Role of arachidonic acid and glutamate in the formation of inositol phosphates induced by noradrenalin in striatal astrocytes.
The noradrenalin-evoked production of [3H]inositol phosphates in mouse striatal astrocytes in primary culture appeared to be the result of the combined stimulation of alpha 1- and alpha 2-adrenergic receptors. Indeed, the noradrenalin (100 microM) response was only partially reproduced by a maximally effective concentration of methoxamine (100 microM), a selective agonist of alpha 1-adrenergic receptors. In addition, the noradrenalin (100 microM)-induced production of [3H]inositol phosphates, which was completely suppressed by the alpha 1-adrenergic antagonist prazosin (1 microM), was also partially inhibited by yohimbine, a selective antagonist of alpha 2-adrenoceptors (maximum inhibition = -57 +/- 11%, measured in the presence of 10 microM yohimbine; six experiments). Finally, UK14.304, a selective alpha 2-adrenergic agonist that was ineffective alone, enhanced the methoxamine-evoked production of [3H] inositol phosphates (EC50 = 86 +/- 21 nM; three experiments). These results suggest that the stimulation of alpha 1-adrenergic receptors is required for the alpha 2-adrenergic receptor-mediated enhancement of phospholipase C activity. The increased production of [3H]inositol phosphates resulting from the stimulation of alpha 2-adrenergic receptors involved pertussis toxin-sensitive G proteins (Gi/o) and depended on extracellular calcium. As shown using the fluorescent dye indo-1, noradrenalin (100 microM) induced a long-lasting increase in cytosolic calcium in striatal astrocytes. Moreover, noradrenalin (100 microM) stimulated [3H]arachidonic acid release from these cells. These two latter responses may result from synergistic effects due to the combined stimulation of alpha 1- and alpha 2-adrenergic receptors, because they were inhibited by either prazosin (1 microM) or yohimbine (10 microM). Finally, the noradrenalin-evoked production of [3H]inositol phosphates seems to result partly from an inhibition by arachidonic acid of glutamate uptake into astrocytes, leading to the stimulation of glutamate metabotropic receptors coupled to phospholipase C. Indeed, the alpha 2-adrenergic component of the noradrenalin response was suppressed by either enzymatic removal of external glutamate or addition of 2-amino-3-phosphonopropionic acid (1 mM), an antagonist of glutamate metabotropic receptors that blocked the glutamate-evoked production of [3H]inositol phosphates in striatal astrocytes, and was reproduced by the direct application of either glutamate or an inhibitor of glutamate uptake, beta-methyl-DL-aspartic acid.
Marin
P
P
Chaire de Neuropharmacologie (INSERM U114), Collège de France, Paris.
Stella
N
N
Cordier
J
J
Glowinski
J
J
Prémont
J
J
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D016718:Arachidonic Acid / Q000502:physiology*; D001253:Astrocytes / Q000187:drug effects* / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D003342:Corpus Striatum / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism; D005971:Glutamates / Q000502:physiology*; D018698:Glutamic Acid; D007295:Inositol Phosphates / Q000096:biosynthesis*; D051379:Mice; D009638:Norepinephrine / Q000494:pharmacology*
D000316:Adrenergic alpha-Agonists; D005971:Glutamates; D007295:Inositol Phosphates; D016718:Arachidonic Acid; D018698:Glutamic Acid; D019204:GTP-Binding Proteins; D002118:Calcium; D009638:Norepinephrine