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

Brainstem Neural Circuits Triggering Vertical Saccades and Fixation

Neurons in the nucleus raphe interpositus have tonic activity that suppresses saccadic burst neurons (BNs) during eye fixations, and that is inhibited before and during saccades in all directions (omnipause neurons, OPNs). We have previously demonstrated via intracellular recording and anatomical staining in anesthetized cats of both sexes that OPNs are inhibited by BNs in the medullary reticular formation (horizontal inhibitory BNs, IBNs). These horizontal IBNs receive monosynaptic input from the caudal horizontal saccade area of the superior colliculus (SC), and then produce monosynaptic inhibition in OPNs, providing a mechanism to trigger saccades. However, it is well known that the neural circuits driving horizontal components of saccades are independent from the circuits driving vertical components. Thus, our previous results are unable to explain how purely vertical saccades are triggered. Here, we again apply intracellular recording to show that a disynaptic vertical IBN circuit exists, analogous to the horizontal circuit. Specifically, we show that stimulation of the SC rostral vertical saccade area produces disynaptic inhibition in OPNs, which is not abolished by midline section between the horizontal IBNs. This excludes the possibility that horizontal IBNs could be responsible for the OPN inhibition during vertical saccades. We then show that vertical IBNs in the interstitial nucleus of Cajal, which receive monosynaptic input from rostral SC, are responsible for the disynaptic inhibition of OPNs. These results indicate that a similarly functioning SC-IBN-OPN circuit exists for both the horizontal and vertical oculomotor pathways. These two IBN-mediated circuits are capable of triggering saccades in any direction.Significance Statement Saccades shift gaze to objects of interest, moving their image to the central retina, where it is maintained for detailed examination (fixation). During fixation, high gain saccade burst neurons (BNs) are tonically inhibited by omnipause neurons (OPNs). Our previous study showed that medullary horizontal inhibitory BNs (IBNs) activated from the caudal superior colliculus (SC) inhibit tonically active OPNs in order to initiate horizontal saccades. The present study addresses the source of OPN inhibition for vertical saccades. We find that OPNs monosynaptically inhibit vertical IBNs in the interstitial nucleus of Cajal during fixation. Those same vertical IBNs are activated by the rostral SC, and inhibit OPN activity to initiate vertical saccades.

Neural Circuits of Inputs and Outputs of the Cerebellar Cortex and Nuclei

This article is dedicated to the memory of Masao Ito. Masao Ito made numerous important contributions revealing the function of the cerebellum in motor control. His pioneering contributions to cerebellar physiology began with his discovery of inhibition and disinhibition of target neurons by cerebellar Purkinje cells, and his discovery of the presence of long-term depression in parallel fiber-Purkinje cell synapses. Purkinje cells formed the nodal point of Masao Ito's landmark model of motor control by the cerebellum. These discoveries became the basis for his ideas regarding the flocculus hypothesis, the adaptive motor control system, and motor learning by the cerebellum, inspiring many new experiments to test his hypotheses. This article will trace the achievements of Ito and colleagues in analyzing the neural circuits of the input-output organization of the cerebellar cortex and nuclei, particularly with respect to motor control. The article will discuss some of the important issues that have been solved and also those that remain to be solved for our understanding of motor control by the cerebellum.

Dentate nucleus as target for deep brain stimulation in dystono-dyskinetic syndromes

PURPOSE

To discuss the potential of deep brain stimulation (DBS) of the dentate nucleus as a treatment for dystono-dyskinetic syndromes.

METHODS

An extensive literature review covered the anatomy and physiology of the dentate nucleus and the experimental evidence for its involvement in the pathophysiology of dystonia and dyskinesia.

RESULTS

Evidence from animal models and from functional imaging in humans is strongly in favor of involvement of the dentate nucleus in dystono-dyskinetic syndromes. Results from previous surgical series of dentate nucleus stimulation were promising but precise description of movement disorders being treated were lacking and outcome measures were generally not well defined.

CONCLUSIONS

In the light of new evidence regarding the involvement of the dentate nucleus in dystono-dyskinetic syndromes, we present a review of the current literature and discuss why the question of dentate nucleus stimulation deserves to be revisited.

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.

Cerebellar afferents originating from the medullary reticular formation that are different from mossy, climbing or monoaminergic fibers in the rat

Integration of cortical Purkinje cell inputs and brain stem inputs is essential in generating cerebellar outputs to the cerebellar nuclei (CN). Currently, collaterals of climbing and mossy fiber axons, noradrenergic, serotoninergic and cholinergic axons, and collaterals of rubrospinal axons are known to innervate the CN from the brain stem. We investigated whether other afferents to the CN from the medulla exist in the rat. Retrograde labeling revealed the presence of neurons that project to the CN but not to the cerebellar cortex in the median reticular formation in the rostrodorsal medulla (tentatively named 'caudal raphe interpositus area', CRI). Anterograde tracer injection into the CRI labeled abundant axonal terminals in the CN, mainly in the ventral parvocellular part of the posterior interposed and lateral nucleus. Axonal reconstruction showed that a single CRI axon projected to the CN with 170-1086 varicosities, more broadly and densely than collaterals of a mossy or climbing fiber axon. CRI axons had no or a few collaterals that projected to the granular and Purkinje cell layers of the cerebellar cortex with some small terminals, indicating that these axons are different from mossy fiber axons. CRI axons also had collaterals that projected to the medial vestibular nucleus and an ascending branch that was not reconstructed. The location of the CRI, electron microscopic observations, and immunostaining results all indicated that CRI axons are not monoaminergic. We conclude that CRI axons form a type of afferent projection to the CN that is different from mossy, climbing or monoaminergic fibers.

