Transporters of the monoamine transporter (MAT) family regulate the uptake of important neurotransmitters like dopamine, serotonin, and norepinephrine. The MAT family functions using the electrochemical gradient of ions across the membrane and comprises three transporters, dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET). MAT transporters have been observed to exist in monomeric states to higher-order oligomeric states. Structural features, allosteric modulation, and lipid environment regulate the oligomerization of MAT transporters. NET and SERT oligomerization are regulated by levels of PIP2 present in the membrane. The kink present in TM12 in the MAT family is crucial for dimer interface formation. Allosteric modulation in the dimer interface hinders dimer formation. Oligomerization also influences the transporters\' function, trafficking, and regulation. This chapter will focus on recent studies on monoamine transporters and discuss the factors affecting their oligomerization and its impact on their function.

The monoamine transporters, including the serotonin transporter (SERT), dopamine transporter (DAT), and norepinephrine transporter (NET), are the therapeutic targets for the treatment of many neuropsychiatric disorders. Despite significant progress in characterizing the structures and transport mechanisms of these transporters, the regulation of their transport functions through dimerization or oligomerization remains to be understood. In the present study, we identified a conserved intramolecular ion-pair at the third extracellular loop (EL3) connecting TM5 and TM6 that plays a critical but divergent role in the modulation of dimerization and transport functions among the monoamine transporters. The disruption of the ion-pair interactions by mutations induced a significant spontaneous cross-linking of a cysteine mutant of SERT and an increase in cell surface expression but with an impaired specific transport activity. On the other hand, similar mutations of the corresponding ion-pair residues in both DAT and NET resulted in an opposite effect on their oxidation-induced dimerization, cell surface expression, and transport function. Reversible biotinylation experiments indicated that the ion-pair mutations slowed down the internalization of SERT but stimulated the internalization of DAT. In addition, cysteine accessibility measurements for monitoring SERT conformational changes indicated that substitution of the ion-pair residues resulted in profound effects on the rate constants for cysteine modification in both the extracellular and cytoplasmatic substrate permeation pathways. Furthermore, molecular dynamics simulations showed that the ion-pair mutations increased the interfacial interactions in a SERT dimer but decreased it in a DAT dimer. Taken together, we propose that the transport function is modulated by the equilibrium between monomers and dimers on the cell surface, which is regulated by a potential compensatory mechanism but with different molecular solutions among the monoamine transporters. The present study provided new insights into the structural elements regulating the transport function of the monoamine transporters through their dimerization.

Psychiatric and substance use disorders inflict major public health burdens worldwide. Their widespread burden is compounded by a dearth of effective treatments, underscoring a dire need to uncover novel therapeutic targets. In this review, we summarize the literature implicating organic cation transporters (OCTs), including three subtypes of OCTs (OCT1, OCT2, and OCT3) and the plasma membrane monoamine transporter (PMAT), in the neurobiology of psychiatric and substance use disorders with an emphasis on mood and anxiety disorders, alcohol use disorder, and psychostimulant use disorder. OCTs transport monoamines with a low affinity but high capacity, situating them to play a central role in regulating monoamine homeostasis. Preclinical evidence discussed here suggests that OCTs may serve as promising targets for treatment of psychiatric and substance use disorders and encourage future research into their therapeutic potential.

To review the literature on the neuropharmacology of synthetic cathinones.
A comprehensive literature search was carried out across multiple databases (mainly PubMed, World Wide Web, and Google Scholar) using relevant keywords.
Cathinones exhibit a broad toxicological profile, mimicking the effects of a wide variety of \'classic drugs\' such as 3,4-methylenedioxymethamphetamine (MDMA), methamphetamine and cocaine. Even small structural changes affect their interactions with key proteins. This article reviews existing knowledge of the mechanisms of action of cathinones at the molecular level, and key findings from research on their structure-activity relationship. The cathinones are also classified according to their chemical structure and neuropharmacological profiles.
Synthetic cathinones represent one of the most numerous and widespread groups among new psychoactive substances. Initially developed for therapeutic purposes, they quickly started to be used recreationally. With a rapidly increasing number of new agents entering the market, structure-activity relationship studies are valuable for assessing and predicting the addictive potential and toxicity of new and potential future substances. The neuropharmacological properties of synthetic cathinones are still not fully understood. A full elucidation of the role of some key proteins, including organic cation transporters, requires detailed studies.

