplify by seeding the aggregation of endogenous Tau. Although key in prion‐like propagation, the binding of exogenous Tau assemblies to the plasma membrane of naïve neurons is not understood. We report that fibrillar Tau forms clusters at the plasma membrane following lateral diffusion. We found that the fibrils interact with the Na+/K+‐ATPase (NKA) and AMPA receptors. The consequence of the clustering is a reduction in the amount of α3‐NKA and an increase in the amount of GluA2‐AMPA receptor at synapses. Furthermore, fibrillar Tau destabilizes functional NKA complexes. Tau and α‐synuclein aggregates often co‐exist in patients’ brains. We now show evidences for cross‐talk between these pathogenic aggregates with α‐synuclein fibrils dramatically enhancing fibrillar Tau clustering and synaptic localization. Our results suggest that fibrillar α‐synuclein and Tau cross‐talk at the plasma membrane imbalance neuronal homeostasis.

Long-term synaptic plasticity is believed to be the cellular substrate of learning and memory. Synaptic plasticity rules are defined by the specific complement of receptors at the synapse and the associated downstream signaling mechanisms. In young rodents, at the cerebellar synapse between granule cells (GC) and Purkinje cells (PC), bidirectional plasticity is shaped by the balance between transcellular nitric oxide (NO) driven by presynaptic N-methyl-D-aspartate receptor (NMDAR) activation and postsynaptic calcium dynamics. However, the role and the location of NMDAR activation in these pathways is still debated in mature animals. Here, we show in adult rodents that NMDARs are present and functional in presynaptic terminals where their activation triggers NO signaling. In addition, we find that selective genetic deletion of presynaptic, but not postsynaptic, NMDARs prevents synaptic plasticity at parallel fiber-PC (PF-PC) synapses. Consistent with this finding, the selective deletion of GC NMDARs affects adaptation of the vestibulo-ocular reflex. Thus, NMDARs presynaptic to PCs are required for bidirectional synaptic plasticity and cerebellar motor learning.

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10Fear conditioning is a form of associative learning that is known to involve brain areas, notably the 11 amygdala, the prefrontal cortex and the periaqueductal grey (PAG). Here, we describe the functional 12 role of pathways that link the cerebellum with the fear network. We found that the cerebellar 13 fastigial nucleus (FN) sends glutamatergic projections to vlPAG that synapse onto glutamatergic and 14 GABAergic vlPAG neurons. Chemogenetic and optogenetic manipulations revealed that the FN-vlPAG 15 pathway controls bi-directionally the strength of the fear memory, indicating a role in the association 16 of the conditioned and unconditioned stimuli, a function consistent with vlPAG encoding of fear 17 prediction error. In addition, we found that a FN -thalamic parafascicular nucleus pathway, which 18 may relay cerebellar influence to the amygdala, is involved in anxiety and fear expression but not in 19 fear memory. Our results reveal the contributions to the emotional system of the cerebellum, which 20 exerts a potent control on the strength of the fear memory through excitatory FN-vlPAG projections. 21 22 23 Keywords: cerebellum; limbic circuits; fear memory, chemogenetics, electrophysiology 24 vermis, in relation to negative emotions (e.g. during recall of self-generated emotional episodes 20 ). 50 Consistent with this, localized cerebellar lesions, principally in the midline vermis, account in large 51 part for the emotional disturbances, inappropriate behavior and changes in affect, which are 52 collectively termed the "cerebellar cognitive affective syndrome" 21 and reported in cerebellar 53 patients. Several studies have shown that the cerebellum has functional connections from fear-54 related areas including the PAG, the amygdala, and the prefrontal cortex [22][23][24][25] . In accordance with the 55 existence of such connections, Pavlovian fear conditioning affects cerebellar plasticity 26 , post-56 conditioning cerebellar inactivation affects memory consolidation 27 , and cerebellar lesions -or 57 inactivation-modulate freezing 24,28 ; however, the pathways by which the cerebellum participates to 58 fear learning or expression remain undefined. 59. The cerebellar vermis, which is most consistently associated with emotional pathologies and fear 60 expression 20,24,29 , projects to the fastigial nucleus (FN), a deep cerebellar nucleus (DCN) which 61 projects to many targets from the spinal cord to the diencephalon 22 . The purpose of the present 62 study is to study the contribution of specific FN output pathways to fear learning. Using 63 neuroanatomical tracings, chemogenetic modulation of the cerebellar input to the vlPAG and to the 64 BLA during fear conditioning, optogenetics and extracellular electrophysiological recordings in awake 65 freely moving animals, we demonstrate the contribution of the cerebellum to fear learning through 66 its inputs to the vlPAG. 67 68 4 69 70Results 71 Neuroanatomical link between the cerebellum and vlPAG 72In order to examine cerebellar projections to areas involve...

Chronic Levodopa therapy, the gold-standard treatment for Parkinson’s Disease (PD), leads to the emergence of involuntary movements, called levodopa-induced dyskinesia (LID). Cerebellar stimulation has been shown to decrease LID severity in PD patients. Here, in order to determine how cerebellar stimulation induces LID alleviation, we performed daily short trains of optogenetic stimulations of Purkinje cells (PC) in freely moving LID mice. We demonstrated that these stimulations are sufficient to suppress LID or even prevent their development. This symptomatic relief is accompanied by the normalization of aberrant neuronal discharge in the cerebellar nuclei, the motor cortex and the parafascicular thalamus. Inhibition of the cerebello-parafascicular pathway counteracted the beneficial effects of cerebellar stimulation. Moreover, cerebellar stimulation reversed plasticity in D1 striatal neurons and normalized the overexpression of FosB, a transcription factor causally linked to LID. These findings demonstrate LID alleviation and prevention by daily PC stimulations, which restore the function of a wide motor network, and may be valuable for LID treatment.

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