Compartmentalization of the deep cerebellar nuclei based on afferent projections and aldolase C expression

The distribution of aldolase C (zebrin II)-positive and -negative Purkinje cells (PCs) can be used to define about 20 longitudinally extended compartments in the cerebellar cortex of the rat, which may correspond to certain aspects of cerebellar functional localization. An equivalent compartmental organization may exist in the deep cerebellar nuclei (DCN). This DCN compartmentalization is primarily represented by the afferent projection pattern in the DCN. PC projections and collateral nuclear projections of olivocerebellar climbing fiber axons have a relatively localized terminal arbor in the DCN. Projections of these axons make a closed olivo-cortico-nuclear circuit to connect a longitudinal stripe-shaped cortical compartment to a small subarea in the DCN, which can be defined as a DCN compartment. The actual DCN compartmentalization, which has been revealed by systematically mapping these projections, is quite different from the cortical compartmentalization. The stripe-shaped alternation of aldolase C-positive and -negative narrow longitudinal compartments in the cerebellar cortex is transformed to the separate clustering of positive and negative compartments in the caudoventral and rostrodorsal DCN, respectively. The distinctive projection of aldolase C-positive and -negative PCs to the caudoventral and rostrodorsal DCN underlies this transformation. Accordingly, the medial cerebellar nucleus is divided into the rostrodorsal aldolase C-negative and caudoventral aldolase C-positive parts. The anterior and posterior interposed nuclei generally correspond to the aldolase C-negative and -positive parts, respectively. DCN compartmentalization is important for understanding functional localization in the DCN since it is speculated that aldolase C-positive and -negative compartments are generally associated with somatosensory and other functions, respectively.

A decrementing form of plasticity apparent in cerebellar learning

Long-term synaptic plasticity is believed to underlie the capacity for learning and memory. In the cerebellum, for example, long-term plasticity contributes to eyelid conditioning and to learning in eye movement systems. We report evidence for a decrementing form of cerebellar plasticity as revealed by the behavioral properties of eyelid conditioning in the rabbit. We find that conditioned eyelid responses exhibit within-session changes that recover by the next day. These changes, which increase with the interstimulus interval, involve decreases in conditioned response magnitude and likelihood as well as increases in latency to onset. Within-subject comparisons show that these changes differ in magnitude depending on the type of training, arguing against motor fatigue or changes in motor pathways downstream of the cerebellum. These phenomena are also observed when stimulation of mossy fibers substitutes for the conditioned stimulus, suggesting that changes take place within the cerebellum or in downstream efferent pathways. Together, these observations suggest a plasticity mechanism in the cerebellum that is induced during training sessions and fades within 23 h. To formalize this hypothesis more specifically, we show that incorporating a short-lasting potentiation at the granule cell to Purkinje cell synapses in a computer simulation of the cerebellum reproduces these behavioral effects. We propose the working hypothesis that the presynaptic form of long-term potentiation observed at these synapses is reversed by time rather than by a corresponding long-term depression. These results demonstrate the utility of eyelid conditioning as a means to identify and characterize the rules that govern input to output transformations in the cerebellum.

Electrophysiological characterization of the cerebellum in the arterially perfused hindbrain and upper body of the rat

In the present study, a non-pulsatile arterially perfused hindbrain and upper body rat preparation is described which is an extension of the brainstem preparation reported by Potts et al., (Brain Res Bull 53(1):59-67), 1. The modified in situ preparation allows study of cerebellar function whilst preserving the integrity of many of its interconnections with the brainstem, upper spinal cord and the peripheral nervous system of the head and forelimbs. Evoked mossy fibre, climbing fibre and parallel fibre field potentials and EMG activity elicited in forelimb biceps muscle by interpositus stimulation provided evidence that both cerebellar inputs and outputs remain operational in this preparation. Similarly, the spontaneous and evoked single unit activity of Purkinje cells, putative Golgi cells, molecular interneurones and cerebellar nuclear neurones was similar to activity patterns reported in vivo. The advantages of the preparation include the ability to record, without the complications of anaesthesia, stabile single unit activity for extended periods (3 h or more), from regions of the rat cerebellum that are difficult to access in vivo. The preparation should therefore be a useful adjunct to in vitro and in vivo studies of neural circuits underlying cerebellar contributions to movement control and motor learning.

