Effect of cerebellar granule cell depletion on spatial learning and memory and in an avoidance conditioning task: studies in postnatally X-irradiated rats

Cerebellum and cognition: Does the rodent cerebellum participate in cognitive functions?

There is a widespread, nearly complete consensus that the human and non-human primate cerebellum is engaged in non-motor, cognitive functions. This body of research has implicated the lateral portions of lobule VII (Crus I and Crus II) and the ventrolateral dentate nucleus. With rodents, however, it is not so clear. We review here approximately 40 years of experiments using a variety of cerebellar manipulations in rats and mice and measuring the effects on executive functions (working memory, inhibition, and cognitive flexibility), spatial navigation, discrimination learning, and goal-directed and stimulus-driven instrumental conditioning. Our conclusion is that there is a solid body of support for engagement of the rodent cerebellum in tests of cognitive flexibility and spatial navigation, and some support for engagement in working memory and certain types of discrimination learning. Future directions will involve determining the relevant cellular mechanisms, cerebellar regions, and precise cognitive functions of the rodent cerebellum.

Neonatal Irradiation Leads to Persistent Proteome Alterations Involved in Synaptic Plasticity in the Mouse Hippocampus and Cortex

Recent epidemiological data indicate that radiation doses as low as those used in computer tomography may result in long-term neurocognitive side effects. The aim of this study was to elucidate long-term molecular alterations related to memory formation in the brain after low and moderate doses of γ radiation. Female C57BL/6J mice were irradiated on postnatal day 10 with total body doses of 0.1, 0.5, or 2.0 Gy; the control group was sham-irradiated. The proteome analysis of hippocampus, cortex, and synaptosomes isolated from these brain regions indicated changes in ephrin-related, RhoGDI, and axonal guidance signaling. Immunoblotting and miRNA-quantification demonstrated an imbalance in the synapse morphology-related Rac1-Cofilin pathway and long-term potentiation-related cAMP response element-binding protein (CREB) signaling. Proteome profiling also showed impaired oxidative phosphorylation, especially in the synaptic mitochondria. This was accompanied by an early (4 weeks) reduction of mitochondrial respiration capacity in the hippocampus. Although the respiratory capacity was restored by 24 weeks, the number of deregulated mitochondrial complex proteins was increased at this time. All observed changes were significant at doses of 0.5 and 2.0 Gy but not at 0.1 Gy. This study strongly suggests that ionizing radiation at the neonatal state triggers persistent proteomic alterations associated with synaptic impairment.

Behavioral effects of neonatal lesions on the cerebellar system

Several rodent models with spontaneous mutations causing cerebellar pathology are impaired in motor functions during the neonatal period, including Grid2(Lc), Rora(sg), Dab1(scm), Girk2(Wv), Lmx1a(dr-sst), Myo5a(dn), Inpp4a(wbl), and Cacna1a(rol) mice as well as shaker and dystonic rats. Deficits are also evident in murine null mutants such as Zic1, Fgfr1/FgFr2, and Xpa/Ercc8. Behavioral deficits are time-dependent following X-irradiated- or aspiration-induced lesions of the cerebellum in rats. In addition, motor functions are deficient after lesions in cerebellar-related pathways. As in animal subjects, sensorimotor disturbances have been described in children with cerebellar lesions. These results underline the importance of the cerebellum and its connections in the development of motor functions.

Long-lasting effects of neonatal ionizing radiation exposure on spatial memory and anxiety-like behavior

Neonatal ionizing radiation exposure has been shown to induce a cerebellar cytoarchitecture disarrangement. Since cerebellar abnormalities have been linked to an impairment of behavioral functions, the aim of the present work was to investigate whether exposure of developing rats to ionizing radiations can produce behavioral deficits in the adult. Male Wistar rats were X-irradiated with 5Gy within 48h after birth and were tested in a radial maze and in an open field at 30 and 90 days post irradiation. Irradiated rats showed significative changes in spatial, exploratory, and procedural parameters in the radial maze, as well as a significative decrease in anxiety-like behavior, assessed in the open field. These results suggest that ionizing radiations can induce long-lasting spatial memory and anxiety-related changes. A relationship with radiation-induced cerebellar cytoarchitecture abnormalities supports the hypothesis that cerebellar integrity seems to be critical to achieve spatial performance and emotional behavior establishment.

