Cerebellar inhibition of inferior olivary transmission in the decerebrate ferret

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

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

Summary 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.     

Inhibition of the inferior olive during conditioned responses in the decerebrate ferret

Output from the interpositus nucleus can inhibit the inferior olive, probably via the GABA-ergic nucleo-olivary pathway. It has been suggested that the function of this inhibition might be to regulate synaptic plasticity resulting from parallel fibre/climbing fibre interaction in cerebellar Purkinje cells, by providing negative feedback information to the olive. Thus, when a learned response, generated by the interpositus nucleus, reaches a sufficient amplitude, the olive would be inhibited and further learning blocked. This suggestion was tested in a classical conditioning paradigm. Decerebrate ferrets were trained using electrical skin stimulation of the forelimb as the conditioned stimulus (CS) and periorbital stimulation as the unconditioned stimulus (US). Climbing fibre responses evoked in Purkinje cells by the US were recorded as surface field potentials in the part of the c3 zone controlling eyeblink. It was found that the CS did not inhibit the olive at the beginning of training, but when conditioned responses were large, the olive was inhibited by the CS in some animals. After a number of unpaired CS presentations, which caused extinction of the conditioned response, the inhibition disappeared. The size of individual conditioned responses correlated negatively with the size of the climbing fibre responses evoked by the US. Climbing fibre responses evoked by direct stimulation of the olive were also inhibited. It was concluded that cerebellar output during performance of a conditioned response inhibits the inferior olive. The results thus support the hypothesis of a cerebellar locus of conditioning and are consistent with the proposed role of cerebello-olivary inhibition.     

Imaging parallel fiber and climbing fiber responses and their short-term interactions in the mouse cerebellar cortex in vivo

A major question in the study of cerebellar cortical function is how parallel fiber and climbing fiber inputs interact to shape information processing. Emphasis has been placed on the long-term effects due to conjunctive stimulation of climbing fibers and parallel fibers. Much less emphasis has been placed on short-term interactions and their spatial nature. To address this question the responses to parallel fiber and climbing fiber inputs and their short-term interaction were characterized using optical imaging with Neutral Red in the anesthetized mouse in vivo. Electrical stimulation of the cerebellar surface evoked an increase in fluorescence consisting of a transverse optical beam. The linear relationship between the optical responses and stimulus parameters, high spatial resolution and close coupling to the electrophysiological recordings show the utility of this imaging methodology. The majority of the optical response was due to activation of postsynaptic alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate (AMPA) and metabotropic glutamate receptors with a minor contribution from the presynaptic parallel fibers. Stimulation of the inferior olive evoked parasagittal bands that were abolished by blocking AMPA glutamate receptors. Conjunctive stimulation of the cerebellar surface and inferior olive resulted in inhibition of the climbing fiber evoked optical responses. This lateral inhibition of the parasagittal bands extended out from both sides of an activated parallel fiber beam and was mediated by GABA(A) but not GABA(B) receptors. One hypothesized role for lateral inhibition of this type is to spatially focus the interactions between parallel fiber and climbing fiber input on Purkinje cells. In summary optical imaging with Neutral Red permitted visualization of cerebellar cortical responses to parallel fiber and climbing fiber activation. The GABA(A) dependent lateral inhibition of the climbing fiber evoked parasagittal bands by parallel fiber stimulation shows that cerebellar interneurons play a short-term role in shaping the responses of Purkinje cells to climbing fiber input.     

Correlations among climbing fiber responses of nearby cerebellar Purkinje cells in the immature rat

