Olivary morphology and olivocerebellar topography in adult lurcher mutant mice

Timing correlations between cerebellar interpositus neuronal firing and classically conditioned eyelid responses in wild-type and Lurcher mice

Classical eyeblink conditioning is an experimental model widely used for the study of the neuronal mechanisms underlying the acquisition of new motor and cognitive skills. There are two principal interpretations of the role of the cerebellum in the learning of eyelid conditioned responses (CRs). One considers that the cerebellum is the place where this learning is acquired and stored, while the second suggests that the cerebellum is mostly involved in the proper performance of acquired CRs, implying that there must be other brain areas involved in the learning process. We checked the timing of cerebellar interpositus nucleus (IPN) neurons’ firing rate with eyelid CRs in both wild-type (WT) and Lurcher (a model of cerebellar cortex degeneration) mice. We used delay and trace conditioning paradigms. WT mice presented a better execution for delay vs. trace conditioning and also for these two paradigms than did Lurcher mice. IPN neurons were activated during CRs following the activation of the orbicularis oculi muscle. Firing patterns of IPN neurons were altered in Lurcher mice. In conclusion, the cerebellum seems to be mostly related with the performance of conditioned responses, rather than with their acquisition.

Smaller Absolute Quantities but Greater Relative Densities of Microvessels Are Associated with Cerebellar Degeneration in Lurcher Mice

Degenerative affections of nerve tissues are often accompanied by changes of vascularization. In this regard, not much is known about hereditary cerebellar degeneration. In this study, we compared the vascularity of the individual cerebellar components and the mesencephalon of 3-month-old wild type mice (n = 5) and Lurcher mutant mice, which represent a model of hereditary olivocerebellar degeneration (n = 5). Paraformaldehyde-fixed brains were processed into 18-μm thick serial sections with random orientation. Microvessels were visualized using polyclonal rabbit anti-laminin antibodies. Then, the stacks comprised of three 5-μm thick optical sections were recorded using systematic uniform random sampling. Stereological assessment was conducted based on photo-documentation. We found that each of the cerebellar components has its own features of vascularity. The greatest number and length of vessels were found in the granular layer; the number of vessels was lower in the molecular layer, and the lowest number of vessels was observed in the cerebellar nuclei corresponding with their low volume. Nevertheless, the nuclei had the greatest density of blood vessels. The reduction of cerebellum volume in the Lurcher mice was accompanied by a reduction in vascularization in the individual cerebellar components, mainly in the cortex. Moreover, despite the lower density of microvessels in the Lurcher mice compared with the wild type mice, the relative density of microvessels in the cerebellar cortex and nuclei was greater in Lurcher mice. The complete primary morphometric data, in the form of continuous variables, is included as a supplement. Mapping of the cerebellar and midbrain microvessels has explanatory potential for studies using mouse models of neurodegeneration.

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.

The reciprocal cerebellar circuitry in human hereditary ataxia

Clinicoanatomic correlation in the spinocerebellar ataxias (SCA) and Friedreich's ataxia (FRDA) is difficult as these diseases differentially affect multiple sites in the central and peripheral nervous systems. A new way to study cerebellar ataxia is the systematic analysis of the "reciprocal cerebellar circuitry" that consists of tightly organized reciprocal connections between Purkinje cells, dentate nuclei (DN), and inferior olivary nuclei (ION). This circuitry is similar to but not identical with the "cerebellar module" in experimental animals. Neurohumoral transmitters operating in the circuitry are both inhibitory (γ-aminobutyric acid in corticonuclear and dentato-olivary fibers) and excitatory (glutamate in olivocerebellar or climbing fibers). Glutamatergic climbing fibers also issue collaterals to the DN. The present study applied five immunohistochemical markers in six types of SCA (1, 2, 3, 6, 7, 17), genetically undefined SCA, FRDA, and FRDA carriers to identify interruptions within the circuitry: calbindin-D28k, neuron-specific enolase, glutamic acid decarboxylase, and vesicular glutamate transporters 1 and 2. Lesions of the cerebellar cortex, DN, and ION were scored according to a guide as 0 (normal), 1 (mild), 2 (moderate), and 3 (severe). Results of each of the five immunohistochemical stains were examined separately for each of the three regions. Combining scores of each anatomical region and each stain yielded a total score as an indicator of pathological severity. Total scores ranged from 16 to 38 in SCA-1 (nine cases); 22 to 39 in SCA-2 (six cases); 9 to 15 in SCA-3 (four cases); and 13 and 25 in SCA-6 (two cases). In single cases of SCA-7 and SCA-17, scores were 16 and 31, respectively. In two genetically undefined SCA, scores were 36 and 37, respectively. In nine cases of FRDA, total scores ranged from 11 to 19. The low scores in SCA-3 and FRDA reflect selective atrophy of the DN. The FRDA carriers did not differ from normal controls. These observations offer a semiquantitative assessment of the critical role of the DN in the ataxic phenotype of SCA and FRDA while other parts of the circuitry appear less important.

Motor learning of mice lacking cerebellar Purkinje cells

The cerebellum plays a key role in the acquisition and execution of motor tasks whose physiological foundations were postulated on Purkinje cells' long-term depression (LTD). Numerous research efforts have been focused on understanding the cerebellum as a site of learning and/or memory storage. However, the controversy on which part of the cerebellum participates in motor learning, and how the process takes place, remains unsolved. In fact, it has been suggested that cerebellar cortex, deep cerebellar nuclei, and/or their combination with some brain structures other than the cerebellum are responsible for motor learning. Different experimental approaches have been used to tackle this question (cerebellar lesions, pharmacological agonist and/or antagonist of cerebellar neurotransmitters, virus tract tracings, etc.). One of these approaches is the study of spontaneous mutations affecting the cerebellar cortex and depriving it of its main input–output organizer (i.e., the Purkinje cell). In this review, we discuss the results obtained in our laboratory in motor learning of both Lurcher (Lc/+) and tambaleante (tbl/tbl) mice as models of Purkinje-cell-devoid cerebellum.

