Ionotropic and metabotropic glutamate receptor agonist-induced [Ca2+]i increase in isolated rat supraoptic neurons

In the present study, the effects of glutamate and of agonists for ionotropic and metabotropic glutamate receptors on intracellular Ca2+ concentration ([Ca2+]i) were investigated in neurons of the rat supraoptic nucleus (SON). We used the intracellular Ca2+ imaging technique with fura-2, in single magnocellular neurons dissociated from the SON of rats. Glutamate (10(-6)-10(-4) M) evoked a dose-dependent increase in [Ca2+]i. The glutamate agonists exerted similar effects, although with some differences in the characteristics of their responses. The [Ca2+]i response to NMDA was smaller than those of glutamate or the non-NMDA receptor agonists, AMPA and kainate, but was significantly enhanced by the removal of extracellular Mg2+. Glutamate, as well as quisqualate, an agonist for both ionotropic and metabotropic glutamate receptors, evoked a [Ca2+]i increase in a Ca2+-free condition, suggesting Ca2+ release from intracellular Ca2+ stores. This was further evidenced by [Ca2+]i increases in response to a more selective metabotropic glutamate receptor agonist, t-ACPD, in the absence of extracellular Ca2+. Furthermore, the quisqualate-induced Ca2+ release was abolished by the selective metabotropic glutamate receptor antagonist, (S)-4-carboxyphenylglycine. The results suggest that metabotropic glutamate receptors as well as non-NMDA and NMDA receptors are present in the SON neurons, and that activation of the first leads to Ca2+ release from intracellular Ca2+ stores and the activation of the latter two types induces Ca2+ entry. These dual mechanisms of Ca2+ signalling may play a role in the regulation of SON neurosecretory cells by glutamate.