Synaptology of the cerebello-olivary pathway. Double labelling with anterograde axonal tracing and GABA immunocytochemistry in the rat

A Systematic Review of Direct Outputs from the Cerebellum to the Brainstem and Diencephalon in Mammals

The cerebellum is involved in many motor, autonomic and cognitive functions, and new tasks that have a cerebellar contribution are discovered on a regular basis. Simultaneously, our insight into the functional compartmentalization of the cerebellum has markedly improved. Additionally, studies on cerebellar output pathways have seen a renaissance due to the development of viral tracing techniques. To create an overview of the current state of our understanding of cerebellar efferents, we undertook a systematic review of all studies on monosynaptic projections from the cerebellum to the brainstem and the diencephalon in mammals. This revealed that important projections from the cerebellum, to the motor nuclei, cerebral cortex, and basal ganglia, are predominantly di- or polysynaptic, rather than monosynaptic. Strikingly, most target areas receive cerebellar input from all three cerebellar nuclei, showing a convergence of cerebellar information at the output level. Overall, there appeared to be a large level of agreement between studies on different species as well as on the use of different types of neural tracers, making the emerging picture of the cerebellar output areas a solid one. Finally, we discuss how this cerebellar output network is affected by a range of diseases and syndromes, with also non-cerebellar diseases having impact on cerebellar output areas.

Cerebellar Modules and Their Role as Operational Cerebellar Processing Units: A Consensus paper [corrected]

The compartmentalization of the cerebellum into modules is often used to discuss its function. What, exactly, can be considered a module, how do they operate, can they be subdivided and do they act individually or in concert are only some of the key questions discussed in this consensus paper. Experts studying cerebellar compartmentalization give their insights on the structure and function of cerebellar modules, with the aim of providing an up-to-date review of the extensive literature on this subject. Starting with an historical perspective indicating that the basis of the modular organization is formed by matching olivocorticonuclear connectivity, this is followed by consideration of anatomical and chemical modular boundaries, revealing a relation between anatomical, chemical, and physiological borders. In addition, the question is asked what the smallest operational unit of the cerebellum might be. Furthermore, it has become clear that chemical diversity of Purkinje cells also results in diversity of information processing between cerebellar modules. An additional important consideration is the relation between modular compartmentalization and the organization of the mossy fiber system, resulting in the concept of modular plasticity. Finally, examination of cerebellar output patterns suggesting cooperation between modules and recent work on modular aspects of emotional behavior are discussed. Despite the general consensus that the cerebellum has a modular organization, many questions remain. The authors hope that this joint review will inspire future cerebellar research so that we are better able to understand how this brain structure makes its vital contribution to behavior in its most general form.

Chloride Homeostasis in Neurons With Special Emphasis on the Olivocerebellar System: Differential Roles for Transporters and Channels

The intraneuronal ionic composition is an important determinant of brain functioning. There is growing evidence that aberrant homeostasis of the intracellular concentration of Cl- ([Cl-]i) evokes, in addition to that of Na+ and Ca2+, robust impairments of neuronal excitability and neurotransmission and thereby neurological conditions. More specifically, understanding the mechanisms underlying regulation of [Cl-]i is crucial for deciphering the variability in GABAergic and glycinergic signaling of neurons, in both health and disease. The homeostatic level of [Cl-]i is determined by various regulatory mechanisms, including those mediated by plasma membrane Cl- channels and transporters. This review focuses on the latest advances in identification, regulation and characterization of Cl- channels and transporters that modulate neuronal excitability and cell volume. By putting special emphasis on neurons of the olivocerebellar system, we establish that Cl- channels and transporters play an indispensable role in determining their [Cl-]i and thereby their function in sensorimotor coordination.

Neocortical Chandelier Cells Developmentally Shape Axonal Arbors through Reorganization but Establish Subcellular Synapse Specificity without Refinement

Diverse types of cortical interneurons (INs) mediate various kinds of inhibitory control mechanisms to balance and shape network activity. Distinct IN subtypes develop uniquely organized axonal arbors that innervate different subcellular compartments of excitatory principal neurons (PNs), which critically contribute to determining their output properties. However, it remains poorly understood how they establish this peculiar axonal organization and synaptic connectivity during development. Here, taking advantage of genetic labeling of IN progenitors, we examined developmental processes of axonal arbors and synaptic connections formed by murine chandelier cells (ChCs), which innervate axon initial segments (AISs) of PNs and thus powerfully regulate their spike generation. Our quantitative analysis by light microscopy revealed that ChCs overgrow and subsequently refine axonal branches as well as varicosities. Interestingly, we found that although a significant number of axonal varicosities are formed off AISs in addition to on AISs, presynaptic markers are predominantly colocalized with those on AISs throughout development. Immunoelectron microscopic (IEM) analysis also demonstrated that only varicosities apposed to AISs contain presynaptic profiles. These results suggest that subcellular synapse specificity of ChCs is genetically predetermined while axonal geometry is shaped through remodeling. Molecular cues localized at AISs may regulate target recognition and synapse formation by ChCs.

Functional correlates of the therapeutic and adverse effects evoked by thalamic stimulation for essential tremor

Thalamic deep brain stimulation (DBS) is an effective therapy for essential tremor. Gibson et al. use functional MRI to reveal patterns of activation that correlate with stimulation-induced therapeutic and adverse effects. Their results suggest that thalamic DBS controls tremor, and induces paraesthesias, through distal modulation of tremor-related network nodes.

Deep brain stimulation is an established neurosurgical therapy for movement disorders including essential tremor and Parkinson’s disease. While typically highly effective, deep brain stimulation can sometimes yield suboptimal therapeutic benefit and can cause adverse effects. In this study, we tested the hypothesis that intraoperative functional magnetic resonance imaging could be used to detect deep brain stimulation-evoked changes in functional and effective connectivity that would correlate with the therapeutic and adverse effects of stimulation. Ten patients receiving deep brain stimulation of the ventralis intermedius thalamic nucleus for essential tremor underwent functional magnetic resonance imaging during stimulation applied at a series of stimulation localizations, followed by evaluation of deep brain stimulation-evoked therapeutic and adverse effects. Correlations between the therapeutic effectiveness of deep brain stimulation (3 months postoperatively) and deep brain stimulation-evoked changes in functional and effective connectivity were assessed using region of interest-based correlation analysis and dynamic causal modelling, respectively. Further, we investigated whether brain regions might exist in which activation resulting from deep brain stimulation might correlate with the presence of paraesthesias, the most common deep brain stimulation-evoked adverse effect. Thalamic deep brain stimulation resulted in activation within established nodes of the tremor circuit: sensorimotor cortex, thalamus, contralateral cerebellar cortex and deep cerebellar nuclei (FDR q < 0.05). Stimulation-evoked activation in all these regions of interest, as well as activation within the supplementary motor area, brainstem, and inferior frontal gyrus, exhibited significant correlations with the long-term therapeutic effectiveness of deep brain stimulation (P < 0.05), with the strongest correlation (P < 0.001) observed within the contralateral cerebellum. Dynamic causal modelling revealed a correlation between therapeutic effectiveness and attenuated within-region inhibitory connectivity in cerebellum. Finally, specific subregions of sensorimotor cortex were identified in which deep brain stimulation-evoked activation correlated with the presence of unwanted paraesthesias. These results suggest that thalamic deep brain stimulation in tremor likely exerts its effects through modulation of both olivocerebellar and thalamocortical circuits. In addition, our findings indicate that deep brain stimulation-evoked functional activation maps obtained intraoperatively may contain predictive information pertaining to the therapeutic and adverse effects induced by deep brain stimulation.

