Inferior Olivary Neurons Research Articles

Firing dynamics of the inferior olive neurons: a simulation study

Firing dynamics of the inferior olive neurons: a simulation study

A model of the olivo-cerebellar system as a temporal pattern generator

The olivo-cerebellar system has been implicated in temporal coordination of task components. Here, we propose a novel model that enables the olivo-cerebellar system to function as a generator of temporal patterns. These patterns could be used for timing of motor, sensory and cognitive tasks. The proposed mechanism for the generation of these patterns is based on subthreshold oscillations in a network of inferior olivary neurons and their control by the cerebellar cortex and nuclei. Our model, which integrates a large body of anatomical and physiological observations, lends itself to simple, testable predictions and provides a new conceptual framework for olivo-cerebellar research.

Synchronization of Animal-Inspired Multiple High-Lift Fins in an Underwater Vehicle Using Olivo–Cerebellar Dynamics

The development of neuroscience-based control methodologies and their integration with the high-lift unsteady hydrodynamics of control surfaces inspired by swimming and flying animals are the subjects of this paper. A biology-inspired rigid autonomous undersea vehicle called the biorobotic autonomous undersea vehicle (BAUV) has been developed at the Naval Undersea Warfare Center (NUWC), Newport, RI. The BAUV is equipped with six simultaneously rolling and pitching fins for generating large unsteady control forces for performing agile maneuvers. First, as an exploratory example, we introduce the van der Pol oscillator as an oscillatory controller for the BAUV and we describe experiments performed to examine the fin forces (thrust and lift) and electric power requirement, and to demonstrate the effectiveness of the oscillator's limit cycle property for disturbance rejection effectiveness. We then describe a BAUV control system that includes six inferior-olive (IO) neuron models for control of the pitch and roll motion of the six foils. These IO neurons exhibit limit cycle oscillation (LCO). For control of the BAUV, these IO neurons must oscillate in synchronism with specific relative phases. We present here four feedback linearizing control systems of varying complexity for control of the relative phases of the IO neurons. It is shown that each of the IO control systems accomplishes asymptotic regulation of the phases and thus enables the foils to produce the required control forces. The first controller has a global synchronization property, but the remaining controllers accomplish local synchronization. We present simulation results for tracking piecewise, time-varying phase angle commands as well as experimental results for control of the BAUV by IO neurons. The results show that with appropriate phasing of the fins, an optimal graceful gait of the BAUV is achieved where no untoward force or moment is present. An analog hardware version of the local controller with a cluster of six IO neurons has also been built, which allows five of the signals to rapidly synchronize to the reference, with or without prescribed phase shift, much like in the simulations. The designed controllers can be used in any platform or multivariate BAUV-like system requiring fast, accurate phase control. Laboratory test results for the phase synchronization of two servomotors (roll and pitch) using the designed analog hardware controller are also shown.

Molecular Mechanisms Controlling Midline Crossing by Precerebellar Neurons

Precerebellar neurons of the inferior olive (IO) and lateral reticular nucleus (LRN) migrate tangentially from the rhombic lip toward the floor plate following parallel pathways. This process is thought to involve netrin-1 attraction. However, whereas the cell bodies of LRN neurons cross the midline, IO neurons are unable to do so. In many systems and species, axon guidance and cell migration at the midline are controlled by Slits and their receptor Robos. We showed previously that precerebellar axons and neurons do not cross the midline in the absence of the Robo3 receptor. To determine whether this signaling by Slits and the two other Robo receptors, Robo1 and Robo2, also regulates precerebellar neuron behavior at the floor plate, we studied the phenotype of Slit1/2 and Robo1/2/3 compound mutants. Our results showed that many IO neurons can cross the midline in absence of Slit1/2 or Robo1/2, supporting a role for midline repellents in guiding precerebellar neurons. We also show that these molecules control the development of the lamellation of the inferior olivary complex. Last, the analysis of Robo1/2/3 triple mutants suggests that Robo3 inhibits Robo1/2 repulsion in precrossing LRN axons but not in IO axons in which it has a dominant and distinct function.

