Imaging Agonist-Induced D2/D3 Receptor Desensitization and Internalization In Vivo with PET/fMRI

Abstract

This study investigated the dynamics of dopamine receptor desensitization and internalization, thereby proposing a new technique for non-invasive, in vivo measurements of receptor adaptations. The D2/D3 agonist quinpirole, which induces receptor internalization in vitro, was administered at graded doses in non-human primates while imaging with simultaneous positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). A pronounced temporal divergence between receptor occupancy and fMRI signal was observed: occupancy remained elevated while fMRI responded transiently. Analogous experiments with an antagonist (prochlorperazine) and a lower-affinity agonist (ropinirole) exhibited reduced temporal dissociation between occupancy and function, consistent with a mechanism of desensitization and internalization that depends upon drug efficacy and affinity. We postulated a model that incorporates internalization into a neurovascular-coupling relationship. This model yielded in vivo desensitization/internalization rates (0.2/min for quinpirole) consistent with published in vitro measurements. Overall, these results suggest that simultaneous PET/fMRI enables characterization of dynamic neuroreceptor adaptations in vivo, and may offer a first non-invasive method for assessing receptor desensitization and internalization.

Figures

Figure 1

Schematic illustrating a compartmental model that describes receptor desensitization and internalization at dopaminergic synapses. The total number of receptors (Bmax) is composed of available receptors at the postsynaptic membrane, those bound by an injected agonist, those bound by endogenous dopamine and desensitized/internalized receptors. Occupied receptors are in exchange with free ligand in the synaptic space. Receptors that are occupied by an agonist trigger desensitization and internalization. As externalization mechanisms are known to be very slow, we assume that kext=0 for the duration of time courses we modeled. The parameters that determine the PET and fMRI signal changes are highlighted in bold. This shows that PET and fMRI time courses contain complementary information about receptor adaptation mechanisms. concentr., concentration; DA, dopamine; fMRI, functional magnetic resonance imaging; PET, positron emission tomography.

Figure 2

Simulation results from the proposed model of receptor desensitization and internalization that show how PET and fMRI signal time courses are affected for different rates of RDI (kDI) due to a D2/D3 agonist injection at time t=0. If no RDI occurs, PET and fMRI signals are matched in time. If RDI occurs with a moderate rate of 0.03/min, PET and fMRI signals start to diverge. With high RDI rates (0.2/min), the fMRI time course is shortened, whereas PET occupancy stays elevated for much longer. CBV, cerebral blood volume; fMRI, functional magnetic resonance imaging; PET, positron emission tomography; RDI, receptor desensitization and internalization.

Figure 3

Parametric maps showing the results from three injection doses of the agonist quinpirole. All maps show imaging data from two animals, analyzed with a mixed-effects model. Upper row: dynamic-binding potential maps (DBPNDpeak) show that specific binding of the radiotracer [11C]raclopride in caudate and putamen decreases with increasing quinpirole. Lower row: voxelwise maps showing %CBVpeak changes, windowed by a P-value map with P<0.03. As quinpirole dose increases, the negative CBV signal that is specific to caudate and putamen increases in magnitude. CBV, cerebral blood volume; DBP, dynamic-binding potential.

Figure 4

Upper: PET time activity curves for the caudate and cerebellum ROIs for the 0.2 mg/kg quinpirole injection at 35 min (for animal M2), with kinetic modeling fits from two-parameter SRTM with the cerebellum as the reference. The arrow at 35 min indicates the time at which the quinpirole challenge was administered. Lower: corresponding CBV time courses show a negative response due to the challenge in the caudate ROI. A second injection of 0.2 mg/kg quinpirole in the same session did not produce a measurable CBV response in the caudate. CBV, cerebral blood volume; PET, positron emission tomography; SRTM, simplified reference tissue model.

Figure 5

Time courses of CBV (dotted line) and occupancy (solid line) resulting from exposure to three different pharmacological challenges in animal M2: (a) the high-affinity agonist quinpirole elicits a very short CBV response, whereas PET occupancy stays elevated for the duration of the experiment. (b) The agonist ropinirole has a lower affinity compared with quinpirole and results in a slightly longer, though still short, CBV response, whereas occupancy peaks at 17.8 min and then starts to decrease. Compared with quinpirole, ropinirole displays a larger CBVpeak signal at lower occupancy. (c) The antagonist prochlorperazine shows that CBV and occupancy time courses are matched, and demonstrates that CBV can stay elevated for longer durations in time. The antagonist showed a reversed CBV sign and the largest magnitude compared with the agonists. Overall, the discrepancy in time between CBV and occupancy and diminished CBV magnitude for the agonists suggests that RDI affects both PET and fMRI, and can vary with drug affinity and potency. CBV, cerebral blood volume; fMRI, functional magnetic resonance imaging; GLM, general linear model; PET, positron emission tomography; RDI, receptor desensitization and internalization