Selective absence of calcium spikes in Purkinje cells of staggerer mutant mice in cerebellar slices maintained in vitro

GIRK1-Mediated Inwardly Rectifying Potassium Current Is a Candidate Mechanism Behind Purkinje Cell Excitability, Plasticity, and Neuromodulation

G-protein-coupled inwardly rectifying potassium (GIRK) channels contribute to the resting membrane potential of many neurons and play an important role in controlling neuronal excitability. Although previous studies have revealed a high expression of GIRK subunits in the cerebellum, their functional role has never been clearly described. Using patch-clamp recordings in mice cerebellar slices, we examined the properties of the GIRK currents in Purkinje cells (PCs) and investigated the effects of a selective agonist of GIRK1-containing channels, ML297 (ML), on PC firing and synaptic plasticity. We demonstrated that GIRK channel activation decreases the PC excitability by inhibiting both sodium and calcium spikes and, in addition, modulates the complex spike response evoked by climbing fiber stimulation. Our results indicate that GIRK channels have also a marked effect on synaptic plasticity of the parallel fiber-PC synapse, as the application of ML297 increased the expression of LTP while preventing LTD. We, therefore, propose that the recruitment of GIRK channels represents a crucial mechanism by which neuromodulators can control synaptic strength and membrane conductance for proper refinement of the neural network involved in memory storage and higher cognitive functions.

Atm deficiency in the DNA polymerase β null cerebellum results in cerebellar ataxia and Itpr1 reduction associated with alteration of cytosine methylation

Genomic instability resulting from defective DNA damage responses or repair causes several abnormalities, including progressive cerebellar ataxia, for which the molecular mechanisms are not well understood. Here, we report a new murine model of cerebellar ataxia resulting from concomitant inactivation of POLB and ATM. POLB is one of key enzymes for the repair of damaged or chemically modified bases, including methylated cytosine, but selective inactivation of Polb during neurogenesis affects only a subpopulation of cortical interneurons despite the accumulation of DNA damage throughout the brain. However, dual inactivation of Polb and Atm resulted in ataxia without significant neuropathological defects in the cerebellum. ATM is a protein kinase that responds to DNA strand breaks, and mutations in ATM are responsible for Ataxia Telangiectasia, which is characterized by progressive cerebellar ataxia. In the cerebella of mice deficient for both Polb and Atm, the most downregulated gene was Itpr1, likely because of misregulated DNA methylation cycle. ITPR1 is known to mediate calcium homeostasis, and ITPR1 mutations result in genetic diseases with cerebellar ataxia. Our data suggest that dysregulation of ITPR1 in the cerebellum could be one of contributing factors to progressive ataxia observed in human genomic instability syndromes.

Inositol 1,4,5-Trisphosphate Receptors in Human Disease: A Comprehensive Update

Inositol 1,4,5-trisphosphate receptors (ITPRs) are intracellular calcium release channels located on the endoplasmic reticulum of virtually every cell. Herein, we are reporting an updated systematic summary of the current knowledge on the functional role of ITPRs in human disorders. Specifically, we are describing the involvement of its loss-of-function and gain-of-function mutations in the pathogenesis of neurological, immunological, cardiovascular, and neoplastic human disease. Recent results from genome-wide association studies are also discussed.

IP3 Receptor Plasticity Underlying Diverse Functions

In the body, extracellular stimuli produce inositol 1,4,5-trisphosphate (IP₃), an intracellular chemical signal that binds to the IP₃ receptor (IP₃R) to release calcium ions (Ca²⁺) from the endoplasmic reticulum. In the past 40 years, the wide-ranging functions mediated by IP₃R and its genetic defects causing a variety of disorders have been unveiled. Recent cryo-electron microscopy and X-ray crystallography have resolved IP₃R structures and begun to integrate with concurrent functional studies, which can explicate IP₃-dependent opening of Ca²⁺-conducting gates placed ∼90 Å away from IP₃-binding sites and its regulation by Ca²⁺. This review highlights recent research progress on the IP₃R structure and function. We also propose how protein plasticity within IP₃R, which involves allosteric gating and assembly transformations accompanied by rapid and chronic structural changes, would enable it to regulate diverse functions at cellular microdomains in pathophysiological states.

