PHARMACOLOGICAL CHARACTERIZATION OF METABOTROPIC GLUTAMATERGIC AND PURINERGIC RECEPTORS LINKED TO CA2+ SIGNALING IN HIPPOCAMPAL ASTROCYTES

Intracellular Ca2+ ([Ca2+](i)) signals induced by metabotropic glutama te receptor (mGluR) agonists and by purinergic agonists in cultured hi ppocampal astrocytes were investigated using [Ca2+]-sensitive fluoroph ores. The mGluR agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) and (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced [Ca2+] (i) responses in 76 and 93% of the cells, respectively. The broad-spec trum mGluR antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) a nd the mGluR1 antagonists (S)-4-carboxy-3-hydroxyphenylglycine (4C3HPG ) and (S)-4-carboxyphenylglycine (4CPC) suppressed the agonist-evoked [Ca2+](i) response in about 25% of the cells completely and in about 6 0% partially, depending on the agonist concentration employed. Togethe r with immunohistochemical receptor localisations these results sugges t the presence of at least two subpopulations of class I mGluRs recrui ted from the truncated splice variants of mGluR1 (mGluR 1b, 1c, 1d) an d/or hitherto unknown glial-specific class I mGluRs. Of the hippocampa l astrocytes 88, 92 or 83% of the cells responded with a [Ca2+](i) ele vation (mostly oscillations) to application of ATP, ADP, or 2-methylth io-ATP (2-MeS-ATP), respectively, whereas only 14 and 5% responded to AMP and adenosine, respectively, indicating the predominance of P2 rec eptors. The ATP-induced [Ca2+](i) signal was suppressed by suramin. Re lease of Ca2+ from intracellular stores was involved in the response t o ATP because the cells also exhibited [Ca2+](i) elevations in Ca2+-fr ee medium. Cells did not respond to 10 mu M UTP. We conclude that the P2Y subtype represents the main [Ca2+](i)-linked purinoceptor in hippo campal astrocytes. Sequential application of ATP and DHPG in Ca-free m edium showed that metabotropic glutamate and purinergic receptors init iate release of Ca2+ from subsets of cyclopiazonic acid-sensitive Ca2 stores which are partly independent. (C) 1998 Elsevier Science Ltd. A ll rights reserved.