The Reelin receptors Apoer2 and Vldlr coordinate the patterning of Purkinje cell topography in the developing mouse cerebellum

The adult cerebellar cortex is comprised of reproducible arrays of transverse zones and parasagittal stripes of Purkinje cells. Adult stripes are created through the perinatal rostrocaudal dispersion of embryonic Purkinje cell clusters, triggered by signaling through the Reelin pathway. Reelin is secreted by neurons in the external granular layer and deep cerebellar nuclei and binds to two high affinity extracellular receptors on Purkinje cells-the Very low density lipoprotein receptor (Vldlr) and apolipoprotein E receptor 2 (Apoer2). In mice null for either Reelin or double null for Vldlr and Apoer2, Purkinje cell clusters fail to disperse. Here we report that animals null for either Vldlr or Apoer2 individually, exhibit specific and parasagittally-restricted Purkinje cell ectopias. For example, in mice lacking Apoer2 function immunostaining reveals ectopic Purkinje cells that are largely restricted to the zebrin II-immunonegative population of the anterior vermis. In contrast, mice null for Vldlr have a much larger population of ectopic Purkinje cells that includes members from both the zebrin II-immunonegative and -immunopositive phenotypes. HSP25 immunoreactivity reveals that in Vldlr null animals a large portion of zebrin II-immunopositive ectopic cells are probably destined to become stripes in the central zone (lobules VI–VII). A small population of ectopic zebrin II-immunonegative Purkinje cells is also observed in animals heterozygous for both receptors (Apoer2^+/−: Vldlr^+/−), but no ectopia is present in mice heterozygous for either receptor alone. These results indicate that Apoer2 and Vldlr coordinate the dispersal of distinct, but overlapping subsets of Purkinje cells in the developing cerebellum.

Stimulation of the inferior olivary complex alters the distribution of the type 1 corticotropin releasing factor receptor in the adult rat cerebellar cortex

In a previous study, it was shown that populations of climbing fibers, derived from the inferior olivary complex (IOC) contain the peptide corticotropin releasing factor (CRF) and that the expression of this peptide in climbing fibers could be modulated by the level of activity in olivary afferents. The intent of this study was to determine if there was comparable plasticity in the distribution of the type 1 CRF receptor (CRF-R1) in the cerebellum of the rat. Our results indicate that CRF-R1 was localized primarily to Purkinje cell somata and their primary dendrites and granule cells. In addition, scattered immunolabeling was present over the somata of Golgi cells, basket cells and stellate cells, as well as Bergmann glial cells and their processes. IOC stimulation for 30 min at 1 Hz increased CRF-R1 expression in molecular layer interneurons and processes of Bergmann glial cells. Little to no effect on CRF receptor distribution was observed in Purkinje cells, granule cells, or Golgi cells. IOC stimulation at 5 Hz however, increased CRF-R1 expression in the processes of Bergmann glial cells while decreasing its expression in basket, stellate and, to some extent, in Purkinje cells. The present results suggest that there is activity-dependent plasticity in CRF-R1 expression that must be considered in defining the mechanism by which the CRF family of peptides modulates activity in cerebellar circuits. The present results also suggest that the primary targets of CRF released from climbing fibers are Bergmann glial cells and interneurons in the molecular layer. Further, interneurons responded with a decrease in receptor expression following more intense levels of stimulation suggesting the possibility of internalization of the receptor. In contrast, Bergmann glial cells showed an increased expression in receptor expression. These data suggest that CRF released from climbing fibers may modulate the physiological properties of basket and stellate cells as well as having a heretofore unidentified and potentially unique effect on Bergmann glia.

