Dopamine D_2/3 Receptor Availabilities in Striatal and Extrastriatal Regions of the Adult Human Brain: Comparison of Four Methods of Analysis

Abstract

Values of binding potentials (BP_ND) of dopamine D_2/3 receptors differ in different regions of the brain, but we do not know with certainty how much of this difference is due either to different receptor numbers, or to different affinities of tracers to the receptors, or to both. We tested the claim that both striatal and extrastriatal dopamine D_2/3 receptor availabilities vary with age in vivo in humans by determining the values of BP_ND of the specific radioligand [¹¹C]raclopride. We determined values of BP_ND in striatal and extrastriatal volumes-of-interest (VOI) with the same specific receptor radioligand. We estimated values of BP_ND in individual voxels of brains of healthy volunteers in vivo, and we obtained regional averages of VOI by dynamic positron emission tomography (PET). We calculated average values of BP_ND in caudate nucleus and putamen of striatum, and in frontal, occipital, parietal, and temporal cortices of the forebrain, by means of four methods, including the ERLiBiRD (Estimation of Reversible Ligand Binding and Receptor Density) method, the tissue reference methods of Logan and Logan-Ichise, respectively, and the SRTM (Simplified Reference Tissue Method). Voxelwise generation of parametric maps of values of BP_ND used the multi-linear regression version of SRTM. Age-dependent changes of the binding potential presented with an inverted U-shape with peak binding potentials reached between the ages of 20 and 30. The estimates of BP_ND declined significantly with age after the peak in both striatal and extrastriatal regions, as determined by all four methods, with the greatest decline observed in posterior (occipital and parietal) cortices (14% per decade) and the lowest decline in caudate nucleus (3% per decade). The sites of the greatest declines are of particular interest because of the clinical implications.

Appendix

The possible mechanism of the inverted-U changes of binding potentials as functions of age determined in the present study can be ascribed to at least two factors that include the receptor density and the availability of receptors as reflection of the concentration of the endogenous ligand (dopamine). We may ascribe the observation that the ratios of BP_ND of cortices to putamen differ somewhat from the ratio of receptor densities to differences of extracellular dopamine concentrations between the cerebral cortices and striatum. Major differences of the dopamine concentration can be estimated by the formula for the concentration of a competitor, derived from the definition of the binding potential and maximum binding capacity,

$${B}_{\mathrm{max}}= {BP}_{\mathrm{ND}} {V}_{\mathrm{T}} {K}_{\mathrm{D}}\left( 1 + \frac{{C}_{\mathrm{i}}}{{K}_{\mathrm{i}}}\right) = {BP}_{\mathrm{ND}} {V}_{\mathrm{T}} {K}_{\mathrm{D}}/\alpha$$

(4)

where ${V}_{\mathrm{T}}$ is the total distribution volume of the tracer, ${C}_{\mathrm{i}}$ and ${K}_{\mathrm{i}}$ the steadystate and half-inhibition concentrations of endogenous ligands or other competitors, respectively, and α the availability of the receptors, defined as the fraction of un-occupied receptors. Availability can then be described by following equation,

$$\alpha = 1 - \sigma = \frac{{K}_{i} }{{K}_{i} + {C}_{i}}= \frac{{BP}_{\mathrm{ND}} {V}_{\mathrm{T}} {K}_{\mathrm{D}}}{{B}_{\mathrm{max}}}$$

(5)

where $\sigma$ is the degree of occupancy of the receptors by all competing ligands. The combination of the approximately linear decline of the maximum binding capacity as a function of age, and the inverse U-shape of the relationship between the actual binding potential and age, suggests that two reciprocally active factors are in operation, as also expressed by the rearrangement of Eqs. 1 and 2,

$${BP}_{\mathrm{ND}}= \frac{{B}_{\mathrm{max}} }{{V}_{\mathrm{T}} {K}_{\mathrm{D}} \left(1 + \frac{{C}_{\mathrm{i}}}{{K}_{\mathrm{i}}}\right) }= \alpha {BP}_{0}$$

