Histogenesis of the cerebellar climbing fiber in the rat

Early climbing fiber metamorphosis has been studied in 250 Golgi preparations of 5–12 day postnatal rat cerebellum, and in less numerous 12–18 day ones. EM controls were provided for the early phase covering maturation of Purkinje cytoplasm, status of axonal neuropil, and synapses between the evanescent nid structure and Purkinje soma membrane. Ramon y Cajal (1890, '11), Athias (1897) and others have contributed extensively to climbing fiber histogenesis. Our results are confirmatory but support more varied developmental detail including that of axonal collateralization within the granular layer, all of which may not persist to maturity. Distinguishing characteristics of climbing fibers are recognized early in histogenesis, including varicosities, redundancies in course, and a manner of branching called cross-over. The curvaceous path a climbing fiber pursues across the granular layer can be corrected at the ganglionic level by a horizontal segment having length equivalent to the offset. These and other redundancies may straighten out with areal expansion of cortical surface as compared to the restricted white matter base. While supporting multiple innervation of nids by local collateralizations proceeding from stem axons in white matter, the study does not preclude a one to one relationship between adult climbing fiber and Purkinje cell as basic design. Unknown factors are the relative amounts of local collateralization proceeding from several versus a single axon stem, and the extent to which branches are issued from a single fiber to different folia.     

Somatosensory representation of the cerebellar climbing fiber system in the rat

The climbing fiber responses of 542 Purkinje cells were isolated in the vermal and intermediate zones of lobules II to VI of the rat cerebellum. Mechanical stimulation successfully elicited 53% of the isolated climbing fiber responses, whereas the remaining units were unresponsive to any stimulation employed. Of the units elicited by the stimulation, 34% required cutaneous and 66% required deep stimulation. Some proportion of the representation of each body region required either cutaneous or deep stimulation. The hind-limb had the largest representation and accounted for 55% (160/288) of the units. In contrast, the forelimb was only represented by 10% of the units, the tail by 16%, the face by 11% and the remaining 6% of the units by surface regions of the spine, chest and abdomen. On the basis of their proportional representation of body regions, 3 different cortical areas were distinguished: (1) a medial vermis, which consisted predominantly of unresponsive units; (2) a lateral vermis, which included representations of the extremities, trunk and tail; and (3) the intermediate zone, where the only representation of the face was evident. Within each area, the representations formed a disjunctive pattern of irregularly shaped patches and areas of overlap. In comparison with the climbing fiber organization of the cat, the medial vermal unresponsive zone and the patch-like representations of various body surfaces in the rat were similar to the cat, but the proportional representation of various body surfaces and effective stimulus modality were different, which may reflect morphological and behavioral differences between the species.     

Reduction of cerebellar norepinephrine alters climbing fiber enhancement of mossy fiber input to the Purkinje cell

Extra-cellular simple and complex spike activity from 58 Purkinje cells were recorded in cats that previously received an intracisternal injection of 6-OHDA which depletes brain catecholamines. The severest catecholamine depletion was noted for cerebellar norepinephrine (21.1% of controls). Less depletion occureed in the brainstem and the visual cortex. Past studies have shown that in normal non-depleted cats, somatosensory stimuli (forepaw tap) evoke both complex and simple spike responses. On those trials where complex spike or climbing fiber responses are evoked, there is an enhancement or increase in responsiveness in the majority of excitatory and inhibitory simple spike responses. In the norepinephrine depleted animal, there is a significant decrease in this climbing fiber enhancement only for the excitatory response components. Furthermore, on those trials where no complex spikes are evoked, there is a significant firing rate is also observed in the depleted animals. Thus, depletion of norepinephrine is associated with a reduction of both response amplitude and climbing fiber induced enhancement of excitatory simple spike responses. The inhibitory responses in these same cells are unchanged when compared to those recorded in the normal non-depleted animals.     

Acute and chronic effects of climbing fiber lesions on cerebellar cyclic guanosine monophosphate

The neurotoxin 3-acetylpyridine was administered to 20-day-old rats to produce lesions of the inferior olive-climbing fiber projection to the cerebellum. Cerebellar cGMP levels were determined 6 h, 24 h, 48 h, 7 days, 14 days and 20 days postlesion. A significant effect on cGMP was found only at 48 h (-28%) and 7 days (-45%) postlesion. The results are discussed with respect to the cellular localization of cGMP and the hypothesized relationship of cGMP to cerebellar Purkinje cell activity.     

