COX-2-Derived Prostaglandin E2 Produced by Pyramidal Neurons Contributes to Neurovascular Coupling in the Rodent Cerebral Cortex

International audienceVasodilatory prostaglandins play a key role in neurovascular coupling (NVC), the tight link between neuronal activity and local cerebral blood flow, but their precise identity, cellular origin and the receptors involved remain unclear. Here we show in rats that NMDA-induced vasodilation and hemodynamic responses evoked by whisker stimulation involve cyclooxygenase-2 (COX-2) activity and activation of the prostaglandin E2 (PgE(2)) receptors EP2 and EP4. Using liquid chromatography-electrospray ionization-tandem mass spectrometry, we demonstrate that PgE(2) is released by NMDA in cortical slices. The characterization of PgE2 producing cells by immunohistochemistry and single-cell reverse transcriptase-PCR revealed that pyramidal cells and not astrocytes are the main cell type equipped for PgE2 synthesis, one third expressing COX-2 systematically associated with a PgE2 synthase. Consistent with their central role in NVC, in vivo optogenetic stimulation of pyramidal cells evoked COX-2-dependent hyperemic responses in mice. These observations identify PgE2 as the main prostaglandin mediating sensory-evoked NVC, pyramidal cells as their principal source and vasodilatory EP2 and EP4 receptors as their targets

Rôle des canaux t dans la dynamique calcique dendritique des neurones du noyau réticulé thalamique

Dernière étape dans le transfert des informations sensorielles vers le cortex, le thalamus joue un rôle privilégié dans l intégration sensorielle pré-corticale et dans la genèse des rythmes lents physiologiques de l EEG. Dans ce réseau, les neurones GABAergiques du Noyau Réticulé (NRT), qui constituent chez le rongeur la seule source d inhibition des neurones thalamocorticaux, sont contactés en retour par ces mêmes neurones et par des fibres corticales. Par conséquent, ces neurones sont idéalement positionnés pour participer à la définition des champs récepteurs, permettre une communication intra-thalamique, ou sélectionner le sous-réseau thalamique traitant l objet d attention dans les processus d attention sélective. Ces fonctions reposent également sur une forte interconnexion entre neurones du NRT mettant en jeu des synapses axo-dendritiques, des jonctions communicantes et des synapses dendrodendritiques dont les modalités d activation restent largement à déterminer. En particulier, il est important d analyser la capacité des potentiels d action à envahir les différents compartiments dendritiques, capacité qui conditionne le recrutement des synapses dendrodendritiques et/ou des jonctions communicantes. Ainsi, les travaux rapportés dans cette thèse montrent que les potentiels d action entraînent un transitoire calcique observable par imagerie bi-photonique jusque dans les compartiments distaux de l arborisation dendritique. De façon surprenante, ces entrées de calcium reposent largement sur le recrutement de canaux calciques de type T bien que ceux-ci soient majoritairement inactivés aux potentiels membranaires associés aux activités toniques.Last step in the transfer of sensory information to the cortex, the thalamus plays a privileged role in sensory integration and pre-cortical in the genesis of slow physiological rhythms of the EEG. In this network, the GABAergic neurons of the nucleus reticularis thalami (NRT), which are the only source of inhibition of thalamocortical neurons in rodents integrate synaptic inputs from both thalamocortical and corticothalamic neurons and project back to the thalamocortical neurons. Therefore, these neurons are ideally positioned to participate in the definition of receptive fields, for an intra-thalamic communication, or select the sub-thalamic network addressing the subject of attention in selective attention processes. These functions are also based on a strong interconnection between NRT neurons involving axo-dendritic synapses, the gap junctions and dendro-dendritic synapses whose activation processes remain largely to be determined. In particular, it is important to analyze the ability of action potentials to invade different dendritic compartments, capacity which determines the recruitment of dendrodendritic synapses and / or gap junctions. Thus, the work reported in this thesis show that action potentials cause a calcium transient observed by two-photon imaging into the distal compartments of the dendritic arborization. Surprisingly, these calcium entries are largely based on the recruitment of T-type calcium channels, although these are mostly inactivated at membrane potentials associated with tonic activityPARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Dynamics of Intrinsic Dendritic Calcium Signaling during Tonic Firing of Thalamic Reticular Neurons

