Extraordinary synapses of the unipolar brush cell: an electron microscopic study in the rat cerebellum

Understanding Cerebellar Input Stage through Computational and Plasticity Rules

A central hypothesis concerning brain functioning is that plasticity regulates the signal transfer function by modifying the efficacy of synaptic transmission. In the cerebellum, the granular layer has been shown to control the gain of signals transmitted through the mossy fiber pathway. Until now, the impact of plasticity on incoming activity patterns has been analyzed by combining electrophysiological recordings in acute cerebellar slices and computational modeling, unraveling a broad spectrum of different forms of synaptic plasticity in the granular layer, often accompanied by forms of intrinsic excitability changes. Here, we attempt to provide a brief overview of the most prominent forms of plasticity at the excitatory synapses formed by mossy fibers onto primary neuronal components (granule cells, Golgi cells and unipolar brush cells) in the granular layer. Specifically, we highlight the current understanding of the mechanisms and their functional implications for synaptic and intrinsic plasticity, providing valuable insights into how inputs are processed and reconfigured at the cerebellar input stage.

A system of feed-forward cerebellar circuits that extend and diversify sensory signaling

Sensory signals are processed by the cerebellum to coordinate movements. Numerous cerebellar functions are thought to require the maintenance of a sensory representation that extends beyond the input signal. Granule cells receive sensory input, but they do not prolong the signal and are thus unlikely to maintain a sensory representation for much longer than the inputs themselves. Unipolar brush cells (UBCs) are excitatory interneurons that project to granule cells and transform sensory input into prolonged increases or decreases in firing, depending on their ON or OFF UBC subtype. Further extension and diversification of the input signal could be produced by UBCs that project to one another, but whether this circuitry exists is unclear. Here we test whether UBCs innervate one another and explore how these small networks of UBCs could transform spiking patterns. We characterized two transgenic mouse lines electrophysiologically and immunohistochemically to confirm that they label ON and OFF UBC subtypes and crossed them together, revealing that ON and OFF UBCs innervate one another. A Brainbow reporter was used to label UBCs of the same ON or OFF subtype with different fluorescent proteins, which showed that UBCs innervate their own subtypes as well. Computational models predict that these feed-forward networks of UBCs extend the length of bursts or pauses and introduce delays-transformations that may be necessary for cerebellar functions from modulation of eye movements to adaptive learning across time scales.

A system of feed-forward cerebellar circuits that extend and diversify sensory signaling

Sensory signals are processed by the cerebellum to coordinate movements. Numerous cerebellar functions are thought to require the maintenance of a sensory representation that extends beyond the input signal. Granule cells receive sensory input, but they do not prolong the signal and are thus unlikely to maintain a sensory representation for much longer than the inputs themselves. Unipolar brush cells (UBCs) are excitatory interneurons that project to granule cells and transform sensory input into prolonged increases or decreases in firing, depending on their ON or OFF UBC subtype. Further extension and diversification of the input signal could be produced by UBCs that project to one another, but whether this circuitry exists is unclear. Here we test whether UBCs innervate one another and explore how these small networks of UBCs could transform spiking patterns. We characterized two transgenic mouse lines electrophysiologically and immunohistochemically to confirm that they label ON and OFF UBC subtypes and crossed them together, revealing that ON and OFF UBCs innervate one another. A Brainbow reporter was used to label UBCs of the same ON or OFF subtype with different fluorescent proteins, which showed that UBCs innervate their own subtypes as well. Computational models predict that these feed-forward networks of UBCs extend the length of bursts or pauses and introduce delays-transformations that may be necessary for cerebellar functions from modulation of eye movements to adaptive learning across time scales.

Incomplete removal of extracellular glutamate controls synaptic transmission and integration at a cerebellar synapse

Synapses of glutamatergic mossy fibers (MFs) onto cerebellar unipolar brush cells (UBCs) generate slow excitatory (ON) or inhibitory (OFF) postsynaptic responses dependent on the complement of glutamate receptors expressed on the UBC's large dendritic brush. Using mouse brain slice recording and computational modeling of synaptic transmission, we found that substantial glutamate is maintained in the UBC synaptic cleft, sufficient to modify spontaneous firing in OFF UBCs and tonically desensitize AMPARs of ON UBCs. The source of this ambient glutamate was spontaneous, spike-independent exocytosis from the MF terminal, and its level was dependent on activity of glutamate transporters EAAT1-2. Increasing levels of ambient glutamate shifted the polarity of evoked synaptic responses in ON UBCs and altered the phase of responses to in vivo-like synaptic activity. Unlike classical fast synapses, receptors at the UBC synapse are virtually always exposed to a significant level of glutamate, which varies in a graded manner during transmission.

Intrinsic and Synaptic Properties Shaping Diverse Behaviors of Neural Dynamics

The majority of neurons in the neuronal system of the brain have a complex morphological structure, which diversifies the dynamics of neurons. In the granular layer of the cerebellum, there exists a unique cell type, the unipolar brush cell (UBC), that serves as an important relay cell for transferring information from outside mossy fibers to downstream granule cells. The distinguishing feature of the UBC is that it has a simple morphology, with only one short dendritic brush connected to its soma. Based on experimental evidence showing that UBCs exhibit a variety of dynamic behaviors, here we develop two simple models, one with a few detailed ion channels for simulation and the other one as a two-variable dynamical system for theoretical analysis, to characterize the intrinsic dynamics of UBCs. The reasonable values of the key channel parameters of the models can be determined by analysis of the stability of the resting membrane potential and the rebound firing properties of UBCs. Considered together with a large variety of synaptic dynamics installed on UBCs, we show that the simple-structured UBCs, as relay cells, can extend the range of dynamics and information from input mossy fibers to granule cells with low-frequency resonance and transfer stereotyped inputs to diverse amplitudes and phases of the output for downstream granule cells. These results suggest that neuronal computation, embedded within intrinsic ion channels and the diverse synaptic properties of single neurons without sophisticated morphology, can shape a large variety of dynamic behaviors to enhance the computational ability of local neuronal circuits.

Selective targeting of unipolar brush cell subtypes by cerebellar mossy fibers

In vestibular cerebellum, primary afferents carry signals from single vestibular end organs, whereas secondary afferents from vestibular nucleus carry integrated signals. Selective targeting of distinct mossy fibers determines how the cerebellum processes vestibular signals. We focused on vestibular projections to ON and OFF classes of unipolar brush cells (UBCs), which transform single mossy fiber signals into long-lasting excitation or inhibition respectively, and impact the activity of ensembles of granule cells. To determine whether these contacts are indeed selective, connectivity was traced back from UBC to specific ganglion cell, hair cell and vestibular organ subtypes in mice. We show that a specialized subset of primary afferents contacts ON UBCs, but not OFF UBCs, while secondary afferents contact both subtypes. Striking anatomical differences were observed between primary and secondary afferents, their synapses, and the UBCs they contact. Thus, each class of UBC functions to transform specific signals through distinct anatomical pathways.

While out jogging, you have no trouble keeping your eyes fixed on objects in the distance even though your head and eyes are moving with every step. Humans owe this stability of the visual world partly to a region of the brain called the vestibular cerebellum. From its position underneath the rest of the brain, the vestibular cerebellum detects head motion and then triggers compensatory movements to stabilize the head, body and eyes.

The vestibular cerebellum receives sensory input from the body via direct and indirect routes. The direct input comes from five structures within the inner ear, each of which detects movement of the head in one particular direction. The indirect input travels to the cerebellum via the brainstem, which connects the brain with the spinal cord. The indirect input contains information on head movements in multiple directions combined with input from other senses such as vision.

By studying the mouse brain, Balmer and Trussell have now mapped the direct and indirect circuits that carry sensory information to the vestibular cerebellum. Both types of input activate cells within the vestibular cerebellum called unipolar brush cells (UBCs). There are two types of UBCs: ON and OFF. Direct sensory input from the inner ear activates only ON UBCs. These cells respond to the arrival of sensory input by increasing their activity. Indirect input from the brainstem activates both ON UBCs and OFF UBCs. The latter respond to the input by decreasing their activity.

The vestibular cerebellum thus processes direct and indirect inputs via segregated pathways containing different types of UBCs. The next step in understanding how the cerebellum maintains a stable visual world is to identify the circuitry beyond the UBCs. Understanding these circuits will ultimately provide insights into balance disorders, such as vertigo.

Mechanisms and functional roles of glutamatergic synapse diversity in a cerebellar circuit

Synaptic currents display a large degree of heterogeneity of their temporal characteristics, but the functional role of such heterogeneities remains unknown. We investigated in rat cerebellar slices synaptic currents in Unipolar Brush Cells (UBCs), which generate intrinsic mossy fibers relaying vestibular inputs to the cerebellar cortex. We show that UBCs respond to sinusoidal modulations of their sensory input with heterogeneous amplitudes and phase shifts. Experiments and modeling indicate that this variability results both from the kinetics of synaptic glutamate transients and from the diversity of postsynaptic receptors. While phase inversion is produced by an mGluR2-activated outward conductance in OFF-UBCs, the phase delay of ON UBCs is caused by a late rebound current resulting from AMPAR recovery from desensitization. Granular layer network modeling indicates that phase dispersion of UBC responses generates diverse phase coding in the granule cell population, allowing climbing-fiber-driven Purkinje cell learning at arbitrary phases of the vestibular input.

