Defective control and adaptation of reflex eye movements in mutant mice deficient in either the glutamate receptor delta2 subunit or Purkinje cells

Population calcium responses of Purkinje cells in the oculomotor cerebellum driven by non-visual input

The climbing fiber input to the cerebellum conveys instructive signals that can induce synaptic plasticity and learning by triggering complex spikes accompanied by large calcium transients in Purkinje cells. In the cerebellar flocculus, which supports oculomotor learning, complex spikes are driven by image motion on the retina, which could indicate an oculomotor error. In the same neurons, complex spikes also can be driven by non-visual signals. It has been shown that the calcium transients accompanying each complex spike can vary in amplitude, even within a given cell, therefore, we compared the calcium responses associated with the visual and non-visual inputs to floccular Purkinje cells. The calcium indicator GCaMP6f was selectively expressed in Purkinje cells, and fiber photometry was used to record the calcium responses from a population of Purkinje cells in the flocculus of awake behaving mice. During visual (optokinetic) stimuli and pairing of vestibular and visual stimuli, the calcium level increased during contraversive retinal image motion. During performance of the vestibulo-ocular reflex in the dark, calcium increased during contraversive head rotation and the associated ipsiverse eye movements. The amplitude of this non-visual calcium response was comparable to that during conditions with retinal image motion present that induce oculomotor learning. Thus, population calcium responses of Purkinje cells in the cerebellar flocculus to visual and non-visual input are similar to what has been reported previously for complex spikes, suggesting that multimodal instructive signals control the synaptic plasticity supporting oculomotor learning.

The architecture of GluD2 ionotropic delta glutamate receptor elucidated by cryo-EM

GluD2 receptor belongs to the orphan delta family of glutamate receptor ion channels. These receptors play key roles in synaptogenesis and synaptic plasticity and are associated with multiple neuronal disorders like schizophrenia, autism spectrum disorder, cerebellar ataxia, intellectual disability, paraplegia, retinal dystrophy, etc. Despite the importance of these receptors in CNS, insights into full-length GluD2 receptor structure is missing till-date. Here we report cryo-electron microscopy structure of the rat GluD2 receptor in the presence of calcium ions and the ligand 7-chlorokynurenic acid, elucidating its 3D architecture. The structure reveals a non-swapped architecture at the extracellular amino-terminal (ATD), and ligand-binding domain (LBD) interface similar to that observed in GluD1; however, the organization and arrangement of the ATD and LBD domains in GluD2 are unique. While our results demonstrate that non-swapped architecture is conserved in the delta receptor family, they also highlight the differences that exist between the two member receptors; GluD1 and GluD2.

Quantitative Proteomics Reveals Distinct Molecular Signatures of Different Cerebellum-Dependent Learning Paradigms

The cerebellum improves motor performance by adjusting motor gain appropriately. As de novo protein synthesis is essential for the formation and retention of memories, we hypothesized that motor learning in the opposite direction would induce a distinct pattern of protein expression in the cerebellum. We conducted quantitative proteomic profiling to compare the level of protein expression in the cerebellum at 1 and 24 h after training from mice that underwent different paradigms of cerebellum-dependent oculomotor learning from specific directional changes in motor gain. We quantified a total of 43 proteins that were significantly regulated in each of the three learning paradigms in the cerebellum at 1 and 24 h after learning. In addition, functional enrichment analysis identified protein groups that were differentially enriched or depleted in the cerebellum at 24 h after the three oculomotor learnings, suggesting that distinct biological pathways may be engaged in the formation of three oculomotor memories. Weighted correlation network analysis discovered groups of proteins significantly correlated with oculomotor memory. Finally, four proteins (Snca, Sncb, Cttn, and Stmn1) from the protein group correlated with the learning amount after oculomotor training were validated by Western blot. This study provides a comprehensive and unbiased list of proteins related to three cerebellum-dependent motor learning paradigms, suggesting the distinct nature of protein expression in the cerebellum for each learning paradigm. The proteomics data have been deposited to the ProteomeXchange Consortium with identifiers <PXD008433>.

GluD1 knockout mice with a pure C57BL/6N background show impaired fear memory, social interaction, and enhanced depressive-like behavior

The GluD1 gene is associated with susceptibility for schizophrenia, autism, depression, and bipolar disorder. However, the function of GluD1 and how it is involved in these conditions remain elusive. In this study, we generated a Grid1 gene-knockout (GluD1-KO) mouse line with a pure C57BL/6N genetic background and performed several behavioral analyses. Compared to a control group, GluD1-KO mice showed no significant anxiety-related behavioral differences, evaluated using behavior in an open field, elevated plus maze, a light-dark transition test, the resident-intruder test of aggression and sensorimotor gating evaluated by the prepulse inhibition test. However, GluD1-KO mice showed (1) higher locomotor activity in the open field, (2) decreased sociability and social novelty preference in the three-chambered social interaction test, (3) impaired memory in contextual, but not cued fear conditioning tests, and (4) enhanced depressive-like behavior in a forced swim test. Pharmacological studies revealed that enhanced depressive-like behavior in GluD1-KO mice was restored by the serotonin reuptake inhibitors imipramine and fluoxetine, but not the norepinephrine transporter inhibitor desipramine. In addition, biochemical analysis revealed no significant difference in protein expression levels, such as other glutamate receptors in the synaptosome and postsynaptic densities prepared from the frontal cortex and the hippocampus. These results suggest that GluD1 plays critical roles in fear memory, sociability, and depressive-like behavior.

