A brainstem integrator for self-location memory and positional homeostasis in zebrafish.

To track and control self-location, animals integrate their movements through space. Representations of self-location are observed in the mammalian hippocampal formation, but it is unknown if positional representations exist in more ancient brain regions, how they arise from integrated self-motion, and by what pathways they control locomotion. Here, in a head-fixed, fictive-swimming, virtual-reality preparation, we exposed larval zebrafish to a variety of involuntary displacements. They tracked these displacements and, many seconds later, moved toward their earlier location through corrective swimming ("positional homeostasis"). Whole-brain functional imaging revealed a network in the medulla that stores a memory of location and induces an error signal in the inferior olive to drive future corrective swimming. Optogenetically manipulating medullary integrator cells evoked displacement-memory behavior. Ablating them, or downstream olivary neurons, abolished displacement corrections. These results reveal a multiregional hindbrain circuit in vertebrates that integrates self-motion and stores self-location to control locomotor behavior.

Structural and Functional Organization of Visual Responses in the Inferior Olive of Larval Zebrafish.

The olivo-cerebellar system plays an important role in vertebrate sensorimotor control. Here, we investigate sensory representations in the inferior olive (IO) of larval zebrafish and their spatial organization. Using single-cell labeling of genetically identified IO neurons, we find that they can be divided into at least two distinct groups based on their spatial location, dendritic morphology, and axonal projection patterns. In the same genetically targeted population, we recorded calcium activity in response to a set of visual stimuli using two-photon imaging. We found that most IO neurons showed direction-selective and binocular responses to visual stimuli and that the functional properties were spatially organized within the IO. Light-sheet functional imaging that allowed for simultaneous activity recordings at the soma and axonal level revealed tight coupling between functional properties, soma location, and axonal projection patterns of IO neurons. Taken together, our results suggest that anatomically defined classes of IO neurons correspond to distinct functional types, and that topographic connections between IO and cerebellum contribute to organization of the cerebellum into distinct functional zones.

Recessive PRDM13 mutations cause fatal perinatal brainstem dysfunction with cerebellar hypoplasia and disrupt Purkinje cell differentiation.

Pontocerebellar hypoplasias (PCHs) are congenital disorders characterized by hypoplasia or early atrophy of the cerebellum and brainstem, leading to a very limited motor and cognitive development. Although over 20 genes have been shown to be mutated in PCHs, a large proportion of affected individuals remains undiagnosed. We describe four families with children presenting with severe neonatal brainstem dysfunction and pronounced deficits in cognitive and motor development associated with four different bi-allelic mutations in PRDM13, including homozygous truncating variants in the most severely affected individuals. Brain MRI and fetopathological examination revealed a PCH-like phenotype, associated with major hypoplasia of inferior olive nuclei and dysplasia of the dentate nucleus. Notably, histopathological examinations highlighted a sparse and disorganized Purkinje cell layer in the cerebellum. PRDM13 encodes a transcriptional repressor known to be critical for neuronal subtypes specification in the mouse retina and spinal cord but had not been implicated, so far, in hindbrain development. snRNA-seq data mining and in situ hybridization in humans show that PRDM13 is expressed at early stages in the progenitors of the cerebellar ventricular zone, which gives rise to cerebellar GABAergic neurons, including Purkinje cells. We also show that loss of function of prdm13 in zebrafish leads to a reduction in Purkinje cells numbers and a complete absence of the inferior olive nuclei. Altogether our data identified bi-allelic mutations in PRDM13 as causing a olivopontocerebellar hypoplasia syndrome and suggest that early deregulations of the transcriptional control of neuronal fate specification could contribute to a significant number of cases.

Olov Oscarsson's Description of Afferent Pathways to the Cerebellum: Excellent Physiology, Base for Anatomy, and Road Toward Understanding Function.

Olov Oscarsson's review on the functional organization of spinocerebellar paths is a prime demonstration of the great skills and huge knowledge base of the electrophysiologists of his era working on communication systems in the brain. Oscarsson describes and characterizes in detail no less than ten different communication lines between the spinal cord and the cerebellum. As such, his work proved to be a highly fertile basis for ongoing physiological and anatomical research. However, even after 50 years of continuing cerebellar research, many questions are still open and even care must be taken that the differentiation in spinocerebellar paths, so carefully demonstrated by Oscarsson, is not lost in present-day research.

