Stable expression of biogenic amine transporters reveals differences in inhibitor sensitivity, kinetics, and ion dependence

De novo generation of dual-target compounds using artificial intelligence

Drugs that interact with multiple therapeutic targets are potential high-value products in polypharmacology-based drug discovery, but the rational design remains a formidable challenge. Here, we present artificial intelligence (AI)-based methods to design the chemical structures of compounds that interact with multiple therapeutic target proteins. The molecular structure generation is performed by a fragment-based approach using a genetic algorithm with chemical substructures and a deep learning approach using reinforcement learning with stochastic policy gradients in the framework of generative adversarial networks. Using the proposed methods, we designed the chemical structures of compounds that would interact with two therapeutic targets of bronchial asthma, i.e., adenosine A2a receptor (ADORA2A) and phosphodiesterase 4D (PDE4D). We then synthesized 10 compounds and evaluated their bioactivities via the binding assays of 39 target human proteins, including ADORA2A and PDE4D. Three of the 10 synthesized compounds successfully interacted with ADORA2A and PDE4D with high specificity.

On the substrate turnover rate of NBCe1 and AE1 SLC4 transporters: structure-function considerations

A transport protein's turnover rate (TOR) is the maximum rate of substrate translocation under saturating conditions. This parameter represents the number of transporting events per transporter molecule (assuming a single transport site) per second (s). From this standpoint, a transporter's TOR is similar to an enzyme's catalytic constant. Knowledge of a transporter's TOR allows comparison of the transport capacity of various transporters at the molecular level as well as the total transport per cell and whole organ levels. Despite this, there is currently a very limited number of transporters, for which TOR has been determined experimentally. In the SLC4 transporter family of CO3 2-/HCO3 - transporters, erythrocyte AE1 (eAE1; SLC4A1) is the only member, for which TOR has been determined (∼50,000 s-1). Whether other SLC4 family members have similar TOR values is currently unknown. Here we report TOR measurements of the electrogenic Na+-CO3 2- cotransporter NBCe1-A (SLC4A4) and the kidney specific AE1 splice variant, kAE1, that play important roles in renal bicarbonate absorption and are mutated in proximal and distal renal tubular acidosis respectively. We have also remeasured the eAE1 TOR value for comparison. NBCe1-A had a TOR value of ∼30,400 s-1 whereas kAE1 and eAE1 had significantly higher values (62,000 s-1 and 60,500 s-1 respectively). We modeled the inward-facing (IF) conformation of NBCe1-A to determine conformational changes during its transport cycle. Comparison of this IF model with our previously determined cryoelectron microscopy (cryoEM) outward-facing (OF) conformation structure, demonstrates that NBCe1-A has an elevator-type transport mechanism with a small vertical ∼5 Å shift of the ion coordination site as we have previously shown for AE1. We speculate that this very small vertical movement plays an important role in contributing to the very high TOR numbers of SLC4 transporters.

Determination of metal ion transport rate of human ZIP4 using stable zinc isotopes

The essential microelement zinc is absorbed in the small intestine mainly by the zinc transporter ZIP4, a representative member of the Zrt/Irt-like protein (ZIP) family. ZIP4 is reportedly upregulated in many cancers, making it a promising oncology drug target. To date, there have been no reports on the turnover number of ZIP4, which is a crucial missing piece of information needed to better understand the transport mechanism. In this work, we used a nonradioactive zinc isotope, 70Zn, and inductively coupled plasma mass spectrometry to study human ZIP4 (hZIP4) expressed in Human embryonic kidney 293 cells. Our data showed that 70Zn can replace the radioactive 65Zn as a tracer in kinetic evaluation of hZIP4 activity. This approach, combined with the quantification of the cell surface expression of hZIP4 using biotinylation or surface-bound antibody, allowed us to estimate the apparent turnover number of hZIP4 to be in the range of 0.08 to 0.2 s-1. The turnover numbers of the truncated hZIP4 variants are significantly smaller than that of the full-length hZIP4, confirming a crucial role for the extracellular domain in zinc transport. Using 64Zn and 70Zn, we measured zinc efflux during the cell-based transport assay and found that it has little effect on the zinc import analysis under these conditions. Finally, we demonstrated that use of laser ablation inductively coupled plasma-TOF-mass spectrometry on samples applied to a solid substrate significantly increased the throughput of the transport assay. We envision that the approach reported here can be applied to the studies of metal transporters beyond the ZIP family.

Structure of the human dopamine transporter and mechanisms of inhibition

The neurotransmitter dopamine has central roles in mood, appetite, arousal and movement1. Despite its importance in brain physiology and function, and as a target for illicit and therapeutic drugs, the human dopamine transporter (hDAT) and mechanisms by which it is inhibited by small molecules and Zn2+ are without a high-resolution structural context. Here we determine the structure of hDAT in a tripartite complex with the competitive inhibitor and cocaine analogue, (-)-2-β-carbomethoxy-3-β-(4-fluorophenyl)tropane2 (β-CFT), the non-competitive inhibitor MRS72923 and Zn2+ (ref. 4). We show how β-CFT occupies the central site, approximately halfway across the membrane, stabilizing the transporter in an outward-open conformation. MRS7292 binds to a structurally uncharacterized allosteric site, adjacent to the extracellular vestibule, sequestered underneath the extracellular loop 4 (EL4) and adjacent to transmembrane helix 1b (TM1b), acting as a wedge, precluding movement of TM1b and closure of the extracellular gate. A Zn2+ ion further stabilizes the outward-facing conformation by coupling EL4 to EL2, TM7 and TM8, thus providing specific insights into how Zn2+ restrains the movement of EL4 relative to EL2 and inhibits transport activity.

Molecular basis of human noradrenaline transporter reuptake and inhibition

Noradrenaline, also known as norepinephrine, has a wide range of activities and effects on most brain cell types1. Its reuptake from the synaptic cleft heavily relies on the noradrenaline transporter (NET) located in the presynaptic membrane2. Here we report the cryo-electron microscopy (cryo-EM) structures of the human NET in both its apo state and when bound to substrates or antidepressant drugs, with resolutions ranging from 2.5 Å to 3.5 Å. The two substrates, noradrenaline and dopamine, display a similar binding mode within the central substrate binding site (S1) and within a newly identified extracellular allosteric site (S2). Four distinct antidepressants, namely, atomoxetine, desipramine, bupropion and escitalopram, occupy the S1 site to obstruct substrate transport in distinct conformations. Moreover, a potassium ion was observed within sodium-binding site 1 in the structure of the NET bound to desipramine under the KCl condition. Complemented by structural-guided biochemical analyses, our studies reveal the mechanism of substrate recognition, the alternating access of NET, and elucidate the mode of action of the four antidepressants.