How the cerebellum may monitor sensory information for spatial representation

The cerebellum has already been shown to participate in the navigation function. We propose here that this structure is involved in maintaining a sense of direction and location during self-motion by monitoring sensory information and interacting with navigation circuits to update the mental representation of space. To better understand the processing performed by the cerebellum in the navigation function, we have reviewed: the anatomical pathways that convey self-motion information to the cerebellum; the computational algorithm(s) thought to be performed by the cerebellum from these multi-source inputs; the cerebellar outputs directed toward navigation circuits and the influence of self-motion information on space-modulated cells receiving cerebellar outputs. This review highlights that the cerebellum is adequately wired to combine the diversity of sensory signals to be monitored during self-motion and fuel the navigation circuits. The direct anatomical projections of the cerebellum toward the head-direction cell system and the parietal cortex make those structures possible relays of the cerebellum influence on the hippocampal spatial map. We describe computational models of the cerebellar function showing that the cerebellum can filter out the components of the sensory signals that are predictable, and provides a novelty output. We finally speculate that this novelty output is taken into account by the navigation structures, which implement an update over time of position and stabilize perception during navigation.

Collateralization of cerebellar output to functionally distinct brainstem areas. A retrograde, non-fluorescent tracing study in the rat

The organization of the cerebellum is characterized by a number of longitudinally organized connection patterns that consist of matching olivo-cortico-nuclear zones. These entities, referred to as modules, have been suggested to act as functional units. The various parts of the cerebellar nuclei (CN) constitute the output of these modules. We have studied to what extent divergent and convergent patterns in the output of the modules to four, functionally distinct brain areas can be recognized. Two retrograde tracers were injected in various combinations of the following nuclei: the red nucleus (RN), as a main premotor nucleus; the prerubral area, as a main supplier of afferents to the inferior olive (IO); the nucleus reticularis tegmenti pontis (NRTP), as a main source of cerebellar mossy fibers; and the IO, as the source of climbing fibers. For all six potential combinations three cases were examined. All nine cases with combinations that involved the IO did not, or hardly, resulted in double labeled neurons. In contrast, all other combinations resulted in at least 10% and up to 67% of double labeled neurons in cerebellar nuclear areas where both tracers were found. These results show that the cerebellar nuclear neurons that terminate within the studied areas represent basically two intermingled populations of projection cells. One population corresponds to the small nucleo-olivary neurons whereas the other consists of medium- to large-sized neurons which are likely to distribute their axons to several other areas. Despite some consistent differences between the output patterns of individual modules we propose that modular cerebellar output to premotor areas such as the RN provides simultaneous feedback to both the mossy fiber and the climbing fiber system and acts in concert with a designated GABAergic nucleo-olivary circuit. These features seem to form a basic characteristic of cerebellar operation.

Strength and timing of motor responses mediated by rebound firing in the cerebellar nuclei after Purkinje cell activation

The cerebellum refines the accuracy and timing of motor performance. How it encodes information to perform these functions is a major topic of interest. We performed whole cell and extracellular recordings of Purkinje cells (PCs) and cerebellar nuclei neurons (CNs) in vivo, while activating PCs with light in transgenic mice. We show for the first time that graded activation of PCs translates into proportional CN inhibition and induces rebound activity in CNs, which is followed by graded motor contractions timed to the cessation of the stimulus. Moreover, activation of PC ensembles led to disinhibition of climbing fiber activity, which coincided with rebound activity in CNs. Our data indicate that cessation of concerted activity in ensembles of PCs can regulate both timing and strength of movements via control of rebound activity in CNs.

Properties of the nucleo-olivary pathway: an in vivo whole-cell patch clamp study

The inferior olivary nucleus (IO) forms the gateway to the cerebellar cortex and receives feedback information from the cerebellar nuclei (CN), thereby occupying a central position in the olivo-cerebellar loop. Here, we investigated the feedback input from the CN to the IO in vivo in mice using the whole-cell patch-clamp technique. This approach allows us to study how the CN-feedback input is integrated with the activity of olivary neurons, while the olivo-cerebellar system and its connections are intact. Our results show how IO neurons respond to CN stimulation sequentially with: i) a short depolarization (EPSP), ii) a hyperpolarization (IPSP) and iii) a rebound depolarization. The latter two phenomena can also be evoked without the EPSPs. The IPSP is sensitive to a GABA_A receptor blocker. The IPSP suppresses suprathreshold and subthreshold activity and is generated mainly by activation of the GABA_A receptors. The rebound depolarization re-initiates and temporarily phase locks the subthreshold oscillations. Lack of electrotonical coupling does not affect the IPSP of individual olivary neurons, nor the sensitivity of its GABA_A receptors to blockers. The GABAergic feedback input from the CN does not only temporarily block the transmission of signals through the IO, it also isolates neurons from the network by shunting the junction current and re-initiates the temporal pattern after a fixed time point. These data suggest that the IO not only functions as a cerebellar controlled gating device, but also operates as a pattern generator for controlling motor timing and/or learning.

Ablation of glutamate receptor GluRδ2 in adult Purkinje cells causes multiple innervation of climbing fibers by inducing aberrant invasion to parallel fiber innervation territory

Glutamate receptor GluRδ2 is exclusively expressed in Purkinje cells (PCs) from early development and plays key roles in parallel fiber (PF) synapse formation, elimination of surplus climbing fibers (CFs), long-term depression, motor coordination, and motor learning. To address its role in adulthood, we previously developed a mouse model of drug-induced GluRδ2 ablation in adult PCs (Takeuchi et al., 2005). In that study, we demonstrated an essential role to maintain the connectivity of PF-PC synapses, based on the observation that both mismatching of presynaptic and postsynaptic specializations and disconnection of PF-PC synapses are progressively increased after GluRδ2 ablation. Here, we pursued its role for CF wiring in adult cerebellum. In parallel with the disconnection of PF-PC synapses, ascending CF branches exhibited distal extension to innervate distal dendrites of the target and neighboring PCs. Furthermore, transverse CF branches, a short motile collateral rarely forming synapses in wild-type animals, displayed aberrant mediolateral extension to innervate distal dendrites of neighboring and remote PCs. Consequently, many PCs were wired by single main CF and other surplus CFs innervating a small part of distal dendrites. Electrophysiological recording further revealed that surplus CF-EPSCs characterized with slow rise time and small amplitude emerged after GluRδ2 ablation, and increased progressively both in number and amplitude. Therefore, GluRδ2 is essential for maintaining CF monoinnervation in adult cerebellum by suppressing aberrant invasion of CF branches to the territory of PF innervation. Thus, GluRδ2 fuels heterosynaptic competition and gives PFs the competitive advantages over CFs throughout the animal's life.

Oculopalatal tremor explained by a model of inferior olivary hypertrophy and cerebellar plasticity

The inferior olivary nuclei clearly play a role in creating oculopalatal tremor, but the exact mechanism is unknown. Oculopalatal tremor develops some time after a lesion in the brain that interrupts inhibition of the inferior olive by the deep cerebellar nuclei. Over time the inferior olive gradually becomes hypertrophic and its neurons enlarge developing abnormal soma-somatic gap junctions. However, results from several experimental studies have confounded the issue because they seem inconsistent with a role for the inferior olive in oculopalatal tremor, or because they ascribe the tremor to other brain areas. Here we look at 3D binocular eye movements in 15 oculopalatal tremor patients and compare their behaviour to the output of our recent mathematical model of oculopalatal tremor. This model has two mechanisms that interact to create oculopalatal tremor: an oscillator in the inferior olive and a modulator in the cerebellum. Here we show that this dual mechanism model can reproduce the basic features of oculopalatal tremor and plausibly refute the confounding experimental results. Oscillations in all patients and simulations were aperiodic, with a complicated frequency spectrum showing dominant components from 1 to 3 Hz. The model’s synchronized inferior olive output was too small to induce noticeable ocular oscillations, requiring amplification by the cerebellar cortex. Simulations show that reducing the influence of the cerebellar cortex on the oculomotor pathway reduces the amplitude of ocular tremor, makes it more periodic and pulse-like, but leaves its frequency unchanged. Reducing the coupling among cells in the inferior olive decreases the oscillation’s amplitude until they stop (at ∼20% of full coupling strength), but does not change their frequency. The dual-mechanism model accounts for many of the properties of oculopalatal tremor. Simulations suggest that drug therapies designed to reduce electrotonic coupling within the inferior olive or reduce the disinhibition of the cerebellar cortex on the deep cerebellar nuclei could treat oculopalatal tremor. We conclude that oculopalatal tremor oscillations originate in the hypertrophic inferior olive and are amplified by learning in the cerebellum.

