Associate editor: A. G. RamageUnfaithful neurotransmitter transporters: Focus on serotonin uptake and implications for antidepressant efficacy
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.
References (161)
- et al.
Neurotransmitter transporters as molecular targets for addictive drugs
Drug Alcohol Depend
(1998) - et al.
Differential pharmacological in vitro properties of organic cation transporters and regional distribution in rat brain
Neuropharmacology
(2006) - et al.
Acceleration of the effect of selected antidepressant drugs in major depression by 5-HT1A antagonists
Trends Neurosci
(1996) - et al.
The human dopamine transporter gene: Gene organization, transcriptional regulation, and potential involvement in neuropsychiatric disorders
Eur Neuropsychopharmacol
(2001) - et al.
Biogenic amine transporters: regulation in flux
Curr Opin Neurobiol
(2000) - et al.
Regulated phosphorylation and trafficking of antidepressant-sensitive serotonin transporter proteins
Biol Psychiatry
(1998) - et al.
Paracrine neurotransmission in the CNS: Involvement of 5-HT
Trends Neurosci
(1999) - et al.
Monoamine neurotransmitter transport mediated by the polyspecific cation transporter rOCT1
FEBS Lett
(1996) - et al.
The chronic effects of desipramine and sertraline on platelet and synaptosomal 5HT uptake in olfactory bulbectomized rats
Prog Neuro-Psychopharmacol Biol Psychiatry
(1988) - et al.
Differences in the in vivo dynamics of neurotransmitter release and serotonin uptake after acute para-methoxyamphetamine and 3,4-methylenedioxymethamphetamine revealed by chronoamperometry
Neurochem Int
(2005)
Effect of antidepressant drugs on the depletion of intraneuronal brain 5-hydroxytryptamine stores caused by 4-methyl-a-ethyl-meta-tyramine
Eur J Pharmacol
Transmitter time course in the synaptic cleft: Its role in central synaptic function
Trends Neurosci
DAncing past the DAT at a DA synapse
Trends Neurosci
Assessing substrates underlying the behavioral effects of antidepressants using the modified rat forced swimming test
Neurosci Behav Rev
Expression and immunolocalization of the plasma membrane monoamine transporter in the brain
Neuroscience
Transport mechanisms governing clearance in vivo revealed by high-speed chronoamperometry
J Neurosci Methods
Dopamine transporter as target for drug development of cocaine dependence medications
Eur J Pharmacol
Identification and characterization of a novel monoamine transporter in the human brain
J Biol Chem
Local inhibition of organic cation transporters increases extracellular serotonin in the medial hypothalamus
Brain Res
From the Golgi-Cajal mapping to the transmitter-based characterization of the neuronal networks leading to two modes of brain communication: Wiring and volume transmission
Brain Res Rev
Neurotransmitter transporters: molecular function of important drug targets
Trends Pharmacol Sci
Molecular characterization of the dopamine transporter
Trends Pharmacol Sci
Primary structure and functional expression of the apical organic cation transporter from kidney epithelial LLC-PK1 cells
J Biol Chem
Genetics of emotional regulation: The role of the serotonin transporter in neural function
Trends Cogn Sci
Evaluation of antidepressant-related behavioral responses in mice lacking the serotonin transporter
Neuropsychopharmacology
Pharmacological characterization of [3H] desipramine binding in rat cerebral cortex
Prog Neuropsychopharmacol
Dynamics of 5-hydroxytryptamine released from dopamine neurons in caudate putamen of the rat
Brain Res
Reuptake of L-DOPA-derived extracellular dopamine in the striatum with dopaminergic denervation via serotonin transporters
Neurosci Lett
Behavioral changes following antisense oligonucleotide-induced reduction of organic cation transporter-3 in mice
Neurosci Lett
Intensification of the central serotonergic processes as a possible determinant of the thymoleptic effect
Lancet
Alcohol dependence and gene x environment interaction in emotion regulation: Is serotonin the link?
Eur J Pharmacol
Estrous cycle modulation of extracellular serotonin in mediobasal hypothalamus: Role of the serotonin transporter and terminal autoreceptors
Brain Res
Gene dose-dependent alterations in extraneuronal serotonin but not dopamine in mice with reduced serotonin transporter expression
J Neurosci Methods
Interaction of cations, anions, and weak base quinine with rat renal cation transporter rOCT2 compared with rOCT1
Am J Physiol Renal Physiol
Early experience and serotonin transporter gene variation interact to influence primate CNS function
Mol Psychiatry
Studies on 5-hydroxytryptamine stores in pineal gland of rat
Acta Physiol Scand Suppl
Cloning and expression of a functional serotonin transporter from rat brain
Nature
Biogenic amine neurotransmitter transporters: just when you thought you knew them
Physiology
Modifications of the serotonin system by antidepressant treatments: Implications for the therapeutic response in major depression
J Clin Psychopharmacol
The norepinephrine transporter in physiology and disease
Handb Exp Pharmacol
Effects of amitriptyline and imipramine on brain amine neurotransmitter metabolites in cerebrospinal fluid
Clin Pharmacol Ther
Quantitative evaluation of 5-hydroxytryptamine (serotonin) neuronal release and uptake: and investigation of extrasynaptic transmission
J Neurosci
The inhibition of noradrenaline uptake by sympathomimetic amines in the rat isolated heart
Br J Pharmacol
Human neurons express the polyspecific cation transporter hOCT2, which translocates monoamine neurotransmitters, amantadine and memantine
Mol Pharmacol
Loss of antidepressant efficacy during maintenance therapy: possible mechanisms and treatments
J Clin Psychiatry
Dopamine reuptake by norepinephrine neurons: exception or rule?
Crit Rev Neurobiol
Cumulative effect of norepinephrine and dopamine carrier blockade on extracellular dopamine increase in the nucleus accumbens shell, bed nucleus of stria terminalis and prefrontal cortex
J Neurochem
Blockade of the noradrenaline carrier increases extracellular dopamine concentrations in the prefrontal cortex: evidence that dopamine is taken up in vivo by noradrenergic terminals
J Neurochem
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