Electrophysiological alterations of the Purkinje cells and deep cerebellar neurons in a mouse model of Alzheimer disease (electrophysiology on cerebellum of AD mice).

Alzheimer's disease is histopathologically well defined by the presence of amyloid deposits and tau-related neurofibrillary tangles in crucial regions of the brain. Interest is growing in revealing and determining possible pathological markers also in the cerebellum as its involvement in cognitive functions is now well supported. Despite the central position of the Purkinje cell in the cerebellum, its electrophysiological behaviour in mouse models of Alzheimer's disease is scarce in the literature. Our first aim was here to focus on the electrophysiological behaviour of the cerebellum in awake mouse model of Alzheimer's disease (APPswe/PSEN1dE9) and the related performance on the water-maze test classically used in behavioural studies. We found prevalent signs of electrophysiological alterations in both Purkinje cells and deep cerebellar nuclei neurons which might explain the behavioural deficits reported during the water-maze test. The alterations of neurons firing were accompanied by a dual (~16 and ~228 Hz) local field potential's oscillation in the Purkinje cell layer of Alzheimer's disease mice which was concomitant to an important increase of both the simple and the complex spikes. In addition, ß-amyloid deposits were present in the molecular layer of the cerebellum. These results highlight the importance of the output firing modification of the AD cerebellum that may indirectly impact the activity of its subcortical and cortical targets.

Input and output organization of the mesodiencephalic junction for cerebro-cerebellar communication.

Most studies investigating the impact of the cerebral cortex (CC) onto the cerebellum highlight the role of the pons, which provides the mossy fibers to the cerebellum. However, cerebro-cerebellar communication may also be mediated by the nuclei of the mesodiencephalic junction (MDJ) that project to the inferior olive (IO), which in turn provides the climbing fibers to the molecular layer. Here, we uncover the precise topographic relations of the inputs and outputs of the MDJ using multiple, classical, and transneuronal tracing methods as well as analyses of mesoscale cortical injections from Allen Mouse Brain. We show that the caudal parts of the CC predominantly project to the principal olive via the rostral MDJ and that the rostral parts of the CC predominantly project to the rostral medial accessory olive via the caudal MDJ. Moreover, using triple viral tracing technology, we show that the cerebellar nuclei directly innervate the neurons in the MDJ that receive input from CC and project to the IO. By unraveling these topographic and prominent, mono- and disynaptic projections through the MDJ, this work establishes that cerebro-cerebellar communication is not only mediated by the pontine mossy fiber system, but also by the climbing fiber system.

Non-cell autonomous control of precerebellar neuron migration by Slit and Robo proteins.

During development, precerebellar neurons migrate tangentially from the dorsal hindbrain to the floor plate. Their axons cross it but their cell bodies stop their ventral migration upon reaching the midline. It has previously been shown that Slit chemorepellents and their receptors, Robo1 and Robo2, might control the migration of precerebellar neurons in a repulsive manner. Here, we have used a conditional knockout strategy in mice to test this hypothesis. We show that the targeted inactivation of the expression of Robo1 and Robo2 receptors in precerebellar neurons does not perturb their migration and that they still stop at the midline. The selective ablation of the expression of all three Slit proteins in floor-plate cells has no effect on pontine neurons and only induces the migration of a small subset of inferior olivary neurons across the floor plate. Likewise, we show that the expression of Slit proteins in the facial nucleus is dispensable for pontine neuron migration. Together, these results show that Robo1 and Robo2 receptors act non-cell autonomously in migrating precerebellar neurons and that floor-plate signals, other than Slit proteins, must exist to prevent midline crossing.

The zinc-finger transcription factor GLI3 is a regulator of precerebellar neuronal migration.