Projection of reconstructed single Purkinje cell axons in relation to the cortical and nuclear aldolase C compartments of the rat cerebellum

Although the overall topography of the cerebellar corticonuclear projection formed by Purkinje cell (PC) axons has been described, only a few studies have dealt with the organization of this projection at the level of individual PC axons. Thus, we reconstructed 65 single PC axons that were labeled with biotinylated dextran amine in the rat. We then analyzed the relationship between the projections of these PCs and the compartmentalization of the cerebellar cortex and nuclei based on the topography of olivocerebellar projection and aldolase C expression in PCs. After giving rise to short local recurrent collaterals near the soma, a PC axon formed a terminal arbor in a specific small area in the cerebellar nuclei (CN). The terminal arbors of vermal PCs were spread more widely than those of hemispheric PCs and sometimes extended to extracerebellar targets. PCs located in any of the aldolase C-positive (Groups I and II) and -negative (Groups III and IV) stripes consistently projected to the caudoventral and rostrodorsal parts of the CN, respectively, precisely in accordance with the compartmentalization of the cortex and nuclei. Mediolateral segregation and rostrocaudal convergence were seen between projections of separate PCs in a single aldolase C compartment. The results revealed a tight link between the projection patterns of individual PC axons, the topography of the olivocerebellar pathway, and the aldolase C expression pattern. Their overall correspondence seems to reflect a basic aspect of cerebellar organization, although some area-dependent variation in the relationship of these three entities was also present.

Commissural mirror-symmetric excitation and reciprocal inhibition between the two superior colliculi and their roles in vertical and horizontal eye movements

The functional roles of commissural excitation and inhibition between the two superior colliculi (SCs) are not yet well understood. We previously showed the existence of strong excitatory commissural connections between the rostral SCs, although commissural connections had been considered to be mainly inhibitory. In this study, by recording intracellular potentials, we examined the topographical distribution of commissural monosynaptic excitation and inhibition from the contralateral medial and lateral SC to tectoreticular neurons (TRNs) in the medial or lateral SC of anesthetized cats. About 85% of TRNs examined projected to both the ipsilateral Forel's field H and the contralateral inhibitory burst neuron region where the respective premotor neurons for vertical and horizontal saccades reside. Medial TRNs received strong commissural excitation from the medial part of the opposite SC, whereas lateral TRNs received excitation mainly from its lateral part. Injection of wheat germ agglutinin-horseradish peroxidase into the lateral or medial SC retrogradely labeled many larger neurons in the lateral or medial part of the contralateral SC, respectively. These results indicated that excitatory commissural connections exist between the medial and medial parts and between the lateral and lateral parts of the rostral SCs. These may play an important role in reinforcing the conjugacy of upward and downward saccades, respectively. In contrast, medial SC projections to lateral SC TRNs and lateral SC projections to medial TRNs mainly produce strong inhibition. This shows that regions representing upward saccades inhibit contralateral regions representing downward saccades and vice versa.

Functional Unity of the Ponto-Cerebellum: Evidence That Intrapontine Communication Is Mediated by a Reciprocal Loop With the Cerebellar Nuclei

The majority of cerebral signals destined for the cerebellum are handed over by the pontine nuclei (PN), which thoroughly reorganize the neocortical topography. The PN maps neocortical signals of wide-spread origins into adjacent compartments delineated by spatially precise distribution of cortical terminals and postsynaptic dendrites. We asked whether and how signals interact on the level of the PN. Intracellular fillings of rat PN cells in vitro did not reveal any intrinsic axonal branching neither within the range of the cells' dendrites nor farther away. Furthermore, double whole cell patch recordings did not show any signs of interaction between neighboring pontine cells. Using simultaneous unit recording in the PN and cerebellar nuclei (CN) in rats in vivo, we investigated whether PN compartments interact via extrinsic reciprocal connections with the CN. Repetitive electrical stimulation of the cerebral peduncle of ≤40 Hz readily evoked rapid sequential activation of PN and CN, demonstrating a direct connection between the structures. Stimulation of the PN gray matter led to responses in neurons ≤600 μm away from the stimulation site at latencies compatible with di- or polysynaptic pathways via the CN. Importantly, these interactions were spatially discontinuous around the stimulation electrode suggesting that reciprocal PN-CN loops in addition reflect the compartmentalized organization of the PN. These findings are in line with the idea that the cerebellum makes use of the compartmentalized map in the PN to orchestrate the composition of its own neocortical input.

Cortico-cerebellar coherence during a precision grip task in the monkey

We studied the synchronization of single units in macaque deep cerebellar nuclei (DCN) with local field potentials (LFPs) in primary motor cortex (M1) bilaterally during performance of a precision grip task. Analysis was restricted to periods of steady holding, during which M1 oscillations are known to be strongest. Significant coherence between DCN units and M1 LFP oscillations bilaterally was seen at approximately 10-40 Hz (contralateral M1: 25/87 units; ipsilateral: 9/87 units). Averaged coherence between DCN units and contralateral M1 LFP showed a prominent approximately 17-Hz coherence peak and an average phase of approximately -pi/2 radians, implying that the DCN units fired around the time of maximal depolarization of M1 cells. The lack of a time delay between DCN and M1 activity suggests that the cerebellum and cortex may form a pair of phase coupled oscillators. Although coherence values were low (mean peak coherence, 0.018), we used a computational model to show that this probably resulted from the nonlinearity of spike generating mechanisms within the DCN. DCN unit discharge and DCN LFPs also showed significant coherence at approximately 10-40 Hz, with similarly low magnitude (mean peak coherence, 0.012). The average coherence phase was -2.5 radians for the 6- to 14-Hz range and -1.1 radians for the 17- to 41-Hz range, suggesting different frequency-specific underlying mechanisms. Finally, 4/40 pairs of simultaneously recorded DCN units showed a significant cross-correlation peak, and 16/40 pairs showed significant unit-unit coherence. The extensive oscillatory synchronization observed between cerebellum and motor cortex may have functional importance in sensorimotor processing.

