Distinctive binding properties of the negative allosteric modulator, [3H]SB269,652, at recombinant dopamine D3 receptors
2018
Abstract Recently, employing radioligand displacement and functional coupling studies, we demonstrated that SB269,652 (N-[(1r,4r)−4-[2-(7-cyano-1,2,3,4-
tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]−1H-indole-2-carboxamide) interacts in an atypical manner with
dopamineD 3 receptor displaying a unique profile reminiscent of a negative allosteric ligand. Here, we characterized the binding of radiolabelled [ 3 H]SB269,652 to human
dopamineD 3 receptor stably expressed in
Chinese Hamster Ovary cells. Under saturating conditions, SB269,652 showed a KD value of ≈ 1 nM. Consistent with high selectivity for human
dopamineD 3 receptor, [ 3 H]SB269,652 binding was undetectable in cells expressing human
dopamineD 1 , D 2L or D 4 receptors and absent in synaptosomes from
dopamineD 3 receptor knockout vs . wild-type mice. In contrast to saturation binding experiments, the dissociation kinetics of [ 3 H]SB269,652 from human
dopamineD 3 receptors initiated with an excess of unlabelled ligand were best fitted by a bi-exponential binding model. Supporting the kinetic data, competition experiments with haloperidol, S33084 (a
dopamineD 3 receptor antagonist) or
dopamine, were best described by a two-site model. In co-transfection experiments binding of SB269,652 to
dopamineD 3 receptor was able to influence the functional coupling of
dopamineD 2 receptor, supporting the notion that SB269,652 is a negative
allosteric modulatoracross receptor dimers. However, because SB269,652 decreases the rate of [ 3 H]
nemonapridedissociation, the present data suggest that SB269,652 behaves as a bitopic antagonist at unoccupied
dopamineD 3 receptor, binding simultaneously to both orthosteric and allosteric sites, and as a pure negative
allosteric modulatorwhen receptors are occupied and it can solely bind to the allosteric site.
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