Habit learning dissociation in rats with lesions to the vermis and the interpositus of the cerebellum

After cerebellar tumors resection, patients show motor skill learning impairments but also cognitive deficits. However, their exact origins remain controversial. Using a rat model of cerebellar injury, we assessed the involvement of two structures often damaged during resection (vermis and interpositus nuclei) on habits development. During extended training of an instrumental task, rats develop response routines that are no longer voluntary or goal-directed but habit-based, evidenced by their insensitivity to changes in the value of the reward. Here we showed that, in contrast to sham or vermis lesioned rats, discrete lesions to interpositus nuclei prevented rats from developing habits with overtraining, without motor difficulties, nor alteration of the instrumental task acquisition. Our results suggest that the role of the cerebellum can be extended from motor skill learning to cognitive routines learning. Similar habit impairment could possibly account for some of the long-term outcome difficulties observed in cerebellar-damaged patients.

Developmental neural plasticity and its cognitive benefits: olivocerebellar reinnervation compensates for spatial function in the cerebellum

The adult mammalian central nervous system displays limited reinnervation and recovery from trauma. However, during development, post-lesion plasticity may generate alternative paths, thus providing models to investigate reinnervation and repair. After unilateral transection of the neonatal rat olivocerebellar path (pedunculotomy), axons from the remaining inferior olive reinnervate the denervated hemicerebellum. Unfortunately, reinnervation to the cerebellar hemisphere is incomplete; therefore, its capacity to mediate hemispheric function (navigation) is unknown. We studied sensorimotor control and spatial cognition of rats with and without transcommissural reinnervation using simple (bridge and ladder) and complex (wire) locomotion tests and the Morris water maze (hidden, probe and cued paradigms). Although pedunculotomized animals completed locomotory tasks more slowly than controls, all groups performed equally in the cued maze, indicating that lesioned animals could orientate to and reach the platform. In animals pedunculotomized on day 3 (Px3), which develop olivocerebellar reinnervation, final spatial knowledge was as good as controls, although they learned more erratically, failing to retain all information from one day to the next. By contrast, animals pedunculotomized on day 11 (Px11), which do not develop reinnervation, did not learn the task, taking less direct routes and more time to reach the platform than controls. In the probe test, control and Px3, but not Px11, animals swam directly to the remembered location. Furthermore, the amount of transcommissural reinnervation to the denervated hemisphere correlated directly with spatial performance. These results show that transcommissural olivocerebellar reinnervation is associated with spatial learning, i.e. even partial circuit repair confers significant functional benefit.

Regional differences in vulnerability of the cerebellar foliations of rats exposed to neonatal X-irradiation

The purpose of the present study was to elucidate regional differences in the vulnerability of cerebellar foliations of rats exposed to X-irradiation. Effects of X-irradiation on foliations were examined with respect to histological changes in Purkinje cells and Bergmann glial fibers by calbindin-D28k (CB) and glial fibrillary acidic protein (GFAP) immunohistochemistry, respectively. Wistar rats were exposed to X-irradiation (1.5 Gy) on postnatal day (PND) 1. At 3 weeks of age, the cerebellum was examined. The cerebella of rats exposed to X-irradiation showed smaller and abnormal foliations compared with controls. Fewer cerebellar foliations due to fusion with neighboring folia were observed in folia I-III and VIa-VII. Moreover, the extent of such abnormalities was more severe in the latter folia. CB-immunoreactive (IR) Purkinje cells exhibited thin, short, disoriented dendrites that had invaded the granular layer or white matter. On the other hand, GFAP-IR Bergmann glial fibers had not extended their processes into the molecular layer perpendicular to the pial surface, and they appeared thin and disoriented. Accordingly, the above cerebellar abnormalities were more severe in folia I-III, VIa-VII and X than in other regions. In contrast to the histological alterations in these folia, there were no apparent qualitative differences in folia IV-V between X-irradiated and controls. These findings indicate regional difference in the vulnerability of cerebellar folia to X-irradiation. Such differences might be attributed to the cerebellar neurogenetic gradient.

The sequelae of cranial irradiation on human cognition

Cranial irradiation (CI) confers remediation of many CNS anomalies. CI, however, carries risks to cognitive performance. A wealth of data describes such deficits specifically in humans. Risk factors that promote increased susceptibility to cognitive decline have also been identified. This paper discusses and grades these risk factors, including age, gender, and the inclusion of chemotherapy, that increase the likelihood of pathologic cognitive development in the human population.

Is the cerebellum ready for navigation?