Tensor-based morphometry and stereology reveal brain pathology in the complexin1 knockout mouse

Complexins (Cplxs) are small, soluble, regulatory proteins that bind reversibly to the SNARE complex and modulate synaptic vesicle release. Cplx1 knockout mice (Cplx1(-/-)) have the earliest known onset of ataxia seen in a mouse model, although hitherto no histopathology has been described in these mice. Nevertheless, the profound neurological phenotype displayed by Cplx1(-/-) mutants suggests that significant functional abnormalities must be present in these animals. In this study, MRI was used to automatically detect regions where structural differences were not obvious when using a traditional histological approach. Tensor-based morphometry of Cplx1(-/-) mouse brains showed selective volume loss from the thalamus and cerebellum. Stereological analysis of Cplx1(-/-) and Cplx1(+/+) mice brain slices confirmed the volume loss in the thalamus as well as loss in some lobules of the cerebellum. Finally, stereology was used to show that there was loss of cerebellar granule cells in Cplx1(-/-) mice when compared to Cplx1(+/+) animals. Our study is the first to describe pathological changes in Cplx1(-/-) mouse brain. We suggest that the ataxia in Cplx1(-/-) mice is likely to be due to pathological changes in both cerebellum and thalamus. Reduced levels of Cplx proteins have been reported in brains of patients with neurodegenerative diseases. Therefore, understanding the effects of Cplx depletion in brains from Cplx1(-/-) mice may also shed light on the mechanisms underlying pathophysiology in disorders in which loss of Cplx1 occurs.

Changes of motor abilities during ontogenetic development in Lurcher mutant mice

Lurcher mutant mice represent a natural model of olivocerebellar degeneration. This degeneration is caused by a mutation of the gene for the delta2 glutamate receptor. Lurcher mutants suffer from cerebellar ataxia and cognitive functions deficiency as a consequence of excitotoxic apoptosis of Purkinje cells in the cerebellar cortex and a secondary decrease of granule cells and inferior olive neurons. This process finishes by the 90th day of postnatal life, but already by 14 days, the Purkinje cells are damaged and the ataxia is fully developed. Purkinje cells die by apoptosis within the first 3 weeks of life. The aim of our work was to study the development of motor functions in the course of the ontogenetic development in Lurcher mutant mice of the B6CBA strain and to compare it with wild type mice of the same strain. Mice aged 2, 3, 6, 9, and 22 weeks were used in our experiment. Motor skills were examined using four standard tests: the horizontal wire, rotating cylinder, footbridge and slanting ladder. Our findings in Lurcher mutant mice show a significant increase of motor abilities up to the sixth postnatal week and selective decrease early after this period. This improvement of motor skills is caused by the physiological development of musculature and the nervous system, probably with some contribution of plasticity of the maturing brain. The cause of the decline of these abilities immediately after the completion of the development is unknown.

The Lurcher mouse: Fresh insights from an old mutant

The Lurcher mouse was first discovered in 1954 as a spontaneously occurring autosomal dominant mutation that caused the degeneration of virtually all cerebellar Purkinje cells and most olivary neurons and granule cells. More recent molecular studies revealed that Lurcher is a gain of function mutation in the delta2 glutamate receptor (GluRdelta2) that converts an alanine to threonine in the highly conserved third hydrophobic segment of GluRdelta2. The mutation converts the receptor into a constitutively leaky cation channel. The GluRdelta2 receptor is predominantly expressed in cerebellar Purkinje cells and in the heterozygous Lurcher mutant (+/Lc). Purkinje cells die due to the mutation in the GluRdelta2 receptor, while olivary neurons and granule cells degenerate due to the loss of their Purkinje cell targets. The purpose of the review is to provide highlights from 5 decades of research on the Lurcher mutant that have provided insights into the developmental mechanisms that regulate cell number during development, cerebellar pattern formation, cerebellar physiology, and the role of the cerebellum in CNS function.

Cerebellar injury: clinical relevance and potential in traumatic brain injury research

A treatment for traumatic brain injury (TBI) remains elusive despite compelling evidence from animal models for a variety of therapeutic targets. Numerous animal models have been developed to address the wide spectrum of mechanisms involved in the progression of secondary injury after TBI. Evidence from well-established models such as the fluid percussion injury (FPI) device, cortical impact model, and the impact acceleration model has demonstrated diffuse pathophysiological mechanisms throughout various brain structures. More specifically, we have recently extended characterization of the FPI model to include pathophysiological changes in the cerebellum following unilateral fluid percussion. Data suggest that the cerebellum is susceptible to selective Purkinje cell loss as well as white matter dysfunction. Despite the cerebellum's low profile in TBI research, there is evidence to warrant further study of the cerebellum to examine mechanisms of neuronal death and traumatic axonal injury. Furthermore, evidence from clinical literature and basic science suggests that some components of TBI pathophysiology have a basis in cerebellar dysfunction. This review highlights some of the recent findings in cerebellar trauma and builds an argument for including the cerebellum as a model to assess mechanisms of secondary injury and its potential contribution to the pathology of TBI.

On the variety of cell death pathways in the Lurcher mutant mouse

Apoptosis as well as autophagy have been implicated in the death of cerebellar Purkinje cells (PCs) in the Lurcher (Lc/+) mutant mouse and at least two different apoptotic pathways participate in the transsynaptic death of granule cells (GC) and inferior olivary (IO) neurones. The relative contribution of these pathways can only be assessed from their momentary involvement at any stage of the complete course of neurodegeneration. Here we used quantitative labelling for activated caspase-3 (Casp-3) and Fluoro-Jade B (FJ-B) to investigate the spatio-temporal pattern of neuronal death from P6 to P67 in Lc/+ mutants. Activated Casp-3 was present only in narrow time intervals (P14 to P22 in PCs; P14 to P28 in GCs) and in small subpopulations of PCs, GCs, and IO neurones. FJ-B positive PCs were detected during a broader period (P14 to P28), and outnumbered Casp-3 labelled PCs by a factor exceeding eight. Nevertheless, FJ-B labelling was restricted to PCs and never found in either GC or IO neurones. In conclusion, we present the first complete time course and extent of Casp-3 activation in Lc/+ mutants and show that the majority of dying neurones in Lc/+ mutants undergo Casp-3 independent cell death. The cellular overload produced by the initial gene defect in Lc/+ mutants apparently activates a variety of apoptotic and non-apoptotic pathways within the same neuronal population. Moreover, we present the first evidence for the ability of FJ-B to selectively label a discrete population of dying PCs, implying a higher selectivity of FJ-B than previously supposed.

