Absolute abundances and affinity states of dopamine receptors in mammalian brain: A review

Abstract

The abundances of dopamine (DA) D(1) and D(2) receptors have been assayed with radioligands in membrane preparations and by autoradiography in vitro, and also in living brain using positron emission tomography (PET). This review compares the saturation binding parameters (B(max) and K(D) ) obtained in striatum by these several methods, and in different species. Some uncertainty in quantitation is derived from the incomplete specificities of commonly used ligands, especially Sch 23,390 for D(1) sites and spiperone for D(2) -like sites. In striatal membrane preparations, the D(1) B(max) ranges from 10 to 139 pmol g(-1) tissue, whereas the D(2) B(max) ranges from 8 to 42 pmol g(-1) tissue. Receptor concentrations in human material, despite the more extended post mortem interval, are roughly similar to those reported in rodent and nonhuman primate. Estimates of B(max) by quantitative autoradiography are generally five times higher than corresponding results for similar ligands in membrane preparations. The saturation binding parameters in living striatum have been estimated by serial PET studies with ligands over a range of specific activities. The few PET estimates of D(1) B(max) , (40-80 pmol g(-1) ) and numerous PET estimates of D(2) B(max) (20-40 pmol g(-1) ) are in general agreement with membrane estimates, but fall far short of the mean of autoradiographic results in vitro. Apparent affinities for D(1) and D(2) ligands in vivo are typically 10 times lower than for corresponding in vitro studies, presumably because the unbound ligand concentration is not corrected for the free fraction in living brain tissue. The disparate B(max) results by method suggest the presence of a large reservoir or reserve of D(1) and D(2) receptors in intact brain sections, which are unavailable to PET ligands in vivo, and which may be lost during the preparation of washed membranes. A subset of receptors existing in a high affinity state for agonists is detected in washed membrane preparations, in which the coupling to intracellular G-proteins may have become artificially limiting. However, in most PET and autoradiographic studies in vitro, agonist and antagonist ligands have similar B(max) . Discrepancies in the literature highlight the need for a better understanding of affinity states in vivo and trafficking of G-protein coupled receptors between plasma membrane and intracellular compartments.

Perturbed Development of Striatal Dopamine Transporters in Fatty Versus Lean Zucker Rats: a Follow-up Small Animal PET Study

“…As such, the present findings of increasing [ 18 F]FP-CMT BP ND in Zucker rats between 6 weeks and 6 months of age are at odds with most biochemical findings; we suppose that PET may detect a specific population of DAT that is distinct from that recovered in membrane preparations or revealed by autoradiography in vitro. This is in partial contrast to findings with ligands for dopamine D1 and D2 receptors, which had similar B max by PET and in striatal membranes, but substantially higher levels as measured by autoradiography in vitro [23].…”

“…These disparate findings suggest that obesity in FZ rats may increase the dynamics of DAT recycling from the plasma membrane, possibly resulting in a net decline in DAT levels, as measured by PET in the present study. However, a [ 123 I]PE2I SPECT study in 33 healthy humans did not reveal any association of striatal DAT binding with BMI (mean 30; range 21-50) [37], nor did a [ 123 I]FP-CIT SPECT study in 123 healthy subjects (BMI mean 25; range 18-41) [38], whereas a [ 99m Tc]-TRODAT-1 study in 50 volunteers did reveal a negative correlation between striatal DAT and BMI (mean 23; range [19][20][21][22][23][24][25][26][27][28][29][30] [39]; the issue may be some threshold for obesity resulting in altered DAT availability in humans, or other factors such as heterogeneity of the causes of human obesity.…”

Altered serotonin and dopamine transporter availabilities in brain of depressed patients upon treatment with escitalopram: A [123I]β-CIT SPECT study

“…It seems plausible that SERT was down-regulated or internalized at follow-up scanning of the present patient group, which would lead to overestimation of the true occupancy. Resolving the competing effects of possible baseline SERT changes and also the competition from endogenous serotonin in vivo would require conducting a saturation binding analysis, such as has been reported for dopamine receptors (Cumming, 2011). Although this technique has not yet been tested for the case of plasma membrane monoamine transporters, the effort might be justified in order to resolve ambiguity of B max and apparent affinity changes in studies of this type.…”

Considerations in the Development of Reversibly Binding PET Radioligands for Brain Imaging

“…Generally, brain is assumed to contain 100 mg of protein per g of wet tissue [57], in which case 1 fmol/g of brain tissue simply equates to 1 nM and 1 fmol/mg protein to 0.1 nM. The range of normal human brain protein densities that has been imaged with PET spans more than two orders of magnitude, for example from about 0.5 nM for the α 4 β 2 subtype of nicotinic acetylcholine receptors (α 4 β 2 nACh) receptors [58, 59] in cortex to over 150 nM for dopamine subtype-1 (D 1 ) receptors in striatum [60] (Table 2). Estimates of human protein target density are recognized to vary considerably with the type of measurement ( e.g., autoradiography in vitro , membrane assay in vitro , or PET) and the choice of radioligand [60].…”