Diverse effects of Purkinje cell loss on deep cerebellar and vestibular nuclei neurons in Purkinje cell degeneration mutant mice: A possible compensatory mechanism

Neuromodulation of the cerebellum rescues movement in a mouse model of ataxia

Deep brain stimulation (DBS) relieves motor dysfunction in Parkinson's disease, and other movement disorders. Here, we demonstrate the potential benefits of DBS in a model of ataxia by targeting the cerebellum, a major motor center in the brain. We use the Car8 mouse model of hereditary ataxia to test the potential of using cerebellar nuclei DBS plus physical activity to restore movement. While low-frequency cerebellar DBS alone improves Car8 mobility and muscle function, adding skilled exercise to the treatment regimen additionally rescues limb coordination and stepping. Importantly, the gains persist in the absence of further stimulation. Because DBS promotes the most dramatic improvements in mice with early-stage ataxia, we postulated that cerebellar circuit function affects stimulation efficacy. Indeed, genetically eliminating Purkinje cell neurotransmission blocked the ability of DBS to reduce ataxia. These findings may be valuable in devising future DBS strategies.

Cerebellar Purkinje cell activity modulates aggressive behavior

Although the cerebellum is traditionally associated with balance and motor function, it also plays wider roles in affective and cognitive behaviors. Evidence suggests that the cerebellar vermis may regulate aggressive behavior, though the cerebellar circuits and patterns of activity that influence aggression remain unclear. We used optogenetic methods to bidirectionally modulate the activity of spatially-delineated cerebellar Purkinje cells to evaluate the impact on aggression in mice. Increasing Purkinje cell activity in the vermis significantly reduced the frequency of attacks in a resident-intruder assay. Reduced aggression was not a consequence of impaired motor function, because optogenetic stimulation did not alter motor performance. In complementary experiments, optogenetic inhibition of Purkinje cells in the vermis increased the frequency of attacks. These results suggest Purkinje cell activity in the cerebellar vermis regulates aggression, and further support the importance of the cerebellum in driving affective behaviors that could contribute to neurological disorders.

Bone marrow transplantation improves motor activity in a mouse model of ataxia

Ataxias are locomotor disorders that can have an origin both neural and muscular, although both impairments are related. Unfortunately, ataxia has no cure, and the current therapies are aimed at motor re-education or muscular reinforcement. Nevertheless, cell therapy is becoming a promising approach to deal with incurable neural diseases, including neuromuscular ataxias. Here, we have used a model of ataxia, the Purkinje Cell Degeneration (PCD) mutant mouse, to study the effect of healthy (wild-type) bone marrow transplantation on the restoration of defective mobility. Bone marrow transplants (from both mutant and healthy donors) were performed in wild-type and PCD mice. Then, a wide battery of behavioural tests was employed to determine possible motor amelioration in mutants. Finally, cerebellum, spinal cord, and muscle were analysed to study the integration of the transplant-derived cells and the origin of the behavioural changes. Our results demonstrated that the transplant of wild-type bone marrow restores the mobility of PCD mice, increasing their capabilities of movement (52-100% of recovery), exploration (20-71% of recovery), speed (35% of recovery), and motor coordination (25% of recovery). Surprisingly, our results showed that bone marrow transplant notably improves the skeletal muscle structure, which is severely damaged in the mutants, rather than ameliorating the central nervous system. Although a multimodal effect of the transplant is not discarded, muscular improvements appear to be the basis of this motor recovery. Furthermore, the results from our study indicate that bone marrow stem cell therapy can be a safe and effective alternative for dealing with movement disorders such as ataxias.

Purkinje cell responses during visually and vestibularly driven smooth eye movements in mice

INTRODUCTION

An essential complement to molecular-genetic approaches for analyzing the function of the oculomotor circuitry in mice is an understanding of sensory and motor signal processing in the circuit. Although there has been extensive analysis of the signals carried by neurons in the oculomotor circuits of species, such as monkeys, rabbits and goldfish, relatively little in vivo physiology has been done in the oculomotor circuitry of mice. We analyzed the contribution of vestibular and nonvestibular signals to the responses of individual Purkinje cells in the cerebellar flocculus of mice.

METHODS

We recorded Purkinje cells in the cerebellar flocculus of C57BL/6 mice during eye movement responses to vestibular and visual stimulation.

