Résumés
Résumé
Aux mécanismes de plasticité cellulaire communs à toutes les cellules, les neurones ajoutent ceux de l’élaboration des formes et des fonctions qui leurs sont spécifiques. L’épigenèse synaptique est l’ensemble des ajustements morphofonctionnels des contacts synaptiques induits par l’environnement, dans la fenêtre de variabilité contrôlée par les réseaux de gènes, eux-mêmes sélectionnés pendant l’évolution du cortex cérébral. Dans le paradigme dominant aujourd’hui, l’épigenèse synaptique constitue le mécanisme matériel du stockage des signaux représentant le monde environnant dans le cortex cérébral. La notion de périodes critiques au cours du développement ouvre l’inscription épigénétique de l’histoire de l’individu dans l’affinage final des formes et des fonctions de ses neurones. Cette « ouverture épigénétique », maximale dans le cerveau humain, est probablement la source de la très grande adaptabilité cognitive de notre espèce, mais peut-être aussi une de ses fragilités.
Summary
Synaptic plasticity, or epigenesis, is present and varies throughout the whole life of the cerebral cortex. The adult synapse is formed of large and variable proteins assemblies acting as molecular switches leading to many distinct functional states. In the flow of activity circulating through the synaptic circuits, these multiple synaptic states transitions are modulated by the levels and sequences of activations of the pre- and post-synaptic domains. The efficiency of synaptic transmission is also modulated by competition and/or cooperativity with neighbouring synapses, and by many neuromodulations. Some transitions eventually lead to synaptogenesis. In the adult cerebral cortex, synaptogenesis remains a local event ; axonal and dendritic arbors are not reshaped. On the contrary, during pre- and post-natal synaptogenesis, the same molecular mechanisms lead to a significant reorganization of the axonal and dendritic arbors. Early in the development, synapses are generated and differentiate under the control of robust mechanisms governed by genes. Then, during the critical periods, extending from the end of gestation to the end of puberty, the refinment of the synaptic architecture becomes experience-expectent. This «epigenetic opening» of synaptogenesis to environment is maximal in the human brain. It is the source of the exceptional cognitive adaptability of our species, and possibly one of its major fragility. Epigenetic manipulations of these critical periods are undertaken, allowing restoration of synaptic plasticity also in the adult brain.
Parties annexes
Références
- 1. Lisman J. Long-term potentiation : outstanding questions and attempted synthesis. Phil Trans R Soc Lond B 2003 ; 358 : 829-42.
- 2. Montgomery JM, Madison DV. Discrete synaptic states define a major mechanism of synapse plasticity. Trends Neurosci 2004 ; 27 : 744-50.
- 3. Voronin LL, Cherubini E. « Deaf, mute and whispering » silent synapses : their role in synaptic plasticity. J Physiol 2004 ; 557 : 3-12.
- 4. Groc L, Heine M, Cognet L, et al. Differential activity-dependent regulation of the lateral mobilities of AMPA and NMDA receptors. Nat Neurosci 2004 ; 7 : 695-6.
- 5. Routtenberg A, Rekart JL. Post-translational protein modification as the substrate for long-lasting memory. Trends Neurosci 2005 ; 28 : 12-9.
- 6. Turrigiano G. A competitive game of synaptic tag. Neuron 2004 ; 44 : 903-4.
- 7. Salgado-Commissariat D, Rosenfield DB, Helekar SA. Nicotine-mediated plasticity in robust nucleus of the archistriatum of the adult zebra finch. Brain Res 2004 ; 1018 : 97-105
- 8. Sallette J, Pons S, Devillers-Thiery A, et al. Nicotine enhances intracellular nicotinic receptor maturation : a novel mechanism of neural plasticity. Neuron 2005 (sous presse).
- 9. Hensch TK, Stryker MP. Columnar architecture sculpted by GABA circuits in developing cat visual cortex. Science 2004 ; 303 : 1678-81.
