Role for ionotropic and metabotropic receptors in quisqualate-stimulated inositol polyphosphate accumulation in rat cerebral cortex.
The actions of the excitatory amino acid quisqualate (QA) on inositol polyphosphate accumulation in cerebral cortex slices have been assessed using both [3H]inositol prelabeling and mass measurements over relatively short incubation periods. QA stimulated accumulation of all the inositol polyphosphates, with similar EC50 values (2.8 +/- 0.7 microM). High performance liquid chromatography analysis of isomeric forms of inositol polyphosphates and specific mass assays revealed that both phosphorylation and dephosphorylation products of inositol-1,4,5-trisphosphate accumulate. A large component of the QA-stimulated inositol polyphosphate accumulation was inhibited by the ionotropic antagonist 6,7-dinitroquinoxaline-2,3-dione in a competitive manner. This implied that the QA response may be due to entry of Ca2+ via voltage-sensitive calcium channels as a consequence of an ionotropic receptor-induced depolarization. In support of this mechanism, the QA-induced response was dependent on the presence of extracellular calcium, whereas the well characterized muscarinic receptor agonist response to carbachol showed only a slight reduction under the same conditions. The concentration-dependent (EC50 8.8 +/- 3 microM) response to the selective ionotropic agonist amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) differed from that to QA or carbachol, in that accumulation of only [3H]inositol mono- and bisphosphates was stimulated, with no increase in the [3H]inositol tris- or tetrakisphosphates. Use of the metabotropic agonist (trans)-(+/-)-1-aminocyclopentyl-1,3-dicarboxylate (ACPD), however, produced concentration-dependent increases in all [3H]inositol polyphosphates. Although both AMPA and ACPD responses alone were smaller in magnitude than that to QA, when present together AMPA and ACPD produced additive responses on [3H]inositol mono- and bisphosphate and a marked synergistic increase in [3H]inositol tetrakisphosphate accumulation, resulting in a response similar to that seen for QA. These data suggest that QA-evoked responses in rat cortex slices are the result of a complex interaction mediated through both ionotropic and metabotropic receptors, in which Ca2+ entry may stimulate accumulation of inositol mono- and bisphosphate directly and divert the metabolism of inositol-1,4,5-trisphosphate to inositol-1,3,4,5-tetrakisphosphate.
Baird
J G
JG
Department of Pharmacology and Therapeutics, University of Leicester, UK.
Challiss
R A
RA
Nahorski
S R
SR
D000818:Animals; D002118:Calcium / Q000502:physiology; D002540:Cerebral Cortex / Q000378:metabolism*; D003515:Cycloleucine / Q000031:analogs & derivatives / Q000494:pharmacology; D007051:Ibotenic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D066298:In Vitro Techniques; D007295:Inositol Phosphates / Q000378:metabolism*; D011810:Quinoxalines / Q000494:pharmacology; D016318:Quisqualic Acid / Q000494:pharmacology*; D051381:Rats; D018091:Receptors, AMPA; D017981:Receptors, Neurotransmitter / Q000502:physiology*; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid
D007295:Inositol Phosphates; D011810:Quinoxalines; D018091:Receptors, AMPA; D017981:Receptors, Neurotransmitter; D003515:Cycloleucine; C036361:1-amino-1,3-dicarboxycyclopentane; D007051:Ibotenic Acid; C056723:FG 9041; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; D016318:Quisqualic Acid; D002118:Calcium