Elisa Galliano

Silencing the majority of cerebellar granule cells uncovers their essential role in motor learning and consolidation.

Cerebellar granule cells (GCs) account for more than half of all neurons in the CNS of vertebrates. Theoretical work has suggested that the abundance of GCs is advantageous for sparse coding during memory formation. Here, we minimized the output of the majority of GCs by selectively eliminating their CaV2.1 (P/Q-type) Ca(2+) channels, which mediate the bulk of their neurotransmitter release. This resulted in reduced GC output to Purkinje cells (PCs) and stellate cells (SCs) as well as in impaired long-term plasticity at GC-PC synapses. As a consequence modulation amplitude and regularity of simple spike (SS) output were affected. Surprisingly, the overall motor performance was intact, whereas demanding motor learning and memory consolidation tasks were compromised. Our findings indicate that a minority of functionally intact GCs is sufficient for the maintenance of basic motor performance, whereas acquisition and stabilization of sophisticated memories require higher numbers of normal GCs controlling PC firing.

Climbing fiber input shapes reciprocity of Purkinje cell firing.

The cerebellum fine-tunes motor activity via its Purkinje cell output. Purkinje cells produce two different types of spikes, complex spikes and simple spikes, which often show reciprocal activity:xa0a periodical increase in complex spikes is associated with a decrease in simple spikes, and vice versa. This reciprocal firing is thought to be essential for coordinated motor behavior, yet how it is accomplished is debated. Here, we show in Ptf1a::cre;Robo3(lox/lox) mice that selectively rerouting the climbing fibers from a contralateral to an ipsilateral projection reversed the complex-spike modulation during sensory stimulation. Strikingly, modulation of simple spikes, which is supposed to be controlled by mossy fibers, reversed as well. Climbing fibers enforce this reciprocity in part by influencing activity of inhibitory interneurons, because the phase of their activity was also converted. Ptf1a::cre;Robo3(lox/lox) mice showed severe ataxia highlighting that climbing fiber input and its impact on reciprocity of Purkinje cell firing play an important role in motor coordination.

A Distinct Subtype of Dopaminergic Interneuron Displays Inverted Structural Plasticity at the Axon Initial Segment

The axon initial segment (AIS) is a specialized structure near the start of the axon that is a site of neuronal plasticity. Changes in activity levels in vitro and in vivo can produce structural AIS changes in excitatory cells that have been linked to alterations in excitability, but these effects have never been described in inhibitory interneurons. In the mammalian olfactory bulb (OB), dopaminergic interneurons are particularly plastic, undergoing constitutive turnover throughout life and regulating tyrosine hydroxylase expression in an activity-dependent manner. Here we used dissociated cultures of rat and mouse OB to show that a subset of bulbar dopaminergic neurons possess an AIS and that these AIS-positive cells are morphologically and functionally distinct from their AIS-negative counterparts. Under baseline conditions, OB dopaminergic AISs were short and located distally along the axon but, in response to chronic 24 h depolarization, lengthened and relocated proximally toward the soma. These activity-dependent changes were in the opposite direction to both those we saw in non-GABAergic OB neurons and those reported previously for excitatory cell types. Inverted AIS plasticity in OB dopaminergic cells was bidirectional, involved all major components of the structure, was dependent on the activity of L-type CaV1 calcium channels but not on the activity of the calcium-activated phosphatase calcineurin, and was opposed by the actions of cyclin-dependent kinase 5. Such distinct forms of AIS plasticity in inhibitory interneurons and excitatory projection neurons may allow considerable flexibility when neuronal networks must adapt to perturbations in their ongoing activity.