Summary Correlations between pairs of spontaneous climbing fiber responses (CFRs) recorded from couples of nearby Purkinje cells (PCs) were studied in immature rats by using cross-correlograms between CFR pairs, and compared to those in adult animals. Correlations were found as early as day 3. Some days later, on PN days 7–9, these correlations were higher than in the adult. In most cases, this was apparently not due to the multiple innervation of PC by climbing fibers (CFs) which normally occurs during this immature stage since: 1) temporal relationships between the paired CFRs varied by more than 30 ms and 2) thresholds for pairs of graded CFRs and additional components of the responses evoked in the 2 PCs by juxtafastigial or olivary stimulation were different. Synchronizing mechanisms were therefore likely to be already located at the olivary level. However, in 3 couples of multiply innervated PCs whose spontaneous CF activities were highly correlated, stimulation experiments revealed a common innervation of the 2 cells by branches of the same CF. In multiply innervated cells, spontaneous responses mediated through distinct CFs were also synchronized, suggesting that these fibers originate from neighboring neurons of the inferior olive. Finally, in 7 to 9-day-old rats, correlations among CFR pairs were much more restricted in the longitudinal axis of the folium than in the transverse one. On the whole, the present study indicates that correlations among CFRs of nearby PCs exist as soon as CF-PC synapses are established and the latter are already organized in sagittal strips at early stages of development.This work was supported by INSERM (C.R.L. Nr. 78.1.003.6)     

Role of climbing fibers in determining the spatial patterns of activation in the cerebellar cortex to peripheral stimulation: an optical imaging study

The spatial patterns of activation in the rat cerebellar cortex evoked by ipsilateral face stimulation were mapped using optical imaging based on the pH sensitive dye, Neutral Red. The aims of the study were to characterize the optical responses evoked by peripheral stimulation and test the hypothesis that the resultant parasagittal banding is due to climbing fiber activation. In the anesthetized rat Crus I and II of the cerebellar cortex were stained with Neutral Red. Epi-fluorescent changes produced by a train of stimuli (5-10s and 4-20 Hz) to the ipsilateral face were monitored in time using a fast, high resolution charge-coupled device camera. The patterns of activation were quantified using a two-dimensional fast Fourier transform analysis that removed signals with high spatial frequencies and minimized the contribution of horizontal structural elements (i.e. blood vessels). The dominant spatial pattern of activation evoked by face stimulation was that of parasagittal bands. The bands were highly frequency-dependent and were elicited most strongly by stimulus frequencies in the range of 6-8 Hz. There was a large fall-off in the response for frequencies above and below. The optical signal evoked by face stimulation built up over a period of 10s and then gradually decayed. Within a folium the individual parasagittal bands exhibited some frequency and temporal specificity. Stimulation of the contralateral inferior olive also resulted in the activation of parasagittal bands with characteristics similar to the bands evoked by face stimulation, including a preferred stimulus frequency which peaked at 10 Hz. Injection of lidocaine into the contralateral inferior olive blocked the parasagittal bands evoked by ipsilateral face stimulation, while control injections of saline had no effect.The results confirm that a parasagittal banding pattern is a dominant feature of the functional architecture of the cerebellar cortex. The parasagittal banding pattern observed with Neutral Red is due primarily to the activation of climbing fiber afferents. The frequency tuning of the responses, with the preference for peripheral stimuli of 6-8 Hz, is in agreement with previous findings that the inferior olive is inherently rhythmic. These observations support the hypothesis that inferior olivary neurons are dynamically coupled into groups that activate parasagittal bands of Purkinje cells in the cerebellar cortex. The frequency tuning also supports the hypothesis that the climbing fiber system is involved with timing. Activation of this afferent system may require stimuli with appropriate frequency content and stimuli synchronized to the rhythmicity of the inferior olive.     

Electrophysiological studies of the projections from the parietal association area to the cerebellar cortex

1. Responses evoked in the cerebellar cortex by stimulation of the parietal association cortex (rostral portions of the middle suprasylvian gyrus) were recorded and analysed in cats, and were compared with those by stimulation of the motor cortex (anterior sigmoid gyrus). 2. The parietal stimulation elicited early mossy fibre and late climbing fibre responses in the cerebellar cortex. The mossy fibre responses appeared at a latency of 2.0--2.5 msec and predominantly in the lateral (hemispherical) part of the contralateral cerebellum (mainly crus I, crus II and paramedian lobules). Cutting of the inferior cerebellar peduncle produced little or no influence upon the mossy fibre responses, which suggests that the mossy fibre responses are mediated chiefly by the pontine nuclei. 3. The climbing fibre responses were recorded at a latency of 17--19 msec and markedly in the contralateral intermediate and medial parts of IV--VI lobules. The responses were easily sppressed by anaesthesia and depended on the conditions of experimental animals. The unstable appearance of the responses and their longer latencies than those of the climbing fibre responses due to stimulation of the motor cortex imply indirect pathways from the parietal association cortex to the inferior olive. 4. The predominant projection of the parietal-induced mossy fibre responses to the lateral part of the cerebellum was compared with the mossy fibre projection from the motor cortex and was discussed as an important component in the cerebrocerebellar loops.     