Generation of RGS8 null mutant mice by Cre/loxP system

Regulators of G-protein signaling (RGS) proteins are negative regulators of heterotrimeric guanine-nucleotide-binding proteins (G-proteins) by activating intrinsic GTPase activity of G alpha and thereby terminating G-protein coupled receptor-associated signal transduction. RGS8 belongs to B/R4 subfamily of RGS proteins and is expressed in the central nervous system, especially dense in cerebellar Purkinje cells. RGS8 binds specifically to G alpha o and G alpha i3 in vitro and activates their intrinsic GTPase activities. To investigate the role of RGS8 in vivo, we generated mice lacking RGS8 by gene targeting in embryonic stem (ES) cells using Cre/loxP system. RGS8 null mutant mice were viable, fertile and showed apparently normal development. Histological analysis showed no apparent abnormalities in morphology of cerebellar layer or Purkinje cells in RGS8 null mutant mice.

[Postnatal development of the cerebellar Purkinje cell shape in guinea pig ontogenesis]

In sagittal cerebellum sections, morphometrical study of cerebellum of mature-born animals - guinea pigs - was performed using Nissl's procedure. A change of shape and volume of Purkinje cells and their nuclei in the course of the guinea pig postnatal ontogenesis was studied. It has been shown that both the growth process itself and the rate of formation of the definite form of Purkinje cells and of their nuclei in the course of ontogenesis proceeds non-uniformly. The most intensive growth of vertical and horizontal diameters of Purkinje cells and of their nuclei is observed during the 1st and 4th weeks of postnatal life. Especially rapid is an increase of horizontal diameters of Purkinje cells and of their nuclei, which impairs the ovoid-bear-like shape to the cerebellar Purkinje cells of adult guinea pigs.

The electro-dynamics of the dendritic space in Purkinje cells of the cerebellum

The functional geometry of the reconstructed dendritic arborization of Purkinje neurons is the object of this work. The combined effects of the local geometry of the dendritic branches and of the membrane mechanisms are computed in passive configuration to obtain the electrotonic structure of the arborization. Steady-currents applied to the soma and expressed as a function of the path distance from the soma form different clusters of profiles in which dendritic branches are similar in voltages and current transfer effectiveness. The locations of the different clusters are mapped on the dendrograms and 3D representations of the arborization. It reveals the presence of different spatial dendritic sectors clearly separated in 3D space that shape the arborization in ordered electrical domains, each with similar passive charge transfer efficiencies. Further simulations are performed in active configuration with a realistic cocktail of conductances to find out whether similar spatial domains found in the passive model also characterize the active dendritic arborization. During tonic activation of excitatory synaptic inputs homogeneously distributed over the whole arborization, the Purkinje cell generates regular oscillatory potentials. The temporal patterns of the electrical oscillations induce similar spatial sectors in the arborization as those observed in the passive electrotonic structure. By taking a video of the dendritic maps of the membrane potentials during a single oscillation, we demonstrate that the functional dendritic field of a Purkinje neuron displays dynamic changes which occur in the spatial distribution of membrane potentials in the course of the oscillation. We conclude that the branching pattern of the arborization explains such continuous reconfiguration and discuss its functional implications.

Induction and survival of binucleated Purkinje neurons by selective damage and aging

Fusion of bone marrow-derived cells with adult Purkinje cells in the cerebellum gives rise to binucleated Purkinje cells. Whether fusion can be modulated by epigenetic factors and whether fused neurons are stable has remained unclear. Here, we show that in mice and rats, partial ablation of Purkinje cells and local microglial activation in the absence of structural damage to the cerebellum increase the rate of fusion. Moreover, mouse Purkinje cells once fused with bone marrow-derived cells are viable for at least 7 months. We also show that cerebellar irradiation is unnecessary for the generation of binucleated Purkinje cells after bone marrow grafting. Moreover, binucleated Purkinje cells can be found in aged mice that did not receive any treatment, suggesting that fusion events occasionally occur throughout the whole lifespan of healthy, unmanipulated individuals. However, in aged chimeric mice that, after bone marrow transplant, have the majority of their nucleated blood cells fluorescent, the number of binucleated fluorescent Purkinje cells is two orders of magnitude less than the total number of binucleated Purkinje cells. This suggests that, in the majority of heterokaryons, either the incoming nucleus is quickly inactivated or fusion is not the only way to generate a binucleated Purkinje cell.

