Investigation of cerebellar nuclei neuronal plasticity and physiological connectivity in vivo

It is a general assumption that the cerebellum plays a pivotal role in learning and memory of sensory-motor information, although the mechanisms through which the cerebellum is able to process information are still largely unknown. Several studies both in in vivo and in in vitro conditions support the existence of synaptic and non-synaptic plasticity in the cerebellum not confined to the sole cerebellar cortex although less is known about plastic changes mechanisms at the level of deep cerebellar nuclei (DCN). Moreover, the discovery that alterations in cerebello-cortical interconnections are related to cognitive diseases such as schizophrenia and autism suggests a cerebellar role in cognition. Further evidence of cerebellar role in cognition comes tracing studies in humans and electrophysiological recordings in rodents in vivo. In this work, putative changes underlying DCN neurons capability to integrate sensory information were investigated by delivering a theta sensory stimulation pattern (TSS) at the level of the peri-oral area of urethane anesthetized mice while recording single-units activity in the Fastigial nucleus. Our results show that TSS is able to evoke several discharge patterns in DCN neurons in vivo, likely depending on the synaptic pathways engaged by the stimulation. Moreover, our results show that long-lasting changes detected in DCN following TSS were related to oscillations in the theta frequency range. The employment of pharmacological tools and optogenetics helped to better characterize some of the processes underlying plasticity in the DCN. The second part of this work is focused on cerebellar involvement in neocortical processing. In particular, we investigated the functional connectivity of fastigial and dentate nuclei (FN and DN, respectively) with the prelimbic area (PrL) of medial prefrontal cortex (mPFC). Herein, we characterized the single-unit pattern changes in PrL neurons following electrical stimulation of the contralateral FN or DN in urethane anesthetized mice. Two main anatomical pathways connect the cerebellum to the prefrontal area: the dopaminergic pathway, passing through the ventral tegmental area, and the glutamatergic pathway arising from the thalamus. Pharmacological approaches helped to discriminate these two pathways and investigate whether and how specific neuromodulators or neurotransmitters affect prefrontal neurons responses to cerebellar stimulation. PrL neurons showed spontaneous activity and responded to electrical stimulation of FN and DN with two response patterns, both characterized by an initial pause, in some cases followed by a rebound excitation detected as a peak in PSTHs. The perfusion of dopamine receptors antagonists did not abolish PrL neurons responses, thus supporting the hypothesis that dopamine released in PrL modulates neuronal responses, presumably affecting neurons intrinsic excitability. Following previous results, we focused our attention on the contribution of GABAergic synaptic inhibition within the PrL, presumably mediated by thalamic projections activated by DN stimulation. The majority of thalamic glutamatergic projections reach the mPFC at the level of interneurons in cortical Layers III/V. Our results showed that for almost all PrL responding units the inhibitory component was abolished by blocking GABAA receptors. Therefore, a reasonable explanation of our data is that the cerebellum is mainly involved in the activation of inhibitory cells regulating pyramidal neurons activity in PrL. Hence, the cerebellum may exert a crucial role in cognition, by preferentially leading to PrL neurons inhibition and therefore controlling the level of excitation/inhibition in mPFC circuitry. Overall, the findings reported in this work are relevant not only for understanding cerebellar functioning itself, but also for understanding cerebellar contribution to higher order cognitive processes.