4-bromohomoibotenic-acid and 1-amino-1-3-dicarboxycyclopentane

Metabotropic glutamate receptors (mGluRs) are a family of glutamate receptors that are coupled to a variety of second messenger systems through GTP-binding proteins. Of the eight subtypes cloned to date, mGluR1 and mGluR5 are coupled to phosphoinositide hydrolysis in expression systems, and both are activated by the glutamate analogue 1-aminocyclopentane-1S,3R-dicarboxylic acid. Previously, we provided evidence that in rat cortical slices, 4-bromohomoibotenic acid (BrHI) and 4-methylhomoibotenic acid (MHI) activate a 1-aminocyclopentane-1S,3R-dicarboxylic acid-insensitive phosphoinositide hydrolysis-coupled mGluR. We further examine these compounds in expression systems. In a stable cell line expressing mGluR1a, BrHI is a weak partial agonist whereas MHI has no agonist activity. In Xenopus oocytes expressing mGluR1a or mGluR5a, BrHI is a weak agonist at mGluR5a whereas MHI is without effect on either receptor. Both BrHI and MHI have weak agonist activity at mGluRs 4a and 7a expressed in stable BHK cell lines whereas neither compound had any activity on BHK cells expressing mGluR2. Finally, we found that the novel mGluR antagonist LY341495 completely blocked the activation of mGluR1 and mGluR5 and blocked the phosphoinositide hydrolysis response to DHPG in rat cortical slices. In contrast, LY341495 did not block the phosphoinositide hydrolysis response to MHI in rat cortical slices. This provides further evidence that the phosphoinositide hydrolysis response to MHI in rat cortical slices is due to activation of a novel receptor that is distinct from the previously cloned mGluRs.

Topics: Animals; Cell Line; Cricetinae; Cycloleucine; Female; Hydrolysis; Ibotenic Acid; Phosphatidylinositols; Rats; Receptors, Metabotropic Glutamate; Xenopus

Glutamate activates a family of receptors, known as metabotropic glutamate receptors (mGluRs), that are coupled to various second messenger systems through G proteins. All mGluR subtypes characterized to date in rat brain slices are activated by the glutamate analogue 1-aminocyclopentane-1S,3R-dicarboxylic acid (1S,3R-ACPD). However, few agonists are available that selectively activate specific mGluR subtypes. We report that the glutamate analogue (R,S)-4-bromohomoibotenate (BrHI) stimulates phosphoinositide hydrolysis in rat cerebral cortical slices in a concentration-dependent manner (EC50 = 190 microM). The response to BrHI is stereoselective and is not blocked by ionotropic glutamate receptor antagonists. It is interesting that the responses to BrHI and 1S,3R-ACPD are completely additive, suggesting that these responses are mediated by different receptor subtypes. Consistent with this, the response to BrHI is insensitive to L-2-amino-3-phosphonopropionic acid (L-AP3), whereas the response to 1S,3R-ACPD is partially blocked by L-AP3. BrHI does not activate metabotropic receptors coupled to changes in cyclic AMP accumulation or activation of phospholipase D. Thus, BrHI seems to activate specifically a phosphoinositide hydrolysis-linked mGluR that is insensitive to 1S,3R-ACPD. This compound may prove useful as a tool for elucidating the roles of different mGluR subtypes in mammalian brain.

Topics: Adenylyl Cyclase Inhibitors; Animals; Cerebral Cortex; Cyclic AMP; Cycloleucine; Drug Resistance; Enzyme Activation; Hydrolysis; Ibotenic Acid; Male; Phosphatidylinositols; Phospholipase D; Rats; Rats, Sprague-Dawley; Receptors, Glutamate

(S)-Bromohomoibotenic acid [(S)-BrHIbo] stereoselectively antagonized glutamate-stimulated phosphoinositide (PI) hydrolysis in baby hamster kidney (BHK) cells expressing mGluR1a in a competitive manner with an IC50 of 250 microM. However, (S)-BrHIbo did not inhibit (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD]-induced PI hydrolysis in rat hippocampal slices (S)- or (R)-BrHIbo did not show any effects on forskolin-stimulated cAMP-formation in BHK cells expressing mGluR2 or mGluR4 but did displace [3H]2-amino-4-phosphonobutyrate ([3H]AP4) binding from rat corticalmembranes with high affinities (IC50 = 1.0 microM and 1.1 microM, respectively). These data suggest that (S)-BrHIbo may interest with multiple PI-coupled glutamate receptors, however, at concentrations that are several fold higher than for displacement of [3H]AP4 binding from rat cortical membranes.

Topics: Aminobutyrates; Animals; Binding, Competitive; Cell Line; Cerebral Cortex; Cricetinae; Cyclic AMP; Cycloleucine; Dose-Response Relationship, Drug; Glutamic Acid; Hippocampus; Hydrolysis; Ibotenic Acid; Kidney; Membranes; Phosphatidylinositols; Quinoxalines; Receptors, AMPA; Receptors, Metabotropic Glutamate; Stereoisomerism