Single Purkinje cell can innervate multiple classes of projection neurons in the cerebellar nuclei of the rat: a light microscopic and ultrastructural triple-tracer study in the rat

Cerebellum Lecture: the Cerebellar Nuclei-Core of the Cerebellum

The cerebellum is a key player in many brain functions and a major topic of neuroscience research. However, the cerebellar nuclei (CN), the main output structures of the cerebellum, are often overlooked. This neglect is because research on the cerebellum typically focuses on the cortex and tends to treat the CN as relatively simple output nuclei conveying an inverted signal from the cerebellar cortex to the rest of the brain. In this review, by adopting a nucleocentric perspective we aim to rectify this impression. First, we describe CN anatomy and modularity and comprehensively integrate CN architecture with its highly organized but complex afferent and efferent connectivity. This is followed by a novel classification of the specific neuronal classes the CN comprise and speculate on the implications of CN structure and physiology for our understanding of adult cerebellar function. Based on this thorough review of the adult literature we provide a comprehensive overview of CN embryonic development and, by comparing cerebellar structures in various chordate clades, propose an interpretation of CN evolution. Despite their critical importance in cerebellar function, from a clinical perspective intriguingly few, if any, neurological disorders appear to primarily affect the CN. To highlight this curious anomaly, and encourage future nucleocentric interpretations, we build on our review to provide a brief overview of the various syndromes in which the CN are currently implicated. Finally, we summarize the specific perspectives that a nucleocentric view of the cerebellum brings, move major outstanding issues in CN biology to the limelight, and provide a roadmap to the key questions that need to be answered in order to create a comprehensive integrated model of CN structure, function, development, and evolution.

Activating Lobule VI PCTH+-Med Pathway in Cerebellum Blocks the Acquisition of Methamphetamine Conditioned Place Preference in Mice

Cerebellum has been implicated in drug addiction; however, its underlying cellular populations and neuronal circuitry remain largely unknown. In the current study, we identified a neural pathway from tyrosine hydroxylase (TH)-positive Purkinje cells (PCTH+) in cerebellar lobule VI to calcium/calmodulin-dependent protein kinase II (CaMKII)-positive glutamatergic neurons in the medial cerebellar nucleus (MedCaMKII), forming the lobule VI PCTH+-MedCaMKII pathway in male mice. In naive male mice, inhibition of PCTH+ neurons activated Med neurons. During conditioned place preference (CPP) training, exposure to methamphetamine (METH) inhibited lobule VI PCTH+ neurons while excited MedCaMKII neurons in mice. Silencing MedCaMKII using a tetanus toxin light chain (tettox) suppressed the acquisition of METH CPP in mice but resulted in motor coordination deficits in naive mice. In contrast, activating lobule VI PCTH+ terminals within Med inhibited the activity of Med neurons and subsequently blocked the acquisition of METH CPP in mice without affecting motor coordination, locomotor activity, and sucrose reinforcements in naive mice. Our findings identified a novel lobule VI PCTH+-MedCaMKII pathway within the cerebellum and explored its role in mediating the acquisition of METH-preferred behaviors.

A Systematic Review of Direct Outputs from the Cerebellum to the Brainstem and Diencephalon in Mammals

The cerebellum is involved in many motor, autonomic and cognitive functions, and new tasks that have a cerebellar contribution are discovered on a regular basis. Simultaneously, our insight into the functional compartmentalization of the cerebellum has markedly improved. Additionally, studies on cerebellar output pathways have seen a renaissance due to the development of viral tracing techniques. To create an overview of the current state of our understanding of cerebellar efferents, we undertook a systematic review of all studies on monosynaptic projections from the cerebellum to the brainstem and the diencephalon in mammals. This revealed that important projections from the cerebellum, to the motor nuclei, cerebral cortex, and basal ganglia, are predominantly di- or polysynaptic, rather than monosynaptic. Strikingly, most target areas receive cerebellar input from all three cerebellar nuclei, showing a convergence of cerebellar information at the output level. Overall, there appeared to be a large level of agreement between studies on different species as well as on the use of different types of neural tracers, making the emerging picture of the cerebellar output areas a solid one. Finally, we discuss how this cerebellar output network is affected by a range of diseases and syndromes, with also non-cerebellar diseases having impact on cerebellar output areas.

Effects of Gestational and Lactational Lead Exposure and High Fat Diet Feeding on Cerebellar Development of Postnatal Rat Offspring

Obesity and heavy metals, such as lead (Pb), are detrimental to the adult brain because they impair cognitive function and structural plasticity. However, the effects of co-administration of Pb and a high-fat diet (HFD) on the developing cerebellum is not clearly elucidated. We investigated the effects of Pb exposure (0.3% lead acetate) on developing cerebellum in the pups of an HFD-fed obese rat model. One week before mating, we fed a chow diet (CD) or HFD to the rats for one week and additionally administered Pb to HFD-fed female SD rats. Thereafter, treatment with Pb and a HFD was continued during the gestational and lactational periods. On postnatal day 21, the pups were euthanized to sample the brain tissue and blood for further analysis. Blood Pb levels were significantly higher in HFD-fed rats than in CD-fed rats. Histologically, the prominent degeneration of Purkinje cells was induced by the co-administration of Pb and HFD. The calbindin-28Kd-, GAD67-, NMDAR1-, and PSD95-immunopositive Purkinje cells and inhibitory synapse-forming pinceau structures were significantly decreased following Pb and HFD co-administration. MBP-immunoreactive myelinated axonal fibers were also impaired by HFD but were significantly damaged by the co-administration of HFD and Pb. Oxidative stress-related Nrf2-HO1 signaling was activated by HFD feeding, and Pb exposure further aggravated oxidative stress, as demonstrated by the consumption of endogenous anti-oxidant in HFD-Pb rats. The pro-inflammatory response was also increased by the co-administration of HFD and Pb in the cerebellum of the rat offspring. The present results suggest that HFD and Pb treatment during the gestational and lactational periods are harmful to cerebellar development.

Role of cerebellum in sleep-dependent memory processes

The activities and role of the cerebellum in sleep have, until recently, been largely ignored by both the sleep and cerebellum fields. Human sleep studies often neglect the cerebellum because it is at a position in the skull that is inaccessible to EEG electrodes. Animal neurophysiology sleep studies have focussed mainly on the neocortex, thalamus and the hippocampus. However, recent neurophysiological studies have shown that not only does the cerebellum participate in the sleep cycle, but it may also be implicated in off-line memory consolidation. Here we review the literature on cerebellar activity during sleep and the role it plays in off-line motor learning, and introduce a hypothesis whereby the cerebellum continues to compute internal models during sleep that train the neocortex.

Gating by Memory: a Theory of Learning in the Cerebellum

This paper presents a model of learning by the cerebellar circuit. In the traditional and dominant learning model, training teaches finely graded parallel fibre synaptic weights which modify transmission to Purkinje cells and to interneurons that inhibit Purkinje cells. Following training, input in a learned pattern drives a training-modified response. The function is that the naive response to input rates is displaced by a learned one, trained under external supervision. In the proposed model, there is no weight-controlled graduated balance of excitation and inhibition of Purkinje cells. Instead, the balance has two functional states-a switch-at synaptic, whole cell and microzone level. The paper is in two parts. The first is a detailed physiological argument for the synaptic learning function. The second uses the function in a computational simulation of pattern memory. Against expectation, this generates a predictable outcome from input chaos (real-world variables). Training always forces synaptic weights away from the middle and towards the limits of the range, causing them to polarise, so that transmission is either robust or blocked. All conditions teach the same outcome, such that all learned patterns receive the same, rather than a bespoke, effect on transmission. In this model, the function of learning is gating-that is, to select patterns that trigger output merely, and not to modify output. The outcome is memory-operated gate activation which operates a two-state balance of weight-controlled transmission. Group activity of parallel fibres also simultaneously contains a second code contained in collective rates, which varies independently of the pattern code. A two-state response to the pattern code allows faithful, and graduated, control of Purkinje cell firing by the rate code, at gated times.

The cerebellum and epilepsy

Epilepsy is the fourth most common neurological disorder, but current treatment options provide limited efficacy and carry the potential for problematic adverse effects. There is an immense need to develop new therapeutic interventions in epilepsy, and targeting areas outside the seizure focus for neuromodulation has shown therapeutic value. While not traditionally associated with epilepsy, anatomical, clinical, and electrophysiological studies suggest the cerebellum can play a role in seizure networks, and importantly, may be a potential therapeutic target for seizure control. However, previous interventions targeting the cerebellum in both preclinical and clinical studies have produced mixed effects on seizures. These inconsistent results may be due in part to the lack of specificity inherent with open-loop electrical stimulation interventions. More recent studies, using more targeted closed-loop optogenetic approaches, suggest the possibility of robust seizure inhibition via cerebellar modulation for a range of seizure types. Therefore, while the mechanisms of cerebellar inhibition of seizures have yet to be fully elucidated, the cerebellum should be thoroughly revisited as a potential target for therapeutic intervention in epilepsy. This article is part of the Special Issue "NEWroscience 2018.