DOI:http://dx.doi.org/10.7554/eLife.15872.001

Whether walking, riding a bicycle or simply standing still, we continually adjust our posture in small ways to prevent ourselves from falling. Our sense of balance depends on a set of structures inside the inner ear called the vestibular system. These structures detect movements of the head and relay this information to the brain in the form of electrical signals. A brain area called the vestibulo-cerebellum then combines these signals with sensory input from the eyes and muscles, before sending out further signals to trigger any adjustments necessary for balance.

One of the main cell types within the vestibulo-cerebellum is the unipolar brush cell (or UBC for short). UBCs pass on signals to another type of neuron called Purkinje cells, which support the learning of motor skills such as adjusting posture. Zampini, Liu et al. set out to test the idea that UBCs transform inputs from the vestibular system into a format that makes it easier for cerebellar Purkinje cells to drive this kind of learning.

First, recordings from slices of rodent brain revealed that UBCs respond in highly variable ways to vestibular input, with both the size and timing of responses varying between cells. This is because vestibular signals trigger the release of a chemical messenger called glutamate onto UBCs, but UBCs possess a variety of different types of glutamate receptors. Vestibular input therefore activates distinct signaling cascades from one UBC to the next. According to a computer model, this variability in UBC responses ensures that a subset of UBCs will always be active at any point during vestibular input. This in turn means that Purkinje cells can fire at any stage of a movement, which boosts the learning of motor skills.

The next steps will be to test this hypothesis using mutant mice that lack specific receptor subtypes in UBCs or UBCs completely. A further challenge for the future will be to build a computer model of the vestibulo-cerebellar system that includes all of its component cell types.

DOI:http://dx.doi.org/10.7554/eLife.15872.002

All the way from the cortex: a review of auditory corticosubcollicular pathways

Enrico Mugnaini has devoted part of his long and fruitful neuroscientific career to investigating the structural similarities between the cerebellar cortex and one of the first relay stations of the mammalian auditory pathway: the dorsal cochlear nucleus. The hypothesis of the cerebellar-like nature of the superficial layers of the dorsal cochlear nucleus received definitive support with the discovery and extensive characterization in his laboratory of unipolar brush cells, a neuron type unique to certain regions of the cerebellar cortex and to the granule cell domains of the cochlear nuclei. Paradoxically, a different line of research carried out in his laboratory revealed that, unlike the mammalian cerebellar cortex, the dorsal cochlear nucleus receives direct projections from the cerebral cortex, a fact that constitutes one of the main differences between the cerebellum and the dorsal cochlear nucleus. In an article published in 1995, Mugnaini's group described in detail the novel direct projections from the rat auditory neocortex to various subcollicular auditory centers, including the nucleus sagulum, the paralemniscal regions, the superior olivary complex, and the cochlear nuclei (Feliciano et al., Auditory Neuroscience 1995; 1:287-308). This review gives Enrico Mugnaini credit for his seminal contribution to the knowledge of auditory corticosubcollicular projections and summarizes how this growing field has evolved in the last 20 years.

Forward signaling by unipolar brush cells in the mouse cerebellum

Unipolar brush cells (UBCs) are glutamatergic interneurons prominently present in the granular layer of the vestibulocerebellum. UBCs engage in extensive synaptic contact with a single presynaptic mossy fiber and signal to downstream granule cells through an elaborate network of mossy fiber-like axons. Ultrastructural examinations and electrophysiological recordings in organotypic slice cultures have indicated that UBCs target not only granule cells but also other UBCs, thus forming chains of two or perhaps more interconnected UBCs. In this report, we show recordings of spontaneous and evoked (di)synaptic events in granule cells and UBCs in fresh cerebellar slices from juvenile mice (5–7 weeks). The patterns of arrival of synaptic events were consistent with the presence of a presynaptic UBC, and recordings from UBCs displayed spontaneous protracted synaptic events characteristic of UBC excitatory synaptic transmission. These results highlight that chains of UBCs could further extend the temporal range of delayed and protracted signaling in the cerebellar cortical network.

Structure, Distribution, and Function of Neuronal/Synaptic Spinules and Related Invaginating Projections

Neurons and especially their synapses often project long thin processes that can invaginate neighboring neuronal or glial cells. These "invaginating projections" can occur in almost any combination of postsynaptic, presynaptic, and glial processes. Invaginating projections provide a precise mechanism for one neuron to communicate or exchange material exclusively at a highly localized site on another neuron, e.g., to regulate synaptic plasticity. The best-known types are postsynaptic projections called "spinules" that invaginate into presynaptic terminals. Spinules seem to be most prevalent at large very active synapses. Here, we present a comprehensive review of all kinds of invaginating projections associated with both neurons in general and more specifically with synapses; we describe them in all animals including simple, basal metazoans. These structures may have evolved into more elaborate structures in some higher animal groups exhibiting greater synaptic plasticity. In addition to classic spinules and filopodial invaginations, we describe a variety of lesser-known structures such as amphid microvilli, spinules in giant mossy terminals and en marron/brush synapses, the highly specialized fish retinal spinules, the trophospongium, capitate projections, and fly gnarls, as well as examples in which the entire presynaptic or postsynaptic process is invaginated. These various invaginating projections have evolved to modify the function of a particular synapse, or to channel an effect to one specific synapse or neuron, without affecting those nearby. We discuss how they function in membrane recycling, nourishment, and cell signaling and explore how they might change in aging and disease.

Variable timing of synaptic transmission in cerebellar unipolar brush cells

The cerebellum ensures the smooth execution of movements, a task that requires accurate neural signaling on multiple time scales. Computational models of cerebellar timing mechanisms have suggested that temporal information in cerebellum-dependent behavioral tasks is in part computed locally in the cerebellar cortex. These models rely on the local generation of delayed signals spanning hundreds of milliseconds, yet the underlying neural mechanism remains elusive. Here we show that a granular layer interneuron, called the unipolar brush cell, is well suited to represent time intervals in a robust way in the cerebellar cortex. Unipolar brush cells exhibited delayed increases in excitatory synaptic input in response to presynaptic stimulation in mouse cerebellar slices. Depending on the frequency of stimulation, delays extended from zero up to hundreds of milliseconds. Such controllable protraction of delayed currents was the result of an unusual mode of synaptic integration, which was well described by a model of steady-state AMPA receptor activation. This functionality extends the capabilities of the cerebellum for adaptive control of behavior by facilitating appropriate output in a broad temporal window.

Local NMDA receptor blockade attenuates chronic tinnitus and associated brain activity in an animal model

Chronic tinnitus has no broadly effective treatment. Identification of specific markers for tinnitus should facilitate the development of effective therapeutics. Recently it was shown that glutamatergic blockade in the cerebellar paraflocculus, using an antagonist cocktail was successful in reducing chronic tinnitus. The present experiment examined the effect of selective N-methyl d-aspartate (NMDA) receptor blockade on tinnitus and associated spontaneous brain activity in a rat model. The NMDA antagonist, D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) (0.5 mM), was continuously infused for 2 weeks directly to the ipsilateral paraflocculus of rats with tinnitus induced months prior by unilateral noise exposure. Treated rats were compared to untreated normal controls without tinnitus, and to untreated positive controls with tinnitus. D-AP5 significantly decreased tinnitus within three days of beginning treatment, and continued to significantly reduce tinnitus throughout the course of treatment and for 23 days thereafter, at which time testing was halted. At the conclusion of psychophysical testing, neural activity was assessed using manganese enhanced magnetic resonance imaging (MEMRI). In agreement with previous research, untreated animals with chronic tinnitus showed significantly elevated bilateral activity in their paraflocculus and brainstem cochlear nuclei, but not in mid or forebrain structures. In contrast, D-AP5-treated-tinnitus animals showed significantly less bilateral parafloccular and dorsal cochlear nucleus activity, as well as significantly less contralateral ventral cochlear nucleus activity. It was concluded that NMDA-mediated glutamatergic transmission in the paraflocculus appears to be a necessary component of chronic noise-induced tinnitus in a rat model. Additionally, it was confirmed that in this model, elevated spontaneous activity in the cerebellar paraflocculus and auditory brainstem is associated with tinnitus.

