Associate editor: A. G. Ramage
Unfaithful neurotransmitter transporters: Focus on serotonin uptake and implications for antidepressant efficacy

https://doi.org/10.1016/j.pharmthera.2008.10.004Get rights and content

Abstract

Biogenic amine transporters for serotonin, norepinephrine and dopamine (SERT, NET and DAT respectively), are the key players terminating transmission of these amines in the central nervous system by their high-affinity uptake. They are also major targets for many antidepressant drugs. Interestingly however, drugs targeted to a specific transporter do not appear to be as clinically efficacious as those that block two or all three of these transporters. A growing body of literature, reviewed here, supports the idea that promiscuity among these transporters (the uptake of multiple amines in addition to their “native” transmitter) may account for improved therapeutic effects of dual and triple uptake blockers. However, even these drugs do not provide effective treatment outcomes for all individuals. An emerging literature suggests that “non-traditional” transporters such as organic cation transporters (OCT) and the plasma membrane monoamine transporter (PMAT) may contribute to the less than hoped for efficacy of currently prescribed uptake inhibitors. OCT and PMAT are capable of clearing biogenic amines from extracellular fluid and may serve to buffer the effects of frontline antidepressants, such as selective serotonin reuptake inhibitors. In addition, polymorphisms that occur in the genes encoding the transporters can lead to variation in transporter expression and function (e.g. the serotonin transporter linked polymorphic region; 5-HTTLPR) and can have profound effects on treatment outcome. This may be accounted for, in part, by compensatory adaptations in other transporters. This review synthesizes the existing literature, focusing on serotonin to illustrate and revive a model for the rationale design of improved antidepressants.

Introduction

Biogenic amine transporters, including the serotonin, norepinephrine and dopamine transporters (SERT, NET and DAT respectively) are pivotal players regulating neurotransmission by the high-affinity uptake of serotonin (5-HT), norepinephrine (NE) and dopamine (DA) released from nerve terminals. While today this statement can be made in an almost cavalier fashion, tremendous advances made in our understanding of biogenic amine transporters since the initial discovery that [3H]NE could be taken up into organs containing sympathetic nerves (Whitby et al., 1961) should not be understated. For example, until relatively recent times (within the last two decades), it was believed that these transporters resided statically in the intrasynaptic plasma membrane of nerve terminals, where they served to faithfully take up neurotransmitter upon its release into the synaptic cleft. With the advent of molecular biological techniques came the cloning of these transporters (Blakely et al., 1991, Hoffman et al., 1991, Pacholczyk et al., 1991, Giros and Caron, 1993) and an explosion in our understanding of how these important proteins work. We now know that these transporters are not static and can be dynamically regulated (for reviews see, Torres et al., 2003b, Blakely et al., 2005, Mandela and Ordway, 2006, Miranda and Sorkin, 2007), including being trafficked in and out of the plasma membrane in response to numerous and diverse stimuli, and by a host of receptor triggered signal transduction pathways (Blakely et al., 1998, Ramamoorthy and Blakely, 1999, Daws et al., 2000; and for reviews see, Blakely and Bauman, 2000, Robinson, 2002, Zahniser and Sorkin, 2004, Steiner et al., 2008). We know that they can form oligomers (Torres et al., 2003a, Chen and Reith, 2008; and for review see, Sitte et al., 2004), that they can operate in different modes (alternating access or channel mode) (Galli et al., 1996, Kahlig et al., 2005) and that polymorphisms in these transporters can determine their expression and activity level (Lesch et al., 1996; and for reviews see Bannon et al., 2001, Bönisch and Brüss, 2006, Murphy and Lesch, 2008). Because these transporters are primary sites of action for both psychotherapeutic drugs, such as those used for the treatment of depression (fluoxetine or “Prozac”) and ADHD (methylphenidate or “Ritalin”), as well as for abused drugs such as 3,4-methylenedioxmethamphetamine (MDMA) and cocaine (Blakely et al., 1998, Ramamoorthy and Blakely, 1999, Dutta et al., 20033; and for reviews see, Amara and Sonders, 1998, Schenk, 2002, Gether et al., 2006, Iversen, 2006, Fleckenstein et al., 2007), the relationship between polymorphisms in these transporters and drug response has come under intense scrutiny.