International audienceThe GABAergic neurons of the nucleus reticularis thalami that control the communication between thalamus and cortex are interconnected not only through axo-dendritic synapses but also through gap junctions and dendro-dendritic synapses. It is still unknown whether these dendritic communication processes may be triggered both by the tonic and the T-type Ca 2+ channel-dependent high frequency burst firing of action potentials displayed by nucleus reticularis neurons during wakefulness and sleep, respectively. Indeed, while it is known that activation of T-type Ca 2+ channels actively propagates throughout the dendritic tree, it is still unclear whether tonic action potential firing can also invade the dendritic arborization. Here, using two-photon microscopy, we demonstrated that dendritic Ca 2+ responses following somatically evoked action potentials that mimic wake-related tonic firing are detected throughout the dendritic arborization. Calcium influx temporally summates to produce dendritic Ca 2+ accumulations that are linearly related to the duration of the action potential trains. Increasing the firing frequency facilitates Ca 2+ influx in the proximal but not in the distal dendritic compartments suggesting that the dendritic arborization acts as a low-pass filter in respect to the back-propagating action potentials. In the more distal compartment of the dendritic tree, T-type Ca 2+ channels play a crucial role in the action potential triggered Ca 2+ influx suggesting that this Ca 2+ influx may be controlled by slight changes in the local dendritic membrane potential that determine the T-type channels' availability. We conclude that by mediating Ca 2+ dynamic in the whole dendritic arborization, both tonic and burst firing of the nucleus reticularis thalami neurons might control their dendro-dendritic and electrical communications

Contribution of T-type current to AP back-propagation.

<p>A. APs (A1) and associated dendritic ΔCa²⁺ (A2) recorded in control condition and after TTA-P2 application (3 µM). Same neuron as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0072275#pone-0072275-g002" target="_blank">figure 2A</a>. Note that TTA-P2 did not modify the tonic discharge of APs. Enlargements of the first and last APs of the train are presented in insets. In contrast, the block of the T-type current decreased the amplitude of ΔCa²⁺ and almost fully abolished Ca²⁺ entry in the distal dendritic region. B1–2. Examples of two neurons in which TTA-P2 had no effect on the ΔCa²⁺ recorded in proximal dendrites. Same calibration as in A2. C. Plots of the maximal fluorescence signal evoked by trains of APs in control condition and after TTA-P2 application for each neuron. The left graph shows that TTA-P2 has little effect on calcium entry monitored in proximal dendrites (mean ΔG/R: control 0.31±0.04, TTA-P2∶0.25±0.04, p>0.05). Right graph: same measurements performed on distal dendrites showed a clear decrease upon TTA-P2 application (mean ΔG/R: control 0.25±0.7, TTA-P2 0.11±0.05, p<005).</p

Sleep Slow Wave-Related Homo and Heterosynaptic LTD of Intrathalamic GABA(A)ergic Synapses: Involvement of T-Type Ca2+ Channels and Metabotropic Glutamate Receptors

International audienceSlow waves of non-REM sleep are suggested to play a role in shaping synaptic connectivity to consolidate recently acquired memories and/or restore synaptic homeostasis. During sleep slow waves, both GABAergic neurons of the nucleus reticularis thalami (NRT) and thalamocortical (TC) neurons discharge high-frequency bursts of action potentials mediated by low-threshold calcium spikes due to T-type Ca2+ channel activation. Although such activity of the intrathalamic network characterized by high-frequency firing and calcium influx is highly suited to modify synaptic efficacy, very little is still known about its consequences on intrathalamic synapse strength. Combining in vitro electrophysiological recordings and calcium imaging, here we show that the inhibitory GABAergic synapses between NRT and TC neurons of the rat somatosensory nucleus develop a long-term depression (I-LTD) when challenged by a stimulation paradigm that mimics the thalamic network activity occurring during sleep slow waves. The mechanism underlying this plasticity presents unique features as it is both heterosynaptic and homosynaptic in nature and requires Ca2+ entry selectively through T-type Ca2+ channels and activation of the Ca2+ -activated phosphatase, calcineurin. We propose that during slow-wave sleep the tight functional coupling between GABA(A) receptors, calcineurin, and T-type Ca2+ channels will elicit LTD of the activated GABAergic synapses when coupled with concomitant activation of metabotropic glutamate receptors postsynaptic to cortical afferences. This I-LTD may be a key element involved in the reshaping of the somatosensory information pathway during sleep

Back-propagating APs produce robust ΔCa²⁺ in distal dendrites of the NRT neurons.