Modulatory Effects of Monoamines and Perineuronal Nets on Output of Cerebellar Purkinje Cells

Classically, the cerebellum has been thought to play a significant role in motor coordination. However, a growing body of evidence for novel neural connections between the cerebellum and various brain regions indicates that the cerebellum also contributes to other brain functions implicated in reward, language, and social behavior. Cerebellar Purkinje cells (PCs) make inhibitory GABAergic synapses with their target neurons: other PCs and Lugaro/globular cells via PC axon collaterals, and neurons in the deep cerebellar nuclei (DCN) via PC primary axons. PC-Lugaro/globular cell connections form a cerebellar cortical microcircuit, which is driven by serotonin and noradrenaline. PCs' primary outputs control not only firing but also synaptic plasticity of DCN neurons following the integration of excitatory and inhibitory inputs in the cerebellar cortex. Thus, strong PC-mediated inhibition is involved in cerebellar functions as a key regulator of cerebellar neural networks. In this review, we focus on physiological characteristics of GABAergic transmission from PCs. First, we introduce monoaminergic modulation of GABAergic transmission at synapses of PC-Lugaro/globular cell as well as PC-large glutamatergic DCN neuron, and a Lugaro/globular cell-incorporated microcircuit. Second, we review the physiological roles of perineuronal nets (PNNs), which are organized components of the extracellular matrix and enwrap the cell bodies and proximal processes, in GABA release from PCs to large glutamatergic DCN neurons and in cerebellar motor learning. Recent evidence suggests that alterations in PNN density in the DCN can regulate cerebellar functions.

Population coding in the cerebellum: a machine learning perspective

The cere resembles a feedforward, three-layer network of neurons in which the "hidden layer" consists of Purkinje cells (P-cells) and the output layer consists of deep cerebellar nucleus (DCN) neurons. In this analogy, the output of each DCN neuron is a prediction that is compared with the actual observation, resulting in an error signal that originates in the inferior olive. Efficient learning requires that the error signal reach the DCN neurons, as well as the P-cells that project onto them. However, this basic rule of learning is violated in the cerebellum: the olivary projections to the DCN are weak, particularly in adulthood. Instead, an extraordinarily strong signal is sent from the olive to the P-cells, producing complex spikes. Curiously, P-cells are grouped into small populations that converge onto single DCN neurons. Why are the P-cells organized in this way, and what is the membership criterion of each population? Here, I apply elementary mathematics from machine learning and consider the fact that P-cells that form a population exhibit a special property: they can synchronize their complex spikes, which in turn suppress activity of DCN neuron they project to. Thus complex spikes cannot only act as a teaching signal for a P-cell, but through complex spike synchrony, a P-cell population may act as a surrogate teacher for the DCN neuron that produced the erroneous output. It appears that grouping of P-cells into small populations that share a preference for error satisfies a critical requirement of efficient learning: providing error information to the output layer neuron (DCN) that was responsible for the error, as well as the hidden layer neurons (P-cells) that contributed to it. This population coding may account for several remarkable features of behavior during learning, including multiple timescales, protection from erasure, and spontaneous recovery of memory.

Integration of Swimming-Related Synaptic Excitation and Inhibition by olig2+ Eurydendroid Neurons in Larval Zebrafish Cerebellum

The cerebellum influences motor control through Purkinje target neurons, which transmit cerebellar output. Such output is required, for instance, for larval zebrafish to learn conditioned fictive swimming. The output cells, called eurydendroid neurons (ENs) in teleost fish, are inhibited by Purkinje cells and excited by parallel fibers. Here, we investigated the electrophysiological properties of glutamatergic ENs labeled by the transcription factor olig2. Action potential firing and synaptic responses were recorded in current clamp and voltage clamp from olig2⁺ neurons in immobilized larval zebrafish (before sexual differentiation) and were correlated with motor behavior by simultaneous recording of fictive swimming. In the absence of swimming, olig2⁺ ENs had basal firing rates near 8 spikes/s, and EPSCs and IPSCs were evident. Comparing Purkinje firing rates and eurydendroid IPSC rates indicated that 1-3 Purkinje cells converge onto each EN. Optogenetically suppressing Purkinje simple spikes, while preserving complex spikes, suggested that eurydendroid IPSC size depended on presynaptic spike duration rather than amplitude. During swimming, EPSC and IPSC rates increased. Total excitatory and inhibitory currents during sensory-evoked swimming were both more than double those during spontaneous swimming. During both spontaneous and sensory-evoked swimming, the total inhibitory current was more than threefold larger than the excitatory current. Firing rates of ENs nevertheless increased, suggesting that the relative timing of IPSCs and EPSCs may permit excitation to drive additional eurydendroid spikes. The data indicate that olig2⁺ cells are ENs whose activity is modulated with locomotion, suiting them to participate in sensorimotor integration associated with cerebellum-dependent learning.SIGNIFICANCE STATEMENT The cerebellum contributes to movements through signals generated by cerebellar output neurons, called eurydendroid neurons (ENs) in fish (cerebellar nuclei in mammals). ENs receive sensory and motor signals from excitatory parallel fibers and inhibitory Purkinje cells. Here, we report electrophysiological recordings from ENs of larval zebrafish that directly illustrate how synaptic inhibition and excitation are integrated by cerebellar output neurons in association with motor behavior. The results demonstrate that inhibitory and excitatory drive both increase during fictive swimming, but inhibition greatly exceeds excitation. Firing rates nevertheless increase, providing evidence that synaptic integration promotes cerebellar output during locomotion. The data offer a basis for comparing aspects of cerebellar coding that are conserved and that diverge across vertebrates.

Developmental Changes in Serotonergic Modulation of GABAergic Synaptic Transmission and Postsynaptic GABAA Receptor Composition in the Cerebellar Nuclei

Outputs from the cerebellar nuclei (CN) are important for generating and controlling movement. The activity of CN neurons is controlled not only by excitatory inputs from mossy and climbing fibers and by γ-aminobutyric acid (GABA)-based inhibitory transmission from Purkinje cells in the cerebellar cortex but is also modulated by inputs from other brain regions, including serotonergic fibers that originate in the dorsal raphe nuclei. We examined the modulatory effects of serotonin (5-HT) on GABAergic synapses during development, using rat cerebellar slices. As previously reported, 5-HT presynaptically decreased the amplitudes of stimulation-evoked inhibitory postsynaptic currents (IPSCs) in CN neurons, with this effect being stronger in slices from younger animals (postnatal days [P] 11-13) than in slices from older animals (P19-21). GABA release probabilities accordingly exhibited significant decreases from P11-13 to P19-21. Although there was a strong correlation between the GABA release probability and the magnitude of 5-HT-induced inhibition, manipulating the release probability by changing extracellular Ca²⁺ concentrations failed to control the extent of 5-HT-induced inhibition. We also found that the IPSCs exhibited slower kinetics at P11-13 than at P19-21. Pharmacological and molecular biological tests revealed that IPSC kinetics were largely determined by the prevalence of α₁ subunits within GABA_A receptors. In summary, pre- and postsynaptic developmental changes in serotonergic modulation and GABAergic synaptic transmission occur during the second to third postnatal weeks and may significantly contribute to the formation of normal adult cerebellar function.

Complex Spike Wars: a New Hope

The climbing fiber-Purkinje cell circuit is one of the most powerful and highly conserved in the central nervous system. Climbing fibers exert a powerful excitatory action that results in a complex spike in Purkinje cells and normal functioning of the cerebellum depends on the integrity of climbing fiber-Purkinje cell synapse. Over the last 50 years, multiple hypotheses have been put forward on the role of the climbing fibers and complex spikes in cerebellar information processing and motor control. Central to these theories is the nature of the interaction between the low-frequency complex spike discharge and the high-frequency simple spike firing of Purkinje cells. This review examines the major hypotheses surrounding the action of the climbing fiber-Purkinje cell projection, discussing both supporting and conflicting findings. The review describes newer findings establishing that climbing fibers and complex spikes provide predictive signals about movement parameters and that climbing fiber input controls the encoding of behavioral information in the simple spike firing of Purkinje cells. Finally, we propose the dynamic encoding hypothesis for complex spike function that strives to integrate established and newer findings.

Purkinje Cell Representations of Behavior: Diary of a Busy Neuron

Fundamental for understanding cerebellar function is determining the representations in Purkinje cell activity, the sole output of the cerebellar cortex. Up to the present, the most accurate descriptions of the information encoded by Purkinje cells were obtained in the context of motor behavior and reveal a high degree of heterogeneity of kinematic and performance error signals encoded. The most productive framework for organizing Purkinje cell firing representations is provided by the forward internal model hypothesis. Direct tests of this hypothesis show that individual Purkinje cells encode two different forward models simultaneously, one for effector kinematics and one for task performance. Newer results demonstrate that the timing of simple spike encoding of motor parameters spans an extend interval of up to ±2 seconds. Furthermore, complex spike discharge is not limited to signaling errors, can be predictive, and dynamically controls the information in the simple spike firing to meet the demands of upcoming behavior. These rich, diverse, and changing representations highlight the integrative aspects of cerebellar function and offer the opportunity to generalize the cerebellar computational framework over both motor and non-motor domains.