IP3 receptor mutations and brain diseases in human and rodents

The inositol 1,4,5-trisphosphate receptor (IP₃ R) is a huge Ca²⁺ channel that is localized at the endoplasmic reticulum. The IP₃ R releases Ca²⁺ from the endoplasmic reticulum upon binding to IP₃ , which is produced by various extracellular stimuli through phospholipase C activation. All vertebrate organisms have three subtypes of IP₃ R genes, which have distinct properties of IP₃ -binding and Ca²⁺ sensitivity, and are differently regulated by phosphorylation and by their associated proteins. Each cell type expresses the three subtypes of IP₃ R in a distinct proportion, which is important for creating and maintaining spatially and temporally appropriate intracellular Ca²⁺ level patterns for the regulation of specific physiological phenomena. Of the three types of IP₃ Rs, the type 1 receptor (IP₃ R1) is dominantly expressed in the brain and is important for brain function. Recent emerging evidence suggests that abnormal Ca²⁺ signals from the IP₃ R1 are closely associated with human brain pathology. In this review, we focus on the recent advances in our knowledge of the regulation of IP₃ R1 and its functional implication in human brain diseases, as revealed by IP₃ R mutation studies and analysis of human disease-associated genes. This article is part of the mini review series "60th Anniversary of the Japanese Society for Neurochemistry".

Role of IP3 receptor signaling in cell functions and diseases

IP3 receptor (IP3R) was found to release Ca(2+) from non-mitochondrial store but the exact localization and the mode of action of IP3 remained a mystery. IP3R was identified to be P400 protein, a protein, which was missing in the cerebellum of ataxic mutant mice lacking Ca(2+) spikes in Pukinje cells. IP3R was an IP3 binding protein and was a Ca(2+) channel localized on the endoplasmic reticulum. Full-length cDNA of IP3R type 1 was initially cloned and later two other isoforms of IP3R (IP3R type 2 and type 3) were cloned in vertebrates. Interestingly, the phosphorylation sites, splicing sites, associated molecules, IP3 binding affinity and 5' promoter sequences of each isoform were different. Thus each isoform of IP3 receptor plays a role as a signaling hub offering a unique platform for matching various functional molecules that determines different trajectories of cell signaling. Because of this distinct role of each isoform of IP3R, the dysregulation of IP3 receptor causes various kinds of diseases in human and rodents such as ataxia, vulnerability to neuronal degeneration, heart disease, exocrine secretion deficit, taste perception deficit. Moreover, IP3 was found not only to release Ca(2+), but also to release IRBIT (IP3receptor binding protein released with inositol trisphosphate) essential for the regulation of acid-base balance, RNA synthesis and ribonucleotide reductase.

Disruption of metabotropic glutamate receptor signalling is a major defect at cerebellar parallel fibre-Purkinje cell synapses in staggerer mutant mice

Staggerer mutant mice have functional loss of a transcription factor, retinoid-related orphan receptor α (RORα), which is abundantly expressed in Purkinje cells (PCs) of the cerebellum.Homozygous staggerer (sg/sg)mice show cerebellar hypoplasia and congenital ataxia. Sg/sg mice serve as an important extreme mouse model of the hereditary spinocerebellar ataxia type 1 (SCA1), since it has been shown that RORα dysfunction is strongly correlated with SCA1 pathogenesis. However, synaptic abnormalities, especially at parallel fibre (PF)-PC synapses, in SCA1-related sg/sg mice have not been examined in detail electrophysiologically. In this study, we report that PFs can still establish functional synapses onto PCs in sg/sg mice in spite of reduction in the number of PF-PC synapses. Compared with PF-evoked EPSCs in the wild-type or heterozygotes, the success rate of the EPSC recordings in sg/sg was quite low (∼40%) and the EPSCs showed faster kinetics and slightly decreased paired pulse facilitation at short intervals. The prominent synaptic dysfunction is that sg/sg mice lack metabotropic glutamate receptor (mGluR)-mediated slow EPSCs completely. Neither intense PF stimulation nor an exogenously applied mGluR agonist, DHPG, could elicit mGluR-mediated responses.Western blot analysis in the sg/sg cerebellum revealed low-level expression of mGluR1 and TRPC3, both of which underlie mGluR-mediated slow currents in PCs. Immunohistochemical data demonstrated marked mislocalization of mGluR1 on sg/sg PCs.We found that mGluR-mediated retrograde suppression of PF-PC EPSCs by endocannabinoid is also impaired completely in sg/sg mice. These results suggest that disruption of mGluR signalling at PF-PC synapses is one of the major synaptic defects in sg/sg mice and may manifest itself in SCA1 pathology.

