Unfaithful neurotransmitter transporters: focus on serotonin uptake and implications for antidepressant efficacy

L-DOPA in Parkinson's Disease: Looking at the "False" Neurotransmitters and Their Meaning

L-3,4-dihydroxyphenylalanine (L-DOPA) has been successfully used in the treatment of Parkinson’s disease (PD) for more than 50 years. It fulfilled the criteria to cross the blood–brain barrier and counteract the biochemical defect of dopamine (DA). It remarkably worked after some adjustments in line with the initial hypothesis, leaving a poor place to the plethora of mechanisms involving other neurotransmitters or mechanisms of action beyond newly synthesized DA itself. Yet, its mechanism of action is far from clear. It involves numerous distinct cell populations and does not mimic the mechanism of action of dopaminergic agonists. L-DOPA-derived DA is mainly released by serotonergic neurons as a false neurotransmitter, and serotonergic neurons are involved in L-DOPA-induced dyskinesia. The brain pattern and magnitude of DA extracellular levels together with this status of false neurotransmitters suggest that the striatal effects of DA via this mechanism would be minimal. Other metabolic products coming from newly formed DA or through the metabolism of L-DOPA itself could be involved. These compounds can be trace amines and derivatives. They could accumulate within the terminals of the remaining monoaminergic neurons. These “false neurotransmitters,” also known for some of them as inducing an “amphetamine-like” mechanism, could reduce the content of biogenic amines in terminals of monoaminergic neurons, thereby impairing the exocytotic process of monoamines including L-DOPA-induced DA extracellular outflow. The aim of this review is to present the mechanism of action of L-DOPA with a specific attention to “false neurotransmission.”

Serotonylation: Serotonin Signaling and Epigenetics

Serotonylation, the covalent linkage of serotonin to proteins has been discovered more than 60 years ago but only recently the mechanisms and first functions have been elucidated. It has been found that transglutaminases (TG) such as TG2 and the blood coagulation factor XIIIa are the enzymes which catalyze the linkage of serotonin and other monoamines to distinct glutamine (Gln) residues of target proteins. The first target proteins, small G-proteins and extracellular matrix constituents, were found in platelets and are pivotally involved in platelet aggregation and the formation of thrombi. The serotonylation of the same proteins is also involved in insulin secretion and in the proliferation of pulmonary vascular smooth muscle cells and thereby in the pathogenesis of pulmonary arterial hypertension (PAH). Recently histones have been described as targets of serotonylation opening the area of transcriptional control to this posttranslational protein modification. Future studies will certainly reveal further target proteins, signaling pathways, cellular processes, and diseases, in which serotonylation or, more general, monoaminylation is important.

Use of selective serotonin reuptake inhibitors is associated with very low plasma-free serotonin concentrations in humans

Background

Selective serotonin reuptake inhibitors (SSRIs) block the serotonin transporter on neurons, but also on platelets, thus decreasing platelet serotonin concentrations in users of SSRIs. Data on plasma-free serotonin concentrations in SSRI users are lacking, while plasma-free serotonin is available for receptor binding and plays a role in several pathophysiological processes. We therefore measured the plasma-free and platelet serotonin concentrations in users of SSRIs and age-matched healthy controls, and we analysed plasma concentrations of the serotonin precursor tryptophan and serotonin metabolite 5-hydroxyindoleamineacetic acid (5-HIAA).

Methods

For this cross-sectional single-centre case control study, participants were recruited at the departments of Psychiatry and General Medicine. High-performance liquid chromatography combined with tandem mass spectrometry (LC-MS/MS) was used to measure plasma-free and platelet serotonin, plasma tryptophan and 5-HIAA concentrations. Preanalytical conditions were optimized by careful blood collection, rapid sample handling, high-speed centrifugation, drug and diet restrictions and age-matched controls.

Results

In 64 SSRI users, median concentrations of plasma-free and platelet serotonin were 10-fold and 14-fold lower, respectively, than in 64 matched controls. Patients using higher dose SSRIs or those with higher affinity for the serotonin transporter had lower plasma-free and platelet serotonin concentrations. Compared with controls, SSRI users had similar median plasma tryptophan concentrations but slightly higher plasma 5-HIAA concentrations.

Conclusion

SSRI users have low platelet serotonin and low plasma-free serotonin. This could not be explained by lower concentrations of its precursor tryptophan, and only partially by increased breakdown to 5-HIAA.

Analysis of Expression and Functional Activity of Aromatic L-Amino Acid Decarboxylase (DDC) and Serotonin Transporter (SERT) as Potential Sources of Serotonin in Mouse Ovary

The origin of serotonin in the ovary is the key question for understanding mechanisms of serotonergic regulation of reproductive function. We performed a study of the expression and functional activity of the serotonin transporter (SERT) and the enzyme for the synthesis of serotonin, aromatic l-amino acid decarboxylase (DDC) in mouse ovary. A pronounced peak of SERT mRNA expression occurs at the age of 14 days, but serotonin synthesis enzymes are expressed at the maximum level in the ovaries of newborn mice. SERT is detected immunohistochemically in all cellular compartments of the ovary with a maximum level of immunostaining in the oocytes of growing ovarian follicles. DDC immunolocalization, in contrast, is detected to a greater extent in primordial follicle oocytes, and decreases at the later stages of folliculogenesis. Serotonin synthesis in all cellular compartments occurs at very low levels, whereas specific serotonin uptake is clearly present, leading to a significant increase in serotonin content in the oocytes of growing primary and secondary follicles. These data indicate that the main mechanism of serotonin accumulation in mouse ovary is its uptake by the specific SERT membrane transporter, which is active in the oocytes of the growing ovarian follicles.

In vivo Hippocampal Serotonin Dynamics in Male and Female Mice: Determining Effects of Acute Escitalopram Using Fast Scan Cyclic Voltammetry

Depression is a highly prevalent psychiatric disorder, impacting females at a rate roughly twice that of males. This disparity has become the focus of many studies which are working to determine if there are environmental or biological underpinnings to depression pathology. The biology of depression is not well understood, but experts agree that a key neurotransmitter of interest is serotonin. Most research on basic serotonin neurochemistry, by us and others, has predominantly focused on male models. Thus, it is now critical to include female models to decipher possible fundamental differences between the sexes that may underlie this disorder. In this paper, we seek to determine any such differences using fast-scan cyclic voltammetry (FSCV) and fast-scan controlled adsorption voltammetry. These techniques allow us to probe the serotonergic system via measurement of evoked and ambient serotonin at carbon fiber microelectrodes (CFMs). Our data reveal no statistical differences, in the hippocampus, in female serotonin chemistry during the different stages of the estrous cycle compared to the mean female response. Furthermore, no difference was observed in evoked serotonin release and reuptake, nor ambient extracellular serotonin levels between male and female mice. We applied a previously developed mathematical model that fits our serotonin signals as a function of several synaptic processes that control the extracellular levels of this transmitter. We used the model to study potential system differences between males and females. One hypothesis brought fourth, that female mice exhibit tighter autoreceptor control of serotonin, is validated via literature and methiothepin challenge. We postulate that this tight regulation may act as a control mechanism against changes in the serotonin signal mediated by estrogen spikes. Importantly, this safety mechanism has no consequence for acutely administered escitalopram’s (ESCIT’s) ability to increase extracellular serotonin between the sexes. This work demonstrates little fundamental differences in in vivo hippocampal serotonin between the sexes, bar control mechanisms in female mice that can be observed under extraneous circumstances. We thus highlight the importance of considering sex as a biological factor in determining pharmacodynamics for personalized medical treatments that involve targeting serotonin receptors.