Parallel fiber receptive fields: a key to understanding cerebellar operation and learning

In several theories of the function of the cerebellum in motor control, the mossy-fiber-parallel fiber input has been suggested to provide information used in the control of ongoing movements whereas the role of climbing fibers is to induce plastic changes of parallel fiber (PF) synapses on Purkinje cells. From studies of climbing fibers during the last few decades, we have gained detailed knowledge about the zonal and microzonal organization of the cerebellar cortex and the information carried by climbing fibers. However, properties of the PF input to Purkinje cells and inhibitory interneurones have been largely unknown. The present review, which focuses on the C3 zone of the cerebellar anterior lobe, will present and discuss recent data of the cutaneous PF input to Purkinje cells, interneurons and Golgi cells as well as novel forms of PF plasticity.     

Activity-induced elevation of cerebellar cyclic GMP occurs in the absence of climbing fiber pathways

The inferior olivary nuclei (ION) of Sprague-Dawley rats were chemically lesioned with 3-acetylpyridine (3-AP), and the completeness verified by the lack of retrograde labeling of the ION following the injection of horseradish peroxidase (HRP) into the cerebellum. The effects of locomotor activity or of immobilization on cerebellar cyclic guanosine monophosphate (cGMP) levels were determined in control saline-treated and experimental, 3-AP-treated rats. Three subgroups of rats from both the control and the experimental groups of rats were (1) required to swim for 60 s, (2) immobilized for 60 s or (3) unmanipulated before being killed by microwave irradiation, and the cerebella were collected for cGMP determination. There was no statistically significant difference in cGMP levels between immobilized and unmanipulated rats in either the experimental or control groups. Pretreatment with 3-AP reduced cerebellar cGMP levels in both the immobilized and the unmanipulated rats to 50% of those observed in the comparably treated groups of saline-treated controls. When compared to the corresponding group of unmanipulated rats, locomotor activity induced a significantly greater elevation of cerebellar cGMP in the experimental animals than in the controls (P < 0.05). These results indicate that while inputs from the ION to the cerebellar Purkinje cells are probably important in maintaining the normal levels of cGMP seen in inactive rats, the locomotor-induced elevation of this parameter occurs in the absence of climbing fibers (from ION). The mechanisms responsible for the greater activity-induced elevation of cGMP levels seen in rats receiving 3-AP over control rats are discussed.     

Changes in the responses of cerebellar nuclear neurons associated with the climbing fiber response of Purkinje cells

Previous studies demonstrated that climbing fiber activity produces a short-term increase in the responsiveness of Purkinje cells to mossy fiber inputs. This led to the hypothesis that there are concomitant alterations in the discharge of cerebellar nuclear neurons. This series of experiments was initiated to test this hypothesis in simultaneously recorded Purkinje cell-nuclear cell pairs in related regions of the cerebellar cortex and nuclei. In decerebrate cats 50 pairs of Purkinje cells and nuclear neurons were identified and simultaneously recorded during spontaneous activity and during peripheral inputs. Auto-correlograms of nuclear cell activity and cross-correlograms of the simple spike and nuclear cell activity triggered on the occurrence of spontaneous complex spikes demonstrated little correlation between these events and the discharge of nuclear neurons. To examine the effect of evoked climbing fiber inputs on the Purkinje cell simple spike and nuclear cell responses, square wave mechanical stimuli which modulated the discharge of both cells of a pair were applied to the forepaw. A separation technique was used to construct one histogram illustrating the responses of the nuclear neuron and Purkinje cell in trials in which the peripheral stimulus evoked a climbing fiber input to the Purkinje cell and another histogram showing their responses in trials in which no climbing fiber input was activated. Using this separation technique it was shown that the amplitude of most Purkinje cell responses increased by 120-1200% in trials in which climbing fiber inputs were activated. The response amplitude of 68% of the nuclear cells was modified for these pairs. Most changes in nuclear cell responses were increases ranging from 120-220%. These changes were felt to reflect the action of many Purkinje cells converging on the isolated nuclear neuron. The modulation of the nuclear neuron was not due only to the effect of the related Purkinje cell, since the gain change of the Purkinje cell and nuclear cell of each pair was not correlated (r = 0.01). The discussion of these findings emphasizes that the increased responses of the nuclear cell are most likely produced by the intracortical action of the climbing fiber system on the responsiveness of Purkinje cells to mossy fiber inputs. Climbing fiber collateral input to nuclear neurons also may contribute to the changes in the nuclear cell responses observed in these experiments.     

