Morphological analysis of the mormyrid cerebellum using immunohistochemistry, with emphasis on the unusual neuronal organization of the valvula

Realistic mossy fiber input patterns to unipolar brush cells evoke a continuum of temporal responses comprised of components mediated by different glutamate receptors

Unipolar brush cells (UBCs) are excitatory interneurons in the cerebellar cortex that receive mossy fiber (MF) inputs and excite granule cells. The UBC population responds to brief burst activation of MFs with a continuum of temporal transformations, but it is not known how UBCs transform the diverse range of MF input patterns that occur in vivo. Here, we use cell-attached recordings from UBCs in acute cerebellar slices to examine responses to MF firing patterns that are based on in vivo recordings. We find that MFs evoke a continuum of responses in the UBC population, mediated by three different types of glutamate receptors that each convey a specialized component. AMPARs transmit timing information for single stimuli at up to 5 spikes/s, and for very brief bursts. A combination of mGluR2/3s (inhibitory) and mGluR1s (excitatory) mediates a continuum of delayed, and broadened responses to longer bursts, and to sustained high frequency activation. Variability in the mGluR2/3 component controls the time course of the onset of firing, and variability in the mGluR1 component controls the duration of prolonged firing. We conclude that the combination of glutamate receptor types allows each UBC to simultaneously convey different aspects of MF firing. These findings establish that UBCs are highly flexible circuit elements that provide diverse temporal transformations that are well suited to contribute to specialized processing in different regions of the cerebellar cortex.

Realistic mossy fiber input patterns to unipolar brush cells evoke a continuum of temporal responses comprised of components mediated by different glutamate receptors

Unipolar brush cells (UBCs) are excitatory interneurons in the cerebellar cortex that receive mossy fiber (MF) inputs and excite granule cells. The UBC population responds to brief burst activation of MFs with a continuum of temporal transformations, but it is not known how UBCs transform the diverse range of MF input patterns that occur in vivo. Here we use cell-attached recordings from UBCs in acute cerebellar slices to examine responses to MF firing patterns that are based on in vivo recordings. We find that MFs evoke a continuum of responses in the UBC population, mediated by three different types of glutamate receptors that each convey a specialized component. AMPARs transmit timing information for single stimuli at up to 5 spikes/s, and for very brief bursts. A combination of mGluR2/3s (inhibitory) and mGluR1s (excitatory) mediates a continuum of delayed, and broadened responses to longer bursts, and to sustained high frequency activation. Variability in the mGluR2/3 component controls the time course of the onset of firing, and variability in the mGluR1 component controls the duration of prolonged firing. We conclude that the combination of glutamate receptor types allows each UBC to simultaneously convey different aspects of MF firing. These findings establish that UBCs are highly flexible circuit elements that provide diverse temporal transformations that are well suited to contribute to specialized processing in different regions of the cerebellar cortex.

Neural connections of the torus semicircularis in the adult Zebrafish

The torus semicircularis (TS) of teleosts is a key midbrain center of the lateral line and acoustic sensory systems. To characterize the TS in adult zebrafish, we studied their connections using the carbocyanine tracers applied to the TS and to other related nuclei and tracts. Two main TS nuclei, central and ventrolateral, were differentiable by their afferent connections. From central TS, (TSc) numerous toropetal cells were labeled bilaterally in several primary octaval nuclei (anterior, magnocellular, descending, and posterior octaval nuclei), in the secondary octaval nucleus, in the caudal octavolateralis nucleus, and in the perilemniscular region. In the midbrain, numerous toropetal cells were labeled in the contralateral TSc. In the diencephalon, toropetal cells labeled from the TSc were observed ipsilaterally in the medial prethalamic nucleus and the periventricular posterior tubercle nucleus. TSc toropetal neurons were also labeled bilaterally in the hypothalamic anterior tuberal nucleus (ATN) and ipsilaterally in the parvicellular preoptic nucleus but not in the telencephalon. Tracer application to the medial octavolateralis nucleus revealed contralateral projections to the ventrolateral TS (TSvl), whereas tracer application to the secondary octaval nucleus labeled fibers bilaterally in TSc and neurons in rostral TSc. The TSc sends ascending fibers to the ipsilateral lateral preglomerular region that, in turn, projects to the pallium. Application of DiI to the optic tectum labeled cells and fibers in the TSvl, whereas application of DiI to the ATN labeled cells and fibers in the TSc. These results reveal that the TSvl and TSc are mainly related with the mechanosensory lateral line and acoustic centers, respectively, and that they show different higher order connections.

A Continuum of Response Properties across the Population of Unipolar Brush Cells in the Dorsal Cochlear Nucleus

Unipolar brush cells (UBCs) in the cerebellum and dorsal cochlear nucleus (DCN) perform temporal transformations by converting brief mossy fiber bursts into long-lasting responses. In the cerebellar UBC population, mixing inhibition with graded mGluR1-dependent excitation leads to a continuum of temporal responses. In the DCN, it has been thought that mGluR1 contributes little to mossy fiber responses and that there are distinct excitatory and inhibitory UBC subtypes. Here, we investigate UBC response properties using noninvasive cell-attached recordings in the DCN of mice of either sex. We find a continuum of responses to mossy fiber bursts ranging from 100 ms excitation to initial inhibition followed by several seconds of excitation to inhibition lasting for hundreds of milliseconds. Pharmacological interrogation reveals excitatory responses are primarily mediated by mGluR1 Thus, UBCs in both the DCN and cerebellum rely on mGluR1 and have a continuum of response durations. The continuum of responses in the DCN may allow more flexible and efficient temporal processing than can be achieved with distinct excitatory and inhibitory populations.SIGNIFICANCE STATEMENT UBCs are specialized excitatory interneurons in cerebellar-like structures that greatly prolong the temporal responses of mossy fiber inputs. They are thought to help cancel out self-generated signals. In the DCN, the prevailing view was that there are two distinct ON and OFF subtypes of UBCs. Here, we show that instead the UBC population has a continuum of response properties. Many cells show suppression and excitation consecutively, and the response durations vary considerably. mGluR1s are crucial in generating a continuum of responses. To understand how UBCs contribute to temporal processing, it is essential to consider the continuous variations of UBC responses, which have advantages over just having opposing ON/OFF subtypes of UBCs.

