Intracellular Ca2+ release mediated by metabotropic glutamate receptor activation in the leech giant glial cell

We have investigated the effects of glutamate and glutamate receptor ligands on the intracellular free Ca2+ concentration ([Ca2+]i) and the 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 the Ca2+ indicator dye Fura-2. Application of glutamate (200-500 mumoll-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-nitroquinoxaline-2,3-dione (CNQX, 50-100 mumol l-1. Metabotropic glutamate receptor (mGluR) ligands had the following rank of potency in inducing a rise in [Ca2+]i: quisqualate (QQ, 200 mumol l-1) > glutamate (200 mumol l-1) > L(+)2-amino-3-phosphonopropionic acid (L-AP3, 200 mumol l-1 > trans-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD, 400 mumol l-1). The mGluR-selective antagonist (RS)-alpha-methyl-4-carboxyphenylglycine [(RS)-MCPG, 1 mmol l-1] significantly reduced glutamate-evoked increases in [Ca2+]i by 20%. Incubation of the ganglia with the endoplasmic ATPase inhibitor cyclopiazonic acid (CPA, 10 mumol l-1) caused a significant (53%) reduction of glutamate-induced [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 Ca(2+)-permeable ionotropic non-NMDA receptors and of metabotropic glutamate receptors leading to Ca2+ release from intracellular Ca2+ stores.