RESULTS

As in other species, most individual Purkinje cells in mice carried both vestibular and nonvestibular signals, and the most common response across cells was an increase in firing in response to ipsiversive eye movement or ipsiversive head movement. When both the head and eyes were moving, the Purkinje cell responses were approximated as a linear summation of head and eye velocity inputs. Unlike other species, floccular Purkinje cells in mice were considerably more sensitive to eye velocity than head velocity.

CONCLUSIONS

The signal content of Purkinje cells in the cerebellar flocculus of mice was qualitatively similar to that in other species. However, the eye velocity sensitivity was higher than in other species, which may reflect a tuning to the smaller range of eye velocities in mice.

Juxtacellular labeling of tonically active neurons and phasically active neurons in the rat striatum

Tonically active neurons (TANs) and phasically active neurons (PANs) are widely believed to be the cholinergic interneurons and GABAergic projection neurons, respectively, in the striatum based on in vivo intracellular recordings coupled with morphological examinations of anesthetized rats, and on histochemical, electrophysiological, and labeling studies of in vitro slice preparations. TANs of alert behaving animals exhibit prolonged pause responses to behaviorally significant events. PANs, on the other hand, are mostly inactive when subjects are quiet and not performing any actions, but exhibit burst discharges in response to external stimuli and/or voluntary actions. Several other types of interneurons have also been identified in the striatum, such as parvalbumin-containing GABAergic interneurons (fast-spiking cells), somatostatin-containing interneurons, and calretinin-containing interneurons. To identify the neurochemical and morphological characteristics of TANs and PANs in a more direct manner, we conducted juxtacellular labeling, combining electrophysiology with immunohistochemistry and morphology in anesthetized rats. All of the juxtacellularly labeled TANs (n=3) among those recorded (n=10) were ChAT-positive and had large cell somata with aspiny dendrites. Thus, although our observations are based on a limited number of neurons, our findings provide the most convincing evidence to date that TANs in the striatum are cholinergic neurons. We also found that the majority of PANs are GABA-immunoreactive (46 of 48 tested) and approximately two-thirds had spiny dendrites (30 of 48 tested), indicating that the majority are medium-sized, spiny, GABAergic projection neurons, consistent with general beliefs. Conversely, the remaining one-third of PANs had aspiny dendrites (n=18), indicating that they were interneurons. Therefore, the present study reveals that TANs are cholinergic neurons and that the majority of PANs are medium-sized, spiny, GABAergic projection neurons, while a smaller number are GABAergic interneurons.

Cerebellum-related characteristics of Scn8a-mutant mice

Among ten sodium channel alpha-subunit genes mapped in human and mouse genomes, the SCN8A gene is primarily expressed in neurons and glia. Mice with two types of Scn8a null mutations--Scn8a ( med ) and Scn8a ( medTg )--live for only 21-24 days, but those with incomplete mutations-Scn8a ( medJ ) and Scn8a ( medJo )--and those with knockout of Scn8a only in cerebellar Purkinje cells live to adult age. We review here previous work on cerebellum and related regions of Scn8a mutant mice and include some newer immunohistochemical and microchemical results. The resurgent sodium current that underlies the repeated firing of Purkinje cells is reduced in Scn8a mutant and knockout mice. Purkinje cells of mutant mice have greatly reduced spontaneous activity, as do the analogous cartwheel cells of the dorsal cochlear nucleus. Up-regulation of GABA(A) receptors in regions to which Purkinje cells project may partially compensate for their decreased activity in the mutant mice. The somata of cerebellar Purkinje cells of Scn8a ( medJ ) and Scn8a ( medJo ) mice, as revealed by PEP-19 immunoreaction, are slightly smaller than normal, and their axons, especially in Scn8a ( medJo ) mice, sometimes show enlargements similar to those in other types of mutant mice. Density of GABA-like immunoreactivity is decreased in Purkinje somata and regions of termination in deep cerebellar and vestibular nuclei of Scn8a ( medJ ) mice, but measured GABA concentration is not significantly reduced in microdissected samples of these regions. The concentrations of taurine and glutamine are significantly increased in cerebellar-related regions of Scn8a ( medJ ) mice, possibly suggesting up-regulation of glial amino acid metabolism.