Hindbrain precerebellar neurons arise from progenitor pools at the dorsal edge of the embryonic hindbrain: the caudal rhombic lip. These neurons follow distinct migratory routes to establish nuclei that provide climbing or mossy fiber inputs to the cerebellum. Gli3, a zinc-finger transcription factor in the Sonic hedgehog signaling pathway, is an important regulator of dorsal brain development. We demonstrate that in Gli3-null mutant mice, disrupted neuronal migratory streams lead to a disorganization of precerebellar nuclei. Precerebellar progenitors are properly established in Gli3-null embryos and, using conditional gene inactivation, we provide evidence that Gli3 does not play a cell-autonomous role in migrating precerebellar neurons. Thus, GLI3 likely regulates the development of other hindbrain structures, such as non-precerebellar nuclei or cranial ganglia and their respective projections, which may in turn influence precerebellar migration. Although the organization of non-precerebellar hindbrain nuclei appears to be largely unaffected in absence of Gli3, trigeminal ganglia and their central descending tracts are disrupted. We show that rostrally migrating precerebellar neurons are normally in close contact with these tracts, but are detached in Gli3-null embryos.

Lessons learned from the syndrome of oculopalatal tremor.

The syndrome of oculopalatal tremor (OPT) featuring the olivo-cerebellar hypersychrony leads to disabling pendular nystagmus and palatal myoclonus. This rare disorder provides valuable information about the motor physiology and offers insights into the mechanistic underpinning of common movement disorders. This focused review summarizes the last decade of OPT research from our laboratory and addresses three critical questions: 1) How the disease of inferior olive affects the physiology of motor learning? We discovered that our brain's ability to compensate for the impaired motor command and implement errors to correct future movements could be affected if the cerebellum is occupied in receiving and transmitting the meaningless signal. A complete failure of OPT patients to adapt to change in rapid eye movements (saccades) provided proof of this principle. 2) Whether maladaptive olivo-cerebellar circuit offers insight into the mechanistic underpinning of the common movement disorder, dystonia, characterized by abnormal twisting and turning of the body part. We discovered that the subgroup of patients who had OPT also had dystonia affecting the neck, trunk, limbs, and face. We also found that the subjects who had tremor predominant neck dystonia (without OPT) also had impaired motor learning on a long and short timescale, just like those with OPT. Altogether, our studies focused on dystonia suggested the evidence for the maladaptive olive-cerebellar system. 3) We discovered that the OPT subjects had difficulty in perceiving the direction of their linear forward motion, i.e., heading, suggesting that olivo-cerebellar hypersynchrony also affects perception.

Gabapentin and memantine increases randomness of oscillatory waveform in ocular palatal tremor.

Syndrome of oculopalatal tremor (OPT) causes pendular nystagmus of the eyes and its disabling consequence on the visual system. Classic pharmacotherapeutic studies revealed reduction in the eye velocity of the oscillatory waveforms. Subjective improvement in vision, however, remains out of proportionately low. Elegant models depicting quasi-sinusoidal coarse oscillations of the eyes highlighted two distinct oscillators; one at the inferior olive causing primary 2 Hz oscillations, while the second, independent oscillator, at the cerebellum adding the randomness to the waveform. Here we examined whether pharmacotherapy affects the randomness of the oscillatory waveform. Horizontal, vertical, and torsional angular eye positions were measured independently from both eyes as six subjects with OPT directed gaze toward a straight-ahead target. The measurements were performed before administration of alpha-2-delta calcium channel blocker (gabapentin) or NMDA receptor antagonist (memantine) and after the subjects were treated with each of these drugs for at least 8 days. Amplitude and velocity of eye oscillations were reduced by gabapentin and memantine, but there was an increase in the waveform randomness. We found that the increase in randomness was proportionate to the amount of reduction in the waveform velocity or amplitude. Hierarchical clustering revealed distinct patterns of oscillatory waveforms, with each subject belonging to a specific cluster group. The pharmacotherapy changed the waveform clustering pattern of the waveform in each subject. We conclude that in addition to incomplete resolution of the oscillation intensity, increased randomness could be one of the reasons why there is not enough clinical difference in the patients' visual quality.