Depressed by Learning-Heterogeneity of the Plasticity Rules at Parallel Fiber Synapses onto Purkinje Cells

Climbing fiber-driven long-term depression (LTD) of parallel fiber synapses onto cerebellar Purkinje cells has long been investigated as a putative mechanism of motor learning. We recently discovered that the rules governing the induction of LTD at these synapses vary across different regions of the cerebellum. Here, we discuss the design of LTD induction protocols in light of this heterogeneity in plasticity rules. The analytical advantages of the cerebellum provide an opportunity to develop a deeper understanding of how the specific plasticity rules at synapses support the implementation of learning.

Long-term sensorimotor adaptation in the ocular following system of primates

The sudden movement of a wide-field image leads to a reflexive eye tracking response referred to as short-latency ocular following. If the image motion occurs soon after a saccade the initial speed of the ocular following is enhanced, a phenomenon known as post-saccadic enhancement. We show in macaque monkeys that repeated exposure to the same stimulus regime over a period of months leads to progressive increases in the initial speeds of ocular following. The improvement in tracking speed occurs for ocular following with and without a prior saccade. As a result of the improvement in ocular following speeds, the influence of post-saccadic enhancement wanes with increasing levels of training. The improvement in ocular following speed following repeated exposure to the same oculomotor task represents a novel form of sensori-motor learning in the context of a reflexive movement.

A saturation hypothesis to explain both enhanced and impaired learning with enhanced plasticity

Across many studies, animals with enhanced synaptic plasticity exhibit either enhanced or impaired learning, raising a conceptual puzzle: how enhanced plasticity can yield opposite learning outcomes? Here, we show that the recent history of experience can determine whether mice with enhanced plasticity exhibit enhanced or impaired learning in response to the same training. Mice with enhanced cerebellar LTD, due to double knockout (DKO) of MHCI H2-K^b/H2-D^b (K^bD^b−/−), exhibited oculomotor learning deficits. However, the same mice exhibited enhanced learning after appropriate pre-training. Theoretical analysis revealed that synapses with history-dependent learning rules could recapitulate the data, and suggested that saturation may be a key factor limiting the ability of enhanced plasticity to enhance learning. Optogenetic stimulation designed to saturate LTD produced the same impairment in WT as observed in DKO mice. Overall, our results suggest that the recent history of activity and the threshold for synaptic plasticity conspire to effect divergent learning outcomes.

DOI:http://dx.doi.org/10.7554/eLife.20147.001

All animals can learn from their experiences. One of the main ideas for how learning occurs is that it involves changes in the strength of the connections between neurons, known as synapses. The ability of synapses to become stronger or weaker is referred to as synaptic plasticity. High levels of synaptic plasticity are generally thought to be good for learning, while low levels of synaptic plasticity make learning more difficult.

Nevertheless, studies have also reported that high levels of synaptic plasticity can sometimes impair learning. To explain these mixed results, Nguyen-Vu, Zhao, Lahiri et al. studied mice that had been genetically modified to show greater synaptic plasticity than normal mice. The same individual mutant animals were sometimes less able to learn an eye-movement task than unmodified mice, and at other times better able to learn exactly the same task. The main factor that determined how well the mice could learn was what the mice had experienced shortly before they began the training.

Nguyen-Vu et al. propose that some experiences change the strength of synapses so much that they temporarily prevent those synapses from undergoing any further changes. Animals with these “saturated” synapses will struggle to learn a new task, even if their brains are normally capable of high levels of synaptic plasticity. Notably, even normal activity appears to be able to put the synapses of the mutant mice into a saturated state, whereas this saturation would only occur in normal mice under a restricted set of circumstances. Consistent with this idea, Nguyen-Vu et al. showed that a specific type of pre-training that desaturates synapses improved the ability of the modified mice to learn the eye-movement task. Conversely, a different procedure that is known to saturate synapses impaired the learning ability of the unmodified mice.

A future challenge is to test these predictions experimentally by measuring changes in synaptic plasticity directly, both in brain slices and in living animals. The results could ultimately help to develop treatments that improve the ability to learn and so could provide benefits to a wide range of individuals, including people who have suffered a brain injury or stroke.

