Responses in the inferior olive to stimulation of the cerebellar and cerebral cortices in the cat

Olivopontocerebellar degeneration associated with 3-hydroxy-3-methylglutaric aciduria in a domestic shorthair cat

CASE SUMMARY

A rescue charity-owned 6-month-old neutered female domestic shorthair cat was presented with progressive tetraparesis, increased extensor muscle tone and signs of spinocerebellar ataxia, including hypermetria. The cat's male sibling, with similar progressive neurological signs, had been euthanased 2 months previously. An inherited metabolic disorder was suspected. Urine for determination of organic acid concentration was obtained and the cat was prescribed carnitine and taurine supplementation. The cat was euthanased 3 months later following progressive neurological signs, including ataxia, tetraparesis, tendency to fall, bilateral absent menace response and intention tremor. A selective post-mortem examination was obtained, taking samples from the brain, cervical spinal cord, tibial branch of the sciatic nerve, muscle, liver and kidneys. Organic acid analysis results received after euthanasia revealed a marked elevation of 3-hydroxy-3-methylglutaric acid (45 mmol/mol creatine [normal range 0-2]) and isovalerylglycine (27 mmol/mol creatinine [normal range 0-2]). 3-Hydroxy-3-methylglutaric acid was deemed clinically relevant as it is a metabolite of 3-hydroxy-3-methylglutaryl-CoA lyase, the enzyme involved in the final step of leucine degradation. Post-mortem examination revealed diffuse, chronic-active, severe olivoponto-(spino)-cerebellar degeneration.

RELEVANCE AND NOVEL INFORMATION

This is the first report of 3-hydroxy-3-methylglutaric aciduria in the veterinary literature and the first description of the neuropathology of this disorder in any species. 3-Hydroxy-3-methylglutaric aciduria in humans occurs rarely and is due to a deficiency in 3-hydroxy-3-methylglutaryl-coenzyme A lyase.

Organization of Excitatory Inputs from the Cerebral Cortex to the Cerebellar Dentate Nucleus

Intracellular recording was made from dentate nucleus neurons (DNNs) in anesthetized cats, to investigate cerebral inputs to DNNs and their responsible pathways. Stimulation of the medial portion of the contralateral pericruciate cortex most effectively produced EPSPs followed by long-lasting IPSPs in DNNs. Stimulation of the pontine nucleus (PN), the nucleus reticularis tegmenti pontis (NRTP) and the inferior olive (IO) produced monosynaptic EPSPs and polysynaptic IPSPs in DNNs. The results indicate that the excitatory input from the cerebral cortex to DNNs is at least partly relayed via the PN, the NRTP and the 10. Intraaxonal injection of HRP visualized the morphology of mossy fibers from the PN to the DN and the cerebellar cortex. The functional significance of the excitatory inputs from the PN and the NRTP to the DN is discussed in relation to the motor control mechanisms of the cerebellum.

Modulation of Purkinje cell complex spike waveform by synchrony levels in the olivocerebellar system

Purkinje cells (PCs) generate complex spikes (CSs) when activated by the olivocerebellar system. Unlike most spikes, the CS waveform is highly variable, with the number, amplitude, and timing of the spikelets that comprise it varying with each occurrence. This variability suggests that CS waveform could be an important control parameter of olivocerebellar activity. The origin of this variation is not well known. Thus, we obtained extracellular recordings of CSs to investigate the possibility that the electrical coupling state of the inferior olive (IO) affects the CS waveform. Using multielectrode recordings from arrays of PCs we showed that the variance in the recording signal during the period when the spikelets occur is correlated with CS synchrony levels in local groups of PCs. The correlation was demonstrated under both ketamine and urethane, indicating that it is robust. Moreover, climbing fiber reflex evoked CSs showed an analogous positive correlation between spikelet-related variance and the number of cells that responded to a stimulus. Intra-IO injections of GABA-A receptor antagonists or the gap junction blocker carbenoxolone produced correlated changes in the variance and synchrony levels, indicating the presence of a causal relationship. Control experiments showed that changes in variance with synchrony were primarily due to changes in the CS waveform, as opposed to changes in the strength of field potentials from surrounding cells. Direct counts of spikelets showed that their number increased with synchronization of CS activity. In sum, these results provide evidence of a causal link between two of the distinguishing characteristics of the olivocerebellar system, its ability to generate synchronous activity and the waveform of the CS.

The olivo-cerebellar system as a neural clock

The cerebellum, and the olivo-cerebellar system in particular, may be the central mechanism of a neural clock that provides a rhythmic neural signal used to time motor and cognitive processes. Several independent lines of evidence support this hypothesis. First, the resting membrane potential of neurons in the inferior olive oscillates at ~10 Hz and the neural input from the olive leads to rhythmic complex spikes in cerebellum Purkinje cells. Second, the repeating modular microstructure of the cerebellum is ideally suited for performing computations underlying a basic neural process such as timing. Third, damage to the cerebellum leads to deficits in the perception of time and in the production of timed movements. Fourth, functional imaging studies in human subjects have shown activation of the inferior olive specifically during time perception. However, additional data on the exact role of rhythmic cerebellar activity during basis motor and sensory processing will be necessary before the hypothesis that the cerebellum is a neural clock is more widely accepted.

Cerebellar output in zebrafish: an analysis of spatial patterns and topography in eurydendroid cell projections

The cerebellum is a brain region responsible for motor coordination and for refining motor programs. While a great deal is known about the structure and connectivity of the mammalian cerebellum, fundamental questions regarding its function in behavior remain unanswered. Recently, the zebrafish has emerged as a useful model organism for cerebellar studies, owing in part to the similarity in cerebellar circuits between zebrafish and mammals. While the cell types composing their cerebellar cortical circuits are generally conserved with mammals, zebrafish lack deep cerebellar nuclei, and instead a majority of cerebellar output comes from a single type of neuron: the eurydendroid cell. To describe spatial patterns of cerebellar output in zebrafish, we have used genetic techniques to label and trace eurydendroid cells individually and en masse. We have found that cerebellar output targets the thalamus and optic tectum, and have confirmed the presence of pre-synaptic terminals from eurydendroid cells in these structures using a synaptically targeted GFP. By observing individual eurydendroid cells, we have shown that different medial-lateral regions of the cerebellum have eurydendroid cells projecting to different targets. Finally, we found topographic organization in the connectivity between the cerebellum and the optic tectum, where more medial eurydendroid cells project to the rostral tectum while lateral cells project to the caudal tectum. These findings indicate that there is spatial logic underpinning cerebellar output in zebrafish with likely implications for cerebellar function.

Interaction between Purkinje cells and inhibitory interneurons may create adjustable output waveforms to generate timed cerebellar output

We develop a new model that explains how the cerebellum may generate the timing in classical delay eyeblink conditioning. Recent studies show that both Purkinje cells (PCs) and inhibitory interneurons (INs) have parallel signal processing streams with two time scales: an AMPA receptor-mediated fast process and a metabotropic glutamate receptor (mGluR)-mediated slow process. Moreover, one consistent finding is an increased excitability of PC dendrites (in Larsell's lobule HVI) in animals when they acquire the classical delay eyeblink conditioning naturally, in contrast to in vitro studies, where learning involves long-term depression (LTD). Our model proposes that the delayed response comes from the slow dynamics of mGluR-mediated IP3 activation, and the ensuing calcium concentration change, and not from LTP/LTD. The conditioned stimulus (tone), arriving on the parallel fibers, triggers this slow activation in INs and PC spines. These excitatory (from PC spines) and inhibitory (from INs) signals then interact at the PC dendrites to generate variable waveforms of PC activation. When the unconditioned stimulus (puff), arriving on the climbing fibers, is coupled frequently with this slow activation the waveform is amplified (due to an increased excitability) and leads to a timed pause in the PC population. The disinhibition of deep cerebellar nuclei by this timed pause causes the delayed conditioned response. This suggested PC-IN interaction emphasizes a richer role of the INs in learning and also conforms to the recent evidence that mGluR in the cerebellar cortex may participate in slow motor execution. We show that the suggested mechanism can endow the cerebellar cortex with the versatility to learn almost any temporal pattern, in addition to those that arise in classical conditioning.

