La turbidité de la rivière Tasiapik (Umiujaq, Nunavik) en lien avec la dégradation du pergélisol

Les causes de la variation de la turbidité des cours d'eau ont été examinées dans le cadre de plusieurs études, et ce,dans de nombreuses régions du monde. Toutefois, les connaissances sur les liens entre la variation de la turbidité et la dégradation du pergélisol sont limitées. Les travaux menés dans les régions arctiques et subarctiques démontrent l'accélération de la dégradation du pergélisol durant les dernières décennies, provoquant des tassements importants du sol et ainsi, un accroissement du fluage d'eau chargée de sédiments vers les lacs et les cours d'eau. Cette étude vise à mieux comprendre la variation spatio-temporelle de la turbidité fluviale en contexte périglaciaire dans le but de contribuer aux connaissances globales des impacts des changements climatiques sur les processus hydrogéomorphologiques des régions nordiques. Des données sur la turbidité d'un cours d'eau et ses affluents, situés à 5 km à l'est du village d'Umiujaq (Nunavik), ont été récoltées lors de l'été 2019 dans six sites présentant des caractéristiques géomorphologiques distinctes. Des analyses statistiques ont été réalisées sur ces enregistrements, en lien avec des données de températures atmosphériques, de précipitations et de débit disponibles. Les résultats ont démontré l'importance de ces paramètres dans la variation de la turbidité de la rivière Tasiapik. Les analyses statistiques indiquent qu'il existe une corrélation significative entre les températures atmosphériques et la turbidité de la rivière, avec des coefficients de corrélation de Pearson atteignant 0,72. Ces relations sont plus ou moins importantes selon l'emplacement échantillonné sur la rivière ainsi que la période de l'année : l'influence des températures atmosphériques sur la turbidité est plus forte au début de l'été, suite à la fonte nivale. Bien que les conditions météorologiques jouent un rôle important dans la variation de la turbidité, il s'est avéré difficile de prédire la charge sédimentaire de la rivière en se basant seulement sur ces paramètres. Les caractéristiques du site telles que la topographie, la nature des dépôts de surface ainsi que la présence de lacs thermokarstiques sont des facteurs importants dans la dynamique du transport sédimentaire de la rivière Tasiapik

Factors affecting river turbidity in a degrading permafrost environment : the Tasiapik River, Umiujaq (Nunavik)

This study focuses on spatiotemporal changes in water turbidity in relation to permafrost to document the impact of meteorological conditions and water flow on hydro-sedimentary processes in northern regions. Starting in June of 2019, water turbidity data were collected at six sites along the Tasiapik River (Nunavik). A statistical analysis was completed based on records of water turbidity, precipitation, water flow, and air temperature. Our results show a significant correlation between air temperatures and turbidity, with a correlation of up to r = 0.59. These correlations depend on the location of the site along the river and the time of the study period (June–October 2019). The flow rate was the primary factor that caused variations in the turbidity of the Tasiapik River. Our results showed that following an increase in flow rate, there was an almost simultaneous increase in turbidity due to erosion of the banks. The duration and intensity of precipitation events are also important factors affecting the process of sediment transport. Even though meteorological conditions play an important role in turbidity variation, other characteristics of the site such as the topography and the existence of thermokarst lakes are additional factors that influence the dynamics of sediment transport in the Tasiapik River.Les travaux menés en Arctique et Subarctique démontrent une accélération de la dégradation du pergélisol durant les dernières décennies, provoquant des tassements importants du sol et par le fait même, un accroissement du fluage d’eau chargée de sédiments vers les lacs et les rivières. Cette étude vise à mieux comprendre la variation spatio-temporelle de la turbidité fluviale en contexte périglaciaire dans le but de faire avancer les connaissances sur les impacts des conditions météorologiques et du débit sur les processus hydrosédimentaires des régions nordiques. Des données de turbidité de l’eau de la rivière Tasiapik, située à 5 km à l’est du village d’Umiujaq (Nunavik), ont été enregistrées de juin à octobre 2019 dans six sites distincts. Des analyses statistiques réalisées sur ces enregistrements indiquent qu’il existe une corrélation significative (r = 0,59) entre les températures de l’air et la turbidité de la rivière. Ces relations sont plus ou moins importantes selon l’emplacement du site le long de la rivière et selon le moment de la période d’étude. Le débit était le principal facteur à l’origine des variations de la turbidité de la rivière Tasiapik. Nos résultats ont montré qu’à la suite d’une augmentation du débit, il y a eu une augmentation presque simultanée de la turbidité due à l’érosion des berges et de la quantité des sédiments en suspension. La durée et l’intensité des précipitations sont également des facteurs importants ayant fait varier la turbidité de la rivière. Bien que les conditions météorologiques jouent un rôle important dans la variation de la turbidité, il s’est avéré que les caractéristiques du site telles que la topographie et la présence de lacs thermokarstiques sont des facteurs importants dans la dynamique du transport sédimentaire de la rivière Tasiapik

