A Specific Trarisduction Mechanism for the Glutamate Action on Phosphoinositide Metabolism via the Quisqualate Metabotropic Receptor in Rat Brain Synaptoneurosomes: I. External Na⁺ Requirement

Abstract

The characteristics of the transduction mechanism(s) activated by glutamate (Glu) via the quisqualate metabotropic receptor, as well as by depolarizing agents, to trigger formation of inositol phosphates (IPs) were investigated in 8-day-old rat forebrain synaptoneurosomes. The replacement of external Na+ by various compounds (Li+, Tris+, N-methyl-D-glucamine+, and sucrose) induces an increase in basal accumulation of IPs and depolarizes synaptoneurosome membranes. Under these conditions, Glu- and K(+)-induced accumulations of IPs are inhibited, whereas the carbachol (Carb)-elicited response of IPs parallels the basal one. Agents increasing Na+ influx, such as veratridine and monensin, depolarize synaptoneurosomes and stimulate formation of IPs. These stimulations are not additive with responses of IPs elicited by Glu or K+. These data suggest that (a) Glu activates phosphoinositide metabolism via a specific mechanism (distinct from that of cholinergic agonists), (b) depolarizing agents and Glu share at least one common intermediate step in their mechanisms of activation of the metabolism of IPs, and (c) the depolarization may correspond to this common step. In addition, Na+ seems to be required for Glu stimulation of metabolism of IPs. The depolarization associated with the action of Glu on formation of IPs results neither from an influx via tetrodotoxin-sensitive voltage-dependent Na+ channels nor from an entry via the classically characterized Na+/Ca2+ or Na+/H+ exchangers. In fact, tetrodotoxin (2 microM) has no effect on the Glu- or K(+)-elicited response of IPs. Amiloride (greater than 50 microM) and some of its derivatives similarly inhibit not only Glu- and K(+)- but also Carb-evoked formation of IPs.

Effect of Thiol Reagents on Phosphoinositide Hydrolysis in Rat Brain Synaptoneurosomes

“…We have reported that depolarizing agents stimulate IP accumulation and mask the IP accumulation triggered by EAA (Guiramand et al, 1989). We have also found that a rise in intracellular Ca2+ by itself stimulates IP accumulation in this model.…”

“…It appears that the stimulatory action of Hg2+ on phosphoinositide metabolism originates from the depolarization that this ion produces. We and others (Gusovsky et al, 1986;Guiramand et al, 1991a) have demonstrated that paired combinations of depolarizing agents never induce an IP accumulation equivalent to the sum of the two maximal IP responses that each of these two substances elicit separately. In order to confirm that depolarization induced by Hg2+ is responsible for the stimulation of basal IP accumulation, we tested the effects on this metabolism of the combination of Hg2+ and another depolarizing agent (K+ or glutamate).…”

“…Indeed, the IP accumulations evoked by carbachol and depolarizing agents are additive ( Fig. 1B) (Guiramand et al, 1991a).…”

A Specific Transduction Mechanism for the Glutamate Action on Phosphoinositide Metabolism via the Quisqualate Metabotropic Receptor in Rat Brain Synaptoneurosomes: II. Calcium Dependency, Cadmium Inhibition

“…In this case, the synaptoneurosomes, following the 1 -h incorporation with ['Hlinositol, were rinsed and then resuspended in a Krebs-Ringer buffer without Ca", in which they were stimulated. Membrane potentials of synaptoneurosomes were determined using the fluorometric probe DiSCz(5) as explained by Guiramand et al (1991).…”

“…The existence of at least one new excitatory amino acid (EAA) receptor of the quisqualate (QA) type coupled to the activation of phosphoinositide metabolism, named the "QA metabotropic receptor," is now well established (Sugiyama et al, 1987;RCcasens et al, 1988; for review, see also Sladeczek et al, 1988; RC- casens et al, 1990~;Schoepp et al, 1990;Watkins et al, 1990;RCcasens and Guiramand, 1991). This QA metabotropic receptor, which has been described in various experimental models, possesses a particular pharmacological profile.…”