A novel site of synaptic relay for climbing fibre pathways relaying signals from the motor cortex to the cerebellar cortical C1 zone

The climbing fibre projection from the motor cortex to the cerebellar cortical C1 zone in the posterior lobe of the rat cerebellum was investigated using a combination of physiological, anatomical and neuropharmacological techniques. Electrical stimulation of the ipsilateral fore- or hindimbs or somatotopically corresponding parts of the contralateral motor cortex evoked climbing fibre field potentials at the same cerebellar recording sites. Forelimb-related responses were located in the C1 zone in the paramedian lobule or lobulus simplex and hindlimb-related responses were located in the C1 zone in the copula pyramidis. Microinjections of anterograde axonal tracer (Fluoro-Ruby or Fluoro-Emerald) were made into the fore- or hindlimb parts of the motor cortex where stimulation evoked the largest cerebellar responses. After a survival period of 7-10 days, the neuraxis was examined for anterograde labelling. No terminal labelling was ever found in the inferior olive, but labelled terminals were consistently found in a well-localized site in the dorso-medial medulla, ventral to the gracile nucleus, termed the matrix region. Pharmacological inactivation of the matrix region (2 mm caudal to the obex) selectively reduced transmission in descending (cerebro-olivocerebellar) but not ascending (spino-olivocerebellar) paths targeting fore- or hindlimb-receiving parts of the C1 zone. Transmission in spino-olivocerebellar paths was either unaffected, or in some cases increased. The identification of a novel pre-olivary relay in cerebro-olivocerebellar paths originating from fore- and hindlimb motor cortex has implications for the regulation of transmission in climbing fibre pathways during voluntary movements and motor learning.

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.

Removal of the inferior olive abolishes myoclonic seizures associated with a loss of olivary serotonin

Several lines of clinical evidence suggest that myoclonus is caused by a reduction of serotonin in the brain and hyperactivity of the inferior olive. We determined whether a change in serotonin content within the olivocerebellar system accompanied a predisposition to myoclonus and investigated the necessity of the inferior olive for a myoclonic seizure. The experiments employed the genetically epilepsy-prone rat that exhibits a profound myoclonic seizure in response to an auditory stimulus. We found that these animals demonstrated a significant reduction in the serotonergic innervation of the inferior olive without a significant change in the serotonergic innervation at any other level of the olivocerebellar circuit. The deficit in olivary serotonin was verified physiologically and pharmacologically by a reduced sensitivity of the genetically epilepsy-prone rat to the tremorogenic effect of harmaline, which is known to produce tremor through a mechanism that requires serotonergic innervation of the inferior olive. We quantified the timing of the myoclonic seizure of the genetically epilepsy-prone rat and found that its large amplitude 2-6 Hz clonus was always preceded by 9-10 Hz tremor that was synchronized among limbs. Ablation of the inferior olive by 3-acetylpyridine abolished the myoclonic seizure. The specificity of the deficit in olivary serotonin, the timing of the seizure, and the demonstration of the necessity of the inferior olive for myoclonus suggest that pathological inferior olivary activity contributes to the genesis of a myoclonic seizure.

Some organizing principles for the control of movement based on olivocerebellar physiology

Motor control is defined as the process of restricting the output of the motor nervous system so that meaningful and coordinated behavior ensues. The high dimensionality of the computation underlying motor control is presented and a simplifying framework is outlined. Evidence that movements are performed non-continuously is reviewed as is the construct of the 'motor synergy' as a fundamental unit of control. It is proposed that the pulsatile nature of movement and the tendency of muscle collectives to be activated as synergies reflect processes that the nervous system has evolved to reduce the dimensionality of motor control. We propose that the inferior olive simplifies the computation underlying motor control by biasing the activities of spinal and cranial motor systems so that discrete collectives of muscles are predisposed to contract at specific times during movement. The well-characterized oscillatory activity of olivary neurons is postulated to provide a pacemaking signal and to restrict the control process to particular moments in time while the process of electrotonic coupling and uncoupling of assemblies of olivary neurons is proposed to underlie the spatial distribution of synergic muscle activations. It is proposed that the olivocerebellar contribution to the control process is to allow movements to be executed rapidly in a feedforward manner, so that the need for sensory guidance and feedback is minimized.

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)

Correspondence between climbing fibre input and motor output in eyeblink-related areas in cat cerebellar cortex

The purpose of the present work was to identify sites in the cerebellar cortex which are likely to control eyeblink. This work was motivated by findings suggesting that the cerebellum is involved in the learning and/or performance of the classically conditioned eyeblink response. The identification was based on climbing fibre input to the cortex and on the effects of electrical stimulation of the cerebellar cortex in cats decerebrated rostral to the red nucleus. The cerebellar surface was searched for areas receiving short latency climbing fibre input on periorbital electrical stimulation. Four such areas were found in the c1 and c3 zones of lobules VI and VII in the anterior lobe of the cerebellum and in the c3 zone in the paramedian lobule. Electrical stimulation of the cerebellar cortex with trains (150-400 Hz) of at least 10 ms duration evoked two types of EMG response in the orbicularis oculi muscle. An early response, time-locked to the onset of the stimulation, was unrelated to climbing fibre input and a delayed response, time-locked to the termination of the stimulation, could only be evoked from areas which received short latency climbing fibre input from the eye, that is, the c1 and c3 zones. The delayed responses had long latencies (up to 50 ms) after the termination of the stimulus train and could be delayed further by prolonging the stimulation. Both types of response were abolished by injections of small amounts of lignocaine into the brachium conjunctivum. A number of characteristics of the delayed responses are described. They could be inhibited by a further shock to the same area of the cerebellar cortex. Their latency could be increased by increasing the stimulation frequency. The period between stimulation and appearance of the response often showed a decrease in spontaneous EMG activity. There was a close topographical correspondence between input and output. Delayed responses could be evoked from all four of the areas in the c1 and c3 zones which have climbing fibre input from the periorbital area. They could not be evoked from other areas. In contrast, early responses were only evoked from areas without such climbing fibre input. It is proposed that the delayed responses were generated by activation of Purkinje cell axons leading to hyperpolarization and a subsequent rebound depolarization and activation of cells in the interpositus nucleus. The cortical areas are therefore probably involved in the control of the orbicularis oculi muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Uniform olivocerebellar conduction time underlies Purkinje cell complex spike synchronicity in the rat cerebellum

