Vol. 40, No. 8 Biol. Pharm. Bull. 40, 1153–1160 (2017) 1153 Current Topics Membrane Transporters as Targets for the Development of Drugs and Therapeutic Strategies

Review Functional Expression of Organic Ion Transporters in Astrocytes and Their Potential as a Drug Target in the Treatment of Central Nervous System Diseases Tomomi Furihata*,a,b and Naohiko Anzaia a Department of Pharmacology, Graduate School of Medicine, Chiba University; 1–8–1 Inohana, Chuou-ku, Chiba 260–8670, Japan: and b Laboratory of Pharmacology and Toxicology, Graduate School of Pharmaceutical Sciences, Chiba University; 1–8–1 Inohana, Chuou-ku, Chiba 260–8675, Japan. Received January 23, 2017

It has become widely acknowledged that astrocytes play essential roles in maintaining physiologi- cal central nervous system (CNS) activities. Astrocytes fulfill their roles partly through the manipulation of their plasma membrane transporter functions, and therefore these transporters have been regarded as promising drug targets for various CNS diseases. A representative example is excitatory amino acid trans- porter 2 (EAAT2), which works as a critical regulator of excitatory signal transduction through its glutamate uptake activity at the tripartite synapse. Thus, enhancement of EAAT2 functionality is expected to acceler- ate glutamate clearance at synapses, which is a promising approach for the prevention of over-excitation of glutamate receptors. In addition to such well-known astrocyte-specific transporters, cumulative evidence suggests that multi-specific organic ion transporters (i.e., organic cation/anion transporters [OCTs/OATs], carnitine/organic cation transporters [OCTNs], and organic anion transporting polypeptides [OATPs]) are also functionally expressed in astrocytes. Even though identification and characterization of their physiologi- cal/pathophysiological roles in astrocytes are in the initial stage, the findings obtained so far indicate that OCT3 and plasma membrane monoamine transporter are significantly involved in the clearance of biogenic amine neurotransmitters in the synaptic cleft, and that OCTN2 and OATP1C1 provide a cellular entry gate for carnitine/acetyl-L-carnitine and thyroxine, respectively. Therefore, organic ion transporters, including those mentioned above, are expected to become emerging pharmacological targets for various CNS diseases. With such expectations in mind, this review will briefly summarize the functional expression of organic ion transporters in astrocytes. Key words astrocyte; transporter; drug development; central nervous system

1. INTRODUCTION enriched transporter glycine transporter-1 (GlyT-1) is reported to be primarily involved in glycine clearance at the synapse. Astrocytes, which are known to be the most abundant cell Since glycine is a known co-agonist of N-methyl-D-aspartate type in the human brain, work as key players in various cen- (NMDA) receptor, GlyT-1 function is indispensable for fine- tral nervous system (CNS) activities. Details of astrocyte CNS tuning NMDA activities. activity have been described in several excellent reviews.1–3) Consistent with their physiological importance in maintain- For example, astrocytes produce lactate from glucose to feed ing CNS homeostasis, functional impairments of astrocyte to neurons as their energy source utilizing astrocytic mono- transporters are often associated with various CNS diseases.5,7) carboxylate transporter 1 (MCT1) and MCT4 (export), and For example, the functional insufficiency of EAAT2 results in neuronal MCT2 (uptake), respectively. This is the so-called an elevation of glutamate concentration at the synapse, even- lactate shuttle. Another example is the formation of the tri- tually inducing excitotoxicity. Therefore, EAAT2 functional partite synapse.4,5) It has been observed that numerous, but enhancers, such as ceftriaxone and LDN/OSU-0212320, are not all, numbers of synapses are surrounded by astrocytes in expected to facilitate glutamate clearance to reduce such tox- the hippocampus,6) whereby astrocytes are considered to play icity. In this way, EAAT2 has drawn significant attention in essential roles in neurotransmitter clearance in the synaptic CNS drug development for the treatment of such diseases as cleft. In the tripartite synapse, two astrocyte-specific trans- amyotrophic lateral sclerosis, Alzheimer’s disease, and Parkin- porters, namely, excitatory amino acid transporter 1 (EAAT1) son’s disease.8) Furthermore, because astrocytes utilize various and EAAT2 (glutamate transporters (GLAST and GLT-1) in types of plasma membrane transporters in order to fulfill their rodents, respectively) significantly contribute to the uptake pleiotropic roles, it can be expected that drug-targetable astro- of the released glutamate into astrocytes, leading to the ter- cyte transporters are not limited to EAAT2. Obviously, GlyT-1 mination of excitation signals. Similarly, another astrocyte- has been regarded as another drug target transporter,9,10) and

