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P Strata

Publications and source records attributed to P Strata.

At least 19 recordsLinked to original sources
PubMed2001

Presynaptic modulation by group III metabotropic glutamate receptors (mGluRs) of the excitatory postsynaptic potential mediated by mGluR1 in rat cerebellar Purkinje cells.

Purkinje neurons were recorded from rat cerebellar slices. Parallel fibres stimulation elicited a fast excitatory postsynaptic potential (EPSP) mediated by ionotropic glutamate (iGluR) -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors followed by the inhibitory gamma-aminobutyric acidA (GABAA)-dependent postsynaptic potential. In the presence of antagonists for iGluRs and for GABAA receptors, brief tetanic activation evoked a slow metabotropic glutamate receptor (mGluR)-dependent EPSP (mGluR-EPSP). This mGluR-EPSP was blocked by the selective mGluR1 antagonists LY367385 and CPCCOEt, but not by the mGluR5 antagonist MPEP. Group II agonists affected neither iGluR-EPSP nor mGluR-EPSP. Conversely, L-AP4 and L-SOP, group III mGluR agonists, inhibited both iGluR- and mGluR-EPSPs. The depolarisations evoked by both AMPA and group I agonists were unaffected, indicating a presynaptic action of group III mGluRs. These data suggest that glutamate released by parallel fibres activates group III mGluR autoreceptors, depressing both iGluR- and mGluR1-mediated EPSPs.

Animals↗
PubMed2001

Role of glutamate delta -2 receptors in activity-dependent competition between heterologous afferent fibers.

A principle that regulates detailed architecture in the brain is that active terminals have a competitive advantage over less active terminals in establishing synaptic connections. This principle is known to apply to fibers within a single neuronal population competing for a common target domain. Here we uncover an additional rule that applies when two neuronal populations compete for two contiguous territories. The cerebellar Purkinje cell dendrites have two different synaptic domains with spines innervated by two separate excitatory inputs, parallel fibers (PFs) and climbing fibers (CFs). Glutamate delta-2 receptors are normally present only on the PF spines where they are important for their innervation. After block of activity by tetrodotoxin, numerous new spines form in the CF domain and become innervated mainly by PFs; all spines, including those still innervated by the CFs, bear delta-2 receptors. Thus, in the absence of activity, PFs gain a competitive advantage over CFs. The entire dendritic arbor becomes a uniform territory with the molecular cues associated with the PFs. To access their proper territory and maintain synaptic contacts, CFs must be active and locally repress the cues of the competitor afferents.

Animals↗
PubMed2001

Characterization of the mGluR(1)-mediated electrical and calcium signaling in Purkinje cells of mouse cerebellar slices.

The metabotropic glutamate receptor 1 (mGluR(1)) plays a fundamental role in postnatal development and plasticity of ionotropic glutamate receptor-mediated synaptic excitation of cerebellar Purkinje cells. Synaptic activation of mGluR(1) by brief tetanic stimulation of parallel fibers evokes a slow excitatory postsynaptic current and an elevation of intracellular calcium concentration ([Ca2+](i)) in Purkinje cells. The mechanism underlying these responses has not been identified yet. Here we investigated the responses to synaptic and direct activation of mGluR(1) using whole cell patch-clamp recordings in combination with microfluorometric measurements of [Ca2+](i) in mouse Purkinje cells. Following pharmacological block of ionotropic glutamate receptors, two to six stimuli applied to parallel fibers at 100 Hz evoked a slow inward current that was associated with an elevation of [Ca2+](i). Both the inward current and the rise in [Ca2+](i) increased in size with increasing number of pulses albeit with no clear difference between the minimal number of pulses required to evoke these responses. Application of the mGluR(1) agonist (S)-3,5-dihydroxyphenylglycine (3,5-DHPG) by means of short-lasting (5-100 ms) pressure pulses delivered through an agonist-containing pipette positioned over the Purkinje cell dendrite, evoked responses resembling the synaptically induced inward current and elevation of [Ca2+](i). No increase in [Ca2+](i) was observed with inward currents of comparable amplitudes induced by the ionotropic glutamate receptor agonist AMPA. The 3,5-DHPG-induced inward current but not the associated increase in [Ca2+](i) was depressed when extracellular Na+ was replaced by choline, but, surprisingly, both responses were also depressed when bathing the tissue in a low calcium (0.125 mM) or calcium-free/EGTA solution. Thapsigargin (10 microM) and cyclopiazonic acid (30 microM), inhibitors of sarco-endoplasmic reticulum Ca2+-ATPase, had little effect on either the inward current or the elevation in [Ca2+](i) induced by 3,5-DHPG. Furthermore, the inward current induced by 3,5-DHPG was neither blocked by 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl)propoxy] ethyl-1H-imidazole, an inhibitor of store operated calcium influx, nor by nimodipine or omega-agatoxin, blockers of voltage-gated calcium channels. These electrophysiological and Ca2+-imaging experiments suggest that the mGluR(1)-mediated inward current, although mainly carried by Na+, involves influx of Ca2+ from the extracellular space.