Reduced expression of the Ca(2+) transporter protein PMCA2 slows Ca(2+) dynamics in mouse cerebellar Purkinje neurones and alters the precision of motor coordination

Cerebellar Purkinje neurones (PNs) express high levels of the plasma membrane calcium ATPase, PMCA2, a transporter protein critical for the clearance of calcium from excitable cells. Genetic deletion of one PMCA2 encoding gene in heterozygous PMCA2 knock-out (PMCA2(+/-) mice enabled us to determine how PMCA2 influences PN calcium regulation without the complication of the severe morphological changes associated with complete PMCA2 knock-out (PMCA2(-/-) in these cells. The PMCA2(+/-) cerebellum expressed half the normal levels of PMCA2 and this nearly doubled the time taken for PN dendritic calcium transients to recover (mean fast and slow recovery times increased from 70 ms to 110 ms and from 600 ms to 1100 ms). The slower calcium recovery had distinct consequences for PMCA2(+/-) PN physiology. The PNs exhibited weaker climbing fibre responses, prolonged outward Ca(2+)-dependent K(+) current (mean fast and slow recovery times increased from 136 ms to 192 ms and from 595 ms to 1423 ms) and a slower mean frequency of action potential firing (7.4 Hz compared with 15.8 Hz). Our findings were consistent with prolonged calcium accumulation in the cytosol of PMCA2(+/-) Purkinje neurones. Although PMCA2(+/-) mice exhibited outwardly normal behaviour and little change in their gait pattern, when challenged to run on a narrow beam they exhibited clear deficits in hindlimb coordination. Training improved the motor performance of both PMCA2(+/-) and wild-type mice, although PMCA2(+/-) mice were always impaired. We conclude that reduced calcium clearance perturbs calcium dynamics in PN dendrites and that this is sufficient to disrupt the accuracy of cerebellar processing and motor coordination.

IP3 receptor/Ca2+ channel: from discovery to new signaling concepts

Inositol 1,4,5-trisphosphate (IP(3)) is a second messenger that induces the release of Ca(2+) from the endoplasmic reticulum (ER). The IP(3) receptor (IP(3)R) was discovered as a developmentally regulated glyco-phosphoprotein, P400, that was missing in strains of mutant mice. IP(3)R can allosterically and dynamically change its form in a reversible manner. The crystal structures of the IP(3)-binding core and N-terminal suppressor sequence of IP(3)R have been identified. An IP(3) indicator (known as IP(3)R-based IP(3) sensor) was developed from the IP(3)-binding core. The IP(3)-binding core's affinity to IP(3) is very similar among the three isoforms of IP(3)R; instead, the N-terminal IP(3) binding suppressor region is responsible for isoform-specific IP(3)-binding affinity tuning. Various pathways for the trafficking of IP(3)R have been identified; for example, the ER forms a meshwork upon which IP(3)R moves by lateral diffusion, and vesicular ER subcompartments containing IP(3)R move rapidly along microtubles using a kinesin motor. Furthermore, IP(3)R mRNA within mRNA granules also moves along microtubules. IP(3)Rs are involved in exocrine secretion. ERp44 works as a redox sensor in the ER and regulates IP(3)R1 activity. IP(3) has been found to release Ca(2+), but it also releases IRBIT (IP(3)R-binding protein released with IP(3)). IRBIT is a pseudo-ligand for IP(3) that regulates the frequency and amplitude of Ca(2+) oscillations through IP(3)R. IRBIT binds to pancreas-type Na, bicarbonate co-transporter 1, which is important for acid-base balance. The presence of many kinds of binding partners, like homer, protein 4.1N, huntingtin-associated protein-1A, protein phosphatases (PPI and PP2A), RACK1, ankyrin, chromogranin, carbonic anhydrase-related protein, IRBIT, Na,K-ATPase, and ERp44, suggest that IP(3)Rs form a macro signal complex and function as a center for signaling cascades. The structure of IP(3)R1, as revealed by cryoelectron microscopy, fits closely with these molecules.