Dopamine reuptake and inhibitory mechanisms in human dopamine transporter

The dopamine transporter has a crucial role in regulation of dopaminergic neurotransmission by uptake of dopamine into neurons and contributes to the abuse potential of psychomotor stimulants1-3. Despite decades of study, the structure, substrate binding, conformational transitions and drug-binding poses of human dopamine transporter remain unknown. Here we report structures of the human dopamine transporter in its apo state, and in complex with the substrate dopamine, the attention deficit hyperactivity disorder drug methylphenidate, and the dopamine-uptake inhibitors GBR12909 and benztropine. The dopamine-bound structure in the occluded state precisely illustrates the binding position of dopamine and associated ions. The structures bound to drugs are captured in outward-facing or inward-facing states, illuminating distinct binding modes and conformational transitions during substrate transport. Unlike the outward-facing state, which is stabilized by cocaine, GBR12909 and benztropine stabilize the dopamine transporter in the inward-facing state, revealing previously unseen drug-binding poses and providing insights into how they counteract the effects of cocaine. This study establishes a framework for understanding the functioning of the human dopamine transporter and developing therapeutic interventions for dopamine transporter-related disorders and cocaine addiction.

Predicting therapeutic and side effects from drug binding affinities to human proteome structures

Evaluation of the binding affinities of drugs to proteins is a crucial process for identifying drug pharmacological actions, but it requires three dimensional structures of proteins. Herein, we propose novel computational methods to predict the therapeutic indications and side effects of drug candidate compounds from the binding affinities to human protein structures on a proteome-wide scale. Large-scale docking simulations were performed for 7,582 drugs with 19,135 protein structures revealed by AlphaFold (including experimentally unresolved proteins), and machine learning models on the proteome-wide binding affinity score (PBAS) profiles were constructed. We demonstrated the usefulness of the method for predicting the therapeutic indications for 559 diseases and side effects for 285 toxicities. The method enabled to predict drug indications for which the related protein structures had not been experimentally determined and to successfully extract proteins eliciting the side effects. The proposed method will be useful in various applications in drug discovery.

Post-translational mechanisms in psychostimulant-induced neurotransmitter efflux

The availability of monoamine neurotransmitters in the brain is under the control of dopamine, norepinephrine, and serotonin transporters expressed on the plasma membrane of monoaminergic neurons. By regulating transmitter levels these proteins mediate crucial functions including cognition, attention, and reward, and dysregulation of their activity is linked to mood and psychiatric disorders of these systems. Amphetamine-based transporter substrates stimulate non-exocytotic transmitter efflux that induces psychomotor stimulation, addiction, altered mood, hallucinations, and psychosis, thus constituting a major component of drug neurochemical and behavioral outcomes. Efflux is under the control of transporter post-translational modifications that synergize with other regulatory events, and this review will summarize our knowledge of these processes and their role in drug mechanisms.

Cholesterol modulation of interactions between psychostimulants and dopamine transporters

The dopamine transporter (DAT) is a key site of action for cocaine and amphetamines. Dysfunctional DAT is associated with aberrant synaptic dopamine transmission and enhanced drug-seeking and taking behavior. Studies in cultured cells and ex vivo suggest that DAT function is sensitive to membrane cholesterol content. Although it is largely unknown whether psychostimulants alter cholesterol metabolism in the brain, emerging evidence indicates that peripheral cholesterol metabolism is altered in patients with psychostimulant use disorder and circulating cholesterol levels are associated with vulnerability to relapse. Cholesterol interacts with sphingolipids forming lipid raft microdomains on the membrane. These cholesterol-rich lipid raft microdomains serve to recruit and assemble other lipids and proteins to initiate signal transduction. There are two spatially and functionally distinct populations of the DAT segregated by cholesterol-rich lipid raft microdomains and cholesterol-scarce non-raft microdomains on the plasma membrane. These two DAT populations are differentially regulated by DAT blockers (e.g. cocaine), substrates (e.g. amphetamine), and protein kinase C providing distinct cholesterol-dependent modulation of dopamine uptake and efflux. In this chapter, we summarize the impact of depletion and addition of membrane cholesterol on DAT conformational changes between the outward-facing and the inward-facing states, lipid raft-associated DAT localization, basal and induced DAT internalization, and DAT function. In particular, we focus on how the interactions of the DAT with cocaine and amphetamine are influenced by membrane cholesterol. Lastly, we discuss the therapeutic potential of cholesterol-modifying drugs as a new avenue to normalize DAT function and dopamine transmission in patients with psychostimulant use disorder.

Structural and functional perspectives on interactions between synthetic cathinones and monoamine transporters

Synthetic cathinone derivatives comprise a family of psychoactive compounds structurally related to amphetamine. Over the last decade, clandestine chemists have synthesized a consistent stream of innovative cathinone derivatives to outpace governmental regulatory restrictions. Many of these unregulated substances are produced and distributed as designer drugs. Two of the principal chemical scaffolds exploited to expand the synthetic cathinone family are methcathinone and α-pyrrolidinopentiophenone (or α-pyrrolidinovalerophenone, α-PVP). These compounds' main physiological targets are monoamine transporters, where they promote addiction by potentiating dopaminergic neurotransmission. This chapter describes techniques used to study the pharmacodynamic properties of cathinones at monoamine transporters in vitro. Biochemical techniques described include uptake inhibition and release assays in rat brain synaptosomes and in mammalian expression systems. Electrophysiological techniques include current measurements using the voltage clamp technique. We describe a Ca2+ mobilization assay wherein voltage-gated Ca2+ channels function as reporters to study the action of synthetic cathinones at monoamine transporters. We discuss results from systematic structure-activity relationship studies on simple and complex cathinones at monoamine transporters with an emphasis on identifying structural moieties that modulate potency and selectivity at these transporters. Moreover, different profiles of selectivity at monoamine transporters directly predict compounds associated with behavioral and subjective effects within animals and humans. In conclusion, clarification of the structural aspects of compounds which modulate potency and selectivity at monoamine transporters is critical to identify and predict potential addictive drugs. This knowledge may allow prompt allocation of resources toward drugs that represent the greatest threats after drugs are identified by forensic laboratories.

Structure-based discovery of conformationally selective inhibitors of the serotonin transporter

The serotonin transporter (SERT) removes synaptic serotonin and is the target of anti-depressant drugs. SERT adopts three conformations: outward-open, occluded, and inward-open. All known inhibitors target the outward-open state except ibogaine, which has unusual anti-depressant and substance-withdrawal effects, and stabilizes the inward-open conformation. Unfortunately, ibogaine's promiscuity and cardiotoxicity limit the understanding of inward-open state ligands. We docked over 200 million small molecules against the inward-open state of the SERT. Thirty-six top-ranking compounds were synthesized, and thirteen inhibited; further structure-based optimization led to the selection of two potent (low nanomolar) inhibitors. These stabilized an outward-closed state of the SERT with little activity against common off-targets. A cryo-EM structure of one of these bound to the SERT confirmed the predicted geometry. In mouse behavioral assays, both compounds had anxiolytic- and anti-depressant-like activity, with potencies up to 200-fold better than fluoxetine (Prozac), and one substantially reversed morphine withdrawal effects.

Stereoselectivity in the Membrane Transport of Phenylethylamine Derivatives by Human Monoamine Transporters and Organic Cation Transporters 1, 2, and 3

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.