Decreased GAD65 mRNA levels in select subpopulations of neurons in the cerebellar dentate nuclei in autism: an in situ hybridization study

The laterally positioned dentate nuclei lie in a key position in the cerebellum to receive input from Purkinje cells in the lateral cerebellar hemisphere participating in both motor and cognitive functions. Although neuropathology of the four cerebellar nuclei using Nissl staining has been qualitatively reported in children and adults with autism, surprisingly the dentate nuclei appeared less affected despite reported reductions in Purkinje cells in the posterolateral cerebellar hemisphere. To determine any underlying abnormalities in the critically important GABAergic system, the rate-limiting GABA synthesizing enzyme, glutamic acid decarboxylase (GAD) type 65 was measured via in situ hybridization histochemistry in dentate somata. GAD65 mRNA labeling revealed two distinct subpopulations of neurons in adult control and autism postmortem brains: small-sized cells (about 10-12 microm in diameter, presumed interneurons) and larger-sized neurons (about 18-20 microm in diameter, likely feedback to inferior olivary neurons). A mean 51% reduction in GAD65 mRNA levels was found in the larger labeled cells in the autistic group compared with the control group (P=0.009; independent t-test) but not in the smaller cell subpopulation. This suggests a disturbance in the intrinsic cerebellar circuitry in the autism group potentially interfering with the synchronous firing of inferior olivary neurons, and the timing of Purkinje cell firing and inputs to the dentate nuclei. Disturbances in critical neural substrates within these key circuits could disrupt afferents to motor and/or cognitive cerebral association areas in the autistic brain likely contributing to the marked behavioral consequences characteristic of autism.

Dorsal cochlear nucleus hyperactivity and tinnitus: are they related?

PURPOSE

Eight lines of evidence implicating the dorsal cochlear nucleus (DCN) as a tinnitus contributing site are reviewed. We now expand the presentation of this model, elaborate on its essential details, and provide answers to commonly asked questions regarding its validity.

CONCLUSIONS

Over the past decade, numerous studies have converged to support the hypothesis that the DCN may be an important brain center in the generation and modulation of tinnitus. Although other auditory centers have been similarly implicated, the DCN deserves special emphasis because, as a primary acoustic nucleus, it occupies a potentially pivotal position in the hierarchy of functional processes leading to the emergence of tinnitus percepts. Moreover, because a great deal is known about the underlying cellular categories and the details of synaptic circuitry within the DCN, this brain center offers a potentially powerful model for probing mechanisms underlying tinnitus.

Cerebellar modulation of frontal cortex dopamine efflux in mice: relevance to autism and schizophrenia

Cerebellar and frontal cortical pathologies have been commonly reported in schizophrenia, autism, and other developmental disorders. Whether there is a relationship between prefrontal and cerebellar pathologies is unknown. Using fixed potential amperometry, dopamine (DA) efflux evoked by cerebellar or, dentate nucleus electrical stimulation (50 Hz, 200 muA) was recorded in prefrontal cortex of urethane anesthetized lurcher (Lc/+) mice with 100% loss of cerebellar Purkinje cells and wildtype (+/+) control mice. Cerebellar stimulation with 25 and 100 pulses evoked prefrontal cortex DA efflux in +/+ mice that persisted for 12 and 25 s poststimulation, respectively. In contrast, 25 pulse cerebellar stimulation failed to evoke prefrontal cortex DA efflux in Lc/+ mice indicating a dependency on cerebellar Purkinje cell outputs. Dentate nucleus stimulation (25 pulses) evoked a comparable but briefer (baseline recovery within 7 s) increase in prefrontal cortex DA efflux compared to similar cerebellar stimulation in +/+ mice. However, in Lc/+ mice 25 pulse dentate nucleus evoked prefrontal cortex DA efflux was attenuated by 60% with baseline recovery within 4 s suggesting that dentate nucleus outputs to prefrontal cortex remain partially functional. DA reuptake blockade enhanced 100 pulse stimulation evoked prefrontal cortex responses, while serotonin or norepinephrine reuptake blockade were without effect indicating the specificity of the amperometric recordings to DA. Results provide neurochemical evidence that the cerebellum can modulate DA efflux in the prefrontal cortex. Together, these findings may explain why cerebellar and frontal cortical pathologies co-occur, and may provide a mechanism that accounts for the diversity of symptoms common to multiple developmental disorders.