Translocation of cytochrome c during cerebellar degeneration in Lurcher and weaver mutant mice

Cytochrome c translocation from the inner mitochondrial membrane into the cytosol is the initial step of the intrinsic apoptotic pathway. As no evidence was ever presented for cytochrome c translocation during cerebellar degeneration in Lurcher (Lc/+) and weaver (wv/wv) mutant mice, we searched for the presence of such a process in cerebellar homogenates of mutant and wild-type mice from postnatal day (P)1 to P56. Here we present the first documented time course of cytochrome c translocation spanning the entire period of neurodegeneration in both mutant types. We identified cytochrome c with Western blotting and monitored cell loss in the cerebellum with Calbindin D-28k immunohistochemistry, Nissl-staining and morphometry. No cytochrome c translocation was ever detected in wild-types at any age investigated. Translocated cytochrome c appeared between P13 and P21 in Lc/+ and between P5 and P6 in wv/wv. These two intervals precisely coincide with the respective periods of maximal neuronal death in the cerebellum. Secondary translocation was also observed at a later stage between P42 and P49 in Lc/+ and from P22 onwards in wv/wv. Since no substantial neuronal loss has ever been observed in Lc/+ and wv/wv mutants at these postnatal ages, the delayed translocation may correspond to cytochrome c of extraneuronal, presumably glial origin. Observations of an increased expression of glial fibrillary acidic protein and sustained remodeling of the astrocytic network in the cerebellum of both mutants, long after the cessation of neuronal death make this assumption rather plausible.

Regional brain variations of cytochrome oxidase activity and motor coordination in Girk2Wv (Weaver) mutant mice

The Girk2(Wv) (weaver) phenotype, caused by a mutated inward rectifying potassium channel, is characterized by degeneration of cerebellar granule cell population as well as midbrain dopamine-containing cells of the nigrostriatal pathway. To investigate the regional brain metabolic consequences of this combined pathology, cytochrome oxidase (CO) activity was measured by histochemistry from brain regions of wild-type and homozygous Girk2(Wv) mutant mice and correlated with motor performances. CO activity of Girk2(Wv) mutants was abnormal in cerebellar cortex, dentate nucleus, and brainstem regions (medial and lateral vestibular nuclei, prepositus, superior colliculus, lateral cuneiform nucleus, and reticular nuclei) implicated in the gaze system. CO activity increased in midbrain dopaminergic regions after correcting for tissue density, regions with severe depletion of tyrosine hydroxylase activity. Forebrain regions were relatively spared in term of CO activity, except for subthalamic nucleus, lateral geniculate nucleus, and cortical eye field. Similarly to the Rora(sg) cerebellar mutant, metabolic alterations in cerebellar and vestibular regions were linearly correlated with poor motor coordination, underlining the sensitivity of these tests to cerebellar dysfunction.

Spontaneous and induced mouse mutations with cerebellar dysfunctions: behavior and neurochemistry

Grid2(Lc) (Lurcher), Grid2(ho) (hot-foot), Rora(sg) (staggerer), nr (nervous), Agtpbp1(pcd) (Purkinje cell degeneration), Reln(rl) (reeler), and Girk2(Wv) (Weaver) are spontaneous mutations with cerebellar atrophy, ataxia, and deficits in motor coordination tasks requiring balance and equilibrium. In addition to these signs, the Dst(dt) (dystonia musculorum) spinocerebellar mutant displays dystonic postures and crawling. More recently, transgenic models with human spinocerebellar ataxia mutations and alterations in calcium homeostasis have been shown to exhibit cerebellar anomalies and motor coordination deficits. We describe neurochemical characteristics of these mutants with respect to regional brain metabolism as well as amino acid and biogenic amine concentrations, uptake sites, and receptors.

Development of Hsp25 expression compartments is not constrained by Purkinje cell defects in the Lurcher mouse mutant

Four transverse zones can be distinguished in the adult mouse cerebellar cortex based on differential expression of cell-specific antigens, termination patterns of mossy fiber afferents, and phenotypes of mouse mutants with cerebellar defects: the anterior zone (AZ), central zone (CZ), posterior zone (PZ), and nodular zone (NZ). In the heterozygous Lurcher (Lc/+) mouse a zonally restricted abnormality in Purkinje cell development is seen. The Purkinje cell-specific antigen zebrin II is normally differentially expressed in all four zones of the adult cerebellum, but in the Lc/+ mutant is confined to the PZ and NZ, caudal to a transverse boundary in the dorsal aspect of lobule VIII. In this study we wanted to understand why zebrin II expression is arrested at this boundary and whether the Lc mutation affects the differentiation of additional Purkinje cell antigens in a similar manner. To determine this, we took advantage of the dynamic developmental timetable of another Purkinje cell antigen, the small heat shock protein Hsp25. Using immunohistochemistry we demonstrate that cerebellar maturation anterior to the CZ/PZ transverse boundary appears to be unaffected by the Lc allele, in that initial progression of Hsp25 expression in the Lc/+ cerebellum was similar to controls. Double-labeling experiments with anti-Hsp25 and anti-calbindin suggest that characteristic banding patterns of Hsp25 in Lc/+ cerebellum develop and are preserved despite cell loss. Thus, since simple temporal or spatial models cannot account for the zonal restriction seen during Lc/+ cerebellar development, the abnormality may be zebrin II-specific.

Regional brain variations of cytochrome oxidase activity inRelnrl-orl mutant mice

Cell malpositioning has been described in laminated structures of the spontaneous mutation, reeler, including the cerebellum, the hippocampus, and the neocortex. Despite the ectopic positions of different neuronal populations, the specificity of synaptic connections is maintained. The metabolic consequences of this form of neuropathology were examined in Reln(rl) mutant mice by quantitative measures of cytochrome oxidase (CO) activity, a mitochondrial enzyme essential for oxidative metabolism in neurons. Despite severe tissue disorganization but in line with the intact synaptic organization, the reeler mutation did not affect global metabolic activity of the laminated structures of the brain. CO activity, however, was altered in specific subregions of the cerebellum, hippocampus, and neocortex, as well as in septum and various brainstem (medial pontine, paramedial reticular, paragigantocellular reticular) regions anatomically related to these structures, attesting to large functional alterations in Reln(rl-orl) brain. Metabolic activity variations were also detected in the ventral tegmental area and ventral neostriatum of the mesolimbic dopaminergic pathway. The results are discussed and compared to the regional CO variations found in other ataxic mice, in regard to the structural defects, the integrity of the connections, and the mutation-specific effects.