Differential distribution of phospholipase C beta isoforms and diaglycerol kinase-beta in rodents cerebella corroborates the division of unipolar brush cells into two major subtypes

Sublineage diversification of specific neural cell classes occurs in complex as well as simply organized regions of the central and peripheral nervous systems; the significance of the phenomenon, however, remains insufficiently understood. The unipolar brush cells (UBCs) are glutamatergic cerebellar interneurons that occur at high density in vestibulocerebellum. As they are classified into subsets that differ in chemical phenotypes, intrinsic properties, and lobular distribution, they represent a valuable neuronal model to study subclass diversification. In this study, we show that cerebellar UBCs of adult rats and mice form two subclasses-type I and type II UBCs-defined by somatodendritic expression of calretinin (CR), mGluR1α, phospholipases PLCβ1 and PLCβ4, and diacylglycerol kinase-beta (DGKβ). We demonstrate that PLCβ1 is associated only with the CR(+) type I UBCs, while PLCβ4 and DGKβ are exclusively present in mGluR1α(+) type II UBCs. Notably, all PLCβ4(+) UBCs, representing about 2/3 of entire UBC population, also express mGluR1α. Furthermore, our data show that the sum of CR(+) type I UBCs and mGluR1α(+) type II UBCs accounts for the entire UBC class identified with Tbr2 immunolabeling. The two UBC subtypes also show a very different albeit somehow overlapping topographical distribution as illustrated by detailed cerebellar maps in this study. Our data not only complement and extend the previous knowledge on the diversity and subclass specificity of the chemical phenotypes within the UBC population, but also provide a new angle to the understanding of the signaling networks in type I and type II UBCs.

The unipolar brush cell: a remarkable neuron finally receiving deserved attention

Unipolar brush cells (UBC) are small, glutamatergic neurons residing in the granular layer of the cerebellar cortex and the granule cell domain of the cochlear nuclear complex. Recent studies indicate that this neuronal class consists of three or more subsets characterized by distinct chemical phenotypes, as well as by intrinsic properties that may shape their synaptic responses and firing patterns. Yet, all UBCs have a unique morphology, as both the dendritic brush and the large endings of the axonal branches participate in the formation of glomeruli. Although UBCs and granule cells may share the same excitatory and inhibitory inputs, the two cell types are distinctively differentiated. Typically, whereas the granule cell has 4-5 dendrites that are innervated by different mossy fibers, and an axon that divides only once to form parallel fibers after ascending to the molecular layer, the UBC has but one short dendrite whose brush engages in synaptic contact with a single mossy fiber terminal, and an axon that branches locally in the granular layer; branches of UBC axons form a non-canonical, cortex-intrinsic category of mossy fibers synapsing with granule cells and other UBCs. This is thought to generate a feed-forward amplification of single mossy fiber afferent signals that would reach the overlying Purkinje cells via ascending granule cell axons and their parallel fibers. In sharp contrast to other classes of cerebellar neurons, UBCs are not distributed homogeneously across cerebellar lobules, and subsets of UBCs also show different, albeit overlapping, distributions. UBCs are conspicuously rare in the expansive lateral cerebellar areas targeted by the cortico-ponto-cerebellar pathway, while they are a constant component of the vermis and the flocculonodular lobe. The presence of UBCs in cerebellar regions involved in the sensorimotor processes that regulate body, head and eye position, as well as in regions of the cochlear nucleus that process sensorimotor information suggests a key role in these critical functions; it also invites further efforts to clarify the cellular biology of the UBCs and their specific functions in the neuronal microcircuits in which they are embedded. High density of UBCs in specific regions of the cerebellar cortex is a feature largely conserved across mammals and suggests an involvement of these neurons in fundamental aspects of the input/output organization as well as in clinical manifestation of focal cerebellar disease.

Neuronal subtype identity in the rat auditory brainstem as defined by molecular profile and axonal projection

The nuclei of the auditory brainstem harbor a diversity of neuronal cell types and are interconnected by excitatory as well as inhibitory ascending, descending, and commissural pathways. Classically, neurons have been characterized by size and shape of their cell body and by the geometry of their dendrites. Our study is based on the use of axonal tracers in combination with immunocytochemistry to identify and distinguish neuronal subtypes by their molecular signature in dorsal and ventral cochlear nucleus, lateral superior olive, medial superior olive, medial nucleus of the trapezoid body, and inferior colliculus of the adult rat. The presumed neurotransmitters glutamate, glycine, and GABA were used alongside the calcium-binding proteins parvalbumin, calretinin, and calbindin-D28k as molecular markers. Our data provide distinct extensions to previous characterizations of neuronal subtypes and reveal regularities and differences across auditory brainstem nuclei that are discussed for their functional implications.

Aspects of the neuroendocrine cerebellum: expression of secretogranin II, chromogranin A and chromogranin B in mouse cerebellar unipolar brush cells

Morphologically distinct neuron classes can be subdivided in sublineages by differential chemical phenotypes that correlate with functional diversity. Here we show by immunocytochemistry that chromogranin A (CgA) chromogranin B (CgB) and secretogranin II (SgII), the principal granins situated in neuronal secretory granules and large dense-core vesicles, are widely but differentially expressed in cells of the mouse cerebellum and terminals of cerebellar afferents. While CgA and CgB were nearly panneuronal, SgII was more restricted in distribution. The cells most intensely immunoreactive for SgII were a class of small, excitatory interneurons enriched in the granular layer of the vestibulocerebellum, the unipolar brush cells (UBCs), although larger neurons likely to be a subset of the Golgi-Lugaro-globular cell population were also distinctly immunopositive; by contrast, Purkinje cells and granule cells were, at best, faintly stained and, stellate, basket cells were unstained. SgII was also present in subsets of mossy fibers, climbing fibers and varicose fibers. Neurons in the cerebellar nuclei and inferior olive were distinctly positive for the three granins. Double-labeling with subset-specific cell class markers indicated that, while both CgA and CgB were present in most UBCs, SgII immunoreactivity was present in the calretinin (CR)-expressing subset, but lacked in metabotropic glutamate receptor 1alpha (mGluR1alpha)-expressing UBCs. Thus, we have identified an additional cell class marker, SgII, which serves to study subtype properties in the UBC population. The abundance of SgII in only one of the two known subsets of UBCs is remarkable, as its expression in other neurons of the cortex was moderate or altogether lacking. The data suggest that the CR-positive UBCs represent a unique neuroendocrine component of the mammalian cerebellar cortex, presumably endowed with transynaptically regulated autocrine or paracrine action/s. Because of the well-known organization of the cerebellar system, several of its neuron classes may represent valuable cellular models to analyze granin functions in situ, in acute slices and in dissociated cell and organotypic slice cultures.

Dynamic metabotropic control of intrinsic firing in cerebellar unipolar brush cells

Neuronal firing is regulated by the complex interaction of multiple depolarizing and hyperpolarizing currents; intrinsic firing, which defines the neuronal ability to generate action potentials in the absence of synaptic excitation, is particularly sensitive to modulation by currents that are active below the action potential threshold. Cerebellar unipolar brush cells (UBCs) are excitatory granule layer interneurons that are capable of intrinsic firing; here we show that, in acute mouse cerebellar slices, barium-sensitive background potassium channels of UBCs effectively regulate intrinsic firing. We also demonstrate that these channels are regulated by group II metabotropic glutamate receptors (mGluRs), which we show to be present in both of the known subsets of UBCs, one of which expresses calretinin and the other mGluR1alpha. Finally, we show that background potassium currents controlling UBCs' firing are mediated by at least two channel types, one of which is sensitive and the other insensitive to the GIRK blocker tertiapin. Thus in UBCs, glutamatergic transmission appears to have a complex bimodal effect: although it increases spontaneous firing through activation of ionotropic receptors, it also has inhibitory effects through the mGluR-dependent activation of tertiapin-sensitive and -insensitive background potassium currents.

Besides Purkinje cells and granule neurons: an appraisal of the cell biology of the interneurons of the cerebellar cortex

Ever since the groundbreaking work of Ramon y Cajal, the cerebellar cortex has been recognized as one of the most regularly structured and wired parts of the brain formed by a rather limited set of distinct cells. Its rather protracted course of development, which persists well into postnatal life, the availability of multiple natural mutants, and, more recently, the availability of distinct molecular genetic tools to identify and manipulate discrete cell types have suggested the cerebellar cortex as an excellent model to understand the formation and working of the central nervous system. However, the formulation of a unifying model of cerebellar function has so far proven to be a most cantankerous problem, not least because our understanding of the internal cerebellar cortical circuitry is clearly spotty. Recent research has highlighted the fact that cerebellar cortical interneurons are a quite more diverse and heterogeneous class of cells than generally appreciated, and have provided novel insights into the mechanisms that underpin the development and histogenetic integration of these cells. Here, we provide a short overview of cerebellar cortical interneuron diversity, and we summarize some recent results that are hoped to provide a primer on current understanding of cerebellar biology.

