title: Activity-dependent mismatch between axo-axonic synapses and the axon initial segment controls neuronal output.
abstract: The axon initial segment (AIS) is a structure at the start of the axon with a high density of sodium and potassium channels that defines the site of action potential generation. It has recently been shown that this structure is plastic and can change its position along the axon, as well as its length, in a homeostatic manner. Chronic activity-deprivation paradigms in a chick auditory nucleus lead to a lengthening of the AIS and an increase in neuronal excitability. On the other hand, a long-term increase in activity in dissociated rat hippocampal neurons results in an outward movement of the AIS and a decrease in the cell's excitability. Here, we investigated whether the AIS is capable of undergoing structural plasticity in rat hippocampal organotypic slices, which retain the diversity of neuronal cell types present at postnatal ages, including chandelier cells. These interneurons exclusively target the AIS of pyramidal neurons and form rows of presynaptic boutons along them. Stimulating individual CA1 pyramidal neurons that express channelrhodopsin-2 for 48 h leads to an outward shift of the AIS. Intriguingly, both the pre- and postsynaptic components of the axo-axonic synapses did not change position after AIS relocation. We used computational modeling to explore the functional consequences of this partial mismatch and found that it allows the GABAergic synapses to strongly oppose action potential generation, and thus downregulate pyramidal cell excitability. We propose that this spatial arrangement is the optimal configuration for a homeostatic response to long-term stimulation.
authors.affiliation: MRC Centre for Developmental Neurobiology, King's College London, London SE1 1UL, United Kingdom;
authors.forename: Winnie
authors.initials: W
authors.lastname: Wefelmeyer
authors.affiliation: Institut de Neurosciences Cognitives et Intégratives d'Aquitaine, Université de Bordeaux, CNRS, 33076 Bordeaux, France.
authors.forename: Daniel
authors.initials: D
authors.lastname: Cattaert
authors.affiliation: MRC Centre for Developmental Neurobiology, King's College London, London SE1 1UL, United Kingdom; juan.burrone@kcl.ac.uk.
authors.forename: Juan
authors.initials: J
authors.lastname: Burrone
mesh_terms: D000200:Action Potentials / Q000502:physiology / Q000528:radiation effects; D000818:Animals; D001369:Axons / Q000502:physiology* / Q000528:radiation effects; D000075402:Channelrhodopsins; D015536:Down-Regulation / Q000528:radiation effects; D006624:Hippocampus / Q000502:physiology; D015640:Ion Channel Gating / Q000528:radiation effects; D008027:Light; D008297:Male; D008822:Mice, Transgenic; D008959:Models, Neurological; D062308:Optogenetics; D017966:Pyramidal Cells / Q000502:physiology / Q000528:radiation effects; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000378:metabolism; D013569:Synapses / Q000502:physiology* / Q000528:radiation effects; D050495:Vesicular Inhibitory Amino Acid Transport Proteins / Q000378:metabolism
chemical_list: D000075402:Channelrhodopsins; D011963:Receptors, GABA-A; D050495:Vesicular Inhibitory Amino Acid Transport Proteins
keywords: axon initial segment; chandelier cells; homeostatic plasticity; intrinsic plasticity; optogenetics