Glutamatergic reinnervation and assembly of glutamatergic synapses in adult rat skeletal muscle occurs at cholinergic endplates.
After denervation of adult rat abdominal muscles, the postsynaptic apparatus of neuromuscular junctions (NMJs) retains its original architecture and clustering of acetylcholine receptors (AChRs). When descending fibers of the spinal cord are surgically diverted to this muscle by a nerve grafting procedure, supraspinal glutamatergic neurons can innervate muscle fibers and restore motor function; the newly formed NMJs switch from a cholinergic to a glutamatergic-type synapse. We show here that regenerating nerve endings contact the fibers in an area occupied by cholinergic endplates. These NMJs are morphologically indistinguishable from those in controls, but they differ in the subunit composition of AChRs. Moreover, by immunofluorescence and immunoelectron microscopy, new NMJs express glutamatergic synapse markers. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluR1 partially colocalizes with AChRs, and vesicular glutamate transporter 2 is localized in the presynaptic compartment. Immunoprecipitation analysis of membranes from reinnervated muscle showed that AMPA receptor subunits GluR1 and GluR2 coimmunoprecipitate with rapsyn, the AChR-anchoring protein at the NMJ. Taken together, these results indicate that cholinergic endplates can be targeted by new glutamatergic projections and that the clustering of AMPA receptors occurs there.
Francolini
Maura
M
Department of Medical Pharmacology, University of Milan and CNR Neuroscience Institute, via Vanvitelli 32, Milan, Italy. maura.francolini@unimi.it
Brunelli
Giorgio
G
Cambianica
Ilaria
I
Barlati
Sergio
S
Barbon
Alessandro
A
La Via
Luca
L
Guarneri
Bruno
B
Boroni
Flora
F
Lanzillotta
Annamaria
A
Baiguera
Cristina
C
Ettorre
Michele
M
Buffelli
Mario
M
Spano
PierFranco
P
Clementi
Francesco
F
Pizzi
Marina
M
D000818:Animals; D005455:Fluorescent Antibody Technique; D018698:Glutamic Acid / Q000378:metabolism*; D047468:Immunoprecipitation; D008297:Male; D016253:Microscopy, Immunoelectron; D009045:Motor Endplate / Q000502:physiology* / Q000648:ultrastructure; D009124:Muscle Proteins / Q000378:metabolism; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology* / Q000648:ultrastructure; D009416:Nerve Regeneration / Q000502:physiology*; D009469:Neuromuscular Junction / Q000502:physiology* / Q000648:ultrastructure; D017729:Presynaptic Terminals / Q000502:physiology / Q000648:ultrastructure; D051381:Rats; D017208:Rats, Wistar; D018091:Receptors, AMPA / Q000378:metabolism; D011950:Receptors, Cholinergic / Q000378:metabolism*; D050599:Vesicular Glutamate Transport Protein 2 / Q000378:metabolism
D018698:Glutamic Acid; D009124:Muscle Proteins; D018091:Receptors, AMPA; D011950:Receptors, Cholinergic; D050599:Vesicular Glutamate Transport Protein 2; C493475:Slc17a6 protein, rat; C051609:peripheral membrane protein 43K; C104722:glutamate receptor ionotropic, AMPA 2