Mapping Neuroreceptors at work: on the Definition and Interpretation of Binding Potentials after 20 years of Progress
Section snippets
Twenty Years of Progress
Positron-emitting radioligands and radiolabeled substrates of transmitter synthesizing enzymes were created as early as 1983 for the in vivo imaging of brain tissue with positron emission tomography (PET) (Wagner et al., 1983) (Fig. 1). Although this class of tracers generally has advanced the search for mechanisms that regulate neurotransmission in the living human brain, the methods of image analysis and interpretation still arouse controversy. We discuss four specific issues in this chapter,
Definition of Binding Potential
The first problem is the uncertain definition of the binding potential, which tends to yield mutually incompatible results in the hands of different users. The term was introduced in one of the earliest models of neuroreceptor kinetics, presented by Mintun et al. (1984). The model demonstrated that the receptor quantity Bmax (i.e., the number of binding sites) and KD−1 (i.e., the binding affinity) were inseparable in vivo. Instead, they introduced the term binding potential for the ratio Bmax⧸KD
Solutions
The analysis of remaining issues and problems suggested a number of solutions. First, the discussion of the different definitions of the binding potential suggests that the most convenient definition is the familiar unitless ratio of the number of bound molecules for each molecule not so bound, commonly known as the B⧸F ratio when it is measured in vitro. Second, the discussion of the uncertain measurements of affinity constants suggests that the solvent to which it is referred must always be
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