Stimulation of Ca(2+)-activated non-specific cationic channels by phospholipase C-linked glutamate receptors in synaptoneurosomes?
The regulation of intracellular Ca2+ concentration ([Ca2+]i) by glutamate metabotropic receptors (mGluR) was studied in 8-day-old rat forebrain synaptoneurosomes using spectrofluorimetric methods. Here we demonstrate that metabotropic glutamate agonists induce in rat brain synaptoneurosomes a Ca2+ influx largely dependent upon the presence of Ca2+ in the external medium. The pharmacological profile of this influx is strongly correlated with the pharmacological profile of the activation of phosphoinositide hydrolysis, i.e. quisqualic acid >> 1S,3R-amino-1-dicarboxylate-1,3 cyclopentane approximately equal to glutamate. This metabotropic glutamate receptor-induced Ca2+ influx is insensitive to voltage-dependent Ca2+ channel antagonists and occurs through a Mn2+ impermeant pathway. The study of the rapid kinetics shows that this influx is triggered after a 300 ms delay compared with that elicited by depolarizing agents and Ca2+ ionophore A23187. In order to assess further if mGluR stimulate this influx through the recruitment of inositol triphosphate (IP3)-sensitive intracellular Ca2+ stores, we have tested the effect of thapsigargin on membrane potential and intracellular Ca2+ simultaneously. Thapsigargin induces a depolarization of the synaptoneurosomal membrane followed by a massive Ca2+ influx, occurring via a Mn2+ nonpermeant route. This depolarizing effect is sensitive to the presence of the intracellular Ca2+ chelator [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetoxymethyl ester], and partially sensitive to extracellular Na+, but insensitive to the presence of extracellular Ca2+. Taken together, our data suggest that mGluR stimulate self-maintained increases of [Ca2+]i in rat forebrain synaptoneurosomes via the activation of a multistep mechanism, sequenced in the following steps: (i) mGluR-induced IP3 synthesis; (ii) IP3-stimulated intracellular Ca2+ release; (iii) Ca(2+)-activated non-specific cation channel, leading to local depolarization and a Ca2+ influx; and (iv) activation of Ca(2+)-sensitive phospholipase C.
Vignes
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M
INSERM U 254, Hôpital Saint-Charles, Montpellier, France.
Blanc
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E
Récasens
M
M
D000818:Animals; D000831:Animals, Newborn; D002118:Calcium / Q000494:pharmacology*; D002412:Cations / Q000494:pharmacology; D018698:Glutamic Acid / Q000494:pharmacology; D007473:Ion Channels / Q000187:drug effects*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D018094:Receptors, Metabotropic Glutamate / Q000187:drug effects*; D013574:Synaptosomes / Q000187:drug effects*; D013997:Time Factors; D010738:Type C Phospholipases / Q000494:pharmacology*
D002412:Cations; D007473:Ion Channels; D018094:Receptors, Metabotropic Glutamate; D018698:Glutamic Acid; D010738:Type C Phospholipases; D011188:Potassium; D002118:Calcium