- 10. Bourgeois JP. Synaptogenesis, heterochrony, and epigenesis in the mammalian neocortex. Acta Paediatr 1997 ; 422 (suppl) : 27-33.
- 11. Bourgeois JP. Synaptogenesis in the neocortex of the newborn. In : Nelson CA, Luciana M, eds. Handbook of developmental cognitive neuroscience. A Bradford book. Cambridge, Massachusetts : MIT Press, 2001 : 23-34.
- 12. Bourgeois JP. Le développement de la connectivité cérébrale : étape ultime de l’individuation ? In : Changeux JP, ed. Gènes et culture. Paris : Éditions Odile Jacob, 2003 : 93-115.
- 13. Goldberg JL. Intrinsic neuronal regulation of axon and dendrite growth. Curr Opin Neurobiol 2004 ; 14 : 551-7.
- 14. Changeux JP, Danchin A. Selective stabilization of developing synapses as a mechanism for the specification of neuronal networks. Nature 1976 ; 264 : 705-12.
- 15. Changeux JP. L’homme de vérité. Paris : Éditions Odile Jacob, 2002.
- 16. Fagiolini M, Pizzorusso T, Berardi N, et al. Functional postnatal development of the rat primary visual cortex and the role of visual experience : dark rearing and monocular deprivation. Vision Res 1994 ; 34 : 709-20.
- 17. Chang EF, Merzenich MM. Environmental noise retards auditory cortical development. Science 2003 ; 300 : 498-502.
- 18. Wang X. The unexpected consequences of a noisy environment. Trends Neurosci 2004 ; 27 : 364-6.
- 19. Helmeke C, Ovtscharoff W Jr, Poeggel G, Braun K. Juvenile emotional experience alters synaptic inputs on pyramidal neurons in the anterior cingulate cortex. Cerebr Cortex 2001 ; 11 : 717-7.
- 20. Berardi N, Pizzorusso T, Maffei L. Critical periods during sensory development. Curr Opin Neurobiol 2000 ; 10 : 138-45.
- 21. Hensch TK. Controling the critical period. Neurosci Res 2003 ; 47 : 17-22.
- 22. Hensch TK. Critical period regulation. Ann Rev Neurosci 2004 ; 27 : 549-79.
- 23. Tropen D, Caleo M, Maffei L. Synergistic effects of brain-derived neurotropic factor and chondroitinase ABC on retinal fiber sprouting after denervation of the superior colliculus in adult rats. J Neurosci 2003 ; 23 : 7034-44.
- 24. Rochefort C, Gheusi G, Vincent JD, Lledo PM. Enriched odor exposure increases the number of newborn neurons in the adult olfactory bulb and improves odor memory. J Neurosci 2002 ; 22 : 2679-89.
- 25. Cotman CW, Berchtold NC. Exercise : a behavioral intervention to enhance brain health and plasticity. Trends Neurosci 2002 ; 25 : 295-301.
- 26. Bartoletti A, Medini P, Berardi N, Maffei L. Environmental enrichment prevents effects of dark-rearing in the rat visual cortex. Nat Neurosci 2004 ; 7 : 215-6.
- 27. Linkenhoker BA, Knudsen EI. Incremental training increases the plasticity of the auditory space map in adult barn owls. Nature 2002 ; 419 : 293-6.
- 28. Paquette V, Lévesque J, Mensour B, et al. Change the mind and you change the brain : effects of cognitive-behavioral therapy on the neural correlates of spider phobia. Neuroimage 2003 ; 18 : 401-9.
- 29. Grant SGN, O’Dell TJ. Multiprotein complex signaling and the plasticity problem. Curr Opin Neurobiol 2001 ; 11 : 363-8.
- 30. Abraham WC, Robins A. Memory retention : the synaptic stability versus plasticity dilemna. Trends Neurosci 2005 ; 28 : 73-8.