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

SERT and OCT mediate 5-HT1B receptor regulation of immobility behavior and uptake of 5-HT and HIS

5-HT clearance, commonly mediated by transporters in the uptake-1 and uptake-2 families, has been linked to 5-HT1B receptor's action on behaviors. Since no specific transporters identified yet, effects of serotonin transporter (SERT) and organic cation transporter (OCTs) on 5-HT1B-elicited immobility phenotype, and 5-HT and HIS uptake were then investigated. Intraperitoneal injections of SERT inhibitor fluoxetine (FLX) and/or OCTs inhibitor decynium (D22) were used prior to local perfusion of 5-HT1B agonist CP93129 into the ventral hippocampus to measure immobility times in the FST and TST, to measure 5-HT uptake efficiencies and HIS uptake efficiencies derived from linear regressions using the transient no-net-flux quantitative microdialysis in C57BL/6 mice. Exogenous 5-HT and HIS uptake were measured following incubation of FLX and/or D22 with CP93129 in the RBL-2H3 cells. Moreover, surface membrane levels of SERT and OCT were detected in response to CP93129. Local CP93129 prolonged immobility times, which were attenuated following pretreatment of either inhibitor. Local CP93129 lowered the slopes obtained from the lineal regressions for 5-HT and HIS (slope is reciprocal to uptake efficiency), which were then weakened following pretreatment of either inhibitor. Similar findings were obtained following CP93129 incubation, and co-incubation of CP93129 with either inhibitor in the RBL-2H3. Moreover, CP93129 dose-dependently moved SERT and OCT3 in the cytosol to the surface membrane. Both SERT and OCT are the target effectors mediating 5-HT1B regulation of immobility time and 5-HT uptake, OCT mediates 5-HT1B regulation of HIS uptake. Their underlying signal transductions need to be further explored.

Kinetic Properties and Pharmacological Modulation of High- and Low-Affinity Dopamine Transport in Striatal Astrocytes of Adult Rats

Astrocytes actively participate in neurotransmitter homeostasis by bidirectional communication with neuronal cells, a concept named the tripartite synapse, yet their role in dopamine (DA) homeostasis remains understudied. In the present study, we investigated the kinetic and molecular mechanisms of DA transport in cultured striatal astrocytes of adult rats. Kinetic uptake experiments were performed using radiolabeled [3H]-DA, whereas mRNA expression of the dopamine, norepinephrine, organic cation and plasma membrane monoamine transporters (DAT, NET, OCTs and PMAT) and DA receptors D1 and D2 was determined by qPCR. Additionally, astrocyte cultures were subjected to a 24 h treatment with the DA receptor agonist apomorphine, the DA receptor antagonist haloperidol and the DA precursor L-DOPA. [3H]-DA uptake exhibited temperature, concentration and sodium dependence, with potent inhibition by desipramine, nortriptyline and decynium-22, suggesting the involvement of multiple transporters. qPCR revealed prominent mRNA expression of the NET, the PMAT and OCT1, alongside lower levels of mRNA for OCT2, OCT3 and the DAT. Notably, apomorphine significantly altered NET, PMAT and D1 mRNA expression, while haloperidol and L-DOPA had a modest impact. Our findings demonstrate that striatal astrocytes aid in DA clearance by multiple transporters, which are influenced by dopaminergic drugs. Our study enhances the understanding of regional DA uptake, paving the way for targeted therapeutic interventions in dopaminergic disorders.

Transporters involved in adult rat cortical astrocyte dopamine uptake: Kinetics, expression and pharmacological modulation

Astrocytes, glial cells in the central nervous system, perform a multitude of homeostatic functions and are in constant bidirectional communication with neuronal cells, a concept named the tripartite synapse; however, their role in the dopamine homeostasis remains unexplored. The aim of this study was to clarify the pharmacological and molecular characteristics of dopamine transport in cultured cortical astrocytes of adult rats. In addition, we were interested in the expression of mRNA of dopamine transporters as well as dopamine receptors D1 and D2 and in the effect of dopaminergic drugs on the expression of these transporters and receptors. We have found that astrocytes possess both Na+-dependent and Na+-independent transporters. Uptake of radiolabelled dopamine was time-, temperature- and concentration-dependent and was inhibited by decynium-22, a plasma membrane monoamine transporter inhibitor, tricyclic antidepressants desipramine and nortriptyline, both inhibitors of the norepinephrine transporter. Results of transporter mRNA expression indicate that the main transporters involved in cortical astrocyte dopamine uptake are the norepinephrine transporter and plasma membrane monoamine transporter. Both dopamine receptor subtypes were identified in cortical astrocyte cultures. Twenty-four-hour treatment of astrocyte cultures with apomorphine, a D1/D2 agonist, induced upregulation of D1 receptor, norepinephrine transporter and plasma membrane monoamine transporter, whereas the latter was downregulated by haloperidol and L-DOPA. Astrocytes take up dopamine by multiple transporters and express dopamine receptors, which are sensitive to dopaminergic drugs. The findings of this study could open a promising area of research for the fine-tuning of existing therapeutic strategies.