Development, circuitry, and function of the zebrafish cerebellum

The cerebellum represents a brain compartment that first appeared in gnathostomes (jawed vertebrates). Besides the addition of cell numbers, its development, cytoarchitecture, circuitry, physiology, and function have been highly conserved throughout avian and mammalian species. While cerebellar research in avian and mammals is extensive, systematic investigations on this brain compartment in zebrafish as a teleostian model organism started only about two decades ago, but has provided considerable insight into cerebellar development, physiology, and function since then. Zebrafish are genetically tractable with nearly transparent small-sized embryos, in which cerebellar development occurs within a few days. Therefore, genetic investigations accompanied with non-invasive high-resolution in vivo time-lapse imaging represents a powerful combination for interrogating the behavior and function of cerebellar cells in their complex native environment.

Histological and Molecular Characterization of the Inferior Olivary Nucleus and Climbing Fibers in the Goldfish, Carassius auratus

The cerebellum receives inputs via the climbing fibers originating from the inferior olivary nucleus in the ventral medulla. In mammals, the climbing fibers entwine and terminate onto both major and peripheral branches of dendrites of the Purkinje cells. In this study, the inferior olivary nucleus and climbing fiber in the goldfish were investigated with several histological techniques. By neural tracer application to the hemisphere of the cerebellum, labeled inferior olivary neurons were found in the ventral edge of the contralateral medulla. Kainate stimulated Co ^{+ +} uptake and gephyrin immunoreactivities were found in inferior olivary neurons, indicating, respectively, that they receive both excitatory (glutamatergic) and inhibitory (GABAergic or glycinergic) inputs. Inferior olivary neurons express vglut2.1 transcripts, suggesting they are glutamatergic. Around 85% of inferior olivary neurons were labeled with anti-calretinin antiserum. Calretinin immunoreactive (ir) climbing fiber terminal-like structures were distributed near the Purkinje cells and in the molecular layer. Double labeling immunofluorescence with anti-calretinin and zebrin II antisera revealed that the calretinin-ir climbing fibers run along and made synaptic-like contacts on the major dendrites of the zebrin II-ir Purkinje cells. In teleost fish, cerebellar efferent neurons, eurydendroid cells, also lie near the Purkinje cells and extend dendrites outward to intermingle with dendrites of the Purkinje cells within the molecular layer. Here we found no contacts between the climbing fiber terminals and the eurydendroid cell dendrites. These results support the idea that Purkinje cells, but not eurydendroid cells, receive strong inputs via the climbing fibers, similar to the mammalian situation.

Morphological analysis of the cerebellum and its efferent system in a basal actinopterygian fish, Polypterus senegalus

Although all vertebrate cerebella contain granule cells, Purkinje cells, and efferent neurons, the cellular arrangement and neural circuitry are highly diverse. In amniotes, cerebellar efferent neurons form clusters, deep cerebellar nuclei, lie deep in the cerebellum, and receive synaptic inputs from Purkinje cells but not granule cells. However, in the cerebellum of teleosts, the efferent neurons, called eurydendroid cells, lie near the cell bodies of Purkinje cells and receive inputs both from axons of Purkinje cells and granule cell parallel fibers. It is largely unknown how the cerebellar structure evolved in ray-finned fish (actinopterygians). To address this issue, we analyzed the cerebellum of a bichir Polypterus senegalus, one of the most basal actinopterygians. We found that the cell bodies of Purkinje cells are not aligned in a layer; incoming climbing fibers terminate mainly on the basal portion of Purkinje cells, revealing that the Polypterus cerebellum has unique features among vertebrate cerebella. Retrograde labeling and marker analyses of the efferent neurons revealed that their cell bodies lie in restricted granular areas but not as deep cerebellar nuclei in the cerebellar white matter. The efferent neurons have long dendrites like eurydendroid cells, although they do not reach the molecular layer. Our findings suggest that the efferent system of the bichir cerebellum has intermediate features between teleosts and amniote vertebrates, and provides a model to understand the basis generating diversity in actinopterygian cerebella.

Patterns of Organization of Cerebellum and Spinal Cord of the Red-Tail Shark (Epalzeorhynchos bicolor): Histological, Morphometrical, and Immunohistochemical Studies

Teleosts exhibit enormous heterogeneity in brain morphology, especially in the patterns of the organization of cerebellum. The cerebellum of a red-tail shark that we analyzed was well-developed and included three main divisions: the valvula cerebelli, the corpus cerebelli, and the vestibulolateral lobe. Characteristically, the cerebellar cortex contained three well-distinct layers: an outer molecular, intermediate ganglionic, and inner granular layer. The ganglionic layer possessed irregularly arranged Purkinje cells and eurydendroid cells that extended their processes into the molecular layer. Both Purkinje cells and eurydendroid cells showed immunoreactivity for iNOS2. Moreover, astrocytes in the cerebellum showed the expression of glial fibrillary acidic protein. The most striking observation in the cerebellum of shark was the lack of deep cerebellar nuclei and a well-identified white matter. On the other hand, the gray substance in the spinal cord displays a characteristic pattern in its organization, in which the dorsal horns lie quite close together, giving the gray substance the shape of an inverted Y and possessing large neurons. Notably, the white matter possessed myelinated nerve fibers. The current study provides the first report on the organization of layers and neurons in the cerebellum and spinal cord of red-tail shark. This research will contribute to the neuroanatomy and evolutionary studies of the brain of Cyprinidae.