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.

Extreme enhancement or depletion of serotonin transporter function and serotonin availability in autism spectrum disorder

A variety of human and animal studies support the hypothesis that serotonin (5-hydroxytryptamine or 5-HT) system dysfunction is a contributing factor to the development of autism in some patients. However, many questions remain about how developmental manipulation of various components that influence 5-HT signaling (5-HT synthesis, transport, metabolism) persistently impair social behaviors. This review will summarize key aspects of central 5-HT function important for normal brain development, and review evidence implicating perinatal disruptions in 5-HT signaling in the pathophysiology of autism spectrum disorder. We discuss the importance, and relative dearth, of studies that explore the possible correlation to autism in the interactions between important intrinsic and extrinsic factors that may disrupt 5-HT homeostasis during development. In particular, we focus on exposure to 5-HT transport altering mechanisms such as selective serotonin-reuptake inhibitors or genetic polymorphisms in primary or auxiliary transporters of 5-HT, and how they relate to neurological stores of serotonin and its precursors. A deeper understanding of the many mechanisms by which 5-HT signaling can be disrupted, alone and in concert, may contribute to an improved understanding of the etiologies and heterogeneous nature of this disorder. We postulate that extreme bidirectional perturbations of these factors during development likely compound or synergize to facilitate enduring neurochemical changes resulting in insufficient or excessive 5-HT signaling, that could underlie the persistent behavioral characteristics of autism spectrum disorder.

The serotonin transporter and nonselective transporters are involved in peripheral serotonin uptake in the Gulf toadfish, Opsanus beta

In mammals, circulating serotonin [5-hydroxytryptamine (5-HT)] is sequestered by platelets via the 5-HT transporter (SERT) to prevent unintended signaling by this potent signaling molecule. Teleost fish appear to lack a similar circulating storage pool, although the diverse effects of 5-HT in teleosts likely necessitate an alternative method of tight regulation, such as uptake by peripheral tissues. Here, a 5-HT radiotracer was used to explore the 5-HT uptake capacity of peripheral tissues in the Gulf toadfish, Opsanus beta, and to elucidate the primary excretion routes of 5-HT and its metabolites. Pharmacological inhibition of SERT and other transporters enabled assessment of the SERT dependence of peripheral 5-HT uptake and excretion. The results indicated a rapid and substantial uptake of 5-HT by the heart atrium, heart ventricle, and gill that was at least partly SERT dependent. The results also supported the presence of a partial blood-brain barrier that prevented rapid changes in brain 5-HT content despite fluctuating plasma 5-HT concentrations. The renal pathway appeared to be the dominant excretory route for 5-HT and its metabolites over shorter time frames (up to ~30 min), but hepatic excretion was substantial over several hours. SERT inhibition ultimately reduced the excretion of 5-HT and its metabolites by urinary, biliary, and/or intestinal pathways. In addition, branchial excretion of 5-HT and its metabolites could not be ruled out. In summary, this study reveals that the toadfish heart and gill play active roles in regulating circulating 5-HT and yields important insights into the control of peripheral 5-HT in this teleost fish.

Inhibitors of MAO-B and COMT: their effects on brain dopamine levels and uses in Parkinson's disease

MAO-B and COMT are both enzymes involved in dopamine breakdown and metabolism. Inhibitors of these enzymes are used in the treatment of Parkinson's disease. This review article describes the scientific background to the localization and function of the enzymes, the physiological changes resulting from their inhibition, and the basic and clinical pharmacology of the various inhibitors and their role in treatment of Parkinson's disease.

A case report SPECT study and theoretical rationale for the sequential administration of ibogaine and 5-MeO-DMT in the treatment of alcohol use disorder

Ibogaine is a plant-derived alkaloid and dissociative psychedelic that demonstrates anti-addictive properties with several substances of abuse, including alcohol. 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) is a naturally occurring psychedelic known to occasion potent mystical-type experiences and also demonstrates anti-addictive properties. The potential therapeutic effects of both compounds in treating alcohol use disorder require further investigation and there are no published human neuroimaging findings of either treatment to date. We present the case of a 31-year-old male military veteran with moderate alcohol use disorder who sought treatment at an inpatient clinic in Mexico that utilized a sequential protocol with ibogaine hydrochloride (1550mg, 17.9mg/kg) on day 1, followed by vaporized 5-MeO-DMT (bufotoxin source 50mg, estimated 5-MeO-DMT content, 5-7mg) on day 3. The patient received SPECT neuroimaging that included a resting-state protocol before, and 3 days after completion of the program. During the patient's ibogaine treatment, he experienced dream-like visions that included content pertaining to his alcohol use and resolution of past developmental traumas. He described his treatment with 5-MeO-DMT as a peak transformational and spiritual breakthrough. On post-treatment SPECT neuroimaging, increases in brain perfusion were noted in bilateral caudate nuclei, left putamen, right insula, as well as temporal, occipital, and cerebellar regions compared to the patient's baseline scan. The patient reported improvement in mood, cessation of alcohol use, and reduced cravings at 5 days post-treatment, effects which were sustained at 1 month, with a partial return to mild alcohol use at 2 months. In this case, serial administration of ibogaine and 5-MeO-DMT resulted in increased perfusion in multiple brain regions broadly associated with alcohol use disorders and known pharmacology of both compounds, which coincided with a short-term therapeutic outcome. We present theoretical considerations regarding the potential of both psychedelic medicines in treating alcohol use disorders in the context of these isolated findings, and areas for future investigation.

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

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

Serotonin uptake is required for Rac1 activation in Kras-induced acinar-to-ductal metaplasia in the pancreas

Pancreatic ductal adenocarcinoma (PDAC), which is the primary cause of pancreatic cancer mortality, is poorly responsive to currently available interventions. Identifying new targets that drive PDAC formation and progression is critical for developing alternative therapeutic strategies to treat this lethal malignancy. Using genetic and pharmacological approaches, we investigated in vivo and in vitro whether uptake of the monoamine serotonin [5-hydroxytryptamine (5-HT)] is required for PDAC development. We demonstrated that pancreatic acinar cells have the ability to readily take up 5-HT in a transport-mediated manner. 5-HT uptake promoted activation of the small GTPase Ras-related C3 botulinum toxin substrate 1 (Rac1), which is required for transdifferentiation of acinar cells into acinar-to-ductal metaplasia (ADM), a key determinant in PDAC development. Consistent with the central role played by Rac1 in ADM formation, inhibition of the 5-HT transporter Sert (Slc6a4) with fluoxetine reduced ADM formation both in vitro and in vivo in a cell-autonomous manner. In addition, fluoxetine treatment profoundly compromised the stromal reaction and affected the proliferation and lipid metabolism of malignant PDAC cells. We propose that Sert is a promising therapeutic target to counteract the early event of ADM, with the potential to stall the initiation and progression of pancreatic carcinogenesis. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Voltammetric evidence for discrete serotonin circuits, linked to specific reuptake domains, in the mouse medial prefrontal cortex