Synaptic organization of the mouse cerebellar cortex in organotypic slice cultures

The cellular and synaptic organization of new born mouse cerebellum maintained in organotypic slice cultures was investigated using immunohistochemical and patch-clamp recording approaches. The histological organization of the cultures shared many features with that observed in situ. Purkinje cells were generally arranged in a monolayer surrounded by a molecular-like neuropil made of Purkinje cell dendritic arborizations. Purkinje cell axons ran between clusters of small round cells identified as granule cells by Kv3.1b potassium channel immunolabelling. The terminal varicosities of the Purkinje cells axons enwrapped presumptive neurons of the cerebellar nuclei whereas their recurrent collaterals were in contact with Purkinje cells and other neurons. Granule cell axons established contacts with Purkinje cell somata and dendrites. Parvalbumin and glutamine acid decarboxylase (GAD) immunohistochemistry revealed the presence of presumptive interneurons throughout the culture. The endings of granule cell axons were observed to be in contact with these interneurons. Similarly, interneurons endings were seen close to Purkinje cells and granule cells. Whole cell recordings from Purkinje cell somata showed AMPA receptor-mediated spontaneous excitatory post-synaptic currents (sEPSCs) and GABAA receptor-mediated spontaneous inhibitory post-synaptic currents (sIPSCs). Similar events were recorded from granule cell somata except that in this neuronal type EPSPs have both a NMDA component and an AMPA component. In addition, pharmacological experiments demonstrated a GABAergic control of granule cell activity and a glutamatergic control of GABAergic neurons by granule cells. This study shows that a functional neuronal network is established in such organotypic cultures even in the absence of the two normal excitatory afferents, the mossy fibers and the climbing fibers.

Control of transcription in the RORa-staggerer mutant mouse cerebellum: glutamate receptor delta2 mRNA

The staggerer (transcription factor RORa-deleted) mutation blocks cerebellar Purkinje cell development shortly after birth. In double mutants, the homozygous staggerer mutation can 'rescue' Purkinje cells carrying a channel-opening mutation in the Glutamate receptor delta2 (Lurcher) from apoptotic death during the third and fourth postnatal weeks. Transcript levels for the glutamate receptor delta2, a channel subunit that is found at both climbing fiber and parallel fiber synapses on cerebellar Purkinje cells, are higher in the staggerer mutant cerebellum than in the wild-type cerebellum at age 14 days. By 21 days, the wild-type level is higher, having increased tremendously while the staggerer increase is modest. The results imply that the mechanism protecting Purkinje cells in staggerer-Lurcher double mutants operates by blocking mutant receptor protein localization, rather than mRNA transcription. Between the ages 10 and 14 days, the climbing fiber innervation of Purkinje cells is known to switch from multiple to single in wild-type, but not in the staggerer mutant. Therefore, the results also suggest that the multiple innervation and the level of the receptor message are coordinated, either directly or indirectly.

Prominent expression of nuclear hormone receptor ROR alpha in Purkinje cells from early development

The ROR alpha is a member of the nuclear hormone receptor gene superfamily, and its deletion causes the staggerer mutation in mice. In the staggerer mutant mouse, Purkinje cells (PCs) are severely affected in the cytology, synapse formation and gene expression. We previously found the presence of mediolateral compartments unique to the staggerer cerebellum, based on different degrees of abnormalities in the cytology and gene expression. In this paper we investigated expression of the ROR alpha mRNA in developing mouse cerebellum, with a particular interest in its regional difference. At embryonic day 15, the ROR alpha mRNA was expressed at the highest level in the PC plate. The prominent expression in PCs was maintained from late embryonic stage through mature stage. At any developmental stages, no apparent regional differences in the ROR alpha mRNA expression were detected in the mediolateral and rostrocaudal axes of the cerebellum. The high expression from early developmental stages provides a molecular-anatomical basis for its important role in phenotypic differentiation of PCs. However, the even distribution in the cerebellum suggests that the unique staggerer compartments are not directly related to the loss of ROR alpha function.

Regional variation in expression of calbindin and inositol 1,4,5-trisphosphate receptor type 1 mRNAs in the cerebellum of the staggerer mutant mouse

The Purkinje cells in the staggerer mutant mouse have various cellular abnormalities, including reduced cell number, ectopia, smaller size and absence of dendritic spines. It is also know that some of these abnormalities exhibit regional variations in the cerebellum. In this paper we have investigated expression in the staggerer Purkinje cells of the calbindin and inositol 1,4, 5-trisphosphate receptor type 1 mRNAs by in situ hybridization. Although the transcription levels of both mRNAs were significantly reduced compared with the wild-type cells, the reduction among the Purkinje cell populations was not even, varying greatly from region to region. Purkinje cells with different transcription levels were distributed in discrete regions and arranged alternately in the mediolateral direction. Moreover, the cell bodies with higher transcription levels were larger in size and aligned in a monolayer between the granular and molecular layers, whereas those with lower levels were smaller in size, fewer in number and dispersed throughout the granular layer. These findings suggest that there is a distinct mediolateral heterogeneity in the staggerer cerebellum with respect to transcription levels of these Purkinje cell-specific molecules, which might correlate with some cytological phenotypes.

