Serotonin (5-hydroxytryptamine, 5-HT) is a neuromodulator underpinning various psychological and physiological processes, with dysregulation implicated in numerous psychiatric disorders. Non-invasive measurement of endogenous 5-HT release in the living human brain is essential to advance understanding of the serotonergic system. The combination of Positron Emission Tomography (PET) neuroimaging of serotonergic receptors with pharmacological and behavioural challenges that stimulate endogenous 5-HT release, offers a unique approach to quantify 5-HT dynamics in vivo. In 2010, Paterson and colleagues concluded in a thorough review that measures of 5-HT release were constrained by limitations in the sensitivity of available tracers and potency of pharmacological challenges. Novel tracers combined with optimised pharmacological challenge paradigms have demonstrated sensitivity to changes in endogenous 5-HT, enabling reproducible detection of acute 5-HT release in both preclinical and human studies in the last 15 years of research. These include the use of agonist radioligands with preferential binding to high-affinity receptor states, such as [11C]AZ10419369 and [11C]Cimbi-36, antagonist tracers, such as [18F]Altanserin, refined challenge designs using pharmacological 5-HT releasers, such as fenfluramine and amphetamine, and the integration of hybrid PET/MR imaging to assess neurovascular aspects. These advances have shifted the field from questioning feasibility to addressing optimal strategies for measuring serotonergic dynamics.
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