In vitro and in vivo pharmacological characterization of dasotraline, a dual dopamine and norepinephrine transporter inhibitor in vivo

Inhibitors of dopamine transporters (DAT), norepinephrine transporters (NET) and serotonin transporters (SERT) are effective treatments for neuropsychiatric diseases. Dasotraline [(1R,4 S)- 4-(3,4-dichlorophenyl)- 1,2,3,4-tetrahydro-1-naphthalenamine, also known as SEP-225289) was evaluated for its inhibitory potency at DAT, NET and SERT using in vitro and in vivo assays. In vitro radiometric functional uptake studies showed preferential inhibition by dasotraline of hDAT (IC₅₀ =3 nM) and hNET (IC₅₀ =4 nM relative to hSERT(IC₅₀ =15 nM). In mouse ex vivo occupancy studies, dasotraline demonstrated total plasma concentration-dependent occupancy at DAT, NET and SERT. Determination of the TO₅₀ (50% transporter occupancy) were 32, 109 and 276 ng/ml, respectively. In SPECT imaging studies in baboons, dasotraline (0.2 mg/kg iv) displaced radiotracer binding to DAT by 87% but only 20% at NET and SERT. Rat microdialysis studies were performed in prefrontal cortex and striatum. Dasotraline produced sustained (>4 h) increases in dopamine and norepinephrine concentrations. Dasotraline was also more potent at increasing synaptic dopamine in the striatum, and norepinephrine in the prefrontal cortex than serotonin in these regions. In summary, dasotraline preferentially inhibits DAT and NET relative to SERT. Together, the occupancy and neurochemical profile of dasotraline provide a mechanistic basis for the treatment of diseases that have an underlying causality involving dopamine and norepinephrine dysfunction.

Uncovering Functional Contributions of PMAT (Slc29a4) to Monoamine Clearance Using Pharmacobehavioral Tools

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.

Early Life Social Stress Causes Sex- and Region-Dependent Dopaminergic Changes that Are Prevented by Minocycline

Early life stress (ELS) is known to modify trajectories of brain dopaminergic development, but the mechanisms underlying have not been determined. ELS perturbs immune system and microglia reactivity, and inflammation and microglia influence dopaminergic transmission and development. Whether microglia mediate the effects of ELS on dopamine (DA) system development is still unknown. We explored the effects of repeated early social stress on development of the dopaminergic system in male and female mice through histological, electrophysiological, and transcriptomic analyses. Furthermore, we tested whether these effects could be mediated by ELS-induced altered microglia/immune activity through a pharmacological approach. We found that social stress in early life altered DA neurons morphology, reduced dopamine transporter (DAT) and tyrosine hydroxylase expression, and lowered DAT-mediated currents in the ventral tegmental area but not substantia nigra of male mice only. Notably, stress-induced DA alterations were prevented by minocycline, an inhibitor of microglia activation. Transcriptome analysis in the developing male ventral tegmental area revealed that ELS caused downregulation of dopaminergic transmission and alteration in hormonal and peptide signaling pathways. Results from this study offer new insight into the mechanisms of stress response and altered brain dopaminergic maturation after ELS, providing evidence of neuroimmune interaction, sex differences, and regional specificity.

Functional characterization of the biogenic amine transporters on human macrophages

Monocyte-derived macrophages are key players in tissue homeostasis and diseases regulated by a variety of signaling molecules. Recent literature has highlighted the ability for biogenic amines to regulate macrophage functions, but the mechanisms governing biogenic amine signaling in and around immune cells remains nebulous. In the central nervous system (CNS), biogenic amine transporters are regarded as the master regulators of neurotransmitter signaling. While we and others have shown that macrophages express these transporters, relatively little is known of their function in these cells. To address these knowledge gaps, we investigated the function of norepinephrine (NET) and dopamine (DAT) transporters on human monocyte-derived macrophages. We found that both NET and DAT are present and can uptake substrate from the extracellular space at baseline. Not only was DAT expressed in cultured monocyte-derived macrophages (MDMs), but it was also detected in a subset of intestinal macrophages in situ. Surprisingly, we discovered a NET-independent, DAT-mediated immuno-modulatory mechanism in response to lipopolysaccharide (LPS). LPS induced reverse transport of dopamine through DAT, engaging an autocrine/paracrine signaling loop that regulated the macrophage response. Removing this signaling loop enhanced the pro-inflammatory response to LPS. Collectively, our data introduce a potential role for DAT in the regulation of innate immunity.

Overlap and Specificity in the Substrate Spectra of Human Monoamine Transporters and Organic Cation Transporters 1, 2, and 3

Human monoamine transporters (MATs) are cation transporters critically involved in neuronal signal transmission. While inhibitors of MATs have been intensively studied, their substrate spectra have received far less attention. Polyspecific organic cation transporters (OCTs), predominantly known for their role in hepatic and renal drug elimination, are also expressed in the central nervous system and might modulate monoaminergic signaling. Using HEK293 cells overexpressing MATs or OCTs, we compared uptake of 48 compounds, mainly phenethylamine and tryptamine derivatives including matched molecular pairs, across noradrenaline, dopamine and serotonin transporters and OCTs (1, 2, and 3). Generally, MATs showed surprisingly high transport activities for numerous analogs of neurotransmitters, but their substrate spectra were limited by molar mass. Human OCT2 showed the broadest substrate spectrum, and also the highest overlap with MATs substrates. Comparative kinetic analyses revealed that the radiotracer meta-iodobenzylguanidine had the most balanced uptake across all six transporters. Matched molecular pair analyses comparing MAT and OCT uptake using the same methodology could provide a better understanding of structural determinants for high cell uptake by MATs or OCTs. The data may result in a better understanding of pharmacokinetics and toxicokinetics of small molecular organic cations and, possibly, in the development of more specific radiotracers for MATs.

Therapeutic targeting of membrane-associated proteins in central nervous system tumors

The activity of the most complex system, the central nervous system (CNS) is profoundly regulated by a huge number of membrane-associated proteins (MAP). A minor change stimulates immense chemical changes and the elicited response is organized by MAP, which acts as a receptor of that chemical or channel enabling the flow of ions. Slight changes in the activity or expression of these MAPs lead to severe consequences such as cognitive disorders, memory loss, or cancer. CNS tumors are heterogeneous in nature and hard-to-treat due to random mutations in MAPs; like as overexpression of EGFRvIII/TGFβR/VEGFR, change in adhesion molecules α5β3 integrin/SEMA3A, imbalance in ion channel proteins, etc. Extensive research is under process for developing new therapeutic approaches using these proteins such as targeted cytotoxic radiotherapy, drug-delivery, and prodrug activation, blocking of receptors like GluA1, developing viral vector against cell surface receptor. The combinatorial approach of these strategies along with the conventional one might be more potential. Henceforth, our review focuses on in-depth analysis regarding MAPs aiming for a better understanding for developing an efficient therapeutic approach for targeting CNS tumors.

Handling of intracellular K+ determines voltage dependence of plasmalemmal monoamine transporter function

The concentrative power of the transporters for dopamine (DAT), norepinephrine (NET), and serotonin (SERT) is thought to be fueled by the transmembrane Na⁺ gradient, but it is conceivable that they can also tap other energy sources, for example, membrane voltage and/or the transmembrane K⁺ gradient. We have addressed this by recording uptake of endogenous substrates or the fluorescent substrate APP⁺(4-(4-dimethylamino)phenyl-1-methylpyridinium) under voltage control in cells expressing DAT, NET, or SERT. We have shown that DAT and NET differ from SERT in intracellular handling of K⁺. In DAT and NET, substrate uptake was voltage-dependent due to the transient nature of intracellular K⁺ binding, which precluded K⁺ antiport. SERT, however, antiports K⁺ and achieves voltage-independent transport. Thus, there is a trade-off between maintaining constant uptake and harvesting membrane potential for concentrative power, which we conclude to occur due to subtle differences in the kinetics of co-substrate ion binding in closely related transporters.

Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression

Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.

Transporter tandems: precise tools for normalizing active transporter in the plasma membrane

The transport efficiency (TE) describes the performance of a transport protein for a specific substrate. To compare the TE of different transporters, the number of active transporters in the plasma membrane must be monitored, as it may vary for each transporter and experiment. Available methods, like LC-MS quantification of tryptic peptides, fail to discriminate inactive intracellular transporters or, like cell-surface biotinylation followed by affinity chromatography and Western blotting, are imprecise and very laborious. We wanted to normalize active transporters by the activity of a second transporter. A transporter tandem, generated by joining two transporter cDNAs into a single open reading frame, should guarantee a 1 : 1 stoichiometry. Here we created a series of tandems with different linkers between the human ergothioneine (ET) transporter ETT (gene symbol SLC22A4) and organic cation transporter OCT2 (SLC22A2). The linker sequence strongly affected the expression strength. The stoichiometry was validated by absolute peptide quantification and untargeted peptide analysis. Compared with wild-type ETT, the normalized ET clearance of the natural variant L503F was higher (f = 1.34); G462E was completely inactive. The general usefulness of the tandem strategy was demonstrated by linking several transporters with ETT; every construct was active in both parts. Transporter tandems can be used - without membrane isolation or protein quantification - as precise tools for transporter number normalization, to identify, for example, relevant transporters for a drug. It is necessary, however, to find suitable linkers, to check the order of transporters, and to verify the absence of functional interference by saturation kinetics.

Human neuronal signaling and communication assays to assess functional neurotoxicity

Prediction of drug toxicity on the human nervous system still relies mainly on animal experiments. Here, we developed an alternative system allowing assessment of complex signaling in both individual human neurons and on the network level. The LUHMES cultures used for our approach can be cultured in 384-well plates with high reproducibility. We established here high-throughput quantification of free intracellular Ca2+ concentrations [Ca2+]i as broadly applicable surrogate of neuronal activity and verified the main processes by patch clamp recordings. Initially, we characterized the expression pattern of many neuronal signaling components and selected the purinergic receptors to demonstrate the applicability of the [Ca2+]i signals for quantitative characterization of agonist and antagonist responses on classical ionotropic neurotransmitter receptors. This included receptor sub-typing and the characterization of the anti-parasitic drug suramin as modulator of the cellular response to ATP. To exemplify potential studies on ion channels, we characterized voltage-gated sodium channels and their inhibition by tetrodotoxin, saxitoxin and lidocaine, as well as their opening by the plant alkaloid veratridine and the food-relevant marine biotoxin ciguatoxin. Even broader applicability of [Ca2+]i quantification as an end point was demonstrated by measurements of dopamine transporter activity based on the membrane potential-changing activity of this neurotransmitter carrier. The substrates dopamine or amphetamine triggered [Ca2+]i oscillations that were synchronized over the entire culture dish. We identified compounds that modified these oscillations by interfering with various ion channels. Thus, this new test system allows multiple types of neuronal signaling, within and between cells, to be assessed, quantified and characterized for their potential disturbance.

Amino Acid Transporter SLC6A14 (ATB0,+) - A Target in Combined Anti-cancer Therapy

Cancer cells are characterized by quick growth and proliferation, demanding constant supply of various nutrients. Several plasma membrane transporters delivering such compounds are upregulated in cancer. Solute carrier family 6 member 14 (SLC6A14), known as amino acid transporter B^0,+ (ATB^0,+) transports all amino acids with exception of the acidic ones: aspartate and glutamate. Its malfunctioning is correlated with several pathological states and it is upregulated in solid tumors. The high expression of SLC6A14 is prognostic and unfavorable in pancreatic cancer, while in breast cancer it is expressed in estrogen receptor positive cells. As many plasma membrane transporters it resides in endoplasmic reticulum (ER) membrane after translation before further trafficking through Golgi to the cell surface. Transporter exit from ER is strictly controlled. The proper folding of SLC6A14 was shown to be controlled from the cytoplasmic side by heat shock proteins, further exit from ER and formation of coatomer II (COPII) coated vesicles depends on specific interaction with COPII cargo-recognizing subunit SEC24C, phosphorylated by kinase AKT. Inhibition of heat shock proteins, known to be upregulated in cancer, directs SLC6A14 to degradation. Targeting proteins regulating SLC6A14 trafficking is proposed as an additional pharmacological treatment of cancer.

Allosteric Modulation of Neurotransmitter Transporters as a Therapeutic Strategy

Neurotransmitter transporters (NTTs) are involved in the fine-tuning of brain neurotransmitter homeostasis. As such, they are implicated in a plethora of complex behaviors, including reward, movement, and cognition. During recent decades, compounds that modulate NTT functions have been developed. Some of them are in clinical use for the management of different neuropsychiatric conditions. The majority of these compounds have been found to selectively interact with the orthosteric site of NTTs. Recently, diverse allosteric sites have been described in a number of NTTs, modulating their function. A more complex NTT pharmacology may be useful in the development of novel therapeutics. Here, we summarize current knowledge on such modulatory allosteric sites, with specific focus on their pharmacological and therapeutic potential.

Synthesis and evaluation of 2-(2'-((dimethylamino)methyl)-4'-(2-fluoroethoxy-substituted)phenylthio)benzenamine derivatives as potential positron emission tomography imaging agents for serotonin transporters

A series of diphenylsulfide derivatives with various substitutions at the 4-position on phenyl ring A and different lengths of the 2-fluoroethoxy-substituted side-chain at the 4'-position on ring B were synthesized and evaluated as potential positron emission tomography (PET) imaging agents for serotonin transporters (SERT). These ligands exhibited high SERT binding affinities (K_i = 0.11-1.3 nM) and the 4-methyl-substituted (4-Me) compounds 7a and 8a displayed excellent selectivity for SERT versus norepinephrine transporters (NET) (392- and 700-fold, respectively). In the parallel artificial membrane permeability assay (PAMPA), these ligands demonstrated moderate to high brain penetration, and the 4-Me analogs showed higher BBB permeability than the corresponding 4-F analogs. The 2-fluoroethoxy-substituted ligands showed higher metabolic stability and lower lipophilicity than 4-F-ADAM. [¹⁸F]7a-c were readily prepared using an automatic synthesizer and exhibited significant uptake and slow washout in rat brains. At 120 min after iv injection, [¹⁸F]7a exhibited the highest uptake in the midbrain, whereas [¹⁸F]7b exhibited the highest uptake in the hypothalamus and midbrain. After treatment with citalopram, a SERT-selective ligand, the uptake of [¹⁸F]7a in the hypothalamus and striatum was significantly decreased. The potent and highly selective SERT binding and the selective and reversible accumulation in SERT-rich brain regions suggested that [¹⁸F]7a is a promising lead for the further development of novel [¹⁸F]-labeled PET imaging agents for SERT binding sites in the brain.

