2025 Theses Doctoral

Advancing the Monoaminergic Neurotransmission Investigative Toolbox: Integrating the Developments of Fluorescent Optical Tracers and Inhibitors

Hwu, Christopher

Monoaminergic neurotransmission regulates numerous fundamental functions of the nervous system, including mood, cognition, learning, motor function, reward, sleep and wakefulness cycles. Its life cycle consists of the synthesis of modulatory neurotransmitters (namely, dopamine, norepinephrine, and serotonin) that are then sequestered into acidic synaptic vesicles (through vesicular monoamine transporter 2, VMAT2, in the brain) and subsequently released to the extracellular space upon vesicular fusion with the presynaptic bouton plasma membrane. These monoamines are then taken up by plasma membrane transporters, including dopamine (DAT), norepinephrine (NET), and serotonin (SERT) transporters, for either cyclic repetition of release and uptake, or degradation. Dysregulation of such processes, resulting in concentration imbalances, have been implicated in numerous pathologies including, depression, drug addiction, and Parkinson’s Disease. Consequently, modern tools, specifically fluorescent optical tracers and monoamine transporter reuptake inhibitors, are necessary for the rigorous investigation of these etiologies.

Chapter 1 highlights the development of novel small molecule fluorescent optical probes that enable the visualization of serotonergic reuptake molecular machinery. Exploring the acridones fluorophore core has yielded the introduction of the first serotonergic fluorescent false neurotransmitter, FFN246, and a bright Uptake 1 transporter system substrate, DD545. The development of a more hydrophilic serotonergic substrate that can be multiplexed with protein-based genetically encoded monoamine biosensors was catalyzed with the discovery of SERTlight after rigorous exploration of the quinolone molecular scaffold.

The second part of the thesis delves into the pharmacological development of novel monoamine transporter reuptake inhibitors amongst different natural product-based scaffolds. In Chapter 2, a thorough investigation of the ibogaine scaffold (including its deconstructed components) has resulted in the introduction of a taxonomic class termed “Synaptic Reuptake Inhibitors”, which introduces concomitant inhibition of two critical serotonergic targets: SERT and VMAT2 inhibition. Chapter 3 details the discovery of speciociliatine as the alkaloid extracted from Mitragyna speciosa that has a sub-micromolar pharmacological as a selective and potent VMAT2 inhibitor.

The final two chapters of the thesis introduce nascent monoamine transporter inhibition structure-and-activity relationships of two molecular scaffolds with members that are becoming a growing societal burden. Chapter 4 examines the amphetamine molecular class and its SAR with respect to monoamine release versus uptake inhibition. Chapter 5 probes whether the adverse effects as witnessed from usage of methylmethcathinone and its derivatives can be attributed to monoamine transporter inhibition.