The Cerebellar GABAAR System as a Potential Target for Treating Alcohol Use Disorder

In the brain, fast inhibitory neurotransmission is mediated primarily by the ionotropic subtype of the gamma-aminobutyric acid (GABA) receptor subtype A (GABA_AR). It is well established that the brain's GABA_AR system mediates many aspects of neurobehavioral responses to alcohol (ethanol; EtOH). Accordingly, in both preclinical studies and some clinical scenarios, pharmacologically targeting the GABA_AR system can alter neurobehavioral responses to acute and chronic EtOH consumption. However, many of the well-established interactions of EtOH and the GABA_AR system have been identified at concentrations of EtOH ([EtOH]) that would only occur during abusive consumption of EtOH (≥40 mM), and there are still inadequate treatment options for prevention of or recovery from alcohol use disorder (AUD, including abuse and dependence). Accordingly, there is a general acknowledgement that more research is needed to identify and characterize: (1) neurobehavioral targets of lower [EtOH] and (2) associated brain structures that would involve such targets in a manner that may influence the development and maintenance of AUDs.Nearly 15 years ago it was discovered that the GABA_AR system of the cerebellum is highly sensitive to EtOH, responding to concentrations as low as 10 mM (as would occur in the blood of a typical adult human after consuming 1-2 standard units of EtOH). This high sensitivity to EtOH, which likely mediates the well-known motor impairing effects of EtOH, combined with recent advances in our understanding of the role of the cerebellum in non-motor, cognitive/emotive/reward processes has renewed interest in this system in the specific context of AUD. In this chapter we will describe recent advances in our understanding of cerebellar processing, actions of EtOH on the cerebellar GABA_AR system, and the potential relationship of such actions to the development of AUD. We will finish with speculation about how cerebellar specific GABA_AR ligands might be effective pharmacological agents for treating aspects of AUD.

In vivo analysis of synaptic activity in cerebellar nuclei neurons unravels the efficacy of excitatory inputs

KEY POINTS

Cerebellar nuclei (CN) neurons can be classified into four groups according to their action potential (AP) waveform, corresponding to four types of neurons previously characterized. Half of the APs are generated by excitatory events, suggesting that excitatory inputs play a key role in generating CN outputs. Analysis of post-synaptic potentials reveals that the probability of excitatory inputs generating an AP is 0.1. The input from climbing fibre collaterals is characterized by a pair of synaptic potentials with a distinct interpair interval of 4.5 ms. The probability of climbing fibre collaterals initiating an AP in CN neurons is 0.15.

ABSTRACT

It is commonly agreed that the main function of the cerebellar system is to provide well-timed signals used for the execution of motor commands or prediction of sensory inputs. This function is manifested as a temporal sequence of spiking that should be expressed in the cerebellar nuclei (CN) projection neurons. Whether spiking activity is generated by excitation or release from inhibition is still a hotly debated issue. In an attempt to resolve this debate, we recorded intracellularly from CN neurons in anaesthetized mice and performed an analysis of synaptic activity that yielded a number of important observations. First, we demonstrate that CN neurons can be classified into four groups. Second, shape-index plots of the excitatory events suggest that they are distributed over the entire dendritic tree. Third, the rise time of excitatory events is linearly related to amplitude, suggesting that all excitatory events contribute equally to the generation of action potentials (APs). Fourth, we identified a temporal pattern of spontaneous excitatory events that represent climbing fibre inputs and confirm the results by direct stimulation and analysis on harmaline-evoked activity. Finally, we demonstrate that the probability of excitatory inputs generating an AP is 0.1 yet half of the APs are generated by excitatory events. Moreover, the probability of a presumably spontaneous climbing fibre input generating an AP is higher, reaching a mean population value of 0.15. In view of these results, the mode of synaptic integration at the level of the CN should be re-considered.

Whole-Cell Properties of Cerebellar Nuclei Neurons In Vivo

Cerebellar nuclei neurons integrate sensorimotor information and form the final output of the cerebellum, projecting to premotor brainstem targets. This implies that, in contrast to specialized neurons and interneurons in cortical regions, neurons within the nuclei encode and integrate complex information that is most likely reflected in a large variation of intrinsic membrane properties and integrative capacities of individual neurons. Yet, whether this large variation in properties is reflected in a heterogeneous physiological cell population of cerebellar nuclei neurons with well or poorly defined cell types remains to be determined. Indeed, the cell electrophysiological properties of cerebellar nuclei neurons have been identified in vitro in young rodents, but whether these properties are similar to the in vivo adult situation has not been shown. In this comprehensive study we present and compare the in vivo properties of 144 cerebellar nuclei neurons in adult ketamine-xylazine anesthetized mice. We found regularly firing (N = 88) and spontaneously bursting (N = 56) neurons. Membrane-resistance, capacitance, spike half-width and firing frequency all widely varied as a continuum, ranging from 9.63 to 3352.1 MΩ, from 6.7 to 772.57 pF, from 0.178 to 1.98 ms, and from 0 to 176.6 Hz, respectively. At the same time, several of these parameters were correlated with each other. Capacitance decreased with membrane resistance (R² = 0.12, P<0.001), intensity of rebound spiking increased with membrane resistance (for 100 ms duration R² = 0.1503, P = 0.0011), membrane resistance decreased with membrane time constant (R² = 0.045, P = 0.031) and increased with spike half-width (R² = 0.023, P<0.001), while capacitance increased with firing frequency (R² = 0.29, P<0.001). However, classes of neuron subtypes could not be identified using merely k-clustering of their intrinsic firing properties and/or integrative properties following activation of their Purkinje cell input. Instead, using whole-cell parameters in combination with morphological criteria revealed by intracellular labelling with Neurobiotin (N = 18) allowed for electrophysiological identification of larger (29.3–50 μm soma diameter) and smaller (< 21.2 μm) cerebellar nuclei neurons with significant differences in membrane properties. Larger cells had a lower membrane resistance and a shorter spike, with a tendency for higher capacitance. Thus, in general cerebellar nuclei neurons appear to offer a rich and wide continuum of physiological properties that stand in contrast to neurons in most cortical regions such as those of the cerebral and cerebellar cortex, in which different classes of neurons operate in a narrower territory of electrophysiological parameter space. The current dataset will help computational modelers of the cerebellar nuclei to update and improve their cerebellar motor learning and performance models by incorporating the large variation of the in vivo properties of cerebellar nuclei neurons. The cellular complexity of cerebellar nuclei neurons may endow the nuclei to perform the intricate computations required for sensorimotor coordination.

Caveats in Transneuronal Tracing with Unmodified Rabies Virus: An Evaluation of Aberrant Results Using a Nearly Perfect Tracing Technique

Apart from the genetically engineered, modified, strains of rabies virus (RABV), unmodified ‘fixed’ virus strains of RABV, such as the ‘French’ subtype of CVS11, are used to examine synaptically connected networks in the brain. This technique has been shown to have all the prerequisite characteristics for ideal tracing as it does not metabolically affect infected neurons within the time span of the experiment, it is transferred transneuronally in one direction only and to all types of neurons presynaptic to the infected neuron, number of transneuronal steps can be precisely controlled by survival time and it is easily detectable with a sensitive technique. Here, using the ‘French’ CVS 11 subtype of RABV in Wistar rats, we show that some of these characteristics may not be as perfect as previously indicated. Using injection of RABV in hind limb muscles, we show that RABV-infected spinal motoneurons may already show lysis 1 or 2 days after infection. Using longer survival times we were able to establish that Purkinje cells may succumb approximately 3 days after infection. In addition, some neurons seem to resist infection, as we noted that the number of RABV-infected inferior olivary neurons did not progress in the same rate as other infected neurons. Furthermore, in our hands, we noted that infection of Purkinje cells did not result in expected transneuronal labeling of cell types that are presynaptic to Purkinje cells such as molecular layer interneurons and granule cells. However, these cell types were readily infected when RABV was injected directly in the cerebellar cortex. Conversely, neurons in the cerebellar nuclei that project to the inferior olive did not take up RABV when this was injected in the inferior olive, whereas these cells could be infected with RABV via a transneuronal route. These results suggest that viral entry from the extracellular space depends on other factors or mechanisms than those used for retrograde transneuronal transfer. We conclude that transneuronal tracing with RABV may result in unexpected results, as not all properties of RABV seem to be ubiquitously valid.

The Periaqueductal Gray Orchestrates Sensory and Motor Circuits at Multiple Levels of the Neuraxis

The periaqueductal gray (PAG) coordinates behaviors essential to survival, including striking changes in movement and posture (e.g., escape behaviors in response to noxious stimuli vs freezing in response to fear-evoking stimuli). However, the neural circuits underlying the expression of these behaviors remain poorly understood. We demonstrate in vivo in rats that activation of the ventrolateral PAG (vlPAG) affects motor systems at multiple levels of the neuraxis through the following: (1) differential control of spinal neurons that forward sensory information to the cerebellum via spino-olivo-cerebellar pathways (nociceptive signals are reduced while proprioceptive signals are enhanced); (2) alterations in cerebellar nuclear output as revealed by changes in expression of Fos-like immunoreactivity; and (3) regulation of spinal reflex circuits, as shown by an increase in α-motoneuron excitability. The capacity to coordinate sensory and motor functions is demonstrated in awake, behaving rats, in which natural activation of the vlPAG in fear-conditioned animals reduced transmission in spino-olivo-cerebellar pathways during periods of freezing that were associated with increased muscle tone and thus motor outflow. The increase in spinal motor reflex excitability and reduction in transmission of ascending sensory signals via spino-olivo-cerebellar pathways occurred simultaneously. We suggest that the interactions revealed in the present study between the vlPAG and sensorimotor circuits could form the neural substrate for survival behaviors associated with vlPAG activation.