Presence of calbindin D28K and GAD67 mRNAs in both orthotopic and ectopic Purkinje cells of staggerer mice suggests that staggerer acts after the onset of cytodifferentiation

We used in situ hybridization to study the expression of GAD67 and calbindin D28K mRNAs in developing mouse cerebellar Purkinje cells. Both genes are expressed prenatally; calbindin D28K mRNAs can be detected in Purkinje cells of embryonic day (E) 15 mice, whereas GAD67 mRNAs first appear slightly later, in E16 mice. The stunted Purkinje cells of staggerer (sg/sg) mutant mice maintain calbindin D28K and GAD67 expression. Our data suggest that the sg/sg mutation does not interfere with the transcriptional activation of these two genes, and might therefore act after the induction of specific gene expression in developing Purkinje cells.

Developmental regulation of Thy 1.2 rate of synthesis in the mouse cerebellum

Thy 1.2 is a well-known major cell surface glycoprotein of the central nervous system (CNS). However, the regulation of the expression of this molecule as well as its function are yet to be determined. To approach these problems we studied the synthesis of the molecule in the developing cerebellum of wild-type and staggerer mutant mice. We found the appearance of a [35S]-methionine-labeled band detected with specific Sepharose 4B-bound monoclonal antibodies (Mabs). The Thy 1.2 activity increases progressively from postnatal day 9 (P9), reaching the highest rate at P12, subsequently decreasing sharply at P13, and remaining relatively low up to P16 in the wild type. Comparison of these data to the rates of total protein synthesis reveals a selective developmental regulation of Thy 1.2 expression, at least at the translational level. This correlates quite well with the timing of synaptic stabilization between parallel fibers and Purkinje cell dendritic spines. Furthermore, at P12 Thy 1.2 protein is preferentially located in the synaptosomal fraction. The parallel fiber:Purkinje cell synapsis is not stabilized in the staggerer mutant mouse. At P12 Thy 1.2 synthesis is 30% of the wild type, indicating that the translational regulation of Thy 1.2 is altered in the staggerer mutation.

Expression of the metabotropic glutamate receptor mGluR1 alpha and the ionotropic glutamate receptor GluR1 in the brain during the postnatal development of normal mouse and in the cerebellum from mutant mice

Expression of the metabotropic glutamate receptor type 1 alpha (mGluR1 alpha) and the non-N-methyl-D-aspartate (NMDA) ionotropic glutamate receptor type 1 (GluR1) in mouse brain was investigated using the antibodies raised against the synthetic peptides corresponding to their C-terminal amino acid sequences. Both receptor proteins are glycosylated predominantly in an asparagine-linked manner, and are abundant in post-synaptic membranes. We showed that mGluR1 alpha and GluR1 expression within the first 3 postnatal weeks undergoes dramatic changes in time and space, i.e., in the hippocampus and cerebellum. These spatio-temporal expression patterns appear to be correlated with the postnatal ontogenesis and establishment of the glutamatergic neurotransmission system in the hippocampus and cerebellum, cell migration, dendritic and axonal growth, spine formation, and synaptogenesis. In the adult cerebellum, mGluR1 alpha is intensely expressed in Purkinje neurons and GluR1 in Bergmann glial cells. Both receptors are expressed to a fair degree in weaver mutant cerebellum despite granule cell degeneration. However, the intrinsic expression levels of both mGluR1 alpha and GluR1 are markedly reduced in the cerebellum of the Purkinje cell-deficient and underdeveloped mutant mice, Purkinje-cell-degeneration, Lurcher, and staggerer, suggesting that GluR1 expression in Bergmann glia cells may be correlated with the sustained interaction with adjacent Purkinje neurons.

Primary structure and functional expression from complementary DNA of a brain calcium channel

The primary structure of a voltage-dependent calcium channel from rabbit brain has been deduced by cloning and sequencing the complementary DNA. Calcium channel activity expressed from the cDNA is dramatically increased by coexpression of the alpha 2 and beta subunits, known to be associated with the dihydropyridine receptor. This channel is a high voltage-activated calcium channel that is insensitive both to nifedipine and to omega-conotoxin. We suggest that it is expressed predominantly in cerebellar Purkinje cells and granule cells.