The medial prefrontal cortex (mPFC) is an important brain region, that controls a variety of behavioral and functional outputs. As an important step in characterizing mPFC functionality, in this paper we focus on chemically defining serotonin transmission in this area. We apply cutting-edge analytical methods, fast-scan cyclic voltammetry (FSCV) and fast-scan controlled adsorption cyclic voltammetry (FSCAV), pioneered in our laboratory, for the first real-time in vivo analysis of serotonin in the mPFC. In prior in vivo work in the substantia nigra, pars reticulata, we found that our sub-second measurements of a single evoked serotonin release were subject to two clearance mechanisms. These mechanisms were readily modeled via Uptake 1, mediated by the serotonin transporters (SERTs), and Uptake 2, mediated by monoamine transporters (dopamine transporters (DATs), norepinephrine transporters (NETs), and organic cation transporters (OCTs)). Here in the mPFC, for the first time to our knowledge, we observe two release events in response to a single stimulation of the medial forebrain bundle (MFB). Of particular note is that each response is tied to a discrete reuptake profile comprising both Uptake 1 and 2. We hypothesize that two distinct populations of serotonin axons traverse the MFB and terminate in different domains with specific reuptake profiles. We test and confirm this hypothesis using a multifaceted pharmacological, histological and mathematical approach. We thus present evidence for a highly elaborate biochemical organization that regulates serotonin chemistry in the mPFC. This knowledge provides a solid foundation on which to base future studies of the involvement of the mPFC in brain function and behavior.

Adrenal serotonin derives from accumulation by the antidepressant-sensitive serotonin transporter

Adrenal chromaffin cells comprise the neuroendocrine arm of the sympathetic nervous system and secrete catecholamines to coordinate the appropriate stress response. Deletion of the serotonin (5-HT) transporter (SERT) gene in mice (SERT^-/- mice) or pharmacological block of SERT function in rodents and humans augments this sympathoadrenal stress response (epinephrine secretion). The prevailing assumption is that loss of CNS SERT alters central drive to the peripheral sympathetic nervous system. Adrenal chromaffin cells also prominently express SERT where it might coordinate accumulation of 5-HT for reuse in the autocrine control of stress-evoked catecholamine secretion. To help test this hypothesis, we have generated a novel mouse model with selective excision of SERT in the peripheral sympathetic nervous system (SERT^ΔTH), generated by crossing floxed SERT mice with tyrosine hydroxylase Cre driver mice. SERT expression, assessed by western blot, was abolished in the adrenal gland but not perturbed in the CNS of SERT^ΔTH mice. SERT-mediated [³H] 5-HT uptake was unaltered in midbrain, hindbrain, and spinal cord synaptosomes, confirming transporter function was intact in the CNS. Endogenous midbrain and whole blood 5-HT homeostasis was unperturbed in SERT^ΔTH mice, contrasting with the depleted 5-HT content in SERT^-/- mice. Selective SERT excision reduced adrenal gland 5-HT content by ≈ 50% in SERT^ΔTH mice but had no effect on adrenal catecholamine content. This novel model confirms that SERT expressed in adrenal chromaffin cells is essential for maintaining wild-type levels of 5-HT and provides a powerful tool to help dissect the role of SERT in the sympathetic stress response.

Constitutive plasma membrane monoamine transporter (PMAT, Slc29a4) deficiency subtly affects anxiety-like and coping behaviours

Originally, uptake-mediated termination of monoamine (e.g., serotonin and dopamine) signalling was believed to only occur via high-affinity, low-capacity transporters ("uptake1 ") such as the serotonin or dopamine transporters, respectively. Now, the important contribution of a second low-affinity, high-capacity class of biogenic amine transporters has been recognised, particularly in circumstances when uptake1 transporter function is reduced (e.g., antidepressant treatment). Pharmacologic or genetic reductions in uptake1 function can change locomotor, anxiety-like or stress-coping behaviours. Comparable behavioural investigations into reduced low-affinity, high-capacity transporter function are lacking, in part, due to a current dearth of drugs that selectively target particular low-affinity, high-capacity transporters, such as the plasma membrane monoamine transporter. Therefore, the most direct approach involves constitutive genetic knockout of these transporters. Other groups have reported that knockout of the low-affinity, high-capacity organic cation transporters 2 or 3 alters anxiety-like and stress-coping behaviours, but none have assessed behaviours in plasma membrane monoamine transporter knockout mice. Here, we evaluated adult male and female plasma membrane monoamine transporter wild-type, heterozygous and knockout mice in locomotor, anxiety-like and stress-coping behavioural tests. A mild enhancement of anxiety-related behaviour was noted in heterozygous mice. Active-coping behaviour was modestly and selectively increased in female knockout mice. These subtle behavioural changes support a supplemental role of plasma membrane monoamine transporter in serotonin and dopamine uptake, and suggest sex differences in transporter function should be examined more closely in future investigations.

Dopamine plasma clearance is increased in piglets compared to neonates during continuous dopamine infusion

AIM

Piglets models have often been used to study the effects of dopamine infusion on hypotension in neonates. However, piglets need higher doses of dopamine than neonates to increase blood pressure. We investigated whether this difference was due to interspecific difference in dopamine pharmacokinetics.

METHODS

Arterial blood samples were drawn from six neonates admitted to the neonatal intensive care unit of Copenhagen University Hospital and 20 newborn piglets during continuous dopamine infusion. Furthermore, to estimate the piglet plasma dopamine half-life, blood samples were drawn at 2.5-minute intervals after the dopamine infusion was discontinued. The plasma dopamine content was analysed by high-performance liquid chromatography with electrochemical detection.

RESULTS

The dopamine displayed first-order kinetics in piglets and had a half-life of 2.5 minutes, while the median plasma clearance was 627.9 mL/kg/minute (interquartile range 452.6-1914.4). Both piglets and neonates showed large interindividual variations in plasma clearance, but the median tended to be lower in neonates (384.9, interquartile range 114.2-480.2 mL/kg/minute).

CONCLUSION

Our results suggest that pharmacokinetic differences may explain the interspecific difference in required doses of dopamine infusion to increase blood pressure. This is important when translating the results obtained in piglet models to treating neonatal hypotension with dopamine.

Triple reuptake inhibitors: Design, synthesis and structure-activity relationship of benzylpiperidine-tetrazoles

Monoamine transporters are important targets in the treatment of various central nervous disorders. Several limitations of traditional reuptake inhibitors, like delayed onset of action, insomnia, and sexual dysfunction, have compelled the search for safer, more effective compounds. In this study, we have sought to identify novel monoamine reuptake inhibitors. Based upon the docking study of compounds that we had reported previously, aromatic rings (A1) were modified to generate a novel series of benzylpiperidine-tetrazoles. Thirty-one compounds were synthesized and evaluated for their triple reuptake inhibition of serotonin, norepinephrine and dopamine. Triple reuptake inhibitor, compound 2q, in particular, showed potent serotonin reuptake inhibition, validating our design approach.

Evaluation of the antidepressant therapeutic potential of isocyanine and pseudoisocyanine analogues of the organic cation decynium-22

Herein we describe the synthesis and evaluation of antidepressant properties of seven analogues (1-7) of the low affinity/high capacity transporter blocker decynium-22 (D-22). All analogues (1-7) were synthesized via base promoted coupling reactions between N-alkylated-2-methylquinolinium iodides or N-alkylated-4-methylquinolinium iodides and electrophilic N-alkylated-2-iodoquinolinium iodides. All final compounds were purified by re-crystallization or preparative HPLC and initial evaluation studies included; 1) screening for in vitro α1-adrenoceptor activity (a property that can lead to unwanted side-effects), 2) measuring antidepressant-like activity in a mouse tail suspension test (TST), and 3) measuring effects upon mouse locomotion. The results showed some analogues have lower affinities at α1-adrenoceptors compared to D-22 and showed antidepressant-like activity without the need for co-administration of SSRIs. Additionally, many analogues did not affect mouse locomotion to the same extent as D-22. Plans for additional evaluations of these promising analogues, including measurement of antidepressant-like activity with co-administration of selective serotonin re-uptake inhibitors (SSRIs), are outlined.

