Organotypic cultures of cerebellar slices as a model to investigate demyelinating disorders

INTRODUCTION

Demyelinating disorders, characterized by a chronic or episodic destruction of the myelin sheath, are a leading cause of neurological disability in young adults in western countries. Studying the complex mechanisms involved in axon myelination, demyelination and remyelination requires an experimental model preserving the neuronal networks and neuro-glial interactions. Organotypic cerebellar slice cultures appear to be the best alternative to in vivo experiments and the most commonly used model for investigating etiology or novel therapeutic strategies in multiple sclerosis. Areas covered: This review gives an overview of slice culture techniques and focuses on the use of organotypic cerebellar slice cultures on semi-permeable membranes for studying many aspects of axon myelination and cerebellar functions. Expert opinion: Cerebellar slice cultures are probably the easiest way to faithfully reproduce all stages of axon myelination/demyelination/remyelination in a three-dimensional neuronal network. However, in the cerebellum, neurological disability in multiple sclerosis also results from channelopathies which induce changes in Purkinje cell excitability. Cerebellar cultures offer easy access to electrophysiological approaches which are largely untapped and we believe that these cultures might be of great interest when studying changes in neuronal excitability, axonal conduction or synaptic properties that likely occur during multiple sclerosis.

Stereotyped spatial patterns of functional synaptic connectivity in the cerebellar cortex

Motor coordination is supported by an array of highly organized heterogeneous modules in the cerebellum. How incoming sensorimotor information is channeled and communicated between these anatomical modules is still poorly understood. In this study, we used transgenic mice expressing GFP in specific subsets of Purkinje cells that allowed us to target a given set of cerebellar modules. Combining in vitro recordings and photostimulation, we identified stereotyped patterns of functional synaptic organization between the granule cell layer and its main targets, the Purkinje cells, Golgi cells and molecular layer interneurons. Each type of connection displayed position-specific patterns of granule cell synaptic inputs that do not strictly match with anatomical boundaries but connect distant cortical modules. Although these patterns can be adjusted by activity-dependent processes, they were found to be consistent and predictable between animals. Our results highlight the operational rules underlying communication between modules in the cerebellar cortex.

Epsilon toxin from Clostridium perfringens acts on oligodendrocytes without forming pores, and causes demyelination

Epsilon toxin (ET) is produced by Clostridium perfringens types B and D and causes severe neurological disorders in animals. ET has been observed binding to white matter, suggesting that it may target oligodendrocytes. In primary cultures containing oligodendrocytes and astrocytes, we found that ET (10(-9) M and 10(-7) M) binds to oligodendrocytes, but not to astrocytes. ET induces an increase in extracellular glutamate, and produces oscillations of intracellular Ca(2+) concentration in oligodendrocytes. These effects occurred without any change in the transmembrane resistance of oligodendrocytes, underlining that ET acts through a pore-independent mechanism. Pharmacological investigations revealed that the Ca(2+) oscillations are caused by the ET-induced rise in extracellular glutamate concentration. Indeed, the blockade of metabotropic glutamate receptors type 1 (mGluR1) prevented ET-induced Ca(2+) signals. Activation of the N-methyl-D-aspartate receptor (NMDA-R) is also involved, but to a lesser extent. Oligodendrocytes are responsible for myelinating neuronal axons. Using organotypic cultures of cerebellar slices, we found that ET induced the demyelination of Purkinje cell axons within 24 h. As this effect was suppressed by antagonizing mGluR1 and NMDA-R, demyelination is therefore caused by the initial ET-induced rise in extracellular glutamate concentration. This study reveals the novel possibility that ET can act on oligodendrocytes, thereby causing demyelination. Moreover, it suggests that for certain cell types such as oligodendrocytes, ET can act without forming pores, namely through the activation of an undefined receptor-mediated pathway.

Attack of the nervous system by Clostridium perfringens Epsilon toxin: from disease to mode of action on neural cells

Epsilon toxin (ET), produced by Clostridium perfringens types B and D, ranks among the four most potent poisonous substances known so far. ET-intoxication is responsible for enterotoxaemia in animals, mainly sheep and goats. This disease comprises several manifestations indicating the attack of the nervous system. This review aims to summarize the effects of ET on central nervous system. ET binds to endothelial cells of brain capillary vessels before passing through the blood-brain barrier. Therefore, it induces perivascular oedema and accumulates into brain. ET binding to different brain structures and to different component in the brain indicates regional susceptibility to the toxin. Histological examination has revealed nerve tissue and cellular lesions, which may be directly or indirectly caused by ET. The naturally occurring disease caused by ET-intoxication can be reproduced experimentally in rodents. In mice and rats, ET recognizes receptor at the surface of different neural cell types, including certain neurons (e.g. the granule cells in cerebellum) as well as oligodendrocytes, which are the glial cells responsible for the axons myelination. Moreover, ET induces release of glutamate and other transmitters, leading to firing of neural network. The precise mode of action of ET on neural cells remains to be determined.

Clostridium perfringens epsilon toxin targets granule cells in the mouse cerebellum and stimulates glutamate release

Epsilon toxin (ET) produced by C. perfringens types B and D is a highly potent pore-forming toxin. ET-intoxicated animals express severe neurological disorders that are thought to result from the formation of vasogenic brain edemas and indirect neuronal excitotoxicity. The cerebellum is a predilection site for ET damage. ET has been proposed to bind to glial cells such as astrocytes and oligodendrocytes. However, the possibility that ET binds and attacks the neurons remains an open question. Using specific anti-ET mouse polyclonal antibodies and mouse brain slices preincubated with ET, we found that several brain structures were labeled, the cerebellum being a prominent one. In cerebellar slices, we analyzed the co-staining of ET with specific cell markers, and found that ET binds to the cell body of granule cells, oligodendrocytes, but not astrocytes or nerve endings. Identification of granule cells as neuronal ET targets was confirmed by the observation that ET induced intracellular Ca(2+) rises and glutamate release in primary cultures of granule cells. In cultured cerebellar slices, whole cell patch-clamp recordings of synaptic currents in Purkinje cells revealed that ET greatly stimulates both spontaneous excitatory and inhibitory activities. However, pharmacological dissection of these effects indicated that they were only a result of an increased granule cell firing activity and did not involve a direct action of the toxin on glutamatergic nerve terminals or inhibitory interneurons. Patch-clamp recordings of granule cell somata showed that ET causes a decrease in neuronal membrane resistance associated with pore-opening and depolarization of the neuronal membrane, which subsequently lead to the firing of the neuronal network and stimulation of glutamate release. This work demonstrates that a subset of neurons can be directly targeted by ET, suggesting that part of ET-induced neuronal damage observed in neuronal tissue is due to a direct effect of ET on neurons.