Central connections of the trigeminal motor command system in the weakly electric Elephantnose fish (Gnathonemus petersii)

The highly mobile chin appendage of Gnathonemus petersii, the Schnauzenorgan, is used to actively probe the environment and is known to be a fovea of the electrosensory system. It receives an important innervation from both the trigeminal sensory and motor systems. However, little is known about the premotor control pathways that coordinate the movements of the Schnauzenorgan, or about central pathways originating from the trigeminal motor nucleus. The present study focuses on the central connections of the trigeminal motor system to elucidate premotor centers controlling Schnauzenorgan movements, with particular interest in the possible connections between the electrosensory and trigeminal systems. Neurotracer injections into the trigeminal motor nucleus revealed bilateral, reciprocal connections between the two trigeminal motor nuclei and between the trigeminal sensory and motor nuclei by bilateral labeling of cells and terminals. Prominent afferent input to the trigeminal motor nucleus originates from the nucleus lateralis valvulae, the nucleus dorsalis mesencephali, the cerebellar corpus C1, the reticular formation, and the Raphe nuclei. Retrogradely labeled cells were also observed in the central pretectal nucleus, the dorsal anterior pretectal nucleus, the tectum, the ventroposterior nucleus of the torus semicircularis, the gustatory sensory and motor nuclei, and in the hypothalamus. Labeled terminals, but not cell bodies, were observed in the nucleus lateralis valvulae and the reticular formation. No direct connections were found between the electrosensory system and the V motor nucleus but the central connections identified would provide several multisynaptic pathways linking these two systems, including possible efference copy and corollary discharge mechanisms.

Melatonin receptor expression in vocal, auditory, and neuroendocrine centers of a highly vocal fish, the plainfin midshipman (Porichthys notatus)

Melatonin plays a central role in entraining activity to the day-night cycle in vertebrates. Here, we investigate neuroanatomical substrates of melatonin-dependent vocal-acoustic behavior in the nocturnal and highly vocal teleost fish, the plainfin midshipman (Porichthys notatus). Using in situ hybridization (ISH) and quantitative real-time PCR (qPCR), we assess the mRNA distribution and transcript abundance of melatonin receptor subtype 1B (mel1b), shown to be important for vocalization in midshipman fish and songbirds. ISH shows robust mel1b expression in major nodes of the central vocal and auditory networks in the subpallium, preoptic area (POA), anterior hypothalamus, dorsal thalamus, posterior tuberculum, midbrain torus semicircularis and periaqueductal gray, and hindbrain. Mel1b label is also abundant in secondary targets of the olfactory, visual, and lateral line systems, as well as telencephalic regions that have been compared to the amygdala, extended amygdala, striatum, septum, and hippocampus of tetrapods. Q-PCR corroborates mel1b abundance throughout the brain and shows significant increases in the morning compared with nighttime in tissue samples inclusive of the telencephalon and POA, but remains stable in other brain regions. Plasma melatonin levels show expected increase at night. Our findings support the hypothesis that melatonin's stimulatory effects on vocal-acoustic mechanisms in midshipman is mediated, in part, by melatonin binding in vocal, auditory, and neuroendocrine centers. Together with robust mel1b expression in multiple telencephalic nuclei and sensory systems, the results further indicate an expression pattern comparable to that in birds and mammals that is indicative of melatonin's broad involvement in the modulation of physiology and behavior.

The Mormyrid Optic Tectum Is a Topographic Interface for Active Electrolocation and Visual Sensing

The African weakly electric fish Gnathonemus petersii is capable of cross-modal object recognition using its electric sense or vision. Thus, object features stored in the brain are accessible by multiple senses, either through connections between unisensory brain regions or because of multimodal representations in multisensory areas. Primary electrosensory information is processed in the medullary electrosensory lateral line lobe, which projects topographically to the lateral nucleus of the torus semicircularis (NL). Visual information reaches the optic tectum (TeO), which projects to various other brain regions. We investigated the neuroanatomical connections of these two major midbrain visual and electrosensory brain areas, focusing on the topographical relationship of interconnections between the two structures. Thus, the neural tracer DiI was injected systematically into different tectal quadrants, as well as into the NL. Tectal tracer injections revealed topographically organized retrograde and anterograde label in the NL. Rostral and caudal tectal regions were interconnected with rostral and caudal areas of the NL, respectively. However, dorsal and ventral tectal regions were represented in a roughly inverted fashion in NL, as dorsal tectal injections labeled ventral areas in NL and vice versa. In addition, tracer injections into TeO or NL revealed extensive inputs to both structures from ipsilateral (NL also contralateral) efferent basal cells in the valvula cerebelli; the NL furthermore projected back to the valvula. Additional tectal and NL connections were largely confirmatory to earlier studies. For example, the TeO received ipsilateral inputs from the central zone of the dorsal telencephalon, torus longitudinalis, nucleus isthmi, various tegmental, thalamic and pretectal nuclei, as well as other nuclei of the torus semicircularis. Also, the TeO projected to the dorsal preglomerular and dorsal posterior thalamic nuclei as well as to nuclei in the torus semicircularis and nucleus isthmi. Beyond the clear topographical relationship of NL and TeO interconnections established here, the known neurosensory upstream circuitry was used to suggest a model of how a defined spot in the peripheral sensory world comes to be represented in a common associated neural locus both in the NL and the TeO, thereby providing the neural substrate for cross-modal object recognition.