Design, synthesis and in vitro activity of 1,4-disubstituted piperazines and piperidines as triple reuptake inhibitors

Monoamine transporters regulate the concentration of monoamine neurotransmitters, which are essential for vital physiological processes, and their dysfunction can cause several central nervous system diseases. Monoamine transporters currently appear to be the potential target in the management of these disorders. In this study, homologation and bioisosterism techniques have been used in the designing of new 1,4-disubstituted piperazines and piperidines. These derivatives were synthesized and evaluated as potential triple reuptake inhibitors for studying the structure-activity relationships. The most advanced compound, 1-(4-(5-benzhydryl-1H-tetrazol-1-yl)butyl)-4-(3-phenylpropyl)piperazine (2i), was able to inhibit monoamine neurotransmitter reuptake in an in vitro test (IC₅₀=158.7nM for 5-HT, 99nM for NE and 97.5nM for DA). These novel potent triple reuptake inhibitor-based 1,4-disubstituted piperazine and piperidine scaffolds deserve further systematic optimization and pharmacological evaluation.

Corticosterone Potentiation of Cocaine-Induced Reinstatement of Conditioned Place Preference in Mice is Mediated by Blockade of the Organic Cation Transporter 3

The mechanisms by which stressful life events increase the risk of relapse in recovering cocaine addicts are not well understood. We previously reported that stress, via elevated corticosterone, potentiates cocaine-primed reinstatement of cocaine seeking following self-administration in rats and that this potentiation appears to involve corticosterone-induced blockade of dopamine clearance via the organic cation transporter 3 (OCT3). In the present study, we use a conditioned place preference/reinstatement paradigm in mice to directly test the hypothesis that corticosterone potentiates cocaine-primed reinstatement by blockade of OCT3. Consistent with our findings following self-administration in rats, pretreatment of male C57/BL6 mice with corticosterone (using a dose that reproduced stress-level plasma concentrations) potentiated cocaine-primed reinstatement of extinguished cocaine-induced conditioned place preference. Corticosterone failed to re-establish extinguished preference alone but produced a leftward shift in the dose-response curve for cocaine-primed reinstatement. A similar potentiating effect was observed upon pretreatment of mice with the non-glucocorticoid OCT3 blocker, normetanephrine. To determine the role of OCT3 blockade in these effects, we examined the abilities of corticosterone and normetanephrine to potentiate cocaine-primed reinstatement in OCT3-deficient and wild-type mice. Conditioned place preference, extinction and reinstatement of extinguished preference in response to low-dose cocaine administration did not differ between genotypes. However, corticosterone and normetanephrine failed to potentiate cocaine-primed reinstatement in OCT3-deficient mice. Together, these data provide the first direct evidence that the interaction of corticosterone with OCT3 mediates corticosterone effects on drug-seeking behavior and establish OCT3 function as an important determinant of susceptibility to cocaine use.

Ontogeny of Norepinephrine Transporter Expression and Antidepressant-Like Response to Desipramine in Wild-Type and Serotonin Transporter Mutant Mice

Depression is a major public health concern with symptoms that are often poorly controlled by treatment with common antidepressants. This problem is compounded in juveniles and adolescents, because therapeutic options are limited to selective serotonin reuptake inhibitors (SSRIs). Moreover, therapeutic benefits of SSRIs are often especially limited in certain subpopulations of depressed patients, including children and carriers of low-expressing serotonin transporter (SERT) gene variants. Tricyclic antidepressants (TCAs) offer an alternative to SSRIs; however, how age and SERT expression influence antidepressant response to TCAs is not understood. We investigated the relation between antidepressant-like response to the TCA desipramine using the tail suspension test and saturation binding of [3H]nisoxetine to the norepinephrine transporter (NET), the primary target of desipramine, in juvenile (21 days postnatal [P21]), adolescent (P28), and adult (P90) wild-type (SERT+/+) mice. To model carriers of low-expressing SERT gene variants, we used mice with reduced SERT expression (SERT+/-) or lacking SERT (SERT-/-). The potency and maximal antidepressant-like effect of desipramine was greater in P21 mice than in P90 mice and was SERT genotype independent. NET expression decreased with age in the locus coeruleus and increased with age in several terminal regions (e.g., the cornu ammonis CA1 and CA3 regions of the hippocampus). Binding affinity of [3H]nisoxetine did not vary as a function of age or SERT genotype. These data show age-dependent shifts for desipramine to produce antidepressant-like effects that correlate with NET expression in the locus coeruleus and suggest that drugs with NET-blocking activity may be an effective alternative to SSRIs in juveniles.

Contrasting Regulation of Catecholamine Neurotransmission in the Behaving Brain: Pharmacological Insights from an Electrochemical Perspective

Catecholamine neurotransmission plays a key role in regulating a variety of behavioral and physiologic processes, and its dysregulation is implicated in both neurodegenerative and neuropsychiatric disorders. Over the last four decades, in vivo electrochemistry has enabled the discovery of contrasting catecholamine regulation in the brain. These rapid and spatially resolved measurements have been conducted in brain slices, and in anesthetized and freely behaving animals. In this review, we describe the methods enabling in vivo measurements of dopamine and norepinephrine, and subsequent findings regarding their release and regulation in intact animals. We thereafter discuss key studies in awake animals, demonstrating that these catecholamines are not only differentially regulated, but are released in opposition of each other during appetitive and aversive stimuli.

Monoaminergic and Histaminergic Strategies and Treatments in Brain Diseases

The monoaminergic systems are the target of several drugs for the treatment of mood, motor and cognitive disorders as well as neurological conditions. In most cases, advances have occurred through serendipity, except for Parkinson's disease where the pathophysiology led almost immediately to the introduction of dopamine restoring agents. Extensive neuropharmacological studies first showed that the primary target of antipsychotics, antidepressants, and anxiolytic drugs were specific components of the monoaminergic systems. Later, some dramatic side effects associated with older medicines were shown to disappear with new chemical compounds targeting the origin of the therapeutic benefit more specifically. The increased knowledge regarding the function and interaction of the monoaminergic systems in the brain resulting from in vivo neurochemical and neurophysiological studies indicated new monoaminergic targets that could achieve the efficacy of the older medicines with fewer side-effects. Yet, this accumulated knowledge regarding monoamines did not produce valuable strategies for diseases where no monoaminergic drug has been shown to be effective. Here, we emphasize the new therapeutic and monoaminergic-based strategies for the treatment of psychiatric diseases. We will consider three main groups of diseases, based on the evidence of monoamines involvement (schizophrenia, depression, obesity), the identification of monoamines in the diseases processes (Parkinson's disease, addiction) and the prospect of the involvement of monoaminergic mechanisms (epilepsy, Alzheimer's disease, stroke). In most cases, the clinically available monoaminergic drugs induce widespread modifications of amine tone or excitability through neurobiological networks and exemplify the overlap between therapeutic approaches to psychiatric and neurological conditions. More recent developments that have resulted in improved drug specificity and responses will be discussed in this review.