Asymmetric recovery in cerebellar-deficient mice following unilateral labyrinthectomy

The term "vestibular compensation" refers to the resolution of motor deficits resulting from a peripheral vestibular lesion. We investigated the role of the cerebellum in the compensation process by characterizing the vestibuloocular reflex (VOR) evoked by head rotations at frequencies and velocities similar to those in natural behaviors in wild-type (WT) versus cerebellar-deficient Lurcher (Lc/+) mice. We found that during exploratory activity, normal mice produce head rotations largely consisting of frequencies < or =4 Hz and velocities and accelerations as large as 400 degrees/s and 5,000 degrees/s2, respectively. Accordingly, the VOR was characterized using sinusoidal rotations (0.2-4 Hz) as well as transient impulses (approximately 400 degrees/s; approximately 2,000 degrees/s2). Before lesions, WT and Lc/+ mice produced similar VOR responses to sinusoidal rotation. Lc/+ mice, however, had significantly reduced gains for transient stimuli. After unilateral labyrinthectomy, VOR recovery followed a similar course for WT and Lc/+ groups during the first week: gain was reduced by 80% for ipsilesionally directed head rotations on day 1 and improved for both strains to values of approximately 0.4 by day 5. Moreover, responses evoked by contralesionally directed rotations returned to prelesion in both strains within this period. However, unlike WT, which showed improving responses to ipsilesionally directed rotations, recovery plateaued after first week for Lc/+ mice. Our results show that despite nearly normal recovery in the acute phase, long-term compensation is compromised in Lc/+. We conclude that cerebellar pathways are critical for long-term restoration of VOR during head rotation toward the lesioned side, while noncerebellar pathways are sufficient to restore proper gaze stabilization during contralesionally directed movements.

Comparison of gamma-aminobutyrate receptors in the medial vestibular nucleus of control and Scn8a mutant mice

The Purkinje cells of the cerebellum provide inhibitory input to vestibular nucleus neurons, with gamma-aminobutyrate (GABA) as neurotransmitter. Using extracellular recordings and bath application of agonists and antagonists, we compared GABA receptors in the medial vestibular nucleus of brain slices from Scn8a mutant mice of med(J) type, in which there is greatly reduced spontaneous and evoked activity of Purkinje cells, to those in slices from control mice. Muscimol, an agonist at GABA(A) receptors, produced a larger reduction of firing rate in neurons of mutant mice than in neurons of control mice, whereas there was no difference for baclofen, an agonist at GABA(B) receptors. In most cases tested, the effects of muscimol and baclofen remained similar when synaptic transmission was blocked, suggesting that the effects were predominantly directly upon GABA receptors of the neurons being recorded from. The up-regulation of GABA(A) receptors was similar in magnitude to that previously found for rats with bilateral transection of the inferior cerebellar peduncle. It may relate in both cases to reduced Purkinje cell input to medial vestibular nucleus neurons. The lack of effect on GABA(B) receptors suggests that the changes found with peduncle transection may have resulted from something more than reduced Purkinje cell activity, such as reduced concentrations of GABA, or that reduction of Purkinje cell activity in Scn8a mutant mice was insufficient to affect GABA(B) receptors. Other possible explanations of the results cannot be excluded since the Scn8a mutation affects other neuron types besides Purkinje cells.

Activity of Vestibular Nuclei Neurons During Vestibular and Optokinetic Stimulation in the Alert Mouse

As a result of the availability of genetic mutant strains and development of noninvasive eye movements recording techniques, the mouse stands as a very interesting model for bridging the gap among behavioral responses, neuronal response dynamics studied in vivo, and cellular mechanisms investigated in vitro. Here we characterized the responses of individual neurons in the mouse vestibular nuclei during vestibular (horizontal whole body rotations) and full field visual stimulation. The majority of neurons (∼2/3) were sensitive to vestibular stimulation but not to eye movements. During the vestibular-ocular reflex (VOR), these neurons discharged in a manner comparable to the “vestibular only” (VO) neurons that have been previously described in primates. The remaining neurons [eye-movement-sensitive (ES) neurons] encoded both head-velocity and eye-position information during the VOR. When vestibular and visual stimulation were applied so that there was sensory conflict, the behavioral gain of the VOR was reduced. In turn, the modulation of sensitivity of VO neurons remained unaffected, whereas that of ES neurons was reduced. ES neurons were also modulated in response to full field visual stimulation that evoked the optokinetic reflex (OKR). Mouse VO neurons, however, unlike their primate counterpart, were not modulated during OKR. Taken together, our results show that the integration of visual and vestibular information in the mouse vestibular nucleus is limited to a subpopulation of neurons which likely supports gaze stabilization for both VOR and OKR.