SIGNIFICANCE STATEMENT

Neural circuits that coordinate survival behaviors remain poorly understood. We demonstrate in rats that the periaqueductal gray (PAG) affects motor systems at the following multiple levels of the neuraxis: (1) through altering transmission in spino-olivary pathways that forward sensory signals to the cerebellum, reducing and enhancing transmission of nociceptive and proprioceptive information, respectively; (2) by alterations in cerebellar output; and (3) through enhancement of spinal motor reflex pathways. The sensory and motor effects occurred at the same time and were present in both anesthetized animals and behavioral experiments in which fear conditioning naturally activated the PAG. The results provide insights into the neural circuits that enable an animal to be ready and able to react to danger, thus assisting in survival.

Anatomical and functional relationships between deep cerebellar nuclei and cerebellar cortical Crus II in vivo in mice

We previously reported that an air-puff stimulation on the ipsilateral whisker pad evoked responses in molecular layer (ML) and Purkinje cell (PC) layer in cerebellar cortex folium Crus II. We used anterograde tracing and electrophysiological methods to investigate the anatomical and functional relationships between the trigeminal tactile response area in the cerebellar cortex Crus II and deep cerebellar nuclei (DCN) in living mice. We found that the axons of tactile activated PCs projected in anterior part (IntA) and posterior part (IntP), and dorsolateral hump (IntDL) of ipsilateral interposed cerebellar nucleus (ICN). In ICN, the tactile stimulus evoked-field potential expressed a sequence of two negative components N1 and N2, while extracellular recordings from ICN neurons revealed that an increase in spike frequency in response to tactile stimulus. When the duration of facial air-puff stimulus were ≥ 30 ms, stimulation off response (Roff) were observed in the ICN, but an increase in the duration of facial air-puff stimulation did not significantly affect the amplitude of Ron (N1 and N2) and Roff. The latency and time to peak of N1 in ICN were significantly shorter than that of N1 in the ML, but the latency and time to peak of N2 in ICN were significantly later than that of P1 in the ML. The present results suggest that the facial sensory information, at least in part, is transferred to ICN by PC axons from Crus II, which evokes excitation in ICN neurons.

Reorganization of Synaptic Connections and Perineuronal Nets in the Deep Cerebellar Nuclei of Purkinje Cell Degeneration Mutant Mice

The perineuronal net (PN) is a subtype of extracellular matrix appearing as a net-like structure around distinct neurons throughout the whole CNS. PNs surround the soma, proximal dendrites, and the axonal initial segment embedding synaptic terminals on the neuronal surface. Different functions of the PNs are suggested which include support of synaptic stabilization, inhibition of axonal sprouting, and control of neuronal plasticity. A number of studies provide evidence that removing PNs or PN-components results in renewed neurite growth and synaptogenesis. In a mouse model for Purkinje cell degeneration, we examined the effect of deafferentation on synaptic remodeling and modulation of PNs in the deep cerebellar nuclei. We found reduced GABAergic, enhanced glutamatergic innervations at PN-associated neurons, and altered expression of the PN-components brevican and hapln4. These data refer to a direct interaction between ECM and synapses. The altered brevican expression induced by activated astrocytes could be required for an adequate regeneration by promoting neurite growth and synaptogenesis.

A novel inhibitory nucleo-cortical circuit controls cerebellar Golgi cell activity

The cerebellum, a crucial center for motor coordination, is composed of a cortex and several nuclei. The main mode of interaction between these two parts is considered to be formed by the inhibitory control of the nuclei by cortical Purkinje neurons. We now amend this view by showing that inhibitory GABA-glycinergic neurons of the cerebellar nuclei (CN) project profusely into the cerebellar cortex, where they make synaptic contacts on a GABAergic subpopulation of cerebellar Golgi cells. These spontaneously firing Golgi cells are inhibited by optogenetic activation of the inhibitory nucleo-cortical fibers both in vitro and in vivo. Our data suggest that the CN may contribute to the functional recruitment of the cerebellar cortex by decreasing Golgi cell inhibition onto granule cells.

Integration of Purkinje cell inhibition by cerebellar nucleo-olivary neurons

Neurons in the cerebellar cortex, cerebellar nuclei, and inferior olive (IO) form a trisynaptic loop critical for motor learning. IO neurons excite Purkinje cells via climbing fibers and depress their parallel fiber inputs. Purkinje cells inhibit diverse cells in the cerebellar nuclei, including small GABAergic nucleo-olivary neurons that project to the IO. To investigate how these neurons integrate synaptic signals from Purkinje cells, we retrogradely labeled nucleo-olivary cells in the contralateral interpositus and lateral nuclei with cholera toxin subunit B-Alexa Fluor 488 and recorded their electrophysiological properties in cerebellar slices from weanling mice. Nucleo-olivary cells fired action potentials over a relatively narrow dynamic range (maximal rate, ∼ 70 spikes/s), unlike large cells that project to premotor areas (maximal rate, ∼ 400 spikes/s). GABA(A) receptor-mediated IPSCs evoked by electrical or optogenetic stimulation of Purkinje cells were more than 10-fold slower in nucleo-olivary cells (decay time, ∼ 25 ms) than in large cells (∼ 2 ms), and repetitive stimulation at 20-150 Hz evoked greatly summating IPSCs. Nucleo-olivary firing rates varied inversely with IPSP frequency, and the timing of Purkinje IPSPs and nucleo-olivary spikes was uncorrelated. These attributes contrast with large cells, whose brief IPSCs and rapid firing rates can permit well timed postinhibitory spiking. Thus, the intrinsic and synaptic properties of these two projection neurons from the cerebellar nuclei tailor them for differential integration and transmission of their Purkinje cell input.

Differential GABAergic and glycinergic inputs of inhibitory interneurons and Purkinje cells to principal cells of the cerebellar nuclei

The principal neurons of the cerebellar nuclei (CN), the sole output of the olivo-cerebellar system, receive a massive inhibitory input from Purkinje cells (PCs) of the cerebellar cortex. Morphological evidence suggests that CN principal cells are also contacted by inhibitory interneurons, but the properties of this connection are unknown. Using transgenic, tracing, and immunohistochemical approaches in mice, we show that CN interneurons form a large heterogeneous population with GABA/glycinergic phenotypes, distinct from GABAergic olive-projecting neurons. CN interneurons are found to contact principal output neurons, via glycine receptor (GlyR)-enriched synapses, virtually devoid of the main GABA receptor (GABAR) subunits α1 and γ2. Those clusters account for 5% of the total number of inhibitory receptor clusters on principal neurons. Brief optogenetic stimulations of CN interneurons, through selective expression of channelrhodopsin 2 after viral-mediated transfection of the flexed gene in GlyT2-Cre transgenic mice, evoked fast IPSCs in principal cells. GlyR activation accounted for 15% of interneuron IPSC amplitude, while the remaining current was mediated by activation of GABAR. Surprisingly, small GlyR clusters were also found at PC synapses onto principal CN neurons in addition to α1 and γ2 GABAR subunits. However, GlyR activation was found to account for <3% of the PC inhibitory synaptic currents evoked by electrical stimulation. This work establishes CN glycinergic neurons as a significant source of inhibition to CN principal cells, forming contacts molecularly distinct from, but functionally similar to, Purkinje cell synapses. Their impact on CN output, motor learning, and motor execution deserves further investigation.

Bidirectional modulation of deep cerebellar nuclear cells revealed by optogenetic manipulation of inhibitory inputs from Purkinje cells

In the mammalian cerebellum, deep cerebellar nuclear (DCN) cells convey all information from cortical Purkinje cells (PCs) to premotor nuclei and other brain regions. However, how DCN cells integrate inhibitory input from PCs with excitatory inputs from other sources has been difficult to assess, in part due to the large spatial separation between cortical PCs and their target cells in the nuclei. To circumvent this problem we have used a Cre-mediated genetic approach to generate mice in which channelrhodopsin-2 (ChR2), fused with a fluorescent reporter, is selectively expressed by GABAergic neurons, including PCs. In recordings from brain slice preparations from this model, mammalian PCs can be robustly depolarized and discharged by brief photostimulation. In recordings of postsynaptic DCN cells, photostimulation of PC axons induces a strong inhibition that resembles these cells' responses to focal electrical stimulation, but without a requirement for the glutamate receptor blockers typically applied in such experiments. In this optogenetic model, laser pulses as brief as 1 ms can reliably induce an inhibition that shuts down the spontaneous spiking of a DCN cell for ∼50 ms. If bursts of such brief light pulses are delivered, a fixed pattern of bistable bursting emerges. If these pulses are delivered continuously to a spontaneously bistable cell, the immediate response to such photostimulation is inhibitory in the cell's depolarized state and excitatory when the membrane has repolarized; a less regular burst pattern then persists after stimulation has been terminated. These results indicate that the spiking activity of DCN cells can be bidirectionally modulated by the optically activated synaptic inhibition of cortical PCs.