Purkinje cell loss affects differentially the execution, acquisition and prepulse inhibition of skeletal and facial motor responses in Lurcher mice

Adult heterozygous Lurcher mice show a degeneration of almost all Purkinje cells and 90% of the granular cells of the cerebellum, resulting in ataxia or general deficits in motor coordination. These mice are therefore an excellent model for studying the role of the cerebellar cortex in motor performance, including the acquisition of new motor abilities. The performance of 3-month-old Lurcher mice was studied in various behavioural (fall, horizontal bar, rotating cylinder, and ladder), spatial orientation (water maze) and associative learning (eyelid classical conditioning) tasks and compared with that of wild-type mice. Behavioural tasks indicated a deficit for motor abilities in Lurcher mice but with some adaptation to the tests and improvement in performance. Wild-type and Lurcher mice performed swimming equally, but the latter learned the task significantly more slowly than the former. The late component of reflex blinks was smaller in amplitude and had a longer latency in Lurcher mice than in controls. Learning curves for Lurcher mice during classical conditioning of eyelid responses were similar to controls, but the amplitude of the learned response in Lurcher mice was significantly lower. The startle response to a severe tone was similar in both control and Lurcher mice but the latter were unable to produce prepulse inhibition. These results suggest that the cerebellar cortex is not indispensable for the performance of this complete set of skeletal and facial tasks, or for the acquisition of new motor abilities, but it is for the appropriate execution and adjustment of any of these motor activities.

Does prenatal stress affect the motoric development of rat pups?

Pregnant rats were exposed to an acute or a repeated stress (presence of a cat) either at the 10th or the 14th gestational day, and the development of their offspring was studied during the first 2 weeks of life. Motor development was measured by different tests: rooting reflex, vibrissae placing response, righting reflex, negative geotaxis. Other landmarks such as eye opening and spontaneous locomotor activity were also recorded. The results showed that, except for the rooting reflex which was most often enhanced (while not significantly) in prenatally stressed rats, the development of the vibrissae placing response, the righting reflex and the negative geotaxis behavior was delayed in the offspring of dams stressed at the 10th gestational day and not (or almost not) in the offspring of dams stressed at the 14th gestational day, the delay being more severe when the prenatal stress was repeated than when it was acutely administered. The spontaneous motor activity was also altered in repeatedly prenatally stressed rats, whatever the day of pregnancy when it was administered, while it was unaffected in acutely prenatally stressed animals. The delay in motor reflexes development was interpreted as alterations in maturation of nervous structures sustaining motor skills, while permanent decrease of spontaneous motor activity was explained by emotional and motivational alterations due to prenatal stress.

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.

Motor coordination in mice with hotfoot, Lurcher, and double mutations of the Grid2 gene encoding the delta-2 excitatory amino acid receptor

Grid2(ho/ho) is a loss of function gene mutation resulting in abnormal dendritic arborizations of Purkinje cells. These mutants were compared in a series of motor coordination tests requiring balance and equilibrium to nonataxic controls (Grid2(ho/+)) and to a double mutant (Grid2(ho/Lc)) with an inserted Lc mutation. The performance of Grid2(ho/ho) mutant mice was poorer than that of controls on stationary beam, coat hanger, unsteady platform, and rotorod tests. Grid2(ho/Lc) did not differ from Grid2(Lc/+) mice. However, the insertion of the Lc mutation in Grid2(ho/Lc) potentiated the deficits found in Grid2(ho/ho) in stationary beam, unsteady platform, and rotorod tests. These results indicate a deleterious effect of the Lc mutation on Grid2-deficient mice.

Patterned Purkinje cell death in the cerebellum

The object of this review is to assemble much of the literature concerning Purkinje cell death in cerebellar pathology and to relate this to what is now known about the complex topography of the cerebellar cortex. A brief introduction to Purkinje cells, and their regionalization is provided, and then the data on Purkinje cell death in mouse models and, where appropriate, their human counterparts, have been arranged according to several broad categories--naturally-occurring and targeted mutations leading to Purkinje cell death, Purkinje cell death due to toxins, Purkinje cell death in ischemia, Purkinje cell death in infection and in inherited disorders, etc. The data reveal that cerebellar Purkinje cell death is much more topographically complex than is usually appreciated.

Motor performance and regional brain metabolism of spontaneous murine mutations with cerebellar atrophy

Three spontaneous mutations with cerebellar atrophy were evaluated for motor coordination and regional brain metabolism, as assessed by cytochrome oxidase (CO) activity. Despite similar neuropathological characteristics, the behavioral phenotype of Lurcher is less severe than that of staggerer, possibly caused by the slower onset of their neuronal degeneration. Although fewer cerebellar cells degenerate in hot-foot than in Lurcher, their motor deficits are more severe, indicating the presence of dysfunctional cells. CO activity in the deep cerebellar nuclei was increased in Lurcher and staggerer but unchanged in hot-foot, probably due to the severe loss of GABAergic input from Purkinje cells in the first two mutants but not the third. Altered CO activity in cerebellar-related pathways was linearly correlated with motor performance, indicating that the activity of this enzyme is associated not only with neuronal activity but also with motor performance.

Distribution of serotonin, its metabolites and 5-HT transporters in the neostriatum of Lurcher and weaver mutant mice

Serotonin (5-HT) uptake sites, or transporters, were measured in the neostriatum (caudate putamen) of wild type (+/+) mice and heterozygous (wv/+) and homozygous (wv/wv) weaver, as well as in heterozygous Lurcher (Lc/+) mutants. These topological surveys were carried out by quantitative ligand binding autoradiography using the uptake site antagonist [3H]-citalopram as a probe of innervation densities in four quadrants of the rostral neostriatum and in two halves of the caudal neostriatum. In addition, tissue concentrations of 5-HT, 5-hydroxyindole-3-acetic acid and 5-hydroxytryptophol were measured by high-performance liquid chromatography with electrochemical detection in these neostriatal divisions. In +/+ mice and in Lc/+ mutants there was a dorso-ventral gradient of increasing 5-HT levels, and they exhibited a similar heterogeneity of [3H]-citalopram labeling. In contrast, the gradients of 5-HT concentrations and [3H]-citalopram binding disappeared in the weaver mutants, suggesting a rearrangement of the 5-HT innervation. This reorganization of the 5-HT system in the neostriatum was more obvious in the wv/wv and is compatible with the hypothesis that the postnatal dopaminergic deficiencies that characterize weaver mutants lead to a sprouting of fibers and thus constitute a genetic model of dopaminergic denervation that leads to a 5-HT hyperinnervation.