Developmental regulation of the membrane properties of central vestibular neurons by sensory vestibular information in the mouse

The effect of the lack of vestibular input on the membrane properties of central vestibular neurons was studied by using a strain of transgenic, vestibular-deficient mutant KCNE1(-/-) mice where the hair cells of the inner ear degenerate just after birth. Despite the absence of sensory vestibular input, their central vestibular pathways are intact. Juvenile and adult homozygous mutant have a normal resting posture, but show a constant head bobbing behaviour and display the shaker/waltzer phenotype characterized by rapid bilateral circling during locomotion. In juvenile mice, the KCNE1 mutation was associated with a strong decrease in the expression of the calcium-binding proteins calbindin, calretinin and parvalbumin within the medial vestibular nucleus (MVN) and important modifications of the membrane properties of MVN neurons. In adult mice, however, there was almost no difference between the membrane properties of MVN neurons of homozygous and control or heterozygous mutant mice, which have normal inner ear hair cells and show no behavioural symptoms. The expression levels of calbindin and calretinin were lower in adult homozygous mutant animals, but the amount of calcium-binding proteins expressed in the MVN was much greater than in juvenile mice. These data demonstrate that suppression of sensory vestibular inputs during a 'sensitive period' around birth can generate the circling/waltzing behaviour, but that this behaviour is not due to persistent abnormalities of the membrane properties of central vestibular neurons. Altogether, maturation of the membrane properties of central vestibular neurons is delayed, but not impaired by the absence of sensory vestibular information.

Changes of amino acid concentrations in the rat vestibular nuclei after inferior cerebellar peduncle transection

Although there is a close relationship between the vestibular nuclear complex (VNC) and the cerebellum, little is known about the contribution of cerebellar inputs to amino acid neurotransmission in the VNC. Microdissection of freeze-dried brain sections and high-performance liquid chromatography (HPLC) were combined to measure changes of amino acid concentrations within the VNC of rats following transection of the cerebellovestibular connections in the inferior cerebellar peduncle. Distributions of 12 amino acids within the VNC at 2, 4, 7, and 30 days after surgery were compared with those for control and sham-lesioned rats. Concentrations of gamma-aminobutyric acid (GABA) decreased by 2 days after unilateral peduncle transection in nearly all VNC regions on the lesioned side and to lesser extents on the unlesioned side and showed partial recovery up to 30 days postsurgery. Asymmetries between the two sides of the VNC were maintained through 30 days. Glutamate concentrations were reduced bilaterally in virtually all regions of the VNC by 2 days and showed complete recovery in most VNC regions by 30 days. Glutamine concentrations increased, starting 2 days after surgery, especially on the lesioned side, so that there was asymmetry generally opposite that of glutamate. Concentrations of taurine, aspartate, and glycine also underwent partially reversible changes after peduncle transection. The results suggest that GABA and glutamate are prominent neurotransmitters in bilateral projections from the cerebellum to the VNC and that amino acid metabolism in the VNC is strongly influenced by its cerebellar connections.

Response of Purkinje neurons to hypobaric hypoxic exposure as shown by alteration in expression of glutamate receptors, nitric oxide synthases and calcium binding proteins

Hypobaric hypoxia is known to impair muscular coordination. It is not known whether hypobaric hypoxia causes any damage to the Purkinje neurons which may be responsible for impairment of muscular coordination. Expression of ionotropic glutamate receptors N-methyl-d-aspartate receptor subunit 1, amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid GluR2/3, calcium binding proteins and nitric oxide synthases in the Purkinje neurons was examined in rats exposed to hypobaric hypoxia. The mRNA expression of N-methyl-d-aspartate receptor subunit 1, GluR2, GluR3 and nitric oxide synthases [neuronal, endothelial and inducible] was upregulated at 3 h peaking at 24 h after the exposure. This was sustained up to 3 days; thereafter, it was comparable to the controls. Immunohistochemical analysis confirmed a marked expression of N-methyl-d-aspartate receptor subunit 1 and GluR2/3 at the above time intervals. Immunoexpression of calbindin-D28k (calbindin) and parvalbumin was intense in the soma of Purkinje neurons in the control rats. It was, however, drastically downregulated up to 3 days after exposure. At 3 days the neuronal dendrites showed intense expression of calbindin which returned to control levels at 7 days. Expression of neuronal nitric oxide synthase and inducible nitric oxide synthase was markedly upregulated from 3 h to 3 days whereas endothelial nitric oxide synthase expression, localized in the blood vessels and Purkinje neurons, remained elevated up to 24 h after the exposure. A progressive darkening of the Purkinje neuron cell bodies was observed at ultrastructural level up to 3 days but degenerating cells were not observed. A salient alteration was the dilation and stacking of smooth endoplasmic reticulum in the dendrites up to 14 days after the exposure. The present results suggest that hypobaric hypoxia leads to overexpression of N-methyl-d-aspartate receptor subunit 1 and GluR2/3 in Purkinje neurons that may be responsive to altered calcium levels as manifested by decreased expression of calcium binding proteins. This together with excess nitric oxide production may have led to transient ultrastructural changes. We propose that the functions of the Purkinje neurons may be altered in response to an acute exposure to hypobaric hypoxia resulting in impairment of motor coordination.