DOI:http://dx.doi.org/10.7554/eLife.20147.002

Psychophysical testing in rodent models of glaucomatous optic neuropathy

Processing of visual information begins in the retina, with photoreceptors converting light stimuli into neural signals. Ultimately, signals are transmitted to the brain through signaling networks formed by interneurons, namely bipolar, horizontal and amacrine cells providing input to retinal ganglion cells (RGCs), which form the optic nerve with their axons. As part of the chronic nature of glaucomatous optic neuropathy, the increasing and irreversible damage and ultimately loss of neurons, RGCs in particular, occurs following progressive damage to the optic nerve head (ONH), eventually resulting in visual impairment and visual field loss. There are two behavioral assays that are typically used to assess visual deficits in glaucoma rodent models, the visual water task and the optokinetic drum. The visual water task can assess an animal's ability to distinguish grating patterns that are associated with an escape from water. The optokinetic drum relies on the optomotor response, a reflex turning of the head and neck in the direction of the visual stimuli, which usually consists of rotating black and white gratings. This reflex is a physiological response critical for keeping the image stable on the retina. Driven initially by the neuronal input from direction-selective RGCs, this reflex is comprised of a number of critical sensory and motor elements. In the presence of repeatable and defined stimuli, this reflex is extremely well suited to analyze subtle changes in the circuitry and performance of retinal neurons. Increasing the cycles of these alternating gratings per degree, or gradually reducing the contrast of the visual stimuli, threshold levels can be determined at which the animal is no longer tracking the stimuli, and thereby visual function of the animal can be determined non-invasively. Integrating these assays into an array of outcome measures that determine multiple aspects of visual function is a central goal in vision research and can be realized, for example, by the combination of measuring optomotor reflex function with electroretinograms (ERGs) and optical coherence tomography (OCT) of the retina. These structure-function correlations in vivo are urgently needed to identify disease mechanisms as potential new targets for drug development. Such a combination of the experimental assessment of the optokinetic reflex (OKR) or optomotor response (OMR) with other measures of retinal structure and function is especially valuable for research on GON. The chronic progression of the disease is characterized by a gradual decrease in function accompanied by a concomitant increase in structural damage to the retina, therefore the assessment of subtle changes is key to determining the success of novel intervention strategies.

Around LTD hypothesis in motor learning

Long-term depression (LTD) at parallel fiber-Purkinje neuron synapses has been regarded as a primary cellular mechanism for motor learning. However, this hypothesis has been challenged. Demonstration of normal motor learning under LTD-suppressed conditions suggested that motor learning can occur without LTD. Synaptic plasticity mechanisms other than LTD have been found at various synapses in the cerebellum. Animals may achieve motor learning using several types of synaptic plasticity in the cerebellum including LTD.

Dynamic characteristics of otolith ocular response during counter rotation about dual yaw axes in mice

The central vestibular system plays an important role in higher neural functions such as self-motion perception and spatial orientation. Its ability to store head angular velocity is called velocity storage mechanism (VSM), which has been thoroughly investigated across a wide range of species. However, little is known about the mouse VSM, because the mouse lacks typical ocular responses such as optokinetic after nystagmus or a dominant time constant of vestibulo-ocular reflex for which the VSM is critical. Experiments were conducted to examine the otolith-driven eye movements related to the VSM and verify its characteristics in mice. We used a novel approach to generate a similar rotating vector as a traditional off-vertical axis rotation (OVAR) but with a larger resultant gravito-inertial force (>1g) by using counter rotation centrifugation. Similar to results previously described in other animals during OVAR, two components of eye movements were induced, i.e. a sinusoidal modulatory eye movement (modulation component) on which a unidirectional nystagmus (bias component) was superimposed. Each response is considered to derive from different mechanisms; modulations arise predominantly through linear vestibulo-ocular reflex, whereas for the bias, the VSM is responsible. Data indicate that the mouse also has a well-developed vestibular system through otoliths inputs, showing its highly conserved nature across mammalian species. On the other hand, to reach a plateau state of bias, a higher frequency rotation or a larger gravito-inertial force was considered to be necessary than other larger animals. Compared with modulation, the bias had a more variable profile, suggesting an inherent complexity of higher-order neural processes in the brain. Our data provide the basis for further study of the central vestibular system in mice, however, the underlying individual variability should be taken into consideration.

Gating of neural error signals during motor learning

Cerebellar climbing fiber activity encodes performance errors during many motor learning tasks, but the role of these error signals in learning has been controversial. We compared two motor learning paradigms that elicited equally robust putative error signals in the same climbing fibers: learned increases and decreases in the gain of the vestibulo-ocular reflex (VOR). During VOR-increase training, climbing fiber activity on one trial predicted changes in cerebellar output on the next trial, and optogenetic activation of climbing fibers to mimic their encoding of performance errors was sufficient to implant a motor memory. In contrast, during VOR-decrease training, there was no trial-by-trial correlation between climbing fiber activity and changes in cerebellar output, and climbing fiber activation did not induce VOR-decrease learning. Our data suggest that the ability of climbing fibers to induce plasticity can be dynamically gated in vivo, even under conditions where climbing fibers are robustly activated by performance errors.

DOI:http://dx.doi.org/10.7554/eLife.02076.001

The cerebellum (or ‘little brain’) is located underneath the cerebral hemispheres. Despite comprising around 10% of the brain’s volume, the cerebellum contains roughly half of the brain’s neurons. Many of the functions of the cerebellum are related to the control and fine-tuning of movement, and people whose cerebellum has been damaged have problems with balance and coordination, and with learning new motor skills.

One of the roles of the cerebellum is to control a reflex known as the vestibulo-ocular reflex, which enables us to keep our gaze fixed on an object as we turn our heads. The cerebellum relays information about head movements to the muscles that control the eyes, instructing the eyes to move in the opposite direction to the head. This keeps the image of the object we are looking at stable on the retina.

