Activation of polyphosphoinositide metabolism as a signal‐transducing system coupled to excitatory amino acid receptors in astroglial cells

Abstract

Excitatory amino acids (EAA) are known to induce an increase in the breakdown of polyphosphoinositides (PI) in brain slices and in dispersed cultures of neurons. We have now used astroglia cultured from newborn rat cerebra to demonstrate that glutamate provokes, in [3H]inositol-labeled cells, an accumulation of inositol phosphates in a time- and concentration-dependent manner. The ED50 value for glutamate was 40 microM. Quisqualate, ibotenate, and kainate were also active, with their relative potencies in the order of quisqualate greater than ibotenate much greater than kainate. No effect was detected with N-methyl-D-aspartate and quinolinic acid in the absence of Mg2+. The nonselective glutamate receptor antagonist gamma-D-glutamylglycine fully inhibited glutamate agonist-induced PI breakdown. A brief pretreatment of the astroglial cells with phorbol esters negated these effects of EAA receptor agonists, suggesting a feedback role for protein kinase C in phospholipase C action. Glutamate also elevated cytosolic free Ca2+ in Fura-2-loaded astroglial cells, as assessed by digital fluorescence imaging microscopy. Since a close metabolic partnership is known to exist between neurons and glia, these findings may have important functional consequences for neural cells in vivo.

Glutamate receptors activate Ca2+ mobilization and Ca2+ influx into astrocytes.

“…[ We also found evidence of QA-induced Ca2+ mobilization in these cells, as would be predicted from previous studies that demonstrated QA-induced InsP3 synthesis in astrocyte cultures (20,21,29,30). Agonist-induced InsP3 synthesis has been shown to be blocked by phorbol ester treatment in many cell types (31).…”

Calcium Dynamics in Cortical Astrocytes and Arterioles During Neurovascular Coupling

“…30,35,43,48 Astrocytes express L-type voltage-dependent Ca 2ϩ channels, 5,13,33 suggesting that these channels may play a role under physiological or pathological conditions. In our study, the effect of L-type Ca 2ϩ channel inhibition on the EFS-induced rise in astrocytic [Ca 2ϩ ] i could be attributed to either a direct effect on astrocytic L-type Ca 2ϩ channels or to indirect effect, for example, modulation of neuronal activity 13 or Ca 2ϩ filling of the astrocytic Ca 2ϩ stores.…”

“…13,5,33 The rise in astrocytic [Ca 2ϩ ] i likely also involves activation of IP 3 Rs. 35,43 Therefore, the effects of the IP 3 R blocker 2-APB (100 mol/L) were tested. This blocker caused a reduction in the EFS-induced rise in [Ca 2ϩ ] i in the astrocytic soma and endfoot by 55Ϯ13% (nϭ6; three experiments) and 44Ϯ10% (nϭ9; three animals), respectively ( Figure 2B).…”

Activation of astrocyte intracellular signaling pathways by interleukin‐1 in rat primary striatal cultures

“…Calcium imaging was used to investigate the percentage of striatal cells in culture that displayed an increase in intracellular calcium to stimulation with N-methyl-D-aspartate (NMDA, 10 M), such as that shown in Figure 1D. This represents a neuronal response, since astrocytes typically display non-NMDA glutamate receptors (Pearce et al, 1986(Pearce et al, , 1990Milani et al, 1989) and occurred in cells with a neuronal morphology. An average of 71% (Ϯ 5 SEM) of cells in culture demonstrated an NMDAinduced calcium response (n ϭ 15 coverslips from three separate culture preparations).…”