Quantitative organization of neurotransmitters in the deep cerebellar nuclei of the Lurcher mutant

The Lurcher mutant mouse is characterized by a primary selective loss of Purkinje cells, leading to the near total apoptotic death of these neurons. In contrast to the subsequent massive secondary degeneration of the granule cells and the inferior olivary neurons, only mild degeneration occurs in the deep cerebellar nuclei (DCN). However, it is not known to what extent the different populations of DCN neurons-glutamatergic principal projection neurons, gamma-aminobutyric acid (GABA)-ergic inferior olivary projection neurons, and glycinergic neurons-are affected in their neurotransmitter composition. To answer this question we studied the neurotransmitter contents (glutamate, GABA, and glycine) of DCN neurons and the size of synaptic boutons immunohistochemically on serial semithin sections in both Lurcher and wild-type mice. Applying the physical dissector counting method, our results confirmed the mild degeneration (a reduction by 20%) of large glutamatergic neurons and a more pronounced degeneration of GABAergic (by 42%) and glycinergic neurons (by 45%). On the other hand, an analysis of neurons colabeled for both GABA and glycine, revealed that this specific colabeling increased in the Lurcher mutant (by 40%). In addition, both the GABA-immunolabeled (IL) (by 56%) and the glycine-IL (by 45%) synaptic boutons showed an increase in diameter in the mutant. The density of these boutons showed a decrease of 30% each. In summary, the increase in the number of neurons colabeled for GABA and glycine, together with the increase in the size of the inhibitory synaptic boutons, could help in providing the minimum inhibition needed to maintain a residual "cerebellar" functionality in the Lurcher DCN.

Horizontal vestibuloocular reflex (VOR) head velocity estimation in Purkinje cell degeneration (pcd/pcd) mutant mice

The horizontal vestibuloocular reflex (VOR) of Purkinje cell degeneration (pcd/pcd) mutant mice, which lack a functional cerebellar cortex, was compared in darkness to that of wild-type animals during constant velocity yaw rotations about an earth-horizontal axis and during sinusoidal yaw rotations about an earth-vertical axis. Both wild-type and pcd/pcd mice showed a compensatory average VOR eye velocity, or bias, during constant velocity horizontal axis rotations, evidence of central neural processing of otolith afferent signals to create a signal proportional to head angular velocity. Eye velocity bias was greater in pcd/pcd mice than in wild-type mice at a low rotational velocity (32 degrees/s), but less at higher velocities (128 and 200 degrees/s). Lesion of the medial nodulus severely attenuated eye velocity bias in two wild-type mice, without attenuating VOR during sinusoidal vertical axis yaw rotations at 0.2 Hz. These results show that while head velocity estimation in mice, as in primates, depends on the cerebellum, pcd/pcd mutant mice develop velocity estimation without a functional cerebellar cortex. We conclude that neural circuits that exclude cerebellar cortex are capable of the signal processing necessary for head angular velocity estimation, but that these circuits are insufficient for normal estimation at high velocities.

