Magnesium ions reveal nanomolar potency of dopamine at [3H]spiperone labelled D-2 receptors in rat corpus striatum.

Preparation and assay of rat striatal membranes in the presence of 10 mM magnesium ions resulted in an IC50 value constant for dopamine of 13 nM at D-2 dopamine receptors labelled by [3H]spiperone. Inclusion of 100 microM GTP under these conditions resulted in a 13 fold decrease in dopamine displacing potency. Displacing potencies for dopamine in the nanomolar range are associated with agonist-specific D-3 receptor binding and it is predicted that the component of D-2 binding with high agonist affinity may play a confounding role in many D-3 receptor studies.