Cerebellar development: afferent organization and Purkinje cell heterogeneity

Olivo- and spinocerebellar maps in the adult cerebellum of small rodents are discontinuous, with sharp boundaries. Cortical Purkinje cells constitute a heterogeneous population, organized into parasagittal, mutually exclusive compartments. The boundaries of the intrinsic cortical compartments and those of the projectional maps are congruent. During development; (i) The incoming olivary fibres, once they penetrate in the cerebellar parenchyma, are attracted toward their ultimate terminal fields, without passing through a stage of random dispersion. (ii) Migrating Purkinje cells and inferior olivary neurons begin, asynchronously, to express cellular markers in an independent manner, giving rise to a transient compartmentation of the cerebellar cortex and the inferior olivary complex respectively. In both instances, the biochemical heterogeneity disappears during the first postnatal week, simultaneously with the acquisition of adult-like cerebellar maps. (iii) The formation of the maps is an early event, prior to the establishment of the synaptology of the cerebellar cortical circuitry. Moreover, the organization of the spinocerebellar projection in adult mutant mice does not depend on the presence of granule cells (staggerer) but on the presence of normal Purkinje cells (weaver), indicating that synaptogenesis with their target neurons is not involved in the process of map formation. The matching of region specific chemical labels between incoming afferent fibres and heterogeneous sets of Purkinje cells is the most appealing mechanism for the formation of cerebellar maps.

Primary structure and functional expression of the inositol 1,4,5-trisphosphate-binding protein P400

Cloning and expression of functional P400 protein from cerebellar Purkinje neurons shows that this protein is a receptor for inositol 1,4,5-trisphosphate, a second messenger that mediates the release of intracellular calcium.

Developmental expression and intracellular location of P400 protein characteristic of Purkinje cells in the mouse cerebellum

The developmental expression and intracellular localization of a cerebellum-characteristic 250-kDa glycoprotein, P400 protein, were studied by immunohistochemical and immunoblot methods using a monoclonal antibody against P400 protein. In the cerebellum of normal mouse, the expression of P400 protein increased from Postnatal Day 3 to Day 21. This enhancement of P400 protein expression occurred only in the Purkinje cells and proceeded with the growth of their dendritic arborization. Electron microscopic analysis indicated that P400 protein is present at the plasma membrane, the endoplasmic reticulum, and the postsynaptic densities of Purkinje cells. Immunohistochemistry of the cerebella of neurological mutant mice indicated that the Purkinje cells of reeler, weaver, and pcd mutant mice retain the ability to produce a large amount of P400 protein. However, the Purkinje cells of staggerer mutant mouse proved to be incapable of enhanced P400 protein expression. These results indicate that P400 protein is a Purkinje cell-characteristic plasma membrane-associated glycoprotein, which is also present at the postsynaptic density and endoplasmic reticulum and that the expression of P400 protein in Purkinje cells is closely associated with the growth of their dendritic arborization.

Intrinsic determinants of firing pattern in Purkinje cells of the turtle cerebellum in vitro

1. The intrinsic response properties of turtle Purkinje cells and the underlying conductances have been investigated with intradendritic and intrasomatic recordings in a slice preparation. 2. The active generation site for fast Na+ spikes was confined to the soma and for slow Ca2+ spikes to the dendrites. The configuration and generation of Ca2+ spikes was more affected by the level of extracellular K+ than were Na+ spikes. 3. Sodium spikes had a lower threshold than Ca2+ spikes at all recording sites. Sodium spike firing was abruptly initiated during depolarizing current pulses and the spike frequency increased from an early minimum to a higher steady-state level over a period of seconds or until the occurrence of Ca2+ spikes. Calcium spikes were always delayed by at least 100 ms from the onset of a depolarizing current pulse from rest. 4. The abrupt onset of Na+ spike firing was due to a tetrodotoxin-sensitive plateau potential. The phase of accelerating firing frequency and the delayed occurrence of Ca2+ spikes was due to a transient hyperpolarization activated by depolarization from rest or from more negative membrane potentials. The transient hyperpolarization was inactivated by depolarized holding potentials and was most probably generate by a rapidly inactivating K+ channel. 5. It is concluded that turtle Purkinje cells display the basic firing properties and underlying conductances known from Purkinje cells of other vertebrates. In turtle Purkinje cells Ca2+ spikes are actively generated in spiny dendrites and it is suggested that spiny dendrites rather than branch points are 'hot spots'. 6. The transient hyperpolarization, not previously described in Purkinje cells, seems particularly important for regulating Ca2+-dependent excitability.