Behavioral and biochemical effects of L-tryptophan and buspirone in a model of cerebellar atrophy

The Lurcher mutant mouse can be considered an adequate model of autosomal dominant spinocerebellar atrophy because of the severe degeneration of its cerebellar cortex and inferior olive. The purpose of this study was to determine whether the motor coordination deficits of Lurcher mutants could be improved after chronic administration of the serotonin (5-hydroxytryptamine; 5-HT) precursor, L-tryptophan, or of the 5-HT(1A) agonist, buspirone. During these treatments, the mice were submitted to behavioral evaluations using the coat hanger and the rotorod tests, as well as an inclined screen and a vertical grid test. At the end of treatments, 5-HT and 5-hydroxindole-3-acetic acid (5-HIAA) were measured in six brain regions. On the coat hanger test, administration of L-tryptophan accelerated movements along the horizontal bar by 44%, while buspirone increased the time spent on the apparatus by 11%. Neither drug had an effect on climbing ability or on the time spent on a rotating beam. Administration of L-tryptophan increased 5-HIAA levels in frontal cortex, neostriatum, thalamus, brainstem, cerebellum and spinal cord, but elevated 5-HT only in neostriatum, brainstem and cerebellum. In contrast, buspirone led to 5-HT increases in cerebellum and augmented 5-HIAA in the spinal cord. The modest test-specific improvements are consistent with some of the clinical data concerning 5-HT pharmacotherapy in patients suffering from cerebellar atrophy.

Motor skills and motor learning in Lurcher mutant mice during aging

Motor learning abilities on the rotorod and motor skills (muscular strength, motor coordination, static and dynamic equilibrium) were investigated in three-, nine-, 15- and 21-month-old Lurcher and control mice. Animals were subjected to motor training on the rotorod before being subjected to motor skills tests. The results showed that control mice exhibited decrease of muscular strength and specific equilibrium impairments in static conditions with age, but were still able to learn the motor task on the rotorod even in old age. These results suggest that, in control mice, efficiency of the reactive mechanisms, which are sustained by the lower transcerebellar loop (cerebello-rubro-olivo-cerebellar loop), decreased with age, while the efficiency of the proactive adjustments, which are sustained by the upper transcerebellar loop (cerebello-thalamo-cortico-ponto-cerebellar loop), did not. In spite of their motor deficits, Lurcher mutants were able to learn the motor task at three months, but exhibited severe motor learning deficits as soon as nine months. Such a deficit seems to be associated with dynamic equilibrium impairments, which also appeared at nine months in these mutants. By two months of age, degeneration of the cerebellar cortex and the olivocerebellar pathway in Lurcher mice has disrupted both lower and upper transcerebellar loops. Disruption of the lower loop could well explain precocious static equilibrium deficits. However, in spite of disruption of the upper loop, motor learning and dynamic equilibrium were preserved in young mutant mice, suggesting that either deep cerebellar nuclei and/or other motor structures involved in proactive mechanisms needed to maintain dynamic equilibrium and to learn motor tasks, such as the striatopallidal system, are sufficient. The fact that, in Lurcher mutant mice, motor learning decreased by the age of nine months suggests that the above-mentioned structures are less efficient, likely due to degeneration resulting from precocious and focused neurodegeneration of the cerebellar cortex. From this behavioral approach of motor skills and motor learning during aging in Lurcher mutant mice, we postulated the differential involvement of two transcerebellar systems in equilibrium maintenance and motor learning. Moreover, in these mutants, we showed that motor learning abilities decreased with age, suggesting that the precocious degeneration of the cerebellar Purkinje cells had long-term effects on motor structures which are not primarily affected. Thus, from these results, Lurcher mutant mice therefore appear to be a good model to study the pathological evolution of progressive neurodegeneration in the central nervous system during aging.

Bax inactivation in lurcher mutants rescues cerebellar granule cells but not purkinje cells or inferior olivary neurons

Lurcher is a gain-of-function mutation in the delta2 glutamate receptor gene (Grid2) that turns the receptor into a leaky ion channel. The expression of the Lurcher gene in heterozygous (Grid2(Lc/+)) mutants induces the death of almost all Purkinje cells starting from the second postnatal week. Ninety percent of the granule cells and 60-75% of the inferior olivary neurons die because of the loss of their target neurons, the Purkinje cells. The apoptotic nature of the neurodegeneration has been demonstrated previously by the presence of activated caspase-3 and DNA fragmentation. Bax, a pro-apoptotic gene of the Bcl-2 family, has been shown to be involved in developmental neuronal death. To study the role of Bax in Grid2(Lc/+) neurodegeneration, double mutants with Grid2(Lc/)+ mice and Bax knock-out mice (Bax-/-) were generated. Bax deletion had no effect on the death of Purkinje cells and inferior olivary neurons, although a temporary rescue of some Purkinje cells could be detected in P15 Grid2(Lc/)+;Bax-/- animals. From postnatal day 15 (P15) to P60, the number of granule cells in Grid2(Lc/)+;Bax-/-mice did not significantly change and was significantly increased compared with the number found in Grid2(Lc/)+;Bax+/+ mice. Granule cell number in P60 Grid2(Lc/)+;Bax-/- mice corresponded to 70% of the number found in wild-type mice. Our results show that Bax inactivation in Grid2(Lc/+) mice does not rescue intrinsic Purkinje cell death or the target-related cell death of olivary neurons, but Bax inactivation does inhibit persistently target-related cell death in cerebellar granule cells.

Autoradiography of glutamate receptor binding in adult Lurcher mutant mice

The mutation Lurcher, resulting from a gain of malfunction of the delta2 glutamate receptor expressed specifically by cerebellar Purkinje cells, causes a primary total loss of these neurons of the cerebellar cortex, as well as the secondary degeneration of cerebellar granule and inferior olive neurons. The distributions of glutamate receptors sensitive to amino-methylisoxazole-propionic acid (AMPA), to kainic acid (KA), and to N-methyl-D-aspartic acid (NMDA) as well as metabotropic sites (MET1 and MET2) were examined in wild type and Lurcher mice by quantitative autoradiography. This study was undertaken to determine the gene effect on the distribution of the various glutamate receptor subtypes, as well as how the cerebellar lesion affects the glutamatergic system in other brain regions. In cerebellum, there were postsynaptic AMPA and metabotropic receptors on Purkinje cells, postsynaptic NMDA receptors on granule cells, as well as KA receptors on granule cells or on parallel fibers. Taking into account surface areas, binding to all receptor subtypes was lower in the cerebellar cortex of Lurcher mutants than in wild type mice, while in the deep cerebellar nuclei only KA receptors were diminished. In other brain regions, the alterations followed always the same pattern characterized by a decrease of NMDA and KA receptors but with an increase of AMPA sites; these reciprocal changes were seen in thalamus. neostriatum, limbic regions, and motor cerebral cortical regions. Comparisons of glutamate receptor distribution in Lurcher mutants and in human autosomal cerebellar ataxia may permit further understanding of the role of glutamate-induced toxicity on neuronal death in these heredo-degenerative diseases.