Animals↗
PubMed2000

Application of neutralizing antibodies against NI-35/250 myelin-associated neurite growth inhibitory proteins to the adult rat cerebellum induces sprouting of uninjured purkinje cell axons.

The myelin-associated proteins NI-35/250 exert a powerful inhibition on axon regeneration, but their function exerted on intact neurons is still unclear. In the adult CNS these proteins are thought to regulate axon growth processes to confine plasticity within restricted regions and to prevent the formation of aberrant connections. We have recently shown that application of neutralizing IN-1 antibody Fab fragment against NI-35/250 proteins to the adult cerebellum induces the expression of injury/growth-associated markers in intact Purkinje cells. Here, we asked whether these cellular modifications are accompanied by growth phenomena of Purkinje neurites. A single intraparenchymal application of IN-1 Fab fragment to the adult cerebellum induces a profuse sprouting of Purkinje axons along their intracortical course. The newly formed processes spread to cover most of the granular layer depth. A significant axon outgrowth is evident 2 d after injection; it tends to increase at 5 and 7 d, but it is almost completely reversed after 1 month. No axonal modifications occur in control Fab-treated cerebella. The IN-1 Fab fragment-induced cellular changes and axon remodeling are essentially reproduced by applying affinity-purified antibody 472 raised against a peptide sequence of the recombinant protein NI-220, thus confirming the specificity of the applied treatments on these myelin-associated molecules. Functional neutralization of NI-35/250 proteins induces outgrowth from uninjured Purkinje neurites in the adult cerebellum. Together with previous observations, this suggests that these molecules regulate axonal plasticity to maintain the proper targeting of terminal arbors within specific gray matter regions.

Age Factors↗
PubMed2000

Atm-deficient mice Purkinje cells show age-dependent defects in calcium spike bursts and calcium currents.

Ataxia telangiectasia in humans results from homozygous loss-of-function mutations in ATM. Neurological deterioration is the major cause of death in ataxia telangiectasia patients: in the cerebellum, mainly Purkinje cells are affected. We have generated Atm-deficient mice which display neurological abnormalities by several tests of motor function consistent with an abnormality of cerebellar function, but without histological evidence of neuronal degeneration. Here we performed a more detailed morphological analysis and an electrophysiological study on Purkinje cells from Atm-deficient mice of different ages. We found no histological or immunohistochemical abnormalities. Electrophysiology revealed no abnormalities in resting membrane potential, input resistance or anomalous rectification. In contrast, there was a significant decrease in the duration of calcium and sodium firing. The calcium deficit became significant between six to eight and 12-20 weeks of age, and appeared to be progressive. By voltage-clamp recording, we found that the firing deficits were due to a significant decrease in calcium currents, while inactivating potassium currents seem unaffected. In other mutant mice, calcium current deficits have been shown to be related to cell death.Our experiments suggest that the electrophysiological defects displayed by Atm-deficient mice are early predegenerative lesions and may be a precursor of Purkinje cell degeneration displayed by ataxia telangiectasia patients.

