| Abstrakt: |
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 (Kd ~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 Kd and Bmax 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 Ki values of 6.8±3.1 µM (mGlu6), 211±43 µM (mGlu7a) and 1.6±0.3 µM (mGlu8a). With L-glutamate, we obtained Ki values of 12.3±3.5, 869±154 and 4.5±0.83 µM, for mGlu6, mGlu7a and mGlu8a, respectively. Ki 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. [ABSTRACT FROM AUTHOR] |
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