<p>A. Top traces: ΔCa²⁺ recorded in response to a somatically evoked burst of 45 APs at 10 Hz (left trace). Amplitude of the ΔCa²⁺ first increased in the most proximal part of the dendrite (compare 10 and 40 µm) before decreasing in the more distal dendritic compartment. Note however that a noticeable ΔCa²⁺ is still present at the more distal location (120 µm). Bottom traces: ΔCa²⁺ recorded in response to a somatically evoked LTS (left trace). Note the increase in ΔCa²⁺ with dendritic distance. B. Summary of ΔCa²⁺ grouped by dendritic location. LTS: n = 19, 13 and 8; APs: n = 21, 15 and 6 for distances <60 µm, 60–120 µm and >120 µm, respectively. *: p<0.05, n.s.: p>0.05. C1. ΔCa²⁺ recorded in response to a somatically evoked burst of 45 APs at 10 (black traces) and 40 Hz (red traces). Increasing the firing frequency produced a clear enhancement in ΔCa²⁺ at proximal and intermediate locations but not at the more distal location (180 µm). C2. Summary of ΔCa²⁺ evoked in the same neurons by trains of 45 APs at 10 and 40 Hz grouped by dendritic location. <60 µm: n = 8; 60–120 µm: n = 7; >120 µm: n = 4. **: p<0.01, *: p<0.05, n.s.: p>0.05. D. Contribution of sodium APs to LTS-evoked dendritic ΔCa²⁺. D1. LTS (left traces) and associated dendritic ΔCa²⁺ (middle traces) measured at 110 µm from the soma in control condition and after TTX application. After blockade of the sodium APs, note the clear increase in amplitude and duration of the LTS and the lack of change in dendritic ΔCa²⁺. An enlargement of ΔCa²⁺ in both conditions is presented in inset. Right traces: at the same dendritic location a clear ΔCa²⁺ was evoked by a train of 45 APs at 10 Hz. Therefore, the lack of changes in LTS-evoked ΔCa²⁺ following TTX application did not result from the inability of APs to back-propagate in this distal dendrite. D2. Top plot shows the maximal fluorescence signal evoked in distal dendrites by LTS in control condition and after TTX application for each neuron. Note that TTX has no clear effect on calcium entry (mean ΔG/R: control 1.2±0.1, TTX: 1.08±0.13, p>0.05). Bottom plot presents the duration of LTSs in control condition and after TTX application (control: 76.1±2.8 ms, TTX: 107.4±5.6 ms, p<0.05). In A and C the scanned dendritic regions are indicated by boxes on the maximal intensity Z projection of the neurons.</p

Dendritic Ca²⁺ responses evoked by tonic APs or LTS in NRT neurons.

<p>A1. Dendritic ΔCa²⁺ triggered by somatically evoked train of 15 APs at 10 Hz (top record). Application of 0.5 µM TTX fully blocked the APs and the ΔCa²⁺ (bottom trace). A2. Dendritic ΔCa²⁺ triggered by a somatically evoked LTS. An enlarged record of the LTS is presented in inset. Application of 3 µM TTA-P2 fully blocked the LTS and the ΔCa²⁺. B. ΔCa²⁺ recorded in three different dendrites of the same neuron. B1. Dendritic ΔCa²⁺ evoked by LTS (left column) and 1, 3, 5 and 10 APs at 10 Hz (right column) in the different dendrites. Examples of the somatically recorded voltage responses are shown underneath the ΔCa²⁺ recorded in location 1 (an enlarged record of the LTS response is illustrated in inset). ΔCa² recorded in response to a single back-propagating AP is presented at higher magnification in inset. B2. Plot of amplitude of ΔCa²⁺ evoked by back-propagating APs (n = 10 neurons). The amplitude was normalized to the ΔCa²⁺ evoked at the time of the 20^th AP of the trains. Note the almost linear summation of the ΔCa²⁺ evoked by the first 5 back-propagating APs. In A and B the scanned dendritic regions are indicated by boxes on the maximal intensity Z projection of the neurons presented in the left panels.</p

Dendritic ΔCa²⁺ in non-bursting NRT neuron.