Dopamine transporter gene expression within the nucleus accumbens plays important role in the acquisition and reinstatement of ethanol-seeking behavior in mice

Alcoholism and alcohol use disorders are chronically relapsing conditions which is a major problem in treating alcohol addiction. In a previous study we showed that the dopamine transporter (DAT) is implicated in voluntary intake and preference. However, its role in modulating ethanol-associated contextual memory remains largely unknown. In this study we have investigated the role of DAT in ethanol-induced conditioned place preference (EtOH-CPP) acquisition and reinstatement in adult male C57BL/6 mice. For this purpose, we used both loss- and gain-of-function approaches to test the effects of central DAT manipulation on EtOH-CPP. We developed a lentiviral-mediated gene transfer approach to examine whether DAT knockdown (shDAT) or overexpression in the nucleus accumbens (Nacc) is enough to impair EtOH-CPP acquisition and reinstatement. In the first experiment, results showed that DAT knockdown blocked, whereas DAT overexpression, exacerbated the acquisition of EtOH-CPP. In the second experiment and after the EtOH-CPP expression, the mice were subjected to a 14-day extinction trials before drug-induced EtOH-CPP reinstatement was induced by a priming injection of 1 g/kg EtOH. Results indicated that reinstatement of EtOH-CPP was considerably decreased after accumbal shDAT injection. However, DAT overexpression significantly increased EtOH-CPP reinstatement. Finally, and following DAT mRNA quantification using RT-PCR, Pearson's correlation showed a strong positive relationship between accumbal DAT mRNA and EtOH-CPP acquisition and reinstatement. These results suggest that DAT expression in the Nacc is involved in the acquisition and retrieval of EtOH contextual memory and that blockade of this transporter can decrease the rewarding properties of EtOH.

Roles Played by the Na+/Ca2+ Exchanger and Hypothermia in the Prevention of Ischemia-Induced Carrier-Mediated Efflux of Catecholamines into the Extracellular Space: Implications for Stroke Therapy

The release of [³H]dopamine ([³H]DA) and [³H]noradrenaline ([³H]NA) in acutely perfused rat striatal and cortical slice preparations was measured at 37 °C and 17 °C under ischemic conditions. The ischemia was simulated by the removal of oxygen and glucose from the Krebs solution. At 37 °C, resting release rates in response to ischemia were increased; in contrast, at 17 °C, resting release rates were significantly reduced, or resting release was completely prevented. The removal of extracellular Ca²⁺ further increased the release rates of [³H]DA and [³H]NA induced by ischemic conditions. This finding indicated that the Na⁺/Ca²⁺ exchanger (NCX), working in reverse in the absence of extracellular Ca²⁺, fails to trigger the influx of Ca²⁺ in exchange for Na⁺ and fails to counteract ischemia by further increasing the intracellular Na⁺ concentration ([Na⁺]_i). KB-R7943, an inhibitor of NCX, significantly reduced the cytoplasmic resting release rate of catecholamines under ischemic conditions and under conditions where Ca²⁺ was removed. Hypothermia inhibited the excessive release of [³H]DA in response to ischemia, even in the absence of Ca²⁺. These findings further indicate that the NCX plays an important role in maintaining a high [Na⁺]_i, a condition that may lead to the reversal of monoamine transporter functions; this effect consequently leads to the excessive cytoplasmic tonic release of monoamines and the reversal of the NCX. Using HPLC combined with scintillation spectrometry, hypothermia, which enhances the stimulation-evoked release of DA, was found to inhibit the efflux of toxic DA metabolites, such as 3,4-dihydroxyphenylacetaldehyde (DOPAL). In slices prepared from human cortical brain tissue removed during elective neurosurgery, the uptake and release values for [³H]NA did not differ from those measured at 37 °C in slices that were previously maintained under hypoxic conditions at 8 °C for 20 h. This result indicates that hypothermia preserves the functions of the transport and release mechanisms, even under hypoxic conditions. Oxidative stress (H₂O₂), a mediator of ischemic brain injury enhanced the striatal resting release of [³H]DA and its toxic metabolites (DOPAL, quinone). The study supports our earlier findings that during ischemia transmitters are released from the cytoplasm. In addition, the major findings of this study that hypothermia of brain slice preparations prevents the extracellular calcium concentration ([Ca²⁺]_o)-independent non-vesicular transmitter release induced by ischemic insults, inhibiting Na⁺/Cl⁻-dependent membrane transport of monoamines and their toxic metabolites into the extracellular space, where they can exert toxic effects.

Palmitoylation by Multiple DHHC Enzymes Enhances Dopamine Transporter Function and Stability

The dopamine transporter (DAT) is a plasma membrane protein that mediates the reuptake of extracellular dopamine (DA) and controls the spatiotemporal dynamics of dopaminergic neurotransmission. The transporter is subject to fine control that tailors clearance of transmitter to physiological demands, and dysregulation of reuptake induced by psychostimulant drugs, transporter polymorphisms, and signaling defects may impact transmitter tone in disease states. We previously demonstrated that DAT undergoes complex regulation by palmitoylation, with acute inhibition of the modification leading to rapid reduction of transport activity and sustained inhibition of the modification leading to transporter degradation and reduced expression. Here, to examine mechanisms and outcomes related to increased modification, we coexpressed DAT with palmitoyl acyltransferases (PATs), also known as DHHC enzymes, which catalyze palmitate addition to proteins. Of 12 PATs tested, DAT palmitoylation was stimulated by DHHC2, DHHC3, DHHC8, DHHC15, and DHHC17, with others having no effect. Increased modification was localized to previously identified palmitoylation site Cys580 and resulted in upregulation of transport kinetics and elevated transporter expression mediated by reduced degradation. These findings confirm palmitoylation as a regulator of multiple DAT properties crucial for appropriate DA homeostasis and identify several potential PAT pathways linked to these effects. Defects in palmitoylation processes thus represent possible mechanisms of transport imbalances in DA disorders.

Thermodynamic first law efficiency of membrane proteins

Proteins are nature's biomolecular machines. Proteins, such as transporters, pumps and motors, have complex function/operating-machinery/mechanisms, comparable to the macro-scaled machines that we encounter in our daily life. These proteins, as it is for their macro-scaled counterparts, convert (part of) other/various forms of energy into work. In this study, we are performing the first law analysis on a set of proteins, including the dopamine transporter, glycine transporters I and II, glutamate transporter, sodium-potassium pump and Ca²⁺ ATPase. Each of these proteins operates on a thermodynamic/mechanic cycle to perform their function. In each of these cycles, they receive energy from a source, convert part of this energy into work and reject the remaining part of the energy to the environment. Conservation of energy principle was applied to the thermodynamic/mechanic cycle of each protein, and thermodynamic first law efficiency was evaluated for each cycle, which shows how much of the energy input per cycle was converted into useful work. Interestingly, calculations based on experimental data indicate that proteins can operate under a range of efficiencies, which vary based on the extracellular and intracellular ion and substrate concentrations. The lowest observed first law efficiency was 50%, which is a very high value if compared to the efficiency of the macro-scaled heat engines we encounter in our daily lives.Communicated by Ramaswamy H. Sarma.