Cortical and Striatal Astrocytes of Neonatal Rats Display Distinct Molecular and Pharmacological Characteristics of Dopamine Uptake

Astrocytes are crucial in the regulation of neurotransmitter homeostasis, and while their involvement in the dopamine (DA) tripartite synapse is acknowledged, it necessitates a more comprehensive investigation. In the present study, experiments were conducted on primary astrocyte cultures from the striatum and cortex of neonatal rats. The pharmacological intricacies of DA uptake, including dependence on time, temperature, and concentration, were investigated using radiolabelled [3H]-DA. The mRNA expression of transporters DAT, NET, PMAT, and OCTs was evaluated by qPCR. Notably, astrocytes from both brain regions exhibited prominent mRNA expression of NET and PMAT, with comparatively lower expression of DAT and OCTs. The inhibition of DA uptake by the DAT inhibitor, GBR12909, and NET inhibitors, desipramine and nortriptyline, impeded DA uptake in striatal astrocytes more than in cortical astrocytes. The mRNA expression of NET and PMAT was significantly upregulated in cortical astrocytes in response to the DA receptor agonist apomorphine, while only the mRNA expression of NET exhibited changes in striatal astrocytes. Haloperidol, a DA receptor antagonist, and L-DOPA, a DA precursor, did not induce significant alterations in transporter mRNA expression. These findings underscore the intricate and region-specific mechanisms governing DA uptake in astrocytes, emphasizing the need for continued exploration to unravel the nuanced dynamics of astrocytic involvement in the DA tripartite synapse.

Organic cation transporters in psychiatric and substance use disorders

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.

Summarizing studies using constitutive genetic deficiency to investigate behavioural influences of uptake 2 monoamine transporters

Burgeoning literature demonstrates that monoamine transporters with high transport capacity but lower substrate affinity (i.e., uptake 2) contribute meaningfully to regulation of monoamine neurotransmitter signalling. However, studying behavioural influences of uptake 2 is hindered by an absence of selective inhibitors largely free of off-target, confounding effects. This contrasts with study of monoamine transporters with low transport capacity but high substrate affinity (i.e., uptake 1), for which there are many reasonably selective inhibitors. To circumvent this dearth of pharmacological tools for studying uptake 2, researchers have instead employed mice with constitutive genetic deficiency in three separate transporters. By studying baseline behavioural shifts, plus behavioural responses to environmental and pharmacological manipulations-the latter primarily targeting uptake 1-investigators have been creatively characterizing the behavioural, and often sex-specific, influences of uptake 2. This non-systematic mini review summarizes current uptake 2 behaviour literature, highlighting emphases on stress responsivity in organic cation transporter 2 (OCT2) work, psychostimulant responsivity in OCT3 and plasma membrane monoamine transporter (PMAT) investigations, and antidepressant responsivity in all three. Collectively, this small but growing body of work reiterates the necessity for development of selective uptake 2-inhibiting drugs, with reviewed studies suggesting that these might advance personalized treatment approaches.

The substrate and inhibitor binding mechanism of polyspecific transporter OAT1 revealed by high-resolution cryo-EM

Organic anion transporters (OATs) of the SLC22 family have crucial roles in the transport of organic anions, including metabolites and therapeutic drugs, and in transporter-mediated drug-drug interactions. In the kidneys, OATs facilitate the elimination of metabolic waste products and xenobiotics. However, their transport activities can lead to the accumulation of certain toxic compounds within cells, causing kidney damage. Moreover, OATs are important drug targets, because their inhibition modulates the elimination or retention of substrates linked to diseases. Despite extensive research on OATs, the molecular basis of their substrate and inhibitor binding remains poorly understood. Here we report the cryo-EM structures of rat OAT1 (also known as SLC22A6) and its complexes with para-aminohippuric acid and probenecid at 2.1, 2.8 and 2.9 Å resolution, respectively. Our findings reveal a highly conserved substrate binding mechanism for SLC22 transporters, wherein four aromatic residues form a cage to accommodate the polyspecific binding of diverse compounds.

Impairment of exocytotic transmitter release by decynium-22 through an inhibition of ion channels

Introduction: In addition to members of the family of Na+/Cl- dependent monoamine transporters, organic cation transporters (OCTs), in particular OCT3, as well as the plasma membrane monoamine transporter (PMAT) may contribute to neuronal reuptake of according neurotransmitters. As opposed to the numerous blockers of monoamine transporters, only a very limited number of specific blockers of OCT3 and PMAT are available. In fact, decynium-22 is the only blocking agent with micromolar affinities for both transport proteins, and this molecule is frequently used to establish roles of OCT3 and/or PMAT as targets for antidepressant drugs and psychostimulants, respectively. Methods/Results: To test for a function of these transporters in the sympathetic nervous system, uptake and release of [3H]1-methyl-4-phenylpyridinium (MPP+) was investigated in primary cultures of rat superior cervical ganglia. Uptake was reduced by cocaine or desipramine, blockers of the noradrenaline transporter, by about 70% and by corticosterone or β-estradiol, blockers of OCT3, by about 30%; decynium-22 achieved complete inhibition of uptake with half maximal effects at 3 μM. Depolarization dependent release was enhanced by corticosterone or β-estradiol, but reduced by decynium-22. As the latter effect is unlikely to be related to actions at OCT3 and/or PMAT, electrophysiological recordings were performed to reveal that decynium-22 inhibits action potential firing and currents through voltage activated calcium channels in superior cervical ganglion neurons. Discussion: These results demonstrate that decynium-22 can impair exocytotic neurotransmitter release by interfering with several types of ion channels. Such transporter-independent effects of decynium-22 that my interfere with basic neuronal functions need to be considered when interpreting results obtained with decynium-22 as prototypic inhibitor of transmitter reuptake via OCT3 and/or PMAT.