The vestibulo-ocular reflex is controlled by a circuit that includes Purkinje cells (which are the main output cells of the cerebellum) and climbing fibres (which originate in the brainstem). Any failure of the vestibulo-ocular reflex to fully compensate for head movements generates an error signal that activates the climbing fibres. These in turn modify the output of Purkinje cells, leading ultimately to adjustments in eye movements.

However, Kimpo et al. have now obtained evidence that Purkinje cells can modulate their response to the instructions they receive from climbing fibres. Monkeys sat in a rotating chair while a visual object they were trained to track with their eyes was moved to induce errors in the vestibulo-ocular reflex. When the object was moved so that a bigger reflexive eye movement was required to stabilize the image, the activation of the climbing fibres in response to the error led to a change in the response of the Purkinje cells, as expected. However, when a smaller reflexive eye movement was needed, the error-driven responses of the climbing fibres did not alter the responses of Purkinje cells. Similar results were obtained using pulses of light to artificially activate climbing fibres and thus simulate error signals.

The work of Kimpo et al. indicates that the cerebellum does not blindly follow the instructions it receives from the brainstem, but can instead modulate its responses to incoming information about performance errors. Further work is now required to identify factors that influence the responsiveness of the cerebellum: such information could ultimately be used to improve learning of motor skills and recovery from injury.

DOI:http://dx.doi.org/10.7554/eLife.02076.002

Regulation and functional roles of rebound potentiation at cerebellar stellate cell-Purkinje cell synapses

Purkinje cells receive both excitatory and inhibitory synaptic inputs and send sole output from the cerebellar cortex. Long-term depression (LTD), a type of synaptic plasticity, at excitatory parallel fiber-Purkinje cell synapses has been studied extensively as a primary cellular mechanism of motor learning. On the other hand, at inhibitory synapses on a Purkinje cell, postsynaptic depolarization induces long-lasting potentiation of GABAergic synaptic transmission. This synaptic plasticity is called rebound potentiation (RP), and its molecular regulatory mechanisms have been studied. The increase in intracellular Ca(2+) concentration caused by depolarization induces RP through enhancement of GABAA receptor (GABAAR) responsiveness. RP induction depends on binding of GABAAR with GABAAR associated protein (GABARAP) which is regulated by Ca(2+)/calmodulin-dependent kinase II (CaMKII). Whether RP is induced or not is determined by the balance between phosphorylation and de-phosphorylation activities regulated by intracellular Ca(2+) and by metabotropic GABA and glutamate receptors. Recent studies have revealed that the subunit composition of CaMKII has significant impact on RP induction. A Purkinje cell expresses both α- and β-CaMKII, and the latter has much higher affinity for Ca(2+)/calmodulin than the former. It was shown that when the relative amount of α- to β-CaMKII is large, RP induction is suppressed. The functional significance of RP has also been studied using transgenic mice in which a peptide inhibiting association of GABARAP and GABAAR is expressed selectively in Purkinje cells. The transgenic mice show abrogation of RP and subnormal adaptation of vestibulo-ocular reflex (VOR), a type of motor learning. Thus, RP is involved in a certain type of motor learning.

A simple machine vision-driven system for measuring optokinetic reflex in small animals

The optokinetic reflex (OKR) is useful to monitor the function of the visual and motor nervous systems. However, OKR measurement is not open to all because dedicated commercial equipment or detailed instructions for building in-house equipment is rarely offered. Here we describe the design of an easy-to-install/use yet reliable OKR measuring system including a computer program to visually locate the pupil and a mathematical procedure to estimate the pupil azimuth from the location data. The pupil locating program was created on a low-cost machine vision development platform, whose graphical user interface allows one to compose and operate the program without programming expertise. Our system located mouse pupils at a high success rate (~90 %), estimated their azimuth precisely (~94 %), and detected changes in OKR gain due to the pharmacological modulation of the cerebellar flocculi. The system would promote behavioral assessment in physiology, pharmacology, and genetics.

Glutamate-receptor-like molecule GluRδ2 involved in synapse formation at parallel fiber-Purkinje neuron synapses

Glutamate-receptor-like molecule δ2 (GluRδ2, GluD2) has been classified as an ionotropic glutamate receptor subunit. It is selectively expressed on the postsynaptic membrane at parallel fiber-Purkinje neuron synapses in the cerebellum. Mutant mice deficient in GluRδ2 show impaired synaptic plasticity, the decrease in the number of parallel fiber-Purkinje neuron synapses, multiple innervation of climbing fibers on a Purkinje neuron, and defects in motor control and learning. Thus, GluRδ2 plays crucial roles in the cerebellar function. Recent studies on GluRδ2 have shown that it has synaptogenic activity. GluRδ2 expressed in a non-neuronal cell induces presynaptic differentiation of granule neurons in a co-culture preparation. This synaptogenic activity depends on an extracellular N-terminal leucine/isoleucine/valine binding protein-like domain of GluRδ2. GluRδ2 plays critical roles in formation, maturation, and/or maintenance of granule neuron-Purkinje neuron synapses.