Using Ca2+-channel biosensors to profile amphetamines and cathinones at monoamine transporters: electro-engineering cells to detect potential new psychoactive substances

BACKGROUND

The appearance of stimulant-class new psychoactive substances (NPS) is a frequent and significant problem in our society. Cathinone variants are often sold illegally as 3,4-methylenedioxy methamphetamine ("ecstasy") or disguised for legal sale using misleading names such as "bath salts" and carry the risk of promoting disruptive mental states, addiction, and fatal overdose. The principal targets of these recreational drugs are monoamine transporters expressed in catecholaminergic and serotonergic neurons. Some transporter ligands can be transported into cells, where they can promote a massive release of neurotransmitters through reverse transport, and others can block uptake. A ligand's dopamine vs. serotonin transporter selectivity, potency, and activity as a substrate or blocker can help elucidate the abuse liability and subjective effects of a drug.

OBJECTIVES

Here, we describe the discovery, development, and validation of an emerging methodology for compound activity assessment at monoamine transporters.

KEY FINDINGS

Substrates generate inward electrical currents through transporters and can depolarize the plasma membrane, whereas blockers work as a "cork in a bottle" and function as antagonists. Voltage-gated Ca²⁺ channels were co-expressed with monoamine transporters in cultured cells and used to measure fluctuations of the membrane electrical potential. In this system, substrates of monoamine transporters produce reliable dose-dependent Ca²⁺ signals, while blockers hinder them.

DISCUSSION

This system constitutes a novel use of voltage-gated Ca²⁺ channels as biosensors for the purpose of characterizing ligand activity at monoamine transporters using fluorimetry. This approach in combination with in vivo evaluations of drugs' abuse-related effects is a powerful strategy for anticipating potential stimulant-class NPS.

Development of a peptide targeting dopamine transporter to improve ADHD-like deficits

Attention-deficit hyperactivity disorder (ADHD) is a neurocognitive disorder characterized by hyperactivity, inattention, working memory deficits and impulsivity. Its worldwide prevalence is estimated to be 3–5% in children and adolescents. The mainstay treatment for ADHD is stimulant medications (e.g. methylphenidate), which increase synaptic dopamine by directly blocking dopamine transporter (DAT). Although these pharmacological agents are effective, they are often associated with various side effects including risks for future substance use disorders in ADHD patients. Here, we investigated an interaction between DAT and dopamine D2 receptor (D2R) as a novel target to develop potential therapeutics for the treatment of ADHD by using an interfering peptide (TAT-DAT_NT) to dissociate this protein complex. We found that TAT-DAT_NT promotes locomotor behavior in Sprague-Dawley rats. Furthermore, using in vivo microdialysis and high-performance liquid chromatography, we found that the disruption of D2R-DAT elevates extracellular dopamine level. More importantly, the interfering peptide, TAT-DAT_NT, attenuates hyperactivity and improves spontaneous alternation behavior in spontaneously hypertensive rats (SHR) ------ a common animal model of ADHD. This work presents a different means (i.e. other than direct blockade by a DAT inhibitor) to regulate the activity of DAT and dopaminergic neurotransmission, and a potential target site for future development of ADHD treatments.

Comparative analysis of novel decynium-22 analogs to inhibit transport by the low-affinity, high-capacity monoamine transporters, organic cation transporters 2 and 3, and plasma membrane monoamine transporter

Growing evidence supports involvement of low-affinity/high-capacity organic cation transporters (OCTs) and plasma membrane monoamine transporter (PMAT) in regulating clearance of monoamines. Currently decynium-22 (D22) is the best pharmacological tool to study these transporters, however it does not readily discriminate among them, underscoring a need to develop compounds with greater selectivity for each of these transporters. We developed seven D22 analogs, and previously reported that some have lower affinity for α1-adrenoceptors than D22 and showed antidepressant-like activity in mice. Here, we extend these findings to determine the affinity of these analogs for OCT2, OCT3 and PMAT, as well as serotonin, norepinephrine and dopamine transporters (SERT, NET and DAT) using a combination of uptake competition with [³H]methyl-4-phenylpyridinium acetate in overexpressed HEK cells and [³H]citalopram, [³H]nisoxetine and [³H]WIN 35428 displacement binding in mouse hippocampal and striatal preparations. Like D22, all analogs showed greater binding affinities for OCT3 than OCT2 and PMAT. However, unlike D22, some analogs also showed modest affinity for SERT and DAT. Dual OCT3/SERT and/or OCT3/DAT actions of certain analogs may help explain their ability to produce antidepressant-like effects in mice and help account for our previous findings that D22 lacks antidepressant-like effects unless SERT function is either genetically or pharmacologically compromised. Though these analogs are not superior than D22 in discriminating among OCTs/PMAT, our findings point to development of compounds with combined ability to inhibit both low-affinity/high-capacity transporters, such as OCT3, and high-affinity/low-capacity transporters, such as SERT, as therapeutics with potentially improved efficacy for treatment of psychiatric disorders.

Model systems for analysis of dopamine transporter function and regulation

The dopamine transporter (DAT) plays a critical role in dopamine (DA) homeostasis by clearing transmitter from the extraneuronal space after vesicular release. DAT serves as a site of action for a variety of addictive and therapeutic reuptake inhibitors, and transport dysfunction is associated with transmitter imbalances in disorders such as schizophrenia, attention deficit hyperactive disorder, bipolar disorder, and Parkinson disease. In this review, we describe some of the model systems that have been used for in vitro analyses of DAT structure, function and regulation, and discuss a potential relationship between transporter kinetic values and membrane cholesterol.

The LeuT-fold neurotransmitter:sodium symporter MhsT has two substrate sites

Crystal structures of the neurotransmitter:sodium symporter MhsT revealed occluded inward-facing states with one substrate (Trp) bound in the primary substrate (S1) site and a collapsed extracellular vestibule, which in LeuT contains the second substrate (S2) site. In n-dodecyl-β-d-maltoside, the detergent used to prepare MhsT for crystallization, the substrate-to-protein binding stoichiometry was determined by using scintillation proximity to be 1 Trp:MhsT. Here, using the same experimental approach, as well as equilibrium dialysis, we report that in n-decyl-β-d-maltoside, or after reconstitution in lipid, MhsT, like LeuT, can simultaneously bind two Trp substrate molecules. Trp binding to the S2 site sterically blocks access to a substituted Cys at position 33 in the S2 site, as well as access to the deeper S1 site. Mutation of either the S1 or S2 site disrupts transport, consistent with previous studies in LeuT showing that substrate binding to the S2 site is an essential component of the transport mechanism.

Reversal of dopamine-mediated firing inhibition through activation of the dopamine transporter in substantia nigra pars compacta neurons

BACKGROUND AND PURPOSE

One of the hallmarks of ventral midbrain dopamine-releasing neurons is membrane hyperpolarization in response to stimulation of somato-dendritic D₂ receptors. At early postnatal age, under sustained dopamine, this inhibitory response is followed by a slow recovery, resulting in dopamine inhibition reversal (DIR). In the present investigation, we aimed to get a better insight into the cellular mechanisms underlying DIR.

EXPERIMENTAL APPROACH

We performed single-unit extracellular recordings with a multi-electrode array device and conventional patch-clamp recordings on midbrain mouse slices.