Ethanol inhibits dopamine uptake via organic cation transporter 3: Implications for ethanol and cocaine co-abuse

Concurrent cocaine and alcohol use is among the most frequent drug combination, and among the most dangerous in terms of deleterious outcomes. Cocaine increases extracellular monoamines by blocking dopamine (DA), norepinephrine (NE) and serotonin (5-HT) transporters (DAT, NET and SERT, respectively). Likewise, ethanol also increases extracellular monoamines, however evidence suggests that ethanol does so independently of DAT, NET and SERT. Organic cation transporter 3 (OCT3) is an emergent key player in the regulation of monoamine signaling. Using a battery of in vitro, in vivo electrochemical, and behavioral approaches, as well as wild-type and constitutive OCT3 knockout mice, we show that ethanol's actions to inhibit monoamine uptake are dependent on OCT3. These findings provide a novel mechanistic basis whereby ethanol enhances the neurochemical and behavioral effects of cocaine and encourage further research into OCT3 as a target for therapeutic intervention in the treatment of ethanol and ethanol/cocaine use disorders.

Faster Serotonin Clearance in CA3 Region of Hippocampus and Antidepressant-like Effect of Decynium-22 in Juvenile Mice Are Putatively Linked to Increased Plasma Membrane Monoamine Transporter Function: Implications for Efficacy of Antidepressants in Juveniles

Selective serotonin reuptake inhibitors (SSRIs) are less efficacious in treating depression in children than in adults. SSRIs block serotonin uptake via the high-affinity, low-capacity serotonin transporter. However, the low-affinity, high-capacity organic cation transporter 3 (OCT3) and plasma membrane monoamine transporter (PMAT) are emerging as important players in serotonin uptake. We hypothesized that OCT3 and/or PMAT are functionally upregulated in juveniles, thereby buffering SSRIs' ability to enhance serotonergic neurotransmission. Unlike in adult mice, we found the OCT/PMAT blocker, decynium-22, to have standalone antidepressant-like effects in juveniles. Using in vivo high-speed chronoamperometry, we found that juveniles clear serotonin from the CA3 region of the hippocampus ~2-fold faster than adult mice. Cell density did not differ between ages, suggesting that faster serotonin clearance in juveniles is unrelated to faster diffusion through the extracellular matrix. Western blot and immunohistochemistry showed that juvenile mice have modestly greater expression of PMAT than adults, whereas OCT3 expression in the CA3 region of the hippocampus was similar between ages. Together, these data suggest that faster serotonin clearance and antidepressant-like effects of decynium-22 in juvenile mice may be due to functionally upregulated PMAT. Faster serotonin clearance via PMAT in juveniles may contribute to reduced therapeutic efficacy of SSRIs in children relative to adults.

Monoamine oxidase A and organic cation transporter 3 coordinate intracellular β1AR signaling to calibrate cardiac contractile function

We have recently identified a pool of intracellular β1 adrenergic receptors (β1ARs) at the sarcoplasmic reticulum (SR) crucial for cardiac function. Here, we aim to characterize the integrative control of intracellular catecholamine for subcellular β1AR signaling and cardiac function. Using anchored Förster resonance energy transfer (FRET) biosensors and transgenic mice, we determined the regulation of compartmentalized β1AR-PKA signaling at the SR and plasma membrane (PM) microdomains by organic cation transporter 3 (OCT3) and monoamine oxidase A (MAO-A), two critical modulators of catecholamine uptake and homeostasis. Additionally, we examined local PKA substrate phosphorylation and excitation-contraction coupling in cardiomyocyte. Cardiac-specific deletion of MAO-A (MAO-A-CKO) elevates catecholamines and cAMP levels in the myocardium, baseline cardiac function, and adrenergic responses. Both MAO-A deletion and inhibitor (MAOi) selectively enhance the local β1AR-PKA activity at the SR but not PM, and augment phosphorylation of phospholamban, Ca2+ cycling, and myocyte contractile response. Overexpression of MAO-A suppresses the SR-β1AR-PKA activity and PKA phosphorylation. However, deletion or inhibition of OCT3 by corticosterone prevents the effects induced by MAOi and MAO-A deletion in cardiomyocytes. Deletion or inhibition of OCT3 also negates the effects of MAOi and MAO-A deficiency in cardiac function and adrenergic responses in vivo. Our data show that MAO-A and OCT3 act in concert to fine-tune the intracellular SR-β1AR-PKA signaling and cardiac fight-or-flight response. We reveal a drug contraindication between anti-inflammatory corticosterone and anti-depressant MAOi in modulating adrenergic regulation in the heart, providing novel perspectives of these drugs with cardiac implications.

High Affinity Decynium-22 Binding to Brain Membrane Homogenates and Reduced Dorsal Camouflaging after Acute Exposure to it in Zebrafish

Organic cation transporters (OCTs) are expressed in the mammalian brain, kidney, liver, placenta, and intestines, where they facilitate the transport of cations and other substrates between extracellular fluids and cells. Despite increasing reliance on ectothermic vertebrates as alternative toxicology models, properties of their OCT homologs transporting many drugs and toxins remain poorly characterized. Recently, in zebrafish (Danio rerio), two proteins with functional similarities to human OCTs were shown to be highly expressed in the liver, kidney, eye, and brain. This study is the first to characterize in vivo uptake to the brain and the high-affinity brain membrane binding of the mammalian OCT blocker 1-1'-diethyl-2,2'cyanine iodide (decynium-22 or D-22) in zebrafish. Membrane saturation binding of [³H] D-22 in pooled zebrafish whole brain versus mouse hippocampal homogenates revealed a high-affinity binding site with a K_D of 5 ± 2.5 nM and Bmax of 1974 ± 410 fmol/mg protein in the zebrafish brain, and a K_D of 3.3 ± 2.3 and Bmax of 704 ± 182 fmol/mg protein in mouse hippocampus. The binding of [³H] D-22 to brain membrane homogenates was partially blocked by the neurotoxic cation 1-methyl-4-phenylpyridinium (MPP+), a known OCT substrate. To determine if D-22 bath exposures reach the brain, zebrafish were exposed to 25 nM [³H] D-22 for 10 min, and 736 ± 68 ng/g wet weight [³H] D-22 was bound. Acute behavioral effects of D-22 in zebrafish were characterized in two anxiety-relevant tests. In the first cohort of zebrafish, 12.5, 25, or 50 mg/L D-22 had no effect on their height in the dive tank or entries and time spent in white arms of a light/dark plus maze. By contrast, 25 mg/L buspirone increased zebrafish dive tank top-dwelling (p < 0.05), an anticipated anxiolytic effect. However, a second cohort of zebrafish treated with 50 mg/L D-22 made more white arm entries, and females spent more time in white than controls. Based on these findings, it appears that D-22 bath treatments reach the zebrafish brain and have partial anxiolytic properties, reducing anti-predator dorsal camouflaging, without increasing vertical exploration. High-affinity binding of [³H] D-22 in zebrafish brain and mouse brain was similar, with nanomolar affinity, possibly at conserved OCT site(s).