Parvalbumin Interneurons of Hippocampus Tune Population Activity at Theta Frequency

SummaryHippocampal theta rhythm arises from a combination of recently described intrinsic theta oscillators and inputs from multiple brain areas. Interneurons expressing the markers parvalbumin (PV) and somatostatin (SOM) are leading candidates to participate in intrinsic rhythm generation and principal cell (PC) coordination in distal CA1 and subiculum. We tested their involvement by optogenetically activating and silencing PV or SOM interneurons in an intact hippocampus preparation that preserves intrinsic connections and oscillates spontaneously at theta frequencies. Despite evidence suggesting that SOM interneurons are crucial for theta, optogenetic manipulation of these interneurons modestly influenced theta rhythm. However, SOM interneurons were able to strongly modulate temporoammonic inputs. In contrast, activation of PV interneurons powerfully controlled PC network and rhythm generation optimally at 8 Hz, while continuously silencing them disrupted theta. Our results thus demonstrate a pivotal role of PV but not SOM interneurons for PC synchronization and the emergence of intrinsic hippocampal theta

Desynchronization of Neocortical Networks by Asynchronous Release of GABA at Autaptic and Synaptic Contacts from Fast-Spiking Interneurons

An activity-dependent long-lasting asynchronous release of GABA from identified fast-spiking inhibitory neurons in the neocortex can impair the reliability and temporal precision of activity in a cortical network

A Characterization of the Electrophysiological and Morphological Properties of Vasoactive Intestinal Peptide (VIP) Interneurons in the Medial Entorhinal Cortex (MEC)

<jats:p>Circuit interactions within the medial entorhinal cortex (MEC) translate movement into a coherent code for spatial location. Entorhinal principal cells are subject to strong lateral inhibition, suggesting that a disinhibitory mechanism may drive their activation. Cortical Vasoactive Intestinal Peptide (VIP) expressing inhibitory neurons are known to contact other interneurons and excitatory cells and are thus capable of providing a local disinhibitory mechanism, yet little is known about this cell type in the MEC. To investigate the electrophysiological and morphological properties of VIP cells in the MEC, we use <jats:italic>in vitro</jats:italic> whole-cell patch-clamp recordings in VIPcre/tdTom mice. We report several gradients in electrophysiological properties of VIP cells that differ across laminae and along the dorsal-ventral MEC axis. We additionally show that VIP cells have distinct morphological features across laminae. Together, these results characterize the cellular and morphological properties of VIP cells in the MEC.</jats:p&gt

The Hippocamposeptal Pathway Generates Rhythmic Firing of GABAergic Neurons in the Medial Septum and Diagonal Bands: An Investigation Using a Complete Septohippocampal Preparation In Vitro

International audienceThe medial septum diagonal band area (MS/DB) projects to the hippocampus through the fornix/fimbria pathway and is implicated in generating hippocampal theta oscillations. The hippocampus also projects back to the MS/DB, but very little is known functionally about this input. Here, we investigated the physiological role of hippocamposeptal feedback to the MS/DB in a complete in vitro septohippocampal preparation containing the intact interconnecting fornix/fimbria pathway. We demonstrated that carbachol-induced rhythmic theta-like hippocampal oscillations recorded extracellularly were synchronized with powerful rhythmic IPSPs in whole-cell recorded MS/DB neurons. Interestingly, we found that these IPSPs evoked rebound spiking in GABAergic MS/DB neurons. In contrast, putative cholinergic and glutamatergic MS/DB neurons responded only weakly with rebound spiking and, as a result, were mostly silent during theta-like oscillations. We next determined the mechanism underlying the rebound spiking that followed the IPSPs in MS/DB GABAergic neurons using phasic electrical stimulation of the fornix/fimbria pathway. We demonstrate that the increased rebound spiking was attributable to the activation of I(h) current, because it was significantly reduced by low concentrations of the I(h) antagonist ZD7288 [4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride]. Together, these results suggest that rhythmical activity in hippocampus is transferred to the MS/DB and can preferentially phase the spiking of GABAergic MS/DB neurons because of their significant expression of I(h) currents. Our data demonstrate that hippocamposeptal inhibition facilitates theta rhythmic discharges in MS/DB GABAergic neurons while favoring the inhibition of most ACh and glutamate neurons

In vitro activation of the medial septum—Diagonal band complex generates atropine-sensitive and atropine-resistant hippocampal theta rhythm: An investigation using a complete septohippocampal preparation

