Dopamine receptors belong to the group of G protein-coupled receptors (GPCRs) . GPCRs are integral membrane proteins that, after binding the respective endogenous messengers, transduce signals into the interior of a cell through interaction with heterotrimeric G proteins. More than 600 different genes coding for GPCRs were discovered on the human genome. Currently, they are the most frequently addressed targets in drug development . Disturbed dopaminergic neurotransmission is linked to a number of diseases of the central nervous system, including Morbus Parkinson and schizophrenia . Therefore, binding assays for dopamine receptors play a crucial role in the development of new drugs for the relevant indications. So far five different subtypes of dopamine receptors (Dj 5) have been found. They can be classified into two groups according to their signal transduction and pharmacology, Dj and D5 on one side and D2 4 on the other. Di and D2 are the subtypes that occur most frequently in the brain . Taking the determination of the affinity of test compounds for dopamine D1 and D2 receptors as a typical issue of the modern drug discovery process, it will be exemplified here how competitive MS binding assays based on the quantitation of the nonbound marker can be accomplished easily.
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