The influence of different cellular environments on PET radioligand binding: An application to D(2/3)-dopamine receptor imaging

Various D(2/3) receptor PET radioligands are sensitive to endogenous dopamine release in vivo. The Occupancy Model is generally used to interpret changes in binding observed in in vivo competition binding studies; an Internalisation Hypothesis may also contribute to these changes in signal. Extension of in vivo competition imaging to other receptor systems has been relatively unsuccessful. A greater understanding of the cellular processes underlying signal changes following endogenous neurotransmitter release may help translate this imaging paradigm to other receptor systems. To investigate the Internalisation Hypothesis we assessed the effects of different cellular environments, representative of those experienced by a receptor following agonist-induced internalisation, on the binding of three D(2/3) PET ligands with previously reported sensitivities to endogenous dopamine in vivo, namely [(3)H]spiperone, [(3)H]raclopride and [(3)H]PhNO. Furthermore, we determined the contribution of each cellular compartment to total striatal binding for these D(2/3) ligands. These studies suggest that sensitivity to endogenous dopamine release in vivo is related to a decrease in affinity in the endosomal environment compared with those found at the cell surface. In agreement with these findings we also demonstrate that ∼25% of total striatal binding for [(3)H]spiperone originates from sub-cellular, microsomal receptors, whereas for [(3)H]raclopride and [(3)H]PhNO, this fraction is lower, representing ∼14% and 17%, respectively. This pharmacological approach is fully translatable to other receptor systems. Assessment of affinity shifts in different cellular compartments may play a crucial role for understanding if a radioligand is sensitive to endogenous release in vivo, for not just the D(2/3), but other receptor systems.