KEY RESULTS

While continuous dopamine (100 μM) perfusion gave rise to firing inhibition that recovered in 10 to 15 min, the same effect was not obtained with the D₂ receptor agonist quinpirole (100 nM). Moreover, firing inhibition caused by the GABA_B receptor agonist baclofen (300 nM) was reversed by dopamine (100 μM), albeit D₂ receptors had been blocked by sulpiride (10 μM). Conversely, the block of the dopamine transporter (DAT) with cocaine (30 μM) prevented firing recovery by dopamine under GABA_B receptor stimulation. Accordingly, in whole-cell recordings from single cells, the baclofen-induced outward current was counteracted by dopamine (100 μM) in the presence of sulpiride (10 μM), and this effect was prevented by the DAT antagonists cocaine (30 μM) and GBR12909 (2 μM).

CONCLUSIONS AND IMPLICATIONS

Our results indicate that the DAT plays a major role in DIR, mediating it under conditions of sustained dopamine exposure, and point to DAT as an important target for pharmacological therapies leading to prolonged enhancement of the dopaminergic signal.

Fluorine-18 labeled diphenyl sulfide derivatives for imaging serotonin transporter (SERT) in the brain

OBJECTIVES

Serotonin transporters (SERT) play an important role in controlling serotonin concentration in the synaptic cleft and in managing postsynaptic signal transduction. Inhibitors of SERT binding are well known as selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine, sertraline, paroxetine, and escitalopram, that are commonly prescribed antidepressants. Positron emission tomography (PET) and single photon emission tomography (SPECT) imaging agents targeting SERT may be useful for studying its function and providing a tool for monitoring drug treatment.

METHODS

A series of novel ¹⁸F-labeled diphenyl sulfide derivatives were prepared and tested for their binding affinity. Among them, 2-((2-((dimethylamino)-methyl)-4-(2-(2-fluoroethoxy)ethoxy)phenyl)thio)aniline, 1, which showed excellent binding toward serotonin transporter (SERT) in the brain (K_i = 0.09 nM), was selected for further evaluation. An active OTs intermediate, 7, was treated with [¹⁸F]F^-/K₂₂₂ to provide [¹⁸F]1 in one step and in high radiochemical yields. This new SERT targeting agent was evaluated in rats by biodistribution studies and animal PET imaging studies.

RESULTS

The radiolabeling reaction led to the desired [¹⁸F]1. After HPLC purification no-carrier-added [¹⁸F]1 was obtained (radiochemical yield, 23-47% (n = 10,); radiochemical purity >99%; molar activity, 15-28 GBq/μmol). Biodistribution studies with [¹⁸F]1 showed good brain uptake (1.04% dose/g at 2 min post-injection), high uptake into the hypothalamus (1.55% dose/g at 30 min), and a high target-to-non-target (hypothalamus to cerebellum) ratio of 6.1 at 120 min post-injection. A PET imaging study in normal rats showed excellent uptake in the midbrain and thalamus regions known to be rich in SERT binding sites at 60 min after iv injection. Chasing experiment with escitalopram (iv, 2 mg/kg) in a rat at 60 min after iv injection caused a noticeable reduction in the regional radioactivity and the target-to-non-target ratio, suggesting binding by [¹⁸F]1 was highly specific and reversible for SERT binding sites in the brain.

CONCLUSIONS

A novel diphenyl sulfide derivative, [¹⁸F]1 for SERT imaging was successfully prepared and evaluated. Results suggest that this new chemical entity is targeting SERT binding sites in the brain, and it is a suitable candidate for future commercial development.

Dopamine signalling in locusts and other insects

Dopamine is an important catecholamine neurotransmitter in invertebrates and vertebrates. It is biochemically derived from tyrosine via L-DOPA. It is most abundant in the central nervous system, but can also be produced in e.g. epidermal cells. Dopamine has conserved roles in the control of movement, pleasure, motivation, arousal and memory between invertebrate and vertebrate animals. It is crucial for melanisation and sclerotisation, important processes for the formation of the exoskeleton of insects and immune function. In this brief review I will discuss some general aspects of insect dopamine biosynthesis and breakdown, dopamine receptors and their pharmacology. In addition, I will provide a glance on the multitude of biological functions of dopamine in insects. More detail is provided concerning the putative roles of dopamine in phase related phenomena in locusts. Finally, molecular and pharmacological adjustments of insect dopamine signalling are discussed in the light of possible approaches towards insect pest management.

Monoamine Transporter and Receptor Interaction Profiles in Vitro Predict Reported Human Doses of Novel Psychoactive Stimulants and Psychedelics

BACKGROUND

Pharmacological profiles of new psychoactive substances can be established rapidly in vitro and provide information on potential psychoactive effects in humans. The present study investigated whether specific in vitro monoamine transporter and receptor interactions can predict effective psychoactive doses in humans.

METHODS

We correlated previously assessed in vitro data of stimulants and psychedelics with human doses that are reported on the Internet and in books.

RESULTS

For stimulants, dopamine and norepinephrine transporter inhibition potency was positively correlated with human doses, whereas serotonin transporter inhibition potency was inversely correlated with human doses. Serotonin 5-hydroxytryptamine-2A (5-HT2A) and 5-HT2C receptor affinity was significantly correlated with psychedelic doses, but 5-HT1A receptor affinity and 5-HT2A and 5-HT2B receptor activation potency were not.

CONCLUSIONS

The rapid assessment of in vitro pharmacological profiles of new psychoactive substances can help to predict psychoactive doses and effects in humans and facilitate the appropriate scheduling of new psychoactive substances.

The dopamine transporter: An unrecognized nexus for dysfunctional peripheral immunity and signaling in Parkinson's Disease

The second-most common neurodegenerative disease, Parkinson's Disease (PD) has three hallmarks: dysfunctional dopamine transmission due, at least in part, to dopamine neuron degeneration; intracellular inclusions of α-synuclein aggregates; and neuroinflammation. The origin and interplay of these features remains a puzzle, as does the underlying mechanism of PD pathogenesis and progression. When viewed in the context of neuroimmunology, dopamine also plays a role in regulating peripheral immune cells. Intriguingly, plasma dopamine levels are altered in PD, suggesting collateral dysregulation of peripheral dopamine transmission. The dopamine transporter (DAT), the main regulator of dopaminergic tone in the CNS, is known to exist in lymphocytes and monocytes/macrophages, but little is known about peripheral DAT biology or how DAT regulates the dopaminergic tone, much less how peripheral DAT alters immune function. Our review is guided by the hypothesis that dysfunctional peripheral dopamine signaling might be linked to the dysfunctional immune responses in PD and thereby suggests a potential bidirectional communication between central and peripheral dopamine systems. This review seeks to foster new perspectives concerning PD pathogenesis and progression.

Quantitative Assessment of the Energetics of Dopamine Translocation by Human Dopamine Transporter

Computational evaluation of the energetics of substrate binding, transport, and release events of neurotransmitter transporters at the molecular level is a challenge, as the structural transitions of these membrane proteins involve coupled global and local changes that span time scales of several orders of magnitude, from nanoseconds to seconds. Here, we provide a quantitative assessment of the energetics of dopamine (DA) translocation through the human DA transporter (hDAT), using a combination of molecular modeling, simulation, and analysis tools. DA-binding and -unbinding events, which generally involve local configurational changes, are evaluated using free-energy perturbation or adaptive biasing force methods. The global transitions between the outward-facing state and the inward-facing state, on the other hand, require a dual-boost accelerated molecular dynamics simulation. We present results on DA-binding/unbinding energetics under different conditions, as well as the conformational energy landscape of hDAT in both DA-bound and -unbound states. The study provides a tractable method of approach for quantitative evaluation of substrate-binding energetics and efficient estimation of conformational energy landscape, in general.