Molecular fMRI of Serotonin Transport

Reuptake of neurotransmitters from the brain interstitium shapes chemical signaling processes and is disrupted in several pathologies. Serotonin reuptake in particular is important for mood regulation and is inhibited by first-line drugs for treatment of depression. Here we introduce a molecular-level fMRI technique for micron-scale mapping of serotonin transport in live animals. Intracranial injection of an MRI-detectable serotonin sensor complexed with serotonin, together with serial imaging and compartmental analysis, permits neurotransmitter transport to be quantified as serotonin dissociates from the probe. Application of this strategy to much of the striatum and surrounding areas reveals widespread nonsaturating serotonin removal with maximal rates in the lateral septum. The serotonin reuptake inhibitor fluoxetine selectively suppresses serotonin removal in septal subregions, whereas both fluoxetine and a dopamine transporter blocker depress reuptake in striatum. These results highlight promiscuous pharmacological influences on the serotonergic system and demonstrate the utility of molecular fMRI for characterization of neurochemical dynamics.

The plasma membrane monoamine transporter (PMAT): Structure, function, and role in organic cation disposition

Plasma membrane monoamine transporter (PMAT) is a new polyspecific organic cation transporter that transports a variety of biogenic amines and xenobiotic cations. Highly expressed in the brain, PMAT represents a major uptake₂ transporter for monoamine neurotransmitters. At the blood-cerebrospinal fluid (CSF) barrier, PMAT is the principal organic cation transporter for removing neurotoxins and drugs from the CSF. Here I summarize our latest understanding of PMAT and its roles in monoamine uptake and xenobiotic disposition.

SSRI Augmentation by 5-Hydroxytryptophan Slow Release: Mouse Pharmacodynamic Proof of Concept

Drugs, notably SSRIs, that elevate brain extracellular 5-HT (5-HTExt) are antidepressants. Unfortunately, most patients fail to remit. Multipronged clinical evidence suggests that elevating 5-HTExt beyond the SSRI effect enhances antidepressant efficacy, but previous such drug strategies had prohibitive limitations. In humans, adjunct treatment with the 5-HT precursor 5-hydroxytryptophan (5-HTP) elevates 5-HTExt beyond the SSRI effect. Small pilot trials suggest that adjunct 5-HTP can confer antidepressant response in treatment-resistant depression (TRD). However, sustained, stable 5-HTExt elevation is required for antidepressant effect; therefore, the rapid absorption and elimination of standard 5-HTP immediate release (IR) likely curtail 5-HTP IR's antidepressant potential. Slow-release (SR) drug delivery can crucially improve efficacy and safety of rapidly absorbed and eliminated compounds. Here we tested in mice the hypothesis that SR delivery will substantially improve 5-HTP's drug properties, by minimizing adverse effects and securing sustained 5-HTExt elevation beyond the SSRI effect. We modeled 5-HTP SR with minipumps, 5-HTP IR with injections, and chronic SSRI with dietary fluoxetine. We tested adjunct 5-HTP SR in wild-type mice and in mice with low brain 5-HT owing to expression of a mutant form of the brain 5-HT synthesis enzyme, tryptophan hydroxylase 2. In both lines of mice, adjunct 5-HTP SR synergized with SSRI to elevate 5-HTExt beyond the SSRI effect. We observed no adverse effect. Adjunct 5-HTP IR could not produce this therapy-like profile, producing transient 5-HTExt spikes and marked adverse effects. Integrated with a body of clinical data, our mouse data suggest that an adjunct 5-HTP SR drug could safely and effectively elevate 5-HTExt beyond the SSRI effect and represent a novel treatment for TRD.

Histamine Clearance Through Polyspecific Transporters in the Brain

Histamine plays an important role as a neurotransmitter in diverse brain functions, and clearance of histamine is essential to avoid excessive histaminergic neuronal activity. Histamine N-methyltransferase, which is an enzyme in the central nervous system that metabolizes histamine, is localized to the cytosol. This suggests that a histamine transport process is essential to inactivate histamine. Previous reports have shown the importance of astrocytes for histamine transport, although neuronal histamine transport could not be ruled out. High-affinity and selective histamine transporters have not yet been discovered, although it has been reported that the following three polyspecific transporters transport histamine: organic cation transporter (OCT) 2, OCT3, and plasma membrane monoamine transporter (PMAT). The K _m values of human OCT2, OCT3, and PMAT are 0.54, 0.64, and 4.4 mM, respectively. The three transporters are expressed in the brain, and their regional distribution is different. Recent studies revealed the contribution of OCT3 and PMAT to histamine transport by primary human astrocytes. Several investigations using mice supported the importance of OCT3 for histamine clearance in the brain. However, further studies are required to elucidate the detailed mechanism of histamine transport in the brain.

Differential Uptake Mechanisms of Fluorescent Substrates into Stem-Cell-Derived Serotonergic Neurons

The actions of the neurotransmitters serotonin, dopamine, and norepinephrine are partly terminated by diffusion and in part by their uptake into neurons via the selective, high-affinity transporters for serotonin (SERT), dopamine (DAT), and norepinephrine (NET), respectively. There is also growing evidence that all three monoamines are taken up into neurons by low-affinity, high-capacity organic cation transporters (OCT) and the plasma membrane monoamine transporter (PMAT). Pharmacological characterization of these low-affinity recombinant transporter proteins in heterologous expression systems has revealed that they are not antagonized by classical inhibitors of SERT, DAT, or NET but that decynium-22 (D22) antagonizes OCT3 and PMAT, whereas corticosterone and progesterone selectively inhibit OCT3. Here, we show that SERT, PMAT, and OCT3, but not OCT1 and OCT2, are coexpressed in murine stem cell-derived serotonergic neurons. Using selective antagonists, we provide evidence that uptake of the fluorescent substrates FFN511, ASP+, and 5-HT into stem cell-derived serotonergic neurons is mediated differentially by these transporters and also involves an as yet unknown transport mechanism.

Organic cation transporter 3 contributes to norepinephrine uptake into perivascular adipose tissue

Perivascular adipose tissue (PVAT) reduces vasoconstriction to norepinephrine (NE). A mechanism by which PVAT could function to reduce vascular contraction is by decreasing the amount of NE to which the vessel is exposed. PVATs from male Sprague-Dawley rats were used to test the hypothesis that PVAT has a NE uptake mechanism. NE was detected by HPLC in mesenteric PVAT and isolated adipocytes. Uptake of NE (10 μM) in mesenteric PVAT was reduced by the NE transporter (NET) inhibitor nisoxetine (1 μM, 73.68 ± 7.62%, all values reported as percentages of vehicle), the 5-hydroxytryptamine transporter (SERT) inhibitor citalopram (100 nM) with the organic cation transporter 3 (OCT3) inhibitor corticosterone (100 μM, 56.18 ± 5.21%), and the NET inhibitor desipramine (10 μM) with corticosterone (100 μM, 61.18 ± 6.82%). Aortic PVAT NE uptake was reduced by corticosterone (100 μM, 53.01 ± 10.96%). Confocal imaging of mesenteric PVAT stained with 4-[4-(dimethylamino)-styrl]-N-methylpyridinium iodide (ASP(+)), a fluorescent substrate of cationic transporters, detected ASP(+) uptake into adipocytes. ASP(+) (2 μM) uptake was reduced by citalopram (100 nM, 66.68 ± 6.43%), corticosterone (100 μM, 43.49 ± 10.17%), nisoxetine (100 nM, 84.12 ± 4.24%), citalopram with corticosterone (100 nM and 100 μM, respectively, 35.75 ± 4.21%), and desipramine with corticosterone (10 and 100 μM, respectively, 50.47 ± 5.78%). NET protein was not detected in mesenteric PVAT adipocytes. Expression of Slc22a3 (OCT3 gene) mRNA and protein in PVAT adipocytes was detected by RT-PCR and immunocytochemistry, respectively. These end points support the presence of a transporter-mediated NE uptake system within PVAT with a potential mediator being OCT3.