Calcium-binding proteins in the cerebellar cortex of the bottlenose dolphin and harbour porpoise

Studying the distribution of Ca2+-binding proteins allows one to discover specific neuron chemotypes involved in the regulation of the activity of various neural elements. While extensive data exist on Ca2+-binding proteins in the nervous system, in particular, in the cerebellar cortex of terrestrial mammals, the localization of these proteins in the cerebellar cortex of marine mammals has not been studied. We studied the localization of calretinin, calbindin, and parvalbumin immunoreactivity in the cerebellar cortex of the bottlenose dolphin Tursiops truncates and harbour porpoise Phocoena phocoena. In both species, most Purkinje cells were calbindin-immunoreactive, while calretinin and parvalbumin were expressed in a small portion of Purkinje cells. In addition, calretinin-immunoreactive unipolar brush and granule cells and calbindin- and parvalbumin-immunoreactive basket, stellate, and Golgi cells were observed. Calretinin-immunoreactive corticopetal (mossy and climbing) fibers were found. Based on the length of the primary dendrite, short-, middle-, and long-dendrite unipolar brush cells could be distinguished. The validity of this classification was supported using cluster analysis suggesting the presence of several natural types of these cells. The distribution of Ca2+-binding proteins in the cerebellar cortex of the cetaceans studied was generally similar to that reported for terrestrial mammals, suggesting that this trait is evolutionarily conservative in mammals.

Distribution and phenotypes of unipolar brush cells in relation to the granule cell system of the rat cochlear nucleus

In most mammals the cochlear nuclear complex (CN) contains a distributed system of granule cells (GCS), whose parallel fiber axons innervate the dorsal cochlear nucleus (DCN). Like their counterpart in cerebellum, CN granules are innervated by mossy fibers of various origins. The GCS is complemented by unipolar brush (UBCs) and Golgi cells, and by stellate and cartwheel cells of the DCN. This cerebellum-like microcircuit modulates the activity of the DCN's main projection neurons, the pyramidal, giant and tuberculoventral neurons, and is thought to improve auditory performance by integrating acoustic and proprioceptive information. In this paper, we focus on the rat UBCs, a chemically heterogeneous neuronal population, using antibodies to calretinin, metabotropic glutamate receptor 1alpha (mGluR1alpha), epidermal growth factor substrate 8 (Eps8) and the transcription factor T-box gene Tbr2 (Tbr2). Eps8 and Tbr2 labeled most of the CN's UBCs, if not the entire population, while calretinin and mGluR1alpha distinguished two largely separate subsets with overlapping distributions. By double labeling with antibodies to Tbr2 and the alpha6 GABA receptor A (GABAA) subunit, we found that UBCs populate all regions of the GCS and occur at remarkably high densities in the DCN and subpeduncular corner, but rarely in the lamina. Although GCS subregions likely share the same microcircuitry, their dissimilar UBC densities suggest they may be functionally distinct. UBCs and granules are also present in regions previously not included in the GCS, namely the rostrodorsal magnocellular portions of ventral cochlear nucleus, vestibular nerve root, trapezoid body, spinal tract and sensory and principal nuclei of the trigeminal nerve, and cerebellar peduncles. The UBC's dendritic brush receives AMPA- and NMDA-mediated input from an individual mossy fiber, favoring singularity of input, and its axon most likely forms several mossy fiber-like endings that target numerous granule cells and other UBCs, as in the cerebellum. The UBCs therefore, may amplify afferent signals temporally and spatially, synchronizing pools of target neurons.

Intrinsic properties and mechanisms of spontaneous firing in mouse cerebellar unipolar brush cells

Neuronal firing patterns are determined by the cell's intrinsic electrical and morphological properties and are regulated by synaptic interactions. While the properties of cerebellar neurons have generally been studied in much detail, little is known about the unipolar brush cells (UBCs), a type of glutamatergic interneuron that is enriched in the granular layer of the mammalian vestibulocerebellum and participates in the representation of head orientation in space. Here we show that UBCs can be distinguished from adjacent granule cells on the basis of differences in membrane capacitance, input resistance and response to hyperpolarizing current injection. We also show that UBCs are intrinsically firing neurons. Using action potential clamp experiments and whole-cell recordings we demonstrate that two currents contribute to this property: a persistent TTX-sensitive sodium current and a ruthenium red-sensitive, TRP-like cationic current, both of which are active during interspike intervals and have reversal potentials positive to threshold. Interestingly, although UBCs are also endowed with a large I(h) current, this current is not involved in their intrinsic firing, perhaps because it activates at voltages that are more hyperpolarized than those associated with autonomous activity.

Postsynaptic enrichment of Eps8 at dendritic shaft synapses of unipolar brush cells in rat cerebellum

Epidermal growth factor receptor pathway substrate 8 (Eps8) is a widely expressed multidomain signaling protein that coordinates two disparate GTPase-dependent mechanisms: actin reorganization via Ras/Rac pathways and receptor trafficking via Rab5. Expression of Eps8, the gene encoding the founding member of the Eps8 family of proteins, was found in cerebellum by virtual Northern analysis and in situ hybridization. Because the cerebellum has a well-known cellular architecture and is a favored model to study synaptic plasticity and actin dynamics, we sought to analyze Eps8 localization in rat cerebellar neurons and synapses by light and electron microscopy. Specificity of Eps8-antibody was demonstrated by immunoblots and in brain sections. In cerebellum, unipolar brush cells (UBCs) were densely Eps8 immunopositive and granule cells were moderately immunostained. In both types of neuron immunoreaction product was localized to the somatodendritic and axonal compartments. Postsynaptic immunostained foci were demonstrated in the glomeruli in correspondence of the synapses formed by mossy fiber terminals with granule cell and UBC dendrites. These foci appeared especially evident in the UBC brush, which contains an extraordinary postsynaptic apparatus of actin microfilaments facing synaptic junctions of the long and segmented varieties. Eps8 immunoreactivity was conspicuously absent in Purkinje cells and their actin-rich dendritic spines, in all types of inhibitory interneurons of the cerebellum, cerebellar nuclei neurons, and astrocytes. In conclusion, Eps8 protein in cerebellum is expressed exclusively by excitatory cortical interneurons and is intracellularly compartmentalized in a cell-class specific manner. This is the first demonstration of the presence of a member of the Eps8 protein family in UBCs and its enrichment at postsynaptic sites.

Moving up or moving down? Malpositioned cerebellar unipolar brush cells in reeler mouse

Cerebellar morphogenesis occurs through a complex interplay of cell proliferation and migration that in mouse and rat begins about midgestation and ends in the third postnatal week. Cerebellar cells derive from germinative matrices in the ventricular zone and the external granular layer. Like granule cells, unipolar brush cells (UBCs) are excitatory interneurons situated in the granular layer of the cortex and innervated by mossy fibers. While granule cells are produced from the external granular layer, the generation of UBCs is still controversial. We utilized the reeler mutant mouse, which has widespread misplacement of neurons due to lack of Reelin protein, to ascertain the origin of UBCs. In the reeler cerebellum, which is small and lacks foliation, Purkinje cells are greatly reduced in number and in large part are located ectopically in deep cerebellar masses. Granule cells are also reduced in number and form an irregular granule cell layer. In this study we demonstrate that the reeler mutation influences the positioning of UBCs and also significantly reduces their number. Both subsets of UBCs identified in normal mouse, the calretinin-positive and the metabotropic glutamate receptor 1alpha-positive subsets, are affected in the reeler. About 40% of the calretinin-positive UBCs are ectopically situated in the deep cerebellar regions and the immediate vicinity of the ependyma of the fourth ventricle. Ectopic UBCs have discrete, although somewhat looser brushes than granular layer UBCs, but form synaptic junctions with complex axon terminals, possibly belonging to mossy fibers and UBC axons, like their normally situated counterpart. The observed displacement of UBCs in the reeler suggests that they originate from the ventricular zone.

Calretinin‐immunoreactive unipolar brush cells in the developing human cerebellum

We have studied the temporal and spatial characteristics of the development of unipolar brush cells (UBCs) in the human cerebellar vermis. Consistently with previous studies in rodents and cat, we have found that unipolar brush cells appear at a relatively late phase of cerebellar development and their development continues up to and beyond the first postnatal year. A series of 23 normal human brains, including 5 adult and 18 fetal or infant brains (between the 24th gestational week and the 11th postnatal month) were used. In order to visualize unipolar brush cells, calretinin-immunocytochemistry was performed on formaldehyde-fixed, paraffin-embedded blocks of the cerebellar vermis. Our results show that calretinin-immunoreactive unipolar brush cells are not yet present in the cerebellar vermis at the 28th gestational week. At birth, they are present in a relatively small number, mostly in the vestibular lobules. At the 3rd, 5th, 8.5th and 11th postnatal months the number of calretinin-immunoreactive unipolar brush cells gradually increase, first appearing in the vestibular lobules, followed by the invasion of the later developing vermal lobules, spreading in a rostro-caudal and proximo-distal direction. Although at the 11th postnatal month unipolar brush cells exhibited adult-like morphological and distributional features, their number appeared to be lower than in the adult cerebellum. The late maturation of unipolar brush cells implies that the cytoarchitectonical development of the human cerebellum is not completed by the end of the first postnatal year.

