Plasticité des voies de communications intercellulaires dans la glande médullosurrénale en réponse au stress

Functional remodeling of intercellular communication pathways in adrenal chromaffin cells in stressed rats

• Auteur / Autrice : Claude Colomer
• Direction : Nathalie Guérineau
• Type : Thèse de doctorat
• Discipline(s) : Neurosciences
• Date : Soutenance en 2008
• Etablissement(s) : Montpellier 2

Mots clés

Mots clés contrôlés

Effets du stress

Medulla surrénale

Mots clés libres

Glande médullo-surrénale

Jonctions gap

Acétylcholine

Nicotine

Transmission synaptique cholinergique

Plasticité

Stress au froid

Vasopressine

Récepteurs nicotiniques

Modélisation

Résumé

Au quotidien, notre organisme est soumis aux variations du milieu environnant perçues comme des facteurs de stress. En réponse à un stress, les catécholamines sécrétées par les cellules chromaffines de la glande médullosurrénale sont parmi les premières hormones à être libérées dans le sang. Leur sécrétion est sous le contrôle principal de l'acétylcholine à la synapse entre le nerf splanchnique et les cellules chromaffines ; la voie de communication intercellulaire médiée par des jonctions gap entre cellules chromaffines est également impliquée. Nous avons montré, dans des tranches de glandes surrénales de rats soumis à un protocole de stress au froid, que transmission synaptique et couplage électrique jonctionnel étaient augmentés. Plus précisément, l'augmentation du couplage électrique jonctionnel se caractérise par l'apparition d'un couplage robuste, qui permet la transmission de potentiels d'action. Par ailleurs, les taux d'expression des connexines 36 et 43 sont augmentés. Afin d'identifier les facteurs impliqués, nous avons caractérisé les effets de la vasopressine, neuropeptide impliqué dans le stress sur les cellules chromaffines et montré que le couplage jonctionnel mis en évidence par la présence de signaux calciques synchrones est augmenté par cette hormone. Nos travaux ont aussi révélé l'implication de nouveaux récepteurs nicotiniques alpha9/alpha10 dans la transmission synaptique. Chez le rat stressé, l'expression de ces récepteurs est augmentée et ils contribuent majoritairement au courant induit par l'acétylcholine. Ce travail décrit pour la première fois une plasticité du couplage jonctionnel de la glande médullosurrénale dans des conditions physiopathologiques. L'augmentation de la communication jonctionnelle entre cellules chromaffines et de l'activité synaptique améliore l'efficacité du couplage stimulation-sécrétion et représente un mécanisme endogène par lequel la médulla assure une libération de catécholamines appropriée en réponse au stress

An increase in circulating catecholamine levels is one of the mechanisms whereby organisms cope with stress. In the periphery, catecholamines mainly originate from the sympathoadrenal system. The secretion of catecholamines by adrenal chromaffin cells is a key event in response to stressors and it is chiefly controlled by trans-synaptically released acetylcholine from the splanchnic nerve endings. As supported by earlier results obtained in the laboratory, in addition to the central control through cholinergic innervation, a local gap junction-delineated route mediating intercellular electrical coupling between chromaffin cells is involved in the hormonal secretory process and represents an efficient complement to synaptic transmission able to amplify catecholamine release after synaptic stimulation. Whether these two communication pathways (i. E. Synaptic neurotransmission and gap junctional coupling) contribute to stress-evoked increased catecholamine secretion still remains unknown. We addressed this issue using acute adrenal slices from stressed rats (5 day-cold exposure). Our results show that in cold exposed rats, gap junctional communication undergoes a functional plasticity, as evidenced by an increased number of dye-coupled cells. Of a physiological interest is that this up-regulation results in the appearance of a robust electrical coupling between chromaffin cells that allows the transmission of action potentials between coupled cells. This enhancement of gap junctional communication parallels an increase in expression levels of connexin36 and connexin43 proteins. Both transcriptional and post-translational mechanisms are involved since Cx36 transcripts are increased in stressed rats and the expression of the scaffolding protein Zonula Occludens-1, known to interact with both Cx36 and Cx43, is also up-regulated. Consistent with an up-regulated coupling in stressed rats, the cytosolic Ca2+ concentration ([Ca2+]i) rise triggered in a single cell by an iontophoretic application of nicotine occurs simultaneously in several neighboring cells. We also showed that in response to stress, both chromaffin cell excitability and chemical transmission at the splanchnic nerve terminal-chromaffin cell synapses are increased. We next investigated whether vasopressin (VP), a neuropeptide known to play a key role in stress response, could be involved in increased gap junctional communication in stressed rats. Our results show that VP and d[Leu4,Lys8]VP, a V1b receptor specific agonist, increase the occurrence of gap junction-mediated synchronized [Ca2+]i transients between chromaffin cells, both in stressed and unstressed rats. This suggests that, although VP can up-regulate gap junctional coupling, it is unlikely the main factor involved in increased gap junctional communication observed in response to stress. Exposure to cold also enhances the synaptic neurotransmission between splanchnic nerve endings and chromaffin cells, as evidenced by an increase in spontaneous excitatory post-synaptic currents (sEPSCs). This correlates with an increased density of nerve fibers innervating the medulla. To go further, we examined whether the nAChR subtype formed by the combination of alpha9 and alpha10 subunits recently identified in isolated rat chromaffin cells is involved in this effect. By using a toxin (alpha-RgIa) to specifically block alpha9/alpha10 nAChRs, we showed that alpha9 /alpha10 nAChRs contribute to synaptic transmission. Interestingly, the expression level of alpha9 receptors is up-regulated in cold exposed rats. In addition, we show that in stressed rats, alpha9 /alpha10 nAChRs mainly contribute to acetylcholine-induced currents, as compared to alpha3 nAChRs that is the main nicotinic receptor activated in response to acetylcholine in control rats. This indicates that stress also induces nAChR plasticity, at least by acting on the expression level. In sum, the functional changes occurring both on gap junctional communication and on synaptic transmission converge to improve the stimulus-secretion coupling efficiency in the adrenal gland and may represent endogenous mechanisms by which the adrenal medullary tissue ensures appropriate increased catecholamine secretion in response to stressors