In vitro bioassays for measuring monoamine transporter function

Monoamine transporters (MATs) - including the dopamine transporter, norepinephrine transporter, and serotonin transporter - are plasma membrane proteins that mediate the uptake of previously released neurotransmitters from the synaptic cleft, thereby terminating neurotransmitter signaling. MATs are implicated in the pathophysiology of numerous neurological and psychiatric disorders, making them key targets for a wide range of psychotherapeutic agents. Furthermore, stimulant drugs, such as cocaine, methamphetamine, and synthetic cathinones, exert their effects by disrupting normal MAT function. The broad structural and functional diversity of compounds targeting MATs underscores the need for comprehensive in vitro functional characterization. This is essential for elucidating a drug's potency, mechanism of action (e.g., uptake inhibition or efflux induction), and selectivity across MATs. Moreover, certain MAT-modulating compounds can serve as tools to study transporter function itself, furthering the understanding of how MAT-mediated uptake and efflux can be modulated. This review focuses on the in vitro assay techniques most commonly employed to assess MAT activity. More specifically, the review will discuss uptake inhibition and efflux assays that are used to assess MAT function and to study the effects of MAT-modulating compounds. Particular emphasis is placed on the underlying mechanisms of these assays, along with their respective benefits and limitations. Key methodological factors that influence functional outcomes and between-assay comparability are also addressed.