Deletion of Cav2.1(alpha1(A)) subunit of Ca2+-channels impairs synaptic GABA and glutamate release in the mouse cerebellar cortex in cultured slices

Deletion of both alleles of the P/Q-type Ca(2+)-channel Ca(v)2.1(alpha(1A)) subunit gene in mouse leads to severe ataxia and early death. Using cerebellar slices obtained from 10 to 15 postnatal days mice and cultured for at least 3 weeks in vitro, we have analysed the synaptic alterations produced by genetically ablating the P/Q-type Ca(2+)-channels, and compared them with the effect of pharmacological inhibition of the P/Q- or N-type channels on wild-type littermate mice. Analysis of spontaneous synaptic currents recorded in Purkinje cells (PCs) indicated that the P/Q-type channels play a prominent role at the inhibitory synapses afferent onto the PCs, with the effect of deleting Ca(v)2.1(alpha(1A)) partially compensated. At the granule cell (GC) to PC synapses, both N- and P/Q-type Ca(2+)-channels were found playing a role in glutamate exocytosis, but with no significant phenotypic compensation of the Ca(v)2.1(alpha(1A)) deletion. We also found that the P/Q- but not N-type Ca(2+)-channel is indispensable at the autaptic contacts between PCs. Tuning of the GC activity implicates both synaptic and sustained extrasynaptic gamma-aminobutyric acid (GABA) release, only the former was greatly impaired in the absence of P/Q-type Ca(2+)-channels. Overall, our data demonstrate that both P/Q- and N-type Ca(2+)-channels play a role in glutamate release, while the P/Q-type is essential in GABA exocytosis in the cerebellum. Contrary to the other regions of the CNS, the effect of deleting the Ca(v)2.1(alpha(1A)) subunit is partially or not compensated at the inhibitory synapses. This may explain why cerebellar ataxia is observed at the mice lacking functional P/Q-type channels.

The mouse cerebellar cortex in organotypic slice cultures: an in vitro model to analyze the consequences of mutations and pathologies on neuronal survival, development, and function

Thin acute slices and dissociated cell cultures taken from different parts of the brain have been widely used to examine the function of the nervous system, neuron-specific interactions, and neuronal development (specifically, neurobiology, neuropharmacology, and neurotoxicology studies). Here, we focus on an alternative in vitro model: brain-slice cultures in roller tubes, initially introduced by Beat Gähwiler for studies with rats, that we have recently adapted for studies of mouse cerebellum. Cultured cerebellar slices afford many of the advantages of dissociated cultures of neurons and thin acute slices. Organotypic slice cultures were established from newborn or 10-15-day-old mice. After 3-4 weeks in culture, the slices flattened to form a cell monolayer. The main types of cerebellar neurons could be identified with immunostaining techniques, while their electrophysiological properties could be easily characterized with the patch-clamp recording technique. When slices were taken from newborn mice and cultured for 3 weeks, aspects of the cerebellar development were displayed. A functional neuronal network was established despite the absence of mossy and climbing fibers, which are the two excitatory afferent projections to the cerebellum. When slices were made from 10-15-day-old mice, which are at a developmental stage when cerebellum organization is almost established, the structure and neuronal pathways were intact after 3-4 weeks in culture. These unique characteristics make organotypic slice cultures of mouse cerebellar cortex a valuable model for analyzing the consequences of gene mutations that profoundly alter neuronal function and compromise postnatal survival.

Glia-induced neuronal differentiation by transcriptional regulation

There is increasing evidence that different phases of brain development depend on neuron-glia interactions including postnatal key events like synaptogenesis. To address how glial cells influence synapse development, we analyzed whether and how glia-derived factors affect gene expression in primary cultures of immunoisolated rat retinal ganglion cells (RGCs) by oligonucleotide microarrays. Our results show that the transcript pattern matched the developmental stage and characteristic properties of RGCs in vitro. Glia-conditioned medium (GCM) and cholesterol up- and downregulated a limited number of genes that influence the development of dendrites and synapses and regulate cholesterol and fatty acid metabolism. The oligonucleotide microarrays detected the transcriptional regulation of neuronal cholesterol homeostasis in response to GCM and cholesterol treatment. Surprisingly, our study revealed neuronal expression and glial regulation of matrix gla protein (Mgp). Together, our results suggest that glial cells promote different aspects of neuronal differentiation by regulating transcription of distinct classes of genes.

IL1-receptor accessory protein-like 1 (IL1RAPL1), a protein involved in cognitive functions, regulates N-type Ca2+-channel and neurite elongation

Null mutations in the IL1-receptor accessory protein-like 1 gene (IL1RAPL1) are responsible for an inherited X-linked form of cognitive impairment. IL1RAPL1 protein physically interacts with neuronal calcium sensor-1 (NCS-1), but the functional impact of the IL1RAPL1/NCS-1 interaction remains unknown. Here, we demonstrate that stable expression of IL1RAPL1 in PC12 cells induces a specific silencing of N-type voltage-gated calcium channels (N-VGCC) activity that explains a secretion deficit observed in these IL1RAPL1 cells. Importantly, this modulation of VGCC activity is mediated by NCS-1. Indeed, a specific loss-of-function of N-VGCC was observed in PC12 cells overexpressing NCS-1, and a total recovery of N-VGCC activity was obtained by a down-regulation of NCS-1 in IL1RAPL1 cells. The functional relevance of the interaction between IL1RAPL1 and NCS-1 was also suggested by the reduction of neurite elongation observed in nerve growth factor (NGF)-treated IL1RAPL1 cells, a phenotype rescued by NCS-1 inactivation. Because both proteins are highly expressed in neurons, these results suggest that IL1RAPL1-related mental retardation could result from a disruption of N-VGCC and/or NCS-1-dependent synaptic and neuronal activities.