Organization of spinocerebellar projection map in three types of agranular cerebellum: Purkinje cells vs. granule cells as organizer element

The organization of the spinocerebellar projection was analysed by the anterograde axonal WGA-HRP (horseradish peroxidase-wheat germ agglutinin conjugate) tracing method in three different types of agranular cerebellar cortex either induced experimentally by X-irradiation or occurring spontaneously in weaver (wv/wv) and staggerer (sg/sg) mutant mice. The results of this study show that in the X-irradiated rat and weaver mouse, in both of which the granule cells are directly affected and die early in development, the spinal axons reproduce, with few differences, the normal spinocerebellar pattern. Conversely, in staggerer mouse, in which the Purkinje cells are intrinsically affected and granule neurons do not seem to be primarily perturbed by the staggerer gene action, the spinocerebellar organization is severely modified. These findings appear somewhat paradoxical because if granule cells, the synaptic targets of mossy spinocerebellar fibers, were necessary for the organization of spinocerebellar projection, the staggerer cerebellum would exhibit a much more normal projectional map than the weaver and the X-irradiated cerebella. It is, therefore, obvious that granule cells, and even specific synaptogenesis, are not essential for the establishment of the normal spinocerebellar topography. On the other hand, the fact that the Purkinje cells are primarily affected in the unique agranular cortex in which the spinocerebellar organization is severely modified suggests that these neurons could be the main element in the organization of the spinocerebellar projection map. This hypothesis is discussed in correlation with already-reported findings on the zonation of the cerebellar cortex by biochemically different clusters of Purkinje cells.

Decreased number of cells in the inferior olivary nucleus of the adult mouse (+/sg) heterozygous for the staggerer gene

Light microscopic study and cell counts of the inferior olivary nucleus were performed in adult mouse (+/sg), i.e. heterozygous for the staggerer gene. Two, six, and twelve-month-old animals were studied and compared to (+/+) C57BL6J mice of the same age. The number of cells and their repartition within the four subnuclei of the inferior olivary nucleus were normal in 2-month-old (+/sg) mice but a cell loss appears afterwards and mostly between 6 and 12 months so that about 30% of the olivary cells are missing in (+/sg) mice aged 12 months. The deficit affects the four inferior olivary nucleus subnuclei but predominates in the dorsal accessory olive. This cell loss is the first described expression of the mutation in the (+/sg) mouse, and it is not yet known if the inferior olivary nucleus represents a primary site of gene action or if the deficit is secondary to a hitherto unnoticed abnormality in the heterozygous (+/sg) mouse.

Cerebellar structure and function in the murine mutant "jolting"

The morphology of the cerebellar cortex of the murine mutant "jolting" and its phenotypically normal littermate was studied. The general organisation of the cerebellum of normal mice was similar in all respects to that described for other vertebrate species. In "jolting" mice aged 4 months or more there was a marked loss of Purkinje cells and spheroids were present on Purkinje cell axons. An ultrastructural examination showed that the spheroids contained randomly orientated neurofilaments, degenerating mitochondria and autophagic vacuoles. The abnormalities in the cerebellum appeared confined to the Purkinje cells. Extracellular recordings were made of the electrical activity of the cerebellar cortex of anaesthetised mice. In the cerebellar cortex of "jolting" animals, Purkinje cells generated little spontaneous activity, but complex discharges resulting from climbing fibre inputs were indistinguishable from normal. The abnormal electrical activity of Purkinje cells in "jolting" mice could be seen in animals as young as 3 weeks. It was concluded that the "jolting" mouse suffers from a cerebellar ataxia.