1. The issue of isochronicity of olivocerebellar fibre conduction time as a basis for synchronizing complex spike activity in cerebellar Purkinje cells has been addressed by latency measurement, multiple-electrode recording and Phaseolus vulgaris leucoagglutinin (PHA-L) tracing of climbing fibres in the adult rat. 2. The conduction time of the olivocerebellar fibres was measured by recording Purkinje cell complex spike (CS) responses from various areas of the cerebellum. The CSs were evoked by stimulating the olivocerebellar fibres near the inferior olive. In spite of a difference in length, as determined directly by light microscopy, the conduction times of different climbing fibres were quite uniform, 3.98 +/- 0.36 ms (mean +/- S.D., n = 660). 3. Multiple-electrode recording of spontaneous Purkinje cell CS activity was employed to study the spatial extent of CS synchronicity in the cerebellar cortex. Recordings of CS were obtained from Purkinje cells located on the surface and along the walls of lobule crus 2a. The rostrocaudal band-like distribution of simultaneous (within 1 ms) CS activity in Purkinje cells extended down the sides of the cerebellar folia to the deepest areas recorded (1.6-2.6 mm deep). As shown in previous experiments, the distribution of simultaneous CS activity did not extend significantly (500 microns) in the mediolateral axis of the cerebellar cortex. 4. In two animals a detailed determination of the length of the olivocerebellar fibre bundles was performed by staining the fibres with PHA-L injected into the contralateral inferior olive. This measurement included fibre bundles terminating in twenty-six different areas, ranging from the tops of the various folia to the bottoms of the fissures in both the hemisphere and the vermis. There was a 47.5% difference between the length of the longest measured fibre bundle (15.8 mm, terminating in lobule 6b, zone A) and the length of the shortest measured fibre bundle (8.3 mm, terminating in the cortex at the base of the primary fissure, zone D), after correction for tissue shrinkage. To attain an isochronous conduction time the conduction velocities for these two fibre bundles were calculated to be 4.22 m/s and 2.37 m/s, respectively. 5. By interpolating between measured points a simple formula was derived to estimate the average length of olivocerebellar fibres terminating in any given area of the cerebellar cortex, excluding the paraflocculus, the flocculus and the most lateral regions of the hemisphere. 6. We investigated the most likely mechanisms by which conduction velocity variations with length could result in global isochronicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Receptive fields of single cells from the face zone of the cat rostral dorsal accessory olive

Natural stimulation was used to map the receptive fields of single cells recorded from the rostral medial portion of the dorsal accessory olive (rDAO) and the subjacent principal olive (PO) of the barbiturate anesthetized cat. Previous reports indicated a somatotopic mapping of the entire contralateral body within the rDAO which included a small face zone and a larger zone with a very precise map for the limbs. While concentrating on the face zone of the rDAO we confirmed the previously reported somatotopy (face: rostral and medial; forelimb: caudal and medial; hindlimb; caudal and lateral; and trunk: rostal and lateral) and found a somatotopy within, and adjacent to, the face zone. At the border between rDAO regions representing forelimb and face, cells with forelimb fields were found to lie dorsally to cells with facial fields. Within the rDAO face region, cells with large facial fields lie dorsally to cells with small facial fields. In both cases, the more ventral cells lie in the ventral lamella of the PO, which suggests a functional as well as physical continuity between rDAO and the ventral lamella of the PO. We therefore conclude that the face zone in the rDAO and the face zone in the PO form one continuous and complete map of the face with an orderly progression of receptive fields. Furthermore, we have found that stimulation of the red nucleus can inhibit rDAO cells with facial receptive fields just as it does cells with receptive fields from the rest of the body.

Cerebro-cerebellar projections from the ventral bank of the anterior ectosylvian sulcus in the cat

1. Stimulation of the ventral bank of the anterior ectosylvian sulcus (AESv) induced marked mossy fibre (MF) and climbing fibre (CF) responses in the cerebellar posterior vermis (lobules VI-VII) and moderate sized ones in the paraflocculus, paramedian lobules and crus I and II of the cat. The relay stations for these responses to the posterior vermis were investigated morphologically and electrophysiologically. 2. It can be considered that the MF responses were relayed at least in part via the dorsolateral, peduncular and paramedian pontine nuclei, since in these nuclei there were units orthodromically responsive to AESv stimulation and antidromically responsive to stimulation of the posterior vermis. The MF responses are thought to be relayed monosynaptically, since the distribution of axon terminals labelled after injection of wheatgerm agglutinin-conjugated peroxidase (WGA-HRP) into the AESv overlapped in these pontine nuclei with that of neurons labelled after injection of WGA-HRP into the posterior vermis. 3. It is thought that the CF responses are relayed in the caudomedial part of the medial accessory olive (MAOcm), because neurons in the MAOcm were orthodromically responsive to AESv stimulation and antidromically responsive to stimulation of the posterior vermis. 4. It is suggested that the cerebro-olivary projection which transmits the orthodromic responses in the MAOcm is indirect, via the superior colliculus (SC), because injection of WGA-HRP into the AESv labelled axon terminals not in the MAOcm but in the SC, and injection of WGA-HRP into the MAOcm gave rise to retrograde labelling of cells in the SC. Synaptic connections between the axon terminals of the cerebrotectal projection and the tecto-olivary neurons were demonstrated by extracellular unit studies in the SC. 5. The hypothesis that the CF responses were transmitted via the SC was supported by the finding that the CF responses disappeared transiently after muscimol or lidocaine was injected into the SC. 6. These findings provide evidence that the MF responses are transmitted at least in part via the cerebro-ponto-cerebellar projection, while the CF responses are relayed via the cerebro-tecto-olivo-cerebellar projection. These cerebro-cerebellar pathways from the AESv are suggested to participate in conducting visual information to the posterior vermis.

Sensory integration in the spino-olivocerebellar pathways of the anaesthetized cat

1. The responses evoked by peripheral nerve stimulation in the c1 and c3 zones of the cerebellar cortex have been examined in barbiturate-anaesthetized cats. The responses evoked via the spino-olivocerebellar pathways (SOCPs), which terminate in the cortex as climbing fibres, were recorded as positive multiunit field potentials from the cerebellar surface of lobule V. 2. Low-strength conditioning stimulation of the superficial radial, ulnar or median nerve frequently modified the climbing fibre-mediated responses evoked by a subsequent test stimulus to one of the other nerves. In most cases this modification involved a depression of the evoked response. The depression was not dependent on the conditioning stimulus evoking climbing fibre-mediated responses in the cortex. 3. The depression of the evoked responses increased as the conditioning stimulus intensity was raised within the range of 1.1-1.5 x threshold (1.1-1.5T). 4. Topical application of bicuculline to the surface of the dorsal column nuclei reduced the depression evoked by conditioning stimulation and it is therefore concluded that GABAergic inhibition in the cuneate nucleus contributes to the depression. 5. The inhibition is discussed in relation to its possible contribution to movement-related regulation of the excitability of SOCPs which occurs during locomotion in awake cats.

Intracellular labeling of neurons in the medial accessory olive of the cat: II. Ultrastructure of dendritic spines and their GABAergic innervation

In order to describe the morphology of dendritic spines of identified neurons in the cat inferior olive together with their gamma-aminobutyric acid (GABA) synaptic input, a technique was used combining intracellular labeling of horseradish peroxidase with postembedding gold-immunocytochemistry. With this technique physiologically identified olivary cells were reconstructed with the light microscope, and the horseradish peroxidase reaction product and immunogold labeling were subsequently examined in serial sections at the ultrastructural level. In addition, a degenerating neuron was observed, resulting in a triple labeling in single ultrathin sections. Quantitative and three-dimensional analysis showed that the dendritic spines were composed of long, thin stalks ending in one or more spine heads. The spines of cells located in the caudal half of the medial accessory olive (type I cells, characterized by dendrites which run away from the soma) were found to be less complex than those of cells located rostrally in this olivary subnucleus (type II cells, characterized by dendrites which tend to turn back towards the soma). Most, if not all, of the spines of both cell types were located within glomeruli. On average, the spines within individual glomeruli originated from 6 different dendrites (with a maximum of 8). Different spines within the same glomerulus were never derived from different dendrites of the same olivary neuron, but single spines frequently gave rise to several spine heads, which could be located either within different glomeruli or inside a single glomerulus. The glomerular spine heads originating from the same spine were rarely located near one another. All spines and most of the spine heads were contacted by both GABAergic and non-GABAergic terminals. Most of the GABAergic terminals contained pleomorphical vesicles and displayed symmetric synapses whereas the non-GABAergic terminals showed usually round to oval vesicles and asymmetric synapses.