MPP+-Induced Changes in Cellular Impedance as a Measure for Organic Cation Transporter (SLC22A1-3) Activity and Inhibition

The organic cation transporters OCT1-3 (SLC22A1-3) facilitate the transport of cationic endo- and xenobiotics and are important mediators of drug distribution and elimination. Their polyspecific nature makes OCTs highly susceptible to drug-drug interactions (DDIs). Currently, screening of OCT inhibitors depends on uptake assays that require labeled substrates to detect transport activity. However, these uptake assays have several limitations. Hence, there is a need to develop novel assays to study OCT activity in a physiological relevant environment without the need to label the substrate. Here, a label-free impedance-based transport assay is established that detects OCT-mediated transport activity and inhibition utilizing the neurotoxin MPP⁺. Uptake of MPP⁺ by OCTs induced concentration-dependent changes in cellular impedance that were inhibited by decynium-22, corticosterone, and Tyrosine Kinase inhibitors. OCT-mediated MPP⁺ transport activity and inhibition were quantified on both OCT1-3 overexpressing cells and HeLa cells endogenously expressing OCT3. Moreover, the method presented here is a valuable tool to identify novel inhibitors and potential DDI partners for MPP⁺ transporting solute carrier proteins (SLCs) in general.

Role of Organic Cation Transporter 3 and Plasma Membrane Monoamine Transporter in the Rewarding Properties and Locomotor Sensitizing Effects of Amphetamine in Male andFemale Mice

A lack of effective treatment and sex-based disparities in psychostimulant addiction and overdose warrant further investigation into mechanisms underlying the abuse-related effects of amphetamine-like stimulants. Uptake-2 transporters such as organic cation transporter 3 (OCT3) and plasma membrane monoamine transporter (PMAT), lesser studied potential targets for the actions of stimulant drugs, are known to play a role in monoaminergic neurotransmission. Our goal was to examine the roles of OCT3 and PMAT in mediating amphetamine (1 mg/kg)-induced conditioned place preference (CPP) and sensitization to its locomotor stimulant effects, in males and females, using pharmacological, decynium-22 (D22; 0.1 mg/kg, a blocker of OCT3 and PMAT) and genetic (constitutive OCT3 and PMAT knockout (−/−) mice) approaches. Our results show that OCT3 is necessary for the development of CPP to amphetamine in males, whereas in females, PMAT is necessary for the ability of D22 to prevent the development of CPP to amphetamine. Both OCT3 and PMAT appear to be important for development of sensitization to the locomotor stimulant effect of amphetamine in females, and PMAT in males. Taken together, these findings support an important, sex-dependent role of OCT3 and PMAT in the rewarding and locomotor stimulant effects of amphetamine.

Interaction Profiles of Central Nervous System Active Drugs at Human Organic Cation Transporters 1-3 and Human Plasma Membrane Monoamine Transporter

Many psychoactive compounds have been shown to primarily interact with high-affinity and low-capacity solute carrier 6 (SLC6) monoamine transporters for norepinephrine (NET; norepinephrine transporter), dopamine (DAT; dopamine transporter) and serotonin (SERT; serotonin transporter). Previous studies indicate an overlap between the inhibitory capacities of substances at SLC6 and SLC22 human organic cation transporters (SLC22A1-3; hOCT1-3) and the human plasma membrane monoamine transporter (SLC29A4; hPMAT), which can be classified as high-capacity, low-affinity monoamine transporters. However, interactions between central nervous system active substances, the OCTs, and the functionally-related PMAT have largely been understudied. Herein, we report data from 17 psychoactive substances interacting with the SLC6 monoamine transporters, concerning their potential to interact with the human OCT isoforms and hPMAT by utilizing radiotracer-based in vitro uptake inhibition assays at stably expressing human embryonic kidney 293 cells (HEK293) cells. Many compounds inhibit substrate uptake by hOCT1 and hOCT2 in the low micromolar range, whereas only a few substances interact with hOCT3 and hPMAT. Interestingly, methylphenidate and ketamine selectively interact with hOCT1 or hOCT2, respectively. Additionally, 3,4-methylenedioxymethamphetamine (MDMA) is a potent inhibitor of hOCT1 and 2 and hPMAT. Enantiospecific differences of R- and S-α-pyrrolidinovalerophenone (R- and S-α-PVP) and R- and S-citalopram and the effects of aromatic substituents are explored. Our results highlight the significance of investigating drug interactions with hOCTs and hPMAT, due to their role in regulating monoamine concentrations and xenobiotic clearance.