The generation and behavioral analysis of ceramide kinase-null mice, indicating a function in cerebellar Purkinje cells

The discovery of ceramide kinase (CerK), which phosphorylates ceramide (Cer) to ceramide 1-phisphate (C1P), established a new pathway for Cer metabolism. Among mouse tissues, brain contains the highest CerK activity. In this study, we found that CerK is highly expressed in cerebellar Purkinje cells. Since Purkinje cells are important for motor-related behaviors, we generated CerK-null mice and performed behavioral analyses. The CerK-null mice were healthy, and displayed no histological abnormalities. The mice lost CerK activity completely, suggesting that CerK is the only enzyme that phosphorylate Cer. However, cellular C1P levels were not different between the CerK-null and wild-type mice, indicating the presence of other C1P-producing pathway. The general motor-coordination was not impaired in the CerK-null mice, but emotional behavior was slightly affected. Our findings suggest that CerK is not necessary for survival at an individual level, but might be involved in higher brain function related to emotion.

Activation of presynaptic GABA(A) receptors induces glutamate release from parallel fiber synapses

The parallel fibers relay information coming into the cerebellar cortex from the mossy fibers, and they form synapses with molecular layer interneurons (MLIs) and Purkinje cells. Here we show that activation of ionotropic GABA receptors (GABA(A)Rs) induces glutamate release from parallel fibers onto both MLIs and Purkinje cells. These GABA-induced EPSCs have kinetics and amplitudes identical to random spontaneous currents (sEPSCs), but, unlike sEPSCs, they occur in bursts of between one and five successive events. The variation in amplitude of events within bursts is significantly less than the variation of all sEPSC amplitudes, suggesting that the bursts result from repetitive activation of single presynaptic fibers. Electron microscopy of immunogold-labeled alpha-1 subunits revealed GABA(A)Rs on parallel fiber terminals. We suggest that the activation of these receptors underlies the increased amplitude of parallel fiber-evoked Purkinje cell EPSCs seen with application of exogenous GABA or after the release of GABA from local interneurons. These results occur only when molecular layer GABA(A)Rs are activated, and the effects are abolished when the receptors are blocked by the GABA(A)R antagonist gabazine (5 microM). From these data, we conclude that GABA(A)Rs located on parallel fibers depolarize parallel fiber terminals beyond the threshold for Na+ channel activation and thereby induce glutamate release onto MLIs and Purkinje cells.

[The number of active Purkinje cells in the microelectrod track as an indicator of maturity of the cerebellum motor function in precocial- and altricial-born animals]

In experiments on 5 age groups of anesthetized guinea pigs (from newborns to 4 weeks of postnatal ontogenesis), activity of cerebellum Purkinje cells (PC) (IV-VII lobules of cerebellar vermis) was studied in the single track of microelectrode passing through cell layers. It has been shown that as early as several hours after birth, in the superficial layer of cerebellar cortex, there are recorded occasional background-active, but functionally mature PC represented by simple and complex spikes and accordingly reflecting synaptic PC activation by afferent inputs of mossy and climbing fibers. The functional manifestation of the guinea pig motor behavior at this period of ontogenesis is act of their standing. At this period of ontogenesis, in the newborn and one-day old guinea pigs, from 1 to rarely 11 active PC are recorded, on average, in the single microelectrode track. At the one-week age, the highest number active PC in the track somewhat increases, predominantly at the expense of the mean from the total number of the cells in the track. In the 2-week old guinea pigs the mean number of active PC in the track somewhat falls, while in the 4-week old and adult animals exceeds again, although slightly of the maximal number of PC in the track of newborn animals. The relatively high number of active PC at the very initial period of postnatal ontogenesis can indicate importance of motor function in the congenital food-procuring reflex.