International audienceThe medial septum and diagonal band complex (MS-DB) is believed to play a key role in generating theta oscillations in the hippocampus, a phenomenon critical for learning and memory. Although the importance of the MS-DB in hippocampal theta rhythm generation is generally accepted, it remains to be determined whether the MS-DB alone can generate hippocampal oscillations or is only a transducer of rhythmic activity from other brain areas. Secondly, it is known that hippocampal theta rhythm can be separated into an atropine-sensitive and insensitive component. However, it remains to be established if the MS-DB can generate both types of rhythm. To answer these questions, we used a new in vitro rat septohippocampal preparation placed in a hermetically separated two side recording chamber. We showed that carbachol activation of the MS-DB generated large theta oscillations in the CA1 and CA3 regions of the hippocampus. These oscillations were blocked by applying either the GABA(A) receptor antagonist bicuculline or the AMPA/kainate antagonist DNQX to the hippocampus. Interestingly, the application of the muscarinic receptor antagonist atropine produced only a partial decrease in the amplitude, without modification of the frequency, of theta. These results show for the first time, that upon optimal excitation, the MS-DB alone is able to generate hippocampal oscillations in the theta frequency band. Moreover, these MS-DB generated theta oscillations are mediated by muscarinic and nonmuscarinic receptors and have a pharmacological profile similar to theta rhythm observed in awake animals

Optogenetic Low-Frequency Stimulation of Specific Neuronal Populations Abates Ictogenesis

Despite many advances made in understanding the pathophysiology of epileptic disorders, seizures remain poorly controlled in approximately one-third of patients with mesial temporal lobe epilepsy. Here, we established the efficacy of cell type-specific low-frequency stimulation (LFS) in controlling ictogenesis in the mouse entorhinal cortex (EC) in an in vitro brain slice preparation. Specifically, we used 1 Hz optogenetic stimulation of calcium/calmodulin-dependent protein kinase II-positive principal cells as well as of parvalbumin- or somatostatin-positive interneurons to study the effects of such repetitive activation on epileptiform discharges induced by 4-aminopyridine. We found that 1 Hz stimulation of any of these cell types reduced the frequency and duration of ictal discharges in some trials, while completely blocking them in others. The field responses evoked by the stimulation of each cell type revealed that their duration and amplitude were higher when principal cells were targeted. Furthermore, following a short period of silence ranging from 67 to 135 s, ictal discharges were re-established with similar duration and frequency as before stimulation; however, this period of silence was longer following principal cell stimulation compared with parvalbumin- or somatostatin-positive interneuron stimulation. Our results show that LFS of either excitatory or inhibitory cell networks in EC are effective in controlling ictogenesis. Although optogenetic stimulation of either cell type significantly reduced the occurrence of ictal discharges, principal cell stimulation resulted in a more prolonged suppression of ictogenesis, and, thus, it may constitute a better approach for controlling seizures.SIGNIFICANCE STATEMENT Epilepsy is a neurological disorder characterized by an imbalance between excitation and inhibition leading to seizures. Many epileptic patients do not achieve adequate seizure control using antiepileptic drugs. Low-frequency stimulation (LFS) is an alternative tool for controlling epileptiform activity in these patients. However, despite the temporal and spatial control offered by LFS, such a procedure lacks cell specificity, which may limit its efficacy. Using an optogenetic approach, we report here that LFS of two interneuron subtypes and, even more so, of principal cells can reliably shorten or abolish seizures in vitro Our work suggests that targeted LFS may constitute a reliable means for controlling seizures in patients presenting with focal seizures

Electrophysiological and Morphological Characterization of Chrna2 Cells in the Subiculum and CA1 of the Hippocampus: An Optogenetic Investigation

The nicotinic acetylcholine receptor alpha2 subunit (Chrna2) is a specific marker for oriens lacunosum-moleculare (OLM) interneurons in the dorsal CA1 region of the hippocampus. It was recently shown using a Chrna2-cre mice line that OLM interneurons can modulate entorhinal cortex and CA3 inputs and may therefore have an important role in gating, encoding, and recall of memory. In this study, we have used a combination of electrophysiology and optogenetics using Chrna2-cre mice to determine the role of Chrna2 interneurons in the subiculum area, the main output region of the hippocampus. We aimed to assess the similarities between Chrna2 subiculum and CA1 neurons in terms of the expression of interneuron markers, their membrane properties, and their inhibitory input to pyramidal neurons. We found that subiculum and CA1 dorsal Chrna2 cells similarly expressed the marker somatostatin and had comparable membrane and firing properties. The somas of Chrna2 cells in both regions were found in the deepest layer with axons projecting superficially. However, subiculum Chrna2 cells displayed more extensive projections with dendrites which occupied a significantly larger area than in CA1. The post-synaptic responses elicited by Chrna2 cells in pyramidal cells of both regions revealed comparable inhibitory responses elicited by GABAA receptors and, interestingly, GABAB receptor mediated components. This study provides the first in-depth characterization of Chrna2 cells in the subiculum, and suggests that subiculum and CA1 Chrna2 cells are generally similar and may play comparable roles in both sub-regions