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.

Mammalian Scribble forms a tight complex with the betaPIX exchange factor

Drosophila Scribble is implicated in the development of normal synapse structure and epithelial tissues, but it remains unclear how it plays a role and which process it controls. The mammalian homolog of Scribble, hScrib, has a primary structure and subcellular localization similar to that of its fly homolog, but its function remains unknown. Here we have used tandem mass spectrometry to identify major components of the hScrib network. We show that it includes betaPIX (also called Cool-1), a guanine nucleotide exchange factor (GEF), and its partner GIT1 (also called p95-APP1), a GTPase activating protein (GAP). betaPIX directly binds to the hScrib PDZ domains, and the hScrib/betaPIX complex is efficiently recovered in epithelial and neuronal cells and tissues. In cerebellar granule cell cultures, hScrib and betaPIX are both partially localized at neuronal presynaptic compartments. Furthermore, we show that hScrib is required to anchor betaPIX at the cell cortex and that dominant-negative betaPIX or hScrib proteins can each inhibit Ca2+-dependent exocytosis in neuroendocrine PC12 cells, demonstrating a functional relationship between these proteins. These data reveal the existence of a tight hScrib/betaPIX interaction and suggest that this complex potentially plays a role in neuronal transmission.

F-actin does not modulate the initial steps of the protein kinase C activation process in living nerve cells

Actin is a major substrate for protein kinase C (PKC) and PKC is considered a modulator of the actin network. In addition in vitro studies (Biochemistry 39 (2000) 271) have suggested that all PKC isoforms bind to actin during the process of activation of the enzyme. To test the physiological significance of such a coupling we used living PC12 cells and primary cultures of cerebellar granule cells. When PC12 cells were treated with either latrunculin B, which impairs actin polymerization, or phalloidin, which stabilizes actin filaments, we observed a significant reduction of the [Ca2+]i response revealed by Fura-2 fluorescence, while the PKC conformational changes followed by Fim-1 fluorescence were unaffected. The responses induced either by cell depolarization or muscarinic receptor activation were similarly affected by the toxin treatment of PC12 cells. In cerebellar granule cells the [Ca2+]i response induced by KCl depolarization was increased by latrunculin treatment, whereas no effect was observed on the PKC response. Latrunculin had no effect on the NMDA-induced responses in these cells. Finally we also show that the response induced by a long-lasting depolarization, which mimics stimulation leading to neuronal plasticity, was not significantly altered by latrunculin or phalloidin treatment of the cells. These results suggest that the actin network is not involved in the initial steps of the PKC activation process in living nerve cells.

A role for phospholipase D1 in neurotransmitter release

Phosphatidic acid produced by phospholipase D (PLD) as a result of signaling activity is thought to play a role in membrane vesicle trafficking, either as an intracellular messenger or as a cone-shaped lipid that promotes membrane fusion. We recently described that, in neuroendocrine cells, plasma membrane-associated PLD1 operates at a stage of Ca(2+)-dependent exocytosis subsequent to cytoskeletal-mediated recruitment of secretory granules to exocytotic sites. We show here that PLD1 also plays a crucial role in neurotransmitter release. Using purified rat brain synaptosomes subjected to hypotonic lysis and centrifugation, we found that PLD1 is associated with the particulate fraction containing the plasma membrane. Immunostaining of rat cerebellar granule cells confirmed localization of PLD1 at the neuronal plasma membrane in zones specialized for neurotransmitter release (axonal neurites, varicosities, and growth cone-like structures). To determine the potential involvement of PLD1 in neurotransmitter release, we microinjected catalytically inactive PLD1(K898R) into Aplysia neurons and analyzed its effects on evoked acetylcholine (ACh) release. PLD1(K898R) produced a fast and potent dose-dependent inhibition of ACh release. By analyzing paired-pulse facilitation and postsynaptic responses evoked by high-frequency stimulations, we found that the exocytotic inhibition caused by PLD1(K898R) was not the result of an alteration in stimulus-secretion coupling or in vesicular trafficking. Analysis of the fluctuations in amplitude of the postsynaptic responses revealed that the PLD1(K898R) blocked ACh release by reducing the number of active presynaptic-releasing sites. Our results provide evidence that PLD1 plays a major role in neurotransmission, most likely by controlling the fusogenic status of presynaptic release sites.

High sensitivity of mouse neuronal cells to tetanus toxin requires a GPI-anchored protein

Tetanus neurotoxin (TeNT) produced by Clostridium tetani specifically cleaves VAMP/synaptobrevin (VAMP) in central neurons, thereby causing inhibition of neurotransmitter release and ensuing spastic paralysis. Although polysialogangliosides act as components of the neurotoxin binding sites on neurons, evidence has accumulated indicating that a protein moiety is implicated as a receptor of TeNT. We have observed that treatment of cultured mouse neuronal cells with the phosphatidylinositol-specific phospholipase C (PIPLC) inhibited TeNT-induced cleavage of VAMP. Also, we have shown that the blocking effects of TeNT on neuroexocytosis can be prevented by incubation of Purkinje cell preparation with PIPLC. In addition, treatment of cultured mouse neuronal cells with cholesterol sequestrating agents such as nystatin and filipin, which disrupt clustering of GPI-anchored proteins in lipid rafts, prevented intraneuronal VAMP cleavage by TeNT. Our results demonstrate that high sensitivity of neurons to TeNT requires rafts and one or more GPI-anchored protein(s) which act(s) as a pivotal receptor for the neurotoxin.

