Monoaminergic nerve terminals in the cerebellar cortex of Purkinje cell degeneration mutant mice: fine structural integrity and modification of cellular environs following loss of Purkinje and granule cells

The Discovery of the Monoaminergic Innervation of the Cerebellum: Convergence of Divergent and Point-to-Point Systems

This Cerebellar Classic highlights the landmark discovery of the innervation of the cerebellar cortex and cerebellar nuclei by noradrenergic and serotoninergic axons emanating, respectively, from the locus coeruleus and the raphé nuclei. Since then, modulation of the activity of cerebellar neurons by the monoamine systems has been studied extensively, as well as their reorganization and modifications during development, plasticity, and disease. The discovery of noradrenergic and serotoninergic innervation of the cerebellum has been a crucial step in understanding the neurochemical relationships between brainstem nuclei and the cerebellum, and the attempts to treat cerebellar ataxias pharmacologically. The large neurochemical repertoire of the cerebellum represents one of the complexities and challenges in the modern appraisal of cerebellar disorders.

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.

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.

Regional distribution of 5-HT transporters in the brain of wild type and 'Purkinje cell degeneration' mutant mice: a quantitative autoradiographic study with [3H]citalopram

The neurological mutant 'Purkinje cell degeneration' (pcd) is characterized by a primary degeneration of Purkinje cells, as well as by retrograde and secondary partial degeneration of cerebellar granule cells and inferior olivary neurons, and can be considered as an animal model of human degenerative ataxias. The serotonin (5-HT) innervation was examined in wild type and pcd mice, by quantifying 5-HT uptake sites, or transporters, using [3H]citalopram binding autoradiography. In both wild type and pcd mutants, the highest densities of 5-HT transporters were in mesencephalic and rostral pontine regions, in limbic structures, in hypothalamus and in discrete thalamic divisions, while the lowest labelling was found in cerebellum and brainstem reticular formation. In pcd mice, although [3H]citalopram labelling was higher in cerebellar cortex and deep cerebellar nuclei, when binding densities were corrected for surface area, the up-regulation of 5-HT transporters was present only in deep cerebellar nuclei. Also, higher labelling was found in nuclei raphe dorsalis and medialis, in ventral divisions of rostral neostriatum, caudal neostriatum, rostral globus pallidus, posteromedial amygdaloid nucleus, septum, olfactory tubercles, vertical limb of Broca's diagonal band, periventricular, latero-ventral and medio-ventral thalamic nuclei, medial geniculate nucleus, anterior hypothalamus and entorhinal cortex. The results indicate a relative integrity of the 5-HT innervation, but with a reorganization of serotoninergic terminals in the cerebellum, in particular in the deep cerebellar nuclei. This suggests that in progressive cerebellar degeneration, as found in the pcd mutant, the modified 5-HT system may still participate in motor functions by exerting an overall modulation of excitatory amino acid neurotransmission, but the availability of 5-HT may be altered in defined brain targets, as is the case for other spontaneous cerebellar mutants, in particular for the 'Lurcher' mutant mouse, a model of human olivopontocerebellar atrophy.

Spatial event processing

The present review advances experimental evidence on the cerebellar involvement in spatial data processing. In particular, data on Morris water maze (MWM) performances of hemicerebellectomized (HCbed) rats indicate a specific cerebellar role within the procedural aspects of spatial functions. In MWM testing, HCbed animals are impaired in developing efficient exploration strategies and display only old and rather ineffective ways for acquiring spatial information, such as peripheral circling around the pool. This behavior is not exhibited if spatial mapping abilities are preoperatively acquired. Thus, MWM experimental data point toward a procedural deficit that specifically impairs the acquisition phase. The characteristics of the cerebellar involvement in affecting the procedures needed for spatial data management are discussed in the light of recent theories on spatial data processing and on cerebellar timing and ordering functions.

Effect of the weaver mutation on the expression of dopamine membrane transporter, tyrosine hydroxylase and vesicular monoamine transporter in dopaminergic neurons of the substantia nigra and the ventral tegmental area