Responses of a spino-olivo-cerebellar pathway in the cat

1. Surface potentials, similar to those found by earlier workers, have been recorded from the vermis of the anterior lobe of the cerebellum following stimulation of muscular, cutaneous and articular nerves of the ipsilateral hind limb. The most conspicuous component of the response consisted of a positive potential succeeded by a smaller negative potential.

2. Micro-electrode recordings showed that this component coincided both with climbing fibre responses in individual Purkinje cells, and with extracellular field potentials within the cerebellar cortex which closely resembled those found by Eccles, Llinás & Sasaki (1966) following electrical stimulation of the inferior olive.

3. Stimulation of the cerebellar surface, in the region where the responses to limb nerve stimulation were largest, led to antidromic invasion of neurones of the contralateral inferior olive. The antidromic action potentials were sometimes followed by up to three orthodromic spikes. Histological techniques were used to show that these neurones were located in the caudal parts of the dorsal and medial accessory olives.

4. Stimulation of nerves of the hind limb evoked discharges of the same neurones of the dorsal accessory olive which were antidromically invaded from the vermis of the anterior lobe. The nerves used (quadriceps, gastrocnemius-soleus, sural and the posterior nerve to the knee joint) were shown to excite heavily overlapping populations of neurones.

5. Those neurones of the medial accessory olive, which were identified antidromically from the anterior lobe vermis, were not discharged by stimulation of hind limb nerves.

6. Simultaneous recording from the surface of the anterior lobe and from the dorsal accessory olive showed that the onset of olive cell discharges occurred about 5 msec before the onset of the positive potential at the cerebellar surface.

     

Olivary branching projections to the flocculus,nodulus and uvula in the rabbit

Summary Olivocerebellar branching projections to the flocculus, nodulus and uvula were studied electrophysiologically in pigmented rabbits anesthetized with pentobarbital and halothane. Neurons in the dorsal cap of the inferior olive were antidromically activated by stimulation of the contralateral flocculus, nodulus and uvula. The antidromic responses in the dorsal cap from the flocculus, nodulus and uvula partially occluded with each other. Electrical stimulation of the nodulus (or uvula) evoked early and late climbing fiber responses in the flocculus with latencies of 4.0 ms and 10.0 ms, respectively. The early response was resistant to repetitive stimulation and not affected by interruption of the olivocerebellar fibers at the lower medulla, while the late response was abolished by these two procedures. This indicated that the early response was evoked through climbing fiber branches to the flocculus and nodulus (or uvula) as an axon reflex, while the late response was evoked via the inferior olive. Of 76 Purkinje cells in the flocculus, 35.6% showed climbing fiber activation through branches to the nodulus and/or uvula, 70% of which were orthodromically activated through climbing fibers from the contralateral optic tract. Electrical stimulation of the flocculus or uvula evoked the early and late climbing fiber responses in the nodulus. Of 60 Purkinje cells in the nodulus, 63.4% showed axon reflex activation from the flocculus and/or uvula, 42% of which were activated through climbing fibers from the contralateral optic tract. These findings demonstrated that the flocculus, nodulus and uvula receive branching climbing fiber projection from the inferior olive, a part of which conveys visual information.     