Diminished interaction of norepinephrine with climbing fiber inputs to cerebellar Purkinje neurons in aged Fischer 344 rats

The ability of norepinephrine (NE) to modulate climbing fiber activation of complex spike discharge in cerebellar Purkinje neurons was compared in young (3-6 months) and aged (18-20 months) Fischer 344 rats. In young rats, NE selectively inhibits spontaneous activity while climbing fiber evoked activity remains intact or increased. NE also increases the probability of observing 4 bursts of full-sized action potentials rather than partially inactivated action potentials in the complex spike. In older rats, both of these modulatory actions of NE on climbing fiber complex spike activation are markedly diminished. These data support the concept that age-related reductions in catecholamine modulation of synaptic inputs may contribute to CNS dysfunction found in senescence.     

Stimulus classification by ensembles of climbing fiber receptive fields.

Although the local structure of the cerebellum is fairly uniform and its inputs are often widely shared, outputs from different regions of the cerebellar cortex reach different parts of the cerebellar and vestibular nuclei, which can affect the rest of the nervous system in different ways. In this review, we explain how different ensembles of climbing fiber responses in the anterior lobe and paramedian lobule can be generated by a tactile stimulus to the distal hindpaw. Apart from differing in degree of activation, the cortical regions differ also in the detailed pattern of the activation transmitted. The anterior lobe can distinguish a greater diversity of stimuli to various skin surfaces than can the paramedian median lobule. This differential classification of particular stimulus arrays by the two cerebellar regions could produce distinct patterns of neuronal activity in various corticonuclear compartments.     

Effects of FAK ablation on cerebellar foliation, Bergmann glia positioning and climbing fiber territory on Purkinje cells

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that is widely expressed in the brain, and plays key roles in various cellular processes in response to both extracellular and intracellular stimuli. Here, we explored the role of FAK in cerebellar development. In the mouse cerebellum, FAK was found to be distributed as tiny cytoplasmic aggregates in various neuronal and glial elements, including Purkinje cells (PCs), Bergmann glia (BG), parallel fiber (PF)-terminals and climbing fiber (CF)-terminals. The neuron/glia-specific ablation of FAK impaired cerebellar foliation, such as variable decreases in foliation sizes and the lack of intercrural and precentral fissures. Some of the BG cells became situated ectopically in the molecular layer. Furthermore, the FAK ablation altered the innervation territories of CFs and PFs on PCs. CF innervation regressed to the basal portion of proximal dendrites and somata, whereas ectopic spines protruded from proximal dendrites and PFs expanded their territory by innervating the ectopic spines. Furthermore, the persistence of surplus CFs innervating PC somata caused multiple innervation. When FAK was selectively ablated in PCs, diminished dendritic innervation and persistent somatic innervation by CFs were observed, whereas cerebellar foliation and cell positioning of BG were normally retained. These results suggest that FAK in various neuronal and glial elements is required for the formation of normal histoarchitecture and cytoarchitecture in the cerebellum, and for the construction of proper innervation territory and synaptic wiring in PCs.     

Propagation of parallel fiber volleys in the cerebellar cortex: a computer simulation.

We have studied the dynamic changes occurring during the propagation of active volleys through the parallel fiber (PF) tract in a portion of molecular layer of the cerebellar cortex, by means of a realistic computer simulation of the fibers (random distribution of PF origin; correlation between length and conduction velocity throughout depth). Surface electrical stimulations as well as mossy fiber stimulations of the PFs were mimicked. From a quantitative analysis of the propagation of the different types of volleys it appears that the constraints enforced by the morphological and functional characteristics of the PFs lead to an important spatial and temporal dispersion of the action potentials during their displacement along the folium. In particular, the time relationship between activity in the deep and superficial fibers as well as the number and location of active fibers at a given time (i.e. locus) will considerably vary during propagation. Moreover, in the case of a mossy fiber stimulation, there exists a striking asymmetry of the volleys travelling on each side of the site of stimulation. Therefore any information coded by the conjunctive activation of more than a few parallel fibers will be spatially and temporally dispersed in such a manner that successive postsynaptic Purkinje cells will receive very different combinations of inputs. This seems in favor of the hypothesis that these neurons function as coincidence detectors.