Essential role for autophagy protein Atg7 in the maintenance of axonal homeostasis and the prevention of axonal degeneration

Autophagy is a regulated lysosomal degradation process that involves autophagosome formation and transport. Although recent evidence indicates that basal levels of autophagy protect against neurodegeneration, the exact mechanism whereby this occurs is not known. By using conditional knockout mutant mice, we report that neuronal autophagy is particularly important for the maintenance of local homeostasis of axon terminals and protection against axonal degeneration. We show that specific ablation of an essential autophagy gene, Atg7, in Purkinje cells initially causes cell-autonomous, progressive dystrophy (manifested by axonal swellings) and degeneration of the axon terminals. Consistent with suppression of autophagy, no autophagosomes are observed in these dystrophic swellings, which is in contrast to accumulation of autophagosomes in the axonal dystrophic swellings under pathological conditions. Axonal dystrophy of mutant Purkinje cells proceeds with little sign of dendritic or spine atrophy, indicating that axon terminals are much more vulnerable to autophagy impairment than dendrites. This early pathological event in the axons is followed by cell-autonomous Purkinje cell death and mouse behavioral deficits. Furthermore, ultrastructural analyses of mutant Purkinje cells reveal an accumulation of aberrant membrane structures in the axonal dystrophic swellings. Finally, we observe double-membrane vacuole-like structures in wild-type Purkinje cell axons, whereas these structures are abolished in mutant Purkinje cell axons. Thus, we conclude that the autophagy protein Atg7 is required for membrane trafficking and turnover in the axons. Our study implicates impairment of axonal autophagy as a possible mechanism for axonopathy associated with neurodegeneration.

Morphological changes in dendritic spines of Purkinje cells associated with motor learning

Experience-dependent changes of spine structure and number may contribute to long-term memory storage. Although several studies demonstrated structural spine plasticity following associative learning, there is limited evidence associating motor learning with alteration of spine morphology. Here, we investigated this issue in the cerebellar Purkinje cells using high voltage electron microscopy (HVEM). Adult rats were trained in an obstacle course, demanding significant motor coordination to complete. Control animals either traversed an obstacle-free runway or remained sedentary. Quantitative analysis of spine morphology showed that the density and length of dendritic spines along the distal dendrites of Purkinje cells were significantly increased in the rats that learned complex motor skills compared to active or inactive controls. Classification of spines into shape categories indicated that the increased spine density and length after motor learning was mainly attributable to an increase in thin spines. These findings suggest that motor learning induces structural spine plasticity in the cerebellar Purkinje neurons, which may play a crucial role in acquiring complex motor skills.

ESCRT-III dysfunction causes autophagosome accumulation and neurodegeneration

Defects in the endosomal-lysosomal pathway have been implicated in a number of neurodegenerative disorders. A key step in the endocytic regulation of transmembrane proteins occurs in a subset of late-endosomal compartments known as multivesicular bodies (MVBs), whose formation is controlled by endosomal sorting complex required for transport (ESCRT). The roles of ESCRT in dendritic maintenance and neurodegeneration remain unknown. Here, we show that mSnf7-2, a key component of ESCRT-III, is highly expressed in most mammalian neurons. Loss of mSnf7-2 in mature cortical neurons caused retraction of dendrites and neuronal cell loss. mSnf7-2 binds to CHMP2B, another ESCRT-III subunit, in which a rare dominant mutation is associated with frontotemporal dementia linked to chromosome 3 (FTD3). Ectopic expression of the mutant protein CHMP2B(Intron5) also caused dendritic retraction prior to neurodegeneration. CHMP2B(Intron5) was associated more avidly than CHMP2B(WT) with mSnf7-2, resulting in sequestration of mSnf7-2 in ubiquitin-positive late-endosomal vesicles in cortical neurons. Moreover, loss of mSnf7-2 or CHMP2B(Intron5) expression caused the accumulation of autophagosomes in cortical neurons and flies. These findings indicate that ESCRT-III dysfunction is associated with the autophagy pathway, suggesting a novel neurodegeneration mechanism that may have important implications for understanding FTD and other age-dependent neurodegenerative diseases.