Projections of glutamate decarboxylase positive and negative cerebellar neurons to the pretectum in the cat

The pretectum is one of the primary visual centers, and plays an important role in the visuomotor reflexes. It also receives projections from the cerebellar nuclei that are considered to regulate these reflexes. Gamma aminobutylic acid (GABA) and glutamate are supposed to be two major neurotransmitters of the projection neurons of the cerebellar nuclei. We double labeled the projecting neurons with a tracer, biotinylated dextran amine (BDA), and with an antiserum to glutamate decarboxylase (GAD), the enzyme that synthesizes GABA. The results indicated that about 40% of the pretectal-projecting neurons of the cerebellar nuclei were GAD immunoreactive. The GAD positive pretectal-projecting neurons were significantly smaller than the GAD negative projecting neurons. Our findings thus suggest the existence of two distinct cerebello-pretectal projection systems: one is mediated by GABAergic inhibitory projections, while the other is mediated by non-GABAergic, probably glutamatergic excitatory ones.

Cerebellar control of the inferior olive

A subpopulation of neurones in the cerebellar nuclei projects to the inferior olive, the source of the climbing fibre input to the cerebellum. This nucleo-olivary projection follows the zonal and, probably also, the microzonal arrangement of the cerebellum so that closed loops are formed between the neurones in the olive, the cerebellar cortex and the nuclei. The nucleo-olivary pathway is GABAergic, but several investigators argue that its main effect is to regulate electrotonic coupling between cells in the inferior olive rather than inhibit the olive. However, there is now strong evidence that the nucleo-olivary fibres do inhibit the olive. Three functions have been suggested for this inhibition: (i) feedback control of background activity in Purkinje cells, (ii) feedback control of learning, and (iii) gating of olivary input in general. Evidence is consistent with (i) and (ii). Activity in the nucleo-olivary pathway suppresses both synaptic transmission and background activity in the olive. When learned blink responses develop, the blink related part of the olive is inhibited while blinks are produced. When the nucleo-olivary pathway is interrupted, there is a corresponding increase in complex spike discharge in Purkinje cells followed by a strong suppression of simple spike firing. Stimulation of the pathway has the opposite results. It is concluded that the nucleo-olivary fibres are inhibitory and that they form a number of independent feedback loops, each one specific for a microcomplex, that regulate cerebellar learning as well as spontaneous activity in the olivo-cerebellar circuit.

Cerebellar inhibition of inferior olivary transmission in the decerebrate ferret

Stimulation around the superior cerebellar peduncle or within the deep cerebellar nuclei is known to inhibit the inferior olive with a very long latency. It has been suggested that this inhibition is mediated by the GABA-ergic nucleo-olivary pathway, but alternative explanations such as activation of an indirect excitatory pathway or a pathway via the red nucleus are possible. A long-latency inhibition via the nucleo-olivary pathway would have profound implications for cerebellar function and the present study was performed to test alternative explanations and to characterize the nucleo-olivary inhibition. Climbing fibre responses (CFRs), evoked by periorbital stimulation and recorded from the cerebellar cortex, could be inhibited by stimulation of two distinct mesencephalic areas. One was located within the superior cerebellar peduncle and the other about 1 mm further ventrally. Inhibition evoked from either area occurred in the inferior olive and was independent of a red nucleus relay. Single Purkinje cell recordings revealed that inhibition from the ventral area was not secondary to olivary activation. It is concluded that stimulation of the ventral area activated nucleo-olivary fibres. The inhibition elicited by stimulation within the peduncle probably resulted from indirect activation on the nucleo-olivary fibres via antidromic activation of the interpositus nucleus. The time courses of the inhibition from the two areas were indistinguishable. The duration of the strongest inhibition was short and had a sharp peak at about 30 ms. It is suggested that the time course of the inhibition is important for temporal regulation of learned responses.

Why do restless legs occur at rest?—pathophysiology of neuronal structures in RLS. Neurophysiology of RLS (part 2)

Restless legs syndrome (RLS) is a heterogeneous disorder encompassing genetically caused types with early onset and acquired varieties occurring later in life. Genetic studies in the near future will most likely discover more than one causative gene. The acquired cases too have different etiologies ranging from idiopathic types to secondary forms with uremia, iron depletion, polyneuropathy and others. Here we aim to correlate typical RLS symptoms, such as the sensory symptoms at rest, the reduction of the complaint in response to movement or other physical stimuli, the dominant involvement of the legs, pain, circadian rhythm, and the responsiveness to dopaminergic drugs with neurophysiological features of the central nervous system. We outline the complexity of the neural structures involved and their connections. A diversity of hypothetical affections of different neuronal levels might lead to various combinations of RLS symptomatology. No single pathophysiological explanation has yet been developed that covers all clinical features.

Adaptive modification of saccade size produces correlated changes in the discharges of fastigial nucleus neurons

Saccade accuracy is known to be maintained by adaptive mechanisms that progressively reduce any visual error that consistently exists when the saccade ends. We used an experimental paradigm known to induce adaptation of saccade size while monitoring the neural correlates of this adaptation. In rhesus monkeys where the medial and lateral recti of one eye were surgically weakened, patching the unoperated eye and forcing the monkey to use the weakened eye induced a gradual increase in saccade size in both eyes until the viewing, weak eye almost acquired the target in one step. Subsequent patching of the weakened eye gradually reversed the situation, so that the saccades in the viewing, normal eye decreased from an initial overshooting to normal. In the caudal fastigial nuclei of unadapted monkeys, neurons typically exhibit an early burst of spikes that is correlated with the onset of contraversive saccades and a later burst of spikes that is correlated with the termination of ipsiversive saccades. Comparing the discharges of the same fastigial neurons recorded before and during adaptation, this basic pattern did not change, but some parameters of the discharges did. The most consistent changes were in the latency of the burst for ipsiversive saccades, which was positively correlated with saccade size (1.28 ms/deg), and in the number of spikes associated with contraversive saccades, which was also positively correlated (0.55 spikes/deg). The former was more important when saccade size was decreasing, and the latter was more important when saccade size was increasing. Based on current knowledge of the anatomical connections of fastigial neurons, as well as on the effects of cerebellar lesions and on recordings in other structures, we argue that these changes are appropriate for causing the associated changes in saccade size.

Fastigiovestibular projections in the rat: retrograde tracing coupled with gammaamino-butyric acid and glutamate immunohistochemistry

In this study, a double labeling technique using retrograde tracing with protein-gold complex (gold-HRP) in conjunction with a gammaamino-butyric acid (GABA) and glutamate immunohistochemical procedure was performed to identify GABA (GABA-IR) and glutamate (Glu-IR) immunoreactive neurons in the cerebellar fastigial nucleus (FN) that projects to the vestibular nuclei (VN). The results show that FN neurons projecting to the VN consist of both GABA-IR and Glu-IR neurons with a predominance of glutamatergic ones. Because GABAergic neurons in the cerebellar nuclei project to the inferior olive (IO), double retrograde labeling experiments were performed with injections of gold-HRP in the IO and of biotilynated dextran amine in the VN. This showed that the GABA-IR fastigiovestibular neurons project by axon collaterals to both the VN and the IO.

Expression of Cx36 in mammalian neurons

Cx36 is the first mammalian member of a novel subgroup of the connexin family, characterized by a long cytoplasmic loop, a peculiar gene structure and a preferential expression in cell types of neural origin. In the present review we summarize the evidence in favour of its predominant expression in neuronal cells in the mammalian central nervous system, such as results from experiments with specific neurotoxins and co-localization of Cx36 mRNA and a neuronal marker. We also report a detailed description of Cx36 mRNA distribution in the rat and human central nervous system by in situ hybridization and, for each brain region, we correlate the novel findings with previous morphological or functional demonstrations of neuronal gap junctions in the same area.