<p>A. The scanned dendritic region is indicated by the box on the maximal intensity Z projection of the neuron (Left panel). Top traces show the dendritic ΔCa²⁺ and the voltage response to a depolarizing pulse from a holding potential of −90 mV. Note the lack of LTS and the presence of a single AP in the enlargement presented in inset. Bottom traces: Dendritic ΔCa²⁺ evoked by a train of 15 APs triggered from a −55 mV holding potential. B. A tonic discharge of 8 APs at 10 Hz was evoked as rebound activity by a 1 s hyperpolarizing pulse to −95 mV. Note the lack of LTS as clearly shown by the enlargement presented in inset. The dendritic ΔCa²⁺ associated to this tonic discharge is presented above. Both tonic discharge and the associated dendritic ΔCa²⁺ were abolished by application of 3 µM TTA-P2 (hyperpolarizing pulse from −55 to −100 mV). ΔG/R: same calibration as in A.</p

Inhibition of Cav3.2 T-type Calcium Channels by Its Intracellular I-II Loop

International audienceVoltage-dependent calcium channels (Cav) of the T-type family (Cav3.1, Cav3.2, and Cav3.3) are activated by low threshold membrane depolarization and contribute greatly to neuronal network excitability. Enhanced T-type channel activity, especially Cav3.2, contributes to disease states, including absence epilepsy. Interestingly, the intracellular loop connecting domains I and II (I-II loop) of Cav3.2 channels is implicated in the control of both surface expression and channel gating, indicating that this I-II loop plays an important regulatory role in T-type current. Here we describe that co-expression of this I-II loop or its proximal region (⌬1-Cav3.2; Ser 423 –Pro 542) together with recombinant full-length Cav3.2 channel inhibited T-type current without affecting channel expression and membrane incorporation. Similar T-type current inhibition was obtained in NG 108-15 neuroblastoma cells that constitutively express Cav3.2 channels. Of interest, ⌬1-Cav3.2 inhibited both Cav3.2 and Cav3.1 but not Cav3.3 currents. Efficacy of ⌬1-Cav3.2 to inhibit native T-type channels was assessed in thalamic neurons using viral transduction. We describe that T-type current was significantly inhibited in the ventrobasal neurons that express Cav3.1, whereas in nucleus reticularis thalami neurons that express Cav3.2 and Cav3.3 channels, only the fast inactivating T-type current (Cav3.2 component) was significantly inhibited. Altogether, these data describe a new strategy to differentially inhibit Cav3 isoforms of the T-type calcium channels

Rome, les Césars et la ville

Cet ouvrage examine les relations complexes entre la personne impériale - et les empereurs successifs - et la Ville de Rome, en tant que ville, espace et capitale, pendant les deux premiers siècles du Principat. Nicole BELAYCHE a rassemblé sur cette question une « équipe » de chercheurs, français et étrangers, fédérant les disciplines du champ historique : histoire politique et idéologique, histoire sociale, histoire culturelle et religieuse, histoire urbaine et topographique. Car les problèmes posés par la/les relation/s entre les Césars et la Ville se situent au confluent de ces approches et c'est précisément le caractère englobant de la question qui la rend utile à notre compréhension de l'Empire romain aux Ier et IIe siècles de notre ère. Cette réflexion collective s'appuie sur le profond renouvellement documentaire offert depuis une génération par les découvertes archéologiques ou la reconsidération de certains sites romains, sur l'affinement de l'approche de la nature du pouvoir impérial et de son expression dans les différents règnes, sur l'explicitation de la subtilité du langage idéologique mis en oeuvre par les empereurs dans leur politique de « communication », enfin sur une appréciation renouvelée de la Rome impériale classique à la lumière des concepts élaborés par les géographes autour de la notion de grande ville