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

Consensus paper: current views on the role of cerebellar interpositus nucleus in movement control and emotion

In the present paper, we examine the role of the cerebellar interpositus nucleus (IN) in motor and non-motor domains. Recent findings are considered, and we share the following conclusions: IN as part of the olivo-cortico-nuclear microcircuit is involved in providing powerful timing signals important in coordinating limb movements; IN could participate in the timing and performance of ongoing conditioned responses rather than the generation and/or initiation of such responses; IN is involved in the control of reflexive and voluntary movements in a task- and effector system-dependent fashion, including hand movements and associated upper limb adjustments, for quick effective actions; IN develops internal models for dynamic interactions of the motor system with the external environment for anticipatory control of movement; and IN plays a significant role in the modulation of autonomic and emotional functions.

Blocking glutamate-mediated inferior olivary signals abolishes expression of conditioned eyeblinks but does not prevent their acquisition

The inferior olive (IO) is considered a crucial component of the eyeblink conditioning network. The cerebellar learning hypothesis proposes that the IO provides the cerebellum with a teaching signal that is required for the acquisition and maintenance of conditioned eyeblinks. Supporting this concept, previous experiments showed that lesions or inactivation of the IO blocked CR acquisition. However, these studies were not conclusive. The drawback of the methods used by those studies is that they not only blocked task-related signals, but also completely shut down the spontaneous activity within the IO, which affects the rest of the eyeblink circuits in a nonspecific manner. We hypothesized that more selective blocking of task-related IO signals could be achieved by using injections of glutamate antagonists, which reduce, but do not eliminate, the spontaneous activity in the IO. We expected that if glutamate-mediated IO signals are required for learning, then blocking these signals during training sessions should prevent conditioned response (CR) acquisition. To test this prediction, rabbits were trained to acquire conditioned eyeblinks to a mild vibrissal airpuff as the conditioned stimulus while injections of the glutamate antagonist γ-d-glutamylglycine were administered to the IO. Remarkably, even though this treatment suppressed CRs during training sessions, the postacquisition retention test revealed that CR acquisition had not been abolished. The ability to acquire CRs with IO unconditioned stimulus signals that were blocked or severely suppressed suggests that mechanisms responsible for CR acquisition are extremely resilient and probably less dependent on IO-task-related signals than previously thought.

Assessing the role of inferior olivary sensory signaling in the expression of conditioned eyeblinks using a combined glutamate/GABAA receptor antagonist protocol

The inferior olive (IO) is a major component of the eyeblink conditioning neural network. The cerebellar learning hypothesis assumes that the IO supplies the cerebellum with a "teaching" unconditioned stimulus input required for the acquisition of the conditioned response (CR) and predicts that inactivating this input leads to the extinction of CRs. Previous tests of this prediction attempted to block the teaching input by blocking glutamatergic sensory inputs in the IO. These tests were inconclusive because blocking glutamate neurotransmission in the IO produces a nonspecific tonic malfunction of cerebellar circuits. The purpose of the present experiment was to examine whether the behavioral outcomes of blocking glutamate receptors in the IO could be counterbalanced by reducing GABA-mediated inhibition in the IO. We found that injecting the IO with the glutamate antagonist γ-d-glutamylglycine (DGG) abolished previously learned CRs, whereas injecting the GABA(A) receptor antagonist gabazine at the same site did not affect CR incidence but shortened CR latencies and produced tonic eyelid closure. To test whether the glutamate antagonist-induced behavioral deficit could be offset by elevating IO activity with GABA(A) antagonists, rabbits were first injected with DGG and then with gabazine in the same training session. While DGG abolished CRs, follow-up injections of gabazine accelerated their recovery. These findings suggest that the level of IO neuronal activity is critical for the performance of CRs, and that combined pharmacological approaches that maintain spontaneous activity at near normal levels hold tremendous potential for unveiling the role of IO-mediated signals in eyeblink conditioning.

Responses to tactile stimulation in deep cerebellar nucleus neurons result from recurrent activation in multiple pathways

In a previous study, we found that neurons in the deep cerebellar nuclei (DCN) respond to 5-ms brief facial tactile stimulation in rats anesthetized with ketamine-xylazine with multiphasic response patterns lasting over 200 ms. It remained unclear, however, to what extent these responses were shaped not only by ascending sensory input from the trigeminal nuclei but also by interactions with other major cerebellar afferent systems, in particular the inferior olive (IO) and cerebral cortex. In the present study, we recorded from the IO, cerebral cortex, cerebellar granule cell layer (GCL), and DCN during the presentation of 5-ms facial tactile stimuli to elucidate potential mechanisms of how extended DCN response patterns are generated. We found that tactile stimulation resulted in robust multiphasic local field potentials responses in the IO as well as in the activation of a wide region of the somatosensory cortex (SI) and the primary motor cortex (MI). DCN neurons responded to electrical stimulation of any of these structures (IO, SI, and MI) with complex temporal patterns strikingly similar to air-puff lip stimulation responses. Simultaneous recordings from multiple structures revealed that long-lasting activation patterns elicited in DCN neurons were based on recurrent network activation in particular between the IO and the DCN with a potential contribution of DCN rebound properties. These results are consistent with the hypothesis that sensory stimulation triggers a feedback network activation of cerebellum, IO, and cerebral cortex to generate temporal patterns of activity that may control the timing of behavior.