Intracellular labeling of neurons in the medial accessory olive of the cat: III. Ultrastructure of axon hillock and initial segment and their GABAergic innervation

The gamma-aminobutyric acid (GABA) synaptic input of identified axons in the cat inferior olive was studied by use of combination of intracellular labeling with horseradish peroxidase and postembedding gold-immunocytochemistry. With this technique olivary cells were physiologically identified and light microscopically reconstructed, and the horseradish peroxidase reaction product and the immunogold labeling were subsequently simultaneously visualized for electron microscopic investigation with the use of serial ultrathin sections. The axons of cell type I (characterized by dendrites which radiate away from the cell body) originated from the soma, whereas those of type II neurons (characterized by dendritic trees which curve back towards the soma) were derived from a primary dendrite. The axons of olivary neurons stand out by the length of their axon hillock (up to 21 microns) and initial segment (up to 40 microns). The hillock forms various spiny appendages which were located within glomeruli together with dendritic spines of other olivary neurons. Axonal spines of type II neurons were more numerous and complex looking than those of type I. The axonal spines, the shaft of the axon hillock, and the transition between the hillock and initial segment were primarily innervated by GABAergic terminals (65%) but non-GABAergic terminals (35%) were present as well. The terminals apposed to the axons of type I neurons contacted mainly the axonal shafts, whereas most of the terminals adjacent to the axons of type II neurons established synaptic contacts with the axonal spines. The initial segments were largely devoid of synaptic input. Distally, the initial segment acquired a myelin sheath.

Mesodiencephalic and cerebellar terminals terminate upon the same dendritic spines in the glomeruli of the cat and rat inferior olive: an ultrastructural study using a combination of [3H]leucine and wheat germ agglutinin coupled horseradish peroxidase anterograde tracing

The mesodiencephalic and cerebellar afferents in the rostral medial accessory and principal olive of the cat and rat were studied following anterograde transport of tritiated leucine combined with anterograde transport of wheat germ agglutinin coupled horseradish peroxidase in the same animals. In all studied areas at least one-third of the labelled glomeruli appeared to contain both mesodiencephalic and cerebellar terminals. In many of these cases it was found that the terminals from both afferent systems contacted the same dendritic spines. Therefore, these olivary spines may be, as will be discussed, well suited for being involved in a timing process.

Organization of cerebral cortico-olivary projections in the rat

Injection of wheatgerm agglutinin-conjugated horseradish peroxidase into electrophysiologically identified portions of the rodent sensorimotor and medial frontal cortex revealed anterograde-labeled projections to the inferior olivary complex. Forelimb motor cortical and supplementary motor cortical regions were found to project predominantly to the principal olive while forelimb sensory cortex fibers distributed mainly within the dorsal accessory olive. Forelimb cortical fibers (both sensory and motor) terminated more rostrally and medially in the medial and dorsal accessory olives than did hindlimb projections. Medial frontal cortex projected to the beta subnucleus, the rostral medial accessory olive (especially the ventral aspect), and to the ventrolateral outgrowth of the dorsal cap. These findings indicate a more extensive origin for cortico-olivary projections than previously described for rats or cats and they demonstrate a rather precise topography for olivary projections from various cortical regions.

Ultrastructural study of the GABAergic, cerebellar, and mesodiencephalic innervation of the cat medial accessory olive: anterograde tracing combined with immunocytochemistry

The rostral medial accessory olive (MAO) of the cat was studied by using an ultrastructural technique combining wheat germ agglutinin-coupled horseradish peroxidase (WGA-HRP) anterograde tracing and postembedding GABA immunocytochemistry. One group of cats received a WGA-HRP injection in the posterior interposed nucleus of the cerebellum and another group received an injection in the nucleus of Darkschewitsch. Based on differences in their morphology three types of GABAergic and three types of nonGABAergic terminals were observed. One type of GABAergic terminal was often GABA/WGA-HRP double-labeled in the cerebellar experiments, and one type of nonGABAergic terminal was often WGA-HRP-labeled in the mesodiencephalic experiments. Following injections of WGA-HRP in the cerebellar nuclei virtually all WGA-HRP-labeled terminals were GABA positive. Quantification of these GABA/WGA-HRP-double-labeled terminals showed that 1) 30% of the GABAergic terminals randomly selected from the entire neuropil were double-labeled, 2) 13% of the GABAergic terminals adjacent to perikarya were double-labeled, and 3) 34% of the GABAergic terminals strategically located next to both of the dendritic elements linked by a gap junction were double-labeled. Statistical analysis of the above data showed that significantly fewer GABAergic terminals adjacent to perikarya were double-labeled (P less than .001) than would be expected from the double-labeled proportion of the randomly selected GABAergic terminals. Following injection of WGA-HRP in the nucleus of Darkschewitsch, all WGA-HRP-labeled terminals were GABA-negative. Quantification of these terminals showed that 1) 26% of the randomly selected nonGABAergic terminals were WGA-HRP labeled, 2) 20% of the nonGABAergic terminals adjacent to perikarya were WGA-HRP labeled, and 3) 23% of the nonGABAergic terminals strategically located next to a gap junction were WGA-HRP labeled. No significant differences were found among these populations. Quantification of terminals of both groups of experiments mentioned above showed that GABAergic terminals composed 1) 38% of the randomly selected terminals, 2) 64% of the terminals apposed to perikarya, and 3) 53% of the terminals strategically located next to gap junctions. Statistical analysis of the above data showed that significantly more GABAergic terminals were located adjacent to perikarya (P less than .001) and strategically next to a gap junction (P less than .05) than would be expected from the random GABAergic innervation. The above findings of the GABAergic, cerebellar, and mesodiencephalic input are discussed with regard to their functional role in the neuronal circuitry of the ros

Electrophysiological analysis of the trigemino-olivo-cerebellar (crura I and II, lobulus simplex) projection in the rat

In albino rats the whisker area was electrically stimulated while climbing fiber responses were surveyed in the cerebellar hemisphere on the ipsilateral side. They were identified both deep in the intercrural sulcus, and in the posterior superior fissure. Histological examination has revealed that the response areas extend longitudinally from the dorsal surface of crus II to the ventral surface of crus I in the intercrural sulcus, and from the rostral surface of crus I to the caudal surface of lobulus simplex in the posterior superior fissure. These are supposed to be transmitted through direct trigemino-olivary projections.