Cell type-specific plasticity at parallel fiber synapses onto Purkinje cells in the posterior caudal lobe of the mormyrid fish cerebellum

It has been demonstrated that there are two morphological subtypes of Purkinje cells (PCs)-fan-shaped Purkinje cells (fPCs) and multipolar Purkinje cells (mPCs)-in the posterior caudal lobe of the mormyrid fish cerebellum, but whether these cell types are also functionally distinct is unknown. Here, we have used electrophysiological and pharmacological tools in a slice preparation to demonstrate that pairing parallel fiber (PF) and climbing fiber (CF) inputs at a low frequency induces long-term depression (LTD) in fPCs but long-term potentiation (LTP) in mPCs. The induction of plasticity in both cell types required postsynaptic Ca²⁺ and type 1α metabotropic glutamate receptors. However, the LTD in fPCs was inducted via a calcium/calmodulin-dependent protein kinase II cascade, whereas LTP induction in mPCs required calcineurin. Moreover, the LTD in fPCs and LTP in mPCs were accompanied by changes to the corresponding paired-pulse ratios and their coefficients of variation, suggesting presynaptic modes of expression for the plasticity at PF terminals for both cell types. Hence, the synaptic plasticity at PF synapses onto PCs in the posterior caudal lobe of the mormyrid cerebellum is cell type specific, with both pre- and postsynaptic mechanisms contributing to its induction and expression. NEW & NOTEWORTHY Much has been learnt about the cerebellar long-term depression (LTD) in the cortex. More recent work has shown that long-term potentiation (LTP) is equally important for cerebellar motor learning. Here we report for the first time that plasticity in the mormyrid cerebellum is cell type specific, e.g., following the conventional pairing of parallel and climbing fiber inputs in an in vitro preparation leads to LTD in one Purkinje cell subtype and LTP in another.

Development of the cerebellum in turbot (Psetta maxima): Analysis of cell proliferation and distribution of calcium binding proteins

The morphogenesis, cell proliferation and neuronal differentiation of the turbot (Psetta maxima) cerebellum has been studied using conventional histological techniques and immunohistochemical methods for proliferating cell nuclear antigen and calcium binding proteins. As in other vertebrates, the cerebellar anlage emerges as proliferative plates of neural tissue during the embryonic period. The anlage of the cerebellum persists without morphological changes until the end of the larval life when the mantle zone is differentiated. The major ontogenetic changesthat drive the formation of the cerebellar subdivisions begin in late premetamorphic larvae when cerebellar plates growth and merge medially. This transformation is accomplished by the reorganization of proliferative zones as well as by the onset of cell differentiation. The cerebellum becomes fully differentiated during metamorphosis when parvalbumin and calretinin were detected in Purkinje and eurydendroid cells. Sustained proliferation is maintained in all subdivisions of the cerebellum and this support the robust growth of this part of the brain that takes place during the metamorphic and juvenile periods.The location and histological organization of the proliferative activity in the turbot mature cerebellum are described and their functional significance was analyzed in light of the information available for other teleosts.

Consensus Paper: Cerebellar Development

The development of the mammalian cerebellum is orchestrated by both cell-autonomous programs and inductive environmental influences. Here, we describe the main processes of cerebellar ontogenesis, highlighting the neurogenic strategies used by developing progenitors, the genetic programs involved in cell fate specification, the progressive changes of structural organization, and some of the better-known abnormalities associated with developmental disorders of the cerebellum.

Relational and procedural memory systems in the goldfish brain revealed by trace and delay eyeblink-like conditioning

The presence of multiple memory systems supported by different neural substrata has been demonstrated in animal and human studies. In mammals, two variants of eyeblink classical conditioning, differing only in the temporal relationships between the conditioned stimulus (CS) and the unconditioned stimulus (US), have been widely used to study the neural substrata of these different memory systems. Delay conditioning, in which both stimuli coincide in time, depends on a non-relational memory system supported by the cerebellum and associated brainstem circuits. In contrast, trace conditioning, in which a stimulus-free time gap separates the CS and the US, requires a declarative or relational memory system, thus depending on forebrain structures in addition to the cerebellum. The distinction between the explicit or relational and the implicit or procedural memory systems that support trace and delay classical conditioning has been extensively studied in mammals, but studies in other vertebrate groups are relatively scarce. In the present experiment we analyzed the differential involvement of the cerebellum and the telencephalon in delay and trace eyeblink-like classical conditioning in goldfish. The results show that whereas the cerebellum lesion prevented the eyeblink-like conditioning in both procedures, the telencephalon ablation impaired exclusively the acquisition of the trace conditioning. These data showing that comparable neural systems support delay and trace eyeblink conditioning in teleost fish and mammals suggest that these separate memory systems and their neural bases could be a shared ancestral brain feature of the vertebrate lineage.

Post-hatching brain morphogenesis and cell proliferation in the pulse-type mormyrid Mormyrus rume proboscirostris

The anatomical organization of African Mormyrids' brain is a clear example of departure from the average brain morphotype in teleosts, probably related to functional specialization associated to electrosensory processing and sensory-motor coordination. The brain of Mormyrids is characterized by a well-developed rhombencephalic electrosensory lobe interconnected with relatively large mesencephalic torus semicircularis and optic tectum, and a huge and complex cerebellum. This unique morphology might imply cell addition from extraventricular proliferation zones up to late developmental stages. Here we studied the ontogeny of these brain regions in Mormyrus rume proboscirostris from embryonic to adult stages by classical histological techniques and 3D reconstruction, and analyzed the spatial-temporal distribution of proliferating cells, using pulse type BrdU labeling. Brain morphogenesis and maturation progressed in rostral-caudal direction, from 4day old free embryos, through larvae, to juveniles whose brain almost attained adult morphological complexity. The change in the relative size of the telencephalon, and mesencephalic and rhombencephalic brain regions suggest a developmental shift in the relative importance of visual and electrosensory modalities. In free embryos, proliferating cells densely populated the lining of the ventricular system. During development, ventricular proliferating cells decreased in density and extension of distribution, constituting ventricular proliferation zones. The first recognizable one was found at the optic tectum of free embryos. Several extraventricular proliferation zones were found in the cerebellar divisions of larvae, persisting along life. Adult M. rume proboscirostris showed scarce ventricular but profuse cerebellar proliferation zones, particularly at the subpial layer of the valvula cerebelli, similar to lagomorphs. This might indicate that adult cerebellar proliferation is a conserved vertebrate feature.