Effects of buspirone on brain indoleamines and catecholamines in wild-type mice and Lurcher mutants

The effects of a chronic serotoninergic stimulation on brain monoamine levels and metabolism were studied in wild-type (+/+) mice and Lurcher (Lc/+) mutants. Endogenous serotonin, dopamine, noradrenaline and some of their major metabolites were measured in the frontal cortex, neostriatum, thalamus, brainstem, cerebellum and spinal cord. In +/+ mice, buspirone (1 mg/kg; i.p.) treatment during 40 days increased indoleamines, albeit with moderate changes in the ratios between tissue serotonin metabolites and endogenous serotonin, augmented noradrenaline contents in the spinal cord, and caused elevations of dopamine metabolites in most regions. In Lc/+ mutants, the effects of buspirone were attenuated, but higher L-tryptophan and indoleamine levels, suggest a storage of serotonin in a non-releasable compartment. In the hypoplastic Lc/+ cerebellum, indoleamine content was accrued, but with a decreased [serotonin metabolites]/[serotonin] ratio, indicating that the reorganized nerve terminals in Lc/+ mutants although they can synthesize and accumulate serotonin, may not utilize it efficiently in synaptic transmission.

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.

Spatial and temporal changes in natural and target deprivation-induced cell death in the mouse inferior olive

The survival of inferior olive neurons is dependent on contact with cerebellar Purkinje cells. There is evidence that this dependence changes with time. Because inferior olivary axons, called climbing fibers, already show significant topographical ordering in cerebellar target zones during late embryogenesis in mice, the question arises as to whether olive neurons are dependent on target Purkinje cells for their survival at this early age. To better characterize this issue, inferior olive development was studied in two transgenic mouse mutants, wnt-1 and L7ADT, with embryonic and early postnatal loss of cerebellar target cells, respectively, and compared to that in the well-studied mutant, Lurcher. Morphological criteria as well as quantitative measures of apoptosis were considered in this developmental analysis. Survival of inferior olive neurons is observed to be independent of Purkinje cells throughout embryogenesis, but dependence begins immediately at birth in both wild types and mutants. Thereafter, wild types and mutants show a rapid increase in olive cell apoptosis, with a peak at postnatal day 4, followed by a period of low-level, but significant, apoptosis that continues to at least postnatal day 11; the main difference is that apoptosis is quantitatively enhanced in the mutants compared to wild types. The multiphasic course of these effects roughly parallels the known phases of climbing fiber synaptogenesis. In addition, despite significant temporal differences among the mutants with respect to absolute numbers of dying cells, there are common spatial features suggestive of distinct intrinsic programs linking different olivary subnuclei to their targets.

Role of an enriched environment on the restoration of behavioral deficits in Lurcher mutant mice

Lurcher mutant mice, characterized by massive degeneration of the cerebellar cortex, and normal littermate controls were reared from birth either in standard conditions or in an enriched environment. The effects of this manipulation on motor functions, landmark water maze learning, exploration, and anxiety were evaluated at 3 months of age. Under standard conditions, Lurcher mutants were impaired in comparison to controls on tests of sensorimotor function and had altered exploratory tendencies. The enriched housing improved the motor coordination of Lurcher mutants and decreased the number of trials before reaching criterion in the landmark water maze. In addition to its effects in Lurcher mutants, enriched rearing also increased some behavioral abilities in normal mice. It is hypothesized that enriched housing altered brain morphology or neurochemistry in both normal and cerebellar-damaged animals.

Inferior olivary-induced expression of Fos-like immunoreactivity in the cerebellar nuclei of wild-type and Lurcher mice

Earlier behavioural studies have shown that the expression of the immediate-early gene c-fos, as visualized by the immunohistochemical detection of Fos, in the inferior olive (IO) correlated closely with expression in related areas of the cerebellar nuclei. It has been speculated that the expression of c-fos within the cerebellar nuclei may be induced by enhanced spiking activity of the immunopositive neurons in the inferior olive. Two potential mechanisms may play a role in this process: a direct induction by way of the collaterals of the olivary climbing fibres to the cerebellar nuclei, or indirectly, by climbing fibre activity-induced depression of mossy fibre-parallel fibre-induced simple spike frequency of the Purkinje cells resulting in a subsequent disinhibition of the related parts of the cerebellar nuclei. In an attempt to distinguish between these possible mechanisms, we analysed Fos immunoreactivity in the olivocerebellar system of wild-type mice and in the mutant mouse Lurcher which lacks Purkinje cells. The tremorgenic agent harmaline, which is known to induce enhanced and rhythmic firing of olivary neurons was given intraperitoneally to anaesthetized mice of both genotypes. Harmaline application coincides with the induction of Fos-immunoreactive neurons in most areas of the IO in both wild-type and Lurcher mice. Both types of mice also showed enhanced expression in the larger neurons of the cerebellar nuclei. However, in the smaller, GABAergic nucleo-olivary neurons, increased c-fos expression was only observed in the wild-type mice. We conclude that: (i) increased olivary activity indeed may result in increased c-Fos expression in related areas of the cerebellar nuclei; (ii) because the indirect mode of induction is not operative in Lurcher mice, the olivary collateral innervation of the cerebellar nuclei is sufficient for c-fos induction in the larger nucleobulbar neurons in Lurcher and potentially also in wild-type mice; however (iii) for the nucleo-olivary cells an intact cerebellar cortical input is necessary to evoke increased expression of c-fos following harmaline application.

Effects of dopaminergic agents and of an NMDA receptor antagonist on motor coordination in Lurcher mutant mice

Lurcher mutant mice, characterized by an ataxic gait and olivocerebellar degeneration, were evaluated for motor coordination in the coat-hanger test after peripheral injections of two doses of dextromethorphan, a noncompetitive N-methyl-D-aspartate receptor antagonist, L-dopa/carbidopa, and SKF 77434, a dopamine D1 receptor agonist. There was an improvement in the distance traveled on the suspended horizontal string after 25 and 50 mg/kg of dextromethorphan and 37.5 mg/kg of L-dopa/carbidopa, but not after SKF 77434. None of the drugs reduced movement times or increased latencies before falling. These results indicate that NMDA receptor antagonism or stimulation of some dopaminergic mechanisms partially improve genetically determined cerebellar ataxia in mice.