* To whom correspondence should be addressed. e-mail: [email protected] © 2017 The Pharmaceutical Society of Japan 1154 Biol. Pharm. Bull. Vol. 40, No. 8 (2017)

Fig. 1. A Summary Illustration of Functional Expression of Organic Ion Transporters in Astrocytes Representative astrocytic organic anion transporters described in this review (OATP1C1, OCT3, OCTN2, and PMAT), and their functions, are summarized. Well- established astrocyte transporters (MCT1, MCT4, EAAT1, EAAT2 and GlyT-1) and their functions are also shown. MCT2 is known to be a neuronal lactate uptake trans- porter, and glucose transporter 1 (GLUT1) is regarded as a primary glucose uptake transporter expressed in astrocytes. The question mark (?) indicates that while several transporters, including MCT8, are thought to be involved in thyroid hormone efflux, their details remain undetermined. Synapse is depicted on the right side. Thick arrows show preferential transport processes. Ac-Car, acetyl-L-carnitine; Car, carnitine; DA, dopamine; DIO2, Type II iodothyronine deiodinase; E, epinephrine; Glc, glucose; Glu, glutamate; Gly, glycine; H, histamine; 5-HT, 5-hydroxytryptamine (serotonin); Lac, lactate; NE, norepinephrine; T3, triiodothyronine; T4, thyroxine (or tetraiodothy- ronine); Pyr, pyruvate.

Table 1. Km Values of OCT3, PMAT, OCTN2, and OATP1C1 for Their Typical Substrates

Transporter Substrate Km (µM) References OCT3 MPP+ 166 (human) 24 Dopamine 1033 (human) 16, 24 1500 (rat) Serotonin 988, 510 (human) 16, 24, 26 500 (rat) Norepinephrine 510, 629, 923, 2630 (human) 15, 16, 24, 26 1900 (rat) Epinephrine 240, 458 (human) 16, 24 1500 (rat) Histamine 220, 641 (human) 16, 24 540 (rat) PMAT MPP+ 33, 111 (human) 24, 73, 77 70 (rat), 50 (mouse) Dopamine 201, 329, 406 (human) 24, 73, 77 271 (rat), 466 (mouse) Serotonin 114, 116, 283 (human) 24, 73, 77 83 (rat), 231 (mouse) Norepinephrine 1078, 2606 (human) 24, 77 Epinephrine 951, 15323 (human) 24, 77 Histamine 4379, 10471 (human) 24, 77 OCTN2 L-Carnitine 4.3 (human) 38, 79 14.8 (rat) D-Carnitine 10.9 (human) 78 Acetyl-L-carnitine 8.5 (human) 78 OATP1C1 Thyroxin 0.09 (human) 59, 60, 80 0.18, 0.34 (mouse) Reverse triiodothyronine 0.13 (human) 59, 80 0.46 (mouse) Vol. 40, No. 8 (2017) Biol. Pharm. Bull. 1155 xCT (the cysteine/glutamate exchanger) seems to be another.11) Mooney have proposed,25) it can be postulated that the inhi- Besides these amino acid transporters, organic ion trans- bition of OCT3 is expected to enhance the efficacy of anti- porters are also promising candidates as drug-targetable astro- depressant or anti-psychotic drugs. cyte transporters. Unlike the above-mentioned astroglia-spe- Details of previous research efforts regarding the above- cific transporters, the expression of organic ion transporters is mentioned proposal has been summarized in the review not always limited to astrocytes (as described later); therefore, article.12) Additionally, a recent study has shown that several their potential as a pharmacological target in astrocytes has anti-depressant drugs, such as desipramine and sertraline, can yet to be established. However, their functions in astrocytes inhibit OCT3-mediated norepinephrine or 5-HT uptake.26) have been demonstrated, and recent advances have pointed to Furthermore, Horton et al.27) have reported that, while dec- 28) the importance of taking into consideration astrocytic organic ynium-22 (D22, a potent inhibitor of “uptake2” ) alone does ion transporter functions in future CNS drug development. not effectively work as an anti-depressant, the compound sig- Thus, with the expectation that astrocytic organic ion nificantly enhances the anti-depressant effects of fluvoxamine transporters will establish their presence in CNS drug devel- (a selective serotonin reuptake inhibitor, SSRI). Furthermore, opment, this review will briefly summarize their expression this incremental effect was significantly attenuated in OCT3 profiles and functions in astrocytes. Additionally, we discuss knock-out mouse, indicating that OCT3 participates in D22’s their potential to become a therapeutic target, while caution- pharmacological action. ary notes in astrocyte organic ion transporter studies are Collectively, the findings obtained so far strongly support discussed. Please see Fig. 1 and Table 1, where representative the idea that the inhibition of OCT3 activity is a promising astrocytic organic ion transporters described in this review are approach for improving