(trans)-1-amino-cyclopentyl-1,3-dicarboxylate stimulates quisqualate phosphoinositide-coupled receptors but not ionotropic glutamate receptors in striatal neurons and Xenopus oocytes.

The effects of a novel glutamate analogue, (trans)-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD), have been tested in striatal neurons in primary culture and in Xenopus oocytes injected with rat brain RNA. Both systems have been previously shown to contain well characterized metabotropic receptors coupled to phospholipase C (Qp), as well as ionotropic glutamate receptors. In striatal neurons, ACPD stimulated inositol phosphate (InsP) accumulation (EC50 = 9.7 +/- 2.5 microM; maximal effect, 184.7 +/- 11.6% of basal accumulation). This effect of ACPD was likely to be mediated by Qp receptors, because maximal ACPD and quisqualate-induced InsP formation were not additive. In contrast, the effects of ACPD and norepinephrine on InsP formation were additive. ACPD-induced InsP formation was not antagonised by antagonists of muscarinic and alpha 1-adrenergic receptors (1 microM atropine and 0.1 microM prazosin, respectively). In Xenopus oocytes, ACPD and quisqualate induced an oscillatory increase of a Ca2(+)-dependent chloride conductance, which is characteristic of the activation of phospholipase C-coupled receptors in this model. The specificity of ACPD on Qp receptors was demonstrated by testing the effect of this drug on quisqualate/kainate as well as on N-methyl-D-aspartate ionotropic receptors. In striatal neurons, the activation of quisqualate/kainate and N-methyl-D-aspartate receptors was tested by measurement of [3H]-gamma-aminobutyric acid release and by electrophysiological recordings using the patch-clamp technique. At concentrations as high as 1 mM, ACPD was inactive on these inotropic receptors, either as agonist or as antagonist. In conclusion, ACPD appeared to be a highly specific agonist of Qp receptors, with no activity on ionotropic glutamate receptors. It will be a useful drug to study the physiological properties of Qp receptors in vertebrate brains.