Effect of Selected Antidepressants on Placental Homeostasis of Serotonin: Maternal and Fetal Perspectives

Depression is a prevalent condition affecting up to 20% of pregnant women. Hence, more than 10% are prescribed antidepressant drugs, mainly serotonin reuptake inhibitors (SSRIs) and selective serotonin and noradrenaline reuptake inhibitors (SNRIs). We hypothesize that antidepressants disturb serotonin homeostasis in the fetoplacental unit by inhibiting serotonin transporter (SERT) and organic cation transporter 3 (OCT3) in the maternal- and fetal-facing placental membranes, respectively. Paroxetine, citalopram, fluoxetine, fluvoxamine, sertraline, and venlafaxine were tested in situ (rat term placenta perfusion) and ex vivo (uptake studies in membrane vesicles isolated from healthy human term placenta). All tested antidepressants significantly inhibited SERT- and OCT3-mediated serotonin uptake in a dose-dependent manner. Calculated half-maximal inhibitory concentrations (IC₅₀) were in the range of therapeutic plasma concentrations. Using in vitro and in situ models, we further showed that the placental efflux transporters did not compromise mother-to-fetus transport of antidepressants. Collectively, we suggest that antidepressants have the potential to affect serotonin levels in the placenta or fetus when administered at therapeutic doses. Interestingly, the effect of antidepressants on serotonin homeostasis in rat placenta was sex dependent. As accurate fetal programming requires optimal serotonin levels in the fetoplacental unit throughout gestation, inhibition of SERT-/OCT3-mediated serotonin uptake may help explain the poor outcomes of antidepressant use in pregnancy.

Decynium-22 affects behavior in the zebrafish light/dark test

Decynium-22 (D-22) is an inhibitor of the uptake2 system of monoamine clearance, resulting in increased levels of dopamine and norepinephrine (and in some cases serotonin) in the nervous system and elsewhere. Uptake2 is mediated by low-affinity, high-capacity transporters that are inhibited by glucocorticoids, suggesting a mechanism of fast glucocorticoid-monoamine interaction in the brain and a possible target for antidepressants. D-22 dose-dependently increased anxiety-like behavior in adult zebrafish exposed to the light/dark test, monotonically increasing scototaxis (dark preference), but affecting risk assessment with an inverted-U-shaped response. These results suggest that the uptake2 system has a role in defensive behavior in zebrafish, presenting a novel mechanism by which stress and glucocorticoids could produce fast neurobehavioral adjustments in vertebrates.

Organic Cation Transporters in Psychiatric Disorders

Selective serotonin reuptake inhibitors (SSRIs) are the most commonly prescribed medications for psychiatric disorders, yet they leave the majority of patients without full symptom relief. Therefore, a major research challenge is to identify novel targets for the improved treatment of these disorders. SSRIs act by blocking the serotonin transporter (SERT), the high-affinity, low-capacity, uptake-1 transporter for serotonin. Other classes of antidepressant work by blocking the norepinephrine or dopamine transporters (NET and DAT), the high-affinity, low-capacity uptake-1 transporters for norepinephrine and dopamine, or by blocking combinations of SERT, NET, and DAT. It has been proposed that uptake-2 transporters, which include organic cation transporters (OCTs) and the plasma membrane monoamine transporter (PMAT), undermine the therapeutic utility of uptake-1 acting antidepressants. Uptake-2 transporters for monoamines have low affinity for these neurotransmitters, but a high capacity to transport them. Thus, activity of these transporters may limit the increase of extracellular monoamines thought to be essential for ultimate therapeutic benefit. Here preclinical evidence supporting a role for OCT2, OCT3, and PMAT in behaviors relevant to psychiatric disorders is presented. Importantly, preclinical evidence revealing these transporters as targets for the development of novel therapeutics for psychiatric disorders is discussed.

Physiology and Pharmacology of Neurotransmitter Transporters

Regulation of the ability of a neurotransmitter [our focus: serotonin, norepinephrine, dopamine, acetylcholine, glycine, and gamma-aminobutyric acid (GABA)] to reach its receptor targets is regulated in part by controlling the access the neurotransmitter has to receptors. Transporters, located at both the cellular plasma membrane and in subcellular vesicles, carry a myriad of responsibilities that include enabling neurotransmitter release and controlling uptake of neurotransmitter back into a cell or vesicle. Driven largely by electrochemical gradients, these transporters move neurotransmitters. The regulation of the transporters themselves through changes in expression and/or posttranslational modification allows for fine-tuning of this system. Transporters have been best recognized as targets for psychoactive stimulants and remain a mainstay target of primarily central nervous system (CNS) acting drugs for treatment of debilitating diseases such as depression and anxiety. Studies reveal, however, that transporters are found and functional in tissues outside the CNS (gastrointestinal and cardiovascular tissues, for example). The importance of neurotransmitter transporters is underscored with discoveries that dysfunction of transporters can cause life-changing disease. This article provides a high-level review of major classes of both plasma membrane transporters and vesicular transporters. © 2021 American Physiological Society. Compr Physiol 11:2279-2295, 2021.