The oculomotor role of the pontine nuclei and the nucleus reticularis tegmenti pontis

Cerebral cortex and the cerebellum interact closely in order to facilitate spatial orientation and the generation of motor behavior, including eye movements. This interaction is based on a massive projection system that allows the exchange of signals between the two cortices. This cerebro-cerebellar communication system includes several intercalated brain stem nuclei, whose eminent role in the organization of oculomotor behavior has only recently become apparent. This review focuses on the two major nuclei of this group taking a precerebellar position, the pontine nuclei and the nucleus reticularis tegmenti pontis, both intimately involved in the visual guidance of eye movements.

Commissural excitation and inhibition by the superior colliculus in tectoreticular neurons projecting to omnipause neuron and inhibitory burst neuron regions

Previous electrophysiological studies have shown that the commissural connections between the two superior colliculi are mainly inhibitory with fewer excitatory connections. However, the functional roles of the commissural connections are not well understood, so we sought to clarify the physiology of tectal commissural excitation and inhibition of tectoreticular neurons (TRNs) in the "fixation " and "saccade " zones of the superior colliculus (SC). By recording intracellular potentials, we identified TRNs by their antidromic responses to stimulation of the omnipause neuron (OPN) and inhibitory burst neuron (IBN) regions and analyzed the effects of stimulation of the contralateral SC on these TRNs in anesthetized cats. TRNs in the caudal SC (saccade neurons) projected to the IBN region, and received mono- or disynaptic inhibition from the entire rostrocaudal extent of the contralateral SC. In contrast, TRNs in the rostral SC projected to the OPN or IBN region and received monosynaptic excitation from the most rostral level of the contralateral SC, and mono- or disynaptic inhibition from its entire rostrocaudal extent. Among the rostral TRNs with commissural excitation, IBN-projecting TRNs also projected to Forel's field H (vertical gaze center), suggesting that they were most likely saccade neurons related to vertical saccades. In contrast, TRNs projecting only to the OPN region were most likely fixation neurons. Most putative inhibitory neurons in the rostral SC had multiple axon branches throughout the rostrocaudal extent of the contralateral SC, whereas excitatory commissural neurons, most of which were rostral TRNs, distributed terminals to a discrete region in the rostral SC.

Cerebellar thalamic activity in the macaque monkey encodes the duration but not the force or velocity of wrist movement

The way in which the cerebellum influences the output of the motor cortex is not known. The aim of this study was to establish whether information about force, velocity or duration of movement is encoded in cerebellar thalamic discharge and could therefore be involved in the modulation of motor cortical activity. Extracellular single cell recordings were made from the cerebellar thalamus (66 neurones) and VPLc (49 neurones) of four conscious macaques performing simple wrist movements with various load and gain conditions imposed. A significant correlation (Spearman's; P<0.05) was found between movement duration and the duration of neuronal discharge of most cerebellar thalamic neurones (65%), the velocity of movement and rate of neuronal discharge of some cerebellar thalamic neurones (23%), but not between force of movement and rate of neuronal discharge of any cerebellar thalamic neurones. Similar relationships were found between the activity of VPLc neurones and these movement parameters. The strength of the correlations increased when many cells were grouped and analysed as an ensemble, suggesting that populations of cerebellar thalamic (and VPLc) neurones can encode a signal with higher fidelity than single neurones alone. The ensemble data confirmed that the most robust association was between the duration of neuronal discharge and movement duration. We propose that the cerebellum does not provide the motor cortex with specific information about movement force or velocity, but rather that its major role is in activating many motor cortical regions for a specific duration, thus influencing the timing of complex movements involving many muscles and joints.

Physiological Characterization of Synaptic Inputs to Inhibitory Burst Neurons From the Rostral and Caudal Superior Colliculus

The caudal superior colliculus (SC) contains movement neurons that fire during saccades and the rostral SC contains fixation neurons that fire during visual fixation, suggesting potentially different functions for these 2 regions. To study whether these areas might have different projections, we characterized synaptic inputs from the rostral and caudal SC to inhibitory burst neurons (IBNs) in anesthetized cats. We recorded intracellular potentials from neurons in the IBN region and identified them as IBNs based on their antidromic activation from the contralateral abducens nucleus and short-latency excitation from the contralateral caudal SC and/or single-cell morphology. IBNs received disynaptic inhibition from the ipsilateral caudal SC and disynaptic inhibition from the rostral SC on both sides. Stimulation of the contralateral IBN region evoked monosynaptic inhibition in IBNs, which was enhanced by preconditioning stimulation of the ipsilateral caudal SC. A midline section between the IBN regions eliminated inhibition from the ipsilateral caudal SC, but inhibition from the rostral SC remained unaffected, indicating that the latter inhibition was mediated by inhibitory interneurons other than IBNs. A transverse section of the brain stem rostral to the pause neuron (PN) region eliminated inhibition from the rostral SC, suggesting that this inhibition is mediated by PNs. These results indicate that the most rostral SC inhibits bilateral IBNs, most likely via PNs, and the more caudal SC exerts monosynaptic excitation on contralateral IBNs and antagonistic inhibition on ipsilateral IBNs via contralateral IBNs. The most rostral SC may play roles in maintaining fixation by inhibition of burst neurons and facilitating saccadic initiation by releasing their inhibition.