Visual Perception of Heading in the Syndrome of Oculopalatal Tremor.

Perception of our linear motion, heading, relies on convergence from multiple sensory systems utilizing visual and vestibular signals. Multisensory convergence takes place in the visuo-vestibular areas of the cerebral cortex and posterior cerebellar vermis. Latter closely connected with the inferior olive may malfunction in disorders of olivo-cerebellar hypersynchrony, such as the syndrome of oculopalatal tremor (OPT). We had recently shown an impairment in vestibular heading perception in the subjects with OPT. Here we asked whether the hypersynchrony in the inferior-olive cerebellar circuit also affects the visual perception of heading, and the impairment is coupled with the deficits in vestibular heading perception. Three subjects with OPT and 11 healthy controls performed a two-alternative forced-choice task in two separate experiments; one when they were moved en bloc in a straight-ahead forward direction or at multiple heading angles to the right or the left; and second when under virtual reality goggle they experienced the movement of star cloud leading to the percept of heading straight, left or to the right at the heading angles similar to those utilized in the vestibular task. The resultant psychometric function curves, derived from the two-alternative-forced-choice task, revealed abnormal threshold to perceive heading direction, abnormal sensitivity to the change in heading direction compared to straight ahead, and a bias towards one side. Although the impairment was present in both visual and vestibular heading perception, the deficits were not coupled.

Increased Purkinje Cell Complex Spike and Deep Cerebellar Nucleus Synchrony as a Potential Basis for Syndromic Essential Tremor. A Review and Synthesis of the Literature.

We review advances in understanding Purkinje cell (PC) complex spike (CS) physiology that suggest increased CS synchrony underlies syndromic essential tremor (ET). We searched PubMed for papers describing factors that affect CS synchrony or cerebellar circuits potentially related to tremor. Inferior olivary (IO) neurons are electrically coupled, with the degree of coupling controlled by excitatory and GABAergic inputs. Clusters of coupled IO neurons synchronize CSs within parasagittal bands via climbing fibers (Cfs). When motor cortex is stimulated in rats at varying frequencies, whisker movement occurs at ~10 Hz, correlated with synchronous CSs, indicating that the IO/CS oscillatory rhythm gates movement frequency. Intra-IO injection of the GABAA receptor antagonist picrotoxin increases CS synchrony, increases whisker movement amplitude, and induces tremor. Harmaline and 5-HT2a receptor activation also increase IO coupling and CS synchrony and induce tremor. The hotfoot17 mouse displays features found in ET brains, including cerebellar GluRδ2 deficiency and abnormal PC Cf innervation, with IO- and PC-dependent cerebellar oscillations and tremor likely due to enhanced CS synchrony. Heightened coupling within the IO oscillator leads, through its dynamic control of CS synchrony, to increased movement amplitude and, when sufficiently intense, action tremor. Increased CS synchrony secondary to aberrant Cf innervation of multiple PCs likely also underlies hotfoot17 tremor. Deep cerebellar nucleus (DCN) hypersynchrony may occur secondary to increased CS synchrony but might also occur from PC axonal terminal sprouting during partial PC loss. Through these combined mechanisms, increased CS/DCN synchrony may plausibly underlie syndromic ET.

Does Inferior-Olive Hypersynchrony Affect Vestibular Heading Perception?

Multisensory integration is critical for resolving ambiguities in isolated sensory systems assuring accurate perception of one's own linear motion, i.e., heading. The vestibular signal, a critical source of information for heading perception, is transformed in appropriate coordinates suitable for multisensory integration-such transformation takes place under cerebellar supervision. Deficiency in cerebellar function due to Purkinje cell loss results in inaccurate multisensory integration and impaired heading perception. Here, we predict that a classic movement disorder, the syndrome of oculopalatal tremor (OPT), also presents with inaccurate heading direction perception. The characteristic feature of oculopalatal tremor is pseudohypertrophic inferior olive that constantly sends spontaneous, hypersynchronous, abnormal, and meaningless signals to the cerebellum. Such malicious olive signal can impair heading perception. We examined vestibular heading perception in 6 individuals with OPT and 9 age-matched healthy controls (HC). We used a two-alternative forced choice task performed during passive en bloc translation. Compared with age-matched HC, OPT group had significantly higher heading direction perception threshold indicating a less sensitive vestibular system to variations in heading direction. Using computational simulations, we show that the addition of the abnormal noise into the cerebellar system results in decreased spatiotemporal tuning behavior of the cerebellar output. Such impairment in spatiotemporal tuning causes reduced ability to perceive heading direction. Hyperactivity in the inferior-olive cerebellar pathway impairs the heading direction perception. We suggest that this impairment stems from abnormal noise into the cerebellum due to hypersynchronized inferior olive.