Ca2+ requirements for cerebellar long-term synaptic depression: role for a postsynaptic leaky integrator

Photolysis of a caged Ca(2+) compound was used to characterize the dependence of cerebellar long-term synaptic depression (LTD) on postsynaptic Ca(2+) concentration ([Ca(2+)](i)). Elevating [Ca(2+)](i) was sufficient to induce LTD without requiring any of the other signals produced by synaptic activity. A sigmoidal relationship between [Ca(2+)](i) and LTD indicated a highly cooperative triggering of LTD by Ca(2+). The duration of the rise in [Ca(2+)](i) influenced the apparent Ca(2+) affinity of LTD, and this time-dependent behavior could be described by a leaky integrator process with a time constant of 0.6 s. A computational model, based on a positive-feedback cycle that includes protein kinase C and MAP kinase, was capable of simulating these properties of Ca(2+)-triggered LTD. Disrupting this cycle experimentally also produced the predicted changes in the Ca(2+) dependence of LTD. We conclude that LTD arises from a mechanism that integrates postsynaptic Ca(2+) signals and that this integration may be produced by the positive-feedback cycle.

LIM-homeodomain proteins Lhx1 and Lhx5, and their cofactor Ldb1, control Purkinje cell differentiation in the developing cerebellum

Purkinje cells are one of the major types of neurons that form the neural circuitry in the cerebellum essential for fine control of movement and posture. During development, Purkinje cells also are critically involved in the regulation of proliferation of progenitors of granule cells, the other major type of neurons in the cerebellum. The process that controls differentiation of Purkinje cells from their early precursors is poorly understood. Here we report that two closely related LIM-homeobox genes, Lhx1 and Lhx5, were expressed in the developing Purkinje cells soon after they exited the cell cycle and migrated out of the cerebellar ventricular zone. Double-mutant mice lacking function of both Lhx1 and Lhx5 showed a severe reduction in the number of Purkinje cells. In addition, targeted inactivation of Ldb1, which encodes a cofactor for all LIM-homeodomain proteins, resulted in a similar phenotype. Our studies thus provide evidence that these transcription regulators are essential for controlling Purkinje cell differentiation in the developing mammalian cerebellum.

Sustained structural change of GABA(A) receptor-associated protein underlies long-term potentiation at inhibitory synapses on a cerebellar Purkinje neuron

Fast inhibitory synaptic transmission is predominantly mediated by GABA(A) receptor (GABA(A)R) in the CNS. Although several types of neuronal activity-dependent plasticity at GABAergic synapses have been reported, the detailed mechanism is elusive. Here we show that binding of structurally altered GABA(A)R-associated protein (GABARAP) to GABA(A)R gamma2 subunit and to tubulin is critical for long-term potentiation [called rebound potentiation (RP)] at inhibitory synapses on a cerebellar Purkinje neuron (PN). Either inhibition of GABARAP association with GABA(A)Rgamma2 or deletion of tubulin binding region of GABARAP impaired RP. Inhibition of tubulin polymerization also suppressed RP. Thus, precise regulation of GABA(A)Rgamma2-GABARAP-microtubule interaction is critical for RP. Furthermore, competitive inhibition of GABARAP binding to GABA(A)Rgamma2 after the RP establishment attenuated the potentiated response, suggesting that GABARAP is critical not only for the induction but also for the maintenance of RP. Fluorescence resonance energy transfer analysis revealed that GABARAP underwent sustained structural alteration after brief depolarization of a PN depending on the activity of Ca2+/calmodulin-dependent protein kinase II (CaMKII), which is required for the RP induction. The susceptibility of GABARAP to undergo structural alteration was abolished by an amino acid replacement in GABARAP. Furthermore, RP was impaired by expression of the mutant GABARAP with the replacement. Together, we conclude that GABA(A)R association with structurally altered GABARAP downstream of CaMKII activation is essential for RP.