Cerebellar inhibitory input to the inferior olive decreases electrical coupling and blocks subthreshold oscillations

GABAergic projection neurons in the cerebellar nuclei (CN) innervate the inferior olive (IO) that in turn is the source of climbing fibers targeting Purkinje neurons in the cerebellar cortex. Anatomical evidence suggests that CN synapses modulate electrical coupling between IO neurons. In vivo studies indicate that they are also involved in controlling synchrony and rhythmicity of IO neurons. Here, we demonstrate using virally targeted channelrhodopsin in the cerebellar nucleo-olivary neurons that synaptic input can indeed modulate both the strength and symmetry of electrical coupling between IO neurons and alter network activity. Similar synaptic modifications of electrical coupling are likely to occur in other brain regions, where rapid modification of the spatiotemporal features of the coupled networks is needed to adequately respond to behavioral demands.

Collateralization of cerebellar output to functionally distinct brainstem areas. A retrograde, non-fluorescent tracing study in the rat

The organization of the cerebellum is characterized by a number of longitudinally organized connection patterns that consist of matching olivo-cortico-nuclear zones. These entities, referred to as modules, have been suggested to act as functional units. The various parts of the cerebellar nuclei (CN) constitute the output of these modules. We have studied to what extent divergent and convergent patterns in the output of the modules to four, functionally distinct brain areas can be recognized. Two retrograde tracers were injected in various combinations of the following nuclei: the red nucleus (RN), as a main premotor nucleus; the prerubral area, as a main supplier of afferents to the inferior olive (IO); the nucleus reticularis tegmenti pontis (NRTP), as a main source of cerebellar mossy fibers; and the IO, as the source of climbing fibers. For all six potential combinations three cases were examined. All nine cases with combinations that involved the IO did not, or hardly, resulted in double labeled neurons. In contrast, all other combinations resulted in at least 10% and up to 67% of double labeled neurons in cerebellar nuclear areas where both tracers were found. These results show that the cerebellar nuclear neurons that terminate within the studied areas represent basically two intermingled populations of projection cells. One population corresponds to the small nucleo-olivary neurons whereas the other consists of medium- to large-sized neurons which are likely to distribute their axons to several other areas. Despite some consistent differences between the output patterns of individual modules we propose that modular cerebellar output to premotor areas such as the RN provides simultaneous feedback to both the mossy fiber and the climbing fiber system and acts in concert with a designated GABAergic nucleo-olivary circuit. These features seem to form a basic characteristic of cerebellar operation.

Clusters of cerebellar Purkinje cells control their afferent climbing fiber discharge

Climbing fibers, the projections from the inferior olive to the cerebellar cortex, carry sensorimotor error and clock signals that trigger motor learning by controlling cerebellar Purkinje cell synaptic plasticity and discharge. Purkinje cells target the deep cerebellar nuclei, which are the output of the cerebellum and include an inhibitory GABAergic projection to the inferior olive. This pathway identifies a potential closed loop in the olivo-cortico-nuclear network. Therefore, sets of Purkinje cells may phasically control their own climbing fiber afferents. Here, using in vitro and in vivo recordings, we describe a genetically modified mouse model that allows the specific optogenetic control of Purkinje cell discharge. Tetrode recordings in the cerebellar nuclei demonstrate that focal stimulations of Purkinje cells strongly inhibit spatially restricted sets of cerebellar nuclear neurons. Strikingly, such stimulations trigger delayed climbing-fiber input signals in the stimulated Purkinje cells. Therefore, our results demonstrate that Purkinje cells phasically control the discharge of their own olivary afferents and thus might participate in the regulation of cerebellar motor learning.

Muscimol microinjection into cerebellar fastigial nucleus exacerbates stress-induced gastric mucosal damage in rats

AIM

To investigate the effects of microinjection of the GABA(A) receptor agonist muscimol into cerebellar fastigial nucleus (FN) on stress-induced gastric mucosal damage and the underlying mechanism in rats.

METHODS

Stress-induced gastric mucosal damage was induced in adult male SD rats by restraining and immersing them in cold water for 3 h. GABA(A) receptor agonist or antagonist was microinjected into the lateral FN. The decussation of superior cerebellar peduncle (DSCP) was electrically destroyed and the lateral hypothalamic area (LHA) was chemically ablated by microinjection of kainic acid. The pathological changes in the gastric mucosa were evaluated using TUNEL staining, immunohistochemistry staining and Western blotting.

RESULTS

Microinjection of muscimol (1.25, 2.5, and 5.0 μg) into FN significantly exacerbated the stress-induced gastric mucosal damage in a dose-dependent manner, whereas microinjection of GABA(A) receptor antagonist bicuculline attenuated the damage. The intensifying effect of muscimol on gastric mucosal damage was abolished by electrical lesion of DSCP or chemical ablation of LHA performed 3 d before microinjection of muscimol. Microinjection of muscimol markedly increased the discharge frequency of the greater splanchnic nerve, significantly increased the gastric acid volume and acidity, and further reduced the gastric mucosal blood flow. In the gastric mucosa, further reduced proliferation cells, enhanced apoptosis, and decreased anti-oxidant levels were observed following microinjection of muscimol.

CONCLUSION

Cerebellar FN participates in the regulation of stress-induced gastric mucosal damage, and cerebello-hypothalamic circuits contribute to the process.

Synchrony and neural coding in cerebellar circuits

The cerebellum regulates complex movements and is also implicated in cognitive tasks, and cerebellar dysfunction is consequently associated not only with movement disorders, but also with conditions like autism and dyslexia. How information is encoded by specific cerebellar firing patterns remains debated, however. A central question is how the cerebellar cortex transmits its integrated output to the cerebellar nuclei via GABAergic synapses from Purkinje neurons. Possible answers come from accumulating evidence that subsets of Purkinje cells synchronize their firing during behaviors that require the cerebellum. Consistent with models predicting that coherent activity of inhibitory networks has the capacity to dictate firing patterns of target neurons, recent experimental work supports the idea that inhibitory synchrony may regulate the response of cerebellar nuclear cells to Purkinje inputs, owing to the interplay between unusually fast inhibitory synaptic responses and high rates of intrinsic activity. Data from multiple laboratories lead to a working hypothesis that synchronous inhibitory input from Purkinje cells can set the timing and rate of action potentials produced by cerebellar nuclear cells, thereby relaying information out of the cerebellum. If so, then changing spatiotemporal patterns of Purkinje activity would allow different subsets of inhibitory neurons to control cerebellar output at different times. Here we explore the evidence for and against the idea that a synchrony code defines, at least in part, the input–output function between the cerebellar cortex and nuclei. We consider the literature on the existence of simple spike synchrony, convergence of Purkinje neurons onto nuclear neurons, and intrinsic properties of nuclear neurons that contribute to responses to inhibition. Finally, we discuss factors that may disrupt or modulate a synchrony code and describe the potential contributions of inhibitory synchrony to other motor circuits.

Harmaline tremor: underlying mechanisms in a potential animal model of essential tremor

BACKGROUND

Harmaline and harmine are tremorigenic β-carbolines that, on administration to experimental animals, induce an acute postural and kinetic tremor of axial and truncal musculature. This drug-induced action tremor has been proposed as a model of essential tremor. Here we review what is known about harmaline tremor.

METHODS

Using the terms harmaline and harmine on PubMed, we searched for papers describing the effects of these β-carbolines on mammalian tissue, animals, or humans.

RESULTS

Investigations over four decades have shown that harmaline induces rhythmic burst-firing activity in the medial and dorsal accessory inferior olivary nuclei that is transmitted via climbing fibers to Purkinje cells and to the deep cerebellar nuclei, then to brainstem and spinal cord motoneurons. The critical structures required for tremor expression are the inferior olive, climbing fibers, and the deep cerebellar nuclei; Purkinje cells are not required. Enhanced synaptic norepinephrine or blockade of ionic glutamate receptors suppresses tremor, whereas enhanced synaptic serotonin exacerbates tremor. Benzodiazepines and muscimol suppress tremor. Alcohol suppresses harmaline tremor but exacerbates harmaline-associated neural damage. Recent investigations on the mechanism of harmaline tremor have focused on the T-type calcium channel.

DISCUSSION

Like essential tremor, harmaline tremor involves the cerebellum, and classic medications for essential tremor have been found to suppress harmaline tremor, leading to utilization of the harmaline model for preclinical testing of antitremor drugs. Limitations are that the model is acute, unlike essential tremor, and only approximately half of the drugs reported to suppress harmaline tremor are subsequently found to suppress tremor in clinical trials.

Functional classification of neurons in the mouse lateral cerebellar nuclei

The deep cerebellar nuclei (DCN) are at the center of the cerebellum not only anatomically but also functionally. Classical anatomical studies have described different types of DCN neurons according to their expression of various marker proteins, but only recently have we begun to characterize these different cell types according to their electrophysiological properties. These efforts have benefited greatly from the availability of transgenic mouse lines that express green fluorescent protein under the control of the glutamic acid decarboxylase (GAD67) and glycine transporter (GlyT2) promoters, which are markers for GABAergic and glycinergic neurons, respectively. These studies have identified several types of neurons within the lateral cerebellar nuclei, each of which exhibits distinct active membrane properties. In addition to their differential use of neurotransmitters (glutamate, GABA, or glycine), these cell types also receive and provide synaptic information from different sources and to different targets.