Inflammation-Induced Histamine Impairs the Capacity of Escitalopram to Increase Hippocampal Extracellular Serotonin

Commonly prescribed selective serotonin reuptake inhibitors (SSRIs) inhibit the serotonin transporter to correct a presumed deficit in extracellular serotonin signaling during depression. These agents bring clinical relief to many who take them; however, a significant and growing number of individuals are resistant to SSRIs. There is emerging evidence that inflammation plays a significant role in the clinical variability of SSRIs, though how SSRIs and inflammation intersect with synaptic serotonin modulation remains unknown. In this work, we use fast in vivo serotonin measurement tools to investigate the nexus between serotonin, inflammation, and SSRIs. Upon acute systemic lipopolysaccharide (LPS) administration in male and female mice, we find robust decreases in extracellular serotonin in the mouse hippocampus. We show that these decreased serotonin levels are supported by increased histamine activity (because of inflammation), acting on inhibitory histamine H3 heteroreceptors on serotonin terminals. Importantly, under LPS-induced histamine increase, the ability of escitalopram to augment extracellular serotonin is impaired because of an off-target action of escitalopram to inhibit histamine reuptake. Finally, we show that a functional decrease in histamine synthesis boosts the ability of escitalopram to increase extracellular serotonin levels following LPS. This work reveals a profound effect of inflammation on brain chemistry, specifically the rapidity of inflammation-induced decreased extracellular serotonin, and points the spotlight at a potentially critical player in the pathology of depression, histamine. The serotonin/histamine homeostasis thus, may be a crucial new avenue in improving serotonin-based treatments for depression.SIGNIFICANCE STATEMENT Acute LPS-induced inflammation (1) increases CNS histamine, (2) decreases CNS serotonin (via inhibitory histamine receptors), and (3) prevents a selective serotonin reuptake inhibitor (SSRI) from effectively increasing extracellular serotonin. A targeted depletion of histamine recovers SSRI-induced increases in extracellular hippocampal serotonin.

The Interaction of Organic Cation Transporters 1-3 and PMAT with Psychoactive Substances

Organic cation transporters 1-3 (OCT1-3, SLC22A1-3) and the plasma membrane monoamine transporter (PMAT, SLC29A4) play a major role in maintaining monoaminergic equilibrium in the central nervous system. With many psychoactive substances interacting with OCT1-3 and PMAT, a growing literature focuses on characterizing their properties via in vitro and in vivo studies. In vitro studies mainly aim at characterizing compounds as inhibitors or substrates of murine, rat, and human isoforms. The preponderance of studies has put emphasis on phenylalkylamine derivatives, but ketamine and opioids have also been investigated. Studies employing in vivo (knockout) models mostly concentrate on the interaction of psychoactive substances and OCT3, with an emphasis on stress and addiction, pharmacokinetics, and sensitization to psychoactive drugs. The results highlight the importance of OCT3 in the mechanism of action of psychoactive compounds. Concerning in vivo studies, a veritable research gap concerning OCT1, 2, and PMAT exists. This review provides an overview and summary of research conducted in this field of research.

General Overview of Organic Cation Transporters in Brain

Inhibitors of Na⁺/Cl^- dependent high affinity transporters for norepinephrine (NE), serotonin (5-HT), and/or dopamine (DA) represent frequently used drugs for treatment of psychological disorders such as depression, anxiety, obsessive-compulsive disorder, attention deficit hyperactivity disorder, and addiction. These transporters remove NE, 5-HT, and/or DA after neuronal excitation from the interstitial space close to the synapses. Thereby they terminate transmission and modulate neuronal behavioral circuits. Therapeutic failure and undesired central nervous system side effects of these drugs have been partially assigned to neurotransmitter removal by low affinity transport. Cloning and functional characterization of the polyspecific organic cation transporters OCT1 (SLC22A1), OCT2 (SLC22A2), OCT3 (SLC22A3) and the plasma membrane monoamine transporter PMAT (SLC29A4) revealed that every single transporter mediates low affinity uptake of NE, 5-HT, and DA. Whereas the organic transporters are all located in the blood brain barrier, OCT2, OCT3, and PMAT are expressed in neurons or in neurons and astrocytes within brain areas that are involved in behavioral regulation. Areas of expression include the dorsal raphe, medullary motoric nuclei, hypothalamic nuclei, and/or the nucleus accumbens. Current knowledge of the transport of monoamine neurotransmitters by the organic cation transporters, their interactions with psychotropic drugs, and their locations in the brain is reported in detail. In addition, animal experiments including behavior tests in wildtype and knockout animals are reported in which the impact of OCT2, OCT3, and/or PMAT on regulation of salt intake, depression, mood control, locomotion, and/or stress effect on addiction is suggested.

Substrates and Inhibitors of Organic Cation Transporters (OCTs) and Plasma Membrane Monoamine Transporter (PMAT) and Therapeutic Implications

The gene products of the SLC22A gene family (hOCT1, hOCT2, and hOCT3) and of the SLC29A4 gene (hPMAT or hENT4) are all polyspecific organic cation transporters. Human OCTs (including hPMAT) are expressed in peripheral tissues such as small intestine, liver, and kidney involved in the pharmacokinetics of drugs. In the human brain, all four transporters are expressed at the blood-brain barrier (BBB), hOCT2 is additionally expressed in neurons, and hOCT3 and hPMAT in glia. More than 40% of the presently used drugs are organic cations. This chapter lists and discusses all known drugs acting as substrates or inhibitors of these four organic cation transporters, independently of whether the transporter is expressed in the central nervous system (CNS) or in peripheral tissues. Of interest is their involvement in drug absorption, distribution, and excretion as well as potential OCT-associated drug-drug interactions (DDIs), with a focus on drugs that act in the CNS.