The excitatory synaptic action of climbing fibres on the Purkinje cells of the cerebellum

1. A single climbing fibre makes an extraordinarily extensive synaptic contact with the dendrites of a Purkinje cell. Investigation of this synaptic mechanism in the cerebellum of the cat has been based on the discovery by Szentagothai & Rajkovits (1959) that the climbing fibres have their cells of origin in the contralateral inferior olive.2. Stimulation in the accessory olive selectively excites fibres that have a powerful synaptic excitatory action on Purkinje cells in the contralateral vermis, evoking a repetitive spike discharge of 5-7 msec duration. Almost invariably this response had an all-or-nothing character. In every respect it corresponds with the synaptic action that is to be expected from climbing fibres.3. Intracellular recording from Purkinje cells reveals that this climbing fibre stimulation evokes a large unitary depolarization with an initial spike and later partial spike responses superimposed on a sustained depolarization.4. Typical climbing fibre responses can be excited, but in a much less selective manner, by stimulation of the olive-cerebellar pathway in the region of the fastigial nucleus, there being often a preceding antidromic spike potential of the Purkinje cell under observation.5. Impaled Purkinje cells rapidly deteriorate with loss of all spike discharge, the climbing fibre response being then reduced to an excitatory post-synaptic potential. This potential shows that stimulation of the inferior olive may evoke two or more discharges at about 2 msec intervals in the same climbing fibre. The complexity of neuronal connexions in the inferior olive is also indicated by the considerable latency range in responses.6. A further complication is that, with stimulation in the region of the fastigial nucleus, the initial direct climbing fibre response is often followed by a reflex discharge, presumably from the inferior olive, which resembles the responses produced by inferior olive stimulation in being often repetitive.7. Typical climbing fibre responses have been evoked by peripheral nerve stimulation and frequently occur spontaneously.8. An account is given of the way in which the responses evoked by climbing fibres in the individual Purkinje cells can account for the potential fields that an inferior olive stimulus evokes on the surface and through the depth of the cerebellar cortex.9. By the application of currents through the recording intracellular electrode it has been possible to effect large changes in the excitatory post-synaptic potential produced by a climbing fibre, it being diminished and even reversed with depolarizing currents and greatly increased by hyperpolarizing currents.     

A Rubro-Olivary pathway II. Simultaneous action on dynamic Fusimotor neurones and the activity of the posterior lobe of the cerebellar cortex

Summary In the preceding paper (Appelberg and Molander 1967) the caudal part of the red nucleus and parts of the inferior olivary nucleus were shown to cause increased dynamic sensitivity of muscle spindles when stimulated repetitively. The results to be presented will show that single shock electrical stimulation in the caudal part of the red nucleus evoked a field potential in the inferior olivary nucleus. This response seemed to be monosynaptically evoked and was observed only in parts of the olive where repetitive stimulation caused increased dynamic sensitivity of muscle spindles. Stimulation in the red nucleus as well as single shock stimulation in the actual part of the inferior olive also caused a potential in the vermis of the posterior cerebellar lobe. In conditioning — test experiments with the two stimuli the conditioning shock was seen to cause alternating periods of decreased and increased responsiveness in the pathway concerned. The same type of interaction was seen between two responses caused by double shock stimulation in the red nucleus.It is concluded that information from the caudal part of the red nucleus reaches dynamic fusimotor neurones in the spinal cord via a relay in the inferior olivary nucleus; an additional relay in the pathway is also predicted. The cerebellum seems to receive information about ongoing activity in the pathway but mesencephalic stimulation was seen to cause spindle effects also in decerebellated animals.     

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.     

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.     

Fluctuation of extracellular potassium and calcium in the cerebellar cortex related to climbing fiber activity

It is known that repetitive stimulation of spino- and cortico-olivary pathways can lead to waxing and waning of the evoked climbing fiber responses of cerebellar Purkinje cells. Extracellular recordings performed with ion-selective microelectrodes showed that the amplitude fluctuations of climbing fiber-related field potentials were accompanied by corresponding changes in ion-activity levels. When the amplitude of the field potentials increased, potassium activity rose by up to 0.7 mmol.l^?1 and calcium activity fell by up to 0.3 mmol.l^?1. Ion activities returned to baseline values when climbing fiber responses failed.The rhythmic fluctuations in ion activities have cycle lengths of 10–20 s, or multiples of it. Presumably they are secondary consequences of slow variations of synaptic transmission in the inferior olive. The generator for the slow fluctuations of olivary synaptic transmission and the consequent variations in ion activities within the cerebellum is probably located within the inferior olive. The ionic modulations could have functional implications for cerebellar information processing. Possible mechanisms of the rhythmogenesis are discussed.     