Ins and outs of cerebellar modules

The modular concept of cerebellar connections has been advocated in the lifetime work of Jan Voogd. In this concept, a cerebellar module is defined as the conglomerate of one or multiple and non-adjacent, parasagittally arranged zones of Purkinje cells, their specific projection to a well-defined region of the cerebellar nuclei, and the climbing fiber input to these zones by a well-defined region of the inferior olivary complex. The modular organization of these olivo-cortico-nuclear connections is further exemplified by matching reciprocal connections between inferior olive and cerebellar nuclei. Because the different regions of the cerebellar nuclei show highly specific output patterns, cerebellar modules have been suggested to constitute functional entities. This idea is strengthened by the observation that anatomically defined modules adhere to the distribution of chemical markers in the cerebellar cortex suggesting that modules not only differ in their input and output relations but also may differ in operational capabilities. Here, I will briefly review some recent data on the establishment of cerebellar modules in rats. Furthermore, some evidence will be shown suggesting that the other main afferent system (i.e., mossy fibers), at least to some extent, also adheres to the modular organization. Finally, using retrograde transneuronal tracing with rabies virus, some evidence will be provided that several cerebellar modules may be involved in the control of individual muscles.

The mysterious microcircuitry of the cerebellar nuclei

The microcircuitry of cerebellar cortex and, in particular, the physiology of its main element, the Purkinje neuron, has been extensively investigated and described. However, activity in Purkinje neurons, either as single cells or populations, does not directly mediate the cerebellar effects on the motor effector systems. Rather, the result of the entire cerebellar cortical computation is passed to the relatively small cerebellar nuclei that act as the final, integrative processing unit in the cerebellar circuitry. The nuclei ultimately control the temporal and spatial features of the cerebellar output. Given this key role, it is striking that the internal organization and the connectivity with afferent and efferent pathways in the cerebellar nuclei are rather poorly known. In the present review, we discuss some of the many critical shortcomings in the understanding of cerebellar nuclei microcircuitry: the extent of convergence and divergence of the cerebellar cortical pathway to the various cerebellar nuclei neurons and subareas, the possible (lack of) conservation of the finely-divided topographical organization in the cerebellar cortex at the level of the nuclei, as well as the absence of knowledge of the synaptic circuitry within the cerebellar nuclei. All these issues are important for predicting the pattern-extraction and encoding capabilities of the cerebellar nuclei and, until resolved, theories and models of cerebellar motor control and learning may err considerably.

Zac1 plays a key role in the development of specific neuronal subsets in the mouse cerebellum

BACKGROUND

The cerebellum is composed of a diverse array of neuronal subtypes. Here we have used a candidate approach to identify Zac1, a tumor suppressor gene encoding a zinc finger transcription factor, as a new player in the transcriptional network required for the development of a specific subset of cerebellar nuclei and a population of Golgi cells in the cerebellar cortex.

RESULTS

We found that Zac1 has a complex expression profile in the developing cerebellum, including in two proliferating progenitor populations; the cerebellar ventricular zone and the external granular layer overlying posterior cerebellar lobules IX and X. Zac1 is also expressed in some postmitotic cerebellar neurons, including a subset of GABAergic interneurons in the medial cerebellar nuclei. Notably, GABAergic interneurons in the cerebellar nuclei are derived from the cerebellar ventricular zone, where Zac1 is also expressed, consistent with a lineage relationship between these two Zac1+ populations. Zac1 is also expressed in a small subset of cells in the posterior vermis, including some neurogranin-immunoreactive (NG+) Golgi cells, which, based on short-term birthdating, are derived from the EGL, where Zac1 is also expressed. However, Zac1+ cells and NG+ Golgi cells in the cerebellar cortex also display unique properties, as they are generated within different, albeit overlapping, time windows. Finally, consistent with the expression profile of Zac1, two conspicuous abnormalities were found in the cerebellum of Zac1 null mice: the medial cerebellar nuclei, and not the others, were significantly reduced in size; and the number of Golgi cells in cerebellar lobule IX was reduced by approximately 60% compared to wild-type littermates.

CONCLUSIONS

The data presented here indicate that the tumor suppressor gene Zac1 is expressed in a complex fashion in the developing cerebellum, including in two dividing progenitor populations and in specific subsets of postmitotic neurons, including Golgi cells and GABAergic neurons in the medial nuclei, which require Zac1 for their differentiation. We thus conclude that Zac1 is a critical regulator of normal cerebellar development, adding a new transcriptional regulator to the growing list of factors involved in generating neuronal diversity in the developing cerebellum.

Zac1 plays a key role in the development of specific neuronal subsets in the mouse cerebellum

Background

The cerebellum is composed of a diverse array of neuronal subtypes. Here we have used a candidate approach to identify Zac1, a tumor suppressor gene encoding a zinc finger transcription factor, as a new player in the transcriptional network required for the development of a specific subset of cerebellar nuclei and a population of Golgi cells in the cerebellar cortex.

Results

We found that Zac1 has a complex expression profile in the developing cerebellum, including in two proliferating progenitor populations; the cerebellar ventricular zone and the external granular layer overlying posterior cerebellar lobules IX and X. Zac1 is also expressed in some postmitotic cerebellar neurons, including a subset of GABAergic interneurons in the medial cerebellar nuclei. Notably, GABAergic interneurons in the cerebellar nuclei are derived from the cerebellar ventricular zone, where Zac1 is also expressed, consistent with a lineage relationship between these two Zac1⁺ populations. Zac1 is also expressed in a small subset of cells in the posterior vermis, including some neurogranin-immunoreactive (NG⁺) Golgi cells, which, based on short-term birthdating, are derived from the EGL, where Zac1 is also expressed. However, Zac1⁺ cells and NG⁺ Golgi cells in the cerebellar cortex also display unique properties, as they are generated within different, albeit overlapping, time windows. Finally, consistent with the expression profile of Zac1, two conspicuous abnormalities were found in the cerebellum of Zac1 null mice: the medial cerebellar nuclei, and not the others, were significantly reduced in size; and the number of Golgi cells in cerebellar lobule IX was reduced by approximately 60% compared to wild-type littermates.

Conclusions

The data presented here indicate that the tumor suppressor gene Zac1 is expressed in a complex fashion in the developing cerebellum, including in two dividing progenitor populations and in specific subsets of postmitotic neurons, including Golgi cells and GABAergic neurons in the medial nuclei, which require Zac1 for their differentiation. We thus conclude that Zac1 is a critical regulator of normal cerebellar development, adding a new transcriptional regulator to the growing list of factors involved in generating neuronal diversity in the developing cerebellum.

Synaptic inhibition, excitation, and plasticity in neurons of the cerebellar nuclei

Neurons of the cerebellar nuclei generate the non-vestibular output of the cerebellum. Like other neurons, they integrate excitatory and inhibitory synaptic inputs and filter them through their intrinsic properties to produce patterns of action potential output. The synaptic and intrinsic features of cerebellar nuclear cells are unusual in several respects, however: these neurons receive an overwhelming amount of basal and driven inhibition from Purkinje neurons, but are also spontaneously active, producing action potentials even without excitation. Moreover, not only is spiking by nuclear cells sensitive to the amount of inhibition, but the strength of inhibition is also sensitive to the amount of spiking, through multiple forms of long-term plasticity. Here, we review the properties of synaptic excitation and inhibition, their short-term plasticity, and their influence on action potential firing of cerebellar nuclear neurons, as well as the interactions among excitation, inhibition, and spiking that produce long-term changes in synaptic strength. The data provide evidence that electrical and synaptic signaling in the cerebellar circuit is both plastic and resilient: the strength of IPSPs and EPSPs readily changes as the activity of cerebellar nuclear cells is modified. Notably, however, many of the identified forms of plasticity have an apparently homeostatic effect, responding to perturbations of input by restoring cerebellar output toward pre-perturbation values. Such forms of self-regulation appear consistent with the role of cerebellar output in coordinating movements. In contrast, other forms of plasticity in nuclear cells, including a long-term potentiation of excitatory postsynaptic currents (EPSCs) and excitation-driven increases in intrinsic excitability, are non-homeostatic, and instead appear suited to bring the circuit to a new set point. Interestingly, the combinations of inhibitory and excitatory stimuli that potentiate EPSCs resemble patterns of activity predicted to occur during eyelid conditioning, suggesting that this form long-term potentiation, perhaps amplified by intrinsic plasticity, may represent a cellular mechanism that is engaged during cerebellar learning.

Implications of functional anatomy on information processing in the deep cerebellar nuclei

The cerebellum has been implicated as a major player in producing temporal acuity. Theories of cerebellar timing typically emphasize the role of the cerebellar cortex while overlooking the role of the deep cerebellar nuclei (DCN) that provide the sole output of the cerebellum. Here we review anatomical and electrophysiological studies to shed light on the DCN's ability to support temporal pattern generation in the cerebellum. Specifically, we examine data on the structure of the DCN, the biophysical properties of DCN neurons and properties of the afferent systems to evaluate their contribution to DCN firing patterns. In addition, we manipulate one of the afferent structures, the inferior olive (IO), using systemic harmaline injection to test for a network effect on activity of single DCN neurons in freely moving animals. Harmaline induces a rhythmic firing pattern of short bursts on a quiescent background at about 8 Hz. Other neurons become quiescent for long periods (seconds to minutes). The observed patterns indicate that the major effect harmaline exerts on the DCN is carried indirectly by the inhibitory Purkinje cells (PCs) activated by the IO, rather than by direct olivary excitation. Moreover, we suggest that the DCN response profile is determined primarily by the number of concurrently active PCs, their firing rate and the level of synchrony occurring in their transitions between continuous firing and quiescence. We argue that DCN neurons faithfully transfer temporal patterns resulting from strong correlations in PCs state transitions, while largely ignoring the timing of simple spikes from individual PCs. Future research should aim at quantifying the contribution of PC state transitions to DCN activity, and the interplay between the different afferent systems that drive DCN activity.