Effect of concurrent organic cation transporter blockade on norepinephrine clearance inhibiting- and antidepressant-like actions of desipramine and venlafaxine

Depression is a major health problem for which most patients are not effectively treated. This underscores a need to identify new targets for the development of antidepressants with improved efficacy. Studies have shown that blockade of low-affinity/high-capacity transporters, such as organic cation transporters (OCTs) and the plasma membrane monoamine transporter (PMAT), with decynium-22 can produce antidepressant-like effects and inhibit serotonin clearance in brain when the serotonin transporter is pharmacologically or genetically compromised. In vitro studies show that OCTs/PMAT are also capable of norepinephrine transport, raising the possibility that decynium-22 might enhance the antidepressant-like effects of norepinephrine transporter inhibitors. Using in vivo electrochemistry, we show that local administration of decynium-22 into dentate gyrus of hippocampus enhanced the ability of the norepinephrine transporter blocker, desipramine, but not the dual norepinephrine/serotonin transporter blocker venlafaxine, to inhibit norepinephrine clearance. In parallel, systemic administration of decynium-22 (0.32 mg/kg) enhanced the antidepressant-like effects of desipramine (32 mg/kg), but not those of venlafaxine, in the tail suspension test, underscoring the heterogeneous response of mice to antidepressants, including those that share similar mechanisms of action. Systemic administration of normetanephrine, a potent blocker of OCT3, failed to potentiate the antidepressant-like effects of desipramine, suggesting that the actions of decynium-22 to augment the antidepressant-like effects of desipramine are likely mediated by another OCT isoform and/or PMAT. Taken together with existing literature, concurrent blockade of OCTs and/or PMAT merits further investigation as an adjunctive therapeutic for desipramine-like antidepressant drugs.

Insight into the Anticancer Activity of Copper(II) 5-Methylenetrimethylammonium-Thiosemicarbazonates and Their Interaction with Organic Cation Transporters

A series of four water-soluble salicylaldehyde thiosemicarbazones with a positively charged trimethylammonium moiety ([H2LR]Cl, R = H, Me, Et, Ph) and four copper(II) complexes [Cu(HLR)Cl]Cl (1-4) were synthesised with the aim to study (i) their antiproliferative activity in cancer cells and, (ii) for the first time for thiosemicarbazones, the interaction with membrane transport proteins, specifically organic cation transporters OCT1-3. The compounds were comprehensively characterised by analytical, spectroscopic and X-ray diffraction methods. The highest cytotoxic effect was observed in the neuroblastoma cell line SH-5YSY after 24 h exposure and follows the rank order: 3 > 2 > 4 > cisplatin > 1 >>[H2LR]Cl. The copper(II) complexes showed marked interaction with OCT1-3, comparable to that of well-known OCT inhibitors (decynium 22, prazosin and corticosterone) in the cell-based radiotracer uptake assays. The work paves the way for the development of more potent and selective anticancer drugs and/or OCT inhibitors.

Serotonin homeostasis in the materno-foetal interface at term: Role of transporters (SERT/SLC6A4 and OCT3/SLC22A3) and monoamine oxidase A (MAO-A) in uptake and degradation of serotonin by human and rat term placenta

AIM

Serotonin is crucial for proper foetal development, and the placenta has been described as a 'donor' of serotonin for the embryo/foetus. However, in later stages of gestation the foetus produces its own serotonin from maternally-derived tryptophan and placental supply is no longer needed. We propose a novel model of serotonin homeostasis in the term placenta with special focus on the protective role of organic cation transporter 3 (OCT3/SLC22A3).

METHODS

Dually perfused rat term placenta was employed to quantify serotonin/tryptophan transport and metabolism. Placental membrane vesicles isolated from human term placenta were used to characterize serotonin transporters on both sides of the syncytiotrophoblast.

RESULTS

We obtained the first evidence that serotonin is massively taken up from the foetal circulation by OCT3. This uptake is concentration-dependent and inhibitable by OCT3 blockers of endogenous (glucocorticoids) or exogenous (pharmaceuticals) origin. Population analyses in rat placenta revealed that foetal sex influences placental extraction of serotonin from foetal circulation. Negligible foetal serotonin levels were detected in maternal-to-foetal serotonin/tryptophan transport and metabolic studies.

CONCLUSION

We demonstrate that OCT3, localized on the foetus-facing membrane of syncytiotrophoblast, is an essential component of foeto-placental homeostasis of serotonin. Together with serotonin degrading enzyme, monoamine oxidase-A, this offers a protective mechanism against local vasoconstriction effects of serotonin in the placenta. However, this system may be compromised by OCT3 inhibitory molecules, such as glucocorticoids or antidepressants. Our findings open new avenues to explore previously unsuspected/unexplained complications during pregnancy including prenatal glucocorticoid excess and pharmacotherapeutic risks of treating pregnant women with OCT3 inhibitors.

Organic Cation Transporter 1 and 3 Contribute to the High Accumulation of Dehydrocorydaline in the Heart

Dehydrocorydaline (DHC), one of the main active components of Corydalis yanhusuo, is an important remedy for the treatment of coronary heart disease. Our previous study revealed a higher unbound concentration of DHC in the heart than plasma of mice after oral administration of C. yanhusuo extract or DHC, but the underlying uptake mechanism remains unelucidated. In our investigations, we studied the transport mechanism of DHC in transgenic cells, primary neonatal rat cardiomyocytes, and animal experiments. Using quantitative real-time polymerase chain reaction and Western blotting, we found that uptake transporters expressed in the mouse heart include organic cation transporter 1/3 (OCT1/3) and carnitine/organic cation transporter 1/2 (OCTN1/2). The accumulation experiments in transfected cells showed that DHC was a substrate of OCT1 and OCT3, with K _m of 11.29 ± 3.3 and 8.96 ± 3.7 μM, respectively, but not a substrate of OCTN1/2. Additionally, a higher efflux level (1.71-fold of MDCK-mock) of DHC was observed in MDCK-MDR1 cells than in MDCK-mock cells. Therefore, DHC is a weak substrate for MDR1. Studies using primary neonatal rat cardiomyocytes showed that OCT1/3 inhibitors (quinidine, decynium-22, and levo-tetrahydropalmatine) prevented the accumulation of DHC, whereas OCTN2 inhibitors (mildronate and l-carnitine) did not affect its accumulation. Moreover, the coadministration of OCT1/3 inhibitors (levo-tetrahydropalmatine, THP) decreased the concentration of DHC in the mouse heart. Based on these findings, DHC may be accumulated partly by OCT1/3 transporters and excreted by MDR1 in the heart. THP could alter the distribution of DHC in the mouse heart. SIGNIFICANCE STATEMENT: We reported the cardiac transport mechanism of dehydrocorydaline, highly distributed to the heart after oral administration of Corydalis yanhusuo extract or dehydrocorydaline only. Dehydrocorydaline (an OCT1/3 and MDR1 substrate) accumulation in primary cardiomyocytes may be related to the transport activity of OCT1/3. This ability, hampered by selective inhibitors (levo-tetrahydropalmatine, an inhibitor of OCT1/3), causes a nearly 40% reduction in exposure of the heart to dehydrocorydaline. These results suggest that OCT1/3 may contribute to the uptake of dehydrocorydaline in the heart.