the therapeutic efficacy of current an- summarized, as well as their functions. ti-depressant drugs. However, specification of the roles played by astrocytic OCT3 is confounded by its region-restricted 2. ORGANIC CATION TRANSPORTERS expression profile, and by the presence of neuronal OCT3. Therefore, characterization of how astrocyte OCT3 contrib- 2.1. Organic Cation Transporter 3 (OCT3) Two ex- utes to the D22’s effects is a challenge for future research. tracellular monoamine clearance systems in monoaminergic On the other hand, the importance of astrocytic OCT3 neurons have been observed. One is the uptake1 system, function has been highlighted in 1-methyl-4-phenyl-1,2,3,6- which consists of high-affinity, low-capacity Na+- and Cl−- tetrahydropyridine (MPTP)-mediated dopaminergic neurotox- dependent norepinephrine, dopamine, and serotonin transport- icity in mice. Cui et al.19) have proposed the following model: + ers (NET, DAT, SERT, respectively). The other is the uptake2 MPTP is metabolized to MPP in astrocytes, which is in turn system, which is a low-affinity, high-capacity Na+- and released from the cells via OCT3, and then taken up by dopa- − Cl -independent monoamine transport process. The uptake2 minergic neurons via DAT. Therefore, understanding the roles system has been thought to be mediated by the molecularly- of OCT3 in astrocytes is important not only for CNS pharma- unidentified extraneuronal monoamine transporter (EMT) pre- cology, but also for CNS toxicology. dominantly expressed in glia.12) However, in the late 1990 s, a 2.2. Plasma Membrane Monoamine Transporter series of molecular cloning and functional expression analysis (PMAT) While OCT3 is a transporter responsible for the of transporters strongly suggested that organic cation trans- uptake2 system (practically synonymous with EMT), the porter 3 (OCT3, SLC22A3) is likely to be a molecular entity of current understanding of the uptake2 system is that it en- the EMT.13–15) compasses a series of low-affinity, high-capacity Na+ and In situ hybridization and immunohistochemical analyses Cl−-independent transporters for the biogenic amines.12) Ac- using rat or mouse brain have demonstrated that, in addition to cordingly, another transporter carrying uptake2 system char- its neuronal expression in the cerebellum, subfornical organ, acteristics has been cloned: the plasma membrane monoamine dorsal raphe, hippocampus, and other brain regions,13,16–19) transporter (PMAT, also known as equilibrative nucleoside OCT3 is also expressed in astrocytes in several brain regions, transporter 4, SLC29A4).29) such as the substantia nigra, striatum, hippocampus, and hy- PMAT is a relatively new organic cation transporter, origi- pothalamic nuclei.17,19) Astrocytic OCT3 expression has also nally cloned from the human brain.29) PMAT has been shown been reported in human brain sections19,20) and in primary to exhibit similar transporter characteristics to those of OCT3 human astrocytes.20–22) However, the absence of OCT3 ex- (for more details, please see the review article30)). Like OCT3, pression in astrocytes has also been observed in some brain PMAT can transport MPP+, dopamine, norepinephrine, epi- areas (such as the cerebellum), and primary human astrocytes nephrine, 5-HT, and histamine, and is strongly inhibited by are not always positive for OCT3 expression.19,23) Therefore, D22.29,30) However, it should be noted that the substrate prefer- OCT3 appears to be expressed in astrocytes heterogeneously. ence of PMAT is different from that of OCT3: OCT3 exhibits OCT3 is a Na+- and Cl−-independent transporter, and has higher transport activity levels towards norepinephrine, epi- shown to be capable of transporting 1-methyl-4-phenylpyri- nephrine, and histamine, whereas PMAT prefers to transport dinium ion (MPP+), dopamine, norepinephrine, epinephrine, dopamine and 5-HT.24) This difference suggests that the role 5-hydroxytryptamine (5-HT), and histamine.16,24) Given these of PMAT in the clearance of biogenic amines at synapses is functions, it is considered likely that, by working together different from that of OCT3. with high-affinity type transporters, astrocytic OCT3 actively As with OCT3, it has been reported that PMAT is ex- participates in the synaptic clearance of these transmitters. pressed in various brain regions in the mouse,29) where its pre- This notion may be related to the findings that OCT3 knock- dominant localization can be found in neurons.24,31) Recently, out mouse, at least slightly, show increased anxiety and sen- however, evidence for PMAT’s expression and functions sitivity to psychostimulants.17) Therefore, as Schildkraut and (histamine and MPP+ uptake) has been provided in primary 1156 