Effects of quisqualic acid analogs on metabotropic glutamate receptors coupled to phosphoinositide hydrolysis in rat hippocampus

Abstract

l-Glutamic acid (L-Glu) and l-aspartic acid (L-Asp) activate several receptor subtypes, including metabotropic Glu receptors coupled to phosphoinositide (PI) hydrolysis. Quisqualic acid (Quis) is the most potent agonist of these receptors. There is evidence that activation of these receptors may cause a long lasting sensitization of neurons to depolarization, a phenomenon called the Quis effect. The purpose of the current studies was to use Quis analogs and the recently identified metabotropic receptor antagonist, (+)-α-methyl-4-carboxy-phenylglycine((+)-MCPG), to define the structural properties required for interaction with the metabotropic receptors coupled to PI hydrolysis and to determine if the Quis effect is mediated by these receptors. The effects of Quis analogs on PI hydrolysis were studied in the absence or presence of the metabotropic receptor-specific agonist 1 SR,3 RS-1-amino-1,3-cyclopentanedicarboxylic acid (1 SR,3 RS-ACPD) in neonatal rat hippocampus. Some of the compounds that induce the Quis effect also stimulate PI hydrolysis, including Quis itself and 9 (homoquisqualic acid). Not all of the Quis analogs that stimulate PI hydrolysis, however, induce the Quis effect, including 7A (EC 50 = 750 ± 150 μM) and ( RS)-4-bromohomoibotenic acid (BrHI) (EC 50 = 130 ± 40 μM). Although (+)-MCPG blocked PI hydrolysis stimulated by Quis (IC 50 = 370 ± 70 μM), it had no effect on the induction of the Quis effect. Other Quis analogs did not stimulate PI hydrolysis but rather blocked the effects of 1 SR,3 RS-ACPD. The IC 50 values were 240 ± 70 μM for 2, 250 ± 90 μM for 3, and 640 ± 200 μM for 4. Data for inhibition by 2 and 3 were consistent with non-competitive mechanisms of action. These studies provide new information about the structural features of Quis required for interaction with metabotropic receptors coupled to PI hydrolysis and provide evidence that the Quis effect is not mediated by (+)-MCPG sensitive subtypes of these receptors.