Potent and Selective Inhibition of Plasma Membrane Monoamine Transporter by HIV Protease Inhibitors

Plasma membrane monoamine transporter (PMAT) is a major uptake-2 monoamine transporter that shares extensive substrate and inhibitor overlap with organic cation transporters 1-3 (OCT1-3). Currently, there are no PMAT-specific inhibitors available that can be used in in vitro and in vivo studies to differentiate between PMAT and OCT activities. In this study, we showed that IDT307 (4-(4-(dimethylamino)phenyl)-1-methylpyridinium iodide), a fluorescent analog of 1-methyl-4-phenylpyridinium (MPP+), is a transportable substrate for PMAT and that IDT307-based fluorescence assay can be used to rapidly identify and characterize PMAT inhibitors. Using the fluorescent substrate-based assays, we analyzed the interactions of eight human immunodeficiency virus (HIV) protease inhibitors (PIs) with human PMAT and OCT1-3 in human embryonic kidney 293 (HEK293) cells stably transfected with individual transporters. Our data revealed that PMAT and OCTs exhibit distinct sensitivity and inhibition patterns toward HIV PIs. PMAT is most sensitive to PI inhibition whereas OCT2 and OCT3 are resistant. OCT1 showed an intermediate sensitivity and a distinct inhibition profile from PMAT. Importantly, lopinavir is a potent PMAT inhibitor and exhibited >120 fold selectivity toward PMAT (IC₅₀ = 1.4 ± 0.2 µM) over OCT1 (IC₅₀ = 174 ± 40 µM). Lopinavir has no inhibitory effect on OCT2 or OCT3 at maximal tested concentrations. Lopinavir also exhibited no or much weaker interactions with uptake-1 monoamine transporters. Together, our results reveal that PMAT and OCTs have distinct specificity exemplified by their differential interaction with HIV PIs. Further, we demonstrate that lopinavir can be used as a selective PMAT inhibitor to differentiate PMAT-mediated monoamine and organic cation transport from those mediated by OCT1-3.

Sex-related differences in small intestinal transit and serotonin dynamics in high-fat-diet-induced obesity in mice

Obesity alters gastrointestinal (GI) motility and 5-HT signalling. Altered 5-HT signalling disrupts control of GI motility. Levels of extracellular 5-HT depend on enterochromaffin (EC) cell release and serotonin transporter (SERT) uptake. We assessed GI transit and 5-HT signalling in the jejunum of normal and obese mice. Male and female mice were fed a control diet (CD; 10% of kilocalories as fat) or a high-fat diet (HFD; 60% of kilocalories as fat). Gastrointestinal transit was increased in male HFD-fed and female CD-fed compared with male CD-fed mice. The 5-HT3 receptor blocker, alosetron, increased gastric emptying in male CD-fed mice, but decreased transit in female CD-fed mice. The 5-HT-induced jejunal longitudinal muscle contractions in vitro were similar in all mice. In contrast to male CD-fed mice, 5-HT uptake (measured using continuous amperometry in vitro) in male HFD-fed mice was fluoxetine insensitive, yet sensitive to cocaine and the dopamine transporter (DAT) blocker GBR 12909. Immunoreactivity for DAT was present in the mucosa, and protein levels were greater in male HFD-fed compared with CD-fed mice. Extracellular 5-HT and mucosal 5-hydroxyindolacetic acid (5-HT metabolite) were similar in male HFD-fed compared with CD-fed mice. 5-Hydroxytryptamine uptake was fluoxetine sensitive in all females. Greater SERT protein, decreased extracellular 5-HT and greater mucosal 5-hydroxyindolacetic acid were observed in female HFD-fed compared with CD-fed mice. Mucosal 5-HT and EC cell numbers were similar in CD-fed and HFD-fed mice of both sexes; female 5-HT and EC cell numbers were increased compared with males. The HFD did not alter plasma sex hormone levels in any mice. Overall, obesity alters GI transit and 5-HT signalling in a sex-dependent manner.

An immunohistochemical analysis of SERT in the blood-brain barrier of the male rat brain

5-Hydroxytryptamine (5-HT) was originally discovered as a vasoconstrictor. 5-HT lowers blood pressure when administered peripherally to both normotensive and hypertensive male rats. Because the serotonin transporter (SERT) can function bidirectionally, we must consider whether 5-HT can be transported from the bloodstream to the central nervous system (CNS) in facilitating the fall in blood pressure. The blood-brain barrier (BBB) is a highly selective barrier that restricts movement of substances from the bloodstream to the CNS and vice versa, but the rat BBB has not been investigated in terms of SERT expression. This requires us to determine whether the BBB of the rat, the species in which we first observed a fall in blood pressure to infused 5-HT, expresses SERT. We hypothesized that SERT is present in the BBB of the male rat. To test this hypothesis, over 500 blood vessels were sampled from coronal slices of six male rat brains. Immunofluorescence of these coronal slices was used to determine whether SERT and RecA-1 (an endothelial cell marker) colocalized to the BBB. Blood vessels were considered to be capillaries if they were between 1.5 and 23 µm (intraluminal diameter). SERT was identified in the largest pial vessels of the BBB (mean ± SEM = 228.70 ± 18.71 µm, N = 9) and the smallest capillaries (mean ± SEM = 2.75 ± 0.12 µm, N = 369). SERT was not identified in the endothelium of blood vessels ranging from 20 to 135 µm (N = 45). The expression of SERT in the rat BBB means that 5-HT entry into the CNS must be considered a potential mechanism when investigating 5-HT-induced fall in blood pressure.

Dopaminergic, serotonergic, and noradrenergic deficits in Parkinson disease

OBJECTIVE

People with Parkinson disease (PD) frequently develop dementia, which is associated with neocortical deposition of alpha-synuclein (α-syn) in Lewy bodies and Lewy neurites. In addition, neuronal loss and deposition of aggregated α-syn also occur in multiple subcortical nuclei that project to neocortical, limbic, and basal ganglia regions. Therefore, we quantified regional deficits in innervation from these PD-affected subcortical nuclei, by measuring the neurotransmitters and neurotransmitter transporter proteins originating from projections of dopaminergic neurons in substantia nigra pars compacta, serotonergic neurons in dorsal raphé nuclei, noradrenergic neurons in locus coeruleus, and cholinergic neurons in nucleus basalis of Meynert.

METHODS

High-performance liquid chromatography and novel enzyme-linked immunosorbent assays were performed to quantify dopaminergic, serotonergic, noradrenergic, and cholinergic innervation in postmortem brain tissue. Eight brain regions from 15 PD participants (with dementia and Braak stage 6 α-syn deposition) and six age-matched controls were tested.