Neural Mechanisms for Predicting the Sensory Consequences of Behavior: Insights from Electrosensory Systems

Perception of the environment requires differentiating between external sensory inputs and those that are self-generated. Some of the clearest insights into the neural mechanisms underlying this process have come from studies of the electrosensory systems of fish. Neurons at the first stage of electrosensory processing generate negative images of the electrosensory consequences of the animal's own behavior. By canceling out the effects of predictable, self-generated inputs, negative images allow for the selective encoding of unpredictable, externally generated stimuli. Combined experimental and theoretical studies of electrosensory systems have led to detailed accounts of how negative images are formed at the level of synaptic plasticity rules, cells, and circuits. Here, I review these accounts and discuss their implications for understanding how predictions of the sensory consequences of behavior may be generated in other sensory structures and the cerebellum.

Modeling latency code processing in the electric sense: from the biological template to its VLSI implementation

Understanding the coding of sensory information under the temporal constraints of natural behavior is not yet well resolved. There is a growing consensus that spike timing or latency coding can maximally exploit the timing of neural events to make fast computing elements and that such mechanisms are essential to information processing functions in the brain. The electric sense of mormyrid fish provides a convenient biological model where this coding scheme can be studied. The sensory input is a physically ordered spatial pattern of current densities, which is coded in the precise timing of primary afferent spikes. The neural circuits of the processing pathway are well known and the system exhibits the best known illustration of corollary discharge, which provides the reference to decoding the sensory afferent latency pattern. A theoretical model has been constructed from available electrophysiological and neuroanatomical data to integrate the principal traits of the neural processing structure and to study sensory interaction with motor-command-driven corollary discharge signals. This has been used to explore neural coding strategies at successive stages in the network and to examine the simulated network capacity to reproduce output neuron responses. The model shows that the network has the ability to resolve primary afferent spike timing differences in the sub-millisecond range, and that this depends on the coincidence of sensory and corollary discharge-driven gating signals. In the integrative and output stages of the network, corollary discharge sets up a proactive background filter, providing temporally structured excitation and inhibition within the network whose balance is then modulated locally by sensory input. This complements the initial gating mechanism and contributes to amplification of the input pattern of latencies, conferring network hyperacuity. These mechanisms give the system a robust capacity to extract behaviorally meaningful features of the electric image with high sensitivity over a broad working range. Since the network largely depends on spike timing, we finally discuss its suitability for implementation in robotic applications based on neuromorphic hardware.

Eppur Si Muove: Evidence for an External Granular Layer and Possibly Transit Amplification in the Teleostean Cerebellum

The secreted signaling factor Sonic Hedgehog (Shh) acts in the floor plate of the developing vertebrate CNS to promote motoneuron development. In addition, shh has dorsal expression domains in the amniote alar plate (i.e., in isocortex, superior colliculus, and cerebellum). For example, shh expressing Purkinje cells act in transit amplification of external granular layer (EGL) cells of the developing cerebellum. Our previous studies had indicated the presence of an EGL in anamniote zebrafish, but a possible role of shh in the zebrafish cerebellar plate remained elusive. Therefore, we used an existing zebrafish transgenic line Tg(2.4shha-ABC-GFP)sb15; Shkumatava et al., 2004) to show this gene activity and its cellular localization in the larval zebrafish brain. Clearly, GFP expressing cells occur in larval alar zebrafish brain domains, i.e., optic tectum and cerebellum. Analysis of critical cerebellar cell markers on this transgenic background and a PH3 assay for mitotic cells reveals that Purkinje cells and eurydendroid cells are completely non-overlapping postmitotic cell populations. Furthermore, shh-GFP cells never express Zebrin II or parvalbumin, nor calretinin. They are thus neither Purkinje cells nor calretinin positive migrating rhombic lip derived cells. The shh-GFP cells also never correspond to PH3 positive cells of the ventral cerebellar proliferative zone or the upper rhombic lip-derived EGL. From this marker analysis and the location of shh-GFP cells sandwiched between calretinin positive rhombic lip derived cells and parvalbumin positive Purkinje cells, we conclude that shh-GFP expressing cells qualify as previously reported olig2 positive eurydendroid cells, which are homologous to the amniote deep cerebellar nuclei. We confirm this using double transgenic progeny of shh-GFP and olig2-dsRed zebrafish. Thus, these zebrafish eurydendroid cells may have the same role in transit amplification as Purkinje cells do in amniotes.

AMPA receptor mediated synaptic excitation drives state-dependent bursting in Purkinje neurons of zebrafish larvae

Purkinje neurons are central to cerebellar function and show membrane bistability when recorded in vitro or in vivo under anesthesia. The existence of bistability in vivo in awake animals is disputed. Here, by recording intracellularly from Purkinje neurons in unanesthetized larval zebrafish (Danio rerio), we unequivocally demonstrate bistability in these neurons. Tonic firing was seen in depolarized regimes and bursting at hyperpolarized membrane potentials. In addition, Purkinje neurons could switch from one state to another spontaneously or with current injection. While GABAAR or NMDAR were not required for bursting, activation of AMPARs by climbing fibers (CFs) was sufficient to trigger bursts. Further, by recording Purkinje neuron membrane potential intracellularly, and motor neuron spikes extracellularly, we show that initiation of motor neuron spiking is correlated with increased incidence of CF EPSPs and membrane depolarization. Developmentally, bistability was observed soon after Purkinje neuron specification and persists at least until late larval stages.

Sensory processing and corollary discharge effects in posterior caudal lobe Purkinje cells in a weakly electric mormyrid fish

Although it has been suggested that the cerebellum functions to predict the sensory consequences of motor commands, how such predictions are implemented in cerebellar circuitry remains largely unknown. A detailed and relatively complete account of predictive mechanisms has emerged from studies of cerebellum-like sensory structures in fish, suggesting that comparisons of the cerebellum and cerebellum-like structures may be useful. Here we characterize electrophysiological response properties of Purkinje cells in a region of the cerebellum proper of weakly electric mormyrid fish, the posterior caudal lobe (LCp), which receives the same mossy fiber inputs and projects to the same target structures as the electrosensory lobe (ELL), a well-studied cerebellum-like structure. We describe patterns of simple spike and climbing fiber activation in LCp Purkinje cells in response to motor corollary discharge, electrosensory, and proprioceptive inputs and provide evidence for two functionally distinct Purkinje cell subtypes within LCp. Protocols that induce rapid associative plasticity in ELL fail to induce plasticity in LCp, suggesting differences in the adaptive functions of the two structures. Similarities and differences between LCp and ELL are discussed in light of these results.