An unsteady platform test for measuring static equilibrium in mice

An unsteady platform test is presented in which mice must remain still on a narrow surface in order to prevent a fall. The mouse spontaneous mutation, Lurcher, causing cerebellar cortical degeneration, was evaluated on the unsteady platform, requiring balance in a stable body position (static equilibrium), as opposed to the stationary beam test, in which the animals are free to move on a larger surface (dynamic equilibrium). Lurcher mutants spent less time and had a higher number of slips than controls on the unsteady platform. In contrast, Lurcher mutants did not differ from controls for latencies before falling and distance travelled on the stationary beam. These results are discussed in terms of the possible involvement of two cerebellar circuits in motor control.

Transplanted neurons alter the course of neurodegenerative disease in Lurcher mutant mice

Embryonic cerebellar, neocortical, and striatal tissues derived from NSE-LacZ transgenic mice were transplanted into the right cerebellar hemisphere of 8- to 10-day-old Lurcher or wild-type mice. Host mice survived for 30-90 days and the transplanted tissue was examined by light microscopy using Nissl staining, X-gal histochemistry, and immunohistochemistry for calcium binding protein and glutamic acid decarboxylase. Transplantation of cerebellar tissue, but not neocortical or striatal progenitors, resulted in robust infiltration of the lurcher mutant host cerebellar cortex by transgenic Purkinje neurons. Deep to the infiltrated molecular layer, the host granular layer was thicker and denser than the mutant granular layer, but transgenic cells did not contribute to the spared granular layer. The host inferior olivary complex consistently exhibited a noticeable bilateral asymmetry in Nissl-stained sections. A quantitative analysis of the olivary complex was performed in 10 90-day-old host mice. The results indicate that the left inferior olivary complex of 90-day-old host mice contained more neurons than the right inferior olive of the host mice and contained more neurons than was observed in 90-day-old Lurcher control mice. Analysis by olivary subdivision indicates that increased neuron numbers were present in all subdivisions of the host left inferior olive. These studies confirm the specific attractive effect of the mutant cerebellar cortex on transplanted Purkinje neuron progenitors and indicate that neural transplants may survive the neurodegenerative period to interact with developing host neural systems. The unilateral rescue of Lurcher inferior olivary neurons in cerebellar transplant hosts indicates that transplanted neurons may interact with diseased host neural circuits to reduce transneuronal degeneration in the course of a neurodegenerative disease.

Recording eye movements in mice: a new approach to investigate the molecular basis of cerebellar control of motor learning and motor timing

The vestibulocerebellum is involved in the control of compensatory eye movements. To investigate its role in learning and timing of motor behavior, we investigated compensatory eye movements in mice with the use of search coils. Wild-type mice showed the ability to increase the gain of their vestibulo-ocular reflex by visuovestibular training. This adaptation did not occur in lurcher mice, a natural mouse mutant that completely lacks Purkinje cells. During the optokinetic reflex the phase of the eye movements of lurcher mice in reference to the stimulus lagged behind that of wild-type littermates, whereas during the vestibulo-ocular reflex it led that of the wild-type mice. During combined optokinetic and vestibular stimulation, the phase of the lurcher mice lagged behind that of the wild-type mice at the low stimulus frequencies, whereas it led the phase of the wild-type mice at the high frequencies. In addition, the optokinetic response of the lurcher mice showed a significantly longer latency during constant-velocity step stimulation than that of the wild-type mice. We conclude that Purkinje cells are necessary for both learning and timing of compensatory eye movements in mice. The present description of gain adaptation and phase dynamics provides the basis for studies in which the molecular mechanisms of cerebellar control of compensatory eye movements are investigated with the use of genetically manipulated mice.

The cerebellum and postural sensorimotor learning in mice and rats

Animals with cerebellar damage caused by gene mutations, surgical ablations and irradiation by X-rays during developmental stages are impaired in maintaining posture and equilibrium. For most tests, even in animals with total cerebellectomy, postural sensorimotor learning is not abolished. Simpler compensatory movements may be adopted. The acquisition of simple sensorimotor skills occurring after massive damage of the cerebellar cortex may be explained by the modulatory role of the cerebellar deep nuclei during learning processes or by the influence of extracerebellar regions taking over lost cerebellar function.

Grooming in Lurcher mutant mice

Lurcher mutant mice, characterized by degeneration of cerebellar Purkinje cells and granule cells, were compared to normal littermate controls for different facets of grooming and nongrooming behaviors after a brief period of water immersion. By comparison to normal controls, the number and the duration of several grooming components were decreased in Lurcher mutant mice, namely, licking the forelimb, the abdomen, the back, and the hindlimb. By contrast, the number and duration of body-shaking episodes were not reduced. Lurcher mutants had fewer grooming elements for bouts with at least five elements. However, the serial organization of grooming, as determined by the order of appearance of grooming elements, was maintained in Lurcher mutants. These results indicate that the cerebellar cortex is involved in the appearance of various grooming elements but not in the organization of the cephalocaudal sequence.

Brain dopamine and amino acid concentrations in Lurcher mutant mice

Lurcher mutant mice are characterized by massive degeneration of the cerebellum, including Purkinje cells and granule cells, as well as for the loss of neurons from the inferior olive. Concentrations of dopamine and two of its metabolites and of several amino acid neurotransmitters were determined in the cerebellum and in other brain regions of these mutants. By comparison to wild-type mice of the same background strain, glutamate and taurine concentrations were reduced in the Lurcher cerebellum. No decrease was found for aspartate, gamma-aminobutyric acid (GABA), glycine, as well as dopamine and its metabolites. Moreover, no neurochemical alterations occurred in the brain stem, thalamus, or neostriatum of Lurcher mutants. A selective reduction of glutamate concentration was found in the hippocampus, while all amino acids measured were decreased in the entorhinal-piriform areas. These results indicate region-selective reductions of neurotransmitter concentrations in a mouse mutant with a defined cerebellar cortical pathology.