Antidepressant efficacy of a selective organic cation transporter blocker in a mouse model of depression

Current antidepressants act principally by blocking monoamine reuptake by high-affinity transporters in the brain. However, these antidepressants show important shortcomings such as slow action onset and limited efficacy in nearly a third of patients with major depression disorder. Here, we report the development of a prodrug targeting organic cation transporters (OCT), atypical monoamine transporters recently implicated in the regulation of mood. Using molecular modeling, we designed a selective OCT2 blocker, which was modified to increase brain penetration. This compound, H2-cyanome, was tested in a rodent model of chronic depression induced by 7-week corticosterone exposure. In male mice, prolonged administration of H2-cyanome induced positive effects on several behaviors mimicking symptoms of depression, including anhedonia, anxiety, social withdrawal, and memory impairment. Importantly, in this validated model, H2-cyanome compared favorably with the classical antidepressant fluoxetine, with a faster action on anhedonia and better anxiolytic effects. Integrated Z-scoring across these depression-like variables revealed a lower depression score for mice treated with H2-cyanome than for mice treated with fluoxetine for 3 weeks. Repeated H2-cyanome administration increased ventral tegmental area dopaminergic neuron firing, which may underlie its rapid action on anhedonia. H2-cyanome, like fluoxetine, also modulated several intracellular signaling pathways previously involved in antidepressant response. Our findings provide proof-of-concept of antidepressant efficacy of an OCT blocker, and a mechanistic framework for the development of new classes of antidepressants and therapeutic alternatives for resistant depression and other psychiatric disturbances such as anxiety.

Bidirectional transport of 2-chloroadenosine by equilibrative nucleoside transporter 4 (hENT4): Evidence for allosteric kinetics at acidic pH

Adenosine has been reported to be transported by equilibrative nucleoside transporter 4 (ENT4), encoded by the SLC29A4 gene, in an acidic pH-dependent manner. This makes hENT4 of interest as a therapeutic target in acidic pathologies where adenosine is protective (e.g. vascular ischaemia). We examined the pH-sensitivity of nucleoside influx and efflux by hENT4 using a recombinant transfection model that lacks the confounding influences of other nucleoside transporters (PK15-NTD). We established that [³H]2-chloroadenosine, which is resistant to metabolism by adenosine deaminase, is a substrate for hENT4. Transport of [³H]2-chloroadenosine at a pH of 6.0 in PK15-NTD cells stably transfected with SLC29A4 was biphasic, with a low capacity (V_max ~ 30 pmol/mg/min) high-affinity component (K_m ~ 50 µM) apparent at low substrate concentrations, which shifted to a high capacity (V_max ~ 500 pmol/mg/min) low affinity system (K_m > 600 µM) displaying positive cooperativity at concentrations above 200 µM. Only the low affinity component was observed at a neutral pH of 7.5 (K_m ~ 2 mM). Efflux of [³H]2-chloroadenosine from these cells was also enhanced by more than 4-fold at an acidic pH. Enhanced influx and efflux of nucleosides by hENT4 under acidic conditions supports its potential as a therapeutic target in pathologies such as ischaemia-reperfusion injury.

Targeting Serotonin Transporters in the Treatment of Juvenile and Adolescent Depression

Depression is a serious public health concern. Many patients are not effectively treated, but in children and adolescents this problem is compounded by limited pharmaceutical options. Currently, the Food and Drug Administration approves only two antidepressants for use in these young populations. Both are selective serotonin reuptake inhibitors (SSRIs). Compounding matters further, they are therapeutically less efficacious in children and adolescents than in adults. Here, we review clinical and preclinical literature describing the antidepressant efficacy of SSRIs in juveniles and adolescents. Since the high-affinity serotonin transporter (SERT) is the primary target of SSRIs, we then synthesize these reports with studies of SERT expression/function during juvenile and adolescent periods. Preclinical literature reveals some striking parallels with clinical studies, primary among them is that, like humans, juvenile and adolescent rodents show reduced antidepressant-like responses to SSRIs. These findings underscore the utility of preclinical assays designed to screen drugs for antidepressant efficacy across ages. There is general agreement that SERT expression/function is lower in juveniles and adolescents than in adults. It is well established that chronic SSRI treatment decreases SERT expression/function in adults, but strikingly, SERT expression/function in adolescents is increased following chronic treatment with SSRIs. Finally, we discuss a putative role for organic cation transporters and/or plasma membrane monoamine transporter in serotonergic homeostasis in juveniles and adolescents. Taken together, fundamental differences in SERT, and putatively in other transporters capable of serotonin clearance, may provide a mechanistic basis for the relative inefficiency of SSRIs to treat pediatric depression, relative to adults.