NGF enhances depolarization effects on SNAP-25 expression: induction of SNAP-25b isoform

The 25 kDa synaptosomal associated protein (SNAP-25), which is implicated in neuronal plasticity and neurosecretion, exists as two isoforms generated by alternative splicing of exons 5a and 5b. The aim of the present study was to characterize factors influencing isoform expression. We report that chronic depolarization of PC12 cells alone or in the presence of NGF induces the expression of isoform-b, in addition to a 1.8- to 3-fold increase in SNAP-25 mRNA and protein as determined by immunoblotting and combined RT-PCR and Southern blot analysis. When cerebellar granule neurons were cultured in elevated K+, the predominant isoform switched from SNAP-25a to SNAP-25b. Taken together these results suggested that chronic depolarization regulates the transcription and processing of SNAP-25 mRNA.

Reversible protein kinase C activation in PC12 cells: effect of NGF treatment

Although protein kinase C (PKC) is a key enzyme in the signal transduction process, there is little information on the mechanism leading to PKC activation in living cells. Using a new fluorescence imaging method, we studied this mechanism and correlated PKC conformational changes with intracellular Ca2+ concentration. PC12 cells were simultaneously loaded with Fura-2-AM and Fim-1, two fluorescent probes, which recognize Ca2+ and PKC, respectively. KCl and carbachol (an agonist to muscarinic receptors) applications induced dose-dependent increases of fluorescence for both probes. Both Ca2+ and PKC responses were observed within seconds following KCl or carbachol application, and were reversible upon stimulus withdrawal. PKC activation kinetics was slightly more rapid than the Ca2+ response after KCl application. After nerve growth factor (NGF) treatment of the cells, the amplitude of the KCl-induced PKC responses was larger indicating an increase in the activated PKC-pool in these cells. This difference between control and NGF-treated cells was not observed following carbachol application, suggesting the involvement of different PKC pools. While the Ca2+ response uniformly occurred in the cytosol, the PKC response displayed a patch pattern with higher intensities in the peripheral zone near the plasma membrane. This heterogeneous distribution of PKC activation sites was similar to the immunocytological localization of Ca2+-dependent and independent PKC isoforms, which suggested that at least several PKC isoforms interacted with intracellular elements. Upon repeated stimulation, the PKC response rapidly desensitized.

Pervanadate-triggered MAP kinase activation and cell proliferation are not sensitive to PD 98059. Evidence for stimulus-dependent differential PD 98059 inhibition mechanism

A tight and stable complex with corresponding protein kinases and phosphatases establishes coupling between activators and inactivators. One such example is emerging from the studies of the Ras-dependent MAP kinase cascade signaling pathway. Pervanadate, a potent inhibitor of protein tyrosine phosphatase, stimulates MAP kinase and elicits cell proliferation in cultured mouse fibroblasts which is insensitive to PD 98059, the major inhibitor of upstream MEK, whereas serum- or TPA-triggered proliferation is sensitive to PD 98059. It is suggested that imbalanced coordination between protein kinase and protein phosphatase determines the cellular responses such as cell proliferation. The PD 98059-insensitive cell proliferation upon protein tyrosine phosphatase inhibition is attributed to a MEK bypass pathway.

Polyethylenimine-mediated DNA transfection of peripheral and central neurons in primary culture: probing Ca2+ channel structure and function with antisense oligonucleotides

To study neuronal ion channel function with antisense oligonucleotides, a reliable method is needed which allows different neuronal cell types to be transfected without artifactual disruptive effects on their electrical properties. Here we report that use of the recently introduced transfecting agent, polyethylenimine, fulfills this requirement. Four days after transfection, in both central and peripheral neurons, an antisense designed to block the synthesis of the Ca2+ channel beta subunits induced a maximal decrease of the Ca2- current amplitude and modification of their kinetics and voltage-dependence. Controls with scrambled oligonucleotides, as well as Na+ current recordings of antisense transfected neurons, confirm both that the transfecting agent does not modify the electrophysiological properties of the neurons and that the effect of the antisense is sequence specific.

Enhancement of NMDA receptor maturation by BDNF in cultured mouse cerebellar granule cells

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor, which is essential during organogenesis for neuronal differentiation and formation of synaptic connections in the developing brain, changes its receptor properties during development by differential expression of multiple subunits. Using a combined electrophysiological and pharmacological approach on primary cultures of mouse cerebellar neurones, we investigated the evolution of the NMDA receptor and the potential effect of the neurotrophin BDNF on its expression. We showed that 1) the current density of NMDA responses increased with time of culture; 2) epsilon 1 subunit expression increased with time in culture relative to epsilon 2 subunit expression; and 3) the time course of the increase in NMDA responses was accelerated by about 2 days in the presence of BDNF.

Voltage-activated ionic currents in differentiating rat cerebellar granule neurons cultured from the external germinal layer

The electrical properties of the precursor cells of the external germinal layer of rat cerebellum were assessed during their differentiation in control medium (Dulbecco's modified Eagle's medium) supplemented or not with either basic fibroblast growth factor (bFGF) or 25 mM potassium chloride (KCl). Resting potential was shown to be -10 mV in all three conditions 3 hours after plating [days in vitro (DIV)0]. By DIV 5, it reached -63 mV for cells cultured in 25 mM KCl but only -28 mV in control and bFGF media. The main voltage-sensitive ionic current measured at DIV 0 under all conditions was a composite IK consisting in a sustained K+ current blocked by tetraethylammonium (IK(TEA)), plus a rapidly activating and inactivating TEA-insensitive IK(A). Both currents increased with time in all conditions, but after 5 days IK(A) became dominant in terms of density. IK(TEA) is likely an IK(Ca), since it was blocked by 67% in 1 mM TEA. On DIV 0, INa and ICa were absent or small in amplitude. By DIV 3, 80% of the cells had currents able to generate a spike. Interestingly, ICa mean amplitude and current density measured at -10 mV in control condition on DIV 1 was significantly larger than those recorded in bFGF and 25 mM KCl. The order of appearance of the ionic currents, IK, ICa, and INa, leads directly to fast spike activity allowing for poor calcium entry. Firing rate likely depends on IK(A), which increased during the first 6 days of development but could be differentially regulated by bFGF.