Action Potentials↗
PubMed2000

Elevation of intradendritic sodium concentration mediated by synaptic activation of metabotropic glutamate receptors in cerebellar Purkinje cells.

Cerebellar Purkinje cells express both ionotropic glutamate receptors and metabotropic glutamate receptors. Brief tetanic stimulation of parallel fibers in rat and mouse cerebellar slices evokes a slow excitatory postsynaptic current in Purkinje cells that is mediated by the mGluR1 subtype of metabotropic glutamate receptors. The effector system underlying this mGluR1 EPSC has not yet been identified. In the present study, we recorded the mGluR1 EPSC using the whole-cell patch-clamp technique in combination with microfluorometric recordings of the intracellular sodium concentration ([Na+]i) by means of the fluorescent sodium indicator SBFI. The mGluR1 EPSC was induced by local parallel fibre stimulation in the presence of the ionotropic glutamate receptor antagonists NBQX and D-APV and the GABAA receptor antagonists bicuculline or picrotoxin. The mGluR1 EPSC was associated with an increase in [Na+]i that was restricted to a specific portion of the dendritic tree. The mGluR1 EPSC as well as the increase in [Na+]i were inhibited by the mGluR antagonist S-MCPG. In the presence of NBQX, D-APV, pictrotoxin and TTX, bath application of the selective mGluR agonist 3,5-DHPG induced an elevation in [Na+]i which extended over the whole dendritic field of the Purkinje cell. This finding demonstrates that the mGluR1-mediated postsynaptic current leads to a significant influx of sodium into the dendritic cytoplasm of Purkinje cells and thereby provides a novel intracellular signalling mechanism that might be involved in mGluR1-dependent synaptic plasticity at this synapse.

2-Amino-5-phosphonovalerate↗
PubMed1999

Control of spine formation by electrical activity in the adult rat cerebellum.

Dendritic spines are a key structure in neuronal plasticity. Enhanced activity is commonly associated with an increase in spine size and density. Purkinje cell dendrites are characterized by a proximal and a distal compartment on which climbing fibers and parallel fibers, respectively, impinge. The proximal region has a very low spine density, whereas the distal region has a high density. Previous experiments showed that after climbing fiber deletion, Purkinje cells become hyperactive, and a large number of spines develop on the proximal dendrites. Here we show that the same hyperspiny transformation occurs in the proximal dendrites of adult Purkinje cells by depressing electrical activity with tetrodotoxin. Thus, spines in different dendritic compartments are created or maintained independently from the level of Purkinje cell-firing rate and when the afferent activity is blocked. This conclusion supports the view that spinogenesis is the expression of an intrinsic program and the two regions of the dendritic tree respond differently to activity block because of differences in the inputs that they receive. On tetrodotoxin treatment, climbing fibers become atrophic and may sprout thin collateral ramifications directed mainly toward the granular layer. All changes are reversible on tetrodotoxin removal. Therefore, Purkinje cells provide a model where spines in different compartments of the same neuron are differently regulated by the activity of their local afferents. In addition, electrical activity is also essential to maintain the full climbing fiber innervation.

Animals↗
PubMed1999

Phenotype changes of inferior olive neurons following collateral reinnervation.

Inferior olive neurons are able to enlarge or retract their axonic terminal fields in response to changes in the extension of their target domain. Following Purkinje cell loss, the retraction of target-deprived climbing fibres is accompanied by a size reduction in the inferior olive neuron cell bodies. Here, we asked whether perikaryal modifications also occur when inferior olivary neurons enlarge their terminal fields to innervate supernumerary targets. To achieve this aim, we carried out a morphometric analysis on the somatic compartment of inferior olive neurons in two experimental conditions known to induce an expansion of their terminal field, i.e. a subtotal 3-acetylpyridine inferior olive lesion in the adult and a unilateral transection of the inferior cerebellar peduncle in newborn rats. In both experimental conditions, the inferior olive neurons that survived the lesion showed a remarkable increase in cell body and nuclear size, although the latter change was less pronounced in the 3-acetylpyridine-treated animals. These results show that both developing and mature inferior olive neurons are capable of adjusting their perikaryal phenotype to match the modifications of their target size.