Projections from the spinal trigeminal nucleus to the cochlear nucleus in the rat

The integration of information across sensory modalities enables sound to be processed in the context of position, movement, and object identity. Inputs to the granule cell domain (GCD) of the cochlear nucleus have been shown to arise from somatosensory brain stem structures, but the nature of the projection from the spinal trigeminal nucleus is unknown. In the present study, we labeled spinal trigeminal neurons projecting to the cochlear nucleus using the retrograde tracer, Fast Blue, and mapped their distribution. In a second set of experiments, we injected the anterograde tracer biotinylated dextran amine into the spinal trigeminal nucleus and studied the resulting anterograde projections with light and electron microscopy. Spinal trigeminal neurons were distributed primarily in pars caudalis and interpolaris and provided inputs to the cochlear nucleus. Their axons gave rise to small (1-3 microm in diameter) en passant swellings and terminal boutons in the GCD and deep layers of the dorsal cochlear nucleus. Less frequently, larger (3-15 microm in diameter) lobulated endings known as mossy fibers were distributed within the GCD. Ventrally placed injections had an additional projection into the anteroventral cochlear nucleus, whereas dorsally placed injections had an additional projection into the posteroventral cochlear nucleus. All endings were filled with round synaptic vesicles and formed asymmetric specializations with postsynaptic targets, implying that they are excitatory in nature. The postsynaptic targets of these terminals included dendrites of granule cells. These projections provide a structural substrate for somatosensory information to influence auditory processing at the earliest level of the central auditory pathways.

Unipolar brush cells--a new type of excitatory interneuron in the cerebellar cortex and cochlear nuclei of the brainstem

Published data and the authors' own studies on the morphology, neurochemical specialization, and spatial organization of unipolar brush neurons (UBN) in the cerebellar cortex and cochlear nuclei of the brainstem are reviewed. UBN represent an exclusive category of excitatory interneurons, with a single dendrite which forms a compact branching with a shape reminiscent of that of a brush in its terminal segment. These cells are characterized by an uneven distribution in the granular layer of the cerebellum, being located mainly in its vestibular zones. UBN synthesize glutamate, calretinin, and metabotropic and ionotropic glutamate receptors. The dendritic brush of UBN form giant synapses with the rosettes of glutamatergic and cholinergic mossy afferent fibers. UBN axons form an intracortical system of mossy fibers which, forming rosettes and glomeruli, make contact with the dendrites of other UBN and granule cells. In the circuits of interneuronal communications, UBN can be regarded as an intermediate component, amplifying the excitatory effects of mossy afferent fibers on granule cells in the cerebellar cortex and cochlear nuclei of the brainstem.

Time of origin of unipolar brush cells in the rat cerebellum as observed by prenatal bromodeoxyuridine labeling

Unipolar brush cells (UBCs) are a class of excitatory, glutamatergic interneurons occurring at high density in the granular layer of the vestibulo-cerebellum. UBCs are intermediate in size between granule cells, which in rat originate postnatally from precursors in the external granular layer, and Golgi cells, which are generated prenatally and postnatally from precursors in the ventricular zone that continue to divide while they migrate toward the cortex. The origin of the UBCs is still poorly understood. In this study, we set forth to ascertain the possible prenatal origin of UBCs, taking advantage of the immunocytochemical 5-bromo-2'-deoxyuridine (BrdU) method to label dividing cells in combination with antisera to cell population markers, that distinguish UBCs from granule and Golgi cells. Pregnant rat dams received six i.p. injections of BrdU (total 36 mg/animal) over 2 successive days at different stages of prenatal development (embryonic day [E]14/15-E20/21). Adult offspring were analyzed for histology. Using antibodies against the ionotropic glutamate receptor GluR2 and the calcium binding protein calretinin we found two populations of UBCs. A subset of about 30% of UBCs was calretinin and GluR2 positive, while the majority of the UBCs were calretinin negative and GluR2 positive. Results indicate that UBCs originate from precursors proliferating between E16 and E21. However, UBCs defined by calretinin immunoreactivity were primarily born in a narrow time window at E17-18. UBCs immunostained with antiserum to GluR2, but not labeled with calretinin were generated later, from E19 to E21. Our data also indicate that a part of GluR2 positive UBCs are born around and after E22. The subset of later born, calretinin negative UBCs may coincide with the pale cells, a group of cerebellar interneurons previously identified by [3H]thymidine labeling.

Vesicular glutamate transporters VGLUT1 and VGLUT2 define two subsets of unipolar brush cells in organotypic cultures of mouse vestibulocerebellum

Different isoforms of a vesicular glutamate transporter (VGLUT) mediate glutamate uptake into synaptic vesicles of excitatory neurons. There is agreement that the VGLUTs are differentially expressed in brain, and that two isoforms, VGLUT1 and VGLUT2, are localized to excitatory axon terminals in the cerebellar cortex. While granule cells express solely VGLUT1, there is no report about the VGLUT(s) of the unipolar brush cell (UBC), the second type of glutamatergic interneuron residing in the cerebellar granular layer. In the mouse, UBCs are particularly numerous in the uvula (lobule IX) and nodulus (lobule X). These folia contain two distinct subsets of UBCs: one kind expresses the calcium-binding protein calretinin (CR), and the other kind expresses the metabotropic glutamate receptor (mGluR) 1alpha. UBCs give rise to an extensive system of intrinsic mossy fibers (MF), whose terminals innervate granule cells and other UBCs, altogether similar to those formed by the extrinsic MFs. The presence of both extrinsic and intrinsic MFs in the vestibulocerebellum makes it difficult to determine which type of VGLUT is contained in MFs formed by the UBC axons. Hence, the nodulus was isolated from sagittal cerebellar slices from postnatal day 10 mice, and cultured for 15-20 days in vitro. Double immunofluorescence and confocal microscopy showed that mossy terminals of CR-positive (CR(+)) UBCs were immunoreactive for VGLUT1 and VGLUT2, while mossy terminals of mGluR1alpha-positive (mGluR1alpha(+)) UBCs were provided with VGLUT1 only. Moreover, CR(+) dendritic brushes were contacted by mossy terminals provided with both transporters, while mGluR1alpha(+) dendritic brushes were contacted by mossy terminals immunopositive for VGLUT1 and immunonegative for VGLUT2. These data indicate that the two UBC subsets use different modalities of vesicular glutamate storage and form separate networks. We consider it possible that expressions of CR with VGLUT1/VGLUT2 and mGluR1alpha(+) with VGLUT1 in the two subsets of vestibulocerebellar UBCs are determined by specific vestibular inputs, carried by groups of primary and/or secondary vestibular afferents.

Differential expression of calretinin and metabotropic glutamate receptor mGluR1alpha defines subsets of unipolar brush cells in mouse cerebellum

The unipolar brush cell (UBC) is a type of glutamatergic interneuron in the granular layer of the cerebellum. The UBC brush and a single mossy fiber (MF) terminal contact each other within a cerebellar glomerulus, forming a giant synapse. Many UBCs receive input from extrinsic MFs, whereas others are innervated by intrinsic mossy terminals formed by the axons of other UBCs. In all mammalian species so far examined, the vestibulocerebellum is enriched of UBCs that are strongly immunoreactive for the calcium binding protein calretinin (CR) in both the somatodendritic and axonal compartment. UBCs have postsynaptic ionotropic glutamate receptors and extrasynaptic metabotropic glutamate receptors that immunocytochemically highlight their somatodendritic compartment and brush, respectively. In this study on the mouse cerebellum, we present evidence that immunoreactivities to CR and mGluR1alpha define two distinct UBC subsets with partly overlapping distributions in lobule X (the nodulus). In sections double-labeled for CR and mGluR1alpha, the patterns of distributions of CR(+)/mGluR1alpha(-) UBCs and CR(-)/mGluR1alpha(+) UBCs differed along the mediolateral and dorsoventral axes of the folium. Moreover, mGluR1alpha(+) UBCs outnumbered CR(+) UBCs. Both UBC subsets were mGluR2/3, GluR2/3, and NMDAR1 immunoreactive. The different distribution patterns of the two UBC subsets within lobule X suggest that expression of CR or mGluR1alpha by UBCs may be afferent-specific and related to the terminal fields of different vestibular MF afferents.