Ionic control of intracellular pH in rat cerebellar Purkinje cells maintained in culture

1. Intracellular pH (pHi) was measured in single rat cerebellar Purkinje cells maintained in primary culture using microspectrofluorescence analysis of the intracellularly trapped pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5 (and -6)-carboxyfluorescein (BCECF). 2. The ratio of the fluorescence signals measured at 530 nm in response to an alternating excitation at 450 and 490 nm was calibrated using the K(+)-H+ ionophore nigericin. This calibration gave a steady-state pHi of 7.06 +/- 0.02 (S.E.M., n = 17) when cells were perfused by a 5% CO2-25 mM-HCO3(-)-buffered solution at an external pH of 7.40 at 37 degrees C. 3. Replacement of external chloride with gluconate in the presence of bicarbonate induced a cytoplasmic alkalinization of about 0.3 pH unit. This alkalinization was independent of external sodium and was greatly reduced by 0.5 mM-DIDS, indicating the presence of a chloride-bicarbonate exchange. 4. In bicarbonate-free (HEPES-buffered) solution the steady-state pHi was 7.37 +/- 0.02 (n = 19), significantly higher than in bicarbonate-buffered solution. Recovery from an intracellular acid load brought about by the ammonium chloride pre-pulse technique was blocked by the removal of external sodium or the addition of 1.5 mM-amiloride, indicating the presence of a sodium-hydrogen exchange. 5. In bicarbonate-buffered solution pHi recovery after an acid load was also completely blocked by addition of 1.5 mM-amiloride indicating the absence of a bicarbonate-dependent acid extrusion mechanism. 6. Addition of 12-O-tetradecanoylphorbol-13-acetate (TPA, 100 nM) induced an amiloride-sensitive alkalinization of about 0.3 pH unit in bicarbonate-buffered solution but had no effect in HEPES-buffered solution. This observation suggests that in cultured Purkinje cells the sodium-hydrogen exchanger could be activated through a protein kinase C pathway only when pHi is maintained at a low physiological value by the activity of the chloride-bicarbonate exchange.

Calcium currents in rat cerebellar Purkinje cells maintained in culture

Calcium permeabilities were examined in large cerebellar neurons maintained in culture, and morphologically identified as Purkinje cells. When cells were supplied with a Dulbecco Minimum Eagle's Medium with 10% horse serum added (5-10 days), somatic recordings revealed complex spikes and these were shown to be generated by Na and Ca components, the Na one being tetrodotoxin-sensitive. At the dendritic level, Ca currents were better resolved than at the soma. In dendrites, Ca entry was shown to occur through at least two distinct currents. The first was a low-threshold transient current (elicited above -60 mV from a holding potential of -80 mV) which was reduced by almost 30% by 50 microM cadmium. The second was a high-threshold current (above -20 mV) which gave rise to (1) a transient component exhibiting a steady-state inactivation and so requiring holding potentials at -80 mV, and (2) a sustained component. Both components were suppressed by 50 microns cadmium. We measured a total Ca current at the dendritic level reaching values of up to 1 nA. In another culture medium (Leibovitz medium) known to allow expression of three types of calcium currents in nodose cells we observed the development of the dendritic tree of Purkinje cells but with no simultaneous expression of the high-threshold Ca current.

Potassium currents in rat cerebellar Purkinje neurones maintained in culture in L15 (Leibovitz) medium

Cerebellar Purkinje cells (PC) can be maintained in culture for one to two weeks in L15, a rich medium known to allow expression of a normal excitability in peripheral neurones. When examined using whole cell recordings, PC proved to be inexcitable in these conditions, and this inexcitability could be related to the presence of large outward K currents. Depolarizing steps of -100 mV revealed a voltage-dependent biphasic K current with a large early transient phase followed by a small plateau phase. The early transient phase could be selectively eliminated by holding the cell at -40 mV or by extracellularly applying 5 mM 4-aminopyridine (4-AP), whereas the plateau was abolished by 15 mM tetraethylammonium (TEA). Hereafter, these currents will be identified as the IA and the delayed current respectively, IA being the predominant current. IA activated between -25 and +65 mV with a midpoint at +3 mV; inactivation occurred between -70 and -20 mV with a midpoint at -57 mV. Current decay followed an exponential time course with a time constant of about 30 ms between -20 and +10 mV. In the cell-attached recording configuration, depolarization elicited openings of two types of K channels, one inactivating and one non-inactivating. The non-inactivating K channel probably corresponded to the delayed K current and had a conductance of 22 pS in a physiological K gradient.

Radioiodinated meta-iodobenzylguanidine uptake in medullary thyroid cancer. A French cooperative study

Fifty meta-iodobenzylguanidine (MIBG) scintiscans were performed in three groups of medullary thyroid cancer (MTC) patients. Group 1 (n = 11) included treated patients with normal calcitonin levels; Group 2 (n = 24) included patients with elevated calcitonin levels due to sporadic and isolated MTC; Group 3 (n = 15) included patients with elevated calcitonin levels due to familial MTC or multiple endocrine neoplasia Type IIA syndrome (MEN). In Group 1 three pheochromocytoma were depicted by MIBG scintiscan. In Group 2 MTC was seen in a small number of patients (3 of 24). In Group 3, besides adrenal hyperplasia and pheochromocytoma four patients, MIBG scintigraphy showed where MTC had localized and spread in almost half of patients (7 of 15). MIBG uptake occurred in patients with relatively high calcitonin level (greater than 0.6 nmol/l). These data indicate that in patients with familial MTC or MEN syndrome, MIBG scintiscan can be useful not only in detecting associated pheochromocytoma, but also in showing MTC.

