yes / Electrophysiological properties and molecular background of the zebrafish (Danio rerio) cardiac inward rectifier current (IK1) were examined. Ventricular myocytes of zebrafish have a robust (−6.7±1.2 pA pF−1 at −120 mV) strongly rectifying and Ba2+-sensitive (IC50=3.8 μM) IK1. Transcripts of six Kir2 channels (drKir2.1a, drKir2.1b, drKir2.2a, drKir2.2b, drKir2.3, and drKir2.4) were expressed in the zebrafish heart. drKir2.4 and drKir2.2a were the dominant isoforms in both the ventricle (92.9±1.5 and 6.3±1.5 %) and the atrium (28.9±2.9 and 64.7±3.0 %). The remaining four channels comprised together less than 1 and 7 % of the total transcripts in ventricle and atrium, respectively. The four main gene products (drKir2.1a, drKir2.2a, drKir2.2b, drKir2.4) were cloned, sequenced, and expressed in HEK cells for electrophysiological characterization. drKir2.1a was the most weakly rectifying (passed more outward current) and drKir2.2b the most strongly rectifying (passed less outward current) channel, whilst drKir2.2a and drKir2.4 were intermediate between the two. In regard to sensitivity to Ba2+ block, drKir2.4 was the most sensitive (IC50=1.8 μM) and drKir2.1a the least sensitive channel (IC50=132 μM). These findings indicate that the Kir2 isoform composition of the zebrafish heart markedly differs from that of mammalian hearts. Furthermore orthologous Kir2 channels (Kir2.1 and Kir2.4) of zebrafish and mammals show striking differences in Ba2+- sensitivity. Structural and functional differences needs to be taken into account when zebrafish is used as a model for human cardiac electrophysiology, cardiac diseases, and in screening cardioactive substances.

The principal neurons of the medial superior olive (MSO) are considered to be responsible for transforming the temporal information present in the binaural acoustic stimulus into an output encoding sound location along the horizontal axis. Spatial resolution of sound localization depends critically on the time resolution with which MSO neurons can detect microsecond differences in the timing of inputs from the two ears. This fast temporal processing is contingent on voltage gated ion channels. The work presented in this thesis demonstrates that two currents, namely a hyperpolarization activated cationic current and low voltage activated potassium current dynamically interact to regulate the intrinsic time resolution of MSO neurons. We observe that the ability of MSO neurons to perform sub-millisecond temporal processing matures after birth, especially around the time of the clearing of the auditory canal. Hyperpolarization activated cationic current was found to be one of the underlying mechanisms transforming slow immature MSO neurons into temporally precise adult MSO neurons. / text

No description available.

<p>During cognitive tasks, synchronicity of neural activity varies and is correlated with performance. However, there may be an upper limit to normal synchronised activity – specifically, epileptogenic activity is characterized byexcess spiking at high synchronicity. An epileptic seizure has a complicated course of events and I therefore focused on the synchronised activity preceding a seizure (fast ripples). These high frequency oscillations (200–1000 Hz) have been identified as possible signature markers of epileptogenic activity and may be involved in generating seizures. Moreover, a range of ionic currents have been suggested to be involved in distinct aspects of epileptogenesis. Based on pharmacological and genetic studies, potassium currents have been implicated, in particular the transient A–type potassium channel (KA). Our first objective was to investigate if KA could suppress synchronized input while minimally affecting desynchronised input. The second objective was to investigate if KA could suppress fast ripple activity. To study this I use a detailed compartmental model of a hippocampal CA1 pyramidal cell. The ion channels were described by Hodgkin–Huxley dynamics.</p><p>The result showed that KA selectively could suppress highly synchronized input. I further used two models of fast ripple input and both models showed a strong reduction in the cellular spiking activity when KA was present. In an ongoing in vitro brain slice experiment our prediction from the simulations is being tested. Preliminary results show that the cellular response was reduced by 30 % for synchronised input, thus confirming our theoretical predictions. By suppressing fast ripples KA may prevent the highly synchronised spiking activity to spread and thereby preventing the seizure. Many antiepileptic drugs down regulate cell excitability by targeting sodium channels or GABA–receptors. These antiepileptic drugs affect the cell during normal brain activity thereby causing significant side effects. KA mainly suppresses the spiking activity when the cell is exposed to abnormally high synchronised input. An enhancement in the KA current might therefore be beneficial in reducing seizures while not affecting normal brain activity.</p>

During cognitive tasks, synchronicity of neural activity varies and is correlated with performance. However, there may be an upper limit to normal synchronised activity – specifically, epileptogenic activity is characterized byexcess spiking at high synchronicity. An epileptic seizure has a complicated course of events and I therefore focused on the synchronised activity preceding a seizure (fast ripples). These high frequency oscillations (200–1000 Hz) have been identified as possible signature markers of epileptogenic activity and may be involved in generating seizures. Moreover, a range of ionic currents have been suggested to be involved in distinct aspects of epileptogenesis. Based on pharmacological and genetic studies, potassium currents have been implicated, in particular the transient A–type potassium channel (KA). Our first objective was to investigate if KA could suppress synchronized input while minimally affecting desynchronised input. The second objective was to investigate if KA could suppress fast ripple activity. To study this I use a detailed compartmental model of a hippocampal CA1 pyramidal cell. The ion channels were described by Hodgkin–Huxley dynamics. The result showed that KA selectively could suppress highly synchronized input. I further used two models of fast ripple input and both models showed a strong reduction in the cellular spiking activity when KA was present. In an ongoing in vitro brain slice experiment our prediction from the simulations is being tested. Preliminary results show that the cellular response was reduced by 30 % for synchronised input, thus confirming our theoretical predictions. By suppressing fast ripples KA may prevent the highly synchronised spiking activity to spread and thereby preventing the seizure. Many antiepileptic drugs down regulate cell excitability by targeting sodium channels or GABA–receptors. These antiepileptic drugs affect the cell during normal brain activity thereby causing significant side effects. KA mainly suppresses the spiking activity when the cell is exposed to abnormally high synchronised input. An enhancement in the KA current might therefore be beneficial in reducing seizures while not affecting normal brain activity.