Postnatal differentiation of unipolar brush cells and mossy fiber-unipolar brush cell synapses in rat cerebellum

The unipolar brush cells are excitatory, cerebellar granular layer interneurons that receive mossy fiber input on their dendritic brushes in the form of a giant glutamatergic synapse. We investigated the postnatal development of the brush of the unipolar brush cell in lobules IX and X by light microscopy and defined the maturation of mossy fiber-unipolar brush cell synapses and mossy fiber-granule cell synapses by electron microscopy using calretinin immunocytochemistry to identify unipolar brush cells. During the first postnatal week, unipolar brush cells possessed one or two short, branched dendrites. The brush differentiated primarily during the successive 21 postnatal (P) days, during which it underwent progressive maturation. This developmental process was subdivided into stages 1-4, which were descriptively termed protodendritic unipolar brush cell (P2-12), filopodial brush (P12-16), intermediate brush (P16-21), and dendriolar brush (P21-28) stages. Electron microscopic measurements of individual mossy fiber-unipolar brush cell and mossy fiber-granule cell synaptic junctions were made at P12, 16, 21, and 28. While the average length of mossy fiber-unipolar brush cell synapses increased during development, that of mossy fiber-granule cell synapses decreased. Comparisons of the lengths of mossy fiber-unipolar brush cell and mossy fiber-granule cell synapses demonstrated that mossy fiber-unipolar brush cell synapses were longer on average than mossy fiber-granule cell synapses for all ages. Frequency distribution histograms also showed that the percentage of mossy fiber-unipolar brush cell synapses longer than 0.5 microm was lower in the pooled P12-P16 groups than in the pooled P21-P28 groups (8 versus 20%). In contrast, mossy fiber-granule cell synapses longer than 0.5 microm were a small minority at P12, 16, and 21, and occurred rarely at P28. The present study indicates that mossy fiber-unipolar brush cell synapses increase in length with the differentiation of the brush dendrioles, while that of mossy fiber-granule cell synapses decrease with differentiation of the granule cell dendritic claws. The finding that mossy fiber-unipolar brush cell synapses were generally longer than mossy fiber-granule cell synapses may indicate that the properties of the postsynaptic targets play a major role in shaping synaptic appositions within cerebellar glomeruli.

IPSC kinetics at identified GABAergic and mixed GABAergic and glycinergic synapses onto cerebellar Golgi cells

In the rat cerebellum, Golgi cells receive serotonin-evoked inputs from Lugaro cells (L-IPSCs), in addition to spontaneous inhibitory inputs (S-IPSCs). In the present study, we analyze the pharmacology of these IPSCs and show that S-IPSCs are purely GABAergic events occurring at basket and stellate cell synapses, whereas L-IPSCs are mediated by GABA and glycine. Corelease of the two transmitters at Lugaro cell synapses is suggested by the fact that both GABA(A) and glycine receptors open during individual L-IPSCs. Double immunocytochemical stainings demonstrate that GABAergic and glycinergic markers are coexpressed in Lugaro cell axonal varicosities, together with the mixed vesicular inhibitory amino acid transporter. Lugaro cell varicosities are found apposed to glycine receptor (GlyR) clusters that are localized on Golgi cell dendrites and participate in postsynaptic complexes containing GABA(A) receptors (GABA(A)Rs) and the anchoring protein gephyrin. GABA(A)R and GlyR/gephyrin appear to form segregated clusters within individual postsynaptic loci. Basket and stellate cell varicosities do not face GlyR clusters. For the first time the characteristics of GABA and glycine cotransmission are compared with those of GABAergic transmission at identified inhibitory synapses converging onto the same postsynaptic neuron. The ratio of the decay times of L-IPSCs and of S-IPSCs is a constant value among Golgi cells. This indicates that, despite a high cell-to-cell variability of the overall IPSC decay kinetics, postsynaptic Golgi cells coregulate the kinetics of their two main inhibitory inputs. The glycinergic component of L-IPSCs is responsible for their slower decay, suggesting that glycinergic transmission plays a role in tuning the IPSC kinetics in neuronal networks.

Unipolar brush cells form a glutamatergic projection system within the mouse cerebellar cortex

Unipolar brush cells (UBCs) of the mammalian vestibulocerebellum receive mossy fiber projections primarily from the vestibular ganglion and vestibular nuclei. Recently, the axons of UBCs have been shown to generate an extensive system of cortex-intrinsic mossy fibers, which resemble traditional cerebellar mossy fiber afferents and synapse with granule cell dendrites and other UBCs. However, the neurotransmitter used by the UBC axon is still unknown. In this study, we used long-term organotypic slice cultures of the isolated nodulus (lobule X) from postnatal day 8 mouse cerebella to identify the neurotransmitter and receptors at synapses of the UBC axon terminals, relying on the notion that, in these cultures, all of the cortex-extrinsic fibers had degenerated during the first few days in vitro. Quantification of glutamate immunogold labeling showed that the UBC axon terminals have the same high gold-particle density as the glutamatergic parallel fiber varicosities. Furthermore, UBCs identified by calretinin immunoreactivity expressed the glutamate receptor subunits GluR2/3, NMDAR1, and mGluR2/3, like they do in the mature mouse cerebellum in situ. Evoked excitatory postsynaptic currents (EPSCs), spontaneous EPSCs, and burst discharges were demonstrated in UBCs and granule cells by patch-clamp recording. Both the evoked and spontaneous EPSCs were blocked by ionotropic glutamate receptor antagonists CNQX and D-AP5. We conclude that neurotransmission at the UBC axon terminals is glutamatergic. Thus, UBCs provide a powerful network of feedforward excitation within the granular layer, which may amplify vestibular signals and synchronize activity in clusters of functionally related granule cells which project vertically to patches of Purkinje cells.

Postsynaptic actin filaments at the giant mossy fiber-unipolar brush cell synapse

The unipolar brush cell (UBC), a small interneuron occurring at high density in the granular layer of the mammalian vestibulocerebellum, receives a giant glutamatergic synapse from a single mossy fiber (MF) rosette, usually on a brush of dendritic branchlets. MF stimulation produces a current in the UBC several orders of magnitude greater in duration than at other glutamatergic synapses. We assumed that the cytoskeleton would have a special role in plasticity of the MF-UBC synapse. Neurofilaments and microtubules are enriched in the UBC somatodendritic compartment but are conspicuously absent in close proximity to the giant synapse, where standard electron microscopy reveals a granulo-flocculent material. Because osmium tetroxide fixation during sample preparation for standard electron microscopy destabilizes actin filaments, we hypothesized that this subsynaptic granulo-flocculent material is actin-based. After actin stabilization, we observed prominent, but loosely organized, bundles of microfilaments at the subsynaptic region of the MF-UBC synapse that linked the postsynaptic density with the cytoskeletal core of the dendritic branchlets. Confocal fluorescence microscopy and pre- and postembedding immunogold labeling with phalloidin and actin antibodies showed that these microfilaments consist of f-actin and contain little beta-actin. This extraordinary postsynaptic actin apparatus is ideally situated to form a dynamic framework for glutamate receptors and other postsynaptic molecules, and to mediate activity-dependent plastic rearrangements of the giant synapse.

Unipolar brush cell: a potential feedforward excitatory interneuron of the cerebellum

Unipolar brush cells are a class of interneurons in the granular layer of the mammalian cerebellum that receives excitatory mossy fiber synaptic input in the form of a giant glutamatergic synapse. Previously, it was shown that the unipolar brush cell axon branches within the granular layer, giving rise to large terminals. Single mossy fiber stimuli evoke a prolonged burst of firing in unipolar brush cells, which would be distributed to postsynaptic targets within the granular layer. Knowledge of the ultrastructure of the unipolar brush cell terminals and of the cellular identity of its postsynaptic targets is required to understand how unipolar brush cells contribute to information processing in the cerebellar circuit. To investigate the unipolar brush cell axon and its targets, unipolar brush cells were patch-clamped in fresh parasagittal slices from rat cerebellar vermis with electrodes filled with Lucifer Yellow and Biocytin, and examined by confocal fluorescence and electron microscopy. Biocytin was localized with diaminobenzidine chromogen or gold-conjugated, silver-intensified avidin. Light microscopic examination revealed a single thin axon emanating from the unipolar brush cell soma that gave rise to 2-3 axon collaterals terminating in mossy fiber-like rosettes in the granular layer, typically within a few hundred microm of the soma. In some cases, axon collaterals crossed the white matter within the same folium before terminating in the adjacent granular layer. Electron microscopic examination of serial ultrathin sections revealed that proximal unipolar brush cell axons and axon collaterals were unmyelinated and devoid of synaptic contacts. However, the rosette-shaped enlargements of each collateral formed the central component of glomeruli where they were surrounded by dendrites of granule cells and/or other unipolar brush cells, with which they formed asymmetric synaptic contacts. A long-latency repetitive burst of polysynaptic activity was observed in granule cells in this cerebellar region following white matter stimulation. The unipolar brush cell axons, therefore, form a system of cortex-intrinsic mossy fibers. The results indicate that synaptic excitation of unipolar brush cells by mossy fibers will drive a large population of granule cells, and thus will contribute a powerful form of distributed excitation within the basic circuit of the cerebellar cortex.