Spontaneous and GABA-evoked chloride channels on pituitary intermediate lobe cells and their internal Ca requirements

On porcine intermediate lobe (IL) endocrine cells, spontaneously opening chloride channels have been studied and compared to GABA-A activated chloride channels. Elementary currents were recorded mainly from outside-out patches excised from IL cells maintained in culture for 1-4 weeks. Spontaneous inward currents were observed in Cs-loaded cells after replacing Na in the extracellular medium by the impermeant ion choline. This activity, at an internal calcium concentration of 10(-8) M corresponded to a channel for chloride ions with a main conductance level of 26 pS, and substates around 11 pS. The sequence of permeabilities to halides was I greater than Br greater than Cl. These conductance characteristics were common to the GABA-operated channels which also showed a main conductance substate of 23-31 pS. The open time of the 26 pS level mostly encountered in spontaneous activity, was distributed along two modes: one, the most frequent, around 1 ms, and the other around 4 ms. This latter mode was the predominant one observed during GABA and isoguvacine applications but in addition a bursting activity of 19 ms duration was also seen. Specific GABA-A receptor antagonists (bicuculline and SR42641, 1 microM) blocked activity evoked by GABA (1-10 microM), but did not affect spontaneous events. These spontaneous Cl events were only observed in a restricted range of internal Ca concentrations, i.e. between 1 nM and 0.1 microM, and were practically abolished at Cai 1 microM. The GABA-induced activity of Cl channels was also Ca-sensitive, being reduced when Cai reached 1 microM.

Postnatal development of the chemosensitivity of rat cerebellar Purkinje cells to excitatory amino acids. An in vitro study

In vitro sagittal slices of immature rat cerebellum were used to study the development of the sensitivity of Purkinje cells (PCs) to L-aspartate (L-Asp), L-glutamate (L-Glu) and related derivatives. As early as postnatal day 0 all PCs already displayed clear excitatory responses to short iontophoretic applications of L-Asp, L-Glu and quisqualate while in the same conditions no effect of N-methyl-D,L-aspartate (NMDLA) was detected. By postnatal day 5, i.e. after the onset of the synaptogenesis, the sensitivity of PCs to L-Asp, L-Glu and quisqualate significantly increased up to values similar to those recorded in adult rat cerebellum and surprisingly nearly all (87%) the recorded cells now also displayed excitatory responses to NMDLA. Although this sensitivity of PCs to NMDLA was significantly lower than that observed with the other drugs, it persisted until the end of the first postnatal month when the adult type of connectivity is already well established but at this stage only 30 per cent of the tested cells were still sensitive to the agonist. During this period, excitatory responses elicited by NMDLA were selectively antagonized by 2-amino-5-phosphonovalerate (2-APV), suggesting that during postnatal development, NMDA receptor types are transiently expressed on PCs membranes since in the adult, NMDLA no longer had an excitatory effect. Instead, this drug now exerted a preferential antagonistic action on the excitatory response elicited by L-Asp. Also in the adult, no major changes occurred in the sensitivity of PCs to L-Asp, L-Glu and quisqualate when these drugs were ejected at a dendritic site whereas, when ejected at the somatic level, the sensitivity of the cell appeared 2-3 times lower.

Intracellular effectors and modulators of GABA-A and GABA-B receptors: a commentary

The inhibitory neurotransmitter GABA activates two receptor subtypes that can be distinguished by their pharmacology. The GABA-A site is competitively antagonized by bicuculline and exclusively coupled to a chloride channel. The GABA-B receptor, for which baclofen is the only specific agonist, is resistant to bicuculline inhibition and, depending upon its localization, will activate K currents and/or inhibit Ca currents. Both electrophysiological and biochemical approaches have been applied to the study of each receptor. The membrane and intracellular components that to date have been implicated in GABA-B activation are discussed: G proteins, adenylate cyclase and intracellular calcium levels. This latter factor is also discussed with respect to GABA-A receptor action.

Effect of internal calcium concentration on calcium currents in rat sensory neurones

Using the patch-clamp technique in whole-cell configuration we have investigated the effect of increasing the internal calcium concentration (Cai) from below 10(-8) M to 10(-6) M on the three calcium currents: ICa,T (T for transient), ICa,S (S for sustained), ICa,N (N for neither), recently described in rat sensory neurones. Increasing Cai led to a dose-dependent reduction of the amplitude of ICa,S and, as Cai reached 5 X 10(-7) M ICa,S was nearly abolished. ICa,N is well evidenced from 5 X 10(-10) M to 10(-7) M where its is a large current. Preliminary observations indicate an increase of its inactivation rate following, as expected for a possible Cai dependent-inactivation, the increase of Cai from 5 X 10(-10) M to 10(-7) M. With Ca = 5 X 10(-7) M, all the cells displayed ICa,T and half of the cells in addition ICa,N, but it was of small amplitude. At Cai = 10(-6) M, most of the recorded cells only exhibited ICa,T.

IA current compared to low threshold calcium current in cranial sensory neurons

Whole-cell recordings of cranial sensory neurons were obtained with CsCl- and tetraethylammonium (TEA)-loaded cells as used for ICa studies. When replacing calcium (Ca) with potassium (K) in the external medium, a transient inward current developed at low threshold, if the holding potential was below -100 mV. Its activation started above -60 mV and reached its maximum at -20 mV. Its reversal potential followed the predicted value of the K equilibrium potential (EK) characteristic of a K current. Pharmacologically, it resembles the classical outward A current since it is insensitive to TEA up to 20 mM and blocked by 4-aminopyridine (4-AP) and 3,4-AP in the millimolar range. This inward A current rises rapidly in less than 5 ms and decays monoexponentially with a time constant of about 60 ms at all potentials studied. Its steady-state inactivation occurred between -100 and -60 mV with a half-inactivation at -86 mV. All these characteristics hold for a voltage-dependent A current devoid of any Ca-dependent component. The activation and inactivation of this A inward current are compared to those of ICa,t, the other low threshold current encountered on those cells. The contribution of the two currents in the control of discharge frequency is discussed.