Hyperventilation Increases the Randomness of Ocular Palatal Tremor Waveforms.

Hyperventilation changes the extracellular pH modulating many central pathologies, such as tremor. The questions that remain unanswered are the following: (1) Hyperventilation modulates which aspects of the oscillations? (2) Whether the effects of hyperventilation are instantaneous and the recovery is rapid and complete? Here we study the effects of hyperventilation on eye oscillations in the syndrome of oculopalatal tremor (OPT), a disease model affecting the inferior olive and cerebellar system. These regions are commonly involved in the pathogenesis of many movement disorders. The focus on the ocular motor system also allows access to the well-known physiology and precise measurement techniques. We found that hyperventilation causes modest but insignificant changes in the intensity of oscillation displacement (i.e., how large the eye excursions are) and velocity (i.e., how fast do the eyes move during oscillations). We found the robust increase in the randomness of the oscillatory waveform during hyperventilation and it instantaneously reverts to the baseline after hyperventilation. The subsequent analysis classified the oscillations according to their waveform shape and randomness into different clusters. The hyperventilation substantially changed the cluster type in 60% of the subjects, but it reverted to the pre-hyperventilation cluster at the conclusion of the hyperventilation. In summary, hyperventilation instantaneously affects the randomness of the oscillatory waveforms but there are less substantial effects on the intensity. The deficits reverse immediately at the end of the hyperventilation.

Pendular Oscillation and Ocular Bobbing After Pontine Hemorrhage.

The pathophysiology of acute, vertical spontaneous eye movements following pontine hemorrhage is not well understood. Here, we present and discuss the video-oculography findings of a patient with acute pontine hemorrhage who developed vertical pendular oscillation and ocular bobbing while comatose. The amplitudes, peak velocities, frequency distribution, and phase planes (velocity versus position) of the eye movements were analyzed. The vertical pendular oscillation was rhythmic with a peak frequency of 1.7 Hz, but amplitudes (mean 1.9°, range 0.2-8.2°) and peak velocities (mean 20.6°/s; range 5.9-60.6°/sec) fluctuated. Overall, their peak velocities were asymmetric, faster with downward than upward. Higher peak velocities were seen with larger amplitudes (downward phase r = 0.95, p < 0.001; upward phase r = 0.91, p < 0.001) and with movements beginning at eye positions lower in the orbit (downward phase r = - 0.64, p < 0.001; upward phase r = - 0.86, p < 0.001). Interspersed were typical ocular bobbing waveforms with a fast (peak velocity 128.8°/s), large-amplitude (17.5°) downward movement, sometimes followed by a flat interphase interval (0.5 s) when the eye was nearly stationary, and then a slow return to mid-position with a decaying velocity waveform. To account for the presence and co-existence of pendular oscillations and bobbing, we present and discuss three hypothetical models, not necessarily mutually exclusive: (1) oscillations originating in the inferior olives due to disruption of the central tegmental tract(s); (2) unstable neural integrator function due to pontine cell group damage involving neurons involved in gaze-holding; (3) low-frequency saccadic intrusions following omnipause neuron damage.

Graded error signals in eyeblink conditioning.

