Céline Roussel

Intracellular calcium regulation by burst discharge determines bidirectional long-term synaptic plasticity at the cerebellum input stage.

Variations in intracellular calcium concentration ([Ca2+]i) provide a critical signal for synaptic plasticity. In accordance with Hebbs postulate (Hebb, 1949), an increase in postsynaptic [Ca2+]i can induce bidirectional changes in synaptic strength depending on activation of specific biochemical pathways (Bienenstock et al., 1982; Lisman, 1989; Stanton and Sejnowski, 1989). Despite its strategic location for signal processing, spatiotemporal dynamics of [Ca2+]i changes and their relationship with synaptic plasticity at the cerebellar mossy fiber (mf)-granule cell (GrC) relay were unknown. In this paper, we report the plasticity/[Ca2+]i relationship for GrCs, which are typically activated by mf bursts (Chadderton et al., 2004). Mf bursts caused a remarkable [Ca2+]i increase in GrC dendritic terminals through the activation of NMDA receptors, metabotropic glutamate receptors (probably acting through IP3-sensitive stores), voltage-dependent calcium channels, and Ca2+-induced Ca2+ release. Although [Ca2+]i increased with the duration of mf bursts, long-term depression was found with a small [Ca2+]i increase (bursts <250 ms), and long-term potentiation (LTP) was found with a large [Ca2+]i increase (bursts >250 ms). LTP and [Ca2+]i saturated for bursts >500 ms and with theta-burst stimulation. Thus, bursting enabled a Ca2+-dependent bidirectional Bienenstock-Cooper-Munro-like learning mechanism providing the cellular basis for effective learning of burst patterns at the input stage of the cerebellum.

Effects of remifentanil on N-methyl-D-aspartate receptor : An electrophysiologic study in rat spinal cord

Background:Remifentanil hydrochloride contained in Ultiva® (GlaxoSmithKline, Genval, Belgium) has been incriminated in difficult postoperative pain management, promotion of hyperalgesia, and direct N-methyl-d-aspartate (NMDA) receptor activation, but the involved mechanisms have remained unclear. In the current study, the authors investigated the effects of remifentanil hydrochloride, with and without its vehicle, glycine, on the activation of NMDA receptors and the modulation of NMDA-induced current on neurons inside the lamina II from the dorsal horn of rat spinal cord. Methods:To test these effects, whole cell patch clamp recordings were conducted on acute rat lumbar spinal cord slices. Considering that both components of Ultiva® (remifentanil hydrochloride and glycine) could be involved in NMDA receptor activation, experiments were performed first with remifentanil hydrochloride, second with glycine, and third with the two components within Ultiva®. Results:Remifentanil hydrochloride does not induce any current, whereas 3 mm glycine induced a current that was abolished by the specific NMDA glutamate site antagonist d-2-amino-5-phosphonovalerate. Ultiva® (remifentanil hydrochloride with its vehicle, glycine) also evoked an inward current that was abolished by d-2-amino-5-phosphonovalerate and not significantly different from the glycine-induced current. Application of remifentanil hydrochloride potentiated the NMDA-induced inward current, and this potentiation was abolished by the &mgr;-opioid receptor antagonist naloxone. Conclusion:These results show that remifentanil hydrochloride does not directly activate NMDA receptors. The NMDA current recorded after application of Ultiva® is related to the presence of glycine. Induced NMDA current is potentiated by application of remifentanil hydrochloride through a pathway involving the &mgr;-opioid receptor.

Targeted calretinin expression in granule cells of calretinin-null mice restores normal cerebellar functions

Ca2 binding proteins such as calretinin, characterized by the presence of EF‐hand motifs that bind Ca2+ ions, are involved in the shaping of intraneuronal Ca2+ fluxes. In the cerebellar cortex, information processing tightly relies on variations in intracellular Ca2+ concentration in Purkinje and granule cells. Calretinin‐deficient (Cr−/−) mice present motor discoordination, suggesting cellular and network cerebellar dysfunctions. To determine the cell specificity of these alterations, we constructed transgenic Cr−/− mice exhibiting a selective reexpression of calretinin in granule cells through the promoter function of the GABAA receptor α6 subunit gene. Normal granule cell excitability and wild‐type Purkinje cell firing behavior in awake mice were restored while the emergence of high‐frequency oscillations was abolished. Behavioral analysis of these calretinin‐rescue mice revealed that normal motor coordination was restored as compared with Cr−/− mice. These results demonstrate that calretinin is required specifically in granule cells for correct computation in the cerebellar cortex and indicate that the finetuning of granule cell excitability through regulation of Ca2+ homeostasis plays a crucial role for information coding and storage in the cerebellum.

Role of calcium binding proteins in the control of cerebellar granule cell neuronal excitability: experimental and modeling studies.

