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.

The mediodorsal nucleus (MD) represents just one piece of a complex relay structure situated within the brain, called the thalamus. MD is characterized by its robust interconnections with other brain areas, especially with limbic-related structures. Given the close anatomo-functional relationship between the MD and the limbic system, this particular thalamic nucleus can directly influence various affective behaviors and participate in cognition. In this work, we review data collected from multiple anatomical studies conducted in rodent, human, and non-human primates, highlighting the complexity of this structure and of the neural networks in which it takes part. We provide proof that the MD is involved in the unification of several anatomical structures, being able to process the information and influence the activity in numerous cortical and subcortical neural circuits. Moreover, we uncover intrinsic and extrinsic mechanisms that offer MD the possibility to execute and control specific high functions of the nervous system. The collected data indicate the great importance of the MD in the limbic system and offer relevant insight into the organization of thalamic circuits that support MD functions.

Fear extinction is a form of inhibitory learning that suppresses the expression of aversive memories and plays a key role in the recovery of anxiety and trauma-related disorders. Here, using male mice, we identify a cerebello-thalamo-cortical pathway regulating fear extinction. The cerebellar fastigial nucleus (FN) projects to the lateral subregion of the mediodorsal thalamic nucleus (MD), which is reciprocally connected with the dorsomedial prefrontal cortex (dmPFC). The inhibition of FN inputs to MD in male mice impairs fear extinction in animals with high fear responses and increases the bursting of MD neurons, a firing pattern known to prevent extinction learning. Indeed, this MD bursting is followed by high levels of the dmPFC 4 Hz oscillations causally associated with fear responses during fear extinction, and the inhibition of FN-MD neurons increases the coherence of MD bursts and oscillations with dmPFC 4 Hz oscillations. Overall, these findings reveal a regulation of fear-related thalamo-cortical dynamics by the cerebellum and its contribution to fear extinction.

The synchronization of neuronal activity in the sensorimotor cortices is crucial for motor control and learning. This synchrony can be modulated by upstream activity in the cerebello-cortical network. However, many questions remain over the details of how the cerebral cortex and the cerebellum communicate. Therefore, our aim is to study the contribution of the cerebellum to oscillatory brain activity, in particular in the case of dystonia, a severely disabling motor disease associated with altered sensorimotor coupling. We used a kainic-induced dystonia model to evaluate cerebral cortical oscillatory activity and connectivity during dystonic episodes. We performed microinjections of low doses of kainic acid into the cerebellar vermis in mice and examined activities in somatosensory, motor and parietal cortices. We showed that repeated applications of kainic acid into the cerebellar vermis, for five consecutive days, generate reproducible dystonic motor behavior. No epileptiform activity was recorded on electrocorticogram (ECoG) during the dystonic postures or movements. We investigated the ECoG power spectral density and coherence between motor cortex, somatosensory and parietal cortices before and during dystonic attacks. During the baseline condition, we found a phenomenon of permanent adaptation with a change of baseline locomotor activity coupled to an ECoG gamma band increase in all cortices. In addition, after kainate administration, we observed an increase in muscular activity, but less signs of dystonia together with modulations of the ECoG power spectra with an increase in gamma band in motor, parietal and somatosensory cortices. Moreover, we found reduced coherence in all measured frequency bands between the motor cortex and somatosensory or parietal cortices compared to baseline. In conclusion, examination of cortical oscillatory activities in this animal model of chronic dystonia caused by cerebellar dysfunction reveals a disruption in the coordination of neuronal activity across the cortical sensorimotor/parietal network, which may underlie motor skill deficits.

The execution of voluntary muscular activity is controlled by the primary motor cortex, together with the cerebellum and basal ganglia. The synchronization of neural activity in the intracortical network is crucial for the regulation of movements. In certain motor diseases, such as dystonia, this synchrony can be altered in any node of the cerebello-cortical network. Questions remain about how the cerebellum influences the motor cortex and interhemispheric communication. This research aims to study the interhemispheric cortical communication between the motor cortices during dystonia, a neurological movement syndrome consisting of sustained or repetitive involuntary muscle contractions. We pharmacologically induced lateralized dystonia to adult male albino mice by administering low doses of kainic acid on the left cerebellar hemisphere. Using electrocorticography and electromyography, we investigated the power spectral densities, cortico-muscular, and interhemispheric coherence between the right and left motor cortices, before and during dystonia, for five consecutive days. Mice displayed lateralized abnormal motor signs, a reduced general locomotor activity, and a high score of dystonia. The results showed a progressive interhemispheric coherence decrease in low-frequency bands (delta, theta, beta) during the first 3 days. The cortico-muscular coherence of the affected side had a significant increase in gamma bands on days 3 and 4. In conclusion, lateralized cerebellar dysfunction during dystonia was associated with a loss of connectivity in the motor cortices, suggesting a possible cortical compensation to the initial disturbances induced by cerebellar left hemisphere kainate activation by blocking the propagation of abnormal oscillations to the healthy hemisphere. However, the cerebellum is part of several overly complex circuits, therefore other mechanisms can still be involved in this phenomenon.

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...

Motor coordination and motor learning are well-known roles of the cerebellum. Recent evidence also supports the contribution of the cerebellum to the oscillatory activity of brain networks involved in a wide range of disorders. Kainate, a potent analog of the excitatory neurotransmitter glutamate, can be used to induce dystonia, a neurological movement disorder syndrome consisting of sustained or repetitive involuntary muscle contractions, when applied on the surface of the cerebellum. This research aims to study the interhemispheric cortical communication between the primary motor cortices after repeated kainate application on cerebellar vermis for five consecutive days, in mice. We recorded left and right primary motor cortices electrocorticograms and neck muscle electromyograms, and quantified the motor behavior abnormalities. The results indicated a reduced coherence between left and right motor cortices in low-frequency bands. In addition, we observed a phenomenon of long-lasting adaptation with a modification of the baseline interhemispheric coherence. Our research provides evidence that the cerebellum can control the flow of information along the cerebello-thalamo-cortical neural pathways and can influence interhemispheric communication. This phenomenon could function as a compensatory mechanism for impaired regional networks.

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