An electrophysiological study of the in vitro, perfused brain stem-cerebellum of adult guinea-pig

1. We describe here a technique which allows the long-term in vitro survival of the perfused isolated brain stem-cerebellum of adult guinea-pig. The viability of this preparation was assessed by comparing the electrophysiological properties of individual neurones and of neuronal pools to those obtained in vivo or in brain slices. The areas investigated included the cerebellar cortex, the inferior olive and the pontine nuclei. 2. Cerebellar field potential and intra- and extracellular single-cell recordings could be obtained for as long as 15 h after the preparation was initially isolated. The waveforms of field potentials recorded at various depths in the cerebellar cortex following surface folial stimulation were similar to those recorded in vivo. Extracellular recordings from single Purkinje cells following white matter stimulation demonstrated antidromic as well as mossy- and climbing fibre-mediated excitation. Stimulation of the cerebellar surface elicited orthodromic parallel fibre excitation of Purkinje cells and basket-stellate and Golgi cell inhibition. 3. Intrasomatic and intradendritic recordings from Purkinje cells reproduced all the phenomenology described earlier under in vivo conditions and in vitro slice preparations. In addition, spontaneous excitatory synaptic potentials generating simple spikes (mossy fibre-parallel fibre-mediated activity) and complex spikes (climbing fibre-mediated activity) were consistently observed. 4. Extracellular field potentials and extra- and intracellular recordings from inferior olive neurones were similar to those previously shown for the mammalian inferior olive. 5. Intracellular recordings were also obtained from pontine nuclei neurones, a major source of mossy fibre afferents to the cerebellum. Stimulation of the contralateral superior cerebellar peduncle produced antidromic invasion of these neurones whereas stimulation of the ipsilateral inferior cerebral peduncle resulted in their orthodromic activation. 6. The preparation responded to pharmacological challenge in a manner which demonstrated a sequential activation of sets of synaptic links in a given pathway. Thus, harmaline generated oscillations of inferior olivary neurones which were similar to those observed in vivo and which produced climbing fibre EPSPs in Purkinje cells at the same frequency as the inferior olivary oscillations. Climbing fibre activation of the Purkinje cells generated powerful inhibitory potentials in the cerebellar nuclear neurones at the same frequency.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional organization of the parvocellular red nucleus in the cat

The experiments were performed on cats under pentobarbital anesthesia. The following results were obtained. (1) Most of the neurons in the rostral part of the red nucleus (RN) were activated by stimulation of the parietal association cortex (P) and the lateral cerebellar nucleus (CN). A number of the neurons were regarded as rubro-olivary since they responded antidromically to inferior olive (IO) stimulation. (2) Stimulation of the P as well as the frontal motor cortex activated IO neurons. A longer and more variable latency in P-induced responses indicated the existence of an indirect connection from the P to the IO. The recording sites for P-induced responses were located in the rostral region of the IO, giving off projection fibers to the cerebellar hemispheral part. After horseradish peroxidase (HRP) injection through recording microelectrodes into the IO where P-induced responses were recorded, small cells labeled retrogradely with HRP were found distributed in the rostral part of the RN. (3) Effects of stimulus to or lesion of the rostral part of the RN revealed that the climbing fiber responses to P stimulation were conveyed through the rubro-olivary pathway originating in the rostral part of the RN. (4) Following HRP injection through recording electrodes into the rostral part of the RN where P-induced responses were recorded, retrogradely labeled cells were seen and located in the P and CN. From these findings, the rostral part of the RN investigated in this study could be regarded as the rubral parvocellular part. The present study suggests that the latero (dentato-)-rubro-olivo-cerebellar circuit forms an internal feedback loop and the P acts on this loop.

Evidence for a GABA-mediated cerebellar inhibition of the inferior olive in the cat

1. Climbing fibres were activated by peripheral nerve stimulation at 'high' frequencies (greater than 3 Hz) for 15-25 s and then at 0.9 Hz for about 1 min. The high frequency activation induced a post-conditioning inhibition, lasting up to about 1 min, of climbing fibre responses recorded from the cerebellar surface. 2. Electrolytic lesions were made in the superior cerebellar peduncle (brachium conjunctivum). After the lesion, the post-conditioning inhibition was completely eliminated. 3. Injections of the GABA-receptor blocker bicuculline methiodide into the inferior olive reversibly blocked the post-conditioning inhibition. 4. The results support the hypothesis proposed by Andersson and Hesslow (1987a), that post-conditioning inhibition is mediated by a GABA-ergic interposito-olivary pathway.

Inferior olive excitability after high frequency climbing fibre activation in the cat

1. Climbing fibre responses (CFRs) were evoked by limb nerve stimulation and recorded from the cerebellar surface in barbiturate anaesthetized cats. Climbing fibres were activated at frequencies of usually 2.5-7.5 Hz for periods of 15-30 s, after which the stimulation frequency was reduced to below 1 Hz. 2. The high-frequency stimulation induced a strong depression of CFR-amplitude, lasting up to 60 s. The magnitude of this depression was dependent on both the frequency and the duration of the high-frequency stimulation. 3. The depression occurred in the c1, c2 and c3 zones of the pars intermedia and in the x zone in the vermis but not in the b zone in the vermis. 4. Recordings of olivary reflex responses demonstrated that the depression occurred in the inferior olive. 5. It is suggested that the inhibition of the inferior olive occurs because the high-frequency stimulation leads to a disinhibition of neurones in the interpositus nucleus which inhibit the olivary neurones.

Excitatory inputs to cerebellar dentate nucleus neurons from the cerebral cortex in the cat

1. In anesthetized cats, we investigated excitatory and inhibitory inputs from the cerebral cortex to dentate nucleus neurons (DNNs) and determined the pathways responsible for mediating these inputs to DNNs. 2. Intracellular recordings were made from 201 DNNs whose locations were histologically determined. These neurons were identified as efferent DNNs by their antidromic responses to stimulation of the contralateral red nucleus (RN). Stimulation of the contralateral pericruciate cortex produced excitatory postsynaptic potentials (EPSPs) followed by long-lasting inhibitory postsynaptic potentials (IPSPs) in DNNs. The most effective stimulating sites for inducing these responses were observed in the medial portion (area 6) and its adjacent middle portion (area 4) of the precruciate gyrus. Convergence of cerebral inputs from area 4 and area 6 to single DNNs was rare. 3. To determine the precerebellar nuclei responsible for mediation of the cerebral inputs to the dentate nucleus (DN), we examined the effects of stimulation of the pontine nucleus (PN), the nucleus reticularis tegmenti pontis (NRTP) and the inferior olive (IO). Systematic mapping was made in the NRTP and the PN to find effective low-threshold stimulating sites for evoking monosynaptic EPSPs in DNNs. Stimulation of either the PN or the NRTP produced monosynaptic EPSPs and polysynaptic IPSPs in DNNs. Using a conditioning-testing paradigm (a conditioning stimulus to the cerebral peduncle (CP) and a test stimulus to the PN or the NRTP) and intracellular recordings from DNNs, we tested cerebral effects on neurons in the PN and the NRTP making a monosynaptic connection with DNNs. Conditioning stimulation of the CP facilitated PN- and NRTP-induced monosynaptic EPSPs in DNNs. This spatial facilitation indicated that the excitatory inputs from the cerebral cortex to DNNs are at least partly relayed via the PN and the NRTP. 4. Stimulation of the contralateral IO produced monosynaptic EPSPs and polysynaptic IPSPs in DNNs. These monosynaptic EPSPs were facilitated by conditioning stimulation of the CP, strongly suggesting that the IO is partly responsible for mediating excitatory inputs from the cerebral cortex to the DN. A comparison was made between the latencies of IO-evoked IPSPs in DNNs and the latencies of IO-evoked complex spikes in Purkinje cells. Such a comparison indicated that the shortest-latency IPSPs evoked from the IO were not mediated via the Purkinje cells and suggested the pathway mediated by inhibitory interneurons in the DN.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential localization of inferior olivary neurons projecting to the tecto-olivo-recipient zones of lobule VII or crus II in the rat cerebellum

In albino rats antidromically evoked potentials by stimulation of the cerebellar cortices of lobule VII or crus II were surveyed in the tectorecipient zone of subnucleus c in the medial accessory olive. Those from lobule VII were localized in the medial region, while those from crus II in the lateral region of the tectorecipient zone. The results confirm the tecto-olivo-cerebellar (lobule VII and crus II) projections via different inferior olivary neurons.