Functional compartmentalization in the flocculus and the ventral dentate and dorsal group y nuclei: an analysis of single olivocerebellar axonal morphology

The cerebellar cortex consists of multiple longitudinal bands defined by their olivocerebellar projection. Single olivocerebellar axons project to a narrow longitudinal band in the cerebellar cortex and to the cerebellar nucleus with their axon collaterals. This olivocortical and olivonuclear organization is related to the functional compartmentalization of the cerebellar system. To reveal the detailed morphologic organization in the flocculus and the cerebellar and vestibular nuclei, we examined olivocerebellar projection by reconstructing the entire trajectories of 19 single olivofloccular axons and by anterograde mapping with biotinylated dextran in the rat. The flocculus was composed of 12 longitudinal band-shaped compartments that subdivided 5 previously described zones. These longitudinal bands were innervated differentially by the caudal and rostral portions of the dorsal cap (DC) and the ventrolateral outgrowth (VLO) and the rostral pole of the medial accessory olive. Single olivofloccular axons with an average of 5.1 climbing fibers usually projected to a single longitudinal band in the flocculus and to the ventral dentate or dorsal group y nucleus with their collaterals. DC neurons projected moderately to the rostrolateral portion of the ventral dentate nucleus, whereas VLO neurons projected densely to the medial portion of the ventral dentate nucleus and the dorsal group y nucleus with rostrocaudal topography. DC and VLO neurons did not project to the vestibular nuclei, although floccular Purkinje cells projected to the vestibular, ventral dentate, and dorsal group y nuclei. The fine morphologically identified longitudinal bands and topographic olivonuclear projections were correlated with previous electrophysiologically defined functional zones in the flocculus and inferior olive.

The entire trajectory of single climbing and mossy fibers in the cerebellar nuclei and cortex

The present study has revealed that OC axons gave rise to a number of thin collaterals. Due to the abundance of these non-CF thin collaterals, it seems better to make a distinction between the terms CFs and OC axons, as was done in the present paper. The present findings on the innervation of PC dendrites by CFs are basically similar to those in previous reports (Ramón y Cajal, 1911; Palay and Chan-Palay, 1974). The number of swellings on a single CF in the present study (n = 250) is comparable to a previously measured value in the rat (n = 288; Rossi et al., 1993) and larger than a value in the frog (n = about 100 beads; Llinás et al., 1969). The average number of CFs per OC axon in this study was close to the number (n = about 7) inferred in the rat by counting the total number of IO neurons and PCs (Schild, 1970). Contact of interneurons by some swellings of CFs in the molecular layer was emphasized by Scheibel and Scheibel (1954) in their study with Golgi staining. Despite the contact of CF terminals on interneurons, the formation of a synaptic structure between them has been excluded in an electron-microscopic study (Hámori and Szentàothai, 1980). On the other hand, electrophysiological studies have demonstrated a weak excitatory effect of CFs on some interneurons (Eccles et al., 1966). Terminals in the granular layer were originated either from thin collaterals of OC axons or from retrograde collaterals of CF terminal arborizations. The former was the main source of swellings in the granular layer. The morphology of the thin collaterals in the present study was consistent with "globose varicosities connected by a fine thread" as described in Golgi preparations and electron micrograms (Chan-Palay and Palay, 1971). Swellings of thin collaterals (about 1.7% of the total number of swellings per OC axon) were most abundant in the upper portion of the granular layer just underneath the PC layer, in which Golgi cells are usually located. Furthermore, some of these swellings were observed to touch presumed Golgi cells in the present study, which is consistent with electron-microscopic findings on the innervation of somata of Golgi cells by thin collaterals (Hámori and Szentàothai, 1980; Chan-Palay and Palay, 1971). Inferior olive stimulation has been shown electrophysiologically to have a weak direct excitatory effect on Golgi cells (Eccles et al., 1966). Ninety-one percent of the OC axons examined had nuclear collaterals; since the possibility of insufficient staining could not be excluded, this percentage may be an underestimation. The ratio of swellings in the cerebellar nuclei versus those of CF terminal arborizations was about 0.036 in individual OC axons in the present study. However, since the volume of the cerebellar nuclei is much smaller than that of the cerebellar cortex, and significant convergence of input from OC axons to cerebellar nucleus neurons is present (Sugihara et al., 1996), cerebellar nucleus projection of OC fibers can still be functionally important. Some swellings seemed to make contact with the soma and the proximal portions of dendrites of large neurons in the present study, which is consistent with the steep rising phase of postsynaptic excitatory potentials in cerebellar nucleus neurons following IO stimulation (Kitai et al., 1977; Shinoda et al., 1987). Although intracellular potentials were presumably recorded only from large output neurons in the cerebellar nuclei, the present study suggested that small neurons were also innervated by OC axons. The present study revealed that virtually all reconstructed LRN axons projected not only to the Cx as mossy fibers, but also to the DCN including the VN by their axon collaterals. None of the LRN neurons specifically projected to the DCN without projecting to the Cx, namely all axon terminals of LRN neurons in the DCN and VN belonged to axon collaterals of mossy fibers projecting to the Cx. (ABSTRACT TRUNCATED)

Timing mechanisms in the cerebellum: testing predictions of a large-scale computer simulation

We used large-scale computer simulations of eyelid conditioning to investigate how the cerebellum generates and makes use of temporal information. In the simulations the adaptive timing displayed by conditioned responses is mediated by two factors: (1) different sets of granule cells are active at different times during the conditioned stimulus (CS), and (2) responding is not only amplified at reinforced times but also suppressed at unreinforced times during the CS. These factors predict an unusual pattern of responding after partial removal of the cerebellar cortex that was confirmed using small, electrolytic lesions of cerebellar cortex. These results are consistent with timing mechanisms in the cerebellum that are similar to Pavlov's "inhibition of delay" hypothesis.