Serotonin Evokes Endocannabinoid Release and Retrogradely Suppresses Excitatory Synapses

5-HT(2)-type serotonin receptors (5-HT(2)Rs) are widely expressed throughout the brain and mediate many of the modulatory effects of serotonin. It has been thought that postsynaptic 5-HT(2)Rs act primarily by depolarizing neurons and thereby increasing their excitability. However, it is also known that 5-HT(2)Rs are coupled to G(q/11)-type G-proteins and that some other types of G(q/11)-coupled receptors can regulate synapses by evoking endocannabinoid release and activating presynaptic cannabinoid-type 1 receptors (CB(1)Rs). Here, we examine whether activation of 5-HT(2)Rs can regulate synapses through such a mechanism by studying excitatory synapses onto cells in the inferior olive (IO). These cells express 5-HT(2)Rs on their soma and dendrites, and the IO receives extensive serotonergic input. We find that the excitatory synaptic inputs onto IO cells are strongly suppressed by serotonin receptor agonists as well as release of endogenous serotonin. Both 5-HT(2)Rs and 5-HT(1B)Rs contribute to this modulation by decreasing the probability of glutamate release from presynaptic boutons. The suppression by 5-HT(2)Rs is of particular interest because it is prevented by CB(1)R antagonists, and 5-HT(2)Rs are thought to be located only postsynaptically on IO cells. Our results indicate that serotonin activates 5-HT(2)Rs on IO neurons, thereby releasing endocannabinoids that act retrogradely to suppress glutamate release by activating presynaptic CB(1)Rs. These findings establish a link between serotonin signaling and endocannabinoid signaling. Based on the extensive distribution of 5-HT(2)Rs and CB(1)Rs, it seems likely that this mechanism could mediate many of the actions of 5-HT(2)Rs throughout the brain.

Altered olivocerebellar activity patterns in the connexin36 knockout mouse.

The inferior olive (IO) has among the highest densities of neuronal gap junctions in the nervous system. These gap junctions are proposed to be the underlying mechanism for generating synchronous Purkinje cell complex spike (CS) activity. Gap junctions between neurons are formed mostly by connexin36 proteins. Thus, the connexin36 knockout (Cx36KO) mouse provides an opportunity to test whether gap junction coupling between IO neurons is the basis of CS synchrony. Multiple electrode recordings of crus 2 CSs were obtained from wildtype (Wt) and Cx36KO mice. Wts showed statistically significant levels of CS synchrony, with the same spatial distribution as has been reported for other species: high CS synchrony levels occurred mostly among Purkinje cells within the same parasagittally-oriented cortical strip. In contrast, in Cx36KOs, synchrony was at chance levels and had no preferential spatial orientation, supporting the gap junction hypothesis. CS firing rates for Cx36KOs were significantly lower than for Wts, suggesting that electrical coupling is an important determinant of IO excitability. Rhythmic CS activity was present in both Wt and Cx36KOs, suggesting that individual IO cells can act as intrinsic oscillators. In addition, the climbing fiber reflex was absent in the Cx36KOs, validating its use as a tool for assessing electrical coupling of IO neurons. Zebrin II staining and anterograde tracing showed that cerebellar cortical organization and the topography of the olivocerebellar projection are normal in the Cx36KO. Thus, the differences in CS activity between Wts and Cx36KOs likely reflect the loss of electrical coupling of IO cells.

Rhythmic Episodes of Subthreshold Membrane Potential Oscillations in the Rat Inferior Olive NucleiIn Vivo

In vitro studies of inferior olive neurons demonstrate that they are intrinsically active, generating periodic spatiotemporal patterns. These self-generated patterns of activity extend the role of olivary neurons beyond that of a deliverer of teaching or error signals. However, autorhythmicity or patterned activity of complex spikes in the cerebellar cortex was observed in only a few studies. This discrepancy between the self-generated rhythmicity in the inferior olive observed in vitro and the sporadic reports on rhythmicity of complex spikes can be reconciled by recording intracellularly from inferior olive neurons in situ. To this end, we recorded intracellularly from olivary neurons of anesthetized rats. We demonstrate that, in vivo, olivary neurons show both slow and fast rhythmic processes. The slow process (0.2-2 Hz) is expressed as rhythmic transitions from quiescent periods to periods of fast rhythm, manifested as subthreshold oscillations of 6-12 Hz. Spikes, if they occur, are locked to the depolarized phase of these subthreshold oscillations and, therefore, hold and transfer rhythmic information. The transient nature of these oscillatory epochs accounts for the difficulties to uncover them by prolonged recordings of complex spikes activity in the cerebellar cortex.