Animals↗
PubMed1999

Mechanisms of axonal plasticity.

GAP-43 plays an important role in axonal plasticity by guiding growth cones rather than supporting axonal elongation. In Purkinje cells that show no regenerative responses and no GAP-43 expression after axotomy, the simple addition of GAP-43 gene induces the formation of branched plexuses typical of sprouting growth. Purkinje cells can express some growth-associated proteins, but never GAP-43, when axotomy is made very close to cell body or when an antibody for the myelin-associated inhibitory molecules is applied to intact cells both in vivo and in vitro. Also in these conditions they are unable to show new axonal profiles even in a permissive environment that allows inferior olive cells to regenerate and reinnervate their target cells. We suggest that GAP-43 is a key molecule to initiate axon growth while other genes are necessary to develop a full regenerative program.

Animals↗
PubMed1998

Retrograde regulation of growth-associated gene expression in adult rat Purkinje cells by myelin-associated neurite growth inhibitory proteins.

Axon regeneration requires that injured neurons reinitiate long-distance growth and upregulate specific genes. To address the question of whether inhibitory environmental cues along the axon could exert a negative, tonic downregulation of growth-associated genes, we have examined adult rat Purkinje cells, which are endowed with poor regenerative capabilities. First we have compared their response to axotomy with that of neurons of the inferior olive, lateral reticular nucleus, and deep cerebellar nuclei, all of which vigorously regenerate into growth-permissive transplants. These injured neurons upregulate the transcription factors c-Jun and JunD, GAP-43, and NADPH diaphorase. In contrast, most axotomized Purkinje cells fail to express any of these markers, showing that the strength of this response parallels the regenerative potential of the examined neuron populations. However, strong upregulation of the same genes can be induced in Purkinje cells after colchicine injection into the uninjured adult cerebellum, indicating that their expression could be controlled by retrograde signals. To assess whether myelin-associated neurite growth inhibitory proteins contribute to this regulation, we applied the neutralizing antibodies IN-1 against one of the main inhibitory components of central myelin (NI-250) either in vivo or in vitro to organotypic cerebellar cultures. Application of IN-1 antibodies induces the upregulation of c-Jun, JunD, and NADPH diaphorase in Purkinje cells, showing that their expression is suppressed constitutively by myelin-associated neurite growth inhibitors. Thus, the inhibitory activity of the IN-1 antigen on axon growth is not restricted to the control of growth cone motility but also involves a retrograde regulation of gene expression in adult central neurons.

Age Factors↗
PubMed1998

Plasticity of the olivocerebellar pathway.

The adult olivocerebellar axons and their terminal arbours, the climbing fibres, are capable of remarkable structural plasticity, regulated through their interaction with Purkinje cells. When these cells are deleted,terminal climbing fibre branches retract. In contrast,there is a vigorous outgrowth of entire terminal arbours when extra postsynaptic neurones are available. The new connections lead to a functional, highly specific pattern of innervation at the single Purkinje cell level and are topographically organized according to the principles of the original projection map.A reversible climbing fibre retraction occurs following depression of electrical activity of the cerebellar cortex. These remarkable plastic properties, together with the fact that these neurones express several growth-associated genes constitutively, suggest that the climbing fibre synapses might be adjusted dynamically to participate in physiological plasticity.

Animals↗
PubMed1998

Postsynaptic current mediated by metabotropic glutamate receptors in cerebellar Purkinje cells.