RESULTS

PD participants compared to controls had widespread reductions of dopamine transporter in caudate, amygdala, hippocampus, inferior parietal lobule (IPL), precuneus, and visual association cortex (VAC) that exceeded loss of dopamine, which was only significantly reduced in caudate and amygdala. In contrast, PD participants had comparable deficits of both serotonin and serotonin transporter in caudate, middle frontal gyrus, IPL, and VAC. PD participants also had significantly reduced norepinephrine levels for all eight brain regions tested. Vesicular acetylcholine transporter levels were only quantifiable in caudate and hippocampus and did not differ between PD and control groups.

INTERPRETATION

These results demonstrate widespread deficits in dopaminergic, serotonergic, and noradrenergic innervation of neocortical, limbic, and basal ganglia regions in advanced PD with dementia.

Multiple functions of neuronal plasma membrane neurotransmitter transporters

Removal from receptors of neurotransmitters just released into synapses is one of the major steps in neurotransmission. Transporters situated on the plasma membrane of nerve endings and glial cells perform the process of neurotransmitter (re)uptake. Because the density of transporters in the membranes can fluctuate, transporters can determine the transmitter concentrations at receptors, thus modulating indirectly the excitability of neighboring neurons. Evidence is accumulating that neurotransmitter transporters can exhibit multiple functions. Being bidirectional, neurotransmitter transporters can mediate transmitter release by working in reverse, most often under pathological conditions that cause ionic gradient dysregulations. Some transporters reverse to release transmitters, like dopamine or serotonin, when activated by 'indirectly acting' substrates, like the amphetamines. Some transporters exhibit as one major function the ability to capture transmitters into nerve terminals that perform insufficient synthesis. Transporter activation can generate conductances that regulate directly neuronal excitability. Synaptic and non-synaptic transporters play different roles. Cytosolic Na(+) elevations accompanying transport can interact with plasmalemmal or/and mitochondrial Na(+)/Ca(2+) exchangers thus generating calcium signals. Finally, neurotransmitter transporters can behave as receptors mediating releasing stimuli able to cause transmitter efflux through multiple mechanisms. Neurotransmitter transporters are therefore likely to play hitherto unknown roles in multiple therapeutic treatments.

Sex- and SERT-mediated differences in stimulated serotonin revealed by fast microdialysis

In vivo microdialysis is widely used to investigate how neurotransmitter levels in the brain respond to biologically relevant challenges. Here, we combined recent improvements in the temporal resolution of online sampling and analysis for serotonin with a brief high-K(+) stimulus paradigm to study the dynamics of evoked release. We observed stimulated serotonin overflow with high-K(+) pulses as short as 1 min when determined with 2-min dialysate sampling in ventral striatum. Stimulated serotonin levels in female mice during the high estrogen period of the estrous cycle were similar to serotonin levels in male mice. By contrast, stimulated serotonin overflow during the low estrogen period in female mice was increased to levels similar to those in male mice with local serotonin transporter (SERT) inhibition. Stimulated serotonin levels in mice with constitutive loss of SERT were considerably higher yet, pointing to neuroadaptive potentiation of serotonin release. When combined with brief K(+) stimulation, fast microdialysis reveals dynamic changes in extracellular serotonin levels associated with normal hormonal cycles and pharmacologic vs genetic loss of SERT function.

Pet-1 Controls Tetrahydrobiopterin Pathway and Slc22a3 Transporter Genes in Serotonin Neurons

Coordinated serotonin (5-HT) synthesis and reuptake depends on coexpression of Tph2, Aadc (Ddc), and Sert (Slc6a4) in brain 5-HT neurons. However, other gene products play critical roles in brain 5-HT synthesis and transport. For example, 5-HT synthesis depends on coexpression of genes encoding the enzymatic machinery necessary for the production and regeneration of tetrahydrobiopterin (BH4). In addition, the organic cation transporter 3 (Oct3, Slc22a3) functions as a low affinity, high capacity 5-HT reuptake protein in 5-HT neurons. The regulatory strategies controlling BH4 and Oct3 gene expression in 5-HT neurons have not been investigated. Our previous studies showed that Pet-1 is a critical transcription factor in a regulatory program that controls coexpression of Tph2, Aadc, and Sert in 5-HT neurons. Here, we investigate whether a common regulatory program determines global 5-HT synthesis and reuptake through coordinate transcriptional control. We show with comparative microarray profiling of flow sorted YFP(+) Pet-1(-/-) and wild type 5-HT neurons that Pet-1 regulates BH4 pathway genes, Gch1, Gchfr, and Qdpr. Thus, Pet-1 coordinates expression of all rate-limiting enzymatic (Tph2, Gch1) and post-translational regulatory (Gchfr) steps that determine the level of mammalian brain 5-HT synthesis. Moreover, Pet-1 globally controls acquisition of 5-HT reuptake in dorsal raphe 5-HT neurons by coordinating expression of Slc6a4 and Slc22a3. In situ hybridizations revealed that virtually all 5-HT neurons in the dorsal raphe depend on Pet-1 for Slc22a3 expression; similar results were obtained for Htr1a. Therefore, few if any 5-HT neurons in the dorsal raphe are resistant to loss of Pet-1 for their full neuron-type identity.

Environmental Enrichment Reduces Anxiety by Differentially Activating Serotonergic and Neuropeptide Y (NPY)-Ergic System in Indian Field Mouse (Mus booduga): An Animal Model of Post-Traumatic Stress Disorder

Exposure to a predator elicits an innate fear response and mimics several behavioral disorders related to post-traumatic stress disorder (PTSD). The protective role of an enriched condition (EC) against psychogenic stressors in various animal models has been well documented. However, this condition has not been tested in field mice in the context of PTSD. In this study, we show that field mice (Mus booduga) housed under EC exhibit predominantly proactive and less reactive behavior compared with mice housed under standard conditions (SC) during exposure to their natural predator (field rat Rattus rattus). Furthermore, we observed that EC mice displayed less anxiety-like behavior in an elevated plus maze (EPM) and light/dark-box after exposure to the predator (7 hrs/7 days). In EC mice, predator exposure elevated the level of serotonin (5-Hydroxytrypamine, [5-HT]) in the amygdala as part of the coping response. Subsequently, the serotonin transporter (SERT) and 5-HT_1A receptor were up-regulated significantly, but the same did not occur in the 5-HT_2C receptor, which is associated with the activation of calmodulin-dependent protein kinase-II (CaMKII) and a transcription factor cAMP response element binding protein (CREB). Our results show that predator exposure induced the activation of CaMKII/CREB, which is accompanied with increased levels of histone acetylation (H3, H4) and decreased histone deacetylases (HDAC1, 2). Subsequently, in the amygdala, the transcription of brain-derived neurotrophic factor (BDNF), neuropeptide Y (NPY) and its Y1 receptor were up-regulated, whereas the Y2 receptor was down-regulated. Therefore, EC facilitated a coping response against a fear associated cue in a PTSD animal model and reduced anxiety by differentially activating serotonergic and NPY-ergic systems.

Role of nitric oxide in the behavioral and neurochemical effects of IB-MECA in zebrafish

RATIONALE

The adenosine A3 receptor and the nitric oxide (NO) pathway regulate the function and localization of serotonin transporters (SERTs). These transporters regulate extracellular serotonin levels, which are correlated with defensive behavior.

OBJECTIVE

The purpose of this study was to understand the role of the A3AR on anxiety and arousal models in zebrafish, and whether this role is mediated by the nitrergic modulation of serotonin uptake.