Spatial navigation required the acquisition of at least two complementary processes: the organization of the spatial representation of the environment (declarative learning) and the acquisition of a motor behaviour adapted to the specific context (procedural learning). The potential role of the cerebellum in spatial navigation is part of the debate concerning its role in cognitive function. Experiments ranging from cerebellar patients to animal models have indicated that cerebellar damage affects the processing of spatial information. The main unresolved issue concern the interpretation of these deficits. Is the cerebellum involved in both declarative and procedural components of navigation? Could all deficits in navigation paradigms be interpreted by a deficit in a motor-dependant process? The purpose of this review is to examine different results coming from anatomical data, experimental paradigms and models in order to give a critical answer to this question.

The effects of cerebellar damage on maze learning in animals

The role of the cerebellum in spatial learning has recently been investigated in genetically and non-genetically lesioned animal models, particularly in water mazes, in view of the minimal impact such lesions exert on swimming movements. A dissociation between place and cued learning in the Morris water maze has been observed in several models, including cerebellar mutant mice (Rora(sg), Nna1(pcd-1J), nervous), rats with lesions of either the lateral cerebellar cortex or the dentate nucleus, and rats with selective Purkinje cell loss caused by intracerebroventricular injections of OX-7-saporin, confirming the hypothesis that cerebellar damage may cause a cognitive deficit independently of fine motor control. In addition, the results of hemicerebellectomized rats indicate the probable involvement of the cerebellum in working memory and the procedural aspect of maze learning. The findings of impaired maze learning in cerebellar-lesioned mice and rats are concordant with those of deficient visuospatial functions in patients with cerebellar atrophy. The spatial deficits may be ascribed to altered metabolic activity in cerebellar-related pathways.

The role of climbing and parallel fibers inputs to cerebellar cortex in navigation

DA-HAN rats with partial or total lesion of climbing (CF) and parallel fibers (PF) inputs of the cerebellum were tested in a water task. Two different protocols were used, requiring to find either a non-visible or a visible platform. These two protocols were, respectively, designed to evaluate visuo-motor guidance (visible platform) and navigation (non-visible platform). Both groups of lesioned rats presented a deficit in the non-visible platform task but not in the visible platform one. The protocol of navigation we used was a fixed start-fixed arrival procedure. Totally lesioned animals were unable to learn to orient their body toward the non-visible platform and adopted instead a circling behavior. Our results suggest a role of cerebellar inputs (climbing (CF) and PF) in navigation.

Lack of effect of moderate Purkinje cell loss on working memory

192 immunoglobulin G-saporin (192-sap) is an immunotoxin which targets the cholinergic basal forebrain after injection into either the ventricular system or the parenchyma of the rat brain. When injected by the i.c.v. route, 192-sap kills some cerebellar Purkinje cells in addition to its more extensive killing of the cholinergic basal forebrain. Behaviorally, i.c.v. injections of 192-sap result in impaired performance in a variety of experimental paradigms of learning and memory including a working memory task in the radial maze. The current study examined the contribution, if any, of immunotoxin-induced Purkinje cell loss to impaired performance in the radial maze. To meet this aim, we used i.c.v. injection of another immunotoxin, OX7-saporin (OX7-sap), at a dose that produced Purkinje cell loss of similar extent to that produced by i.c.v. 192-sap. We then compared these OX7-sap-injected rats with 192-sap-injected rats in a radial maze working memory task. We found a working memory impairment only in the 192-sap-injected rats. These data show that moderate Purkinje cell loss alone is insufficient to impair working memory. Furthermore, the data are consistent with the idea that the working memory deficit observed in 192-sap-injected animals is likely due to lesioning of the cholinergic basal forebrain.

Effects of early midline cerebellar lesion on cognitive and emotional functions in the rat

Midline lesion of the cerebellum was performed in young 10-day-old DA/HAN strained (pigmented) rats. Once adults, the lesioned animals were subjected to a series of behavioral tests and their performances were compared with those of control (nonlesioned) rats. Compared with controls, the spontaneous motor activity of the lesioned rats was higher, they showed persevering behavior and did not pay attention to environmental distractors. In anxiety and social discrimination tests, disinhibition tendencies were obvious, which suggested that the animals were less dependent on the context. These abnormalities were most likely due to early midline lesion of the cerebellum and not to a deficit in maternal care before weaning, since the dams took care of the lesioned and control pups similarly. From these results, it can be concluded that the cerebellar vermis is involved in motor control, attentional capabilities and emotional behavior. Given that the lesioned rats observed in this study presented obvious autistic-like symptoms, and since a number of autistic subjects have cerebellar deficits and, particularly, a hypoplasia of vermal lobules, our results may strengthen the idea that the cerebellar vermis is involved in autism, as already suggested in the guinea pig (Caston J, et al. Eur J Neurosci 1998;10:2677-2684).