In rat cerebellar slices, repetitive parallel fiber stimulation evokes an inward, postsynaptic current in Purkinje cells with a fast component mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors and a slower component mediated by metabotropic glutamate receptors (mGluR). The mGluR-mediated excitatory postsynaptic current (mGluR-EPSC) is evoked selectively by parallel fiber stimulation; climbing fiber stimulation is ineffective. The mGluR-EPSC is elicited most effectively with increasing frequencies of parallel fiber stimulation, from a threshold of 10 Hz to a maximum response at approximately 100 Hz. The amplitude of the mGluR-EPSC is a linear function of the number of stimulus pulses without any apparent saturation, even with >10 pulses. Thus mGluRs at the parallel fiber-Purkinje cell synapse can function as linear detectors of the number of spikes in a burst of activity in parallel fibers. The mGluR-EPSC is present from postnatal day 15 and persists into adulthood. It is inhibited by the generic mGluR antagonist (RS)-a-methyl-4-carboxyphenylglycine and by the group I mGluR antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid at a concentration selective for mGluR1. Although the intracellular transduction pathway involves a G protein, the putative mediators of mGluR1 (phospholipase C and protein kinase C) are not directly involved, indicating that the mGluR-EPSC studied here is mediated by a different and still unidentified second-messenger pathway. Heparin, a nonselective antagonist of inositol-trisphosphate (IP3) receptors, has no significant effect on the mGluR-EPSC, suggesting that also IP3 might be not required for the response. Buffering intracellular Ca2+ with a high concentration of bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid partially inhibits the mGluR-EPSC, indicating that Ca2+ is not directly responsible for the response but that resting Ca2+ levels exert a tonic potentiating effect on the mGluR-EPSC.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1997

Targeted overexpression of the neurite growth-associated protein B-50/GAP-43 in cerebellar Purkinje cells induces sprouting after axotomy but not axon regeneration into growth-permissive transplants.

B-50/GAP-43 is a nervous tissue-specific protein, the expression of which is associated with axon growth and regeneration. Its overexpression in transgenic mice produces spontaneous axonal sprouting and enhances induced remodeling in several neuron populations (; ). We examined the capacity of this protein to increase the regenerative potential of injured adult central axons, by inducing targeted B-50/GAP-43 overexpression in Purkinje cells, which normally show poor regenerative capabilities. Thus, transgenic mice were produced in which B-50/GAP-43 overexpression was driven by the Purkinje cell-specific L7 promoter. Uninjured transgenic Purkinje cells displayed normal morphology, indicating that transgene expression does not modify the normal phenotype of these neurons. By contrast, after axotomy numerous transgenic Purkinje cells exhibited profuse sprouting along the axon and at its severed end. Nevertheless, despite these growth phenomena, which never occurred in wild-type mice, the severed transgenic axons were not able to regenerate, either spontaneously or into embryonic neural or Schwann cell grafts placed into the lesion site. Finally, although only a moderate Purkinje cell loss occurred in wild-type cerebella after axotomy, a considerable number of injured transgenic neurons degenerated, but they could be partially rescued by the different transplants placed into the lesion site. Thus, B-50/GAP-43 overexpression substantially modifies Purkinje cell response to axotomy, by inducing growth processes and decreasing their resistance to injury. However, the presence of this protein is not sufficient to enable these neurons to accomplish a full program of axon regeneration.

Animals↗
PubMed1997

Reestablishment of the olivocerebellar projection map by compensatory transcommissural reinnervation following unilateral transection of the inferior cerebellar peduncle in the newborn rat.

It is unclear whether reparative processes in the injured mammalian brain are able to restore the topographic organisation of neuronal connections. To address this question, we have investigated the plasticity of the olivocerebellar system. This pathway has a precise topographic arrangement, in which subsets of inferior olivary neurons project to parasagittally oriented Purkinje cell compartments. Following unilateral transection of the inferior cerebellar peduncle in newborn rats, axons from the contralateral projection cross the cerebellar midline and reinnervate the deafferented hemicerebellum. By this experimental approach, we first analysed the behaviour of calcitonin gene-related peptide (CGRP)-immunoreactive climbing fibres. This marker is transiently expressed by a subset of developing inferior olivary axons, which terminate in the cerebellar cortex into several parasagittal strips. We show that transcommissural axons reestablish the original pattern of climbing fibre bands within a few days after lesion. Then, in adult animals injured at birth, we assessed whether the newly formed climbing fibre bands align with zebrin II+/- Purkinje cell compartments, as in normal conditions. The newly formed projection is organised in parasagittally oriented strips which mirror the distribution of their counterparts on the intact side and are precisely aligned to the heterogeneous Purkinje cell compartments. In addition, the patchy distribution of olivo-nuclear fibres suggests that specific reinnervation is also achieved in the deep nuclei. Thus, transcommissural olivocerebellar reinnervation is not random, but it is regulated by selective interactions between distinct subsets of olivocerebellar axons and target neurons aimed at reestablishing the correct projection map.