Le système rénine-angiotensine est impliqué dans le remodelage structurel et électrique caractérisant la fibrillation auriculaire (FA). L’angiotensine II (ANG II) induit le développement de fibrose et d’hypertrophie au niveau des oreillettes, prédisposant à la FA. Or, les mécanismes électrophysiologiques par lesquels l’ANG II pourrait promouvoir la FA sont peu connus. L’objectif de ce projet de recherche est d’évaluer l’effet de l’ANG II sur les courants potassiques et calciques au niveau auriculaire indépendamment du remodelage structurel. Pour ce faire, nous avons utilisé la technique de patch-clamp avec un modèle de souris surexprimant le récepteur de type 1 à l’angiotensine II (AT1R) spécifiquement au niveau cardiaque. Pour distinguer les effets directs de la surexpression d’AT1R des effets induits par le remodelage cardiaque, nous avons étudié des souris âgées de 180 jours, qui présentent du remodelage structurel, et des souris âgées de 50 jours, qui n’en présentent pas. Des études précédentes sur ce modèle ont montré qu’au niveau des myocytes ventriculaires, l’ANG II réduit le courant potassique global (Ipeak) et rectifiant entrant (IK1) ainsi que le courant calcique de type L (ICaL). Ainsi, notre hypothèse est que l’ANG II modulera aussi ces courants au niveau auriculaire, pouvant ainsi augmenter l’hétérogénéité de repolarisation auriculaire et de ce fait le risque de développer et maintenir la FA. Nous avons observé une diminution significative de la densité d’IK1 dans l’oreillette gauche des souris transgéniques sans changement d’Ipeak. De plus, la densité d’ ICaL n’est pas réduite chez les souris transgéniques âgées de 50 jours. En conclusion, l’effet de l’ANG II sur les courants potassiques et calciques semble dépendre de la chambre cardiaque. En effet, nous savions que l’ANGII réduisait Ipeak, IK1 et ICaL au niveau ventriculaire, mais nos résultats ont montré qu’il ne les affectait pas directement au niveau des oreillettes. Ceci suggère des mécanismes de régulation impliquant des voies de signalisation distinctes selon les chambres cardiaques. Enfin, nos résultats montrant l’absence de l’influence directe de la surexpression d’AT1R sur les canaux K+ et Ca2+ au niveau des myocytes auriculaires renforcent l’importance d’approfondir nos connaissances sur les effets de l’angiotensine II sur le développement de la fibrose, sur le remodelage structurel et sur la conduction électrique cardiaque. / The renin-angiotensin-aldosterone system contributes to the structural and electrical remodelling that characterise atrial fibrillation (AF). Angiotensin II (ANG II) induces fibrosis and hypertrophy in the atrium, creating a substrate for AF development. Whether or not ANG II promotes electrophysiological remodelling in the atrium and by which mechanisms is not known. The objective of this research project is to evaluate the effect of ANG II on potassium and calcium currents in atrial myocytes independently of structural remodelling. We used the patch-clamp technique to measure ionic currents in a mouse model overexpressing the angiotensin II type 1 receptor (AT1R) locally in the heart. To differentiate the direct effects of AT1R overexpression with those related to structural remodelling, we studied mice aged of 180 days that are characterised by structural remodelling and mice aged of 50 days that aren’t. Previous studies have shown that this mouse presented with reduced total potassium current (Ipeak) and inward rectifier potassium current (IK1) as well as with a decrease in L-type calcium currents (ICaL) in ventricular myocytes. Therefore, our hypothesis was that ANG II would also regulate those currents at the atrial level, possibly leading to an increase in heterogeneity in atrial repolarisation and to an increase in AF initiation and maintenance. We show in this project a significant reduction of the IK1 in the left atrium of transgenic mice without any changes of the Ipeak. Furthermore, we show no modulation of ICaL density in 50 days old transgenic mice. We conclude that ANG II effect on potassium and calcium currents depends on the cardiac chamber. Indeed, although we already knew ANG II reduced Ipeak, IK1 and ICaL in ventricular myocytes of transgenic mice, in this project we found ANG II did not affect Ipeak and ICaL at the atrial level. These findings suggest distinctive regulation pathways by which ANG II affects the different cardiac chambers. Furthermore, the absence of direct influence of ANG II on potassium and calcium currents in atrial myocytes reinforces the importance to better understand ANG II’s effect on cardiac fibrosis development, structural remodelling and electric conduction.

No description available.

No description available.