Selective metabotropic receptor agonists distinguish non-ionotropic glutamate binding sites

Metabotropic glutamate receptors (mGluRs) are thought to mediate diverse processes in brain including synaptic plasticity and excitotoxicity. These receptors are often divided into three groups by their pharmacological profiles. [3H]Glutamate binding in the presence of compounds selective for ionotropic glutamate receptors can be used as a general assay for these receptors; subtypes of this non-ionotropic [3H]glutamate binding differ in both pharmacology and anatomical distribution, and are differentially sensitive to quisqualate. The characteristics of these binding sites are consistent with those of group 1 (high-affinity quisqualate) and group 2 (low-affinity quisqualate) mGluRs. Under our assay conditions, no [3H]glutamate binding to group 3-like (L-AP4 sensitive) sites could be demonstrated. We have attempted to characterize particular agents which may selectively measure [3H]glutamate binding to mGluR subtypes. We used two isomers of 2-(carboxycyclopropyl)glycine, L-CCG-I and L-CCG-II, and the (2S,1'R,2'R,3'R) isomer of 2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) as competitors of non-ionotropic [3H]glutamate binding sites. DCG-IV clearly distinguishes two binding sites. Quantitative levels of DCG-IV binding by anatomic region correlate with quisqualate-defined binding subtypes: high-affinity DCG-IV binding correlates with low-affinity quisqualate binding, whereas low-affinity DCG-IV binding correlates with high-affinity quisqualate binding. L-CCG-II displaces only one type of non-ionotropic [3H]glutamate binding, corresponding to high-affinity quisqualate binding. Therefore DCG-IV and L-CCG-II at appropriate concentrations appear to distinguish binding to putative group 2 vs. group 1 mGluRs. L-CCG-I displaces both high- and low-affinity quisqualate binding sites, but unlike the other two compounds, does not clearly distinguish between them.