Treadmill performance of mice with cerebellar lesions: 2. Lurcher mutant mice

The sensorimotor skills of a spontaneous mouse mutant with cerebellar cortical atrophy, Lurcher, were examined on either a fast or a slow treadmill inclined at one of three slopes, requiring forward movements in order to avoid footshocks. During the early part of acquisition, Lurcher mutants had lower latencies before falling on either treadmill than normal mice, but not during a retention test. For both Lurcher mutants and controls, the amount of time spent walking as a function of time spent on the belt increased with an increase in belt speed. Lurcher mutants had higher walking time/total time ratios on the slow but not on the fast treadmill. It is concluded that cerebellar cortical degeneration impaired the time course of acquisition but not long-term retention of the treadmill task.

Impaired acquisition of a Morris water maze task following selective destruction of cerebellar purkinje cells with OX7-saporin

Spatial learning in the Morris water maze task is believed to be dependent on an intact hippocampal system. However, evidence from human studies and animal experiments suggests a potential cerebellar involvement in spatial processing, place learning, and other types of 'higher-order' cognition. In order to investigate this possibility, intraventricular injections (ICV) of the anti-neuronal immunotoxin OX7-saporin were used to selectively destroy cerebellar Purkinje cells, without affecting other brain areas believed to be critically involved in spatial learning and memory. Bilateral ICV injections of 2 microg OX7-saporin (4 microg total) in adult male rats produced substantial loss of Purkinje cells (56%) throughout the cerebellum without affecting hippocampal morphology or biochemical indices of cholinergic, serotonergic, or catecholaminergic function in the hippocampus, frontal cortex, or striatum. ICV OX7-saporin significantly impaired acquisition and performance of the standard Morris water maze task (though the impairment was less severe than reported in earlier studies that used alternate lesion methods or mutant mice species), but did not alter performance on the cued version of the task, or locomotor activity. In addition, lesioned animals spent significantly less time in the target quadrant on probe trial days 4 and 7 and the average distance to target scores (ADT) were significantly greater than controls on those days. Swim speed was not affected. Based on the specificity of the behavioral and neurobiological alterations, these data support the hypothesis that the cerebellum is involved in spatial processing and place learning.

The differentiation of cerebellar interneurons is independent of their mitotic history

A narrow time window centered around the terminal mitosis of their precursors has been recognized to be critical for the determination and/or realization of the developmental fate of a variety of neuronal phenotypes. In contrast, individual cell lineages in the cerebellum get separated early during embryonic development, and at least precursors for granule neurons have been found to be specified while still proliferating. We utilized primary dissociated cultures to address the issue of whether the faithful development of cerebellar granule cells and basket/stellate cells is dependent on their mitotic history and on the completion of a fixed number of cell cycles. Neuroblasts derived from embryonic cerebellar anlagen and transferred into primary dissociated cultures stopped proliferating as assessed by a loss of expression of the cell proliferation marker, Ki-67, and a failure to incorporate 5-bromo-2'-deoxyuridine. Although these cells had been forced to leave the proliferating cell pool prematurely, they developed into granule neurons or basket/stellate cells as judged by their distinct pattern of expression of specific molecular markers and the acquisition of a typical morphology. This included the cell intrinsic capacity of granule neurons to position their afferent synapses specifically to their dendrites. Thus, the competence of cerebellar interneurons to differentiate appropriately is independent of the precise timing of their final mitosis; however, their sensitivity towards extrinsic developmental signals appears to vary in a cell cycle-dependent manner, as suggested by the failure to survive of those cells that were in S-phase at the time of cultivation.

Developmental brain injury associated with abnormal play behavior in neonatally Borna disease virus-infected Lewis rats: a model of autism

Play behavior, nonsocial exploratory activity, and nonplay social interaction were observed in male juvenile Lewis rats with brain developmental injury following neonatal infection with Borna disease virus (BDV). These behaviors were tested using the 'intruder-resident' paradigm, with social isolation of residents for six days prior to testing. Four experimental pairings of infected (BDV) and uninfected (NL) rats were studied as follows: NL-NL; NL BDV; BDV NL; and BDV-BDV (the first member is the resident, the second member is the intruder). Observation of social activities was carried out for 10 min on two consecutive days. Nonsocial exploratory activity (e.g. ambulation and rearing) was similar in BDV and NL residents. Duration of nonplay social investigation (e.g. sniffing, approach, and follow) was higher in BDV residents as compared to NL residents when tested on the first test day. On the second day, all rats showed similar level of nonplay social interaction. When confronted with NL intruders, NL residents exhibited significantly more play behavior compared to the NL-BDV, BDV NL and BDV-BDV pairs, when play behavior was measured by the number of 'pins'. Moreover, irrespective of a type of intruder, NL residents demonstrated higher play soliciting behavior than BDV residents, indicating attenuated readiness to play in BDV-infected rats. The number of pins and play solicitations in BDV-NL pairs significantly increased over the two days of testing, while play activity in NL-BDV pairs declined on the second test day. This pattern suggests that the degree of social reinforcement on the first day of testing affected the level of play on the second day. These data demonstrate deficits in play behavior and other social interactions following BDV-associated developmental brain injury, thus supporting the value of the neonatally BDV-infected rat as an animal model of autism.