Localization of glutamic-acid-decarboxylase-immunoreactive axon terminals in the inferior olive of the rat, with special emphasis on anatomical relations between GABAergic synapses and dendrodendritic gap junctions

Immunocytochemical and electron microscopic methods were used to examine the GABAergic innervation of the inferior olivary nucleus in adult rats. This neuronal system was visualized with an antibody against glutamic acid decarboxylase (GAD, EC 4.1.1.15), the GABA-synthesizing enzyme. A GAD-positive reaction product was encountered only in short segments of preterminal axons and in axon terminals. Their relative number per unit area of neuropil was very similar in all olivary subnuclei. Despite this homogeneity in density, obvious intraregional differences existed. Some regions were strongly immunoreactive (the "c" subgroup, the beta nucleus, and the mediolateral outgrowth of the medial accessory olive), whereas others were weakly labeled (the dorsomedial cell column and the central zones of the medial accessory and principal olives). The strongly immunoreactive areas contained the largest and most intensively labeled axon terminals. Areas of weak labeling were filled with small, weakly immunoreactive nerve terminals. Thus, variations in size and in intensity of labeling create a specific pattern of GABA innervation, revealed by an almost continuous gradient between the above-mentioned extremes. The GAD-positive axon terminals established conventional synapses with dendrites (94% of the samples) or with cell bodies (6%). The vast majority of these synapses were type II (84%) and only a small proportion formed type I synaptic contacts (16%), regardless of the nature of the postsynaptic element. Immunoreactive terminals were also involved in the complex synaptic arrangements--the glomeruli, which characterize the olivary neuropil. Within these formations, olivary neurons were electrotonically coupled through dendrodendritic gap junctions. There was a constant association between GAD-positive axon terminals and small dendritic appendages linked by gap junctions. This association was revealed not only by the systematic presence of immunolabeled terminals directly apposed to the dendritic appendages but, more importantly, by the frequent presence of type II synapses straddling both elements. These synapses were in close proximity to the low-resistance pathways represented by the gap junctions. The strategic location of these GABA synapses is discussed in relation to recent findings indicating the possibility of a synaptic modulation of the electrical coupling: the release of GABA, by increasing nonjunctional membrane conductance, could shunt the coupling between olivary neurons. The functional decoupling of selected gap junctions would be responsible for the spatial organization of the olivary electrotonic coupling.

The secondary spikes of climbing fibre responses recorded from Purkinje cell axons in cat cerebellum

Responses evoked in Purkinje cells by climbing fibre activity were investigated by recording from Purkinje cell axons in the cerebellum of anaesthetized cats. Purkinje cell axons were identified by firing pattern and by latency of responses to stimulation of peripheral nerve and of the inferior olive. Axonal climbing fibre responses usually consisted of one to two spikes, suggesting that normally only the initial spike or, at most, this and one of the secondary spikes are propagated down the Purkinje cell axon. When two successive climbing fibre responses were evoked, the number of spikes in the second response was increased, usually up to three to five. This effect could be obtained at stimulation intervals of up to 100 ms. In a few cases it was possible for a climbing fibre response to be preceded by a parallel fibre volley evoked by stimulation of the cerebellar surface. This increased the number of spikes in the axonal climbing fibre response. The results suggest that the number of propagated spikes in the climbing fibre response can be modified by a preceding input to the Purkinje cell.

The secondary spikes of climbing fibre responses recorded from Purkinje cell somata in cat cerebellum

Extracellularly recorded climbing fibre responses in Purkinje cell somata in the cerebellar cortex were investigated in cats deeply anaesthetized with barbiturate. The effects on the amplitude of initial and secondary spikes of preceding climbing fibre activation, on-beam parallel fibre activation and off-beam parallel fibre activation were studied. When a climbing fibre response was preceded by climbing fibre activation there was a decrease in the amplitude of the initial spike of the second response at intervals up to 25 ms and little effect at longer intervals. Secondary spike amplitude was greatly increased at intervals up to 100 ms. When a complex spike was preceded by on-beam parallel fibre activation there was a decrease in the initial spike amplitude at short intervals and an increase in the amplitude at long intervals. Secondary spike amplitude was increased up to 150 ms after an on-beam parallel fibre volley. When a complex spike was preceded by off-beam parallel fibre stimulation there was an increase in initial spike amplitude at intervals up to about 200 ms and a decrease in secondary spike amplitude at intervals up to about 150 ms. The results show that the amplitude of the secondary spikes can be modified by a preceding input to the Purkinje cell. The results also suggest that the secondary spikes are generated in the Purkinje cell dendrites and the initial spike in the soma.

Synaptic organization of the cerebello-thalamo-cerebral pathway in the cat. III. Cerebellar input to corticofugal neurons destined for different subcortical nuclei in areas 4 and 6

To analyze the cerebellar effects on corticofugal neurons destined for different subcortical nuclei, intracellular recordings were made from corticofugal neurons in areas 4 and 6 of the cat. Corticonuclear neurons to the red nucleus (RN) and the pontine nucleus (PN), and pyramidal tract neurons (PTNs) with collaterals to these nuclei were identified by their antidromic responses to the stimulation of these nuclei and the pyramid. Three types of RN-projecting neurons (corticorubral neurons (CRNs), corticopontine neurons (CPNs) with a collateral to the RN and PTNs with a collateral to the RN) and two types of PN-projecting neurons (CPNs and PTNs with a collateral to the PN) were differentiated. Furthermore, these corticofugal neurons were classified as fast and slow neurons on the basis of a critical axonal conduction velocity of 20 m/s. About 80% of 98 RN-projecting neurons in area 4 were PTNs, and among the rest, CPNs were more common than CRNs. A similar tendency of the frequency distribution of 37 RN-projecting neurons was also observed in area 6. In area 4, about 70% of 158 PN-projecting neurons were PTNs (80 fast and 30 slow PTNs) and the rest were CPNs, while in area 6, only 35% of 99 PN-projecting neurons were PTNs (10 fast and 25 slow PTNs). Among the CPNs in areas 4 and 6, slow CPNs were more frequently encountered. Cerebellar effects on these identified corticofugal neurons were investigated, using electrical stimulation of the brachium conjunctivum (BC). In both areas 4 and 6, a substantial number of fast conducting CRNs, CPNs and PTNs projecting to the RN or the PN received short-latency (predominantly disynaptic), large-amplitude EPSPs from the BC, and a considerable number of slow conducting neurons to the RN and/or the PN received longer-latency, smaller-amplitude EPSPs from the BC.

Spatial distribution of evoked potentials in the inferior olivary nucleus by stimulation of the visual afferents in the rat

Visual pathways (optic disc, optic nerve and pretectal regions) were electrically stimulated and evoked potentials were explored throughout the inferior olive in the anesthetized rat. Responsive areas were identified as the caudal half of the dorsal cap, nucleus beta and the most caudal region of subnucleus c of the medial accessory olive. No field potentials were identified in the rostral half of the dorsal cap, its ventrolateral outgrowth or the dorsomedial cell column. Contralateral retinal afferents were only effective all over the responsive areas.