Altered axonal transport of cytoskeletal proteins in the mutant diabetic mouse

Polypeptides in the motor axons of the sciatic nerve in 120-day-old normal and diabetic mice C57BL/Ks (db/db) were labeled by injection of [35S]methionine into the ventral horn of the spinal cord. At 8, 15, and 25 days after the injection, the distribution of radiolabeled polypeptides along the sciatic nerve was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Four major radiolabeled polypeptides, tentatively identified as actin, tubulin, and the two lightest subunits of the neurofilament triplet, were studied in both diabetic and control mice. In the diabetic animals, the two polypeptides identified as actin and tubulin showed a reduction of average velocity of migration along the sciatic nerve, resulting in a higher fraction of radioactivity in the proximal part of the sciatic nerve, whereas the front of radioactivity (advancing at maximal velocity) moved at a normal rate. In contrast, both the average and maximal velocities of the two neurofilament subunits were slower in the diabetic mice than in the control mice. These results indicate that the axonal transport of the cytoskeletal proteins is differentially affected in the course of diabetic neuropathy, and may suggest that the impairment concerns mainly the proteins carried by the slowest component of axonal transport.

Electrophysiological studies on the postnatal development of intracerebellar nuclei neurons in rat cerebellar slices maintained in vitro. I. Postsynaptic potentials

The development of the synaptic responses of intracerebellar nuclei neurons was studied in the rat by the use of thick sagittal cerebellar slices maintained in vitro. It has been shown that functional excitatory synapses are present on these neurons from birth, probably due to climbing and/or mossy fiber collaterals; functional inhibitory synapses, due to monosynaptic projections of Purkinje cell axons onto intracerebellar nuclei, are present as early as postnatal day 2; and a more complex pattern of synaptic responses, including short latency excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs), longer latency IPSPs, and late depolarizing responses, can be elicited in nuclear neurons as early as postnatal day 3, indicating an early development of some complete functional cerebellar circuits involving the intracerebellar nuclei.

Effect of excitatory amino acids on Purkinje cell dendrites in cerebellar slices from normal and staggerer mice

The sensitivity of Purkinje cells to short pulse applications of L-aspartate, L-glutamate and related derivatives in their dendritic fields was tested in normal and staggerer mutant mice using cerebellar slices maintained in vitro. In normal mice, the response of Purkinje cells to L-aspartate and L-glutamate consisted of a transient and dose-dependent increase of their firing of simple spikes. The potency of L-aspartate in exciting Purkinje cells was lower than that of L-glutamate when the two drugs were released from adjacent barrels of the same iontophoretic electrode. Quisqualate was an even more potent excitant of these cells than L-aspartate and L-glutamate, whereas N-methyl-DL-aspartate had little or no effect. In staggerer mutant mice, the sensitivity of Purkinje cells to L-aspartate, L-glutamate and quisqualate was not significantly altered. On the contrary, N-methyl-DL-aspartate had a much stronger potency than normal in exciting Purkinje cells although this was still smaller than that of the other agonists tested. These results suggest that the sensitivity of Purkinje cells to L-aspartate and L-glutamate, i.e. the putative neurotransmitters of the climbing and parallel fibers respectively, remains largely normal in staggerer mice. In contrast, in the mutant, N-methyl-D-aspartate receptors are likely to be much more developed than normal.

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

The bioelectrical properties of Purkinje cells (intrasomatic recordings) were studied in sagittal cerebellar slices of both adult staggerer and control mice. Mean input resistances of Purkinje cells were 25 +/- 4 M omega and 48 +/- 7 M omega in normal and staggerer mice respectively. In both groups, time-dependent inward rectifications were apparent in the hyperpolarizing voltage-responses. In normal mice, tetrodotoxin (TTX)-sensitive simple spikes and slower-rising multiphasic spikes, abolished when Ca was replaced by Cd in the bath, spontaneously occurred in Purkinje cells. These Na- and Ca-dependent spikes were also elicited by depolarizing current pulses. In the mutant, Ca spikes were never observed, even in strongly depolarized cells. On the contrary, TTX-sensitive simple spikes occurred spontaneously or were elicited by depolarizing current pulses. When Ca was replaced by Ba in the bath, the Ca spikes evoked in normal Purkinje cells by direct stimulation were first enhanced and then replaced by prolonged action potentials (1-6 s in duration) which were TTX-resistant and Cd-sensitive. These (Ba) action potentials were also triggered by climbing fibre activation of the cells. In staggerer mice, Ca spikes were never elicited by direct stimulation in Ba-containing medium, although in a few cells prolonged action potentials were occasionally elicited by depolarizing current pulses. However, this latter type of response was never evoked by climbing fibre activation of Purkinje cells. In the mutant, extracellular application of tetraethylammonium (TEA) generated prolonged action potentials, the plateaux of depolarization of which were less positive than those elicited by Ba in control mice. These plateaux were abolished by TTX and left unaffected by the substitution of Ca by Cd in the bath, suggesting that they were due to a non-inactivating Na conductance. On the whole, the present study strongly suggests that voltage-dependent Ca channels are missing in most staggerer Purkinje cells or at least that their characteristics and/or distribution are such that they cannot be activated. Na channels appear unaffected.

Voltage clamp analysis of the effect of excitatory amino acids and derivatives on Purkinje cell dendrites in rat cerebellar slices maintained in vitro

A voltage clamp analysis of the effects of L-aspartate, L-glutamate and related derivatives on Purkinje cell dendrites was performed in rat cerebellar slices maintained in vitro. Short iontophoretic pulse applications of L-aspartate and L-glutamate in the dendritic field of Purkinje cells induced dose-dependent inward currents with fast onset and recovery. Quisqualate application also gave rise to well developed inward currents with fast onset and slow recovery, whereas N-methyl-D,L-aspartate had no or little effect on Purkinje cell membranes unless prolonged (several seconds) applications were used. Steady applications of low doses of N-methyl-D,L-aspartate much more severely depressed L-aspartate than L-glutamate mediated responses, whereas inward currents due to quisqualate were unaffected. Inward currents due to quisqualate were often more reduced than those due to L-aspartate by steady applications of 2-amino-5-phosphonovalerate, and the antagonistic action of this drug on responses due to L-glutamate was very weak. These results suggest that receptors of Purkinje cells for glutamate and aspartate are different, and are also different from N-methyl-D-aspartate and quisqualate receptors.