Inward rectification and low threshold calcium conductance in rat cerebellar Purkinje cells. An in vitro study

The bioelectrical properties of Purkinje cells were analysed in sagittal slices of adult rat cerebellum by the use of intracellular recordings performed at a somatic level in current or in voltage clamp. The passive electrical constants of Purkinje cells were determined by measuring the time course and the amplitude of the voltage responses induced by hyperpolarizing current pulses. The mean value of input resistance was 21 +/- 1 M omega. Mean values of the membrane time constant and of the total electrotonic length of Purkinje cells were 19.5 +/- 1.7 ms and 0.59 +/- 0.01 ms respectively. A time dependent inward rectification was present in all cells. In current-clamp experiments it appeared as a sag in hyperpolarizing voltage responses which were followed by well developed anodal breaks. In voltage-clamped cells, the inward relaxation induced by hyperpolarizing commands fitted to a single exponential. It was already present near resting potential and could reach an amplitude of up to 4 nA for jumps near to -120 mV. This relaxation was provisionally termed Ih. Tail current relaxations also fitted to a single exponential when they were recorded in the presence of tetrodotoxin (TTX) and of Co. The inward relaxation induced by hyperpolarizing commands was readily blocked by Cs, whereas it was unaffected when Ba replaced Ca in the bath, except near rest where it was strongly reduced. The Ca channel blockers Cd, Co and D600 also markedly depressed or even suppressed the inward rectification near resting potential, and up to about -85 mV, whereas this blocking effect was much less apparent or even absent at more negative potentials. Ih was clearly enhanced when the external K concentration was raised up to 20 mM. In the presence of TTX and Co in the bath, inward relaxations induced by hyperpolarizing jumps were unaffected in Na-free solution, whereas the amplitude of tail currents was reduced. Furthermore, the reversal potential of Ih which ranged between -45 and -56 mV in the Co plus TTX containing solution, shifted toward more negative values in the Na-free medium. In contrast, Ih remained unchanged in low Cl solution. From these experiments, it is likely that K and Na are the main charge carriers of Ih. Furthermore, this current seems to be contaminated near resting potential by a Ca-dependent K current. Anodal breaks following hyperpolarizing commands were slightly attenuated when Cd or TTX were added to the bath.(ABSTRACT TRUNCATED AT 400 WORDS)

28 K cholecalcin (CaBP) levels in abnormal cerebella: studies on mutant mice and harmaline- and 3-acetylpyridine-treated rats

The cerebellar Purkinje cells of both birds and mammals contain a specific calcium-binding protein, 28 K cholecalcin (CaBP). This is the same protein as the vitamin D-dependent kidney CaBP, but its Purkinje cell level is apparently vitamin D independent. The cerebellar CaBP contents of 3-acetylpyridine- and harmaline-treated rats and 5 mutant mouse strains (Purkinje cell degeneration, reeler, weaver, staggerer and nervous) were measured using a specific radioimmunoassay. The results indicate that the level of cerebellar CaBP is not dependent on the physiological state of the Purkinje cells but is an intrinsic measure of the size of the Purkinje cell population.

Electrophysiological studies on the postnatal development of intracerebellar nuclei neurons in rat cerebellar slices maintained in vitro. II. Membrane conductances

The development of membrane conductances of intracerebellar nuclei neurons was studied in the rat since birth up to the weaning period by the use of thick sagittal cerebellar slices maintained in vitro. Mature nuclear neurons express fast sodium and slowly inactivating sodium conductances, as well as calcium conductances. As early as birth, fast sodium and calcium conductances appear well developed whereas slowly inactivating sodium conductances mature within the first postnatal week.

Electrophysiological properties of in vitro hippocampal pyramidal cells from normal and staggerer mutant mice

Electrophysiological properties of intracellularly recorded CA1 pyramidal cells from normal and staggerer mice were compared by using hippocampal slices maintained in vitro. In staggerer mice, the passive membrane properties of these neurons as well as their synaptic potentials elicited by stratum radiatum stimulation were very similar to those observed in normal mice. In control and mutant mice and in standard Krebs solution, CA1 pyramidal cells mainly fired tetrodoxin (TTX)-sensitive fast spikes but could also generate slow spikes. In both groups, replacement of calcium (Ca) by barium (Ba) or introduction of TEA in the bathing medium prolonged the repolarization of the fast spikes and suppressed the brief spike afterhyperpolarization which normally followed them, thus suggesting that both events involve fast potassium conductances. Furthermore, in both groups of animals, TEA and Ba enhanced the slow spikes and induced the appearance of prolonged depolarizations. These slow events were TTX-resistant and were abolished by the Ca channel blockers cadmium or cobalt, thus suggesting that they are Ca-dependent. On the whole, the present results indicate that the staggerer mutation which yields marked abnormalities in the bioelectrical properties of cerebellar Purkinje cells has no such effect on CA1 pyramidal cells.