Functional Recovery in a Friedreich's Ataxia Mouse Model by Frataxin Gene Transfer Using an HSV-1 Amplicon Vector

Functional Recovery in a Friedreich's Ataxia Mouse Model by Frataxin Gene Transfer Using an HSV-1 Amplicon Vector

Rodent models of tremor

This review focuses on rodent models of tremor, particularly those induced by pharmacological agents. Harmaline is one of the most frequently used tremor-generating drugs and harmaline-induced tremor is regarded as a model of essential tremor. Harmaline acts on inferior olive neurons, causing enhanced neuronal synchrony and rhythmicity in the olivocerebellar system. In addition, it selectively induces cerebellar Purkinje cell death, speculatively because of excessive glutamate release from nerve terminals of the olivocerebellar system onto Purkinje cells. Systemic administration of cholinomimetics can also produce generalized tremor, and muscarinic receptors on striatal neurons are thought to be the best candidate for the tremor-generating mechanism. On the other hand, dopaminergic neurotoxins, which are used in models of parkinsonism, have yet to be used for experimental analysis of tremor, because tremors induced by dopamine depletion in rodents are less remarkable than those induced by harmaline or cholinomimetics. Recently developed gamma-aminobutyric acid (GABA)(A) receptor alpha-1 subunit knockout mice exhibit postural and kinetic tremors, and clearly reproduce the features of essential tremors. Although from a phenomenological point of view, rodent models of tremor cannot entirely mimic human tremor disorders, they have useful advantages in the analysis of pathophysiological mechanisms underlying tremor. Development of convenient and reproducible methods for evaluating rodent tremor is therefore recommended.

Somatomotor and oculomotor inferior olivary neurons have distinct electrophysiological phenotypes.

The electrophysiological properties of rat inferior olive (IO) neurons in the dorsal cap of Kooy (DCK) and the adjacent ventrolateral outgrowth (VLO) were compared with those of IO neurons in the principal olive (PO). Whereas DCK/VLO neurons are involved in eye movement control via their climbing fiber projection to the cerebellar flocculus, PO neurons control limb and digit movements via their climbing fiber projection to the lateral cerebellar hemisphere. In vitro patch recordings from DCK/VLO neurons revealed that low threshold calcium currents, Ih currents, and subthreshold oscillations are lacking in this subset of IO neurons. The recordings of activity in DCK neurons obtained by using voltage-sensitive dye imaging showed that activity is not limited to a single neuron, but rather that clusters of DCK neurons can be active in unison. These electrophysiological results show that the DCK/VLO neurons have unique properties that set them apart from the neurons in the PO nucleus. This finding indicates that motor control, from the perspective of the olivocerebellar system, is fundamentally different for the oculomotor and the somatomotor systems.

Clinicopathologic investigation of a family with expanded SCA8 CTA/CTG repeats

We investigated a family manifesting progressive ataxia, with expanded SCA8 CTA/CTG repeats. Neuropathologically, degeneration of Purkinje, inferior olivary, and nigral neurons and periaqueductal gliosis were evident. The sites of Purkinje cell loss were occupied by fibrillary accumulations. The remaining Purkinje cells showed somatic sprouts, and intracytoplasmic 1C2-positive granular structures were recognizable. This characteristic distribution of neurodegeneration and Purkinje cell cytopathology were distinct from those of other hereditary spinocerebellar ataxias previously reported.

