Evidence against AMPA receptor-lacking glutamatergic synapses in the superficial dorsal horn of the rat spinal cord.
Pure NMDA receptor (NMDAr)-mediated EPSCs, thought to correspond to "silent" glutamatergic synapses that lack AMPA receptors (AMPArs), have been observed in superficial spinal dorsal horn of neonatal but not adult rats. Recent anatomical studies suggest that AMPArs are present at virtually all glutamatergic synapses in this region in adults. We used antigen retrieval to examine colocalization of AMPArs and PSD-95 (a marker for glutamatergic synapses) in laminae I-II of neonatal and adult rats. We found a high degree of colocalization in all cases, which suggests that AMPArs are present in the great majority of glutamatergic synapses even in neonatal animals. We therefore reexamined evidence for silent synapses by performing blind whole-cell recordings from superficial dorsal horn neurons in slices from neonatal or adult rats, with focal stimulation to activate glutamatergic synapses. On some occasions in both neonatal (10 of 109, 9%) and adult (9 of 77, 12%) slices, NMDAr-mediated EPSCs were observed when the holding potential was raised to +50 mV at a stimulus strength that had failed to evoke AMPAr-mediated EPSCs. However, in all cases tested, AMPAr-mediated EPSCs were then observed when the cell was returned to -70 mV; this and other properties of the EPSCs suggest that they do not represent genuine silent synapses. When compared with previous findings, our results indicate that the appearance of silent synapses depends on experimental protocol. This suggests that pure NMDAr-mediated EPSCs seen in previous studies do not correspond to AMPAr-lacking synapses but result from another mechanism, for example, loss of labile AMPArs from recently formed synapses.
Neuroscience and Molecular Pharmacology, Faculty of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom.
Toshiharu
T
Yasaka
David I
DI
Hughes
Erika
E
Polgár
Gergely G
GG
Nagy
Masahiko
M
Watanabe
John S
JS
Riddell
Andrew J
AJ
Todd
D000367:Age Factors; D000818:Animals; D000831:Animals, Newborn; D001703:Biophysics; D002614:Chelating Agents / Q000494:pharmacology; D000076122:Disks Large Homolog 4 Protein; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004558:Electric Stimulation / Q000379:methods; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology; D005260:Female; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D018408:Patch-Clamp Techniques / Q000379:methods; D020671:Posterior Horn Cells / Q000187:drug effects / Q000502:physiology*; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D018091:Receptors, AMPA / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013116:Spinal Cord / Q000166:cytology* / Q000254:growth & development; D013569:Synapses / Q000145:classification / Q000502:physiology*; D014633:Valine / Q000031:analogs & derivatives / Q000494:pharmacology
D002614:Chelating Agents; D000076122:Disks Large Homolog 4 Protein; C501449:Dlg4 protein, rat; D018691:Excitatory Amino Acid Antagonists; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D011810:Quinoxalines; D018091:Receptors, AMPA; C062865:2,3-dioxo-6-nitro-7-sulfamoylbenzo(f)quinoxaline; D004533:Egtazic Acid; C056639:2-amino-5-phosphopentanoic acid; D014633:Valine; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid