Measurement of changes in endogenous serotonin level by positron emission tomography with [¹⁸F]altanserin

Objective: Positron emission tomography (PET) has been used to investigate changes in the concentration of endogenous neurotransmitters. Recently, this technique has been applied to the imaging of serotonin_2A receptors using [¹⁸F]altanserin. In these measurements, a reduction in binding potential (BP) suggests an increase in endogenous serotonin levels caused by pharmacological or cognitive stimulations, and the sensitivity of BP reduction depends on the characteristics of [¹⁸F]altanserin. In this study, we evaluated an analytical method for estimating the changes in endogenous serotonin levels based on PET scans with [¹⁸F]altanserin at baseline and stimulated states and validated it using simulations and small animal PET studies. Methods: First, in the simulations, the time-activity curves at baseline and the stimulated states were generated using an extended compartment model including the competition for the receptors between the administered [¹⁸F]altanserin and endogenous serotonin. In the stimulated state, the magnitude and onset of the endogenous serotonin elevation were altered to varying degrees. In these time-activity curves, BP was estimated using the simplified reference tissue model (SRTM), and the reduction in BP was evaluated by comparison with that of the baseline state. Next, the proposed method was applied to mouse PET studies. Endogenous serotonin levels were elevated by treatment with selective serotonin reuptake inhibitors (SSRIs), and PET studies were performed twice, once with and once without treatment. In both scans, BP was estimated using the SRTM with the cerebellum as a reference region, and the reduction in BP after SSRI treatment was evaluated. Results: In the simulations, the BP estimate of the stimulated state was smaller than that of the baseline state, and their reduction was related to the amount of change in the serotonin concentration. BP reduction was also affected by the onset of serotonin elevation. In the mouse studies, the BP of the cerebral cortex decreased in the scans with SSRI treatment. Conclusions: The reduction in BP estimated using the SRTM from [¹⁸F]altanserin-PET studies at baseline and in stimulated states can detect changes in the binding conditions of serotonin_2A receptors. This may be useful for investigating the elevation of endogenous serotonin levels caused by stimulations.