Animals↗
PubMed1997

Italian grants.

Explore the source record for details and available documents.

Financing, Organized↗
PubMed1997

The early phase of horizontal optokinetic responses in the pigmented rat and the effects of lesions of the visual cortex.

Horizontal optokinetic responses of pigmented rats were studied both in intact animals and in animals that had received lesions of the visual area of the cerebral cortex. In response to uniform velocity stimulation, there was an initial phase of rapid acceleration, larger than that reported in earlier studies, followed by a period of fairly uniform acceleration until the eye velocity approached that of the stimulus. As reported previously, responses to monocular stimulation were highly asymmetric, with the responses to nasotemporal stimulation being much weaker than those to temporonasal stimulation. Responses to sinusoidal stimulation were also studied. No significant effect of cortical lesions on the responses was seen.

Animals↗
PubMed1997

Reciprocal trophic interactions between climbing fibres and Purkinje cells in the rat cerebellum.

In the adult cerebellum both the climbing fibre arbour and the Purkinje cell are very plastic and each element is able to exert a remarkable action on the other one. The adult phenotype of the Purkinje cell is strictly dependent on the presence of its climbing fibre arbour. When the climbing fibre is missing, the Purkinje cell undergoes a hyperspiny transformation and becomes hyperinnervated by the parallel fibres. However, this change is fully reversible. The climbing fibre-deprived Purkinje cell is able to elicit sprouting of nearby located intact climbing fibres and the new arbour is able to fully restore synaptic connections which appear normal both morphologically and functionally. Multiple climbing fibre innervation of a single Purkinje cell persists in the adult hypogranular cerebellum. The different fibres are distributed to separate dendritic regions, suggesting a local competition between the different arbours for their territory. It is postulated that in the intact rat, an activity dependent mechanism of the parallel fibre favours the predominance of one arbour with the elimination of its competitors. When the Purkinje cell is deleted, the climbing fibre arbour becomes heavily atrophic and reduced in size. The analysis of the pattern of this atrophy indicates that the climbing fibre arbour is made by two compartments: a proximal one, whose survival depends on the integrity of the inferior olive, and a distal one, which represents the true pre-synaptic site, which strictly depends on the target. The climbing fibre terminal arbour is able to extend its territory of innervation not only when adult intact climbing fibres are confronted with nearby denervated Purkinje cells, but also when an embryonic cerebellum is grafted onto the surface of an adult unlesioned cerebellum. In this case, collaterals of intact climbing fibre arbours elongate through the pial surface, enter the graft to innervate the Purkinje cells. This growth is likely under the influence of a tropic signal released by the embryonic Purkinje cells. This suggests that the sprouting observed in the adult rat following a subtotal inferior olive lesion is also triggered by a similar factor. The axonal elongation and the consequent synaptogenesis are likely guided by local cues. In this condition, the distribution of the new collateral reinnervation occurs within its projectional map. In addition, when the inferior cerebellar peduncle is sectioned at birth, the climbing fibres of the non-deafferented hemicerebellum emit collaterals which cross the midline and innervate cerebellar strips which are symmetrically positioned relative to the intact side. In the grafting experiments, both the migrated and non-migrated Purkinje cells show the typical electrophysiological properties of the mature cerebellum. These data show that the disappearance of neuronal elements is not a necessary prerequisite to allow new neurones to become fully morphologically and functionally integrated into an adult brain. The reciprocal trophic influence between the climbing fibres and the Purkinje cells shown in the present series of experiments are likely operative in the adult brain not only in pathological conditions and they could give a basic contribution to the synaptic plasticity underlying learned behaviour.

Animals↗