Comparative neuronal morphology of the cerebellar cortex in afrotherians, carnivores, cetartiodactyls, and primates

Although the basic morphological characteristics of neurons in the cerebellar cortex have been documented in several species, virtually nothing is known about the quantitative morphological characteristics of these neurons across different taxa. To that end, the present study investigated cerebellar neuronal morphology among eight different, large-brained mammalian species comprising a broad phylogenetic range: afrotherians (African elephant, Florida manatee), carnivores (Siberian tiger, clouded leopard), cetartiodactyls (humpback whale, giraffe) and primates (human, common chimpanzee). Specifically, several neuron types (e.g., stellate, basket, Lugaro, Golgi, and granule neurons; N = 317) of the cerebellar cortex were stained with a modified rapid Golgi technique and quantified on a computer-assisted microscopy system. There was a 64-fold variation in brain mass across species in our sample (from clouded leopard to the elephant) and a 103-fold variation in cerebellar volume. Most dendritic measures tended to increase with cerebellar volume. The cerebellar cortex in these species exhibited the trilaminate pattern common to all mammals. Morphologically, neuron types in the cerebellar cortex were generally consistent with those described in primates (Fox et al., 1967) and rodents (Palay and Chan-Palay, 1974), although there was substantial quantitative variation across species. In particular, Lugaro neurons in the elephant appeared to be disproportionately larger than those in other species. To explore potential quantitative differences in dendritic measures across species, MARSplines analyses were used to evaluate whether species could be differentiated from each other based on dendritic characteristics alone. Results of these analyses indicated that there were significant differences among all species in dendritic measures.

A temporal basis for predicting the sensory consequences of motor commands in an electric fish

Mormyrid electric fish are a model system for understanding how neural circuits predict the sensory consequences of motor acts. Medium ganglion cells in the electrosensory lobe create negative images that predict sensory input resulting from the fish's electric organ discharge (EOD). Previous studies have shown that negative images can be created through plasticity at granule cell-medium ganglion cell synapses, provided that granule cell responses to the brief EOD command are sufficiently varied and prolonged. Here we show that granule cells indeed provide such a temporal basis and that it is well-matched to the temporal structure of self-generated sensory inputs, allowing rapid and accurate sensory cancellation and explaining paradoxical features of negative images. We also demonstrate an unexpected and critical role of unipolar brush cells (UBCs) in generating the required delayed responses. These results provide a mechanistic account of how copies of motor commands are transformed into sensory predictions.

Immunohistochemical distribution of calretinin and calbindin (D-28k) in the brain of the cladistian Polypterus senegalus

Polypteriform fishes are believed to be basal to other living ray-finned bony fishes, and they may be useful for providing information of the neural organization that existed in the brain of the earliest ray-finned fishes. The calcium-binding proteins calretinin (CR) and calbindin-D28k (CB) have been widely used to characterize neuronal populations in vertebrate brains. Here, the distribution of the immunoreactivity against CR and CB was investigated in the olfactory organ and brain of Polypterus senegalus and compared to the distribution of these molecules in other ray-finned fishes. In general, CB-immunoreactive (ir) neurons were less abundant than CR-ir cells. CR immunohistochemistry revealed segregation of CR-ir olfactory receptor neurons in the olfactory mucosa and their bulbar projections. Our results confirmed important differences between pallial regions in terms of CR immunoreactivity of cell populations and afferent fibers. In the habenula, these calcium-binding proteins revealed right-left asymmetry of habenular subpopulations and segregation of their interpeduncular projections. CR immunohistochemistry distinguished among some thalamic, pretectal, and posterior tubercle-derived populations. Abundant CR-ir populations were observed in the midbrain, including the tectum. CR immunoreactivity was also useful for characterizing a putative secondary gustatory/visceral nucleus in the isthmus, and for distinguishing territories in the primary viscerosensory column and octavolateral region. Comparison of the data obtained within a segmental neuromeric context indicates that some CB-ir and CR-ir populations in polypteriform fishes are shared with other ray-finned fishes, but other positive structures appear to have evolved following the separation between polypterids and other ray-finned fishes.

Differential roles of ventral pallidum subregions during cocaine self-administration behaviors

The ventral pallidum (VP) is necessary for drug-seeking behavior. VP contains ventromedial (VPvm) and dorsolateral (VPdl) subregions, which receive projections from the nucleus accumbens shell and core, respectively. To date no study has investigated the behavioral functions of the VPdl and VPvm subregions. To address this issue, we investigated whether changes in firing rate (FR) differed between VP subregions during four events: approaching toward, responding on, or retreating away from a cocaine-reinforced operandum and a cocaine-associated cue. Baseline FR and waveform characteristics did not differ between subregions. VPdl neurons exhibited a greater change in FR compared with VPvm neurons during approaches toward, as well as responses on, the cocaine-reinforced operandum. VPdl neurons were more likely to exhibit a similar change in FR (direction and magnitude) during approach and response than VPvm neurons. In contrast, VPvm firing patterns were heterogeneous, changing FRs during approach or response alone, or both. VP neurons did not discriminate cued behaviors from uncued behaviors. No differences were found between subregions during the retreat, and no VP neurons exhibited patterned changes in FR in response to the cocaine-associated cue. The stronger, sustained FR changes of VPdl neurons during approach and response may implicate VPdl in the processing of drug-seeking and drug-taking behavior via projections to subthalamic nucleus and substantia nigra pars reticulata. In contrast, the heterogeneous firing patterns of VPvm neurons may implicate VPvm in facilitating mesocortical structures with information related to the sequence of behaviors predicting cocaine self-infusions via projections to mediodorsal thalamus and ventral tegmental area.