Inferior olivary inactivation abolishes conditioned eyeblinks: extinction or cerebellar malfunction?

The inferior olive (IO) is a required component of neural circuits controlling the classical conditioning of eyeblink responses. Previous reports indicated that lesioning or inactivating the IO abolishes conditioned eyeblinks (CRs), but there was disagreement regarding the timing of the CR performance deficit. As a result, it was not clear whether IO inactivation produces unlearning of CRs or a non-specific dysfunction of cerebellar circuits. Since most of these studies used methods that could block unrelated axons passing through the IO region, additional experiments are required to further elucidate IO function, using inactivating agents that act selectively on cell bodies. In the present study, the IO was inactivated using the glutamate receptor antagonist DGG and the GABA-A receptor agonist muscimol in rabbits performing well-learned CRs. Effects of inactivating the IO on CR expression and on neuronal activity in the anterior cerebellar interposed nucleus (IN) were examined. We found that either blocking excitatory glutamate inputs or activating inhibitory GABA inputs to the IO abolished CRs. This effect occurred with variable delay following drug injections. Additional experiments, in which post-injection testing was delayed to allow for drug diffusion, revealed invariably immediate suppression of CRs. This demonstrated that suppressing IO activity using DGG or muscimol does not induce unlearning of CRs. Single-unit recording during DGG injections revealed that CR suppression was paralleled by a dramatic suppression of IN neuronal activity. We concluded that inactivating the rostral parts of the IO complex abolishes CRs by producing a tonic malfunction of cerebellar eyeblink conditioning circuits.

Developmental changes in evoked Purkinje cell complex spike responses

The development of synaptic interconnections between the cerebellum and inferior olive, the sole source of climbing fibers, could contribute to the ontogeny of certain forms of motor learning (e.g., eyeblink conditioning). Purkinje cell complex spikes are produced exclusively by climbing fibers and exhibit short- and long-latency activity in response to somatosensory stimulation. Previous studies have demonstrated that evoked short- and long-latency complex spikes generally occur on separate trials and that this response segregation is regulated by inhibitory feedback to the inferior olive. The present experiment tested the hypothesis that complex spikes evoked by periorbital stimulation are regulated by inhibitory feedback from the cerebellum and that this feedback develops between postnatal days (PND) 17 and 24. Recordings from individual Purkinje cell complex spikes in urethan-anesthetized rats indicated that the segregation of short- and long-latency evoked complex spike activity emerges between PND17 and PND24. In addition, infusion of picrotoxin, a GABAA-receptor antagonist, into the inferior olive abolished the response pattern segregation in PND24 rats, producing evoked complex spike response patterns similar to those characteristic of younger rats. These data support the view that cerebellar feedback to the inferior olive, which is exclusively inhibitory, undergoes substantial changes in the same developmental time window in which certain forms of motor learning emerge.

Spinal projections of the cat parvicellular red nucleus

Traditionally, the red nucleus of the cat is divided into two parts: a large-celled, magnocellular, division (RNm) and a small-celled, parvicellular, division (RNp). The RNm projects to the spinal cord and receives input from the cerebellar interpositus nucleus. The RNp projects to the inferior olive and receives input from the cerebellar dentate nucleus. In this report, we reexamine the connections of the red nucleus using the bidirectional tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Our findings demonstrate that the cat RNp has a large caudal and lateral region that projects to contralateral spinal cord and not to the inferior olive. The spinally projecting region of RNp receives input from the dentate nucleus and a lateral segment of anterior interpositus. Cervical projections from the red nucleus show a topography with the rostral portion of RNp favoring upper segments and the caudal portion of RNm favoring lower segments. The results show that dentate output can influence spinal activity without passing through the cerebral cortex. For the control of movements such as reaching and grasping, we suggest that RNp and dentate focus on the control of proximal limb musculature, whereas RNm and the anterior interpositus focus on the control of distal limb musculature. We also suggest that other species are likely to have a small-celled area of red nucleus projecting to the spinal cord.