χ-Conotoxins are known for their ability to selectively inhibit norepinephrine transporters, an ability that makes them potential leads for treating various neurological disorders, including neuropathic pain. PnID, a peptide isolated from the venom of Conus pennaceus, shares high sequence homology with previously characterized χ-conotoxins. Whereas previously reported χ-conotoxins seem to only have a single native disulfide bonding pattern, PnID has three native isomers due to the formation of different disulfide bond patterns during its maturation in the venom duct. In this study, the disulfide connectivity and three-dimensional structure of these disulfide isomers were explored using regioselective synthesis, chromatographic coelution, and solution-state nuclear magnetic resonance spectroscopy. Of the native isomers, only the isomer with a ribbon disulfide configuration showed pharmacological activity similar to other χ-conotoxins. This isomer inhibited the rat norepinephrine transporter (IC50 = 10 ± 2 µM) and has the most structural similarity to previously characterized χ-conotoxins. In contrast, the globular isoform of PnID showed more than ten times less activity against this transporter and the beaded isoform did not display any measurable biological activity. This study is the first report of the pharmacological and structural characterization of an χ-conotoxin from a species other than Conus marmoreus and is the first report of the existence of natively-formed conotoxin isomers.

The antidepressant-like activity of (+)-catharanthine and (-)-18-methoxycoronaridine [(-)-18-MC] was studied in male and female mice using forced swim (FST) and tail suspension tests (TST). The underlying molecular mechanism was assessed by electrophysiological, radioligand, and functional experiments. The FST results showed that acute administration (40 mg/kg) of (+)-catharanthine or (-)-18-MC induces similar antidepressant-like activity in male and female mice at 1 h and 24 h, whereas the TST results showed a lower effect for (-)-18-MC at 24 h. Repeated treatment at lower dose (20 mg/kg) augmented the efficacy of both congeners. The FST results showed that (-)-18-MC reduces immobility and increases swimming times without changing climbing behavior, whereas (+)-catharanthine reduces immobility time, increases swimming times more markedly, and increases climbing behavior. To investigate the contribution of the serotonin and norepinephrine transporters in the antidepressant effects of (+)-catharanthine and (-)-18-MC, we conducted in vitro radioligand and functional studies. Results obtained demonstrated that (+)-catharanthine inhibits norepinephrine transporter with higher potency/affinity than that for (-)-18-MC, whereas both congeners inhibit serotonin transporter with similar potency/affinity. Moreover, whereas no congener activated/inhibited/potentiated the function of serotonin receptor 3A or serotonin receptor 3AB, both increased serotonin receptor 3A receptor desensitization. Depletion of serotonin decreased the antidepressant-like activity of both congeners, whereas norepinephrine depletion only decreased (+)-catharanthine\'s activity. Our study shows that coronaridine congeners induce antidepressant-like activity in a dose- and time-dependent, and sex-independent, manner. The antidepressant-like property of both compounds involves serotonin transporter inhibition, without directly activating/inhibiting serotonin receptors 3, while (+)-catharanthine also mobilizes norepinephrinergic neurotransmission.

Fluorescence microscopy has revolutionized the visualization of physiological processes in live-cell systems. With the recent innovations in super resolution microscopy, these events can be examined with high precision and accuracy. The development of fluorescently labelled small molecules has provided a significant advance in understanding the physiological relevance of targeted proteins that can now be visualized at the cellular level. One set of physiologically important target proteins are the monoamine transporters (MATs) that play an instrumental role in maintaining monoamine signalling homeostasis. Understanding the mechanisms underlying their regulation and dysregulation is fundamental to treating several neuropsychiatric conditions such as attention deficit hyperactivity disorder (ADHD), anxiety, depression and substance use disorders. Herein, we describe the rationale behind the small molecule design of fluorescently labelled ligands (FLL) either as MAT substrates or inhibitors as well as their applications to advance our understanding of this class of transporters in health and disease.