One aspect of biogenic amine transporter function that has received relatively less attention is that of their “promiscuous nature”. The idea of transporter promiscuity heralds back to the 1960s when several groups provided evidence that 5-HT could be accumulated by catecholaminergic neurons (Bertler et al., 1964, Burgen and Iversen, 1965, Lichtensteiger et al., 1967, Fuxe and Ungerstedt, 1967). Uptake of 5-HT into rat brain slices was defined by at least two processes. These were termed “uptake-1” and “uptake-2”. Uptake-1, the “SERT”, had a high affinity but low capacity to take up [3H]5-HT and uptake-2, then thought to be the “NET”, had low affinity but high capacity to take up [3H]5-HT into brain slices (Burgen and Iversen, 1965, Shaskan and Snyder, 1970). Conclusions drawn from such studies were that 5-HT, even in relatively low concentrations, may enter catecholaminergic neurons in significant amounts. This line of investigation was given additional impetus with the discovery that many antidepressants were inhibitors of both NE and 5-HT uptake (Glowinski and Axelrod, 1966, Ross and Renyi, 1969). These early discoveries form the basis of this review, which describes recent advances in our understanding of biogenic amine transporter promiscuity and implications for the development of therapeutics for psychiatric disorders.

Section snippets

Transporter location sets the scene for transporter promiscuity

The concept of paracrine or volume transmission (Fuxe and Agnati, 1991, Fuxe et al., 2007) as the primary mode for monoamine neurotransmission is now well entrenched in our current thinking. In contrast to wiring transmission, where there exists “quasi contact” between neurons, volume transmission is mediated by diffusion of neurotransmitter through the extracellular matrix to its target (for detailed review and historical detail see Fuxe et al., 2007). Ultrastructural studies reveal SERT, NET

What can affinity values tell us?

These new estimates of synaptic neurotransmitter concentration bring an interesting twist to the way we think about biogenic amine transporters in terms of their affinity (Km) and maximal capacity to transport (Vmax) various substrates. For example, in vitro preparations made from homogenates, synaptosomes or transfected cell lines provide an excellent means by which to estimate Km and Vmax values, particularly in terms of the relative selectivity of a transporter for various substrates. These

Heterologous uptake of dopamine and norepinephrine

Existence of heterologous uptake of DA by the NET is arguably the most comprehensively documented to date (see Carboni & Silvagni, 2004 for additional review). Several groups have independently shown that extracellular DA is increased following blockade of the NET. For example, NET blockade in prefrontal cortex leads to a significantly greater increase in extracellular DA than blockade of DAT (Carboni et al., 2006), consistent with the greater density of NET relative to DAT in this brain region

Biological significance of transporter promiscuity for serotonin

While there is no doubt that the SERT is the “prime-mover” of active 5-HT uptake from extracellular fluid, as just discussed, substantial evidence for uptake of 5-HT by other transporters, including the NET and DAT has amassed over the past 5 decades (e.g. Burgen and Iversen, 1965, Shaskan and Snyder, 1970, Jackson and Wightman, 1995, Daws et al., 1998, Daws et al., 2005, Pan et al., 2001, Zhou et al., 2002, Zhou et al., 2005, Callaghan et al., 2005). The studies discussed herein suggest that

Old ideas, new discoveries

Promiscuous uptake of biogenic amine transporters is not a new idea, as evidenced by the pioneering work of several groups in the 1960s and 1970s (Bertler et al., 1964, Lichtensteiger et al., 1967, Fuxe and Ungerstedt, 1967, Shaskan and Snyder, 1970). However, it took until the 1990s and 2000s for this notion to become accepted as physiologically significant and of potential impact to the design of therapeutics such as antidepressants. In 2004, Schildkraut and Mooney proposed the extraneuronal

“Old dog, new tricks”: new vistas for therapeutics

The over 40-year-old monoamine hypothesis of depression, postulating a functional deficiency of noradrenergic or serotonergic transmission in brain, stemmed from studies examining the acute pharmacologic actions of tricyclic antidepressants (TCAs) and MAO inhibitors (MAOIs) (Prange, 1964, Schildkraut, 1965, Coppen, 1967, Lapin and Oxenkrug, 1969, Carlsson et al., 1969, Bowden et al., 1985 and see Frazer, 1997 for review). It has been proposed that drugs that augment the increase in

Concluding remarks

The focus of this review has centered on 5-HT, but could be applied to any of the biogenic amine neurotransmitters. Clearly there are converging lines of evidence that novel mediators of 5-HT (and other biogenic amines) uptake may be critically important in human disease. OCT and PMAT provide an exciting new direction for research efforts directed toward discovery and design of improved therapeutics for depression. In addition, the relatively high affinity of abused drugs such as

Acknowledgments

Supported in part from USPHS grant MH64489, NARSAD and the Alcoholic Beverage Medical Research Foundation. Dr. Glenn M. Toney is gratefully acknowledged for his comments on this manuscript.

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