The adult homozygous weaver mutant mouse (wv/wv) is characterized by a loss of dopamine (DA) neurons in the nigrostriatal pathway. Quantitative in situ hybridization of three different dopaminergic markers: dopamine membrane transporter (DAT), tyrosine hydroxylase (TH), and vesicular monoamine transporter (VMAT2) was performed on individual dopaminergic cells of the substantia nigra pars compacta (SNC) and the ventral tegmental area (VTA) in 2-month-old wv/wv mice, in order to investigate the metabolic state of remaining dopaminergic cell bodies and gain further insight into modifications observed on dopaminergic nerve terminals in the striatum and the nucleus accumbens. Cellular expression of DAT mRNA in remaining dopaminergic cells of both the SNC and the VTA was decreased in the wv/wv mice compared to the wild-type mice (+/+). In contrast, the expression of TH and VMAT2 mRNA remained unchanged in the wv/wv mice. Furthermore, in 7-day-old wv/wv mice, before the onset of cell death in the midbrain. DAT mRNA levels were reduced in dopaminergic neurons in both the SNC and VTA. In these animals, the cellular expression of TH mRNA remained unchanged. These results taken together indicate that DAT expression is one of the first targets in the ventral mesencephalon of the wv mutation, inducing a specific decrease of DA uptake in the striatum and the nucleus accumbens. The alteration of the DA membrane transporter could play a role in the progression of DA neuronal death in the wv mice.

Cerebellar contribution to spatial event processing: Morris water maze and T-maze

Recently, a cognitive function of cerebellar networks has been challenging the traditional view of the cerebellum as a motor control centre. Among the cognitive abilities reported to be affected by cerebellar deficits is the capacity to solve a spatial problem. We investigated the influence of a cerebellar lesion on spatial abilities by behavioural analysis of rats that had undergone surgical hemicerebellectomy (HCb; HCbed rats). Experiments were performed with a Morris water maze (MWM) and a water T-maze in both cue and place versions (visible or hidden platform respectively). Results indicate a severe impairment in coping with spatial information in all phases of MWM testing as well as in the T-maze paradigm. However, if the MWM cue phase was prolonged, HCbed rats displayed some ability to learn platform position, although at a level significantly different from controls. They succeeded in finding the platform, even in a pure place paradigm, such as finding a hidden platform with the starting points sequentially changed. Retention testing was also performed, demonstrating that HCb affects acquisition but not retention of spatial information. HCbed animals exhibit such disrupted exploration behaviour that they can display only peripheral circling, and they can acquire spatial relations only when proximal cues are available. Furthermore, in all phases of testing, platform finding for HCbed animals is essentially based on place strategies. Thus, a specific pattern of spatial behaviour, markedly different from that displayed following hippocampal or cortical lesions, characterizes cerebellar lesioned rats. These results are discussed taking into account the role in procedural learning recently assigned to cerebellar networks, demonstrating that the cerebellar circuits represent the keystone of the procedural components of spatial event processing.

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.

The cerebellar model of neural grafting: structural integration and functional recovery

A synopsis is presented of the recent history of cerebellar tissue transplantation over the past 25 years. The properties of growth and differentiation of cerebellar grafts placed intraocularly or intracranially are reviewed, as well as the interaction of heterotopic and orthotopic grafts with the host brain. Particular emphasis is placed on the use of ataxic mouse mutants as recipients of donor cerebellar tissue for the correction of their structural deficits and the functional recovery of behavioural responses.

CNS monoamine levels and motoric behaviors in the hotfoot ataxic mutant

Mice that were homozygous recessive for the single-gene mutation, hotfoot, showed profound and progressive motor disturbances in an open field after approximately the 4th postnatal week. Studies were undertaken to examine the role of the monoaminergic system in the behavioral and developmental expression of this neurological mutation. Relative to controls, 10-and 30-day-old hotfoot mice demonstrated a significantly attenuated response to the stimulating locomotor effects of amphetamine while adult hotfoot mice were motorically unaffected by amphetamine administration. 30-day-old and adult hotfoot mice also were hypothermic relative to phenotypically normal mice after amphetamine administration. Examination of monoamine levels and turnover revealed that hotfoot mice had significantly greater concentrations of norepinephrine associated with lower turnover in cerebellum and greater levels of serotonin in cerebellum and striatum, relative to phenotypic controls. In addition, mice born and raised by hotfoot dams demonstrated neurochemical alterations regardless of genotype. Both the neurochemical data and the developmental response to the general catecholamine agonist, amphetamine, suggest that the monaminergic neurotransmitter system may be altered as a consequence of the hotfoot mutation.

Plastic changes in Ara C-treated and "transplanted" coeruleocerebellar cultures

Locus coeruleus axons project to cerebellar cortex in coeruleocerebellar cultures, where they make functional contacts, and also appear as fine fibers in the outgrowth zones. The predominant catecholamine of locus coeruleus neurons in culture is dopamine. When coeruleocerebellar cultures are exposed to cytosine arabinoside to destroy cerebellar granule cells and functionally compromise glia, there is a resultant increase of Purkinje cell survival and a sprouting of Purkinje cell recurrent axon collaterals, plus an increase of catecholaminergic axons accompanied by a doubling of tissue dopamine content. If such reorganized cultures are transplanted with granule cells and glia, a second round of plastic changes ensues in which the Purkinje cell population and the recurrent axon collaterals are reduced to control levels, but catecholaminergic axons and dopamine content remain increased. The maintenance of catecholaminergic axons does not appear to depend on the persistence of target neurons.

