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Differences in agonist and antagonist activities for two indices of metabotropic glutamate receptor‐stimulated phosphoinositide turnover
Author(s) -
Mistry Rajendra,
Challiss R.A. John
Publication year - 1996
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1996.tb15347.x
Subject(s) - acpd , metabotropic glutamate receptor , metabotropic receptor , agonist , ionotropic effect , glutamate receptor , nmda receptor , antagonist , inositol phosphate , biology , medicine , endocrinology , chemistry , receptor , inositol , biochemistry
1 The abilities of the four diastereoisomers of 1‐aminocyclopentane‐1,3‐dicarboxylic acid (ACPD) to stimulate, and the metabotropic glutamate receptor (mG1uR) antagonist (±)‐α‐methylcarboxyphenylglycine (MCPG) to inhibit, phosphoinositide turnover in neonatal rat cerebral cortex have been studied. Two indices of phosphoinositide cycle activity were assessed; inositol 1,4,5‐trisphosphate (Ins(1,4,5)P 3 ) mass accumulation, and total inositol phosphate [ 3 H]‐InsP x accumulation (in the presence of Li + ) in myo ‐[ 3 H]‐inositol prelabelled slices. 2 The diastereoisomers of ACPD stimulated each response with a rank order of potency of 1S, 3R > 1R, 3R > 1S, 3S > > 1R, 3S. The response to 1R, 3R‐ACPD was largely prevented by pre‐addition of the NMDA‐receptor antagonist, MK‐801, or omission of extracellular Ca 2+ , suggesting that this isomer acts indirectly on phosphoinositide responses through activation of NMDA‐type ionotropic glutamate receptors. In contrast, the responses to 1S, 3R‐ and 1S, 3S‐ACPD were unaffected by prior addition of MK‐801, but were blocked by MCPG. 3 The concentration of 1S, 3R‐ACPD required to half‐maximally stimulate the Ins(1,4,5)P 3 response (‐log EC 50 (M), −4.09 ± 0.10) was significantly higher than that required to exert a similar effect on [ 3 H]‐InsP x accumulation (‐log EC 50 (M), −4.87 ±0.07; P <0.01; n =4). A similar marked 8–9 fold discrepancy between these two values was observed for the 1S, 3S isomer, which elicited similar maximal responses to those caused by 1S, 3R‐ACPD. 4 Significant differences were also observed with respect to the ability of (±)‐MCPG (1 mM) to cause a rightward shift in the concentration‐response relationships for 1S, 3R‐ACPD‐stimulated Ins(1,4,5)P 3 (5.59±0.24 fold shift) and [ 3 H]‐InsP x (3.04±0.34 fold shift; P <0.01; n =4) responses, giving rise to K d values of 218 and 490 μ m for (±)‐MCPG antagonism of the respective responses. 5 The potency difference between the 1S, 3R‐ACPD‐stimulated Ins(1,4,5)P 3 and [ 3 H]‐InsP x responses was reduced when experiments were performed in nominally calcium‐free medium ([Ca 2+ ] e = 2–5 μ m ) and EC 50 values were almost identical when extracellular calcium was reduced further by EGTA addition ([Ca 2+ ] e ≤ 100 nM). Similarly, the K d value for (±)‐MCPG antagonism of the 1S, 3R‐ACPD‐stimulated [ 3 H]‐InsP x response decreased under [Ca 2+ ] e ‐free conditions, approaching those obtained for the 1S, 3R‐ACPD‐stimulated Ins(1,4,5)P 3 response in the presence of normal [Ca 2+ ] e . 6 These data suggest that estimates of the activities of mGluR agonists and antagonists, derived by measuring phosphoinositide turnover, can differ significantly depending on whether Ins(1,4,5)P 3 mass or [ 3 H]‐InsP x responses are measured. In particular, the possibility that the mGluR‐mediated [ 3 H]‐InsP x response may not simply reflect direct receptor/G protein/phosphoinositidase C (PIC) activation, but may also be the consequence of stimulation of a facilitatory Ca 2+ ‐influx pathway is discussed.