Mutant mice as a model for cerebellar ataxia

Not later than two synapses after their arrival in the cerebellar cortex all excitatory afferent signals are subsequently transformed into inhibitory ones. Guaranteed by the exceedingly ordered and stereotyped synaptic arrangement of its cellular elements, the cerebellar cortex transmits this inhibitory result of cerebellar integration exclusively via Purkinje cells (PCs) in a precise temporal succession directly onto the target neurons of the deep cerebellar and vestibular nuclei. Thus the cerebellar cortex seems to produce a temporal pattern of inhibitory influence on these target neurons that modifies their excitatory action in such a way that an activation of muscle fibers occurs which progressively integrates the intended motion into the actual condition of the motoric inventory. In consequence, disturbances that affect this cerebellar inhibition will cause uncoordinated, decomposed and ataxic movements, commonly referred to as cerebellar ataxia. Electrophysiological investigations using different cerebellar mouse mutants have shown that alterations in the cerebellar inhibitory input in the target nuclei lead to diverse neuronal responses and to different consequences for the behavioural phenotype. A dependence between the reconstitution of inhibition and the behavioural outcome seems to exist. Obviously two different basic mechanisms are responsible for these observations: (1) ineffective inhibition on target neurons by surviving PCs; and (2) enhancement of intranuclear inhibition in the deep cerebellar and vestibular nuclei. Which of the two strategies evolves is dependent upon the composition of the residual cell types in the cerebellum and on the degree of PC input loss in a given area of the target nuclei. Motor behaviour seems to deteriorate under the first of these mechanisms whereas it may benefit from the second. This is substantiated by stereotaxic removal of the remaining PC input, which eliminates the influence of the first mechanism and is able to induce the second strategy. As a consequence, motor performance improves considerably. In this review, results leading to the above conclusions are presented and links forged to human cerebellar diseases.

Purkinje cell degeneration and control mice: responses of single units in the dorsal cochlear nucleus and the acoustic startle response

The cartwheel cell is the most numerous inhibitory interneuron of the dorsal cochlear nucleus (DCN). It is expected to be an important determinant of DCN function. To assess the contribution of the cartwheel cell, we examined the discharge characteristics of DCN neurons and behavioral measures in the Purkinje cell degeneration (pcd) mice, which lack cartwheel cells, and compared them to those of the control mice. Distortion product otoacoustic emissions and auditory brainstem-evoked response thresholds were similar between the two groups. Extracellularly recorded DCN single units in ketamine/xylazine-anesthetized mice were classified according to post-stimulus time histogram (PSTH) and excitatory-inhibitory response area (EI-area) schemes. PSTHs recorded in mouse DCN included chopper, pauser/buildup, onset, inhibited and nondescript types. EI-areas recorded included Types I, II, III, I/III, IV and V. There were no significant differences in the proportions of various unit types between the pcd and control mice. The pcd units had slightly lower thresholds to characteristic frequency tones; however, they had spontaneous rates, thresholds to noise, and maximum driven rates to noise that were similar to those of the control units. Pcd mice had smaller startle amplitudes, but startle latency, prepulse inhibition/augmentation and facilitation by a background tone were comparable between the two groups. From these results, we conclude that DCN function in response to relatively simple acoustic stimuli is minimally affected by the absence of the cartwheel cells. Future studies employing more complex and/or multimodal stimuli should help assess the role of the cartwheel cells.

Vermectomy enhances parvalbumin expression and improves motor performance in weaver mutant mice: an animal model for cerebellar ataxia

In the Weaver mutant mouse (wv/wv), an animal model for hereditary cerebellar ataxia, electrophysiological experiments have revealed a disorganized output of cerebellar Purkinje cells (the latter using GABA as an inhibitory transmitter) which, by a cascade of mechanisms, was thought to be the cause of the poor motor abilities. In Purkinje cell degeneration mice (pcd/pcd) lacking nearly all Purkinje cells and displaying milder motor deficiencies than wv, in comparison to wild-type mice, a strong increase in parvalbumin- and (co-localized with parvalbumin) glycine-immunopositive somata in the deep cerebellar and vestibular nuclei has recently been found. It was therefore intriguing to investigate whether motor performance in weaver mutants could be ameliorated by applying cerebellar lesions to eliminate the faulty output and to look for a change in transmitter weighting, indicated by a strong increase in parvalbumin-positive somata in areas (the respective target areas) which were formerly devoid of it. Ten Weaver mutants were subjected to cerebellar lesions. After removal of the vermis a total abolition of tremor, a definite improvement in the balance of affected body parts, an increase in locomotor activity when tested in an open-field matrix, and a strong increase in parvalbumin expression in Weaver mutant deep cerebellar and vestibular nuclei in comparison to wild-types have indeed been found. Increase in motor activity (or explorative behaviour) has been placed in relation to learning mechanisms. The increase in parvalbumin expression and the observed improvement in motor abilities and mechanisms probably related to learning underline the hypothesis that any change in the physiological equilibrium of the brain function by removal of input or output related to an assembly of nerve cells leads to a cascade of changes at the transmitter and neuronal level in near or distant connected brain structures.