On the variety of cell death pathways in the Lurcher mutant mouse

Apoptosis as well as autophagy have been implicated in the death of cerebellar Purkinje cells (PCs) in the Lurcher (Lc/+) mutant mouse and at least two different apoptotic pathways participate in the transsynaptic death of granule cells (GC) and inferior olivary (IO) neurones. The relative contribution of these pathways can only be assessed from their momentary involvement at any stage of the complete course of neurodegeneration. Here we used quantitative labelling for activated caspase-3 (Casp-3) and Fluoro-Jade B (FJ-B) to investigate the spatio-temporal pattern of neuronal death from P6 to P67 in Lc/+ mutants. Activated Casp-3 was present only in narrow time intervals (P14 to P22 in PCs; P14 to P28 in GCs) and in small subpopulations of PCs, GCs, and IO neurones. FJ-B positive PCs were detected during a broader period (P14 to P28), and outnumbered Casp-3 labelled PCs by a factor exceeding eight. Nevertheless, FJ-B labelling was restricted to PCs and never found in either GC or IO neurones. In conclusion, we present the first complete time course and extent of Casp-3 activation in Lc/+ mutants and show that the majority of dying neurones in Lc/+ mutants undergo Casp-3 independent cell death. The cellular overload produced by the initial gene defect in Lc/+ mutants apparently activates a variety of apoptotic and non-apoptotic pathways within the same neuronal population. Moreover, we present the first evidence for the ability of FJ-B to selectively label a discrete population of dying PCs, implying a higher selectivity of FJ-B than previously supposed.

Neuroprotective effect of mifepristone involves neuron depolarization

In several regions of the developing nervous system, neurons undergo programmed cell death. In the rat cerebellum, Purkinje cell apoptosis is exacerbated when cerebellar slices are cultured during the first postnatal week. To understand the mechanism of this developmental apoptosis, we took advantage of its inhibition by the steroid analog mifepristone. This effect did not involve the classical steroid nuclear receptors. Microarray analysis revealed that mifepristone down-regulated mRNA levels of the Na+/K+-ATPase alpha3 subunit more than three times. Consistent with the down-regulation of the Na+/K+-ATPase, mifepristone caused Purkinje cell membrane depolarization. Depolarizing agents like ouabain (1 microM), tetraethylammonium (2 mM), and veratridine (2 microM) protected Purkinje cells from apoptosis. These results suggest a role of excitatory inputs in Purkinje cell survival during early postnatal development. Indeed, coculturing cerebellar slices with glutamatergic inferior olivary neuron preparations allowed rescue of Purkinje cells. These findings reveal a new neuroprotective mechanism of mifepristone and support a pivotal role for excitatory inputs in the survival of Purkinje neurons. Mifepristone may be a useful lead compound in the development of novel therapeutic approaches for maintaining the resting potential of neurons at values favorable for their survival under neuropathological conditions.

Role of the Olivo-Cerebellar System in Timing

Timing has been proposed as a basic function of the cerebellar cortex (particularly the climbing fiber afferents and their sole source, the inferior olive) that explains the contribution of the cerebellum to both motor control and nonmotor cognitive functions. However, whether the olivo-cerebellar system mediates time perception without motor behavior remains controversial. We used event-related functional magnetic resonance imaging to dissociate the neural correlates of the perceptual from the motor aspects of timing. The results show activation of multiple areas within the cerebellar cortex during both perception and motor performance of temporal sequences. The results further show that the inferior olive was activated only when subjects perceived the temporal sequences without motor activity. This finding is most consistent with electrophysiological studies showing decreased responsiveness of the inferior olivary neurons to sensory input during expected, self-produced movement. Our results suggest that the primary role of the inferior olive and the climbing fiber system in timing is the encoding of temporal information independent of motor behavior.

Continuous Electrical Oscillations Emerge from a Coupled Network: A Study of the Inferior Olive using Lentiviral Knockdown of Connexin36

Do continuous subthreshold oscillations in membrane potential within an electrically coupled network depend on gap junctional coupling? For the inferior olive (IO), modeling and developmental studies suggested that the answer is yes, although physiological studies of connexin36 knock-out mice lacking electrical coupling suggested that the answer is no. Here we addressed the question differently by using a lentivirus-based vector to express, in the IO of adult rats, a single amino acid mutation of connexin36 that disrupts the intracellular trafficking of wild-type connexin36 and blocks gap junctional coupling. Confocal microscopy of green fluorescence protein-labeled dendrites revealed that the mutant connexin36 prevented wild-type connexin36 from being expressed in dendritic spines of IO neurons. Intracellular recordings from lentivirally transduced IO networks revealed that robust and continuous subthreshold oscillations require gap junctional coupling of IO neuron somata within 40 microm of one another. Topological studies indicated that the minimal coupled network for supporting such oscillations may be confined to the dendritic arbor of a single IO neuron. Occasionally, genetically uncoupled IO neurons showed transient oscillations; however, these were not sustained longer than 3 s and were 69% slower and 71% smaller than the oscillations of normal IO neurons, a finding replicated with carbenoxolone, a pharmacological antagonist of gap junctions. The experiments provided the first direct evidence that gap junctional coupling between neurons, specifically mediated by connexin36, allows a continuous network oscillation to emerge from a population of weak and episodic single-cell oscillators. The findings are discussed in the context of the importance of gap junctions for cerebellar rhythms involved in movement.