Discovery and rediscoveries of Golgi cells

When Camillo Golgi invented the black reaction in 1873 and first described the fine anatomical structure of the nervous system, he described a ‘big nerve cell’ that later took his name, the Golgi cell of cerebellum (‘Golgischen Zellen’, Gustaf Retzius, 1892 ). The Golgi cell was then proposed as the prototype of type‐II interneurons, which form complex connections and exert their actions exclusively within the local network. Santiago Ramón y Cajal (who received the Nobel Prize with Golgi in 1906) proceeded to a detailed description of Golgi cell morphological characteristics, but functional insight remained very limited for many years. The first rediscovery happened in the 1960s, when neurophysiological analysis in vivo revealed that Golgi cells are inhibitory interneurons. This finding promoted the development of two major cerebellar theories, the ‘beam theory’ of John Eccles and the ‘motor learning theory’ of David Marr, in which the Golgi cells regulate the spatial organisation and the gain of input signals to be processed and learned by the cerebellar circuit. However, the matter was not set and a series of pioneering observations using single unit recordings and electron microscopy raised new issues that could not be fully explored until the 1990s. Then, the advent of new electrophysiological and imaging techniques in vitro and in vivo demonstrated the cellular and network activities of these neurons. Now we know that Golgi cells, through complex systems of chemical and electrical synapses, effectively control the spatio‐temporal organisation of cerebellar responses. The Golgi cells regulate the timing and number of spikes emitted by granule cells and coordinate their coherent activity. Moreover, the Golgi cells regulate the induction of long‐term synaptic plasticity along the mossy fibre pathway. Eventually, the Golgi cells transform the granular layer of cerebellum into an adaptable spatio‐temporal filter capable of performing several kinds of logical operation. After more than a century, Golgis intuition that the Golgi cell had to generate under a new perspective complex ensemble effects at the network level has finally been demonstrated.

Synaptic transmission and plasticity at inputs to murine cerebellar Purkinje cells are largely dispensable for standard nonmotor tasks

In addition to its well established role in motor coordination, the cerebellum has been hypothesized to be involved in the control of cognitive and emotional functions. Although a cerebellar contribution to nonmotor functions has been supported by recent studies in human and monkey, it remains to be clarified with an in-depth, systematic approach in mouse mutants. Here we subjected four different cerebellar cell-specific mouse lines whereby the excitatory or inhibitory input to Purkinje cells (PCs) and/or PC postsynaptic plasticity were compromised, to a wide battery of standard cognitive and emotional tests. The four lines, which have all been shown to suffer from impaired motor learning without being ataxic, were tested for social behavior using a sociability task, for spatial navigation using the Morris watermaze, for fear responses using contextual and cued conditioning, and general anxiety using the open-field task. None of the four cerebellum-specific genetic perturbations showed significantly impaired cognitive or emotional behavior. In fact, even without correction for multiple comparisons, only 5 of 154 statistical comparisons showed a marginally significant deficit. Therefore, our data indicate that none of the perturbations of cerebellar functioning studied here affected the cognitive or emotional tests we used. This suggests that there may be a differential impact of the murine and human cerebellum on nonmotor functions. We hypothesize that these differences could be a consequence of the remarkable enlargement of the cerebellar hemispheres during the latest phase of vertebrate phylogeny, which occurred in parallel with the evolution of the cerebral cortex.

Questioning the Cerebellar Doctrine

The basic principles of cerebellar function were originally described by Flourens, Cajal, and Marr/Albus/Ito, and they constitute the pillars of what can be considered to be the classic cerebellar doctrine. In their concepts, the main cerebellar function is to control motor behavior, Purkinje cells are the only cortical neuron receiving and integrating inputs from climbing fiber and mossy-parallel fiber pathways, and plastic modification at the parallel fiber synapses onto Purkinje cells constitutes the substrate of motor learning. Yet, because of recent technical advances and new angles of investigation, all pillars of the cerebellar doctrine now face regular re-examination. In this review, after summarizing the classic concepts and recent disputes, we attempt to synthesize an integrated view and propose a revisited version of the cerebellar doctrine.

Anatomical investigation of potential contacts between climbing fibers and cerebellar Golgi cells in the mouse.