Characterization of transsynaptic tracing with central application of pseudorabies virus

Although transsynaptic tracing with peripheral injection of pseudorabies virus (PRV) has been extensively characterized, several methodological issues related to central application of this tracer have not been addressed. In the present study, we addressed the following three issues by using microinjection of a cocktail containing PRV (Bartha strain) and cholera toxin subunit B (CTb) into different sites in the rat brain. First, we estimated PRV diffusion by examining injection sites at different times after application. Second, we tested whether PRV is taken up by fibers of passage following injections into the olivocerebellar pathway. Third, we developed criteria for leakage of PRV into cerebral ventricles. Our data indicate that (i) centrally injected PRV diffuses very little and produces focal injection sites; (ii) PRV is taken up and transported by fibers of passage, although less prominently than found for Ctb; (iii) PRV produces specific and easily identifiable ependymal cell as well as neuronal labeling following ventricular injection. This labeling can be used as a criterion for determining if labeling obtained was due to injected tracer leaking into brain ventricles. In summary, the present study provides new and important information about using PRV to trace central multisynaptic circuitry.

Movement-related gating of climbing fibre input to cerebellar cortical zones

The inferior olive climbing fibre projection and associated spino-olivocerebellar paths (SOCPs) have been studied intensively over the last quarter of a century yet precisely what information they signal to the cerebellar cortex during movements remains unclear. A different approach is to consider the times during a movement when afferent signals are likely to be conveyed via these paths. Central regulation (gating) of afferent transmission during active movements is well documented in sensory pathways leading to the cerebral cortex and the present review examines the possibility that a similar phenomenon also occurs in SOCPs during movements such as locomotion and reaching. Several lines of evidence are considered which suggest that SOCPs are not always open for transmission. Instead, flow of sensory information to the cerebellum via climbing fibre paths is powerfully modulated during active movements. The findings are discussed in relation to the parasagittal zonal organization of the cerebellar cortex and, in particular, evidence is presented that different cerebellar zones are subject to similar patterns of gating during reaching but can differ appreciably in the pattern of modulation their SOCPs exhibit during locomotion. Furthermore, differences in gating can occur at different rostrocaudal loci within the same zone, suggesting that in the awake behaving animal, individual cerebellar zones are not functionally homogeneous. Finally, the data are interpreted in relation to the error detector hypothesis of climbing fibre function and the possibility explored that the gating serves as a task-dependent mechanism that operates to prevent self-generated 'irrelevant' sensory inputs from being relayed via the SOCPs to the cerebellar cortex, while behaviourally 'relevant' signals are selected for transmission.

Red nucleus stimulation inhibits within the inferior olive

Red nucleus stimulation inhibits within the inferior olive. J. Neurophysiol. 80: 3127-3136, 1998. In the anesthetized cat, electrical stimulation of the magnocellular red nucleus (RNm) inhibits responses of rostral dorsal accessory olive (rDAO) neurons to cutaneous stimulation. We tested the hypothesis that RNm-mediated inhibition occurs within the inferior olive by using stimulation of the ventral funiculus (VF) of the spinal cord in place of cutaneous stimulation of the hindlimb. Fibers in the VF terminate on hindlimb rDAO neurons, so inhibition of this input would have to occur within the olive. rDAO responses elicited by VF stimulation were inhibited by prior stimulation of the RNm, indicating that inhibition occurs within the olive. In contrast, evoked potentials recorded from the VF or dorsal columns following hindlimb stimulation were not affected by prior stimulation of RNm, indicating that stimulation of the RNm does not inhibit olivary afferents at spinal levels. RNm stimulation that inhibited rDAO responses had little effect on evoked somatosensory responses in thalamus, indicating that inhibition generated by activity in RNm may be specific to rDAO. To test limb specificity of RNm-mediated inhibition, conditioning stimulation was applied to the dorsolateral funiculus at thoracic levels, which selectively activates RNm neurons projecting to the lumbar cord. Stimulation at thoracic levels inhibited evoked responses from hindlimb but not forelimb regions of rDAO, suggesting that inhibitory effects of RNm activity are limb specific. Several studies have reported that olivary neurons have reduced sensitivity to peripheral stimulation during movement; it is likely that RNm-mediated inhibition occurring within the olive contributes to this reduction of sensitivity. Inhibition of rDAO responses by descending motor pathways appears to be a salient feature of olivary function.

The nature of reinforcement in cerebellar learning

In a now classic study, W. J. Brogden and W. H. Gantt (1942, American Journal of Physiology, 119, 277-278) demonstrated that movements (limbs, head, eyelid) elicited by direct electrical stimulation of certain regions of the cerebellum (particularly the ansiform lobe) could be trained to respond to neutral auditory or visual conditioned stimuli with appropriate pairing. In recent work we have replicated these results in detail and presented considerable evidence that the reinforcing or "teaching" pathway so activated for the learning of discrete movements is the inferior olive-climbing fiber projection system to the cerebellum. Very strong evidence now indicates that the memory traces for this "skilled response" learning are formed and stored in the cerebellum. The climbing fiber system and inhibitory pathway from the interpositus nucleus to the inferior olive appear to form a neural instantiation of the Resorla-Wagner formulation of classical conditioning and accounts for the "cognitive" phenomenon of blocking. It is concluded that reinforcement in this form of learning is not due simply to contiguity/contingency or to unconditioned stimulus aversiveness, per se, but rather to activation of a particular brain circuit, here the climbing fiber system, a circumstance that may apply to other forms of learning, with other reinforcement circuits, as well.

staggerer phenotype in retinoid-related orphan receptor alpha-deficient mice

Retinoid-related orphan receptor alpha (RORalpha) is a member of the nuclear receptor superfamily. To study its physiological role we generated null-mutant mice by targeted insertion of a lacZ reporter gene encoding the enzyme beta-galactosidase. In heterozygous RORalpha+/- mice we found beta-galactosidase activity, indicative of RORalpha protein expression, confined to the central nervous system, skin and testis. In the central nervous system, the RORalpha gene is expressed in cerebellar Purkinje cells, the thalamus, the suprachiasmatic nuclei, and retinal ganglion cells. In skin, RORalpha is strongly expressed in the hair follicle, the epidermis, and the sebaceous gland. Finally, the peritubular cells of the testis and the epithelial cells of the epididymis also strongly express RORalpha. Recently, it was reported that the ataxic mouse mutant staggerer (sg/sg) is caused by a deletion in the RORalpha gene. The analysis of the cerebellar and the behavioral phenotype of homozygous RORalpha-/- mice proves identity to sg/sg mice. Although the absence of RORalpha causes dramatic developmental effects in the cerebellum, it has no apparent morphological effect on thalamus, hypothalamus, and retina. Similarly, testis and skin of RORalpha-/- mice display a normal phenotype. However, the pelage hair of both sg/sg and RORalpha-/- is significantly less dense and when shaved shows reluctance to regrow.

A mathematical model of the cerebellar-olivary system I: self-regulating equilibrium of climbing fiber activity

We use a mathematical model to investigate how climbing fiber-dependent plasticity at granule cell to Purkinje cell (gr-->Pkj) synapses in the cerebellar cortex is influenced by the synaptic organization of the cerebellar-olivary system. Based on empirical studies, gr-->Pkj synapses are assumed to decrease in strength when active during a climbing fiber input (LTD) and increase in strength when active without a climbing fiber input (LTP). Results suggest that the inhibition of climbing fibers by cerebellar output combines with LTD/P to self-regulate spontaneous climbing fiber activity to an equilibrium level at which LTP and LTD balance and the expected net change in gr-->Pkj synaptic weights is zero. The synaptic weight vector is asymptotically confined to an equilibrium hyperplane defining the set of all possible combinations of synaptic weights consistent with climbing fiber equilibrium. Results also suggest restrictions on LTP/D at gr-->Pkj synapses required to produce synaptic weights that do not drift spontaneously.