Psychostimulants As Cognitive Enhancers in Adolescents: More Risk than Reward?

Methylphenidate and other psychostimulants, originally developed to treat attention deficit-hyperactivity disorder, are increasingly abused by healthy adolescents and adults seeking an advantage in scholastic performance and work productivity. However, how these drugs may affect cognitive performance, especially in the young brain, remains unclear. Here, we review recent literature and emphasize the risks of abuse of psychostimulants in healthy adolescents and young adults. We conclude that while the desire for cognitive enhancement, particularly with rising costs of education and increasingly competitive nature of scholarship programs, is unlikely to diminish in the near future, it is crucial for the scientific community to thoroughly examine the efficacy and safety of these stimulants in healthy populations across development. The current dearth of knowledge on the dose–response curve, metabolism, and cognitive outcomes in adolescents following methylphenidate or other psychostimulant exposure may be perpetuating a perception of these drugs as “safe” when that might not be true for developing brains.

L-Tyrosine availability affects basal and stimulated catecholamine indices in prefrontal cortex and striatum of the rat

We previously found that L-tyrosine (L-TYR) but not D-TYR administered by reverse dialysis elevated catecholamine synthesis in vivo in medial prefrontal cortex (MPFC) and striatum of the rat (Brodnik et al., 2012). We now report L-TYR effects on extracellular levels of catecholamines and their metabolites. In MPFC, reverse dialysis of L-TYR elevated in vivo levels of dihydroxyphenylacetic acid (DOPAC) (L-TYR 250-1000 μM), homovanillic acid (HVA) (L-TYR 1000 μM) and 3-methoxy-4-hydroxyphenylglycol (MHPG) (L-TYR 500-1000 μM). In striatum L-TYR 250 μM elevated DOPAC. We also examined L-TYR effects on extracellular dopamine (DA) and norepinephrine (NE) levels during two 30 min pulses (P2 and P1) of K+ (37.5 mM) separated by t = 2.0 h. L-TYR significantly elevated the ratio P2/P1 for DA (L-TYR 125 μM) and NE (L-TYR 125-250 μM) in MPFC but lowered P2/P1 for DA (L-TYR 250 μM) in striatum. Finally, we measured DA levels in brain slices using ex-vivo voltammetry. Perfusion with L-TYR (12.5-50 μM) dose-dependently elevated stimulated DA levels in striatum. In all the above studies, D-TYR had no effect. We conclude that acute increases within the physiological range of L-TYR levels can increase catecholamine metabolism and efflux in MPFC and striatum. Chronically, such repeated increases in L-TYR availability could induce adaptive changes in catecholamine transmission while amplifying the metabolic cost of catecholamine synthesis and degradation. This has implications for neuropsychiatric conditions in which neurotoxicity and/or disordered L-TYR transport have been implicated.

Electrophysiological Actions of Synthetic Cathinones on Monoamine Transporters

Products containing psychoactive synthetic cathinones, such as mephedrone and 3,4-methylenedioxypyrovalerone (MDPV) are prevalent in our society. Synthetic cathinones are structurally similar to methamphetamine, and numerous synthetics have biological activity at dopamine, serotonin, and norepinephrine transporters. Importantly, monoamine transporters co-transport sodium ions along with their substrate, and movement of substrates and ions through the transporter can generate measurable ionic currents. Here we review how electrophysiological information has enabled us to determine how synthetic cathinones affect transporter-mediated currents in cells that express these transporters. Specifically, drugs that act as transporter substrates induce inward depolarizing currents when cells are held near their resting membrane potential, whereas drugs that act as transporter blockers induce apparent outward currents by blocking an inherent inward leak current. We have employed the two-electrode voltage-clamp technique in Xenopus laevis oocytes overexpressing monoamine transporters to determine whether synthetic cathinones found in the so-called bath salts products behave as blockers or substrates. We also examined the structure-activity relationships for synthetic cathinone analogs related to the widely abused compound MDPV, a common constituent in "bath salts" possessing potent actions at the dopamine transporter.

A novel mode of operation of SLC22A11: Membrane insertion of estrone sulfate versus translocation of uric acid and glutamate

Estrone sulfate alias estrone-3-sulfate (E3S) is considerably larger and much more hydrophobic than typical substrates of SLC22 transporters. It is puzzling that many otherwise unrelated transporters have been reported to transport E3S. Here we scrutinized the mechanism of transport of E3S by SLC22A11 (alias OAT4), by direct comparison with uric acid (UA), an important physiological substrate. Heterologous expression of SLC22A11 in human 293 cells gave rise to a huge unidirectional efflux of glutamate (Glu) and aspartate, as determined by LC-MS/MS. The uptake of E3S was 20-fold faster than the uptake of UA. Yet, the outward transport of Glu was inhibited by extracellular E3S, but not by UA. The release of E3S after preloading was trans-stimulated by extracellular dehydroepiandrosterone sulfate (DHEAS), but neither by UA nor 6-carboxyfluorescein (6CF). The equilibrium accumulation of E3S was enhanced 3-fold by replacement of chloride with gluconate, but the opposite effect was observed for UA. These results establish that SLC22A11 provides entirely different transport mechanisms for E3S and UA. Therefore, E3S must not be used as a substitute for UA to assay the function of SLC22A11. In equilibrium accumulation experiments, the transporter-mediated uptake was a linear function of the concentration of UA and 6CF. By contrast, in the same concentration range the graph for E3S was hyperbolic. This suggests that SLC22A11 inserts E3S into a small volume with limited capacity, the plasma membrane. Our data support the notion that the reverse process, extraction from the membrane, is also catalyzed by the carrier.

What Mechanisms Are Responsible for the Reuptake of Levodopa-Derived Dopamine in Parkinsonian Striatum?

Levodopa is the most effective medication for motor symptoms in Parkinson's disease. However, various motor and non-motor complications are associated with levodopa treatment, resulting from altered levodopa-dopamine metabolism with disease progression and long-term use of the drug. The present review emphasizes the role of monoamine transporters other than the dopamine transporter in uptake of extracellular dopamine in the dopamine-denervated striatum. When dopaminergic neurons are lost and dopamine transporters decreased, serotonin and norepinephrine transporters compensate by increasing uptake of excessive extracellular dopamine in the striatum. Organic cation transporter-3 and plasma membrane monoamine transporter, low affinity, and high capacity transporters, also potentially uptake dopamine when high-affinity transporters do not work normally. Selective serotonin reuptake inhibitors and serotonin norepinephrine reuptake inhibitors are often administered to patients with Parkinson's disease presenting with depression, pain or other non-motor symptoms. Thus, it is important to address the potential of these drugs to modify dopamine metabolism and uptake through blockade of the compensatory function of these transporters, which could lead to changes in motor symptoms of Parkinson's disease.