Biol. Pharm. Bull. Vol. 40, No. 8 (2017) and immortalized human astrocytes.20,22,23) Therefore, it is determinant role of carnitine availability for the fatty acid probable that PMAT also serves as an astrocyte organic cation β-oxidation process. transporter. In addition to carnitine, acetyl-L-carnitine, which is an- Considering the neurotransmitter uptake activity of PMAT, other endogenous OCTN2 substrate, is also known to be a it is not surprising to assume PMAT may be a target of cer- physiologically multi-talented molecule, not simply an energy tain CNS drugs, in a similar manner to that of OCT3. For source. It can be used for amino acid synthesis,46) and acts as example, in addition to OCT3, the functional inhibition of a nutraceutical with anti-oxidative effects.47,48) Consistently, PMAT by D22 may also be involved in its anti-depressant acetyl-L-carnitine is expected to ameliorate several brain dis- augmenting effects.27) However, currently it is difficult to ease conditions, such as depression49) and ischemia.50) Consid- separately evaluate the contribution of each transporter to the ering that the important target site of these acetyl-L-carnitine action of D22. actions are astrocytes, the functional activation of astrocytic To summarize, it is considered likely that astrocytic PMAT OCTN2 should be a beneficial approach for the treatment of plays a crucial role in biogenic amine clearance in both the several brain diseases. normal and pathological brain, thus suggesting that PMAT Meanwhile, OCTN1 exhibits differential substrate specific- has significant potential to become a pharmacological target ity (e.g., significant transport ability toward ergothioneine, in several brain diseases. In order to further enhance the fea- but not to carnitine51)). Thus, physiological roles of OCTN1 sibility of uptake2 system-targeted drugs (e.g., anti-depressant are assumed to be clearly different from those of OCTN2. It drugs), it is important to develop ways whereby astrocytic has been shown that OCTN1 is expressed in neurons,52,53) but OCT3 and PMAT functions are independently evaluated. In there is no report describing its function in astrocytes to date. this view, the identification of selective inhibitors (like lopi- However, in our unpublished observation, OCTN1 mRNA navir, a recently identified PMAT-specific inhibitor32)), are ex- expression has been found in human primary astrocytes. It is pected to contribute to the clarification of transporter-specific worth further examining OCTN1 existence in human astro- function in astrocytes. cytes. 2.3. Other Organic Cation Transporters Regarding other members of the OCT family, results obtained so far sug- 3. ORGANIC ANION TRANSPORTERS gest that OCT1 (SLC22A1) is not significantly expressed in the brain, if at all, but OCT2 (SLC22A2) appears to be expressed. Representative multi-specific organic anion transporter Although OCT2 mRNA expression in rat cultured astrocytes families include organic anion transporters (OATs), organic has been reported,33) several studies have identified that OCT2 anion transporting polypeptides (OATPs), and MCTs. It has is primarily expressed in neurons.34–36) Therefore, it is pos- been well described that several MCT members are function- sible that significance of OCT2 function in astrocytes might ally expressed in the brain; see also other excellent reviews for be only marginal. Nevertheless, its neuronal expression and details.54,55) By contrast, to date there is no clear evidence for functional redundancy with OCT3 and PMAT should be noted the functional expression of well-characterized OAT members in relevant studies. (namely, OAT1–3) in astrocytes. Thus, this section focuses OCT asides, carnitine/organic cation transporters (OCTN1 solely on OATPs expressed in astrocytes. and OCTN2, SLC22A4 and SLC22A5, respectively) are the OATP1C1 (also known as OATP14 or OATP-F, SLCO1C1) different family of organic cation transporters. With different has exhibited a functional expression in astrocytes, as substrate selectivity from those of OCTs and PMAT, OCTN1 evidenced during efforts to explore the identification of a and OCTN2 can transport various cationic/amphipathic com- sulforhodamine 101 (SR101, an anionic fluorescence com- pounds, such as D-carnitine, L-carnitine (carnitine), acetyl-L- pound) uptake transporter in hippocampus astrocytes. It has carnitine, ergothioneine, and drugs like verapamil and oxali- been shown that hippocampal astrocytes can be efficiently platin.37) labeled with SR101, but the uptake system involved had been OCTN2 is a Na+-dependent high-affinity carnitine trans- unknown.56) However, Hülsmann’s group has recently clarified porter.38) Carnitine is an essential molecule for