Estrous hormones enhance coupled, rhythmic olivary discharge in correlation with facilitated limb stepping

Synchronized oscillatory behaviour of neurons within the dorsal accessory olive of the rat were monitored across spontaneous changes in rapid, co-ordinated limb movements associated with the estrous (hormone) and circadian cycles as well as following local administration of estradiol. Facilitation of rapid alterations in limb movement is observed following increases in circulating estradiol and progesterone on the night of behavioural estrus, as assessed by the ability of a rat to maintain a consistent position on a treadmill in response to variable changes in acceleration. Synchronized olivary activity was determined using chronically implanted bundles of microwires (50 microns diameter) to record from as many as 23 individual olivary neurons simultaneously across several four to five day estrous cycles, and in some cases after injection of the estrous hormones, estradiol and/or progesterone, either by systemic or local administration. Rats were tested during non-movement, constant speed locomotion or using a variable acceleration paradigm. Olivary oscillations were most commonly observed during treadmill locomotion with variable changes in acceleration. Local administration of estradiol, in combination with systemic progesterone administration, enhanced rhythmic olivary oscillations during this paradigm. At this time, at least a six-fold increase in the size of the coupled cluster of rhythmically discharging olivary neurons was observed compared with values obtained on diestrus, a low hormone state. Similar facilitating effects on olivary oscillations were observed on estrus, or following systemic injection of both estrous hormones. Administration of either steroid alone was not as effective. During the dark phase of the light:dark cycle, the oscillatory behaviour of these neurons was enhanced more than five-fold than during the light phase, and coupling diameter was maximal on the night of behavioural estrus. The synchronized oscillatory discharge of neurons within the olive is a putative timing mechanism which may underlie hormone-associated facilitation of rapid limb movements. The results from the present study provide evidence that both hormonal and circadian factors can enhance olivary rhythmicity in association with behavioural rhythmicity.

A model of long-term memory storage in the cerebellar cortex: a possible role for plasticity at parallel fiber synapses onto stellate/basket interneurons

By evoking changes in climbing fiber activity, movement errors are thought to modify synapses from parallel fibers onto Purkinje cells (pf*Pkj) so as to improve subsequent motor performance. Theoretical arguments suggest there is an intrinsic tradeoff, however, between motor adaptation and long-term storage. Assuming a baseline rate of motor errors is always present, then repeated performance of any learned movement will generate a series of climbing fiber-mediated corrections. By reshuffling the synaptic weights responsible for any given movement, such corrections will degrade the memories for other learned movements stored in overlapping sets of synapses. The present paper shows that long-term storage can be accomplished by a second site of plasticity at synapses from parallel fibers onto stellate/basket interneurons (pf*St/Bk). Plasticity at pf*St/Bk synapses can be insulated from ongoing fluctuations in climbing fiber activity by assuming that changes in pf*St/Bk synapses occur only after changes in pf*Pkj synapses have built up to a threshold level. Although climbing fiber-dependent plasticity at pf*Pkj synapses allows for the exploration of novel motor strategies in response to changing environmental conditions, plasticity at pf*St/Bk synapses transfers successful strategies to stable long-term storage. To quantify this hypothesis, both sites of plasticity are incorporated into a dynamical model of the cerebellar cortex and its interactions with the inferior olive. When used to simulate idealized motor conditioning trials, the model predicts that plasticity develops first at pf*Pkj synapses, but with additional training is transferred to pf*St/Bk synapses for long-term storage.

Projection from the cerebellar lateral nucleus to precerebellar nuclei in the mossy fiber pathway is glutamatergic: a study combining anterograde tracing with immunogold labeling in the rat

The pontine nuclei (PN) and the nucleus reticularis tegmenti pontis (NRTP) are sources of an excitatory projection to the cerebellar cortex via mossy fibers and a direct excitatory projection to the cerebellar nuclei. These precerebellar nuclei, in turn, receive a feedback projection from the cerebellar nuclei, which mostly originate in the lateral nucleus (LN). It has been suggested that the feedback projection from the LN partially uses gamma-aminobutyric acid (GABA) as a transmitter. We tested this hypothesis by using a combination of anterograde tracing (biotinylated dextran amine injection into the LN) and postembedding GABA and glutamate immunogold histochemistry. The pattern of labeling in the PN and the NRTP was compared with that of cerebellonuclear terminals in two other target structures, the parvocellular part of the nucleus ruber (RNp) and the ventromedial and ventrolateral thalamus (VM/VL). The projection to the inferior olive (IO), which is known to be predominantly GABAergic, served as a control. A quantitative analysis of the synaptic terminals labeled by the tracer within the PN, the NRTP, and the VL/VM revealed no GABA immunoreactivity. Only one clearly labeled terminal was found in the RNp. In contrast, 72% of the terminals in the IO were clearly GABA immunoreactive, confirming the reliability of our staining protocol. Correspondingly, glutamate immunohistochemistry labeled the majority of the cerebellonuclear terminals in the PN (88%), the NRTP (90%), the RNp (93%), and the VM/VL (63%) but labeled only 5% in the IO. These data do not support a role for GABAergic inhibition either in the feedback systems from the LN to the PN and the NRTP or within the projections to the RNp and the VM/VL.

A model of Pavlovian eyelid conditioning based on the synaptic organization of the cerebellum

We present a model based on the synaptic and cellular organization of the cerebellum to derive a diverse range of phenomena observed in Pavlovian eyelid conditioning. These phenomena are addressed in terms of critical pathways and network properties, as well as the sites and rules for synaptic plasticity. The theory is based on four primary hypotheses: (1) Two cerebellar sites of plasticity are involved in conditioning: (a) bidirectional long-term depression/potentiation at granule cell synapses onto Purkinje cells (gr-->Pkj) in the cerebellar cortex and (b) bidirectional plasticity in the interpositus nucleus that is controlled by inhibitory inputs from Purkinje cells; (2) climbing fiber activity is regulated to an equilibrium level at which the net strength of gr-->Pkj synapses remains constant unless an unexpected unconditioned stimulus (US) is presented or an expected US is omitted; (3) a time-varying representation of the conditioned stimulus (CS) in the cerebellar cortex permits the temporal discrimination required for conditioned response timing; and (4) the ability of a particular segment of the CS to be represented consistently across trials varies as a function of time since CS onset. This variation in across-trials consistency is thought to contribute to the ISI function. The model suggests several empirically testable predictions, some of which have been tested recently.

Association between dendritic lamellar bodies and complex spike synchrony in the olivocerebellar system

Dendritic lamellar bodies have been reported to be associated with dendrodendritic gap junctions. In the present study we investigated this association at both the morphological and electrophysiological level in the olivocerebellar system. Because cerebellar GABAergic terminals are apposed to olivary dendrites coupled by gap junctions, and because lesions of cerebellar nuclei influence the coupling between neurons in the inferior olive, we postulated that if lamellar bodies and gap junctions are related, then the densities of both structures will change together when the cerebellar input is removed. Lesions of the cerebellar nuclei in rats and rabbits resulted in a reduction of the density of lamellar bodies, the number of lamellae per lamellar body, and the density of gap junctions in the inferior olive, whereas the number of olivary neurons was not significantly reduced. The association between lamellar bodies and electrotonic coupling was evaluated electrophysiologically in alert rabbits by comparing the occurrence of complex spike synchrony in different Purkinje cell zones of the flocculus that receive their climbing fibers from olivary subnuclei with different densities of lamellar bodies. The complex spike synchrony of Purkinje cell pairs, that receive their climbing fibers from an olivary subnucleus with a high density of lamellar bodies, was significantly higher than that of Purkinje cells, that receive their climbing fibers from a subnucleus with a low density of lamellar bodies. To investigate whether the complex spike synchrony is related to a possible synchrony between simple spikes, we recorded simultaneously the complex spike and simple spike responses of Purkinje cell pairs during natural visual stimulation. Synchronous simple spike responses did occur, and this synchrony tended to increase as the synchrony between the complex spikes increased. This relation raises the possibility that synchronously activated climbing fibers evoke their effects in part via the simple spike response of Purkinje cells. The present results indicate that dendritic lamellar bodies and dendrodendritic gap junctions can be downregulated concomitantly, and that the density of lamellar bodies in different olivary subdivisions is correlated with the degree of synchrony of their climbing fiber activity. Therefore these data support the hypothesis that dendritic lamellar bodies can be associated with dendrodendritic gap junctions. Considering that the density of dedritic lamellar bodies in the inferior olive is higher than in any other area of the brain, this conclusion implies that electrotonic coupling is important for the function of the olivocerebellar system.