Inhibition of inferior olivary transmission by mesencephalic stimulation in the cat

Cerebellar climbing fiber responses (CFRs) evoked in anesthetized cats by stimulation of peripheral nerves, contralateral inferior olive and cerebellar white matter were investigated by recording unit activity and surface field responses in anterior lobe of cerebellar cortex. When nerve and olive stimulation was preceded at long intervals (greater than 35 ms) by weak electrical stimulation of an ipsilateral mesencephalic area close to the locus coeruleus and brachium conjunctivum, CFRs could be virtually abolished in the pars intermedia but not in the vermis. White-matter evoked CFRs were not affected; thus the site of the inhibition was the inferior olive.

Rhythmic discharge of climbing fibre afferents in response to natural peripheral stimuli in the cat

The rhythmicity of inferior olivary neurones evoked by natural ipsilateral forepaw inputs was evaluated in the climbing fibre afferent discharge of Purkinje cells recorded in the cerebellar cortex of the decerebrate, unanaesthetized cat. Almost 50% of all Purkinje cells responding to the forepaw stimulus with an increase in complex spike activity exhibited periodic discharge, with the dominant periodicity being between 100 and 160 ms. In ten of twenty-five neighbouring, simultaneously recorded Purkinje cells the forepaw stimulus evoked similar periodicity in their complex spike discharge. For some cells two peaks of complex spike activity were evoked by a forepaw stimulus without an obvious third peak. By altering the stimulus duration the second peak of the response was shown to be temporally uncoupled to the 'off' phase of the displacement for many cells. The interdependence of the trials contributing to the periodic peaks in the peristimulus time histogram (p.s.t.h.) was examined by a 'separation technique'. This analysis indicated that the complex spikes contributing to a specific peak in the p.s.t.h. were generated with a high degree of independence (i.e. in different trials) from the complex spikes contributing to any other peak. It was hypothesized that the independence of the rhythmic complex spike peaks is due to the long relative refractoriness following a complex spike in a single cell. Therefore, the probability of a complex spike occurring at the next one or two cycles is decreased significantly. As a consequence, an inferior olivary neurone fires usually at only one of the various peaks in response to a single presentation of the forepaw stimulus. This hypothesis predicts that stimuli evoking a complex spike at the initial peak in a high percentage of trials should give rise to less periodicity. This prediction was tested by comparing the presence or absence of evoked oscillation with the probability of evoking a complex spike in the first peak of the p.s.t.h. Cells exhibiting a probability for complex spike discharge of over 50% in the first peak showed much less periodicity than cells with a complex spike occurring in less than 50% of the trials in the first peak. These results are discussed in the context of the inferior olive being viewed as a population of coupled elements with a tendency to oscillate. The natural forepaw stimulus is hypothesized as synchronizing the phases of spontaneously oscillating climbing fibre afferents, resulting in the observed periodicity in the complex spike p.s.t.h.

Mutual inhibition between olivary cell groups projecting to different cerebellar microzones in the cat

This study was carried out predominantly on the b zone in the lateral vermis of the cerebellar anterior lobe. This zone is divided into sagittally oriented microzones which receive a somatotopically organized climbing fibre input. It was shown that the climbing fibre input to one microzone is inhibited by stimulation of a nerve that projects to an adjacent microzone. The degree of inhibition was related to the proximity of the microzones involved. The latency of the inhibition was short and the duration 70-110 ms. The inhibition of climbing fibre responses occurred in the inferior olive and was presumably due to post-synaptic inhibition of the olivo-cerebellar neurones. The mutual inhibition could be produced by antidromic activation of olivo-cerebellar neurones. An inhibition with similar properties as in the b zone, but weaker, was observed between forelimb and hindlimb inputs to the c1 and c3 zones in the pars intermedia. In the c3 zone, an inhibition between adjacent forelimb microzones also occurred.

The origin of the reticulo-olivary projection in the rat: a retrograde horseradish peroxidase study

Electrophoretic injections of horseradish peroxidase were made in different parts of the rat inferior olivary complex using a ventral approach. Data from these injections provide anatomical evidence for the existence of a projection to the inferior olive which takes origin from reticular nuclei in the brainstem. The majority of reticulo-olivary neurons are located in the nucleus raphe obscurus and nucleus raphe pallidus. Other reticular nuclei which contribute to this projection include the nucleus reticularis ventralis and nucleus reticularis gigantocellularis. Analysis of injections confined to specific parts of the olivary complex reveals a topographical pattern in the reticulo-olivary projection. Caudal parts of the complex receive input primarily from the nucleus reticularis ventralis. As more rostral and medial parts of the inferior olive are included in the injection, there is concomitant shifting of labeled neurons to the nucleus reticularis gigantocellularis and the raphe nuclei. The reticulo-olivary neurons may serve several non-mutually exclusive roles in olivary circuitry. They may be the source of serotonin and/or substance P to the nucleus. Physiologically, they may provide the inhibitory input observed in the nucleus. Finally, some of these neurons may be the brainstem relay of the lateral funiculus and dorsolateral funiculus spino-olivo-cerebellar pathway proposed by Larson and his co-workers (J. Physiol., Lond. 203, 611-640, 641-649).

The projection from the motor cortex to the inferior olive in the cat. An experimental study using axonal transport techniques

The cortico-olivary projection has been investigated in the cat with the methods of retrograde transport of horseradish peroxidase and wheat-germ agglutinin conjugated with horseradish peroxidase as well as autoradiographic techniques using tritium-labelled amino acids. The projection arises from cells in cortical layer V and terminates mainly ipsilaterally and less densely contralaterally. The strongest termination site is in the caudal medial accessory olive adjacent to subnucleus beta. Projections to that area originate in the medial portions of areas 4 and 6 rostral to the cruciate sulcus. Regions of the motor cortex related to axial back and neck, proximal forelimb and face musculature plus the frontal eye field are represented in largely overlapping areas of the caudal medial accessory olive. A second zone of termination is present in the rostral olive at the junction of the ventral lamella of the principal olive and the medial border of the dorsal accessory olive. Projections to that area arise from a central portion of area 4 at the border between the anterior sigmoid gyrus and the presylvian gyrus. This area contains portions of the representation of the muscle groups controlling the neck and proximal forelimb (shoulder and elbow) only. The frontal eye field, which in the cat influences both extraocular and neck musculature, is also an important direct source of input to this portion of the inferior olive. Contralateral terminations are distributed symmetrically. Combining this information with the olivocerebellar distribution, cerebellar cortical areas corresponding to this direct cortical input are defined. Taking into account that the cortico-olivary fibers appear to arise only from those portions of the motor cortex involved in the control of axial and proximal forelimb muscles, it is suggested that the cortico-olivo-cerebellar projections play a preponderant role in the cerebellar control of posture.