Climbing fibers (CFs) originating in the inferior olive (IO) constitute one of the main inputs to the cerebellum. In the mammalian cerebellar cortex each of them climbs into the dendritic tree of up to 10 Purkinje cells (PCs) where they make hundreds of synaptic contacts and elicit the so-called all-or-none complex spikes controlling the output. While it has been proven that CFs contact molecular layer interneurons (MLIs) via spillover mechanisms, it remains to be elucidated to what extent CFs contact the main type of interneuron in the granular layer, i.e., the Golgi cells (GoCs). This issue is particularly relevant, because direct contacts would imply that CFs can also control computations at the input stage of the cerebellar cortical network. Here, we performed a systematic morphological investigation of labeled CFs and GoCs at the light microscopic level following their path and localization through the neuropil in both the granular and molecular layer. Whereas in the molecular layer the appositions of CFs to PCs and MLIs were prominent and numerous, those to cell-bodies and dendrites of GoCs in both the granular layer and molecular layer were virtually absent. Our results argue against the functional significance of direct synaptic contacts between CFs and interneurons at the input stage, but support those at the output stage.

Impact of NMDA Receptor Overexpression on Cerebellar Purkinje Cell Activity and Motor Learning

Abstract In many brain regions involved in learning NMDA receptors (NMDARs) act as coincidence detectors of pre- and postsynaptic activity, mediating Hebbian plasticity. Intriguingly, the parallel fiber (PF) to Purkinje cell (PC) input in the cerebellar cortex, which is critical for procedural learning, shows virtually no postsynaptic NMDARs. Why is this? Here, we address this question by generating and testing independent transgenic lines that overexpress NMDAR containing the type 2B subunit (NR2B) specifically in PCs. PCs of the mice that show larger NMDA-mediated currents than controls at their PF input suffer from a blockage of long-term potentiation (LTP) at their PF-PC synapses, while long-term depression (LTD) and baseline transmission are unaffected. Moreover, introducing NMDA-mediated currents affects cerebellar learning in that phase-reversal of the vestibulo-ocular reflex (VOR) is impaired. Our results suggest that under physiological circumstances PC spines lack NMDARs postsynaptically at their PF input so as to allow LTP to contribute to motor learning.

Embryonic and postnatal neurogenesis produce functionally distinct subclasses of dopaminergic neuron

Most neurogenesis in the mammalian brain is completed embryonically, but in certain areas the production of neurons continues throughout postnatal life. The functional properties of mature postnatally generated neurons often match those of their embryonically produced counterparts. However, we show here that in the olfactory bulb (OB), embryonic and postnatal neurogenesis produce functionally distinct subpopulations of dopaminergic (DA) neurons. We define two subclasses of OB DA neuron by the presence or absence of a key subcellular specialisation: the axon initial segment (AIS). Large AIS-positive axon-bearing DA neurons are exclusively produced during early embryonic stages, leaving small anaxonic AIS-negative cells as the only DA subtype generated via adult neurogenesis. These populations are functionally distinct: large DA cells are more excitable, yet display weaker and – for certain long-latency or inhibitory events – more broadly tuned responses to odorant stimuli. Embryonic and postnatal neurogenesis can therefore generate distinct neuronal subclasses, placing important constraints on the functional roles of adult-born neurons in sensory processing.

Editorial: The Olivo-Cerebellar System

Studies on the olivo-cerebellar system have rapidly advanced over the past decade, leading to new insight in the structural and functional properties of its synapses, neurons, intrinsic circuits, and connectivity with the rest of the brain. As in many other fields of neuroscience, it is becoming more and more appropriate to try to bring our understanding at the level of individual synapses and neurons to that of ensemble activity, circuits, and behavior. This Editorial aims to facilitate this process by ordering the 26 contributions of this special issue of Frontiers in Brain Microcircuits Series from studies on the development and structure of synaptic contacts to those on the function of local microcircuits and network plasticity as well as the olivo-cerebellar system as a whole. More specifically, we highlight here the main points of the chapters on development, circuit organization and structural plasticity of various types of neurons in the olivo-cerebellar system (A); the chapters on their basic activity and synaptic plasticity (B); the chapters on the relevance of the emerging network patterns in the olivo-cerebellar system (C); the chapters on current high-level theories of motor learning (D); and the chapters on the overall role of the olivo-cerebellar system in the integration of sensorimotor control and cognition (E).