2.4. Brain single-photon emission computed tomography (SPECT) imaging

Hypothalamic and thalamic SERT and striatal DAT availability were assessed using SPECT imaging and the radiotracer ¹²³I-Nω-fluoropropyl-2β-carboxymethoxy-3β-(4-iodophenyl)nortropane (¹²³I-FP-CIT), a method that we and others have previously validated [40–44]. SERTs and DATs facilitate the synaptic reuptake of serotonin and dopamine, respectively, into the pre-synaptic neuron and thereby regulate the amounts of neurotransmitter available for signaling [45]. In vivo-assessed SERT and DAT availability reflect a combination of the expression of the transporter (B_max) and the affinity of the radiotracer for the given transporter (1/K_d [46]). In other words, an increase in B_max and a decrease in K_d can both increase ¹²³I-FP-CIT binding (B_max/K_d). The K_d can thus be influenced by changing the synaptic levels of neurotransmitters or a change in the conformational state of the transporter [47]. An increase in transporter availability (i.e., higher binding of ¹²³I-FP-CIT) reflects higher transporter expression (i.e., higher B_max) or higher affinity (i.e., lower K_d) induced by a decrease in the concentration of endogenous neurotransmitter, a conformational change, or a combination of these factors. Because ¹²³I-FP-CIT binding in the hypothalamus/thalamus and the striatum can be blocked in vivo by selective SERT and DAT reuptake inhibitors, respectively, binding of ¹²³I-FP-CIT in the hypothalamus/thalamus and the striatum predominantly reflect SERT and DAT, respectively [40,41]. We have previously demonstrated that hypothalamic/thalamic SERT and striatal DAT availability can be optimally imaged 2 and 3 h after the administration of the radiotracer, respectively [43,48].

Subjects were administered an intravenous bolus of approximately 100 MBq ¹²³I-FP-CIT (specific activity >750 MBq/nmol; radiochemical purity >98%; produced in accordance with GMP guidelines; GE Healthcare, Eindhoven, The Netherlands). Next, SPECT imaging was performed using the InSPira HD system, a brain-dedicated SPECT camera (Neurologica, Boston, USA) with the following parameters: acquisition time per slice, 180 s; slice thickness, 4 mm. Slices were acquired from the level of the cerebellum up to the striatum. SPECT images were reconstructed with an iterative expectation maximization algorithm and corrected for attenuation by manually aligning an adult head template [41,49]. To limit thyroid uptake of free radioactive iodide, all subjects were pre-treated with potassium iodide.