The climbing fibers of the cerebellar cortex,their origin and pathways in cat

Summary The sources and pathways of the climbing fibers to the cerebellar posterior vermis were studied with combined electrophysiological and anatomical methods in cats.Recording from identified cerebellar Purkinje cells, monosynaptic climbing fiber (CF) responses have been obtained both for stimulation of the inferior olive (IO) and various parts of the brain stem (BS). CF responses were found to be of three types, IO only, BS only or both IO and BS. However the responses to BS stimulation were very few in number in comparison with IO or IO and BS types of responses. The latencies of the responses were shorter for the BS cases consistent with their distance from the cerebellum.A comparison of latencies and the relative responsiveness of the different area of the brain stem which were studied, indicate that part of the CF ascend through the pontine region and enter the cerebellum by way of the medium and superior peduncles. This finding is confirmed by the results of anatomical studies in which degenerating fibers were found in the molecular layer (using the Nauta technique) after lesion of the brachium pontis but not after lesions of the medial portion of the pons. Similarly, injection of radioactive leucine into the pontine nuclei failed to show any labeled fibers in the molecular layer.Horseradish peroxidase (HRP) was injected into localized regions of the posterior vermis after total bilateral destruction of the inferior peduncles. Large numbers of positive, marked cells were still found in the inferior olive.It is concluded that nearly all, if not all, the climbing fibers originate in the inferior olive and that they ascend to the cerebellum by way of all the peduncles.     

Topographical organization of climbing fiber pathway from the superior colliculus to cerebellar vermal lobules VI-VII in the cat.

Topographical distribution of the climbing fiber responses induced by stimulation of the superior colliculus was investigated in the cerebellar posterior vermis (lobules VI-VII) of the cat. The climbing fiber-responsive areas were distributed longitudinally forming sagittal zones. The sagittal zones responsive to stimulation of the left and right superior colliculus were located on the side ipsilateral to the stimulation, and they were completely segregated. The sagittal zones responsive to stimulation of the caudal superior colliculus were distributed more laterally than those responsive to stimulation of the rostral superior colliculus. Study of the extracellular unit in the inferior olive demonstrated that the climbing fiber responses were relayed in the caudomedial part of the medial accessory olive contralateral to the stimulation.     

Responses evoked in the cerebellar cortex by stimulation of the caudate nucleus in the cat

1. Responses evoked in the cerebellar cortex following stimulation of caudate nucleus are described.

2. The evoked responses recorded from the surface of the cerebellar cortex were found to be of two types, one with a short (4-6 msec) latency and one with a longer (12-17 msec) latency.

3. The short latency response was maximal in the lobulus simplex, the longer latency response was maximal in paramedian lobule.

4. Following lesions in the inferior olive the longer latency response was absent.

5. Recordings from within the cerebellar cortex showed that the short latency response was uniformly distributed throughout the grey matter, the longer latency response was maximal in the region of the Purkinje cell bodies.

6. It was concluded that the short latency response was due to activation via the mossy fibres and the longer latency response to activation via the climbing fibres.

7. It was found that responses could be evoked in the cerebellum following stimulation of only the latero-ventral part of the caudate nucleus; stimulation of the rest of the nucleus caused no response in the cerebellum. This division of the caudate nucleus into two parts is similar to the subdivision of the caudate nucleus made by other workers using different criteria.

     

Neuronal mechanisms of the interaction of Deiters' nucleus with some brainstem structures