Diversity and complexity of roles of granule cells in the cerebellar cortex. Editorial

The cerebellar granule cell, the most numerous neurons in the brain, forms the main excitatory neuron of the cerebellar cortical circuitry. Granule cells are synaptically connected with both mossy fibers and Golgi cells inside specialized structures called glomeruli, and thereby, they are subject to both feed-forward and feed-back inhibition. Their unique architecture with about four dendrites and a single axon ascending in the cerebellar cortex to bifurcate into two parallel fibers making synapses with Purkinje neurons has attracted numerous scientists. Recent advances show that they are much more than just relays of mossy fibers. They perform diverse and complex transformations in the spatiotemporal domain. This special issue highlights novel avenues in our understanding of the roles of this key neuronal population of the cerebellar cortex, ranging from developmental up to physiological and pathological points of view.

Changes in Purkinje cell firing and gene expression precede behavioral pathology in a mouse model of SCA2

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominantly inherited disorder, which is caused by a pathological expansion of a polyglutamine (polyQ) tract in the coding region of the ATXN2 gene. Like other ataxias, SCA2 most overtly affects Purkinje cells (PCs) in the cerebellum. Using a transgenic mouse model expressing a full-length ATXN2^Q127-complementary DNA under control of the Pcp2 promoter (a PC-specific promoter), we examined the time course of behavioral, morphologic, biochemical and physiological changes with particular attention to PC firing in the cerebellar slice. Although motor performance began to deteriorate at 8 weeks of age, reductions in PC number were not seen until after 12 weeks. Decreases in the PC firing frequency first showed at 6 weeks and paralleled deterioration of motor performance with progression of disease. Transcription changes in several PC-specific genes such as Calb1 and Pcp2 mirrored the time course of changes in PC physiology with calbindin-28 K changes showing the first small, but significant decreases at 4 weeks. These results emphasize that in this model of SCA2, physiological and behavioral phenotypes precede morphological changes by several weeks and provide a rationale for future studies examining the effects of restoration of firing frequency on motor function and prevention of future loss of PCs.

Cerebellar LTD vs. motor learning-lessons learned from studying GluD2

Synaptic plasticity, such as long-term potentiation and long-term depression (LTD), is believed to underlie learning and memory processes in vivo. The cerebellum is an ideal brain region to obtain definitive proof for this hypothesis. The current belief is that the acquisition of motor learning is stored by LTD at the parallel fiber (PF)-Purkinje cell synapse in the cerebellar cortex. Recently, however, several lines of mutant mice that display normal motor learning in the absence of cerebellar LTD have been reported. A similar dichotomy between synaptic plasticity at the circuitry level and learning at the behavioral level has also been reported in the hippocampus. One possible explanation for this dichotomy is that compensatory pathways at the molecular and circuitry levels play an important role in mice that have been genetically modified for their entire lives. Mice that are genetically modified to be deficient in or to express mutant versions of the δ2 glutamate receptor (GluD2) serve as an interesting model due to the predominant expression of GluD2 at PF-Purkinje cell synapses. Furthermore, two major functions of GluD2-PF synapse formation and LTD induction-can be mechanistically dissociated so that the role of LTD in motor learning can be investigated in the absence of morphological abnormalities caused by altered synapse formation. Therefore, genetic manipulations of GluD2 will help to clarify the relationship between LTD and motor learning in the cerebellum.

Influence of parallel fiber-Purkinje cell synapse formation on postnatal development of climbing fiber-Purkinje cell synapses in the cerebellum

The climbing fiber (CF) to Purkinje cell (PC) synapse in the cerebellum provides an ideal model for the study of developmental rearrangements of neural circuits. At birth, each PC is innervated by multiple CFs. These surplus CFs are eliminated during postnatal development, and mono innervation is attained by postnatal day 20 (P20) in mice. Earlier studies on spontaneous mutant mice and animals with "hypogranular" cerebella indicate that regression of surplus CFs requires normal generation of granule cells and their axons, parallel fibers (PFs), and normal formation of PF-PC synapses. Our understanding of how PF-PC synapse formation affects development of CF-PC synapse has been greatly advanced by analyses of mutant mice deficient in glutamate receptor delta2 subunit (GluRdelta2), an orphan receptor expressed selectively in PCs. Deletion of GluRdelta2 results in impairment of PF-PC synapse formation, which leads to defects in development of CF-PC synapses. In this article, we review how impaired PF-PC synapse formation affects wiring of CFs to PCs based mostly on our data on GluRdelta2 knockout mice. We propose a new scheme that CF-PC synapses are shaped by the three consecutive events, namely functional differentiation of multiple CFs into one strong and a few weak inputs from P3 to P7, "early phase" of CF synapse elimination from P7 to around P11, and "late phase" of CF synapse elimination from around P12. Normal PF-PC synapse formation is required for the "late phase" of CF synapse elimination.

To gate or not to gate: are the delta subunits in the glutamate receptor family functional ion channels?