Cerebellar nuclei: the olivary connection

This chapter gives an overview of the relation between the inferior olive and the cerebellar nuclei based on tracing and electrophysiological experiments in rats and cats. The olivary and cerebellar nuclear masses appear to maintain a precise topographical relationship. The olivary projection to the cerebellar nuclei is strictly contralateral, originates as climbing fiber collaterals, and is excitatory. The cerebellar projection to the inferior olive is exactly reciprocal to the olivonuclear projection, but, in addition, also has a variable but definite ipsilateral component, and has been demonstrated to be GABAergic. Stimulation and lesion experiments carried out in cat imply that the cerebellar nucleo-olivary pathway may be involved in regulating the oscillatory capabilities of olivary cells and as such may also have an effect on the synchrony of olivary firing. Subsequent stimulation experiments performed in rats suggests that synchronous firing of olivary neurons may result in profound effects in cerebellar nuclear neurons.

Signal processing in the C2 module of the flocculus and its role in head movement control

The major novel findings described and reviewed in the present study have all been demonstrated in the C2 module, which is formed by the rostral medial accessory olive, posterior interposed nucleus of the cerebellum, and zone C2. We show (1) that expression of dendritic lamellar bodies and dendrodendritic gap junctions in the rostral medial accessory olive are both down regulated by removal of the GABAergic input from the posterior interposed nucleus of the cerebellum to electrotonically coupled olivary dendrites; (2) that the high density of dendritic lamellar bodies in the rostral medial accessory olive can be correlated with a relatively high level of CS synchrony in the C2 zone of the flocculus; and (3) that the C2 zone of the flocculus is involved in head movements and probably gaze control. These results support the hypothesis that dendritic lamellar bodies are associated with dendrodentritic gap junctions, and they suggest that appropriate executions of compensatory head and eye movements require particular levels of complex spike synchrony in the flocculus.

The compartmentalization of the cerebellum

The concept of developmental compartments originated in studies of Drosophila embryogenesis. This review examines the hypothesis that the modular structure of the vertebrate cerebellum is strongly analogous to this earlier scheme. The pattern of cerebellar development, the adult circuitry, a variety of molecular markers expressed in specific subdivisions, and the phenotypes of several neurological mutations all provide abundant evidence that the vertebrate cerebellum is organized into modules. We present the case that, as a group, these markers reveal distinct boundaries that partition the cerebellum into true developmental compartments. Although this reductionist viewpoint advances our understanding of cerebellar organization, the relationship between these compartments and the functional behavior of the cerebellum remains a mystery.

Morphological correlates of bilateral synchrony in the rat cerebellar cortex

Simultaneous recordings of the left and right crus IIA of the cerebellar cortex in the rat have demonstrated that Purkinje cells of both sides can be activated synchronously by their climbing fibers. Because climbing fibers arise exclusively from the contralateral inferior olive (IO), this physiological finding seems to contradict the anatomy. To define the structural basis responsible for the bilateral synchrony, we examined the possibilities that bilateral common afferent inputs to the IO and interolivary connections form the underlying mechanisms. The bilaterality of the major afferents of the olivary regions that project to crus IIA was studied using Phaseolus vulgaris leucoagglutinin as an anterograde tracer. We found that the excitatory and inhibitory projections from the spinal trigeminal nucleus and dorsolateral hump of the interposed cerebellar nucleus to the transition area between the principal olive and dorsal accessory olive were bilateral. A second possible mechanism for bilateral synchrony, which is the possibility that axons of olivary neurons provide collaterals to the contralateral side, was investigated using biotinylated dextran amine as an anterograde tracer. Labeled axons were traced and reconstructed from the principal olive and dorsal and medial accessory olive up to the entrance of the contralateral restiform body. None of these axons gave rise to collaterals. The possibility that neurons in the left and right IO are electronically coupled via dendrodendritic connections was investigated by examining the midline region of the IO. The neuropil of the left and right IO is continuous in the dorsomedial cell column. Examination of Golgi impregnations of this subdivision demonstrated that (1) many dendrites cross from one side to the other, (2) neurons close to the midline give rise to dendrites that extend into both olives, and (3) dendrites of neurons in the dorsomedial cell column frequently traverse into adjacent olivary subdivisions such as the medial accessory olive and the transition area between the principal olive and dorsal accessory olive. Sections immunostained for dendritic lamellar bodies or GABAergic terminals showed the same pattern: the neuropils of the dorsomedial cell columns on both sides form a continuum with each other as well as with the neuropil of other adjacent olivary subdivisions. Ultrastructural examination of the dorsomedial cell column demonstrated that the midline area includes many complex glomeruli that contain dendritic spines linked by gap junctions. To verify whether the complex spike synchrony observed between left and right crus IIA could indeed be mediated in part through coupled neurons in the dorsomedial cell column, we recorded simultaneously from crus IIA areas and from left and right vermal lobule IX, which receives climbing fibers from the dorsomedial cell column. In these experiments we demonstrated that the climbing fibers of all four areas, i.e., the left and right crus IIA as well as the left and right lobule IX, can fire synchronously. The present results indicate that synchronous climbing fiber activation of the left and right crus IIA in the rat can be explained by (1) bilateral inputs to the transition areas between the principal olive and dorsal accessory olive and (2) dendrodendritic electrotonic coupling between neurons of the left and right dorsomedial cell column and between neurons of the dorsomedial cell column and adjacent olivary subdivisions.

The effect of ethanol on alcohol-responsive essential tremor: a positron emission tomography study

We used H2 15O positron emission tomography (PET) to investigate the effect of ethyl alcohol on regional cerebral blood flow in 6 patients with alcohol-responsive essential tremor and 6 age-matched control subjects. The patients were scanned while at rest and during involuntary postural tremor of the extended right arm. Normal control subjects were scanned at rest and during passive wrist oscillation of the right arm at tremor frequency. Regional cerebral blood flow associated with these conditions was measured before and after oral administration of 2 to 3 units of alcohol. The mean blood alcohol level was 35.3 +/- 20.0 mg/dl in the patient group and caused marked suppression of tremor; it was 33.9 +/- 12.9 mg/dl in the control group. Similar to previous PET studies on essential tremor patients, tremor compared with rest was associated with bilateral cerebellar activation including the cerebellar vermis. This pattern of activation differed from passive wrist oscillation where ipsilateral cerebellar activation was observed. Ethanol ingestion led to bilateral decreases of cerebellar blood flow in both tremor patients and normal subjects, and this was associated with suppression of tremor in the patients. Alcohol-associated increases of regional cerebral blood flow were observed in the inferior olivary nuclei in the patients but not in the control subjects. We conclude that alcohol-induced suppression of essential tremor is mediated via a reduction of cerebellar synaptic overactivity resulting in increased afferent input to the inferior olivary nuclei.

Enkephalin-immunoreactive fastigial neurons in the rat cerebellum project to upper cervical cord segments

By using enkephalin immunohistochemistry combined with retrograde fluorescent labelling, a great majority of neurons in the rat cerebellum sending their axons to the spinal cord were shown to contain enkephalin immunoreactivity. These neurons were numerous and clustered in the fastigial nucleus but far less abundant in other cerebellar nuclei. Enkephalin-immunoreactive fibers present in the ventral horn and the central cervical nucleus of upper cervical cord segments almost completely disappeared contralaterally following kainic acid-induced cell loss in the fastigial nucleus. The results indicate that fastigial and some other cerebellar nucleus neurons provide enkephalin-containing projections toward these spinal sites.