Isochrony in the olivocerebellar system underlies complex spike synchrony

In a recent issue of The Journal of PhysiologyBaker & Edgley (2006) published results which they interpreted as showing a non-uniform olivocerebellar conduction time in the rat. Baker and Edgley used the climbing fibre reflex to measure conduction time to different points along a folial wall. This reflex is initiated by electrical stimulation of the cerebellar white matter, which triggers antidromic spikes in olivocerebellar axons that invade the inferior olive (IO). Current generated by these spikes spreads to neighbouring IO neurones via the gap junctions that electrically couple IO neurones (Llinas et al. 1974). This spreading excitation can trigger orthodromic spikes that return to the cerebellar cortex and trigger Purkinje cell complex spikes (CSs). Baker and Edgley found that climbing fibre reflex latency varied with recording electrode depth, and inferred from this that conduction time between the IO and cerebellar cortex varies systematically with cortical location. This study conflicts with results by us and others that indicate a near uniform olivocerebellar conduction time in rats and turtles (Sugihara et al. 1993; Lang & Rosenbluth, 2003; Ariel, 2005). Below we discuss several troubling issues with this new study, but first we would like to make the point that spontaneous CS synchrony patterns almost necessitate a uniform olivocerebellar conduction time.

Organization and remodeling of the olivocerebellar climbing fiber projection

Climbing fibers, terminal portions of the axons of inferior olive neurons, form strong synaptic connections to Purkinje cells in an exclusive one-to-one relationship. This projection is established during development by drastic reshaping in each climbing fiber and in overall axonal arborization. Early climbing fibers form loose 'creeper'-type terminal arbors that seem to make weak contact with many Purkinje cells in the first postnatal week. The terminal arbor then becomes focused on a single Purkinje cell with the aggregation of swellings ('transitional' type), and eventually tightly surrounds the Purkinje cell soma ('nest' type) in the second postnatal week. The terminal arbor is then displaced upward to the stem of the apical dendrite of the Purkinje cell ('capuchon' or 'hood') and eventually to the proximal portion of the dendritic tree (mature climbing fiber). Single-axon morphology in rats has shown that olivocerebellar axons in the creeper stage branch more frequently and have many more climbing fibers than those in adults. The climbing fibers that originate from an axon are largely organized into microzones as in adults. Concomitant with this remodeling of climbing fibers, the number of climbing fibers per olivocerebellar axon is significantly decreased by the putative retraction of climbing fibers during development from the creeper to nest stage. Due to additional retraction after the nest stage, an olivocerebellar axon in an adult has about seven climbing fibers. The above morphological remodeling and retraction during development can be closely compared to the changes in climbing fiber-Purkinje cell synaptic interaction observed in rats and mice. Generation and aggregation of the swellings in the terminal arbor between the creeper and nest stages are correlated with maturation of the synaptic connection. The decrease in climbing fibers in the same and following periods is correlated with the elimination of overabundant synapses to establish one-to-one connections.