Postnatal development of unipolar brush cells in the cerebellar cortex of cat

The postnatal developmental distribution pattern of metabotropic glutamate receptor (mGluR1a) immunoreactive unipolar brush cells (UBCs) was studied in the cerebellar cortex of kittens. On the day of birth (P0) UBCs are already present in the white matter in lobule X of the vermis, but only a few of these cell seemed to migrate to the deeper region of the internal granular layer. By the end of the first week (P8) UBCs were seen to invade the white matter + internal granular layer of lobules IX, VIII, I, and II of the vermis, and they spread further in the transitory area medio-laterally from the vermis toward the cerebellar hemispheres. By P15, UBCs appeared in lobules III and VII of the vermis, as well as in corresponding lobules of the neocerebellum, with especially high numbers in lobule VII. By P22, UBCs migrated further after their medio-lateral course in the neocerebellum, and began to invade lobules V and VI. At P62 the amount of UBCs in midsagittal planes of early developing vermal lobules (I, II, VII-X) resembled the P132 or adult pattern. The medio-lateral migration and incorporation of UBCs into the late-developing cerebellar lobules V and VI was completed only by P132, when the spatial distribution of UBCs in both the vermal and neocerebellar lobules was comparable to that seen in the 1 year old young adult cat. Although by P132 the postnatal migration of the vast majority of UBCs seemed to be completed, in the cerebellum of adult cats a few migrating UBCs could still be observed in the white matter of the cerebellar lobules, and beneath the ependyma of the fourth ventricle. It is concluded that during ontogenesis the migration course of UBCs follows essentially the developmental sequence of cerebellar lobules, although the incorporation of UBCs into the internal granular layer continues until 4 months postnatally, i.e., much beyond the apparent completion (about two months postnatally) of cytoarchitectonic built up of the cerebellar cortex of kittens.

Unipolar brush cell axons form a large system of intrinsic mossy fibers in the postnatal vestibulocerebellum

The unipolar brush cells (UBCs), a class of neurons recently identified in the granular layer of the vestibulocerebellum, receive excitatory synaptic input from mossy fibers (MFs) in the form of a giant glutamatergic synapse. UBCs are provided with axons that bear synaptic endings situated at the center of glomeruli, similar to cerebellar MF afferents. A single MF stimulus evokes a prolonged train of action potentials in the UBC (Rossi et al., 1995), which is presumably distributed to postsynaptic targets. Knowledge of the synaptic connections of UBC axons is essential to define the role of these cells in the integration of vestibular signals in the cerebellar circuitry. To evaluate these connections, the nodulus (folium X) was isolated from vermal slices of postnatal day 8 mice, cultured for 2-4 or 15-30 days in vitro, and studied by electron and fluorescence microscopy. The peak of degeneration of extrinsic MF terminals, which have been severed from the parent cell bodies, was observed at 2 days in vitro (DIV). Quantification of degenerating and nondegenerating (e.g., intrinsic) MF terminals indicated that about half of the MF terminals were provided by local UBC axons synapsing on dendrites of granule cells and other UBCs. The proportion of nondegenerating vs. degenerating MF terminals terminating on UBCs also indicated that approximately two-thirds of the intrinsic MFs are involved in UBC-UBC connections. In long-term cultures, the granular layer appeared well preserved and the UBC axons formed an extensive system of MF collaterals. It is suggested that UBCs may act by spatially amplifying vestibular inputs carried by extrinsic MFs.

Cerebellar involvement in Pick's disease: affliction of mossy fibers, monodendritic brush cells, and dentate projection neurons

Pick's disease chiefly is characterized by progressive degeneration of specific telencephalic cortical areas and associated subcortical nuclei. Components of the cerebellum also are affected. Immunoreactions for abnormally hyperphosphorylated tau protein, indicating the development of cytoskeletal anomalies in a few susceptible neuroectodermal cell types, permit visualization and identification of the pathology. Initially, accumulations of nonargyrophilic material appear in the perikarya and cellular processes of susceptible nerve cells. In some neuronal types, the abnormal deposits are transformed into more condensed inclusions, so-called Pick bodies in perikarya and Pick neurites in cellular processes, some of which become argyrophilic in the course of the disease. This study employs silver techniques and immunoreactions to draw attention to Pick's disease-associated lesions in the cerebellar cortex and cerebellar nuclei. Immunoreactive rosettes, which correspond to the terminal synaptic boutons of mossy fibers, frequently are encountered in the cerebellar granule cell layer. Some cases of Pick's disease also exhibit afflicted monodendritic brush cells in this layer. Single immunopositive Purkinje cells occasionally are seen as well. The brunt of the alterations is borne by cerebellar subdivisions receiving dense input from the telencephalic cortex through the pontocerebellar pathway (neocerebellum). The dentate nucleus shows immunoreactive axons with numerous varicose thickenings which remain confined to the reaches of this band-like nuclear gray and probably represent collaterals of altered mossy fibers. A large number of the dentate projection cells also contain the abnormal material in the perikarya, as well as in all of the neuronal processes. Many of these cells develop spherical nonargyrophilic condensations of this material. Output of the neocerebellum is conveyed to extended territories of the telencephalic cortex via the dentate nucleus and thalamus. Therefore, all of the cerebellar territories which receive major input from and generate output chiefly to the telencephalic cortex (pontocerebellum or neocerebellum) are notably afflicted in Pick's disease. Other subdivisions with preponderant input from the spinal cord and/or other noncortical sources remain intact or else are only minimally involved. It is concluded that the pattern of cerebellar involvement reflects Pick's disease-associated neocortical destruction.

Metabotrop glutamate receptor type 1a expressing unipolar brush cells in the cerebellar cortex of different species: a comparative quantitative study

Morphology, distribution and number of unipolar brush cells (UBCs) was studied in the cerebellar vermal lobules I-X of the chicken, rat, guinea pig, cat, and monkey using monoclonal mGluR1a antibody as a marker to visualise these recently described nerve cells (Mugnaini and Floris [1994] J. Comp. Neurol. 339:174-180; Mugnaini et al. [1994] Synapse 16:284-311). The morphological appearance of mGluR1a immunopositive UBCs is similar in all species investigated: they are small cells, having a single, relatively short and thick dendrite, terminating in brush-like dendrioles. Although this, probably excitatory, cell type can be found all over the cerebellar cortex, highest density of UBCs can be seen in the vermal cortex. The present study, therefore, was focused on the quantitative morphology and distribution of UBCs in the 10 lobules of the vermis. Calculating the number of UBCs/l Purkinje cell (PC), we have found differences in this value (average in vermal lobules I-X) from 1.04 in rat, 1.10 in chicken, 1.16 in guinea pig, 2.27 in monkey, and up to 2.44 in cat. The highest density of UBCs was observed in lobules I, IX, and X, whereas the lowest number of UBCs/l PC was found in lobules IV-VI (in the mammals) and in lobules VII-VIII (in the chicken). In mammals, particularly the monkey and cat, an increased presence of UBCs was observed in vermal sub-lobules VIc-VIIb,c, a region defined as the oculomotor vermis because of its role in the control of saccadic eye movement. There is also a basic difference between chicken and mammals in the distribution of UBCs within the lobules: in mammals, the lowest density of these nerve cells was found in the peripheral portion of the lobules, near to the pia, while in the chicken, in contrast, the density of UBCs was the highest subpially with fewer UBCs located in the deepest curvature of the lobules. Finally, the functional significance of the differences in the density and in the distribution pattern of UBCs in the cerebellar vermis between the phylogenetically different species investigated is briefly discussed.

Nitric oxide in the flocculus works the inhibitory neural circuits after unilateral labyrinthectomy

We previously reported that nitric oxide (NO) production in the unipolar brush (UB) cells is involved in vestibular compensation [T. Kitahara, N. Takeda, P.C. Emson, T. Kubo, H. Kiyama, Changes in nitric oxide synthase-like immunoreactivities in unipolar brush cells in the rat cerebellar flocculus after unilateral labyrinthectomy, Brain Res. 765 (1997) 1-6]. To further elucidate the role of NO-mediated signaling in flocculus after unilateral labyrinthectomy (UL), we examined UL-induced Fos expression, a marker of neural activity, in vestibular brainstem with continuous floccular infusions of Nomega-nitro-l-arginine methyl ester (l-NAME), an inhibitor of NO synthase (NOS). After UL with floccular l-NAME infusions, Fos expression appeared in bilateral medial vestibular (MVe) and prepositus hypoglossal (PrH) nuclei. After UL with floccular saline infusions, however, Fos expression was observed only in the ipsi-MVe and contra-PrH. Furthermore, it has been revealed that UL with l-NAME infusions caused more severe vestibulo-ocular disturbances than UL with saline infusions at the initial stage [Kitahara et al. Brain Res. 765 (1997) 1-6]. Therefore, it is suggested that UL with floccular l-NAME infusions activates the contra-MVe and ipsi-PrH neurons and causes more severe imbalance between intervestibular nuclear activities at the initial stage. NO-mediated signaling in flocculus could be a possible driving force of the flocculus-mediated inhibition on the contra-MVe and ipsi-PrH at the initial stage of vestibular compensation.

