LY341495 is a highly potent and selective antagonist for group II mGlu receptors (mGlu2 and mGlu3). High affinity binding of [³H]LY341495 to recombinant human group II mGlu receptors (mGlu2 and mGlu3), and in rat brain homogenates (K_d ~1 nM), has been previously described.

Although LY341495 is a very selective nM-potent antagonist for group II mGlu receptors, it is also a relatively potent antagonist for group III mGlu receptors at high nanomolar to low micromolar concentrations.

In this study we examined and characterized the binding of [³H]LY341495 to membranes of cells expressing recombinant human group III mGlu receptors. Using up to 100 nM of [³H]LY341495, the level of specific binding in human mGlu4a receptor-expressing cell membranes was not appreciable and binding to this site was not examined further. In contrast, we demonstrated sufficient specific binding of [³H]LY341495 to human mGlu6, mGlu7a and mGlu8a receptor-expressing cell membranes to allow for further characterizations. [³H]LY341495 binding was saturable and rapidly reversible. [³H]LY341495 bound to a single site in each cell line, with K_d and B_max values of 31.6-6.8 nM and 3.3-0.7 pmol/mg protein (mGlu6), 72.7-22.0 nM and 3.7-0.4 pmol/mg protein (mGlu7a), and 14.0-1.1 nM and 3.0-0.2 pmol/mg protein (mGlu8a). [³H]LY341495 binding to mGlu6, 7a and 8a was displaceable by compounds which interact functionally with group III mGlu receptors. For example, L-AP4 displaced [³H]LY341495 with K_i values of 6.8-3.1 µM (mGlu6), 211-43 µM (mGlu7a) and 1.6-0.3 µM (mGlu8a). With L-glutamate, we obtained K_i values of 12.3-3.5, 869-154 and 4.5-0.83 µM, for mGlu6, mGlu7a and mGlu8a, respectively. K_i values for unlabelled LY341495 were 0.058-0.008, 0.22-0.05 and 0.029-0.008 µM, respectively. These studies demonstrated that [³H]LY341495 is a useful radioligand for studying the pharmacology and expression of recombinant mGlu6, 7a and 8a receptors in cell lines.