Serotonin-immunoreactivity in the cerebellum of two neurological mutant mice and the corresponding wild-type genetic stocks

Using a serotonin (5-HT)-specific antibody, we examined the 5-HTergic innervation of the cerebellum in the normal mouse (+/+) and in two neurological mutants: weaver (wv/wv), which are characterized by a genetically determined loss of granule cells, and 'Purkinje cell degeneration' (pcd/pcd), which are characterized by a genetically determined loss of Purkinje cells. In normal cerebellum, serotonin-immunoreactive (5-HT-ir) fibers are discrete and ascend to all three layers of the cerebellar cortex. Serotonin-immunoreactive fibers have a much higher density in the atrophic cerebella of both weaver and pcd mutants, where they form multidirectional contours. These anatomical findings provide a profile of 5-HT axon innervation of mouse cerebellum and extend previous neurochemical observations on the metabolic state of cerebellar 5-HT in neurological mutants.

Modification of noradrenergic innervation in the cerebellum of mutant rats with Purkinje cell degeneration (jaundiced Gunn rats)

In heterozygous (Jj) and homozygous Gunn rats (jj), cerebellar noradrenergic innervation was examined using immunohistochemical, neurochemical and electrophysiological techniques. Immunohistochemical analysis using an antiserum against tyrosine hydroxylase (TH) revealed a marked enhancement in immunoreactivity largely in the granular layer and the whole nuclei in the jj cerebellum, resulting from an increase in TH-immunoreactive varicose fibers forming synapse-like structures on the somata and dendrites of granule cells or nuclear neurons. The concentration of norepinephrine in both the cortical and nuclear regions of the jj cerebellum was significantly higher than that in the control, whereas no significant difference of this total amount was observed between the jj and Jj cerebella. Injection of norepinephrine into the Jj cerebellar nuclei reduced the firing rate of spontaneous unitary discharges of neurons in the interpositus nucleus. These findings suggest that the the jj cerebellum causes an enhancement of the noradrenergic innervation which may possibly be one of its characteristic alterations.

Increased activity of tyrosine hydroxylase in the cerebellum of the X-irradiated dystonic rat

The exposure of the cephalic end of rats to repeated doses of X-irradiation (150 rad) immediately after birth induces a long-term increase in the noradrenaline (NA) content of cerebellum (CE) (+ 37.8%), and a decrease in cerebellar weight (65.2% of controls), which results in an increased NA concentration (+ 109%). This increase in the neurotransmitter level is accompanied by a dystonic syndrome and histological abnormalities: Purkinje cells (the target cells for NA afferents to CE) fail to arrange in a characteristic monolayer, and their primary dendritic tree appears randomly oriented. The injection of reserpine 0.9 and 1.2 mg/kg ip to adult rats for 18 h depletes cerebellar NA content in both controls (15.7 +/- 4 ng/CE and 2.8 +/- 1.5 ng/CE, respectively) and X-irradiated rats (17.1 +/- 1 ng/CE and 8.3 +/- 2 ng/CE, respectively). The activity of tyrosine hydroxylase (TH) in CE of adult rats, measured by an in vitro assay, is significantly increased in neonatally X-irradiated animals when compared to age-matched controls (16.4 +/- 1.4 vs 6.32 +/- 0.6 nmol CO2/h/mg prot., p less than 0.01). As observed for NA levels, a net increase in TH activity induced by the ionizing radiation is also measured: 308.9 +/- 23.8 vs 408.2 +/- 21.5 nmol CO2/h/CE, p less than 0.01 (controls and X-treated, respectively). These results suggest that X-irradiation at birth may induce an abnormal sprouting of noradrenergic afferents to CE. The possibility that these changes represent a response of the NA system to the dystonic syndrome is discussed.

Differential effects of granule cell transplantation on two sprouted axonal systems in granuloprival coeruleocerebellar cultures

Exposure of coeruleocerebellar cultures to cytosine arabinoside for the first 5 days in vitro destroyed granule cells and induced sprouting of cortical neurites (Purkinje cell recurrent axon collaterals) and catecholaminergic fibers. Transplantation of such granuloprival cultures with kainic acid-treated cerebellar explants as a source of granule cells resulted in a reduction of silver-positive cortical neurites, but not of histofluorescent catecholaminergic axons. Tissue levels of catecholamines were similar in transplanted and nontransplanted cultures. Differences in types of contacts made with target Purkinje cells in vitro may account for the difference in response to granule cell transplantation by the two axonal groups.