The two delta receptor subunits remain the most puzzling enigma within the ionotropic glutamate receptor family. Despite the recent elucidation of the ligand-binding domain structure of delta2, many fundamental questions with regard to the subunits' mechanism of function still remain unanswered. Of necessity, the majority of studies on delta receptors focused on the metabotropic function of delta2, since electrophysiological approaches to date are limited to the characterization of spontaneous currents through the delta2-lurcher mutant. Indeed, accumulated evidence primarily from delta2-deficient transgenic mice suggest that major physiological roles of delta2 are mediated via metabotropic signaling by the subunit's C terminus. Why then would the subunits retain a conserved ion channel domain if they do not form functional ion channels? Any progress with regard to ionotropic function of the two delta subunits has been hampered by their largely unknown pharmacology. Even now that a pharmacological profile for delta2 is being established on the basis of the ligand-binding domain structure, wild-type delta2 channels in heterologous expression systems stay closed in the presence of molecules that have been demonstrated to bind to the receptor's ligand-binding domain. In this paper, we review the current knowledge of delta subunits focusing on the disputed ionotropic function.

Asymmetric recovery in cerebellar-deficient mice following unilateral labyrinthectomy

The term "vestibular compensation" refers to the resolution of motor deficits resulting from a peripheral vestibular lesion. We investigated the role of the cerebellum in the compensation process by characterizing the vestibuloocular reflex (VOR) evoked by head rotations at frequencies and velocities similar to those in natural behaviors in wild-type (WT) versus cerebellar-deficient Lurcher (Lc/+) mice. We found that during exploratory activity, normal mice produce head rotations largely consisting of frequencies < or =4 Hz and velocities and accelerations as large as 400 degrees/s and 5,000 degrees/s2, respectively. Accordingly, the VOR was characterized using sinusoidal rotations (0.2-4 Hz) as well as transient impulses (approximately 400 degrees/s; approximately 2,000 degrees/s2). Before lesions, WT and Lc/+ mice produced similar VOR responses to sinusoidal rotation. Lc/+ mice, however, had significantly reduced gains for transient stimuli. After unilateral labyrinthectomy, VOR recovery followed a similar course for WT and Lc/+ groups during the first week: gain was reduced by 80% for ipsilesionally directed head rotations on day 1 and improved for both strains to values of approximately 0.4 by day 5. Moreover, responses evoked by contralesionally directed rotations returned to prelesion in both strains within this period. However, unlike WT, which showed improving responses to ipsilesionally directed rotations, recovery plateaued after first week for Lc/+ mice. Our results show that despite nearly normal recovery in the acute phase, long-term compensation is compromised in Lc/+. We conclude that cerebellar pathways are critical for long-term restoration of VOR during head rotation toward the lesioned side, while noncerebellar pathways are sufficient to restore proper gaze stabilization during contralesionally directed movements.

Enhancement of both long-term depression induction and optokinetic response adaptation in mice lacking delphilin

In the cerebellum, Delphilin is expressed selectively in Purkinje cells (PCs) and is localized exclusively at parallel fiber (PF) synapses, where it interacts with glutamate receptor (GluR) delta2 that is essential for long-term depression (LTD), motor learning and cerebellar wiring. Delphilin ablation exerted little effect on the synaptic localization of GluRdelta2. There were no detectable abnormalities in cerebellar histology, PC cytology and PC synapse formation in contrast to GluRdelta2 mutant mice. However, LTD induction was facilitated at PF-PC synapses in Delphilin mutant mice. Intracellular Ca(2+) required for the induction of LTD appeared to be reduced in the mutant mice, while Ca(2+) influx through voltage-gated Ca(2+) channels and metabotropic GluR1-mediated slow synaptic response were similar between wild-type and mutant mice. We further showed that the gain-increase adaptation of the optokinetic response (OKR) was enhanced in the mutant mice. These findings are compatible with the idea that LTD induction at PF-PC synapses is a crucial rate-limiting step in OKR gain-increase adaptation, a simple form of motor learning. As exemplified in this study, enhancing synaptic plasticity at a specific synaptic site of a neural network is a useful approach to understanding the roles of multiple plasticity mechanisms at various cerebellar synapses in motor control and learning.

Rotational responses of vestibular-nerve afferents innervating the semicircular canals in the C57BL/6 mouse

Extracellular recordings were made from vestibular-nerve afferents innervating the semicircular canals in anesthetized C57BL/6 mice ranging in age from 4-24 weeks. A normalized coefficient of variation was used to divide afferents into regular (CV*<0.1) and irregular (CV*>0.1) groups. There were three overall conclusions from this study. First, mouse afferents resemble those of other mammals in properties such as resting discharge rate and dependence of response dynamics on discharge regularity. Second, there are differences in mouse afferents relative to other mammals that are likely related to the smaller size of the semicircular canals. The rotational sensitivity of mouse afferents is approximately threefold lower than that reported for afferents in other mammals. One consequence of the lower sensitivity is that mouse afferents have a larger linear range for encoding head velocity. The long time constant of afferent discharge, which is a measure of low-frequency response dynamics, is shorter in mouse afferents than in other species. Third, juvenile mice (age 4-7 weeks) appear to lack a class of low-sensitivity, highly irregular afferents that are present in adult animals (age 10-24 weeks). By analogy to studies in the chinchilla, these irregular afferents with low sensitivities for lower rotational frequencies correspond to calyx-only afferents. These findings suggest that, although the calyx ending on to type I hair cells is morphologically complete in mice by the age of about 1 month, the physiological response properties in these juvenile animals are not equivalent to those in adults.

