Opioid receptors also belong to the group of G protein-coupled receptors. Besides controlling pain perception, they control functions as widely different as breathing, gastrointestinal motility, diuresis, temperature regulation, or cognitive processes. Binding assays for opioid receptors are therefore indispensable for the search for potential drugs in many indication areas. Opioid receptors can be divided into three subtypes, m-, k- and d-receptors. The pharmacologic effects mediated by these receptor subtypes are considerably different. Therefore, radioligand binding assays characterizing the affinity of test compounds at opioid receptor subtypes represent an essential primary screening tool in the search for, e.g. new analgesics [63, 64].
The competitive MS binding assays for m-opioid receptors were established using a membrane fraction of CHO-K1 cells that express human m-receptors in contrast to the native brain membrane preparations used in the dopamine receptor MS binding assays. The use of a heterologously expressed target offers the advantage that a relatively high concentration of a comparatively pure target can be employed, which in turn means that the requirement on the marker's affinity and selectivity as well as the sensitivity required for the quantitation of the marker is less demanding.
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