Short-lasting conditioned stimulus applied to the middle cerebellar peduncle elicits delayed conditioned eye blink responses in the decerebrate ferret

In delay eye blink conditioning, the conditioned stimulus (CS) ends at the time of the unconditioned stimulus (US). If the CS duration is decreased, there will be a 'trace' period with no ongoing CS before the onset of the US. During this period some neural activity has to continue after the CS offset to: (i) permit association between the CS and the US; and (ii) elicit a conditioned response appearing after the CS offset. In this study we test the role of the cerebellum in maintaining CS activity required for eliciting a conditioned response after the CS offset. Decerebrate ferrets were trained in a delay conditioning paradigm with an electrical stimulation of the forelimb as CS and of the periorbital area as US. The conditioned responses in the upper eyelid were monitored with electromyographical techniques. In well-trained animals, test CSs of short duration down to 0.2 ms were applied to the forelimb or the middle cerebellar peduncle, while the interstimulus interval between CS onset and US onset was kept constant at 300 ms. Test CSs of short duration applied to the forelimb elicited conditioned responses. More importantly, also a short-lasting CS to the middle cerebellar peduncle could elicit conditioned responses. The results indicate that precerebellar CS pathways are not required for maintaining the neural activity that elicits conditioned responses after the CS offset. It is suggested that neurons maintaining such activity are located in the cerebellum, either the cortex alone or the cortex and the deep nuclei.

Learned movements elicited by direct stimulation of cerebellar mossy fiber afferents

Definitive evidence is presented that the conditioned stimulus (CS) in classical conditioning reaches the cerebellum via the mossy fiber system. Decerebrate ferrets received paired forelimb and periocular stimulation until they responded with blinks to the forelimb stimulus. When direct mossy fiber stimulation was then given, the animals responded with conditioned blinks immediately, that is, without ever having been trained to the mossy fiber stimulation. Antidromic activation was prevented by blocking mossy fibers with lignocaine ventral to the stimulation site. It could be excluded that cerebellar output functioned as the CS. Analysis of latencies suggests that conditioned responses (CRs) are not generated by mossy fiber collaterals to the deep nuclei. Hence, the memory trace is probably located in the cerebellar cortex.

Cerebellar output channels

The cerebellum has long been regarded as involved in the control of movement, in part through its connections with the cerebral cortex. These connections were thought to combine inputs from widespread regions of the cerebral cortex and "funnel" them into the motor system at the level of the primary motor cortex. Retrograde transneuronal transport of herpes simplex virus type I has recently been used to identify areas of the cerebral cortex that are "directly" influenced by the output of the cerebellum. Results suggest that cerebellar output projects via the thalamus to multiple cortical areas, including premotor and prefrontal cortex, as well as the primary motor cortex. In addition, the projections to different cortical areas appear to originate from distinct regions of the deep cerebellar nuclei. These observations have led to the proposal that cerebellar output is composed of a number of separate "output channels." Evidence from functional imaging studies in humans and single neuron recording studies in monkeys suggests that individual output channels are concerned with different aspects of motor or cognitive behavior.

Ultrastructural study of the GABAergic and cerebellar input to the nucleus reticularis tegmenti pontis

The nucleus reticularis tegmenti pontis is an intermediate of the cerebrocerebellar pathway and serves as a relay centre for sensorimotor and visual information. The central nuclei of the cerebellum provide a dense projection to the nucleus reticularis tegmenti pontis, but it is not known to what extent this projection is excitatory or inhibitory, and whether the terminals of this projection contact the neurons in the nucleus reticularis tegmenti pontis that give rise to the mossy fibre collaterals innervating the cerebellar nuclei. In the present study the nucleus reticularis tegmenti pontis of the cat was investigated at the ultrastructural level following anterograde and retrograde transport of wheat germ agglutinin coupled to horseradish peroxidase (WGA-HRP) from the cerebellar nuclei combined with postembedding GABA immunocytochemistry. The neuropil of this nucleus was found to contain many WGA-HRP labeled terminals, cell bodies and dendrites, but none of these pre- or postsynaptic structures was double labeled with GABA. The vast majority of the WGA-HRP labeled terminals contained clear spherical vesicles, showed asymmetric synapses, and contacted intermediate or distal dendrites. Many of the postsynaptic elements of the cerebellar afferents in the nucleus reticularis tegmenti pontis were retrogradely labeled with WGA-HRP, while relatively few were GABAergic. We conclude that all cerebellar terminals in the nucleus reticularis tegmenti pontis of the cat are nonGABAergic and excitatory, and that they contact predominantly neurons that project back to the cerebellum. Thus, the reciprocal circuit between the cerebellar nuclei and the nucleus reticularis tegmenti pontis appears to be well designed to function as an excitatory reverberating loop.

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.