the transport that SR101-labelling intensity is severely reduced in the pres- of fatty acid into mitochondria, where fatty acids are oxidized ence of rifampicin and dehydroepiandrosterone sulfate, which to produce acetyl-CoA as an energy source. Therefore, it is are OATP substrates/inhibitors,57) and SR101-labelled astro- considered likely that OCTN2 activity is closely linked with cytes mostly disappeared in Oatp1c1-knockout mice.58) tissue energy homeostasis. OATP1C1 is a multi-specific organic anion transporter, Brain distribution of OCTN2 is not clearly known at pres- but its primary substrates are thyroid hormones, with the ent, but Inazu et al.39) first showed functional OCTN2 expres- highest activity towards 3,5,3′,5′-tetraiodothyronine (thy- sion in rat astrocytes. This has been further supported by roxine, T4).59,60) The thyroid hormone transporting ability subsequent reports of OCTN2 in rats40,41) and humans.23) The of OATP1C1 is in good agreement with the critical roles of presence of OCTN2 in astrocytes seems reasonable, since as- astrocytes in CNS thyroid hormone homeostasis. Brain cells trocytes require substantial amounts of energy to fulfill their utilize thyroid hormones of both peripheral and CNS origin. roles. Panov et al.42) have hypothesized that, in line with the Because brain type 2 deiodinase expression is restricted in fact that perisynaptic processes of astrocytes hold large num- astrocytes,61) T4 is converted to the active form 3,5,3′-triio- bers of mitrochondria,43) fatty acid β-oxidation is the prefer- dothyronine (T3), primarily in astrocytes in the brain. Then, able process for acetyl-CoA production, by which astrocytes T3 either binds to thyroid hormone receptors (TRα and TRβ) accomplish large-scale production of lactate and ATP at the in astrocytes or is released into extracellular fluids, whereby same time. And in fact, fatty acid oxidation in astrocytes has other brain cells can take up the hormone. been proven to date.44,45) Therefore, OCTN2 may play the It has been known that thyroid hormones regulate a variety Vol. 40, No. 8 (2017) Biol. Pharm. Bull. 1157 of physiological functions of astrocytes.62,63) Briefly, thyroid larly, although to a lesser extent, differences in the inhibition hormones promote astrocyte maturation and regulate extra- properties of D22 have been observed between human and cellular matrix synthesis and growth factor secretions.64,65) mouse PMATs.73) As suggested by these reports, even though T3 enhances the expression levels of astrocytic glutamate the physiological roles of OCT3 and PMAT appear to be most- transporters (GLAST and GLT-1).66) Furthermore, thyroid ly conserved among species, their inhibitor (and substrate) hormones indirectly affect neuronal differentiation and func- recognition preference must be, at least slightly, different, de- tion through their action in astrocytes. For example, it has pending on their intrinsic amino acid sequences. been observed that T3-primed astrocytes express higher levels The second difference is in expression profiles. This issue of heparan sulfate proteoglycans, which provide neurite out- has not been clearly appreciated in the study of astrocytic growth effects.67) transporters, and therefore, currently, no appropriate example Therefore, OATP1C1 expressed in astrocytes is likely to can be provided. However, it has been shown that OATP1C1 is be a molecule actively taking part in brain thyroid hormone abundantly expressed in rodent, but not in human, blood–brain action, and its functional modification may have therapeu- barrier.74,75) This fact invites attention to quantitative species tic potential in some brain diseases. However, it is also true differences in other organic ion transporters in astrocytes. that other thyroid hormone transporters have been identified, Finally, the existence of species-specific family members such as MCT8, MCT10 and L-type amino acid transporters should be noted. A clear example is that while OCTN3 is a (LATs)68); in all of these, mRNAs are expressed in astro- rodent member of the OCTN family, and expressed in rat as- cytes (unpublished observation). Thus, the clarification of trocytes,76) humans do not appear to have this member.37) OATP1C1-specific roles in CNS thyroid hormone homeostasis Currently, the understanding of species differences of as- will be an important challenge in future studies. trocytic organic ion transporters is premature. However, since In addition to OATP1C1, other OATP members may work this is critically important in the appropriate translation of as astrocyte organic anion transporters. While there is no pre-clinical results into clinical trials in drug development, ex- report showing the expression of