Activity of Vestibular Nuclei Neurons During Vestibular and Optokinetic Stimulation in the Alert Mouse

As a result of the availability of genetic mutant strains and development of noninvasive eye movements recording techniques, the mouse stands as a very interesting model for bridging the gap among behavioral responses, neuronal response dynamics studied in vivo, and cellular mechanisms investigated in vitro. Here we characterized the responses of individual neurons in the mouse vestibular nuclei during vestibular (horizontal whole body rotations) and full field visual stimulation. The majority of neurons (∼2/3) were sensitive to vestibular stimulation but not to eye movements. During the vestibular-ocular reflex (VOR), these neurons discharged in a manner comparable to the “vestibular only” (VO) neurons that have been previously described in primates. The remaining neurons [eye-movement-sensitive (ES) neurons] encoded both head-velocity and eye-position information during the VOR. When vestibular and visual stimulation were applied so that there was sensory conflict, the behavioral gain of the VOR was reduced. In turn, the modulation of sensitivity of VO neurons remained unaffected, whereas that of ES neurons was reduced. ES neurons were also modulated in response to full field visual stimulation that evoked the optokinetic reflex (OKR). Mouse VO neurons, however, unlike their primate counterpart, were not modulated during OKR. Taken together, our results show that the integration of visual and vestibular information in the mouse vestibular nucleus is limited to a subpopulation of neurons which likely supports gaze stabilization for both VOR and OKR.

Synaptic signalling in cerebellar plasticity

A major goal of learning and memory research is to correlate the function of molecules with the behaviour of organisms. The beautiful laminar structure of the cerebellar cortex lends itself to the study of synaptic plasticity, because its clearly defined patterns of neurons and their synapses form circuits that have been implicated in simple motor behaviour paradigms. The best understood in terms of molecular mechanism is the parallel fibre-Purkinje cell synapse, where presynaptic long-term potentiation and postsynaptic long-term depression and potentiation finely tune cerebellar output. Our understanding of these forms of plasticity has mostly come from the electrophysiological and behavioural analysis of knockout mutant mice, but more recently the knock-in of synaptic molecules with mutated phosphorylation sites and binding domains has provided more detailed insights into the signalling events. The present review details the major forms of plasticity in the cerebellar cortex, with particular attention to the membrane trafficking and intracellular signalling responsible. This overview of the current literature suggests it will not be long before the involvement of the cerebellum in certain motor behaviours is fully explained in molecular terms.

Increased occurrence of climbing fiber inputs to the cerebellar flocculus in a mutant mouse is correlated with the timing delay of optokinetic response

The cerebellum plays an essential role in motor control, and its dysfunction may delay the onset of action and disrupt smooth and efficient movement. A Purkinje neuron (PN), the sole output cell type in the cerebellar cortex, receives two distinct types of excitatory synaptic inputs, numerous weak inputs from granule neurons (GNs) and occasional strong inputs from a climbing fiber (CF). The role of each input and the significance of low firing rate of CF have been studied. Here we show that the increased occurrence of CF inputs altered the firing pattern of a PN, which was correlated with timing of a reflex. We used the mutant mice deficient in the glutamate receptor delta2 subunit, a molecule related to ionotropic glutamate receptor specifically expressed at GN-PN synapses. The mutant mouse shows more frequent CF inputs and longer timing delay in optokinetic response (OKR), reflex eye movement that follows slow motion of a large visual field. A PN shows two types of action potentials: complex spikes (CS) induced by CF inputs; and simple spikes. They changed respective firing rates during sinusoidal optokinetic stimulation, and the timing of each firing rate modulation was similar in wild-type and mutant mice. However, increased occurrence of CS in the mutant altered the total firing pattern of a PN in the flocculus, which was correlated with the timing delay of OKR. These results support the functional merit of low firing rate of CF in motor control.

Impaired motor learning in the vestibulo-ocular reflex in mice with multiple climbing fiber input to cerebellar Purkinje cells

A unique feature of the cerebellar architecture is that Purkinje cells in the cerebellar cortex each receive input from a single climbing fiber. In mice deficient in the gamma isoform of protein kinase C (PKCgamma-/- mice), this normal architecture is disrupted so that individual Purkinje cells receive input from multiple climbing fibers. These mice have no other known abnormalities in the cerebellar circuit. Here, we show that PKCgamma-/- mice are profoundly impaired in vestibulo-ocular reflex (VOR) motor learning. The PKCgamma-/- mice exhibited no adaptive increases or decreases in VOR gain at training frequencies of 2 or 0.5 Hz. This impairment was present across a broad range of peak retinal slip speeds during training. We compare the results for VOR motor learning with previous studies of the performance of PKCgamma-/- mice on other cerebellum-dependent learning tasks. Together, the results suggest that single climbing fiber innervation of Purkinje cells is critical for some, but not all, forms of cerebellum-dependent learning, and this may depend on the region of the cerebellum involved, the organization of the relevant neural circuits downstream of the cerebellar cortex, as well as the timing requirements of the learning task.