Borna disease virus-induced hippocampal dentate gyrus damage is associated with spatial learning and memory deficits

In neonatally inoculated rats, Borna disease virus (BDV) leads to a persistent infection of the brain in the absence of an inflammatory response and is associated with neuroanatomic, developmental, physiologic, and behavioral abnormalities. One of the most dramatic sites of BDV-associated damage in the neonatal rat brain is the dentate gyrus, a neuroanatomic region believed to play a major role in spatial learning and memory. The absence of a generalized inflammatory response to neonatal BDV infection permits direct effects of viral damage to the dentate gyrus to be examined. In this report, neonatally BDV-infected rats at various stages of dentate gyrus degeneration were evaluated in the Morris water maze, a swimming test that assesses the rats' capacity to navigate by visual cues. Our data demonstrate progressive spatial learning and memory deficits in BDV-infected rats that coincided with a gradual decline in the estimated hippocampal dentate gyrus neuron density.

Role of the cerebellum in exploration behavior

Compared to +/+ mice, Lurcher (+/Lc) mutant mice whose cerebellar cortex is lacking almost all Purkinje cells and granule cells, exhibit a low level of exploration; this deficit is not due to a low level of activity but to both a decreased motivation to explore a novel environment and to spatial deficits. The characteristics of exploration in cerebellectomized +/+ and +/Lc mice suggest that the cerebellum is involved not only in cognitive but also in motivational processes.

Timed active avoidance learning in lurcher mutant mice

Lurcher mutant mice (+/Lc) which exhibit a massive loss of neurons in the cerebellar cortex and in the inferior olivary nuclei were subjected to an active avoidance learning task; the animals' avoidance response must occur within a small time window after a short or a long delay. The control mice needed a mean of 8.3 sessions of 10 trials (short delay group) and of 11.8 sessions (long delay group) and showed good retention after a 24 h interval. When subjected to the same number of sessions, the +/Lc mice were unable to learn the timing task. However, a subgroup of lurcher mutants was able to learn after a high number of sessions (25.4 sessions as a mean). There was no intergroup difference in the standard version of one-way active avoidance. These results indicate that the cerebellar cortex is involved in time processing during active avoidance. The cerebellum may be part of a loop including the cerebral cortex known to be involved in time perception. An alternative explanation is that the cerebellar mutant animals had persevering tendencies acquired during performance of the one-way avoidance task.

Effect of cerebellar granule cell depletion on learning of the equilibrium behaviour: study in postnatally X-irradiated rats

To assess the role of the mossy fibre-granule cell pathway in learning, the cerebellum of young DA/HAN strain rats was irradiated to make the cortex completely or partially agranular. The X-rays were delivered according to two different schedules, between 5-14 postnatal days (early group) and between 10-14 postnatal days (late group). Histological controls at 35 days showed a mean loss of granule cells of 96 +/- 1% in the early group and of 61 +/- 3% in the late group. The irradiated animals were subjected, from day 23 to day 35, to daily sensorimotor training on a rotorod. The scores and the strategy used (walking or hanging) by the rats were noted. The results demonstrate that a partial loss of granule cells due to a late X-irradiation schedule induced mild motor disabilities but no learning deficit, the only problem being difficulty in elaborating rapidly an efficient strategy to solve a novel problem. A sub-total loss of the granule cells, due to an early X-irradiation schedule, induced gross motor disabilities and the animals used hanging > 90% of the time. Due to the discrepancy between the learning abilities, which were preserved at least in part, and the gross motor impairments, the animals elaborated a novel strategy (jumping from the beam), allowing them to escape the experimental situation. This avoidance behaviour may be due to a decrease of anxiety, a lack of behavioural inhibition and/or attentional deficits that have been already observed in several other examples of cerebellar abnormalities.