Serotonin concentration and turnover in cerebellum and other brain regions of pcd mutant mice

The Purkinje cell degeneration (pcd) mutant mouse is characterized by a loss of Purkinje cells in the cerebellum. Loss of granule cells occurs and is severe in pcd mutants after 9 months of age. Since Purkinje cells and granule cells represent two groups of target cells for serotonin neurons projecting from raphe nuclei and other brain areas, the content and turnover of serotonin in the cerebellum were determined in pcd mice aged 3-15 months. The content of serotonin was not decreased in pcd mouse cerebellum but tended to be slightly increased after 7 months. The ratio of 5-hydroxyindoleacetic acid (5-HIAA) to serotonin was significantly decreased in cerebellum at 7-15 months but not at 3 or 6 months. The decrease in this ratio is indicative of decreased serotonin turnover. Similar changes were not seen in brainstem or hypothalamus in mice up to 14 months old, but slight decreases were observed at 15 months. Another index of turnover, the accumulation of 5-HIAA after administration of probenecid to block its efflux from brain, was decreased by 46% in 7-month-old pcd mice in the cerebellum but not in the brainstem or hypothalamus. The decrease in serotonin turnover in pcd mouse cerebellum occurs subsequent to and perhaps due to the loss of the target Purkinje and granule cells.

Noradrenergic innervation of the cerebellar cortex in normal and in Purkinje cell degeneration mutant mice: evidence for long term survival following loss of the two major cerebellar cortical neuronal populations

Purkinje cell degeneration mutant mice were examined during the course of Purkinje cell death (26 and 35 days old) and at 3, 5, 9 and 12 months of age. Glyoxylic acid fluorescence histochemistry for catecholamines was used to investigate possible alterations or reorganization of the noradrenergic fibers from the coeruleo-cerebellar system in response to the degeneration of two major cell types in the cerebellar cortex, of which one, the Purkinje cell, is reported to be the major target neuron. In control mice, noradrenergic fibers traveled in linear and tortuous profiles through the granule cell layer, formed pericellular arrays alongside Purkinje cell somata, and branched profusely into both radially oriented and longitudinally oriented chains. The density of noradrenergic varicosities diminished in the molecular layer, there was with age. In the mutants, concomitant with the progressive shrinkage of the molecular layer, there was a progressive increase in the density of noradrenergic varicosities. This was most conspicuous at 9 and 12 months of age, at which time the molecular layer has been depleted not only of Purkinje cell dendrites, but also of parallel fibers. Noradrenergic fibers in these zones formed dense parallel bundles of varicose profiles whose density reached 621.3 +/- 122.8% (mean +/- SD, n = 4) at 9-12 months of age, compared with age-matched controls. Neurochemical measurement of norepinephrine content in whole cerebellum of the Purkinje cell degeneration mutants revealed no change compared with age-matched controls. We conclude that noradrenergic innervation persists in the cerebellar cortex despite the death of Purkinje cells and most of the granule cells. Although we found an increased density of varicosities in the molecular layer of mutant mice, progressing with age, we believe that this can be explained on the basis of the resultant geometry of the altered cerebellar cortex. It appears that the health of the environment surrounding the noradrenergic fibers in cerebellar cortex has little influence on their anatomical integrity.

Altered histofluorescent pattern of noradrenergic innervation of the cerebellum of the mutant mouse Purkinje cell degeneration

The Purkinje target cells for noradrenergic fibers originating in the locus coeruleus are considered to be of importance in the regulation of noradrenergic input to the cerebellum. The availability of a mouse mutant, Purkinje cell degeneration provides a non-surgical means for studying cellular regulation of innervation. Using a glyoxylic acid histofluorescent method for visualizing noradrenergic fibers, the observations have been made that the density of green histofluorescent neurites is markedly increased in both the granule and molecular layers of the cerebellum of Purkinje cell degeneration mice, following spontaneous degeneration of the Purkinje cells. However, because of tissue shrinkage, tissue concentration of norepinephrine also increases, but total tissue content of norepinephrine is unchanged in whole cerebellum and outer cerebellar cortex. These findings indicate that the relative number of noradrenergic afferents to the molecular layer of the cerebellum is not reduced following spontaneous degeneration of Purkinje cells. Therefore, Purkinje target cells do not appear to be essential for maintenance of afferent inputs in mature cerebellum.