Activation of a metabotropic glutamate receptor increases intracellular calcium concentrations in neurons of the avian cochlear nucleus

Abstract

Metabotropic glutamate receptors have been shown to stimulate phosphatidylinositol metabolism, and subsequently liberate Ca2+ from intracellular stores, in a variety of tissue and cell types. We previously demonstrated that glutamate could stimulate phosphatidylinositol metabolism, generating inositol-1,4,5-trisphosphate (IP3), in isolated cochlear nucleus tissue from the chick. Using the calcium indicator dye fura-2 and ratiometric fluorescent imaging, this study examined the ability of glutamate and its analogs to liberate Ca2+ from intracellular stores of neurons of the avian cochlear nucleus, and qualitatively characterized the pharmacological profile of such an action. In normal, Ca(2+)-containing medium, glutamate, kainate (KA), alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), NMDA, quisqualate (QUIS), and (+/-)-aminocyclopentane-trans-dicarboxylate (ACPD) elicited increases in intracellular calcium concentrations ([Ca2+]i). In the absence of external Ca2+, glutamate, quisqualate, and ACPD evoked increases in [Ca2+]i. In normal medium, the ionotropic glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the NMDA receptor antagonist 2-amino-5-phosphonovalerate (APV) attenuated but did not abolish the glutamate-evoked response and had no effect on the ACPD-evoked response. The putative metabotropic glutamate receptor antagonist 2-amino-3-phosphonopropionate (AP3) was without effect on the glutamate- and ACPD-evoked increases in [Ca2+]i in Ca(2+)-free medium. We conclude that a metabotropic glutamate receptor (mGluR) is present on cochlear nucleus neurons and is able to stimulate the phosphatidylinositol metabolism--Ca2+ signal transduction cascade.