Visuospatial abilities

The importance of the hippocampus and its anatomical connections, including the medial septum, thalamic nuclei, and neocortical regions in many spatial tasks including the Morris water maze, has been emphasized. Studies in mutant mice with cerebellar atrophy and in rats with electrolytic lesions of the cerebellum have indicated that the cerebellum has a role in visuospatial and visuomotor processes in the Morris maze. Directional deficits in the water have also been noted in rats whose cerebellum was exposed to X-rays during different developmental stages. Cerebellar interactions with the superior colliculus, the hippocampus, and the neocortex via thalamic nuclei are suggested to be the basis of the cerebellar modulation of directional sense in maze tests.

Neurobehavioral evaluation of lurcher mutant mice during ontogeny

Lurcher mutant mice were compared to normal littermate controls for body weight, body righting, negative geotropism, sensorimotor coordination (rotating grid, wire suspension, rotorod), and visuomotor coordination requiring swimming toward a pole during postnatal (P) days 0-30. Lurcher mutants had a lower body weight on P20-P30 and were slower before performing the complete body righting response on P13-P30. Because of postural instability during the negative geotropism test, lurcher mutants turned quicker up the slope than normal mice. The mutants fell sooner from the rotating grid on P11-P14, from the horizontal wire on P15-P16, and from the rotorod on P14-P30. Lurcher mutants were also slower before swimming to the pole or climbing to the top of the pole and were inferior in pole climbing height on P22-P30. These results indicate test-selective and time-selective neurobehavioral deficits during ontogeny in a spontaneous cerebellar mutant.

The numerical matching of source and target populations in the CNS: the inferior olive to Purkinje cell projection

During a defined critical period of development, if the target of a neuronal population is removed, there is a massive decrease in the number of neurons that survive into adulthood. Previous studies have found that source neuron number is a strictly linear function of target size. The current work extends these observations to the inferior olive-->Purkinje cell projection. Three distinct model systems have been used: (i) lurcher<-->wild-type aggregation chimeras, (ii) staggerer<-->wild-type chimeras and (iii) naturally occurring polymorphisms in Purkinje cell number found in different inbred mouse strains. Total neuron numbers were counted in the inferior olive and plotted as a function of the number of Purkinje cells in the contralateral cerebellar cortex. In lurcher mutants and chimeras, the relationship between these values is well described by a straight line. This suggests that, like the granule-->Purkinje cell circuit, the olive-->Purkinje cell circuit uses a linear algorithm to achieve a numerical balance. The results from the two other model systems were not as clear cut. In the staggerer chimeras, we found only a rough correlation between neuron and target numbers and in the inbred strains there was no discernible relationship at all. These findings indicate that in the final analysis, there are multiple factors involved in the determination of the number of olive cells surviving into adulthood. The potential contribution of sustaining collaterals and afferent inputs is discussed as well as the possible existence of different subcircuits of olivocerebellar connections, each with its own numerical matching function.

Regional distribution of the 5-HT innervation in the brain of normal and lurcher mice as revealed by [3H]citalopram quantitative autoradiography

The neurological cerebellar mutant lurcher is characterized by a primary degeneration of Purkinje cells as well as retrograde secondary partial degeneration of cerebellar granule cells and inferior olivary neurons. Since serotonin (5-HT) has been implicated in the modulation of excitatory amino acid systems of the cerebellum, the 5-HT innervation of the normal and lurcher mice was examined by quantifying uptake sites using [3H]citalopram autoradiography, and by biochemical assays of the indoles 5-HT, 5-hydroxy-L-tryptophan and 5-hydroxyindole-3-acetic acid using high-performance liquid chromatography. Comparable results were found between [3H]citalopram binding and 5-HT tissue concentrations in different brain regions. The highest [3H]citaslopram labelling was observed in defined structures of the mesencephalic and upper pontine regions, in limbic strutures, in hypothalamus and in discrete thalamic divisions, while the lowest labelling of uptake sites was documented in cerebellum and brainstem reticular formation. In lurcher mutants, the histology confirmed cell degeneration and the reduction in width, leading to 65%, 45% and 25% atrophies of total cerebellum, deep nuclei and inferior olivary nucleus, respectively. The [3H]citalopram labelling corrected for surface loss was 45% and 20% higher to cerebellar deep nuclei and red nucleus, respectively, but remained unchanged in the cerebellar cortex and inferior olivary nucleus. Moreover, higher labelling was found in nucleus raphe dorsalis, ventral tegmental area, inferior colliculus, locus coeruleus, pontine central grey and anterior thalamic nuclei, areas known to be part of cerebellar afferent and efferent systems. The present results indicate that in such pathological conditions as described for the lurcher mutant, the 5-HT system may modulate motor function not only at the level of the cerebellum, but also in other forebrain structures functionally related to the motor system.

Beam sensorimotor learning and habituation to motor activity in lurcher mutant mice

Lurcher mutant mice lose cerebellar granule cells and Purkinje cells. The mutants were compared to normal mice in a beam-walking task. Normal mice were placed on a slippery bridge while lurchers, because of their severe ataxia, were placed on a bridge with the same diameter, but enveloped with surgical tape to improve traction. The performance of both groups improved with repeated trials. In an activity box, lurcher mutants were as active as normal mice, showed normal intrasession habituation, and emerged from a toy object as easily as normal mice. These results indicate that the cerebellar damage in lurchers does not prevent the acquisition of a motor skill task requiring balance in an immobile apparatus. Ataxia was not accompanied by hypoactivity, inhibition or disturbances in intrasession habituation.

Climbing sensorimotor skills in Lurcher mutant mice

Lurcher mutant mice, characterized by massive degeneration of cerebellar granule and Purkinje cells, were compared to normal mice of the same background strain in three sensorimotor learning tasks requiring climbing skills. In the coat-hanger test, Lurcher mutants were deficient in forepaw contact time and four paw latencies. Improvement over time was weak for both groups. For latencies before reaching the halfway point or the top of the diagonal bar in the same apparatus, Lurcher mutants showed gradual improvement while normal mice appeared to lose motivation to perform the task. In a rope-climbing test, a slight reduction in movement time was detected in Lurcher mutants with repeated trials, whereas the performance of normal mice was stable over time. In a water escape pole-climbing test, Lurcher mutants were impaired relative to controls. Both groups decreased the time required before grasping the pole across days of testing. The number of segments climbed was increased in Lurcher mutants across days. These results illustrate that massive cerebellar damage does not eliminate the possibility of acquiring climbing skills, but under some conditions limit performance.