Current state of agonist radioligands for imaging of brain dopamine D2/D3 receptors in vivo with positron emission tomography.

Dopamine (DA) is known to play an important role in numerous brain functions and has been suggested to be involved in several neuropsychiatric and neurological disorders. From early on, positron emission tomography (PET) studies of the DA system have yielded high interest. Currently, several aspects of the functionality of DA neurons can be imaged, including the DA synthesis rate and the expression of DA-related proteins (receptors and transporters). A more recent application of radioligands targeting DA receptors is to study the endogenous neurotransmitter levels in vivo in brain. In vitro binding studies have suggested that in general DA receptors, as many other G-protein coupled receptors, exist in two affinity states for agonist binding. The high affinity state is thought to represent the functional state of the receptor, and the proportion between high and low affinity states may change with the development of disease. PET imaging with agonist radioligands may provide information on the existence of the high affinity state in vivo. In addition, DA receptor agonist radioligands may be superior tools for measuring changes in endogenous DA levels as antagonist radioligands inherently bind to both the low and the high affinity state. This review primarily summarizes the present status of the D(2)/D(3) receptor agonist PET radioligands. In addition several PET studies evaluating the utility of the agonist concept are discussed.