Topographic distribution of output neurons in cerebellar nuclei and cortex to somatotopic map of primary motor cortex

To investigate the somatotopic organization of the cerebellum, we analysed multisynaptic inputs to the primary motor cortex (MI) using retrograde transneuronal transport of rabies virus. At 3 days after rabies injections into proximal forelimb, distal forelimb and hindlimb representations of the macaque MI, second-order neurons via the thalamus were labeled in the deep cerebellar nuclei, including the dentate (DN), anterior interpositus (AIN) and posterior interpositus nuclei. In the DN, the labeling of both the forelimb and hindlimb was seen mainly in the dorsal aspect. The labeling of the hindlimb was located rostral to that of the forelimb and the labeling of the proximal forelimb was located slightly rostral to that of the distal forelimb. The same rostrocaudal arrangement was observed in the AIN. In the posterior interpositus nucleus, however, labeling from the MI hindlimb and forelimb representations largely overlapped. At the 4-day postinjection period, third-order labeling occurred in Purkinje cells of the cerebellar hemisphere. The Purkinje cell labeling from the forelimb representation, including the proximal and distal regions, was observed primarily in lobules IV-VI and crus I. The proximal forelimb labeling was both rostral and lateral to that of the distal forelimb within lobules IV-VI. However, the hindlimb labeling was seen both rostral and lateral to that of the proximal forelimb within lobules III-VI. These results indicate that the hindlimb, proximal forelimb and distal forelimb are arranged rostrocaudally in the DN and AIN, whereas there is dual somatotopy along the rostrocaudal and lateromedial axes in the cerebellar cortex.

Physiology of morphologically identified cells in the posterior caudal lobe of the mormyrid cerebellum

The cerebellum of the mormyrid fish consists of three major divisions: the valvula, the central lobes, and the caudal lobes. Several studies have focused on the central lobes and the valvula, but little is known about the caudal lobes. The mormyrid caudal lobe includes anterior and posterior components. The anterior caudal lobe is associated with the lateral line and eighth nerve end organs, whereas the posterior caudal lobe is associated with the electrosensory system. The present study examines the physiology and pharmacology of morphologically identified Purkinje cells and efferent cells in an in vitro slice preparation of the posterior caudal lobe. We found that the Purkinje cells in the posterior caudal lobe can be classified into three subtypes based on both their morphology and on their physiological responses to intracellular current injection and to synaptic inputs from parallel fibers and climbing fibers. Similarities and differences between the physiology of the caudal lobe and that of other regions of the mormyrid cerebellum and the mammalian cerebellum are discussed.

CD3 and immunoglobulin G Fc receptor regulate cerebellar functions

The immune and nervous systems display considerable overlap in their molecular repertoire. Molecules originally shown to be critical for immune responses also serve neuronal functions that include normal brain development, neuronal differentiation, synaptic plasticity, and behavior. We show here that FcgammaRIIB, a low-affinity immunoglobulin G Fc receptor, and CD3 are involved in cerebellar functions. Although membranous CD3 and FcgammaRIIB are crucial regulators on different cells in the immune system, both CD3epsilon and FcgammaRIIB are expressed on Purkinje cells in the cerebellum. Both CD3epsilon-deficient mice and FcgammaRIIB-deficient mice showed an impaired development of Purkinje neurons. In the adult, rotarod performance of these mutant mice was impaired at high speed. In the two knockout mice, enhanced paired-pulse facilitation of parallel fiber-Purkinje cell synapses was shared. These results indicate that diverse immune molecules play critical roles in the functional establishment in the cerebellum.

Oxidative stress, nitric oxide, and the mechanisms of cell death inLurcherPurkinje cells

