Reduced blockade by extracellular Mg(2+) is permissive to NMDA receptor activation in cerebellar granule neurons that model a migratory phenotype.
NMDA receptors (NMDAR) contribute to neuronal development throughout the CNS. However, their mode(s) of activation preceding synaptic maturation is unclear, as they are not co-localized with alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPARs) which normally provide sufficient depolarization to relieve voltage-dependent blockade by Mg(2+). We used cerebellar granule neurons (CGNs) cultured at a near-physiological KCl concentration to examine maturation-dependent changes in NMDAR responses. In contrast, most studies use KCl-supplemented medium to promote survival. At 2-4 days in vitro CGNs: (i) express developmental markers resembling the in vivo migratory phenotype; (ii) maintain a basal amount of calcium responsive element-binding protein phosphorylation that requires NMDARs and calcium/calmodulin-dependent kinases, but not AMPARs; (iii) exhibit NMDA-mediated Ca(2+) influx not effectively blocked by ambient Mg(2+) (0.75 mM) or AMPARs; (iv) maintain a more depolarized resting membrane potential and increased resistance compared to synaptically-connected CGNs. Moreover, migrating CGNs in explant cultures demonstrate NMDA-mediated Ca(2+) influx not effectively blocked by 0.75 mM Mg(2+), and NMDAR but not AMPAR antagonists slow migration. These data suggest the biophysical properties of immature CGNs render NMDARs less sensitive to Mg(2+) blockade, enhancing the likelihood of activation in the absence of AMPAR depolarization.
Gerber
Adam M
AM
Department of Neuroscience and Physiology, SUNY Upstate Medical University, Syracuse, New York 13210, USA.
Beaman-Hall
Carol M
CM
Mathur
Anjili
A
Vallano
Mary Lou
ML
D000818:Animals; D000831:Animals, Newborn; D015415:Biomarkers / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000502:physiology; D002465:Cell Movement; D002478:Cells, Cultured; D002531:Cerebellum / Q000166:cytology / Q000378:metabolism*; D017362:Cyclic AMP Response Element-Binding Protein / Q000502:physiology; D005110:Extracellular Space / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism*; D008564:Membrane Potentials; D009474:Neurons / Q000378:metabolism*; D010766:Phosphorylation; D051381:Rats; D018091:Receptors, AMPA / Q000037:antagonists & inhibitors / Q000502:physiology; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors / Q000502:physiology*; D015398:Signal Transduction; D013569:Synapses / Q000502:physiology
D015415:Biomarkers; D017362:Cyclic AMP Response Element-Binding Protein; D018091:Receptors, AMPA; D016194:Receptors, N-Methyl-D-Aspartate; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D008274:Magnesium; D002118:Calcium