Glycinergic projection neurons of the cerebellum

The cerebellum funnels its entire output through a small number of presumed glutamatergic premotor projection neurons in the deep cerebellar nuclei and GABAergic neurons that feed back to the inferior olive. Here we use transgenic mice selectively expressing green fluorescent protein in glycinergic neurons to demonstrate that many premotor output neurons in the medial cerebellar (fastigial) nuclei are in fact glycinergic, not glutamatergic as previously thought. These neurons exhibit similar firing properties as neighboring glutamatergic neurons and receive direct input from both Purkinje cells and excitatory fibers. Glycinergic fastigial neurons make functional projections to vestibular and reticular neurons in the ipsilateral brainstem, whereas their glutamatergic counterparts project contralaterally. Together, these data suggest that the cerebellum can influence motor outputs via two distinct and complementary pathways.

The periaqueductal grey modulates sensory input to the cerebellum: a role in coping behaviour?

The paths that link the periaqueductal grey (PAG) to hindbrain motor circuits underlying changes in behavioural responsiveness to external stimuli are unknown. A major candidate structure for mediating these effects is the cerebellum. The present experiments test this directly by monitoring changes in size of cerebellar responses evoked by peripheral stimuli following activation of the PAG. In 22 anaesthetized adult Wistar rats, climbing fibre field potentials were recorded from the C1 zone in the paramedian lobule and the copula pyramidis of the cerebellar cortex evoked, respectively, by electrical stimulation of the ipsilateral fore- and hindlimb. An initial and a late response were attributable to activation of Abeta and Adelta peripheral afferents respectively (hindlimb onset latencies 16.9 and 23.8 ms). Chemical stimulation at physiologically-identified sites in the ventrolateral PAG (a region known to be associated with hyporeactive immobility) resulted in a significant reduction in size of both the Abeta and Adelta evoked field potentials (mean reduction relative to control +/- SEM, 59 +/- 7.5 and 66 +/- 11.9% respectively). Responses evoked by electrical stimulation of the dorsal or ventral funiculus of the spinal cord were also reduced by PAG stimulation, suggesting that part of the modulation may occur at supraspinal sites (including at the level of the inferior olive). Overall, the results provide novel evidence of descending control into motor control centres, and provide the basis for future studies into the role of the PAG in regulating motor activity in different behavioural states and in chronic pain.

Compartmentation of the cerebellar nuclei of the mouse

The cerebellar nuclei integrate inhibitory input from Purkinje cells with excitatory input from mossy and climbing fiber collaterals and are the sole cerebellar output. Numerous studies have shown that the cerebellar cortex is highly compartmentalized into hundreds of genetically determined, reproducible topographic units--transverse zones and parasagittal stripes--that can be identified through the expression patterns of numerous molecules. The Purkinje cell stripes project to the cerebellar nuclei. However, there is no known commensurate topographic complexity in the cerebellar nuclei. Rather, conventional anatomical descriptions identify four major subdivisions--the medial, anterior and posterior interposed, and lateral nuclei--together with a few intranuclear subdivisions. To begin to address the apparent complexity gap, we have used a panel of antigens and transgenes to reveal a reproducible molecular heterogeneity in the mouse cerebellar nuclei. Based on the differential expression patterns, singly and in combination, a new cerebellar nuclear topographic map has been constructed. This reveals the subdivision of the cerebellar nuclei into at least 12 reproducible expression domains. We hypothesize that such heterogeneity is the counterpart of the zones and stripes of the cerebellar cortex.

Decreased GAD65 mRNA levels in select subpopulations of neurons in the cerebellar dentate nuclei in autism: an in situ hybridization study

The laterally positioned dentate nuclei lie in a key position in the cerebellum to receive input from Purkinje cells in the lateral cerebellar hemisphere participating in both motor and cognitive functions. Although neuropathology of the four cerebellar nuclei using Nissl staining has been qualitatively reported in children and adults with autism, surprisingly the dentate nuclei appeared less affected despite reported reductions in Purkinje cells in the posterolateral cerebellar hemisphere. To determine any underlying abnormalities in the critically important GABAergic system, the rate-limiting GABA synthesizing enzyme, glutamic acid decarboxylase (GAD) type 65 was measured via in situ hybridization histochemistry in dentate somata. GAD65 mRNA labeling revealed two distinct subpopulations of neurons in adult control and autism postmortem brains: small-sized cells (about 10-12 microm in diameter, presumed interneurons) and larger-sized neurons (about 18-20 microm in diameter, likely feedback to inferior olivary neurons). A mean 51% reduction in GAD65 mRNA levels was found in the larger labeled cells in the autistic group compared with the control group (P=0.009; independent t-test) but not in the smaller cell subpopulation. This suggests a disturbance in the intrinsic cerebellar circuitry in the autism group potentially interfering with the synchronous firing of inferior olivary neurons, and the timing of Purkinje cell firing and inputs to the dentate nuclei. Disturbances in critical neural substrates within these key circuits could disrupt afferents to motor and/or cognitive cerebral association areas in the autistic brain likely contributing to the marked behavioral consequences characteristic of autism.

Modulatory effects of serotonin on GABAergic synaptic transmission and membrane properties in the deep cerebellar nuclei

Cerebellar outputs from the deep cerebellar nuclei (DCN) are critical for generating and controlling movement. DCN neuronal activity is primarily controlled by GABAergic inhibitory transmission by Purkinje cells in the cerebellar cortex and is also modulated by nerve inputs originating from other brain regions within and outside the cerebellum. In this study, we examined the modulatory effects of 5-HT on GABAergic synapses in the DCN. 5-HT decreased the amplitude of stimulation-evoked inhibitory postsynaptic currents (eIPSCs) in DCN neurons, and this effect was abolished by a 5-HT(1B) antagonist, SB 224289. The decrease in IPSC amplitude was associated with an increased paired-pulse ratio of the IPSC. 5-HT also decreased the frequency of miniature IPSCs without altering the amplitude. These data suggest that 5-HT presynaptically inhibited GABA release. Furthermore, 5-HT elicited a slow inward current in DCN neurons. Pharmacological studies showed that 5-HT activated the 5-HT(5) receptor, which is positively coupled to G protein and elicited the slow inward current through enhancement of hyperpolarization-activated cation channel activation. Finally, we examined the effects of 5-HT on the spike generation that accompanies repetitive stimulation of inhibitory synapses. 5-HT increased the spontaneous firing rate in DCN neurons caused by depolarization. Increase in the 5-HT-induced tonic firing relatively decreased the contrast difference from the rebound depolarization-induced firing. However, the inhibitory transmission-induced silencing of DCN firing remained during the conditioning stimulus. These results suggest that 5-HT plays a regulatory role in spike generation and contributes to the gain control of inhibitory GABAergic synapses in DCN neurons.

Purkinje cell input to cerebellar nuclei in tottering: ultrastructure and physiology

Homozygous tottering mice are spontaneous ataxic mutants, which carry a mutation in the gene encoding the ion pore of the P/Q-type voltage-gated calcium channels. P/Q-type calcium channels are prominently expressed in Purkinje cell terminals, but it is unknown to what extent these inhibitory terminals in tottering mice are affected at the morphological and electrophysiological level. Here, we investigated the distribution and ultrastructure of their Purkinje cell terminals in the cerebellar nuclei as well as the activities of their target neurons. The densities of Purkinje cell terminals and their synapses were not significantly affected in the mutants. However, the Purkinje cell terminals were enlarged and had an increased number of vacuoles, whorled bodies, and mitochondria. These differences started to occur between 3 and 5 weeks of age and persisted throughout adulthood. Stimulation of Purkinje cells in adult tottering mice resulted in inhibition at normal latencies, but the activities of their postsynaptic neurons in the cerebellar nuclei were abnormal in that the frequency and irregularity of their spiking patterns were enhanced. Thus, although the number of their terminals and their synaptic contacts appear quantitatively intact, Purkinje cells in tottering mice show several signs of axonal damage that may contribute to altered postsynaptic activities in the cerebellar nuclei.

Ionic Factors Governing Rebound Burst Phenotype in Rat Deep Cerebellar Neurons

Large diameter cells in rat deep cerebellar nuclei (DCN) can be distinguished according to the generation of a transient or weak rebound burst and the expression of T-type Ca2+ channel isoforms. We studied the ionic basis for the distinction in burst phenotypes in rat DCN cells in vitro. Following a hyperpolarization, transient burst cells generated a high-frequency spike burst of ≤450 Hz, whereas weak burst cells generated a lower-frequency increase (<140 Hz). Both cell types expressed a low voltage–activated (LVA) Ca2+ current near threshold for rebound burst discharge (−50 mV) that was consistent with T-type Ca2+ current, but on average 7 times more current was recorded in transient burst cells. The number and frequency of spikes in rebound bursts was tightly correlated with the peak Ca2+ current at −50 mV, showing a direct relationship between the availability of LVA Ca2+ current and spike output. Transient burst cells exhibited a larger spike depolarizing afterpotential that was insensitive to blockers of voltage-gated Na+ or Ca2+ channels. In comparison, weak burst cells exhibited larger afterhyperpolarizations (AHPs) that reduced cell excitability and rebound spike output. The sensitivity of AHPs to Ca2+ channel blockers suggests that both LVA and high voltage–activated (HVA) Ca2+ channels trigger AHPs in weak burst compared with only HVA Ca2+ channels in transient burst cells. The two burst phenotypes in rat DCN cells thus derive in part from a difference in the availability of LVA Ca2+ current following a hyperpolarization and a differential activation of AHPs that establish distinct levels of membrane excitability.