Minimizing errors is an important aspect of learning. However, it is not enough merely to record if an error occurred. For efficient learning, information about the magnitude of errors is critical. Did my tennis swing completely miss the target or did I hit the ball, but not quite in the sweet spot? How can neurons - which have traditionally been thought of as binary units - signal the magnitude of an error? Here I review evidence that eyeblink conditioning - a basic form of motor learning - depends on graded signals from the inferior olive which guides plasticity in the cerebellum and ultimately tunes behavior. Specifically, evidence suggests that: (1) Error signals are conveyed to the cerebellum via the inferior olive; (2) Signals from the inferior olive are graded; (3) The strength of the olivary signal affects learning; (4) Cerebellar feedback influences the strength of the olivary signal. I end the review by exploring how graded error signals might explain some behavioral learning phenomena.

A meta-analysis of case studies and clinical characteristics of hypertrophic olivary degeneration secondary to brainstem infarction.

Transsynaptic degeneration in the cerebellum and brainstem may give rise to a rare neurological condition with various clinical manifestations, namely hypertrophic olivary degeneration. The classical manifestations of hypertrophic olivary degeneration comprise myoclonus, palatal tremor, ataxia, and ocular symptoms. Any lesions interrupting the dentate-rubro-olivary pathway, referred to as the anatomic Guillain-Mollaret triangle, contribute to the broad aetiologies of hypertrophic olivary degeneration. The clinical diagnosis depends primarily on the associated symptoms and the characteristic magnetic resonance imaging findings. Concerning treatment and prognosis, there are no widely accepted guidelines. Here, we identified 11 cases of hypertrophic olivary degeneration secondary to brainstem infarction from 1964 to the present. Combined with two of our cases, the clinical and imaging findings of 13 patients with hypertrophic olivary degeneration secondary to brainstem infarction were studied. A meta-analysis of case studies gives the correlation coefficient between infraction location and time to develop hypertrophic olivary degeneration as 0.217 (P = 0.393, P > 0.05). At the significance level of P < 0.05, there was no significant correlation between information location and time to develop hyperophic olivary degeneration. The χ2 between infraction location and magnetic resonance imaging findings of hypertrophic olivary degeneration was 8.750 (P = 0.364, P > 0.05). At the significance level of P < 0.05, there was no significant correlation between infraction location and magnetic resonance imaging findings of hypertrophic olivary degeneration. Conclusion based on the analysis of available data suggests that when newly developed or progressive worsening motor symptoms are presented in patients with previous brainstem infarction, a diagnosis of hypertrophic olivary degeneration should be investigated.

Sox14 Is Required for a Specific Subset of Cerebello-Olivary Projections.

We identify Sox14 as an exclusive marker of inhibitory projection neurons in the lateral and interposed, but not the medial, cerebellar nuclei. Sox14+ neurons make up ∼80% of Gad1+ neurons in these nuclei and are indistinguishable by soma size from other inhibitory neurons. All Sox14+ neurons of the lateral and interposed cerebellar nuclei are generated at approximately E10/10.5 and extend long-range, predominantly contralateral projections to the inferior olive. A small Sox14+ population in the adjacent vestibular nucleus "Y" sends an ipsilateral projection to the oculomotor nucleus. Cerebellar Sox14+ and glutamatergic projection neurons assemble in non-overlapping populations at the nuclear transition zone, and their integration into a coherent nucleus depends on Sox14 function. Targeted ablation of Sox14+ cells by conditional viral expression of diphtheria toxin leads to significantly impaired motor learning. Contrary to expectations, associative learning is unaffected by unilateral Sox14+ neuron elimination in the interposed and lateral nuclei.SIGNIFICANCE STATEMENT The cerebellar nuclei are central to cerebellar function, yet how they modulate and process cerebellar inputs and outputs is still primarily unknown. Our study gives a direct insight into how nucleo-olivary projection neurons are generated, their projections, and their function in an intact behaving mouse. These neurons play a critical conceptual role in all models of cerebellar function, and this study represents the first specific analysis of their molecular identity and function and offers a powerful model for future investigation of cerebellar function in motor control and learning.

Neurons of the inferior olive respond to broad classes of sensory input while subject to homeostatic control.