Oxidative stress is postulated to play a role in cell death in many neurodegenerative diseases. As a model of neonatal neuronal cell death, we have examined the role of oxidative stress in Purkinje cell death in the heterozygous Lurcher mutant (+/Lc). Lurcher is a gain of function mutation in the delta2 glutamate receptor (GluRdelta2) that turns the receptor into a leaky membrane channel, resulting in chronic depolarization of +/Lc Purkinje cells starting around the first week of postnatal development. Virtually, all +/Lc Purkinje cells die by the end of the first postnatal month. To investigate the role of oxidative stress in +/Lc Purkinje cell death, we have examined nitric oxide synthase (NOS) activity and the expression of two markers for oxidative stress, nitrotyrosine and manganese super oxide dismutase (MnSOD), in wild type and +/Lc Purkinje cells at P10, P15, and P25. The results show that NOS activity and immunolabeling for nitrotyrosine and MnSOD are increased in +/Lc Purkinje cells. To determine whether peroxynitrite formation is a prerequisite for +/Lc Purkinje cell death, +/Lc mutants were crossed with an alpha-nNOS knockout mutant (nNOSalpha(-/-)) to reduce the production of NO. Analysis of the double mutants showed that blocking alpha-nNOS expression does not rescue +/Lc Purkinje cells. However, we present evidence for sustained NOS activity and nitrotyrosine formation in the GluRdelta2(+/Lc):nNOS(-/-) double mutant Purkinje cells, which suggests that the failure to rescue GluRdelta2(+/Lc):nNOS(-/-) Purkinje cells may be explained by the induction of alternative nNOS isoforms.

Cell cycle inhibition and retinoblastoma protein overexpression prevent Purkinje cell death in organotypic slice cultures

Purkinje cells are vulnerable to a number of physical, chemical, and genetic insults during development and maturity. Normal development of these cells depends on the cell-cell interactions between granule and astroglial cell populations. Apoptotic death in Purkinje neurons had been shown to be associated with cell cycle activation, and new DNA synthesis is associated with Purkinje cell death in staggerer and lurcher mutant mice. Here using an in vitro organotypic slice culture model from 9 (P9) and 4 days (P4) old postnatal rats we show that the cyclin dependent kinase (cdk) inhibitors (roscovitine, olomoucine, and flavopiridol) protect the Purkinje cells from cell death. The results are more pronounced in the cerebellar sections from P4 rats. Analysis of Purkinje neurons in sections from P4 rats after 1 week of culturing showed that while there were very limited calbindin positive neurons in the untreated sections the cdk inhibitor treated sections had a notably higher number. Although treatment with cdk inhibitors inhibited Purkinje cell loss significantly, the morphology of these neurons was abnormal, with stunted dendrites and axons. Since the retinoblastoma protein (Rb) is the major pocket protein involved in determining the differentiated state of neurons we examined the effect of over-expressing Rb in the organotypic cultures. Rb overexpression significantly inhibited the Purkinje cell death and these neurons maintained their normal morphology. Thus our studies show that the cell death in Purkinje neurons observed in organotypic cultures is cell cycle dependent and the optimal survival requires Rb.

The neuropeptide corticotropin-releasing factor regulates excitatory transmission and plasticity at the climbing fibre-Purkinje cell synapse

The climbing fibre (CF) input controls cerebellar Purkinje cell (PC) activity as well as synaptic plasticity at parallel fibre (PF)-PC synapses. Under high activity conditions, CFs release not only glutamate, but also the neuropeptide corticotropin-releasing factor (CRF). Brief periods of such high CF activity can lead to the induction of long-term depression (LTD) at CF-PC synapses. Thus, we have examined for the first time the role of CRF in regulating excitatory postsynaptic currents (EPSCs) and long-term plasticity at this synapse. Exogenous application of CRF alone transiently mimicked three aspects of CF-LTD, causing reductions in the CF-evoked excitatory postsynaptic current, complex spike second component and complex spike afterhyperpolarization. The complex spike first component is unaffected by CF-LTD induction and was similarly unaffected by CRF. Application of a CRF receptor antagonist reduced the expression amplitude and induction probability of CF-LTD monitored at the EPSC level. Collectively, these results suggest that under particular sensorimotor conditions, co-release of CRF from climbing fibres could down-regulate excitatory transmission and facilitate LTD induction at CF-PC synapses. Inhibition of either protein kinase C (PKC) or protein kinase A (PKA) attenuated the effects of CRF upon CF-EPSCs. We have previously shown that CF-LTD induction is PKC-dependent, and here demonstrate PKA-dependence as well. These results suggest that both the acute effects of CRF on CF-EPSCs as well as the facilitating effect of CRF on CF-LTD induction can be explained by a CRF-mediated recruitment of PKC and PKA.

