Stimulatory effects of the putative metabotropic glutamate receptor antagonist L‐AP3 on phosphoinositide turnover in neonatal rat cerebral cortex

1 The effects of the metabotropic glutamate receptor (mGluR) antagonist, L‐2‐amino‐3‐phosphono‐propionate (L‐AP3) on phosphoinositide turnover in neonatal rat cerebral cortex slices has been investigated. 2 At concentrations of ≤300 μm, L‐AP3 inhibited total [3H]‐inositol phosphate ([3H]‐InsPx) and Ins(1,4,5)P3 mass responses stimulated by the selective mGluR agonist, 1‐amino‐cyclopentane‐1S, 3R‐dicarboxylic acid (1S, 3R‐ACPD). Comparison with the competitive mGluR antagonist (±)‐α‐methyl‐4‐carboxyphenylglycine ((±)‐MCPG) clearly demonstrated that L‐AP3 caused inhibition by a mechanism that was not competitive, as L‐AP3 decreased the maximal response to 1S, 3R‐ACPD (by ∼40% at 300 μm L‐AP3) without significantly affecting the concentration of 1S, 3R‐ACPD required to cause half‐maximal stimulation of the [3H]‐InsPx response. 3 In contrast, at a higher concentration L‐AP3 (1 mM) caused a large increase in [3H]‐InsPx accumulation which was similar in magnitude in both the absence and presence of 1S, 3R‐ACPD (300 μm). D‐AP3 (1 mM) had no stimulatory effect alone and did not affect the response evoked by 1S, 3R‐ACPD. L‐AP3 (1 mM) also caused a large increase in Ins(1,4,5)P3 accumulation. The magnitude of the response (4–5 fold increase over basal) approached that evoked by a maximally effective concentration of 1S, 3R‐ACPD, but differed substantially in the time‐course of the response. The stimulatory effects of 1S, 3R‐ACPD and L‐AP3 on Ins(1,4,5)P3 accumulation were also similarly affected by decreases in extracellular calcium concentration. 4 Detailed analysis of the inositol phospholipid labelling pattern and the inositol (poly)phosphate isomeric species generated following addition of L‐AP3 was also performed. In the continued presence of myo‐[3H]‐inositol, L‐AP3 (1 mM) stimulated a significant increase in phosphatidylinositol labelling, but not that of the polyphosphoinositides, and the inositol (poly)phosphate profile suggested that substantial Ins(1,4,5)P3 metabolism occurs via both 5‐phosphatase and 3‐kinase routes. 5 A significant stimulatory effect of L‐AP3 (1 mM) on [3H]‐InsPx accumulation was also observed in neonatal rat hippocampus, and cerebral cortex and hippocampus slices prepared from adult rat brain. 6 These data demonstrate that whilst L‐AP3 antagonizes mGluR‐mediated phosphoinositide responses at concentrations of ≤ 300 μm, higher concentrations substantially stimulate this response. The ability of (±)‐MCPG (1 mM) to attenuate significantly