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) antagonis t, L-2-amino-3-phosphonopropionate (L-AP3) on phosphoinositide turnove r in neonatal rat cerebral cortex slices has been investigated. 2 At c oncentrations of less than or equal to 300 mu M, L-AP3 inhibited total [H-3]-inositol phosphate ([H-3]-InsP(x)) and Ins(1,4,5)P-3 mass respo nses stimulated by the selective mGluR agonist, 1-amino-cyclopenrane-1 S, 3R-dicarboxylic acid (1S, 3R-ACPD). Comparison with the competitive mGluR antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine ((+/-)-MCP G) clearly demonstrated that L-AP3 caused inhibition by a mechanism th at was not competitive, as L-AP3 decreased the maximal response to 1S, 3R-ACPD (by similar to 40% at 300 mu M L-AP3) without significantly a ffecting the concentration of 1S, 3R-ACPD required to cause half-maxim al stimulation of the [H-3]-InsP(x) response. 3 In contrast, at a high er concentration L-AP3 (1 mM) caused a large increase in [H-3]-InsP(x) accumulation which was similar in magnitude in both the absence and p resence of 1S, 3R-ACPD (300 mu M). D-AP3 (1 mM) had no stimulatory eff ect 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)P-3 accumulation. Th e 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 stimul atory effects of 1S, 3R-ACPD and L-AP3 on Ins(1,4,5)P-3 accumulation w ere also similarly affected by decreases in extracellular calcium conc entration. 4 Detailed analysis of the inositol phospholipid labelling pattern and the inositol (poly)phosphate isomeric species generated fo llowing addition of L-AP3 was also performed. In the continued presenc e of myo-[H-3]-inositol, L-AP3 (1 mM) stimulated a significant increas e in phosphatidylinositol labelling, but not that of the polyphosphoin ositides, and the inositol (poly)phosphate profile suggested that subs tantial Ins(1,4,5)P-3 metabolism occurs via both 5-phosphatase and 3-k inase routes. 5 A significant stimulatory effect of L-AP3 (1 mM) on [H -3]-InsP(x) accumulation was also observed in neonatal rat hippocampus , and cerebral cortex and hippocampus slices prepared from adult rat b rain. 6 These data demonstrate that whilst L-AP3 antagonizes mGluR-med iated phosphoinositide responses at concentrations of less than or equ al to 300 mu M, higher concentrations substantially stimulate this res ponse. The ability of (+/-)-MCPG (1 mM) to attenuate significantly L-A P3-stimulated [H-3]-InsP(x) accumulation, suggests that both the inhib itory and stimulatory effects of L-AP3 may be mediated by mGluRs.