Linking synaptic plasticity and spike output at excitatory and inhibitory synapses onto cerebellar Purkinje cells

Understanding the relationship between synaptic plasticity and neuronal output is essential if we are to understand how plasticity is encoded in neural circuits. In the cerebellar cortex, motor learning is thought to be implemented by long-term depression (LTD) of excitatory parallel fiber (PF) to Purkinje cell synapses triggered by climbing fiber (CF) input. However, theories of motor learning generally neglect the contribution of plasticity of inhibitory inputs to Purkinje cells. Here we describe how CF-induced plasticity of both excitatory and inhibitory inputs is reflected in Purkinje cell spike output. We show that coactivation of the CF with PF input and interneuron input leads not only to LTD of PF synapses but also to comparable, "balanced" LTD of evoked inhibitory inputs. These two forms of plasticity have opposite effects on the spike output of Purkinje cells, with the number and timing of spikes sensitively reflecting the degree of plasticity. We used dynamic clamp to evaluate plasticity-induced changes in spike responses to sequences of excitation and feedforward inhibition of varied relative and absolute amplitude. Balanced LTD of both excitatory and inhibitory components decreased the net spike output of Purkinje cells only for inputs with small inhibitory components, whereas for inputs with a larger proportion of feedforward inhibition CF-triggered LTD resulted in an increase in the net spike output. Thus, the net effect of CF-triggered plasticity on Purkinje cell output depends on the balance of excitation and feedforward inhibition and can paradoxically increase cerebellar output, contrary to current theories of cerebellar motor learning.

Impaired motor learning in the vestibulo-ocular reflex in mice with multiple climbing fiber input to cerebellar Purkinje cells

A unique feature of the cerebellar architecture is that Purkinje cells in the cerebellar cortex each receive input from a single climbing fiber. In mice deficient in the gamma isoform of protein kinase C (PKCgamma-/- mice), this normal architecture is disrupted so that individual Purkinje cells receive input from multiple climbing fibers. These mice have no other known abnormalities in the cerebellar circuit. Here, we show that PKCgamma-/- mice are profoundly impaired in vestibulo-ocular reflex (VOR) motor learning. The PKCgamma-/- mice exhibited no adaptive increases or decreases in VOR gain at training frequencies of 2 or 0.5 Hz. This impairment was present across a broad range of peak retinal slip speeds during training. We compare the results for VOR motor learning with previous studies of the performance of PKCgamma-/- mice on other cerebellum-dependent learning tasks. Together, the results suggest that single climbing fiber innervation of Purkinje cells is critical for some, but not all, forms of cerebellum-dependent learning, and this may depend on the region of the cerebellum involved, the organization of the relevant neural circuits downstream of the cerebellar cortex, as well as the timing requirements of the learning task.

Prenatal corticosterone influences the trajectory of neuronal development, delaying or accelerating aspects of the Purkinje cell differentiation

The nervous system developmental programs proceed in orderly fashion following strict timetables. However, the mechanisms regulating developmental timing remain largely unknown. Increases or decreases in glucocorticoids in the fetal brain can be detrimental. We present evidence supporting that corticosterone forwards the migration of cerebellar granule neurons when applied acutely during pregnancy. This change in developmental tempo enhances dendritic growth of Purkinje neurons, increases the nuclear area, accelerates perinucleolar rosette appearance and decreases the development of Nissl bodies. Our observations thus support that forwarding the occurrence of developmental events does not always arrest neuronal growth, as some heterochronic developmental models imply. We suggest that prenatal glucocorticoids alter the trajectory of Purkinje neurons development soon after birth. These changes could represent a transient condition or could produce medium or long-term later consequences. More studies are needed to evaluate these intriguing possibilities.