INTRACELLULAR CA2-RECEPTOR ACTIVATION IN THE LEECH GIANT GLIAL-CELL( RELEASE MEDIATED BY METABOTROPIC GLUTAMATE)

We have investigated the effects of glutamate and glutamate receptor l igands on the intracellular free Ca2+ concentration ([Ca2+](i)) and th e membrane potential (Em) of single, identified neuropile glial cells in the central nervous system of the leech Hirudo medicinalis, Exposed glial cells of isolated ganglia were filled iontophoretically with th e Ca2+ indicator dye Fura-2, Application of glutamate (200-500 mu mol l(-1)) caused biphasic membrane potential shifts and increases in [Ca2 +](i), which were only partly reduced by either removing extracellular Ca2+ or blocking ionotropic glutamate receptors with 6-cyano-7-nitroq uinoxaline-2,3-dione (CNQX, 50-100 mu mol l(-1)), Metabotropic glutama te receptor (mGluR) ligands had the following rank of potency in induc ing a rise in [Ca2+](i): quisqualate (QQ, 200 mu mol l(-1)) > glutamat e (200 mu mol l(-1)) > L(+)2-amino-3-phosphonopropionic acid (L-AP3,20 0 mu mol l(-1)) > trans-1-aminocyclopentane-1,3-dicarboxylic acid (t-A CPD, 400 mu mol l(-1)). The mGluR-selective antagonist (RS)-alpha-meth yl-4-carboxyphenylglycine [(RS)MCPG, 1 mmol l(-1)] significantly reduc ed glutamate-evoked increases in [Ca2+](i) by 20%, Incubation of the g anglia with. the endoplasmic ATPase inhibitor cyclopiazonic acid (CPA, 10 mu mol l(-1)) caused a significant (53%) reduction of glutamate-in duced [Ca2+](i) transients, while incubation with lithium ions (2 mmol l(-1)) resulted in a 46% reduction, The effects of depleting the Ca2 stores with CPA and of CNQX were additive, We conclude that glutamate -induced [Ca2+](i) transients were mediated by activation of both Ca2-permeable ionotropic non-NMDA receptors and of metabotropic glutamate receptors leading to Ca2+ release from intracellular Ca2+ stores.