Calcium binding proteins, such as calretinin, are abundantly expressed in distinctive patterns in the central nervous system but their physiological function remains poorly understood. Calretinin is expressed in cerebellar granule cells which provide the major excitatory input to Purkinje cells through parallel fibers. Calretinin deficient mice exhibit dramatic alterations in motor coordination and in Purkinje cell firing recorded in vivo through unknown mechanisms. In the present paper, we review the results obtained with the patch clamp recording techniques in acute slice preparation. This data allow us to investigate the effect of a null mutation of the calretinin gene on the intrinsic electroresponsiveness of cerebellar granule cells at a mature developmental stage. Calretinin deficient granule cells exhibit faster action potentials and generate repetitive spike discharge showing an enhanced frequency increase with injected currents. These alterations disappear when 0.15 mM of the exogenous fast calcium buffer BAPTA is infused in the cytosol to restore the calcium buffering capacity. Furthermore, we propose a mathematical model demonstrating that the observed alterations of granule cell excitability can be explained by a decreased cytosolic calcium buffering capacity due to the absence of calretinin. We suggest that calcium binding proteins modulate intrinsic neuronal excitability and may therefore play a role in the information processing in the central nervous system.

Activation of protein kinase C and inositol 1,4,5-triphosphate receptors antagonistically modulate voltage-gated sodium channels in striatal neurons

Regulation of voltage-gated sodium channels is crucial to firing patterns that constitute the output of medium spiny neurons (MSN), projecting neurons of the striatum. This modulation is thus critical for the final integration of information processed within the striatum. It has been shown that the adenylate cyclase pathway reduces sodium currents in MSN through channel phosphorylation by cAMP-dependent protein kinase. However, it is unknown whether a phospholipase C (PLC)-mediated signaling cascade could also modulate voltage-gated sodium channels within MSN. Using the whole-cell patch clamp technique, we investigated the effects of activation of two key components in PLC-mediated signaling cascades: protein kinase C (PKC) and inositol-1,4,5-triphosphate (IP(3)) receptors on voltage-dependent sodium current. Cellular dialysis with phorbol 12-myristate 13-acetate, an activator of PKC, significantly reduced peak sodium current amplitude, while adenophostin A, an activator of IP(3) receptors, significantly increased peak sodium current amplitude. This effect of adenophostin was abolished by calcium chelation or by FK506, an inhibitor of calcineurin. These results suggest an antagonistic role of PKC and IP(3) in the modulation of striatal voltage-gated sodium channels, peak current amplitude being decreased through phosphorylation by PKC and increased through dephosphorylation by calcineurin.

Dynamic Control of Neuronal Firing Threshold by Calcium Buffering: A New Role for Calcium Binding Proteins

We have investigated the detailed regulation of neuronal firing threshold by the cytosolic calcium buffering capacity using a combination of mathematical modeling and patch clamp recording in acute slice. Theoretical results show that, at similar free calcium concentration, increased calcium buffer concentration lowers the firing threshold of cerebellar granule cells. We show that this effect is a direct consequence of the major slowdown of calcium dynamics. Patch clamp recordings on cerebellar granule cells loaded with a high concentration of the fast calcium buffer BAPTA (15 mM) reveal alterations in the excitability threshold as compared to cells loaded with 0.15 mM BAPTA. In high calcium buffering conditions, granule cells exhibit a significative lower firing threshold. These results suggest that cytosolic calcium buffering capacity can tightly modulate neuronal firing threshold and therefore that calcium-binding proteins may play a critical role in the information processing in the central nervous system.

Morphological Alterations in the Cerebellar Granule Cell Layer of Mice Lacking Calretinin

Calcium binding proteins, such as calretinin, are abundantly expressed in distinctive patterns in the central nervous system, but their physiological functions remain poorly understood. Calretinin is expressed in cerebellar granule cells and calretinin deficient mice suffer from alterations in motor coordination. Using confocal microscopy, we demonstrate that calretinin deficient mice exhibit a significantly decreased density of granule cells at the level of the cerebellar cortex. Moreover, it has been shown that migration of granule cells is tighly associated with intracellular calcium fluctuations. Therefore, we hypothesize that the perturbation of the calcium dynamics in calretinin deficient mice may be the cause of the observed morphological alterations. To test this assumption, we are currently developping two strategies.First, using confocal microscopy and cerebellar microexplant cultures, we are studying calcium transients occuring during granule cell migration in wild type and calretinin knock-out mice. On the other hand, we are developping a dedicated computational model for [Ca2+]i transients that takes into account calcium fluxes through the plasma and ER membrane. This model will shed light on the possible mechanism responsible for the modulation by calretinin of calcium dynamics during granule cell migration.