Glutamate receptors on myelinated spinal cord axons: I. GluR6 kainate receptors.
OBJECTIVE
The deleterious effects of glutamate excitotoxicity are well described for central nervous system gray matter. Although overactivation of glutamate receptors also contributes to axonal injury, the mechanisms are poorly understood. Our goal was to elucidate the mechanisms of kainate receptor-dependent axonal Ca(2+) deregulation.


METHODS
Dorsal column axons were loaded with a Ca(2+) indicator and imaged in vitro using confocal laser-scanning microscopy.


RESULTS
Activation of glutamate receptor 6 (GluR6) kainate receptors promoted a substantial increase in axonal [Ca(2+)]. This Ca(2+) accumulation was due not only to influx from the extracellular space, but a significant component originated from ryanodine-dependent intracellular stores, which, in turn, depended on activation of L-type Ca(2+) channels: ryanodine, nimodipine, or nifedipine blocked the agonist-induced Ca(2+) increase. Also, GluR6 stimulation induced intraaxonal production of nitric oxide (NO), which greatly enhanced the Ca(2+) response: quenching of NO with intraaxonal (but not extracellular) scavengers, or inhibition of neuronal NO synthase with intraaxonal Nomega-nitro-L-arginine methyl ester, blocked the Ca(2+) increase. Loading axons with a peptide that mimics the C-terminal PDZ binding sequence of GluR6, thus interfering with the coupling of GluR6 to downstream effectors, greatly reduced the agonist-induced axonal Ca(2+) increase. Immunohistochemistry showed GluR6/7 clusters on the axolemma colocalized with neuronal NO synthase and Ca(v)1.2.


CONCLUSIONS
Myelinated spinal axons express functional GluR6-containing kainate receptors, forming part of novel signaling complexes reminiscent of postsynaptic membranes of glutamatergic synapses. The ability of such axonal "nanocomplexes" to release toxic amounts of Ca(2+) may represent a key mechanism of axonal degeneration in disorders such as multiple sclerosis where abnormal accumulation of glutamate and NO are known to occur.
Ouardouz
Mohamed
M
Ottawa Health Research Institute, University of Ottawa, Ontario, Canada.
Coderre
Elaine
E
Basak
Ajoy
A
Chen
Andrew
A
Zamponi
Gerald W
GW
Hameed
Shameed
S
Rehak
Renata
R
Yin
Xinghua
X
Trapp
Bruce D
BD
Stys
Peter K
PK
D000818:Animals; D001369:Axons / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000502:physiology; D003545:Cysteine / Q000378:metabolism; D004533:Egtazic Acid / Q000031:analogs & derivatives; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D018698:Glutamic Acid / Q000494:pharmacology; D006879:Hydroxocobalamin / Q000494:pharmacology; D008297:Male; D018613:Microscopy, Confocal / Q000379:methods; D009211:Myoglobin / Q000494:pharmacology; D009413:Nerve Fibers, Myelinated / Q000187:drug effects / Q000378:metabolism*; D009569:Nitric Oxide / Q000378:metabolism / Q000494:pharmacology; D054731:PDZ Domains / Q000502:physiology; D010455:Peptides / Q000378:metabolism; D055503:Protein Multimerization / Q000502:physiology; D051381:Rats; D020318:Rats, Long-Evans; D018092:Kainic Acid Receptors / Q000737:chemistry / Q000502:physiology*; D012433:Ryanodine / Q000494:pharmacology; D013119:Spinal Cord Injuries / Q000378:metabolism; D013126:Spinal Nerve Roots / Q000166:cytology*; D000097806:GluK2 Kainate Receptor
D002118:Calcium; D020746:Calcium Channels, L-Type; D003545:Cysteine; D004533:Egtazic Acid; D018691:Excitatory Amino Acid Antagonists; D018698:Glutamic Acid; D006879:Hydroxocobalamin; D009211:Myoglobin; D009569:Nitric Oxide; D010455:Peptides; D018092:Kainic Acid Receptors; D012433:Ryanodine; D000097806:GluK2 Kainate Receptor; C419385:L-type calcium channel alpha(1C); C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester