Early synapse formation in developing interneurons of the adult olfactory bulb.
New olfactory bulb granule cells (GCs) are GABAergic interneurons continuously arising from neuronal progenitors and integrating into preexisting bulbar circuits. They receive both GABAergic and glutamatergic synaptic inputs from olfactory bulb intrinsic neurons and centrifugal afferents. Here, we investigated the spatiotemporal dynamic of newborn GC synaptogenesis in adult mouse olfactory bulb. First, we established that GABAergic synapses onto mature GC dendrites contain the GABA(A) receptor alpha2 subunit along with the postsynaptic scaffolding protein gephyrin. Next, we characterized morphologically and electrophysiologically the development of GABAergic and glutamatergic inputs onto newborn GCs labeled with eGFP (enhanced green fluorescent protein) using lentiviral vectors. Already when reaching the GC layer (GCL), at 3 d post-vector injection (dpi), newborn GCs exhibited tiny voltage-dependent sodium currents and received functional GABAergic and glutamatergic synapses, recognized immunohistochemically by apposition of specific presynaptic and postsynaptic markers. Thereafter, GABAergic and glutamatergic synaptic contacts increased differentially in the GCL, and at 7 dpi, PSD-95 clusters outnumbered gephyrin clusters. Thus, the weight of GABAergic input was predominant at early stages of GC maturation, but not later. Newborn GC dendrites first reached the external plexiform layer at 4 dpi, where they received functional GABAergic contacts at 5 dpi. Reciprocal synapses initially were formed on GC dendritic shafts, where they might contribute to spine formation. Their presence was confirmed ultrastructurally at 7 dpi. Together, our findings unravel rapid synaptic integration of newborn GCs in adult mouse olfactory bulb, with GABAergic and glutamatergic influences being established proximally before formation of output synapses by apical GC dendrites onto mitral/tufted cells.
Panzanelli
Patrizia
P
Department of Anatomy, Pharmacology and Forensic Medicine, National Institute of Neuroscience-Italy, University of Turin, 10126 Turin, Italy. patrizia.panzanelli@unito.it
Bardy
Cedric
C
Nissant
Antoine
A
Pallotto
Marta
M
Sassoè-Pognetto
Marco
M
Lledo
Pierre-Marie
PM
Fritschy
Jean-Marc
JM
D000704:Analysis of Variance; D000818:Animals; D001703:Biophysics; D002352:Carrier Proteins / Q000378:metabolism; D002454:Cell Differentiation; D003712:Dendrites / Q000378:metabolism / Q000648:ultrastructure; D004558:Electric Stimulation; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005968:Glutamate Decarboxylase / Q000235:genetics; D049452:Green Fluorescent Proteins / Q000235:genetics; D066298:In Vitro Techniques; D007395:Interneurons / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D016253:Microscopy, Immunoelectron / Q000379:methods; D016202:N-Methylaspartate / Q000494:pharmacology; D055495:Neurogenesis / Q000187:drug effects / Q000502:physiology*; D009830:Olfactory Bulb / Q000166:cytology* / Q000648:ultrastructure; D018408:Patch-Clamp Techniques / Q000379:methods; D017729:Presynaptic Terminals / Q000378:metabolism / Q000648:ultrastructure; D011963:Receptors, GABA-A / Q000378:metabolism; D013569:Synapses / Q000502:physiology* / Q000648:ultrastructure; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism / Q000494:pharmacology
D002352:Carrier Proteins; D018691:Excitatory Amino Acid Antagonists; D008565:Membrane Proteins; D011963:Receptors, GABA-A; C485184:enhanced green fluorescent protein; C073441:gephyrin; D049452:Green Fluorescent Proteins; D005680:gamma-Aminobutyric Acid; D016202:N-Methylaspartate; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; D005968:Glutamate Decarboxylase; C402565:glutamate decarboxylase 1