Developmental changes in eye-blink conditioning and neuronal activity in the inferior olive

Neuronal activity was recorded in the dorsal accessory inferior olive in infant rats during classical conditioning of the eye-blink response. The percentage and amplitude of eye-blink conditioned responses (CRs) increased as a function of age. The magnitude of the neuronal response to the unconditioned stimulus (US) decreased with age. There were also age-specific modifications of US-elicited inferior olive neuronal activity during paired trials in which a conditioned eye-blink response was performed. The results indicate that the development of the conditioned eye-blink response may depend on dynamic interactions between multiple developmental processes within the eye-blink circuitry. Differences in the functional maturity of olivo-cerebellar pathways may limit the induction of plasticity in the cerebellum and thereby limit the development of eye-blink conditioned responses.

Gracile, cuneate, and spinal trigeminal projections to inferior olive in rat and monkey

In the cat, somatosensory nuclei send substantial projections to the inferior olive, where they terminate in a somatotopic fashion. Although the organization of the cat inferior olive has been used to interpret data from other species, published data suggest this organization may not occur universally. The present study investigated whether the inferior olive in albino rats and cynomolgus monkeys receives the same brainstem somatosensory inputs, whether these inputs are organized somatotopically and, if so, how the organization compares with that in the cat. Projections from the gracile, cuneate and spinal trigeminal nuclei were labeled with wheat germ agglutinin conjugated to horseradish peroxidase or with biotinylated dextran. The results were compared with data from cats (Berkley and Hand [1978] J. Comp. Neurol. 180:253-264). In the rat and monkey, the gracile, cuneate and spinal trigeminal nuclei all project to the contralateral inferior olive, where each nucleus has a distinct preferred terminal field. As in the cat, projections to the medial accessory olive and caudal dorsal accessory olive did not terminate in a precisely organized fashion. Projections to the rostral dorsal accessory olive, however, formed a clear somatotopic map. These somatotopic maps differed from those in the cat in that input from the trigeminal nucleus was confined rostrally, so that the caudal end only received input from the gracile and cuneate nuclei. These data indicate that similar organizational principles characterize the somatosensory projections to the inferior olives of the three species. Nevertheless, distinct species differences occur with regard to the details of this organization.

The estrous cycle and the olivo-cerebellar circuit. II. Enhanced selective sensory gating of responses from the rostral dorsal accessory olive

This study demonstrates that gating of responses of the rostral dorsal accessory olive (rDAO) to somatosensory stimulation varies across the estrous hormone cycle of the rat. The rDAO has been suggested as an "error" or event signal generator for the cerebellar cortex. Selective sensory gating of input to this structure may underlie this error signalling function. In the present study, as many as 23 single neurons were recorded simultaneously from either the forepaw or the snout areas of the rDAO. Responses of these neurons to electrical stimulation of peripheral afferents were determined during active movement or non-movement conditions. These results were then compared across the estrous cycle or after administration of the estrous hormones 17 beta-estradiol (E2) and/or progesterone (P) to rats on diestrus or following E2 priming. Elevations in circulating estrous hormones produced greater excitatory responses of rDAO neurons to stimulation during non-movement, and, conversely, enhanced inhibition of rDAO activity during active movement of the stimulated peripheral area compared with control diestrous conditions, suggesting that selective gating processes to the rDAO are enhanced by estrous hormones. The results of this study suggest that the night of behavioral estrus is associated with enhanced selective sensory gating processes associated with improved detection and processing of error signals.

Trigeminocerebellar and trigemino-olivary projections in rats

Retrograde and anterograde neuronal tracers (HRP, biocytin, biotinylated dextran-amine) were used to study the organisation of trigeminocerebellar and trigemino-olivary connections, focusing on the connectivity between trigeminal nuclear regions and the sagittal zones of the rat cerebellar cortex. Trigeminocerebellar projections were bilateral, but mostly ipsilateral. Direct trigeminocerebellar fibres originated mostly in the principal trigeminal nucleus (VP) and pars oralis (Vo), pars interpolaris (Vi), and to a lesser extent in pars caudalis (Vc) of the spinal trigeminal nucleus. Consistent projections were found from the Vc to cerebellar lobules IX and X. The trigeminal fibres terminated in the cerebellum in an organised fashion. The ventral part of the VP, Vo and Vi projected to the medial A zone and the C3 and D2 subzones, whereas the dorsal part of the nuclei projected to the lateral A zone and the C2, D0 and D1 subzones. In lobules IX and X, the organisation was different. The medial half of the VP, Vo, Vi and Vc projected to the lateral aspects of these lobules whereas their lateral part projected to their medial aspects. Trigeminal projections to the deep cerebellar nuclei were also present. Projections to a given sagittal zone concomitantly reached its corresponding nuclear target. Trigemino-olivary projections were principally contralateral. The Vo, Vi and Vc projected to the rostromedial dorsal accessory olive, the adjacent dorsal leaf and the dorsomedial part of the ventral leaf of the principal olive, which are known to project subzones C3, D0 and D1 of the rat cerebellar cortex.

Morphological correlates of bilateral synchrony in the rat cerebellar cortex

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

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.