Morphology of axon collaterals of single climbing fibers in the deep cerebellar nuclei of the rat

Projection of inferior olive (IO) neurons to the deep cerebellar nuclei (CN) was investigated in the rat by reconstructing single axons that were labeled with biotinylated dextran amine injected into the IO. All reconstructed terminal arborizations in the CN (n = 18) arose as collaterals from climbing fibers (CFs). One to six nuclear collaterals were given off from each of six CFs that were reconstructed along the nearly entire pathway backward from cortical terminal arborizations to the IO. Nuclear collaterals were much thinner (0.2-0.3 micron in diameter) than stem axons projecting to Purkinje cells (0.7-1.4 microns). The number of swellings per a single nuclear collateral ranged from 24 to 118 (n = 18). Terminal arborizations of nuclear collateral originating from a single CF spread for some hundreds of micrometers and occupied a localized portion within the CN.

A projection linking motor cortex with the LM-suprageniculate nuclear complex through the periaqueductal gray area which surrounds the nucleus of Darkschewitsch in the cat

Whereas a previous study by one of us (Hicks et al., 1986) suggested that periaqueductal gray (PAG) neurons projecting to the lateralis medialis-suprageniculate (LM-SG) complex might mediate transmission of affective-related nociceptive information, our present work suggests instead, a function in processes related to movement. Cells of the nucleus of Darkschewitsch (ND) are known to have reciprocal projections with the motor cortex (MX), in particular with the hand area of MX, and also to project to the rostral medial accessory olivary (MAO) nucleus (Onodera and Hicks, 1995a). That the ND might be related to saccadic oculomotor function, as well as to the control of hand movements through its connections via the olivo-cerebellar circuit, is indicated by the fact that ND receives a strong projection from the substantia nigra pars reticulata and zona incerta (SNR/ZI) and projects directly and/or indirectly to eye movement nuclei (Onodera and Hicks, 1995b). Thus, ND may function in permitting integration of eye-hand motor coordination. This study focussed on the area of PAG surrounding ND. WGA-HRP was injected into MX and many labelled terminals and large neurones were in ND, with lesser numbers being observed in the area of the PAG surrounding ND. After injections into ND and closely adjacent areas, labelled terminals were observed sparsely distributed with a restricted area of the LM-SG complex. After injections into LM-SG area, small neuronal somata were seen in the area of the PAG surrounding ND, but no labelled somata were detected in ND. Thus if the cells of this PAG area, like those of ND, have similar functions owing to their common reciprocal connections with MX, then the small neurones in PAG projecting to LM-SG may constitute an important link in the circuitry subserving visual processing and/or the regulation of orienting movements of the hand, head and eye.

On the cerebellum, cutaneomuscular reflexes, movement control and the elusive engrams of memory

This review focuses on the role of the cerebellum in regulating cutaneomuscular reflexes and provides a hypothesis regarding the way in which this action contributes to the coordination of goal-directed movements of the extremities. Specific attention is directed towards the cerebellum's role in conditioned and unconditioned eyeblink reflexes and limb withdrawal reflexes as models of its interactions with the cutaneomuscular reflex systems. The implications regarding the cerebellum as a storage site for motor engrams also is discussed in the context of these two behaviors. The proposed hypothesis suggests that the cerebellum regulates important features of the cutaneomuscular reflex circuits including the integration of their activity with descending pathways in a manner that implements these fundamental reflex circuits in the organization and control of goal-directed movements of the extremities.

Projections from the lateral and interposed cerebellar nuclei to the thalamus of the rat: a light and electron microscopic study using single and double anterograde labelling

The lateral and interposed cerebellar nuclei may have different functions in the control of movement. Efferent fibres from both nuclei project predominantly to areas of the thalamus, which in turn project to the motor cortex. In this study, single and double anterograde-tracing techniques have been used to examine and compare the pathways from the lateral and interposed nuclei to the thalamus in the rat by using both light and electron microscopy to look for evidence of organisational or structural features that may underlie the proposed functional differences between these nuclei. Terminals from the lateral nucleus were found to be located most medially in the thalamus, predominantly in the ventral lateral nucleus and the rostral pole of the posterior nuclear group. Terminals from the posterior interposed nucleus were located slightly rostral and lateral to those from the lateral nucleus, mainly around the border between the ventral lateral nucleus and the ventral posterior medial nucleus. Terminals from the anterior interposed nucleus were located slightly rostral and lateral to those from the posterior interposed nucleus, predominantly in the rostral pole of the ventral posterior lateral nucleus. Terminals from the lateral and interposed nuclei were also found in double anterograde-tracing experiments to be nonoverlapping in the regions between these main areas of termination. The structure of terminals from the lateral and interposed nuclei, however, as well as their synaptic relationship with thalamic neurones, were found to be similar. The terminals are large and form synapses with proximal dendrites of thalamic neurones. They contained round vesicles and formed multiple synaptic contacts with dendritic shafts, as well as dendritic spines. The findings indicate that information from the lateral and interposed nuclei is processed in separate regions of the thalamus but that the mode of synaptic transfer to thalamic neurones is likely to be similar for the two projections.

Identification of pontocerebellar axon collateral synaptic boutons in the rat cerebellar nuclei

Injections of the orthogradely transported tracer PHA-L into the basilar pontine nuclei or reticulotegmental nucleus of hooded rats produced labeling of pontocerebellar axons that distributed to the cerebellar cortex and nuclei. EM examination of the lateral and interposed cerebellar nuclei revealed that labeled pontocerebellar axon terminals formed synaptic boutons in the cerebellar nuclei that were morphologically different from the characteristic mossy fiber terminals observed in the cerebellar cortex.