The organization of piriform cortex and the lateral olfactory tract following the loss of mitral cells in PCD mice

Homozygous Purkinje Cell Degeneration (PCD) mice exhibit a selective loss of olfactory bulb mitral cells (MCs) after 4 months of age. This selective degeneration leaves a subpopulation of denervated granule cells which establish new reciprocal dendro-dendritic synapses with unaffected tufted cells (TCs) (14). This suggests a capacity for plasticity in TCs and raises the question of whether a comparable degree of reorganization occurs in their axonal terminals in piriform cortex (PC) following the loss of MCs. Homozygous (experimental) and heterozygous (control) PCD mice were routinely perfused and processed for electron microscopy. A quantitative electron microscopic analysis was performed on radially oriented micrograph montages spanning from the pia into layer II of PC. After MC loss in the experimental animals there was a decrease in density of larger myelinated axons in the lateral olfactory tract (LOT). Myelinated axons in the LOT had a mean cross-sectional diameter of 1.26 +/- 0.04, and 0.81 +/- 0.025 microm in the control and experimental mice, respectively. In superficial layer I of PC, control mice had presynaptic axonal terminals from mitral and tufted cells with characteristic electron lucent (light) profiles establishing asymmetric synapses with pyramidal cell dendrites. In contrast, the experimental mice showed a decrease in electron lucent terminals and a robust increase in electron dense (dark) presynaptic associational terminals. Although the overall synaptic density did not differ between the control and experimental mice (16.40 +/- 0.94 and 18.10 +/- 0.96 synapses/100 microm2, respectively), an overall decrease in the thickness of Layer 1 suggests that the total number of synapses decreases following MC loss. In addition to the apparent increase of associational terminals, the diameter of terminal enlargements increased as well as the number of multiple synaptic contact per terminals in the experimental animal, suggesting further compensatory mechanisms for the loss of MC presynaptic terminals.

Improvement in motor performance of Weaver mutant mice following lesions of the cerebellum

In Weaver mutants (B6CBA wv/wv) cerebellar granule cells degenerate almost completely postnatally. A partial loss of Purkinje cells (PC) and a degeneration of dopaminergic cells in the substantia nigra have also been found. Weaver mice suffer from striking motor symptoms, including difficulty in walking without toppling over. In an attempt to influence the poor motor performance, the cerebellum in young animals was removed, thus eliminating the faulty output of surviving PCs, demonstrated electrophysiologically. Unoperated Weaver, lesioned wildtypes and one sham mouse were used as controls. Before and after operation, a battery of behavioural tests was performed. In Weaver mice, tumbling to the side (t) and the relation of t to the motor activity (k) while traversing an open-field matrix, (t/k), improved considerably, as did manoeuvring on a slanted wire mesh, but keeping balance on a wooden bench did not to the same degree. Locomotor activity alone improved in some animals. In wildtypes no significant changes occurred after operation, with the exception of a strong reduction in locomotor activity. The experiments demonstrate that the motor performance in Weaver mutant mice benefits from removal of their cerebellum.

Dependence of parvalbumin expression on Purkinje cell input in the deep cerebellar nuclei

A complete loss of Purkinje cell (PC) input leads to an increase in expression of the calcium-binding protein parvalbumin (Parv) in neurons of the deep cerebellar nuclei (DCN) of PC degeneration (pcd) mutants. To verify this apparent dependence of Parv expression on PC input in the DCN, the patterns of expression in five other cerebellar mutants (weaver, staggerer, leaner, nervous, and lurcher) with differing grades and chronologies of PC loss were compared. Degree and time course of PC loss and the subsequent denervation of DCN neurons were monitored by using Calbindin D-28k (Calb) immunocytochemistry. Similar to pcd mice, somatal Parv in lurcher mutants increased massively throughout the cerebellar nuclei. In nervous and leaner mutants, somatal Parv was restricted to almost completely denervated nuclear areas, whereas areas with appreciable remnants of PC input were spared. The first appearance of Parv+ somata was closely correlated with the time course of PC degeneration--postnatal day 19 in lurcher mutants and postnatal day 23 in nervous mutants. In staggerer mice, neurons immunopositive for Parv as well as for Calb were present in outer DCN areas, likely representing ectopic PCs rather than DCN neurons. No Parv+ DCN somata were found in weaver mutants at any time. In conclusion, increased expression of somatal Parv in DCN neurons is not restricted to the specific histopathology in pcd mutants but is a common mechanism that is dependent on the topography and severeness of PC-input loss. The functional significance of the Parv increase and its possible contribution to the degree of motor disability among the different mutants are discussed.