Differential coupling of G-protein-linked receptors to Ca2+ mobilization through inositol(1,4,5)trisphosphate or ryanodine receptors in cerebellar granule cells in primary culture

Rat cerebellar granule cells in primary culture possess muscarinic, metabot ropic glutamatergic, histaminergic and alpha-adrenergic receptors which cou ple to phosphoinositide-specific phospholipase C. We have determined the ab ility of these receptors to elevate inositol(1,4,5)trisphosphate and to rel ease intracellular calcium, in order to establish the correlation between t hese two responses. In resting cerebellar granule cells, only the muscarini c agonist carbachol evoked significant increases in both inositol(1,4,5)tri sphosphate and cytoplasmic free Ca2+. Mild depolarization (20 mM KCl) enhan ced inositol(1,4,5)trisphosphate elevation by carbachol and histamine, but not by noradrenaline or the metabotropic glutamate agonist 1S,3R ACPD. In c ontrast, Ca2+-release responses were modified differently by 20 mM KCl-depo larization: the responses to carbachol, histamine and 1S,3R ACPD, but not t he responses to noradrenaline, were markedly enhanced. The contribution of ryanodine-sensitive Ca2+-release channels (ryanodine receptors) to the calc ium release signal in depolarized cells was determined. Ryanodine (10 mu M) inhibited most effectively the cytoplasmic Ca2+ elevation evoked by 1S,3R ACPD (> 90%), while Ca2+ release upon stimulation by carbachol and histamin e was only inhibited by approximate to 60% and remained larger than in the absence of KCl. Our data are consistent with a specific coupling between me tabotropic glutamate receptors and ryanodine-sensitive Ca2+-release channel s which may not require generation of inositol(1,4,5)trisphosphate.