The synaptic basis of GABAA,slow

Although two kinetically distinct evoked GABAA responses (GABAA,fast and GABAA,slow) have been observed in CA1 pyramidal neurons, studies of spontaneous IPSCs (sIPSCs) in these neurons have reported only a single population of events that resemble GABAA,fast in their rise and decay kinetics. The absence of slow sIPSCs calls into question the synaptic basis of GABAA,slow. We present evidence here that both evoked responses are synaptic in origin, because two classes of minimally evoked, spontaneous and miniature IPSCs exist that correspond to GABAA,fast and GABAA,slow. Slow sIPSCs occur infrequently, suggesting that the cells underlying these events have a low spontaneous firing rate, unlike the cells giving rise to fast sIPSCs. Like evoked GABAA,fast and GABAA,slow, fast and slow sIPSCs are modulated differentially by furosemide, a subtype-specific GABAA antagonist. Furosemide blocks fast IPSCs by acting directly on the postsynaptic receptors, because it reduces the amplitude of both miniature IPSCs and the responses of excised patches to applied GABA. Thus, in the hippocampus, parallel inhibitory circuits are composed of separate populations of interneurons that contact anatomically segregated and pharmacologically distinct postsynaptic receptors.

Calcium-binding proteins in primate cerebellum

Single and double antigen localization procedures were used to study the distribution of the calcium-binding proteins calretinin, calbindin and parvalbumin in the cerebellum of the squirrel monkey (Saimiri sciureus). The immunostaining experiments have revealed that each of the three calcium-binding proteins occurred, either alone or in various combinations, in many neuronal types of the monkey cerebellum, including the Purkinje cells. Immunoreactivity for calbindin was detected in virtually all Purkinje cells, whereas immunoreactivity for calretinin and parvalbumin was encountered only in some subpopulations of Purkinje cells. In the vermal region, parvalbumin immunostaining appeared in the form of typical weak and strong alternating parasagittal bands. Calretinin immunoreactivity was found in virtually all neurons and fiber systems related to the granular layer, including the monodendritic cells, the granule cells and their parallel fibers, the Golgi and Lugaro cells and the mossy fibers. The Golgi cells also displayed calbindin and parvalbumin immunoreactivity. Parvalbumin was found to labeled both the climbing and mossy fibers, as well as the basket and stellate cells lying in the molecular layer. These results reveal that virtually all the different neuronal types in the primate cerebellum contain at least one of three calcium-binding proteins investigated in the present study. Furthermore, calretinin appears to be a particularly reliable molecular maker for all the neuronal elements associated with the granular layer in the primate cerebellum.

Prolonged physiological entrapment of glutamate in the synaptic cleft of cerebellar unipolar brush cells

The cellular mechanism underlying the genesis of the long-lasting alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptor-mediated excitatory postsynaptic currents (EPSCs) at the mossy fiber (MF)-unipolar brush cell (UBC) synapse in rat vestibular cerebellum was examined with the use of whole cell and excised patch-clamp recording methods in thin cerebellar slices. Activation of MFs evokes an all-or-none biphasic AMPA-receptor-mediated synaptic current with a late component that peaks at 100-800 ms, which has been proposed to originate from an entrapment of glutamate in the MF-UBC synaptic cleft and is generated by the steady-state activation of AMPA receptors. Bath application of cyclothiazide, which blocks desensitization of AMPA receptors, produced a dose-dependent enhancement of the amplitude of the synaptic current (median effective dose 30 microM) and slowing of the rise time of the fast EPSC. N-methyl-D-aspartate-receptor-mediated EPSCs in UBCs were not potentiated in amplitude or time course by cyclothiazide (100 microM). The dose-response relations for the steady-state current evoked by glutamate acting at AMPA receptors in excised outside-out patches from UBC and granule somatic membranes was biphasic, peaking at 50 microM and declining to 50-70% of this value at 1 mM glutamate. When glutamate was slowly washed from patches to simulate the gradual decline of glutamate in the synapse, a late hump in the transmembrane current was observed in patches from both cell types. The delivery of a second MF stimulus at the peak of the slow EPSC evoked a fast EPSC of reduced amplitude followed by an undershoot of the subsequent slow current, consistent with the hypothesis that the peak of the slow EPSC reflects the peak of the biphasic steady-state dose-response curve. Estimates of receptor occupancy and glutamate concentration derived from the ratio of fast EPSC amplitudes, and the amplitude and polarity of the initial steady-state current in paired-pulse experiments, predict a slow decline of glutamate with a time constant of 800 ms, declining to ineffective concentrations at 5.4 s. Manipulation of cleft glutamate concentration by lowered extracellular calcium or delivery of brief stimulus trains abolished the slow EPSC and restored the undershoot to paired stimuli, respectively, in a manner consistent with a prolonged lifetime of glutamate in the cleft. The slow component of the EPSC was prolonged in duration by the glutamate reuptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate, suggesting that glutamate transport contributes to the time course of the synaptic current in UBCs. The data support the notion that the MF-UBC synapse represents an ultrastructural specialization to effectively entrap glutamate for unusually prolonged periods of time following release from MF terminals. The properties of the postsynaptic receptors and constraints on diffusional escape of glutamate imposed by synaptic ultrastructure and glutamate transporters act in concert to sculpt the time course of the resulting slow EPSC. This in turn drives a long-lasting train of action potentials in response to single presynaptic stimuli.

Changes in nitric oxide synthase-like immunoreactivities in unipolar brush cells in the rat cerebellar flocculus after unilateral labyrinthectomy

To elucidate the role of nitric oxide (NO)-mediated signaling in vestibular compensation, we examined effects of unilateral labyrinthectomy (UL) on the neuronal isoform of NO synthase (NOS) expression in the rat central vestibular system using immunohistochemical techniques. After UL, a substantial number of NOS-like immunoreactive (-LIR) neurons were observed in the granule cell layer in bilateral flocculi, and these neurons were determined to be unipolar brush cells (UB cells) by their unique morphology and location. NOS-LIR UB cells appeared by 12 h with a maximum increase in number 24 h after UL, and then gradually disappeared in accordance with the development of vestibular compensation. Continuous floccular infusion of N(omega)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NOS, or 2-phenyl-4,4,5,5-tetramethyl-1-oxyl-3-oxide (PTIO), an inhibitor of NO, caused more severe vestibulo-ocular deficits at the initial stage after UL and slightly delayed the recovery from these symptoms. All these findings suggest that up-regulation of NO production in floccular UB cells facilitates vestibular compensation, especially at the initial stage after UL.

Physiological identification of the targets of cartwheel cells in the dorsal cochlear nucleus

The integrative contribution of cartwheel cells of the dorsal cochlear nucleus (DCN) was assessed with intracellular recordings from anatomically identified cells. Recordings were made, in slices of the cochlear nuclei of mice, from 58 cartwheel cells, 22 fusiform cells, 3 giant cells, 5 tuberculoventral cells, and 1 cell that is either a superficial stellate or Golgi cell. Cartwheel cells can be distinguished electrophysiologically from other cells of the cochlear nuclei by their complex spikes, which comprised two to four rapid action potentials superimposed on a slower depolarization. The rapid action potentials were blocked by tetrodotoxin (n = 17) and were therefore mediated by voltage-sensitive sodium currents. The slow spikes were eliminated by the removal of calcium from the extracellular saline (n = 3) and thus were mediated by voltage-sensitive calcium currents. The spontaneous and evoked firing patterns of cartwheel cells were distinctive. Cartwheel cells usually fired single and complex spikes spontaneously at irregular intervals of between 100 ms and several seconds. Shocks to the DCN elicited firing that lasted tens to hundreds of milliseconds. With the use of these distinctive firing patterns, together with a pharmacological dissection of postsynaptic potentials (PSPs), possible targets of cartwheel cells were identified and the function of the connections was examined. Not only cartwheel and fusiform cells, but also giant cells, received patterns of synaptic input consistent with their having originated from cartwheel cells. These cell types responded to shocks of the DCN with variable trains of PSPs that lasted hundreds of milliseconds. PSPs within these trains appeared both singly and in bursts of two to four, and were blocked by 0.5 or 1 microM strychnine (n = 4 cartwheel, 4 fusiform, and 2 giant cells), indicating that cartwheel cells are likely to be glycinergic. In contrast with cartwheel cells, which are weakly excited by glycinergic input, glycinergic PSPs consistently inhibited fusiform and giant cells. Tuberculoventral cells and the putative superficial stellate cell received little or no spontaneous synaptic activity. Shocks to the DCN evoked synaptic activity that lasted approximately 5 ms. These cells therefore probably do not receive input from cartwheel cells. In addition, the brief firing of tuberculoventral cells and of the putative superficial stellate cell in response to shocks indicates that these cells are unlikely to contribute to the late, glycinergic synaptic potentials observed in cartwheel, fusiform, and giant cells.