The effects of stimulation of the interstitial nucleus of Ramón y Cajal, as well as the nucleus of Darkschewitsch, inferior olive and nucleus reticularis tegmenti pontis on the neuronal activity of the lateral vestibular nucleus of Deiters were studied by means of an intracellular recording technique. Stimulation of these structures is shown to lead to antidromic and orthodromic activation of Deiters' neurons. Collaterals of vestibulospinal neurons entering these structures are revealed electrophysiologically. It was shown that stimulation of the rostral part of the inferior olive evoked in Deiters' neurons predominantly antidromic responses, whereas stimulation of the caudal part of the inferior olive leads mostly to synaptic activation. Stimulation of Ramón y Cajal's and Darkschewitsch's nuclei evokes mono- and/or polysynaptic excitatory and inhibitory postsynaptic potentials in Deiters' neurons. Mono-, oligo- and/or polysynaptic inhibitory postsynaptic potentials were also evoked by stimulation of nucleus reticularis tegmenti pontis, as well as the rostral and particularly, caudal parts of the inferior olive. Stimulation of the caudal part of the inferior olive evoked mono-, oligo- and/or polysynaptic excitatory postsynaptic potentials in Deiters' neurons. Convergence of influences from the stimulated structures on the vestibular neurons under study was shown. A topical correlation between Deiters' nucleus and the brainstem nuclei mentioned above was found. The presence of inhibitory and excitatory interaction of these structures with Deiters' nucleus was established.     

Memory trace of motor learning shifts transsynaptically from cerebellar cortex to nuclei for consolidation

Adaptation of ocular reflexes is a prototype of motor learning. While the cerebellum is acknowledged as the critical site for motor learning, the functional differences between the cerebellar cortex and nuclei in motor memory formation are not precisely known. Two different views are proposed: one that the memory is formed within the cerebellar flocculus, and the other that the memory is formed within vestibular nuclei. Here we developed a new paradigm of long-term adaptation of mouse horizontal optokinetic response eye movements and examined the location of its memory trace. We also tested the role of flocculus and inferior olive in long-term adaptation by chronic lesion experiments. Reversible bilateral flocculus shutdown with local application of 0.5 microl-5% lidocaine extinguished the memory trace of day-long adaptation, while it very little affected the memory trace of week-long adaptation. The responsiveness of vestibular nuclei after week-long adaptation was examined by measuring the extracellular field responses to the electrical stimulation of vestibular nerve under trichloroacetaldehyde anesthesia. The amplitudes and slopes of evoked monosynaptic field response (N1) of week-long adapted mice were enhanced around the medial vestibular nucleus compared with those of control mice. Chronic flocculus or inferior olive lesions abolished both day and week-long adaptations. These results suggest that the functional memory trace of short-term adaptation is formed initially within the cerebellar cortex, and later transferred to vestibular nuclei to be consolidated to a long-term memory. Both day and week-long adaptations were markedly depressed when neural nitric oxide was pharmacologically blocked locally and when neuronal nitric oxide synthase was ablated by gene knockout, suggesting that cerebellar long-term depression underlies both acquisition and consolidation of motor memory.     

Maturation of evoked climbing fiber input to rat cerebellar Purkinje cells (I.)

Summary An analysis of evoked responses of Purkinje cells in developing rat cerebellum to climbing fiber input was conducted to determine which identifying properties of this afferent system are established early in development and which specific features mature with age. Rat pups at various ages were anesthetized with 0.5% halothane and unit recordings made with glass micropipettes. By the third postnatal day, electrical stimulation of the sensorimotor cortex and limbs at low stimulation rates (<1/sec) could elicit distinct burst responses appearing at long latencies (180 msec), indicating that pathways of both ascending and descending climbing fiber systems are intact early in cerebellar cortical development. A distinctive feature maturing over the first 1–1.5 weeks was the characteristic of the all-or-none burst response since before about day 11 the interspike interval, amount of inactivation, and number of spikes in evoked burst responses all varied from stimulation to stimulation. Mean latencies decreased from 180 msec at day 3 to 50 msec by day 10, but did not achieve the adult value of 20 msec until the fourth week. Typically, climbing fiber responses could only follow at stimulation rates of less than 0.2/sec at day 3, but by day 12 could follow up to 10/sec, which is the same as in the adult. The data indicate that the climbing fiber system establishes connectivity from diverse sources and exhibits identifying characteristics similar to the adult early in cerebellar development. Most aspects of the maturation of transmission can be explained if there were a decrease in the time scale of function of the synapses involved, mainly those in the inferior olive.Supported by N.I.H. Grant 5-R01-6M00133 and N.S.F. Grant GB 43301