Calcium influx via L-type voltage-gated channels mediates the delayed, elevated increases in steady-state c-fos mRNA levels in cerebellar granule cells exposed to excitotoxic levels of glutamate.
The altered kinetics of steady-state c-fos mRNA production in cultured cerebellar granule cells under excitotoxic conditions was investigated in neurons subjected to depolarising stimuli, namely, high KCl and L-glutamate (Glu), in which Ca2+ influx occurs by differing routes. Increases in intracellular-free calcium levels ([Ca2+]i) stimulated by nontoxic or toxic levels of Glu were blocked by selective N-methyl-D-aspartate (NMDA) receptor antagonism; were blocked only partially by the L-type channel blocker, nifedipine; and were unaffected by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptor antagonists. Glu-induced cell death was prevented only by NMDA receptor blockade. Exposure of cells to nontoxic levels of Glu resulted in a transient increase in c-fos mRNA levels, whereas an excitotoxic dose produced a delay in the appearance of c-fos mRNA but a subsequent, progressive, and sustained (>4 hr) increase. An excitotoxic dose of Glu in combination with either nifedipine or selective NMDA receptor antagonists resulted in the normal, transient increase of c-fos mRNA levels. Chronic exposure to 55 mM KCl caused no cytotoxicity, although it resulted in a delayed, elevated increase in c-fos mRNA levels that was unaffected by NMDA receptor blockade but reverted to the normal, transient profile of c-fos mRNA formation when it was coadministered with nifedipine. The KCl-induced increase in [Ca2+]i levels was inhibited dramatically by nifedipine but was unaffected by any of the ionotropic Glu receptor antagonists. The results support the notion that the appearance of a delayed but elevated increase in steady-state c-fos mRNA levels following exposure to excitotoxic doses of Glu is mediated specifically by calcium influx via L-type voltage-gated channels.
Neurochemistry Group, School of Biomedical Sciences, University of St. Andrews, Fife, Scotland, United Kingdom. rg3@st-andrews.ac.uk
R
R
Griffiths
L
L
Ritchie
K
K
Lidwell
A
A
Grieve
C S
CS
Malcolm
M
M
Scott
C
C
Meredith
D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology; D002470:Cell Survival / Q000187:drug effects; D002531:Cerebellum / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D004594:Electrophysiology; D016762:Genes, fos / Q000235:genetics*; D018698:Glutamic Acid / Q000494:pharmacology*; D006706:Homeostasis / Q000502:physiology; D007425:Intracellular Membranes / Q000378:metabolism; D015640:Ion Channel Gating / Q000502:physiology; D051379:Mice; D008815:Mice, Inbred Strains; D009498:Neurotoxins / Q000494:pharmacology*; D009994:Osmolar Concentration; D011189:Potassium Chloride / Q000494:pharmacology; D012333:RNA, Messenger / Q000378:metabolism*; D013997:Time Factors
D015220:Calcium Channels; D009498:Neurotoxins; D012333:RNA, Messenger; D018698:Glutamic Acid; D011189:Potassium Chloride; D002118:Calcium