Cerebellar nuclear projections from the basilar pontine nuclei and nucleus reticularis tegmenti pontis as demonstrated with PHA-L tracing in the rat

Small iontophoretic placements of the orthogradely transported axonal tracer Phaseolus vulgaris leucoagglutinin (PHA-L) were made in portions of the basilar pontine nuclei (BPN) or nucleus reticularis tegmenti pontis (NRTP) to determine if these cell groups provide projections to the cerebellar nuclei (CN) in the rat and if so, to visualize the morphology of the axons and terminals and illustrate any topographical organization in this system. Axons that originated from BPN or NRTP neurons and contained PHA-L were visualized by an immunohistochemical procedure that involved sequential incubation of tissue sections with goat anti-PHA-L antibody, biotinylated rabbit anti-goat immunoglobulin, and a biotin-avidin-peroxidase conjugate. Following injections of PHA-L restricted to ventral and medial portions of the BPN, labeled fibers were observed in the brachium pontis, the white matter dorsal to the CN, and to a lesser extent, in the white matter of the parafloccular stalk. Labeled preterminal axons entered the CN and gave rise to beaded axons that arborized primarily within dorsal portions of the lateral, interposed, and medial cerebellar nuclei. Injections of PHA-L restricted to either lateral portions of the BPN or ventrolateral regions of NRTP produced labeled fibers in the cerebellum that most frequently involved the parafloccular stalk and ventral portions of the CN. In contrast, dorsomedial NRTP injections resulted in the presence of labeled fibers both in the dorsal cerebellar white matter and the parafloccular stalk as well as dorsal and ventral portions of the CN. With the exception of the rostral and medial territory of interpositus anterior which received very sparse input, all portions of each CN subdivision seemed to exhibit some degree of terminal labeling. The density of labeled axon terminals in the CN appeared to be somewhat greater in the NRTP-injected cases compared to BPN-injected animals. These observations indicate that in the rat, both the BPN and NRTP contain neurons whose axons distribute to the CN. It is likely that most of the axons which project to the CN are collaterals of fibers that continue into the cerebellar cortex as mossy fibers but confirmation of this point must await further investigation. In light of the extensive projections from the cerebral cortex to the BPN and NRTP, this axonal system provides the cerebral cortex with a relatively direct route of access to the CN via one synapse in the BPN or NRTP.

Organization of the pontine nuclei

The pontine nuclei provide the cerebellar hemispheres with the majority of their mossy fiber afferents, and receive their main input from the cerebral cortex. Even though the vast majority of pontine neurons send their axons to the cerebellar cortex, and are contacted monosynaptically by (glutamatergic) corticopontine fibers, the information-processing taking place is not well understood. In addition to typical projection neurons, the pontine nuclei contain putative GABA-ergic interneurons and complex synaptic arrangements. The corticopontine projection is characterized by a precise but highly divergent terminal pattern. Large and functionally diverse parts of the cerebral cortex contribute; in the monkey the most notable exception is the almost total lack of projections from large parts of the prefrontal and temporal cortices. Within corticopontine projections from visual and somatosensory areas there is a de-emphasis of central vision and distal parts of the extremities as compared with other connections of these sensory areas. Subcorticopontine projections provide only a few percent of the total input to the pontine nuclei. Certain cell groups, such as the reticular formation, project in a diffuse manner whereas other nuclei, such as the mammillary nucleus, project to restricted pontine regions only, partially converging with functionally related corticopontine connections. The pontocerebellar projection is characterized by a highly convergent pattern, even though there is also marked divergence. Neurons projecting to a single cerebellar folium appear to be confined to a lamella-shaped volume in the pontine nuclei. The organization of the pontine nuclei suggests that they ensure that information from various, functionally diverse, parts of the cerebral cortex and subcortical nuclei are brought together and integrated in the cerebellar cortex.

Direction of transneuronal transport of herpes simplex virus 1 in the primate motor system is strain-dependent

We examined the axonal transport of two strains of herpes simplex virus 1 (HSV-1) within the central nervous system of cebus monkeys. Each strain was injected into the "arm area" of the primary motor cortex. One strain, HSV-1(McIntyre-B), was transported transneuronally in the retrograde direction. It infected neurons at sites known to project to the arm area of the primary motor cortex (e.g., ventrolateral thalamus). In addition, "second-order" neurons were labeled in the deep cerebellar nuclei (dentate and interpositus) and in the globus pallidus (internal segment). This result supports the concept that the arm area of the primary motor cortex is a target of both cerebellar and basal ganglia output. In contrast, the other strain, HSV-1(H129), was transported transneuronally in the anterograde direction. It infected neurons at sites known to receive input from the arm area of the primary motor cortex (e.g., putamen, pontine nuclei). In addition, "third-order" neurons were labeled in the cerebellar cortex (granule and Golgi cells) and in the globus pallidus (largely the external segment). Our observations suggest that strain differences have an important impact on the direction of transneuronal transport of HSV-1. Furthermore, it should be possible to examine the organization of cerebellar and basal ganglia loops with cerebral cortex by exploiting transneuronal transport of HSV-1 and virus strain differences.