KEY POINTS: Purkinje cells in the cerebellum integrate input from sensory organs with that from premotor centres. Purkinje cells use a variety of sensory inputs relaying information from the environment to modify motor control. Here we investigated to what extent the climbing fibre inputs to Purkinje cells signal mono- or multi-sensory information, and to what extent this signalling is subject to recent history of activity. We show that individual climbing fibres convey multiple types of sensory information, together providing a rich mosaic projection pattern of sensory signals across the cerebellar cortex. Moreover, firing probability of climbing fibres following sensory stimulation depends strongly on the recent history of activity, showing a tendency to homeostatic dampening. ABSTRACT: Cerebellar Purkinje cells integrate sensory information with motor efference copies to adapt movements to behavioural and environmental requirements. They produce complex spikes that are triggered by the activity of climbing fibres originating in neurons of the inferior olive. These complex spikes can shape the onset, amplitude and direction of movements and the adaptation of such movements to sensory feedback. Clusters of nearby inferior olive neurons project to parasagittally aligned stripes of Purkinje cells, referred to as 'microzones'. It is currently unclear to what extent individual Purkinje cells within a single microzone integrate climbing fibre inputs from multiple sources of different sensory origins, and to what extent sensory-evoked climbing fibre responses depend on the strength and recent history of activation. Here we imaged complex spike responses in cerebellar lobule crus 1 to various types of sensory stimulation in awake mice. We find that different sensory modalities and receptive fields have a mild, but consistent, tendency to converge on individual Purkinje cells, with climbing fibres showing some degree of input-specificity. Purkinje cells encoding the same stimulus show increased events with coherent complex spike firing and tend to lie close together. Moreover, whereas complex spike firing is only mildly affected by variations in stimulus strength, it depends strongly on the recent history of climbing fibre activity. Our data point towards a mechanism in the olivo-cerebellar system that regulates complex spike firing during mono- or multi-sensory stimulation around a relatively low set-point, highlighting an integrative coding scheme of complex spike firing under homeostatic control.

The anatomical pathway from the mesodiencephalic junction to the inferior olive relays perioral sensory signals to the cerebellum in the mouse.

KEY POINTS: Perioral tactile signals are transmitted via the infraorbital nerve (ION) to trigeminal nuclei. Each cerebellar Purkinje cell (PC) receives this signal as complex spikes (CSs) via a climbing fibre (CF) emerging from the inferior olive (IO). The anatomical pathway from trigeminal nuclei to the IO is not clearly identified. In the present study, we examined candidate anatomical pathways for perioral sensory signalling by analysing CSs recorded from PCs in male mice by single unit recording. CS generation by ION stimulation was inhibited by injection of a GABAA receptor agonist, muscimol, into the contralateral mesodiencephalic junction, which is referred to as the area parafascicularis prerubralis (PfPr). The number of CSs evoked by mechanical whisker stimulation was also decreased by contralateral PfPr inhibition. These results suggest the existence of a sensory signalling pathway to the IO via the PfPr in mice. ABSTRACT: Perioral tactile signals are transmitted via the infraorbital nerve (ION) to trigeminal nuclei. Each cerebellar Purkinje cell receives this signal as complex spikes (CSs) via a climbing fibre emerging from the inferior olive (IO). However, the anatomical pathway from the trigeminal nuclei to the IO is not clearly identified. In the present study, we recorded CSs from Purkinje cells in male mice by single unit recording, and examined the signal transduction pathway. CSs were evoked by electrical stimulation of the ipsilateral or contralateral ION with a latency of 20-70 ms. CS generation by ipsilateral ION stimulation was inhibited by injection of a GABAA receptor agonist, muscimol, into the contralateral mesodiencephalic junction, ranging from around the fasciculus retroflexus to the interstitial nucleus of Cajal, which is referred to as the area parafascicularis prerubralis (PfPr). CSs evoked by contralateral ION stimulation were also suppressed by muscimol injection into the PfPr, although the effective area was more restricted. Furthermore, CSs evoked by mechanical stimulation around the whisker region were suppressed by PfPr inhibition. We also found that the primary motor cortex plays a role to suppress this signalling pathway. These results indicate the existence of an anatomical pathway for conducting perioral sensory signals to the IO via the PfPr.