Pattern of calbindin-D28k and calretinin immunoreactivity in the brain of Xenopus laevis during embryonic and larval development

The present study represents a detailed spatiotemporal analysis of the localization of calbindin-D28k (CB) and calretinin (CR) immunoreactive structures in the brain of Xenopus laevis throughout development, conducted with the aim to correlate the onset of the immunoreactivity with the development of compartmentalization of distinct subdivisions recently identified in the brain of adult amphibians and primarily highlighted when analyzed within a segmental paradigm. CR and CB are expressed early in the brain and showed a progressively increasing expression throughout development, although transient expression in some neuronal subpopulations was also noted. Common and distinct characteristics in Xenopus, as compared with reported features during development in the brain of mammals, were observed. The development of specific regions in the forebrain such as the olfactory bulbs, the components of the basal ganglia and the amygdaloid complex, the alar and basal hypothalamic regions, and the distinct diencephalic neuromeres could be analyzed on the basis of the distinct expression of CB and CR in subregions. Similarly, the compartments of the mesencephalon and the main rhombencephalic regions, including the cerebellum, were differently highlighted by their specific content in CB and CR throughout development. Our results show the usefulness of the analysis of the distribution of these proteins as a tool in neuroanatomy to interpret developmental aspects of many brain regions.

Development and evolution of cerebellar neural circuits

The cerebellum controls smooth and skillful movements and it is also involved in higher cognitive and emotional functions. The cerebellum is derived from the dorsal part of the anterior hindbrain and contains two groups of cerebellar neurons: glutamatergic and gamma-aminobutyric acid (GABA)ergic neurons. Purkinje cells are GABAergic and granule cells are glutamatergic. Granule and Purkinje cells receive input from outside of the cerebellum from mossy and climbing fibers. Genetic analysis of mice and zebrafish has revealed genetic cascades that control the development of the cerebellum and cerebellar neural circuits. During early neurogenesis, rostrocaudal patterning by intrinsic and extrinsic factors, such as Otx2, Gbx2 and Fgf8, plays an important role in the positioning and formation of the cerebellar primordium. The cerebellar glutamatergic neurons are derived from progenitors in the cerebellar rhombic lip, which express the proneural gene Atoh1. The GABAergic neurons are derived from progenitors in the ventricular zone, which express the proneural gene Ptf1a. The mossy and climbing fiber neurons originate from progenitors in the hindbrain rhombic lip that express Atoh1 or Ptf1a. Purkinje cells exhibit mediolateral compartmentalization determined on the birthdate of Purkinje cells, and linked to the precise neural circuitry formation. Recent studies have shown that anatomy and development of the cerebellum is conserved between mammals and bony fish (teleost species). In this review, we describe the development of cerebellar neurons and neural circuitry, and discuss their evolution by comparing developmental processes of mammalian and teleost cerebellum.

Synaptic dynamics and long-term plasticity at synapses of Purkinje cells onto neighboring Purkinje cells of a mormyrid fish: a dual cell recording study

The input synapses of cerebellar Purkinje cells (PCs) have been extensively studied and much has been learned about their dynamics, plasticity and functionality. In contrast there is limited information available about PC output synapses. This study uses dual cell recording methods to investigate synaptic dynamics and plasticity at individual PC synapses onto neighboring PCs in in vitro preparations of the mormyrid cerebellum. This synaptic connectivity may be strong or weak. For strong connections, inhibitory postsynaptic potentials (IPSPs) or currents (IPSCs) are synchronized with the action potentials of the presynaptic cell. For weak connections, however, the pre- and postsynaptic potentials are no longer synchronized, and presynaptic burst firing at intraburst rates of ∼50 Hz or higher is required to reliably induce the postsynaptic inhibition. A depression of this postsynaptic inhibition was observed for both types of connectivity following repeated presynaptic bursts, which was subsequently largely reversed following pairings of the presynaptic burst-induced IPSPs/IPSCs with evoked burst firing of the postsynaptic cell. Moreover, the original postsynaptic depression was found to be either augmented or reversed depending on the temporal order of each pair of additional pre- and postsynaptic cell activations, hence demonstrating a reversible and spike timing-dependent plasticity (STDP) at this synapse.

Functional circuitry of a unique cerebellar specialization: the valvula cerebelli of a mormyrid fish

The valvula cerebelli of the mormyrid electric fish is a useful site for the study of cerebellar function. The valvula forms a part of the electrosensory-electromotor system of this fish, a system that offers many possibilities for the study of sensory-motor integration. The valvula also has a number of histological features not present in mammals which facilitate investigation of cerebellar circuitry and its plasticity. This initial study characterizes the basic physiology and pharmacology of cells in the valvula using an in vitro slice preparation. Intrinsic properties and synaptic responses of Purkinje cells and other cell types were examined. We found that Purkinje cells fire a small narrow Na(+) spike and a large broad Ca(2+) spike, generated in the axon initial segment and dendritic-soma region, respectively. Purkinje cells respond to parallel fiber inputs with graded excitatory postsynaptic potentials (EPSPs) and to climbing fiber inputs with all-or-none EPSPs. Efferent cells, Golgi cells, and deep stellate cells all fire a single type of large narrow spike and respond only to parallel fiber inputs. Both parallel fiber and climbing fiber responses in Purkinje cells appear to be entirely mediated by AMPA-type glutamate receptors, whereas parallel fiber responses in efferent cells and stellate cells include AMPA and NMDA components. In addition, a strong synaptic inhibition was uncovered in both Purkinje cells and efferent cells in response to the focal stimulation of parallel fibers. Dual cell recordings indicate that deep stellate cells contribute at least partially to this inhibition. We conclude that despite its unique histology, the local functional circuitry of the mormyrid valvula cerebelli is largely similar to that of the mammalian cerebellum. Thus, what is learned concerning the functioning of the mormyrid valvula cerebelli may be expected to be informative about cerebellar function in general.