The Lurcher mouse: Fresh insights from an old mutant

The Lurcher mouse was first discovered in 1954 as a spontaneously occurring autosomal dominant mutation that caused the degeneration of virtually all cerebellar Purkinje cells and most olivary neurons and granule cells. More recent molecular studies revealed that Lurcher is a gain of function mutation in the delta2 glutamate receptor (GluRdelta2) that converts an alanine to threonine in the highly conserved third hydrophobic segment of GluRdelta2. The mutation converts the receptor into a constitutively leaky cation channel. The GluRdelta2 receptor is predominantly expressed in cerebellar Purkinje cells and in the heterozygous Lurcher mutant (+/Lc). Purkinje cells die due to the mutation in the GluRdelta2 receptor, while olivary neurons and granule cells degenerate due to the loss of their Purkinje cell targets. The purpose of the review is to provide highlights from 5 decades of research on the Lurcher mutant that have provided insights into the developmental mechanisms that regulate cell number during development, cerebellar pattern formation, cerebellar physiology, and the role of the cerebellum in CNS function.

Motor Deficits in Homozygous and Heterozygous P/Q-Type Calcium Channel Mutants

P/Q-type voltage-dependent Ca2+channels (VDCCs) are highly expressed in the cerebellum, and mutations of these channels are associated with disrupted motor function. Several allelic variants of the α1A pore-forming subunit of P/Q-type VDCCs have been described, and mice homozygous for these mutations exhibit gait ataxia, as do α1A knockout mice. Here we report that heterozygous α1A mutants also have a motor phenotype. Mice heterozygous for the leaner and α1A knockout mutations exhibit impaired motor learning in the vestibulo-ocular reflex (VOR), suggesting that subtle disruption of P/Q Ca2+currents is sufficient to disrupt motor function. Basal VOR and optokinetic reflex performance were normal in the heterozygotes but severely impaired in the leaner and α1A knockout homozygotes.

Sparse cerebellar innervation can morph the dynamics of a model oculomotor neural integrator

The oculomotor integrator is a brainstem neural network that converts velocity signals into the position commands necessary for eye-movement control. The cerebellum can independently adjust the amplitude of eye-movement commands and the temporal characteristics of neural integration, but the percentage of integrator neurons that receive cerebellar input is very small. Adaptive dynamic systems models, configured using the genetic algorithm, show how sparse cerebellar inputs could morph the dynamics of the oculomotor integrator and independently adjust its overall response amplitude and time course. Dynamic morphing involves an interplay of opposites, in which some model Purkinje cells exert positive feedback on the network, while others exert negative feedback. Positive feedback can be increased to prolong the integrator time course at virtually any level of negative feedback. The more these two influences oppose each other, the larger become the response amplitudes of the individual units and of the overall integrator network.

Ho15J: a new hotfoot allele in a hot spot in the gene encoding the delta2 glutamate receptor

Hotfoot, a recessive mouse mutation characterized by ataxia and jerky movements of the hindlimbs, is caused by various mutations in the gene (Grid2) encoding the delta2 glutamate receptor (GluRdelta2). So far, at least 20 alleles, arising either spontaneously or through the random insertion of transgenes, have been described. Interestingly, most hotfoot mutants have deletions of one or more exons coding for portions of the most amino-terminal domain of GluRdelta2. However, because live mice colonies are no longer available for most hotfoot mutants, the possibility that the loss of a part of an intron might affect the splicing of other exons or the general efficiency of transcription could not be ruled out. Here, we report that a newly identified hotfoot mutant, ho15J, was caused by an intragenic deletion of the Grid2 gene, which indeed resulted in a new type of 52-amino-acid deletion in the most amino-terminal domain of GluRdelta2. Like GluRdelta2 proteins in ho4J mutants, GluRdelta2 proteins in ho15J mice were retained in the soma of Purkinje cells, where they were degraded. Long-term depression, a form of synaptic plasticity underlying information storage in the cerebellum, was abrogated, and ho15J mice showed severe motor discoordination on rotarod tests. The agreement between the PCR results for genomic DNA and the RT-PCR results for the ho15J allele supports the view that PCR analyses of grid2 genomic DNA can predict alterations in mRNA and protein. In addition, the present findings underscore the importance of the most amino-terminal domain in GluRdelta2 signaling and cerebellar functions.

Time and tide in cerebellar memory formation

The notion that the olivocerebellar system is crucial for motor learning is well established. In recent years, it has become evident that there can be many forms of both synaptic and non-synaptic plasticity within this system and that each might have a different role in developing and maintaining motor learning across a wide range of tasks. There are several possible molecular and cellular mechanisms that could underlie adaptation of the vestibulo-ocular reflex and eyeblink conditioning. Although causal relationships between particular cellular processes and individual components of a learned behaviour have not been demonstrated unequivocally, an overall picture is emerging that the different types and sites of cellular plasticity relate importantly to the stage of learning and/or its temporal specifics.