Multiple Mechanisms for Inhibition of Excitatory Amino Acid Receptors Coupled to Phosphoinositide Hydrolysis

Abstract

Excitatory amino acid (EAA) analogues activate receptors that are coupled to the increased hydrolysis of phosphoinositides (PIs). In these studies, hippocampal slices were prepared from neonatal rats (6-11 days old) to characterize the effects of EAA analogues on these receptors. The concentrations of ibotenate and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) required to evoke half-maximal stimulation (EC50 values) were 28 and 51 microM, respectively. Although the data for stimulation of PI hydrolysis by ibotenate and trans-ACPD were best fit to theoretical curves that had Hill slopes of 1, data for stimulation of PI hydrolysis by quisqualate were best fit to two sites. The EC50 values were 0.43 and 44 microM. The high-affinity sites were 70% of the total. A number of EAA analogues were tested for inhibition of PI metabolism. One of these, L-aspartate-beta-hydroxamate (L-A beta HA), was identified as a novel inhibitor of this response. L-A beta HA was equipotent as an inhibitor of PI metabolism stimulated by ibotenate, quisqualate, and trans-ACPD. The data for this inhibition were best fit to two sites. Between 32 and 48% of the total sites had high affinity with IC50 values in the range of 1.2-6.3 microM. The low-affinity sites had IC50 values between 610 and 2,700 microM. DL-2-Amino-3-phosphonopropionate (DL-AP3) was also equipotent as an inhibitor of PI hydrolysis stimulated by ibotenate, quisqualate, and trans-ACPD (IC50 values were 480-850 microM). In contrast to the data for L-A beta HA, the data for DL-AP3 were best fit to a single site. Both of these inhibitors reduced the maximal response caused by the agonists, consistent with noncompetitive mechanisms of action. Several experiments were designed to examine potential mechanisms for these noncompetitive effects. These studies suggest that either L-A beta HA and DL-AP3 bind to a site on the receptor and irreversibly block activation of the receptor, or that these inhibitors act via a distinct site that specifically regulates EAA receptors coupled to PI hydrolysis.

Endogeous sulfur dioxide protects against oleic acid-induced acute lung injury in association with inhibition of oxidative stress in rats

“…47,48 Moreover, HDX could inhibit matrix metalloproteinases and AAT activities, in addition to reversing the increased AAT levels in plasma as demonstrated by a few rat experiments. [49][50][51] Furthermore, HDX could inhibit AAT activities and the generation of endogenous SO 2 in both pulmonary tissues and myocardium.…”

Swelling of Müller cells induced by AP3 and glutamate transport substrates in rat retina

“…Although AP3 has been shown to inhibit 3[H]-glutamate uptake into synaptosomes (Littman et al, 1992), it is not clear whether AP3 is a substrate for transport. We addressed this issue using whole-cell patch clamp recordings in microcultures of hippocampal astrocytes (Mennerick and Zorumski, 1994).…”

“…Previous studies indicate that AF' 3 inhibits glutamate transport (Littman et al, 1992). To determine whether effects on glutamate transporters contribute t o AP3-induced damage, we examined the effects of the glutamate transport inhibitors D,L-threo-3-hydroxyaspartate (THA), pyrrolidine dicarboxylic acid (PDC), D,L-aminoadipic acid (AAA), and dihydrokainate (DHK) ( Table 1).…”

“…In support of this possibility, recent studies indicate that the severe neurotoxicity produced by repeated AP3 injections in neonatal rats is mimicked by certain agents that antagonize metabotropic receptors (Price et al, 1995), and 2-amino-4-phosphonobutyrate (AP4), a structural analog of AP3 that also has partial agonistantagonist effects at metabotropic receptors, causes C) 1996 Wiley-Liss, Inc. astrocyte swelling (Bender et al, 1995;Hansson, 1994). However, other studies indicate that AP3 inhibits glutamate transport in hippocampal synaptosomes (Littman et al, 1992), suggesting that effects on glial glutamate transporters might contribute to the swelling. Indeed, the Muller cell swelling produced by glutamate appears to result from glutamate transport into cells (Casper et al, 1982).…”

Effects of pharmacological inhibition of glutamate-uptake on ischaemia-induced glutamate efflux and anoxic depolarization latency

“…The fact that L-trans-PDC is transportable also implies that, when it is applied extracellularly, it may also produce carrier-mediated efflux of glutamate (heteroexchange) (Waldmeier et al 1993;Griffiths et al 1994). Some studies have suggested that L-trans-PDC may have agonistic actions on metabotropic receptors (mGluR) (Littman et al 1992;Miller et al 1994). However, there is some controversy on the origin of these effects, because they could actually be secondary to heteroexchange (Thomsen et al 1994;Desai et al 1995).…”