(6)

where ${BP}_{0}$ is the theoretically highest achievable binding potential, i.e., the binding potential in the absence of competitors, equal to ${B}_{\mathrm{max}}/({V}_{\mathrm{T}} {K}_{\mathrm{D}}$). It is possible to assess the concentration of competitors as function of age by regression of Eq. 6 to the inverted U-shape of the relationship between binding potential and age, on the basis of three claims: The maximum binding capacity declines linearly with age [60], the inverted-U shape of the binding potential as function of age dictates a linear increase of the availability of the receptors with age- associated loss of dopamine [55, 61], and the average occupancy by dopamine is 10% at the age of 30 [62], consistent with an availability of 90%. In another study [63], the authors estimated D_2/3 receptor occupancy by dopamine to be 21% at an average age of 25, with the assumption that it might be even higher due to an incomplete depletion of synaptic dopamine, after administration of the tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine (AMPT). Here, we set the occupancy level at the age of 25 at 50% corresponding to an availability of 50%. The resulting regression equation rearranges to,

$${BP}_{\mathrm{ND}}= 0.5 {BP}_{0.25} \left(\frac{1 + b_{\alpha} A}{1 + 25{b}_{\alpha }}\right)\left(\frac{1 - {b}_{\mathrm{BP}} }{1 + 25{b}_{\mathrm{BP}}}\right)$$

(7)

where ${BP}_{0.25}$ is the value of ${BP}_{0}$ at age 25, ${b}_{\alpha }$ the rate of increase of availability as function of age, A the age of the subject, and ${b}_{\mathrm{BP}}$ the rate of decline of the maximum binding capacity (${BP}_{0}$) as function of age. The regression parameter ${b}_{\alpha }$ was chosen to be 0.1, providing the optimal R² values for the regression to be fitted to the data (see Fig. 

Fig. 5

Mean R² values as a function of b_α. The R² is coefficient of determination, which determines convergence of model with experimental data and the higher value of R² results in higher accuracy, which is seen b_α equal 0.1. Abscissa: Log10 rescaling of b values, the rate of increase of availability as function of age

5).

The results of the non-linear regression with Eq. 3 shown in Figs. 1 and 2 include the estimates of dopamine concentrations, calculated from Eq. 3, the decline of BP₀, and the estimation of availability as function of age. The values calculated by the ERLiBiRD method were the lowest among those tested by all four methods in all regions, explained by the inclusion of the contents of the vascular volume into the calculation of the quantity of non-displaceable tracer. It is known that the ratio of precision to accuracy of the ERLiBiRD method is higher than in the reference tissue methods of Logan and SRTM [30], as also revealed by the coefficient of variation determined in the present study, but the accuracy is not directly quantifiable without consideration of the volume of the vascular bed and the partition coefficient of the radioligand raclopride. With a partition coefficient of 0.5 [29] and a vascular volume of 5% of the whole-brain volume, inclusion of the radioactivity in the vascular bed in the calculation of the binding potential with ERLiBiRD accounts for 10% in the steady-state, enough to explain the lower binding potentials obtained with the ERLiBiRD method.

The parametric mapping demonstrated significant foci of age-related decline bilaterally in putamen, in the right insula, and in regions of the temporal cortex (superior temporal gyrus, middle temporal gyrus) and regions of the frontal cortex (precentral gyrus). In the present study, we observed significant declines of values in insula and frontal cortex in the VOI analysis, as well by the parametric mapping analysis. In the putamen of patients with early Parkinson’s disease, D_2/3 receptors undergo up-regulation [55], whereas in the prefrontal cortex of individuals with advanced Parkinson’s disease, D_2/3 receptors appear to decline [22]. The combination of increased receptor density in the caudate nucleus and decreased values of BP_ND in the cortex has been reported for patients with schizophrenia [56, 57]. In patients with Parkinson’s disease and schizophrenia, estimates of BP_ND with [¹¹C]raclopride may therefore reveal changes of dopamine concentrations in the cerebral cortex, as well as in the striatum. Another interesting aspect to investigate would be the comparison between genders, in whom in a recent study, Fazio et al. [58] found a negative correlation between BP_ND and age, and an effect of gender with higher values of BP_ND in females [58], perhaps related to period phases. In the present study, because of the comparatively low number of women investigated (one quarter of all subjects tested), exclusion of women from the analysis did not significantly change the results.