Developmental changes in the expression of gamma-aminobutyric acidA/benzodiazepine receptor subunit mRNAs in the murine inferior olivary complex

The pharmacological and physiological properties of ligand-gated ion channels are dependent on their subunit composition; spontaneously occurring changes in subunit composition during neuronal development may result in dramatic functional differences between embryonic and adult forms of the receptor complex. In the present study, in situ hybridization with antisense cRNA probes was used to examine the subunit composition of the gamma-aminobutyric acidA/benzodiazepine (GABAA/BZ) receptor in the developing inferior olivary complex. This receptor is thought to be a pentameric chloride channel comprised of selected alpha, beta, gamma, delta, and rho subunits, the majority of which have several isoforms: alpha 1-6, beta 1-4, gamma 1-4, and rho 1,2. Among the 13 subunit variants present in the mammalian central nervous system, alpha 2-5, beta 3, and gamma 1,2 mRNAs are expressed at significant levels in the inferior olivary complex. Two clearly different temporal patterns of GABAA/BZ receptor subunit mRNA expression were observed: The expression of alpha 3, alpha 5, beta 3, and gamma 2 mRNAs was at a peak during embryonic and early postnatal development followed by rapid down-regulation thereafter. Conversely, alpha 2, alpha 4, and gamma 1 mRNA expression was very low or absent during early development, and a pronounced increase was observed at the end of postnatal week 1. These studies suggest that there are developmental changes in the subunit composition of the GABAA/BZ receptor in inferior olivary neurons. These changes in subunit expression, which occur during a period of major alterations in afferent and efferent synaptic connections, may subserve a change in the role of GABA from its function as a neurotrophic factor to that of an inhibitory neurotransmitter.

GABA immunoreactivity in the nucleus isthmo-opticus of the centrifugal visual system in the pigeon: a light and electron microscopic study

The present study examined GABA immunoreactivity within the retinopetal nucleus isthmo-opticus (NIO) of the pigeon centrifugal visual system (CVS) using light- (immunohistofluorescence, peroxidase anti-peroxidase: PAP) and electron- (postembedding GABA immunogold) microscopic techniques. In some double-labeling experiments, the retrograde transport of the fluorescent dye rhodamine beta-isothiocyanate (RITC) after its intraocular injection was combined with GABA immunohistofluorescence. GABA-immunoreactive (-ir) somata were demonstrated within the neuropilar zone of the NIO adjacent to the centrifugal cell laminae whereas the centrifugal neurons were always immunonegative. A quantitative ultrastructural analysis was performed which distinguished five categories of axon terminal profiles (P1-5) on the basis of various cytological criteria: type of synaptic contact (symmetrical or asymmetrical); shape, size, and density of synaptic vesicles as well as the immunolabeling (positive or negative), size of profile and appearance of hyaloplasm. Numerous GABA-ir afferents to centrifugal neurons via axon terminal types P2a, P2c, and P3 were observed which comprised 47.1% of the total input. Moreover, the data suggest that some of the P2a terminals, which make up 26.4% of the input, stem from the intrinsic GABA-ir interneurons, whereas the latter receive P1, P3, but also P2 terminal input, indicating that interneurons may contact other interneurons via type P2a axon terminals. The results also suggest that the GABA-ir P3 or the immunonegative P1b and P5 axon terminals are of extrinsic origin arising from cells in the optic tectum whereas the P2c and P4 axon terminals are associated with extra-tectal input to the NIO. The GABAergic innervation of centrifugal neurons within the NIO may be the basis for the demonstrated facilitatory effect of the centrifugal output upon ganglion cell responses. This is relevant to hypotheses regarding CVS involvement in attentional mechanisms through selective enhancement of retinal sensitivity depending on the location of meaningful or novel stimuli.

Cerebellar influence on olivary excitability in the cat

This study examines the influence of the cerebellum on the excitability of inferior olivary neurons in the cat. Two major pathways from the cerebellar nuclei to the inferior olive have been investigated by electrophysiological and anatomical techniques. The first, excitatory pathway connects the cerebellar nuclei through nuclei at the mesodiencephalic junction with the inferior olive. The second is the direct, GABAergic, nucleo-olivary pathway. Intra- as well as extracellular recordings obtained in the rostral part of the medial accessory and principal olives revealed that electrical stimulation with a short burst of three pulses delivered at the mesodiencephalic junction results in short-latency activation (4-8 ms) of most olivary neurons. More than half of the units showed, in addition to the short-latency activation, a consistent response with a much longer latency (approximately 180 ms). Many units (66%) that responded to mesodiencephalic stimulation could also be activated by superior cerebellar peduncle stimulation with a similar stimulation paradigm (latency 9-15 ms). However, in such cases consistent long-latency responses were only rarely recorded (7%). To distinguish between the effect of the two pathways, both of which are activated by superior cerebellar peduncle stimulation, an electrolytic lesion of the nucleo-olivary fibres was made in the brainstem in six experiments. The effect of this lesion was verified in three cases by retrograde horseradish peroxidase tracing from the rostral inferior olive at the end of the experiment. This time only extracellular recordings were made. Stimulation of the mesodiencephalic junction still resulted in easily activated olivary units which showed an increased probability of firing a long-latency action potential. Stimulation of the superior cerebellar peduncle now resulted in a 50% decrease in probability of activating olivary units in the short-latency range. However, a five-fold increase in the chance of triggering action potentials in the long-latency interval was noted, implying that many units reacted only with a long-latency action potential. The results obtained with our experimental paradigm appear enigmatic since it is well established that the nucleo-olivary pathway is GABAergic and thus, by convention, should be inhibitory to the olivary neurons. However, it is possible to explain these results in terms of dynamic coupling of olivary neurons. This concept ascribes an important role to the nucleo-olivary pathway in regulating the degree of electrotonic coupling between olivary neurons (probably by a shunting mechanism) and as such may be an important instrument in the regulation of synchronous and rhythmic olivary discharges.(ABSTRACT TRUNCATED AT 400 WORDS)

GABAergic innervation of rat abducens motoneurons retrogradely labelled with HRP: quantitative ultrastuctural analysis of cell bodies and proximal dendrites

In this quantitative electron microscopic study we investigated the distribution of GABA axon terminals on rat abducens motoneurons by combining retrograde labelling of montoneurons with post-embedding immunodetection of GABA. We analysed the synapses on 13 cell bodies and 60 proximal dendritic profiles distributed along the entire rostro-caudal extent of the nucleus. For each of these two compartments, we analysed 1754 and 1176 axon terminals in contact with 6042 and 3299 microns of postsynaptic membrane. The axon terminals were classified as Sv-type (containing spherical vesicles) or Pv-type (containing pleomorphic vesicles). The GABAergic terminals contained pleomorphic vesicles and established mainly symmetrical synaptic contacts. Their apposition lengths were greater than those of unlabelled terminals. On cell bodies, the percentage of GABAergic synaptic covering varied from 2.5% to 14.1% and the synaptic frequency of GABAergic axon terminals varied from 0.6% to 8.9%. These two parameters were significantly correlated with the diameter of the motoneurons. The percentage of synaptic covering and synaptic frequency were smaller on dendrites of small motoneurons than on those of large ones. The proximal dendrites of small motoneurons had a lesser GABAergic innervation than large ones. The total synaptic covering and frequency were smaller on somata than on dendrites. However, the percentage of synaptic covering by GABA terminals was higher on cell bodies than on proximal dendrites.

Projections of individual Purkinje cells of identified zones in the flocculus to the vestibular and cerebellar nuclei in the rabbit

The rabbit flocculus can be divided into five zones (zones 1, 2, 3, 4, and C2) with the use of acetylcholinesterase histochemistry. The projections of individual Purkinje cells in these zones to the vestibular and cerebellar nuclei were studied by using biocytin as an anterograde tracer. The zones were physiologically identified in terms of the Purkinje cell complex spike modulation occurring in response to optokinetic stimulation. In zones 1 and 3 neurons respond best to rotation about a horizontal axis that is close to perpendicular to the ipsilateral anterior semicircular canal, whereas in zones 2 and 4 neurons respond best to rotation about the vertical axis. Complex spike activity in zone C2 is unresponsive to optokinetic stimulation. Collectively, Purkinje cells of zone 1 projected to the ventral dentate nucleus, dorsal group y, and superior vestibular nucleus; Purkinje cells of zones 2 and 4 projected to the magnocellular and parvicellular parts of the medial vestibular nucleus; Purkinje cells of zone 3 projected to dorsal group y, ventral group y, and the superior vestibular nucleus; and Purkinje cells of zone C2 projected to the interposed posterior nucleus and dorsal group y. Some of the labeled Purkinje cell axons branched and innervated two nuclei. Branching axons from zone 1 either innervated both the ventral dentate nucleus and the superior vestibular nucleus or both dorsal group y and the superior vestibular nucleus. Branching axons from zones 2 and 4 innervated both the magnocellular and the parvicellular parts of the medial vestibular nucleus. Branching axons from zone 3 innervated both dorsal group y and the superior vestibular nucleus, or both ventral group y and the superior vestibular nucleus. Branching axons from zone C2 innervated both the interposed posterior nucleus and dorsal group y. Some of the target nuclei of the floccular Purkinje cell axons (e.g., dorsal group y and interposed posterior nucleus) project to the part of the inferior olive that, in turn, projects to the corresponding floccular zone, thus completing a closed pathway consisting of the inferior olive, the cerebellar cortex, and the cerebellar and vestibular nuclei. Other target nuclei (e.g., superior vestibular nucleus and medial vestibular nucleus) do not project back to the olivary subnuclei that innervate the flocculus and are part of an open olivofloccular pathway. An individual Purkinje cell thus can innervate a nucleus in the closed pathway as well as a nucleus in the open pathway.(ABSTRACT TRUNCATED AT 400 WORDS)