Block of Inferior Olive Gap Junctional Coupling Decreases Purkinje Cell Complex Spike Synchrony and Rhythmicity

Inferior olivary (IO) neurons are electrotonically coupled by gap junctions. This coupling is thought to underlie synchronous complex spike (CS) activity generated by the olivocerebellar system in Purkinje cells, and also has been hypothesized to be necessary for IO neurons to generate spontaneous oscillatory activity. These characteristics of olivocerebellar activity have been proposed to be central to the role of this system in motor coordination. However, the relationship of gap junction coupling between IO neurons to synchronous and rhythmic CS activity has never been directly tested. Thus, to address this issue, multiple electrode recordings were obtained from crus 2a Purkinje cells, and carbenoxolone, a gap junction blocker, was injected into the IO. Carbenoxolone reduced CS synchrony by 50% overall, but in some experiments, >80% reductions were achieved. Carbenoxolone also reduced the average firing rate by 50%, suggesting that electrical coupling is a significant source of excitation for IO neurons. Moreover, carbenoxolone caused a reduction in the approximately 10 Hz rhythmicity of CS activity, and this reduction was correlated with the extent to which the injection reduced CS synchrony. Lastly, carbenoxolone was found to reverse or prevent changes in synchrony that are normally induced by injection of GABAA and glutamate receptor antagonists into the IO, suggesting that the effects of these drugs on CS synchrony patterns require electrical coupling of IO neurons. In sum, our results provide direct evidence that electrical coupling of IO neurons underlies synchronous CS activity, and suggest important roles for this coupling in shaping other aspects of IO spiking patterns.

Olivocerebellar modulation of motor cortex ability to generate vibrissal movements in rat

The vibrissal movements known as whisking are generated in a pulsatile, or non-continuous, fashion and comprise sequences of brief regularly spaced movements. These rhythmic timing sequences imply the existence of periodically issued motor commands. As inferior olivary (IO) neurones generate periodic synchronous discharges that could provide the underlying timing signal, this possibility was tested by determining whether the olivocerebellar system modulates motor cortex (MCtx)-triggered whisker movements in rats. Trains of current pulses were applied to MCtx, and the resulting whisker movements were recorded using a high speed video camera. The evoked movement patterns demonstrated properties consistent with the existence of an oscillatory motor driving rhythm. In particular, movement amplitude showed a bell-shaped dependence on stimulus frequency, with a peak at 11.5+/-2.3 Hz. Moreover, movement trajectories showed harmonic and subharmonic entrainment patterns within specific stimulus frequency ranges. By contrast, movements evoked by facial nerve stimulation showed no such frequency-dependent properties. To test whether the IO was the oscillator in question, IO neuronal properties were modified in vivo by intra-IO picrotoxin injection, which enhances synchronous oscillatory IO activity and reduces its natural frequency. The ensuing changes in the evoked whisker patterns were consistent with these pharmacological effects. Furthermore, in cerebellectomized rats, oscillatory modulation of MCtx-evoked movements was greatly reduced, and intra-IO picrotoxin injections did not affect the evoked movement patterns. Additionally, multielectrode recording of Purkinje cell complex spikes showed a temporal correlation of olivocerebellar activity during MCtx stimulus trains to evoked movement patterns. In sum, the results indicate that MCtx's ability to generate movements is modulated by an oscillatory signal arising in the olivocerebellar system.

Topography of olivo‐cortico‐nuclear modules in the intermediate cerebellum of the rat

This study provides a detailed anatomical description of the relation between olivo-cortico-nuclear modules of the intermediate cerebellum of the rat and the intrinsic zebrin pattern of the Purkinje cells. Strips of climbing fibers were labeled using small injections of biotinylated dextran amine into either the medial or dorsal accessory olives, while, in some cases, simultaneous retrograde labeling of Purkinje cells was obtained using gold-lectin injections into selected parts of the interposed nuclei. Our data are represented in a new, highly detailed, cortical surface reconstruction of the zebrin pattern and in relation to the collateral labeling of the climbing fibers to the cerebellar nuclei. We show that the somatotopic regions of the dorsal accessory olive behave differently in their projections to essentially zebrin-negative regions that represent the C1 and C3 zones of the anterior and posterior parts of the cortex. The rostral part of the medial accessory olive projects to zebrin-positive areas, in particular to the P4+ band of the anterior lobe and lobule VI and to the P5+ band of the posterior lobe, indicating that C2 has two noncontiguous representations in the SL and crus 1. By relating the areas of overlap that resulted from the injections in the accessory olives, i.e., labeling of climbing fiber strips and patches of climbing fiber nuclear collaterals, with the results from the injections in the interposed nuclei, i.e., retrograde labeling of Purkinje cells and of inferior olivary neurons, direct verification of the concept of modular cerebellar connections was obtained.