A light microscope-based double retrograde tracer strategy to chart central neuronal connections

This protocol describes a double retrograde tracing method to chart divergent projections in the CNS using light microscope techniques. It is based on immunohistochemical visualization of retrograde transport of cholera toxin b-subunit (CTb) and silver enhancement of a gold-lectin conjugate. Production of the gold-lectin is explained in detail, and a technique is offered to record through the injection pipettes, to help guide accurate placement of injections. Visualization of the two tracers results in light brown staining of CTb-labeled neurons and labeling by black particles of gold-lectin-containing neurons. Both types of label are easily recognized in the same neuron. The labeling is permanent and is well suited for studies in which large areas of the brain need to be surveyed. The whole procedure (excluding survival time) takes approximately 5-7 d to complete.

Lock-and-key mechanisms of cerebellar memory recall based on rebound currents

A basic question for theories of learning and memory is whether neuronal plasticity suffices to guide proper memory recall. Alternatively, information processing that is additional to readout of stored memories might occur during recall. We formulate a "lock-and-key" hypothesis regarding cerebellum-dependent motor memory in which successful learning shapes neural activity to match a temporal filter that prevents expression of stored but inappropriate motor responses. Thus, neuronal plasticity by itself is necessary but not sufficient to modify motor behavior. We explored this idea through computational studies of two cerebellar behaviors and examined whether deep cerebellar and vestibular nuclei neurons can filter signals from Purkinje cells that would otherwise drive inappropriate motor responses. In eyeblink conditioning, reflex acquisition requires the conditioned stimulus (CS) to precede the unconditioned stimulus (US) by >100 ms. In our biophysical models of cerebellar nuclei neurons this requirement arises through the phenomenon of postinhibitory rebound depolarization and matches longstanding behavioral data on conditioned reflex timing and reliability. Although CS-US intervals<100 ms may induce Purkinje cell plasticity, cerebellar nuclei neurons drive conditioned responses only if the CS-US training interval was >100 ms. This bound reflects the minimum time for deinactivation of rebound currents such as T-type Ca2+. In vestibulo-ocular reflex adaptation, hyperpolarization-activated currents in vestibular nuclei neurons may underlie analogous dependence of adaptation magnitude on the timing of visual and vestibular stimuli. Thus, the proposed lock-and-key mechanisms link channel kinetics to recall performance and yield specific predictions of how perturbations to rebound depolarization affect motor expression.

Precise spatial relationships between mossy fibers and climbing fibers in rat cerebellar cortical zones

Classically, mossy fiber and climbing fiber terminals are regarded as having very different spatial distributions in the cerebellar cortex. However, previous anatomical studies have not studied these two major cerebellar inputs with sufficient resolution to confirm this assumption. Here, we examine the detailed pattern of collateralization of both types of cerebellar afferent using small injections of the bidirectional tracer cholera toxin b subunit into the posterior cerebellum. The cortical and zonal location of these injections was characterized by mapping climbing fiber field potentials, the distribution of retrogradely labeled olivary neurons, and the intrinsic zebrin pattern of Purkinje cells. Labeled climbing fiber collaterals were distributed as longitudinal strips and were always accompanied by clusters of labeled mossy fiber rosettes in the subjacent granular layer. Two- and three-dimensional reconstructions and quantitative analysis showed that mossy fibers also collateralized to other stripe-like regions usually below Purkinje cells with the same zebrin-positive or zebrin-negative characteristics as that of the injection site and associated climbing fiber collaterals. The distribution of retrogradely labeled neurons in two major sources of mossy fibers, the lateral reticular and basilar pontine nuclei, revealed interlobular and some interzonal differences. These data indicate that nonadjacent cerebellar zones, sharing the same climbing fiber input and zebrin identity, also share a common mossy fiber input. Other cerebellar cortical regions that receive collaterals from the same mossy fibers usually also have the same zebrin signature. Together with the distribution of neurons in precerebellar centers, the findings suggest a revision of the modular hypothesis for information processing in the cerebellar cortex.

Morphological and electrophysiological properties of GABAergic and non-GABAergic cells in the deep cerebellar nuclei

The deep cerebellar nuclei (DCN) integrate inputs from the brain stem, the inferior olive, and the spinal cord with Purkinje cell output from cerebellar cortex and provide the major output of the cerebellum. Despite their crucial function in motor control and learning, the various populations of neurons in the DCN are poorly defined and characterized. Importantly, differences in electrophysiological properties between glutamatergic and GABAergic cells of the DCN have been largely elusive. Here, we used glutamate decarboxylase (GAD) 67-green fluorescent protein (GFP) knock-in mice to unambiguously identify GABAergic (GAD-positive) and non-GABAergic (GAD-negative, most likely glutamatergic) neurons of the DCN. Morphological analysis of DCN neurons patch-clamped with biocytin-containing electrodes revealed a significant overlap in the distributions of the soma sizes of GAD-positive and GAD-negative cells. Compared with GAD-negative DCN neurons, GAD-positive DCN neurons fire broader action potentials, display stronger frequency accommodation, and do not reach as high firing frequencies during depolarizing current injections. Furthermore, GAD-positive cells display slower spontaneous firing rates and have a more shallow frequency-to-current relationship than the GAD-negative cells but exhibit a longer-lasting rebound depolarization and associated spiking after a transient hyperpolarization. In contrast to the rather homogeneous population of GAD-positive cells, the GAD-negative cells were found to consist of two distinct populations as defined by cell size and electrophysiological features. We conclude that GABAergic DCN neurons are specialized to convey phasic spike rate information, whereas tonic spike rate is more faithfully relayed by the large non-GABAergic cells.

A novel site of synaptic relay for climbing fibre pathways relaying signals from the motor cortex to the cerebellar cortical C1 zone

The climbing fibre projection from the motor cortex to the cerebellar cortical C1 zone in the posterior lobe of the rat cerebellum was investigated using a combination of physiological, anatomical and neuropharmacological techniques. Electrical stimulation of the ipsilateral fore- or hindimbs or somatotopically corresponding parts of the contralateral motor cortex evoked climbing fibre field potentials at the same cerebellar recording sites. Forelimb-related responses were located in the C1 zone in the paramedian lobule or lobulus simplex and hindlimb-related responses were located in the C1 zone in the copula pyramidis. Microinjections of anterograde axonal tracer (Fluoro-Ruby or Fluoro-Emerald) were made into the fore- or hindlimb parts of the motor cortex where stimulation evoked the largest cerebellar responses. After a survival period of 7-10 days, the neuraxis was examined for anterograde labelling. No terminal labelling was ever found in the inferior olive, but labelled terminals were consistently found in a well-localized site in the dorso-medial medulla, ventral to the gracile nucleus, termed the matrix region. Pharmacological inactivation of the matrix region (2 mm caudal to the obex) selectively reduced transmission in descending (cerebro-olivocerebellar) but not ascending (spino-olivocerebellar) paths targeting fore- or hindlimb-receiving parts of the C1 zone. Transmission in spino-olivocerebellar paths was either unaffected, or in some cases increased. The identification of a novel pre-olivary relay in cerebro-olivocerebellar paths originating from fore- and hindlimb motor cortex has implications for the regulation of transmission in climbing fibre pathways during voluntary movements and motor learning.

Organization of pontocerebellar projections to identified climbing fiber zones in the rat

The organization of pontocerebellar projections to the paravermis and hemisphere of the posterior cerebellum of the rat was studied in relation to the organization of climbing fibers. Small injections of cholera toxin subunit B were placed in the cerebellar cortex at locations predetermined by evoked climbing fiber potentials from selected body parts or based on coordinates. The injection site was characterized with respect to the zebrin pattern and by the distribution of retrogradely labeled neurons in the inferior olive. The following zones were studied: hindlimb-related zones C1 and C2 of lobule VIII; forelimb-related zones C1, C2, and D0/D1 of the paramedian lobule; and face-related zones A2 of the paramedian lobule and C2 and D0 of crus 2B. The results show that the distribution of pontine neurons is closely related to the climbing fiber somatotopy. Injections centered on face-related zones result in distribution of pontine neurons within the pontine core region. Forelimb regions surround this core, whereas hindlimb regions are mostly supplied by caudal pontine regions and by a single patch of more rostrally located neurons. This distribution fits well with published data on the somatotopy of the corticopontine projection from the rat primary somatosensory cortex. However, apart from differences in the participation of ipsilaterally projecting cells, the distribution of pontine neurons does not change significantly when the injection covers different zones of the same lobule such as C1 and C2 of lobule VIII; C1, C2, and D0/D1 of the paramedian lobule; A2 of the paramedian lobule; and C2 and D0 of crus 2B.