Olivary projecting neurons in the nucleus of Darkschewitsch in the cat receive excitatory monosynaptic input from the cerebellar nuclei

The direct projection from the cerebellar nuclei to the inferior olive is GABAergic. In the present study, we investigated the projection from the cerebellar nuclei to the mesodiencephalic junction which is known to provide an excitatory projection to the inferior olive. The mesodiencephalic junction was studied in cat following anterograde transport of wheatgerm agglutinated horseradish peroxidase from the cerebellar nuclei in combination with: (1) retrograde transport of gold-lectin conjugate from the inferior olive; and (2) postembedding GABA-immunocytochemistry. Light microscopic analysis revealed that overlap of the anterograde and retrograde labeling was especially prominent in the nucleus of Darkschewitsch. Electron microscopic examination of this area showed: (1) that many cerebellar terminals made synaptic contacts with neurons that project to the inferior olive; (2) that virtually all cerebellar terminals were non-GABAergic and displayed an excitatory morphology; and (3) olivary projecting neurons were non-GABAergic. It is concluded that the indirect cerebellar projection to the inferior olive via the nucleus of Darkschewitsch is disynaptic and excitatory.

The rostral dorsal cap and ventrolateral outgrowth of the rabbit inferior olive receive a GABAergic input from dorsal group Y and the ventral dentate nucleus

The dorsal cap and ventrolateral outgrowth of the inferior olive are involved in the control of eye movements. The caudal dorsal cap is predominantly involved in the horizontal optokinetic reflex; it receives most of its GABAergic input from the nucleus prepositus hypoglossi. In the present study, we determined the source of a major inhibitory input to the rostral dorsal cap and the ventrolateral outgrowth, which are the olivary subnuclei mainly involved in the "vertical" optokinetic reflexes. We studied these subnuclei in the rabbit with the use of retrograde tracing of horseradish peroxidase and anterograde tracing of wheat germ agglutinin-coupled horseradish peroxidase combined with postembedding immunocytochemistry. The ventral dentate nucleus of the cerebellum and dorsal group y project contralaterally to the rostral dorsal cap and ventrolateral outgrowth; this projection is entirely GABAergic. The terminals of this input form predominantly symmetric synapses with extraglomerular and intraglomerular dendrites; the remaining terminals are axosomatic. In addition, the dorsal cap and ventrolateral outgrowth contain significantly more crest synapses than any other olivary subnucleus. The terminals that form these crest synapses are derived from dorsal group y and/or the ventral dentate nucleus. None of the terminals in the dorsal cap or ventrolateral outgrowth was glycinergic.

Fine structure of the dorsal cap of the inferior olive and its GABAergic and non-GABAergic input from the nucleus prepositus hypoglossi in rat and rabbit

The dorsal cap of the inferior olive is involved in the control of eye movements and is excited by inputs from the midbrain. In the present study we attempted to determine the inhibitory input to this nucleus in rat and rabbit. The projection from the nucleus prepositus hypoglossi to the dorsal cap was studied in the light microscope by anterograde tracing of Phaseolus vulgaris-leucoagglutinin and lesion-induced depletion of glutamic acid decarboxylase immunoreactivity, and in the electron microscope by anterograde tracing of wheat germ agglutinin-coupled horseradish peroxidase combined with GABA immunocytochemistry. We show that the nucleus prepositus hypoglossi projects bilaterally to the dorsal cap, contralaterally to the ventrolateral outgrowth, and ipsilaterally to the medial accessory olive. After lesioning of the nucleus prepositus hypoglossi, the caudal dorsal cap was depleted of most of its glutamic acid decarboxylase-immunoreactive terminals while the rostral dorsal cap and the ventrolateral outgrowth were depleted of a minor part. Ultrastructural analysis indicates that the majority, but not all, of the terminals from the nucleus prepositus hypoglossi in the dorsal cap are GABA-positive. These GABA-positive and GABA-negative terminals form predominantly symmetric and asymmetric synapses; most of them synapse on dendrites outside and inside glomeruli, frequently in association with dendrodendritic gap junctions, while a small minority are axosomatic. None of the terminals from the nucleus prepositus hypoglossi was found to form a crest synapse, although synapses of this kind were predominantly formed by GABAergic terminals. This study shows that the dorsal cap receives a major inhibitory input from the nucleus prepositus hypoglossi, the terminals of which are located at strategic positions on the olivary neurons.

GABAergic neurons in the mammalian inferior olive and ventral medulla detected by glutamate decarboxylase immunocytochemistry

Neurons containing glutamic acid decarboxylase (GAD) (presumed GABAergic neurons) were mapped by immunocytochemistry in the ventral medulla of rat, rabbit, cat, rhesus monkey, and human, with emphasis on the inferior olive. In all species, three categories of GABAergic neurons were identified: periolivary neurons in the gray matter and the white matter surrounding the inferior olive, internuclear neurons located in the white matter between the subnuclei of the inferior olive, and intranuclear neurons located within the olivary gray matter. The intranuclear GABAergic neurons of the inferior olive had a characteristic morphology which differed from non-GABAergic olivary neurons; they were usually smaller, and, wherever their processes were stained, they had radiating, sparsely branching dendrites. They were also usually distinguished from the other GABAergic neurons by their smaller size. The intraolivary GABAergic neurons constituted only a minor proportion of the total olivary neuronal population, but they were concentrated in regions of the olive that varied by species. In the rat, they were situated in the rostral tip of the medial accessory olive and in the caudal subdivision of the dorsal accessory olive, while in the rabbit, they were located in the caudal two-thirds of the medial accessory olive, in the dorsal cap, and in the ventral lateral outgrowth. Such neurons were extremely rare in the cat; only a few were found in the rostral parts of the principal olive, the medial accessory olive, and the dorsal accessory olive. In the rhesus monkey, the principal olive and the lateral region of the rostral medial accessory olive contained most of the intranuclear GABAergic neurons, but some were also present in the dorsal accessory olive. In the human, such neurons occurred in the principal olive, the dorsal accessory olive and the rostral medial accessory olive, but as in the rhesus monkey, most were observed in the principal olive.

The tectorecipient zone in the inferior olivary nucleus in the rat

Tecto-olivary and olivocerebellar projections in the rat were investigated in order to identify the tectorecipient zone in the inferior olivary nucleus and to determine whether inferior olivary neurons projecting to the cerebellar tecto-olivo-recipient zones (lobule VII, crus II, lobulus simplex, and paramedian lobule) originate in the different regions within the tectorecipient zone. An electrophysiological method and an axonal transport technique of wheat-germ agglutinin-conjugated horseradish peroxidase were used. The tectorecipient zone was identified in the caudomedial region of the medial accessory olive. Neurons projecting to lobule VII originated in the caudomedial region of the tectorecipient zone, but those to crus II, lobulus simplex, and paramedian lobule originated in its rostrolateral region. These observations suggest that there are two independent tecto-olivo-cerebellar systems: 1) superior colliculus--the medial region of the tectorecipient zone--lobule VII--the caudomedial region of the fastigial nucleus; and 2) superior colliculus--the rostralateral region of the tectorecipient region--crus II, lobulus simplex, and paramedian lobule--the dorsolateral protuberance of the fastigial nucleus.