OATP1B/Oatp1b members tensive research efforts should be made to fully clarify these in astrocytes, recently Oatp1a4 and Oatp1a5 mRNA, and inter-species distinctions. OATP2B1 mRNA expression has been reported in cultured rat and human astrocytes, respectively.23,69) Moreover, OATP3A1 5. CONCLUSION and 4A1 mRNA can also be detected in primary human as- trocytes (unpublished observation). While characterization of Accumulating evidence of the functional expression of or- their function in astrocytes awaits further studies, it is pos- ganic ion transporters in astrocytes makes it likely that these sible that the functions of such OATP members may contrib- transporters have significant potential to become pharmaco- ute to the pharmacological action of both current and future logical targets for various CNS diseases. The multi-substrate CNS drugs. nature of organic ion transporters, along with non-fatal phe- notypes identified by knockout mouse, imply that these trans- 4. CAUTIONARY NOTES ON ASTROCYTE ORGANIC porters would not play a fatal, but rather a fine-tuning role in ION TRANSPORTER STUDIES various CNS activities. Furthermore, it is expected that not only functional boosting, but also the inhibition of the trans- For future astrocytic organic ion transporter studies aimed porter functions are feasible approaches in the development of at drug discovery, it is important to provide two important new CNS drugs. cautionary notes: one is astrocyte heterogeneity, and the other However, the identification and characterization of astro- inter-species differences of the transporters. cytic organic ion transporters is still in the initial stage. Even As for astrocyte heterogeneity, several lines of evidence for EMT transporters (OCT3 and PMAT), their physiological have shown that morphologically different subsets of astrocyte and pathophysiological roles remain to be fully established. It populations exist within the brain (such as protoplasmic and is true that broad substrate specificity of transporters, as well fibrous astrocytes), and their functions are also assumed to as their yet unestablished intra-brain expression characteris- be different.70–72) Even in a morphologically-similar astrocyte tics, have made it difficult to precisely understand how they subset, roles may differ in a region-dependent or develop- work in astrocytes and which diseases they are involved in. mental stage-dependent manner. Given these points, it is not Therefore, in order to further specify their roles, elaborative surprising to see that the astrocytic expression of organic ion research efforts, with the above-described cautionary notes in transporters is region- or stage-specific. However, to date, mind, are unequivocally necessary. These include the develop- only limited information regarding brain distribution profiles ment of astrocyte-specific transporter(s) knockout mouse and of astrocytic organic ion transporters is available, even in transporter-specific inhibitors, scrutiny of their region- and rodents. Therefore, further detailed investigation regarding cell type-specific expression profiles, and human-oriented ex- the matter is strongly encouraged, the results of which will amination. In these efforts, extra-astrocytic expression of the be fundamental information for the precise characterization transporters should be taken into consideration. of the physiological/pathophysiological roles of organic ion Despite the various challenges that lie in ahead, we hope transporters. this review will stimulate additional astrocytic organic ion Roughly speaking, there are three types of species differ- transporter studies that will contribute to further understand- ences. The first is a functional (qualitative) difference. It has ing of their physiological/pathophysiological roles, as well as been shown that rat OCT3 is much more sensitive to inhibi- to the development of manipulation strategies to modify their tion by D-amphetamine and ketamine (>100-fold), but less functions. We believe that such research advances will in turn sensitive to MK-801, compared with human OCT3.16) Simi- lead to the discovery of new CNS drugs targeted to astrocytic 1158 Biol. Pharm. Bull. Vol. 40, No. 8 (2017) organic ion transporters. serotonin in serotonin-transporter-deficient mice. Proc. Natl. Acad. Sci. U.S.A., 105, 18976–18981 (2008). Conflict of Interest The authors declare no conflict of 19) Cui M, Aras R, Christian WV, Rappold PM, Hatwar M, Panza J, Jackson-Lewis V, Javitch JA, Ballatori N, Przedborski S, Tieu K. interest. The organic cation transporter-3 is a pivotal modulator of neurode- generation in the nigrostriatal dopaminergic pathway. Proc. Natl. 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