Bioelectrical properties of cerebellar Purkinje cells in reeler mutant mice

Electroresponsive properties of intracellularly recorded Purkinje cells (PCs) in reeler mice were studied by using sagittal cerebellar slices maintained in vitro. In normally as well as in abnormally located PCs, fast spikes and slower rising multiphasic spikes were elicited by depolarizing currents, and they were abolished by tetrodotoxin and by cadmium respectively. This demonstrates that these neurons, as PCs in normal animals, do have sodium- and calcium-dependent spikes, thus indicating that these bioelectrical properties do not depend on the connectivity of PCs.

Olivocerebellar projections in control and staggerer mutant mice. An autoradiographic study

Olivocerebellar projections were studied in normal and in staggerer mutant mice by means of autoradiographic methods following [3H]leucine injections in the inferior olivary complex. In normal mice, a sagittal banding pattern of labeled climbing fibers was apparent in the contralateral cerebellar cortex, thus confirming previous observations in rodents. In staggerer mice, olivocerebellar projections also displayed a clear sagittal zonation, thus indicating that this organization may be achieved in the mutant, in spite of pronounced abnormalities of the cerebellar cortex.

Autoradiographic study of the distribution of olivocerebellar connections during the involution of the multiple innervation of Purkinje cells by climbing fibers in the developing rat

The distribution of olivocerebellar projections was studied in 6--13-day-old rats using an autoradiographic method, to determine whether the adult-type sagittal banding pattern of these projections was already settled. Clear sagittal-labeled bands were seen within the cerebellar cortex as already as 6 days when all Purkinje cells are multiinnervated by climbing fibers and in older animals, where most Purkinje cells are monoinnervated. These data suggest that the sagittal zonation of the olivocerebellar connections is already settled in early development.

Fate of the multiple innervation of cerebellar Purkinje cells by climbing fibers in immature control, x-irradiated and hypothyroid rats

The fate of the multiple innervation of Purkinje cells (PCs) by climbing fibers (CFs) was studied as a function of age in immature rats rendered agranular by X-irradiation, in immature hypothyroid rats, and compared to that in controls. This was done by examining in each group the intracellular activities of PCs mediated via CFs throughout maturation. From the third day in control rats, CF responses of PCs evoked by juxta fastigial region (JF) stimulation or occurring spontaneously already resembled the adult responses with, however, some important differences: (1) most of these responses were graded by steps with the intensity of the stimulation before day 13, due to the multiple innervation of PCs by CFs (see below); (2) immature CF responses exhibited a longer duration and their initial spike started near the peak of the EPSP instead of near the baseline later on. Finally, an anlage of CF response was already present in most PCs on day 2, and consisted of a single fast spike elicited near the peak of an underlying all-or-none EPSP. In the 3 groups of rats, CF EPSPs already closely resembled the adult ones as early as 3 days, although their total duration and especially their time to peak were longer. In control rats, these CF EPSPs reversed with depolarizing currents from day 3 and currents for reversal were much lower than in the adult. 'Dual' CF EPSPs of PCS37 were encountered in immature 7- to 10-day-old controls, and persisted in hypothyroid rats until the end of the third postnatal week. The mono- or the multiple innervation of PCs by CFs was ascertained in th 3 groups according to the graded or the all-or-none character of CF EPSPs, and the number of CFs impinging on a given PC was estimated by the number of steps in the response. In control rats, most of PCs were already multiply innervated by CFs as early as 3 days. The multiple innervation culminated on day 5 with an average number of 3.4 CFs for PC, and rapidly regressed later on, so that the adult-type monoinnervation was the rule after day 13. In hypothyroid rats, the establishment of the redundancy and its regression was delayed by 2--3 days. In X-irradiated rats, the settlement and the involution of the multiple innervation of PCs by CFs was exactly superimposed with that seen in controls until day 8. Later on, regression of the supernumerary contacts no longer occurred so that most PCs remained multiply innervated until adulthood. Finally, the first clear-cut IPSPs were detected in PCs on day 9 in control and X-irradiated rats and 2--3 days later in hypothyroid animals.

Correlations among climbing fiber responses of nearby cerebellar Purkinje cells in the immature rat

Correlations between pairs of spontaneous climbing fiber responses (CFRs) recorded from couples of nearby Purkinje cells (PCs) were studied in immature rats by using cross-correlograms between CFR pairs, and compaired to those in adult animals. Correlations were found as early as day 3. Some days later, on PN days 7--9, these correlations were higher than in the adult. In most cases, this was apparently not due to the multiple innervation of PC by climbing fibers (CFs) which normally occurs during this immature stage since: 1) temporal relationships between the paired CFRs varied by more than 30 ms and 2) thresholds for pairs of graded CFRs and additional components of the responses evoked in the 2 PCs by juxtafastigial or olivary stimulation were different. Synchronizing mechanisms were therefore likely to be already located at the olivary level. However, in 3 couples of multiply innervated PCs whose spontaneous CF activities were highly correlated, stimulation experiments revealed a common innervation of the 2 cells by branches of the same CF. In multiply innervated cells, spontaneous responses mediated through distinct CFs were also synchronized, suggesting that these fibers originate from neighboring neurons of the inferior olive. Finally, in 7 to 9-day-old rats, correlations among CFR pairs were much more restricted in the longitudinal axis of the folium than in the transverse one. On the whole, the present study indicates that correlations among CFRs of nearby PCs exist as soon as CF-PC synapses are established and the latter are already organized in sagittal strips at early stages of development.