Atypical protein kinase C regulates primary dendrite specification of cerebellar Purkinje cells by localizing Golgi apparatus

Neurons have highly polarized structures that determine what parts of the soma elaborate the axon and dendrites. However, little is known about the mechanisms that establish neuronal polarity in vivo. Cerebellar Purkinje cells extend a single primary dendrite from the soma that ramifies into a highly branched dendritic arbor. We used the zebrafish cerebellum to investigate the mechanisms by which Purkinje cells acquire these characteristics. To examine dendritic morphogenesis in individual Purkinje cells, we marked the cell membrane using a Purkinje cell-specific promoter to drive membrane-targeted fluorescent proteins. We found that zebrafish Purkinje cells initially extend multiple neurites from the soma and subsequently retract all but one, which becomes the primary dendrite. In addition, the Golgi apparatus specifically locates to the root of the primary dendrite, and its localization is already established in immature Purkinje cells that have multiple neurites. Inhibiting secretory trafficking through the Golgi apparatus reduces dendritic growth, suggesting that the Golgi apparatus is involved in the dendritic morphogenesis. We also demonstrated that in a mutant of an atypical protein kinase C (aPKC), Prkci, Purkinje cells retain multiple primary dendrites and show disrupted localization of the Golgi apparatus. Furthermore, a mosaic inhibition of Prkci in Purkinje cells recapitulates the aPKC mutant phenotype. These results suggest that the aPKC cell autonomously controls the Golgi localization and thereby regulates the specification of the primary dendrite of Purkinje cells.

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.

Electrophysiological characteristics of cells in the anterior caudal lobe of the mormyrid cerebellum

We have examined the basic electrophysiology and pharmacology of cells in the anterior caudal lobe (CLa) of the mormyrid cerebellum. Intracellular recordings were performed in an in vitro slice preparation using the whole-cell patch recording method. The responses of cells to parallel fiber (PF) and climbing fiber (CF) stimulation and to somatic current injection were recorded, and then characterized by bath application of receptor and ion channel blockers. Using biocytin or neurobiotin, these cells were also morphologically identified after recording to ensure their classification. Efferent cells and two subtypes of Purkinje cells were identified on the basis of their physiology and morphology. While the majority of Purkinje cells fire a single type of spike that is mediated by Na(+), some fire a large broad spike mediated by Ca(2+) and a narrow spike mediated by Na(+) at resting potential levels. By patching one recording electrode to the soma and another to one of the proximal dendrites of the same cell simultaneously, it was found that the Na(+) spike has an axonal origin and the Ca(2+) spike is generated in the soma-dendritic region of Purkinje cells. Efferent cells fire a single type of Na(+) spike only. Despite variations in their physiology and morphology, all cell types responded to PF stimulation with graded excitatory postsynaptic potentials (EPSPs) mediated by AMPA receptors. However, none of the efferent cells and only some of the Purkinje cells responded to CF activation with a large, AMPA receptor-mediated all-or-none EPSPs. We conclude that the functional circuitry of the CLa resembles that of other regions of the mormyrid cerebellum and is largely similar to that of the mammalian cerebellum.

Anatomy of zebrafish cerebellum and screen for mutations affecting its development

The cerebellum is important for the integration of sensory perception and motor control, but its structure has mostly been studied in mammals. Here, we describe the cell types and neural tracts of the adult zebrafish cerebellum using molecular markers and transgenic lines. Cerebellar neurons are categorized to two major groups: GABAergic and glutamatergic neurons. The Purkinje cells, which are GABAergic neurons, express parvalbumin7, carbonic anhydrase 8, and aldolase C like (zebrin II). The glutamatergic neurons are vglut1(+) granule cells and vglut2(high) cells, which receive Purkinje cell inputs; some vglut2(high) cells are eurydendroid cells, which are equivalent to the mammalian deep cerebellar nuclei. We found olig2(+) neurons in the adult cerebellum and ascertained that at least some of them are eurydendroid cells. We identified markers for climbing and mossy afferent fibers, efferent fibers, and parallel fibers from granule cells. Furthermore, we found that the cerebellum-like structures in the optic tectum and antero-dorsal hindbrain show similar Parvalbumin7 and Vglut1 expression profiles as the cerebellum. The differentiation of GABAergic and glutamatergic neurons begins 3 days post-fertilization (dpf), and layers are first detectable 5 dpf. Using anti-Parvalbumin7 and Vglut1 antibodies to label Purkinje cells and granule cell axons, respectively, we screened for mutations affecting cerebellar neuronal development and the formation of neural tracts. Our data provide a platform for future studies of zebrafish cerebellar development.

The neuronal organization of a unique cerebellar specialization: the valvula cerebelli of a mormyrid fish

The distal valvula cerebelli is the most prominent part of the mormyrid cerebellum. It is organized in ridges of ganglionic and molecular layers, oriented perpendicular to the granular layer. We have combined intracellular recording and labeling techniques to reveal the cellular morphology of the valvula ridges in slice preparations. We have also locally ejected tracer in slices and in intact animals to examine its input fibers. The palisade dendrites and fine axon arbors of Purkinje cells are oriented in the horizontal plane of the ridge. The dendrites of basal efferent cells and large central cells are confined to the molecular layer but are not planar. Basal efferent cell axons are thick and join the basal bundle leaving the cerebellum. Large central cell axons are also thick, and they traverse long distances in the transverse plane, with local collaterals in the ganglionic layer. Vertical cells and small central cells also have thick axons with local collaterals. The dendrites of Golgi cells are confined to the molecular layer, but their axon arbors are either confined to the granular layer or proliferate in both the granular and ganglionic layers. Dendrites of deep stellate cells are distributed in the molecular layer, with fine axon arbors in the ganglionic layer. Granule cell axons enter the molecular layer as parallel fibers without bifurcating. Climbing fibers run in the horizontal plane and terminate exclusively in the ganglionic layer. Our results confirm and extend previous studies and suggest a new concept of the circuitry of the mormyrid valvula cerebelli.