Albizia julibrissin Durazz is one of the most common herbs used for depression and anxiety treatment, but its mechanism of action as an antidepressant or anxiolytic drug have not been fully understood. We previously isolated and identified one lignan glycoside compound from Albizia Julibrissin Durazz, (-)-syringaresinol-4-O-β-D-apiofuranosyl-(1→2)-β-D-glucopyranoside (SAG), that inhibited all three monoamine transporters with a mechanism of action different from that of the conventional antidepressants. In this study, we generated homology models for human dopamine transporter and human norepinephrine transporter, based on the X-ray structure of Drosophila dopamine transporter, and conducted the molecular docking of SAG to all three human monoamine transporters. Our computational results indicated that SAG binds to an allosteric site (S2) that has been demonstrated to be formed by an aromatic pocket positioned in the scaffold domain in the extracellular vestibule connected to the central site (S1) in these monoamine transporters. In addition, we demonstrated that SAG stabilizes a conformation of serotonin transporter with both the extracellular and cytoplasmic pathways closed. Furthermore, we performed mutagenesis of the residues in both the allosteric and orthosteric sites to biochemically validate SAG binding in all three monoamine transporters. Our results are consistent with the molecular docking calculation and support the association of SAG with the allosteric site. We expect that this herbal molecule could become a lead compound for the development of new therapeutic agents with a novel mechanism of action.

Stereoselectivity is well known and very pronounced in drug metabolism and receptor binding. However, much less is known about stereoselectivity in drug membrane transport. Here, we characterized the stereoselective cell uptake of chiral phenylethylamine derivatives by human monoamine transporters (NET, DAT, and SERT) and organic cation transporters (OCT1, OCT2, and OCT3). Stereoselectivity differed extensively between closely related transporters. High-affinity monoamine transporters (MATs) showed up to 2.4-fold stereoselective uptake of norepinephrine and epinephrine as well as of numerous analogs. While NET and DAT preferentially transported (S)-norepinephrine, SERT preferred the (R)-enantiomer. In contrast, NET and DAT showed higher transport for (R)-epinephrine and SERT for (S)-epinephrine. Generally, MAT stereoselectivity was lower than expected from their high affinity to several catecholamines and from the high stereoselectivity of some inhibitors used as antidepressants. Additionally, the OCTs differed strongly in their stereoselectivity. While OCT1 showed almost no stereoselective uptake, OCT2 was characterized by a roughly 2-fold preference for most (R)-enantiomers of the phenylethylamines. In contrast, OCT3 transported norphenylephrine and phenylephrine with 3.9-fold and 3.3-fold preference for their (R)-enantiomers, respectively, while the para-hydroxylated octopamine and synephrine showed no stereoselective OCT3 transport. Altogether, our data demonstrate that stereoselectivity is highly transporter-to-substrate specific and highly diverse even between homologous transporters.

Plasma membrane monoamine transporter (PMAT, Slc29a4) transports monoamine neurotransmitters, including dopamine and serotonin, faster than more studied monoamine transporters, e.g., dopamine transporter (DAT), or serotonin transporter (SERT), but with ~400-600-fold less affinity. A considerable challenge in understanding PMAT\'s monoamine clearance contributions is that no current drugs selectively inhibit PMAT. To advance knowledge about PMAT\'s monoamine uptake role, and to circumvent this present challenge, we investigated how drugs that selectively block DAT/SERT influence behavioral readouts in PMAT wildtype, heterozygote, and knockout mice of both sexes. Drugs typically used as antidepressants (escitalopram, bupropion) were administered acutely for readouts in tail suspension and locomotor tests. Drugs with psychostimulant properties (cocaine, D-amphetamine) were administered repeatedly to assess initial locomotor responses plus psychostimulant-induced locomotor sensitization. Though we hypothesized that PMAT-deficient mice would exhibit augmented responses to antidepressant and psychostimulant drugs due to constitutively attenuated monoamine uptake, we instead observed sex-selective responses to antidepressant drugs in opposing directions, and subtle sex-specific reductions in psychostimulant-induced locomotor sensitization. These results suggest that PMAT functions differently across sexes, and support hypotheses that PMAT\'s monoamine clearance contribution emerges when frontline transporters (e.g., DAT, SERT) are absent, saturated, and/or blocked. Thus, known human polymorphisms that reduce PMAT function could be worth investigating as contributors to varied antidepressant and psychostimulant responses.