The inferior olive of a prosimian primate, Galago senegalensis. II. Olivocerebellar projections to the vestibulocerebellum

Olivocerebellar projections to the vestibulocerebellum of Galago senegalensis, a prosimian primate, were studied by using a horseradish peroxidase (HRP) method. Following implants of HRP into the flocculus, labeled cells are found in subgroup d of contralateral medial accessory olive (MAO), while labeled somata are present throughout subgroups c and d after injections of HRP into the nodulus and adjacent parts of the uvula. Labeled cells are also located throughout the dorsomedial cell column (DMCC) and a small contingent of HRP-filled cells are seen in dorsomedial areas of the ventral lamellae of the principal olivary nucleus (PO) after HRP placements in nodulus and uvula. Subsequent to injections of paraflocculus, labeled cells are found in subgroup a of MAO, in the lateral bend of PO, and in its ventral lamella (VLPO). Collectively these data indicate a topographically organized projection of olivocerebellar fibers to the flocculonodular lobe and adjacent areas of paraflocculus and uvula. The paraflocculus receives input from subgroup a of MAO, the lateral bend of PO, and VLPO. Subgroup d of MAO projects to the flocculus and nodulus with a relatively minor contribution to uvula. Neurons in subgroups c and DMCC of MAO project to specific portions of nodulus and adjacent uvula. These data are in general agreement with those from other species. In contrast to earlier reports, however, this study describes a projection from VLPO to lateral areas of the contralateral nodulus and uvula. A zonal organization of olivocerebellar terminations in the nodulus and ventral portions of uvula is suggested. A medial zone is subdivided into medial and lateral parts which receive projections from subgroups c and d of MAO, and from cells in DMCC, respectively. A more lateral area receives axons from cells in VLPO.

Projections from the reticular formation of the medulla, the spinal trigeminal and lateral reticular nuclei to the inferior olive

Injections of tritiated L-leucine were placed in the reticular formation of the medulla, the spinal trigeminal and lateral reticular nuclei of cats and silver grain accumulations in the inferior olivary nucleus were demonstrated by autoradiography. Cells of the reticular formation located at the junction of nuclei reticularis magnocellularis and reticularis parvocellularis in the rostral medulla and within nucleus reticularis ventralis in the caudal medulla contribute four distinct projections to the olive. Three projections are distributed ipsilaterally in the caudal part of the medial accessory olive, at mid-level of the dorsal accessory olive and in the ventrolateral bend of the principal olive, at rostral levels. There is also a small controlateral projection to the caudal part of the medial accessory olive. the spinal trigeminal nucleus sends crossed projections to the rostral part of the dorsal accessory olive and adjacent ventral lamella as well as to the caudal part of the medial accessory olive. The lateral reticular nucleus sends an extensive ipsilateral projection to the caudal part of the medial accessory olive and provides a small contribution to the same subdivision, contralaterally. All these projections converge with other known afferents to the olive.

Somatosensory properties of the inferior olive of the cat

We examined the somatosensory properties of 391 neurons in the inferior olive in 20 cats that were anesthetized with barbiturate or decerebrated. A response consisted of a single spike with a variable number of wavelets followed by a long refractory period. Neurons responsive to natural somatosensory stimuli were recorded in all olivary subdivisions. The dorsal accessory olive (DAO) contained the highest proportion of responsive units (96%), compared with 66% for the medial accessory olive (MAO) and 43% for the principal olivary (PO) nucleus. Within the rostral DAO we found a refined cutaneous map of the entire contralateral body surface. In the caudal DAO responsiveness to manipulation of deep tissues became prominent, and both individual limbs and bilateral pairs were represented. In the medial region of the PO responsiveness to taps predominated and bilaterally symmetrical fields were frequent. The lateral PO was unresponsive under the conditions of these experiments. The MAO was distinguished by a greater complexity of receptive field and by a preponderance of deep over cutaneous modality. The lateral part of caudal MAO contained cells with interesting spatial patterns of excitation and inhibition, whereas most cells in the rostral MAO had purely excitatory fields. A teleceptive area receiving visual and auditory input was recognized in the medial MAO and nearby structures such as the dorsal cap. Contact and proprioceptive signals arriving via climbing fibers may provide the cerebellum with information necessary to relate the body to external objects.

Origin and sagittal termination areas of cerebro-cerebellar climbing fibre paths in the cat

Climbing fibre responses were recorded in the cerebellar anterior lobe on stimulation of the cerebral cortex. A zonal pattern was demonstrated in the cortical projection, which was related to the cerebellar sagittal zones, as identified from peripheral climbing fibre input. In all zones, except c2, a co-variation of the responses evoked on peripheral nerve stimulation and on stimulation of the corresponding part of the sensorimotor cortex was found. There was a bilateral projection to the a, b, c2 and d1 zones which also, to a varying extent, receive a bilateral peripheral input. The x, c1 and c3 zones, receiving an ipsilateral peripheral input, were activated exclusively from the contralateral cortex. Stimulation of the posterior sigmoid gyrus (p.s.g.) evoked responses in all the zones. These responses had, in all zones except d1, lower thresholds and shorter latencies than the responses from other cortical areas. Two separate p.s.g. areas were shown to project to the pars intermedia zones (c1, c2, c3 and d1), the lateral area to the caudal parts and the medial area to the rostral parts of the zones. In contrast, the b zone received a projection from only one p.s.g. area, centred between, but overlapping, the two areas projecting to the pars intermedia zones. Stimulation of the anterior sigmoid gyrus evoked short-latency responses in the d1 zone and long-latency responses in all other zones. Stimulation of the first and second somatosensory areas (SI and SII) was generally less effective in evoking climbing fibre responses than was stimulation of the p.s.g. The only exception was the c2 zone, in which responses were evoked from the SII with nearly as low thresholds and short latencies as on p.s.g. stimulation. From the parietal cortex, long-latency responses were regularly evoked in the d1 zone and less frequently in the a, b and c2 zones.

Climbing fibre modification of cerebellar Purkinje cell responses to parallel fibre inputs

Electrical stimulation has been used to activate, separately and independently, the climbing fibre inputs to Purkinje cells of the cat cerebellum. The effects of climbing fibre impulses on Purkinje cell discharges evoked by parallel fibre stimulation has been examined. It was found that impulses in the climbing fibre could block or reduce, in a graded manner, the Purkinje cell response to parallel fibre inputs. It has previously been shown that climbing a fibre inputs do not suppress the antidromic spike response of the Purkinje cell to cerebellar white matter stimulation. This suggests that the climbing fibre impulses modify the Purkinje cell response to parallel fibre inputs by reducing the excitatory action of parallel fibre impulses on the Purkinje cell.

Suppression of simple spike discharges of cerebellar Purkinje cells by impulses in climbing fibre afferents

Electrical stimulation has been used to activate climbing fibres (CFs) projecting to the cat cerebellar cortex, and the effects of low frequency (up to 10 Hz) trains of CF inputs on the discharges of cerebellar cortical neurons has been examined. Repetitive activation of the CF innervating a Purkinje (P) cell could reduce or completely suppress the ongoing simple spike discharges (SS) of the P cell. Suppression of SS was very readily produced by trains of short bursts of impulses in the CF. SS could remain suppressed for up to 4 min after such a train of CF impulses. Recording obtained from Golgi, basket and granule cells indicates that these cortical neurones did not mediate the suppression. It is proposed that the SS are suppressed as the result of a direct action of CF impulses on the P cell's dendritic tree which blocks the ongoing transmission of excitation from parallel fibres.

Modulation of harmaline-induced rhythmic discharge of the inferior olive by juxtafastigial stimulation

We previously reported induction or suppression by juxtafastigial stimulation of the rhythmic complex spike discharge of Purkinje cells in harmaline treated rats. In this paper we show that this modulation of the cerebellar rhythmic activity implies the involvement of inferior olive neurons. These results are discussed in the general framework of the olivo-cerebello-bulbar circuitry. A modulatory control of the inferior olive neuron activity by the raphe system is suggested to explain part of these results.