METHODS

The effects of IB-MECA (0.01 and 0.1 mg/kg) were assessed in a series of behavioral tasks in adult zebrafish, as well as on extracellular serotonin levels in vivo and serotonin uptake in brain homogenates. Finally, the interaction between IB-MECA and drugs blocking voltage-dependent calcium channels (VDCCs), NO synthase, and SERT was analyzed.

RESULTS

At the lowest dose, IB-MECA decreased bottom dwelling and scototaxis, while at the highest dose, it also decreased shoaling, startle probability, and melanophore responses. These effects were accompanied by an increase in brain extracellular serotonin levels. IB-MECA also concentration-dependently increased serotonin uptake in vitro. The effects of IB-MECA on extracellular 5-HT, scototaxis, and geotaxis were blocked by L-NAME, while only the effects on 5-HT and scototaxis were blocked by verapamil. In vitro, the increase in 5-HT uptake was dependent on VDCCs and NO. Finally, fluoxetine blocked the effect of IB-MECA on scototaxis, but not geotaxis.

CONCLUSION

These results suggest that the effect of IB-MECA on scototaxis are mediated by a VDCC-NO-SERT pathway. While NO seems to mediate the effects of IB-MECA on geotaxis, neither VDCCs nor SERT seems to be involved in this process.

5-HT2C receptor desensitization moderates anxiety in 5-HTT deficient mice: from behavioral to cellular evidence

BACKGROUND

Desensitization and blockade of 5-HT2C receptors (5-HT2CR) have long been thought to be central in the therapeutic action of antidepressant drugs. However, besides behavioral pharmacology studies, there is little in vivo data documenting antidepressant-induced 5-HT2CR desensitization in specific brain areas.

METHODS

Mice lacking the 5-HT reuptake carrier (5-HTT(-/-)) were used to model the consequences of chronic 5-HT reuptake inhibition with antidepressant drugs. The effect of this mutation on 5-HT2CR was evaluated at the behavioral (social interaction, novelty-suppressed feeding, and 5-HT2CR-induced hypolocomotion tests), the neurochemical, and the cellular (RT-qPCR, mRNA editing, and c-fos-induced expression) levels.

RESULTS

Although 5-HTT(-/-) mice had an anxiogenic profile in the novelty-suppressed feeding test, they displayed less 5-HT2CR-mediated anxiety in response to the agonist m-chlorophenylpiperazine in the social interaction test. In addition, 5-HT2CR-mediated inhibition of a stress-induced increase in 5-HT turnover, measured in various brain areas, was markedly reduced in 5-HTT(-/-) mutants. These indices of tolerance to 5-HT2CR stimulation were associated neither with altered levels of 5-HT2CR protein and mRNA nor with changes in pre-mRNA editing in the frontal cortex. However, basal c-fos mRNA production in cells expressing 5-HT2CR was higher in 5-HTT(-/-) mutants, suggesting an altered basal activity of these cells following sustained 5-HT reuptake carrier inactivation. Furthermore, the increased c-fos mRNA expression in 5-HT2CR-like immune-positive cortical cells observed in wild-type mice treated acutely with the 5-HT2CR agonist RO-60,0175 was absent in 5-HTT(-/-) mutants.

CONCLUSIONS

Such blunted responsiveness of the 5-HT2CR system, observed at the cell signaling level, probably contributes to the moderation of the anxiety phenotype in 5-HTT(-/-) mice.

Integrating the monoamine, neurotrophin and cytokine hypotheses of depression--a central role for the serotonin transporter?

Monoamine, in particular serotonergic neurotransmission has long been recognized as an important factor in the aetiology of depression. The serotonin transporter (SERT) is the primary regulator of serotonin levels in the brain and a key target for widely used antidepressant drugs, such as selective serotonin reuptake inhibitors (SSRIs). In realising the limitations of current antidepressant therapy, depression research has branched out to encompass other areas such as synaptic plasticity, neurogenesis and brain structural remodelling as factors which influence mood and behaviour. More recently, the immune system has been implicated in the development of depression and various intriguing observations have inspired the cytokine hypothesis of depression. Over the past two decades evidence of in vitro and in vivo regulation of SERT function by pro-inflammatory cytokines as well as by mechanisms of synaptic plasticity has been accumulating, offering a mechanistic link between the monoamine, neurotrophin and cytokine theories of depression. This review will focus firstly on the interconnected roles of serotonin and neurotrophins in depression and antidepressant therapy, secondly on the impact of the immune system on serotonin transporter regulation and neurotrophin signalling and finally we propose a model of reciprocal regulation of serotonin and neurotrophin signalling in the context of inflammation-induced depression.

Serotonin in the ventral hippocampus modulates anxiety-like behavior during amphetamine withdrawal

Withdrawal from amphetamine is associated with increased anxiety and sensitivity to stressors which are thought to contribute to relapse. Rats undergoing amphetamine withdrawal fail to exhibit stress-induced increases in serotonin (5-HT) release in the ventral hippocampus and show heightened anxiety-like behaviors. Therefore, we tested the hypothesis that reducing 5-HT levels in the ventral hippocampus is a causal mechanism in increasing anxiety-like behaviors during amphetamine withdrawal. First, we tested whether reducing 5-HT levels in the ventral hippocampus directly increases anxiety behavior. Male rats were bilaterally infused with 5,7-dihydroxytryptamine (5,7-DHT) into the ventral hippocampus, which produced a 83% decrease in ventral hippocampus 5-HT content, and were tested on the elevated plus maze (EPM) for anxiety-like behavior. Reducing ventral hippocampus 5-HT levels decreased the time spent in the open arms of the maze, suggesting that diminished ventral hippocampus 5-HT levels increases anxiety-like behavior. Next, we tested whether increasing 5-HT levels in the ventral hippocampus reverses anxiety behavior exhibited by rats undergoing amphetamine withdrawal. Rats were treated daily with either amphetamine (2.5-mg/kg, i.p.) or saline for 2weeks, and at 2weeks withdrawal, were infused with the selective serotonin reuptake inhibitor paroxetine (0.5μM) bilaterally into the ventral hippocampus and tested for anxiety-like behavior on the EPM. Rats pre-treated with amphetamine exhibited increased anxiety-like behavior on the EPM. This effect was reversed by ventral hippocampus infusion of paroxetine. Our results suggest that 5-HT levels in the ventral hippocampus are critical for regulating anxiety behavior. Increasing 5-HT levels during withdrawal may be an effective strategy for reducing anxiety-induced drug relapse.

Serotonin and sensory nerves: meeting in the cardiovascular system

Blood pressure regulation by 5-HT has proven to be a complex story to unravel. The work by Cuesta et al., in this issue of Vascular Pharmacology adds another layer of complexity by providing sound in vivo data that 5-HT, through the 5-HT7 receptor, can inhibit the vasodepressor actions of the sensory nervous system and thereby promote blood pressure maintenance. This interaction of 5-HT with the sensory nervous system is inhibitory, whereas 5-HT is understood to be stimulatory in other systems. Moreover, activation of the 5-HT7 receptor has been linked to both reduction and elevation of blood pressure. These interactions are discussed in this mini-review, as are potential steps forward in understanding the interplay of 5-HT, the sensory nervous system and blood pressure.

Transendothelial Transport and Its Role in Therapeutics

Present review paper highlights role of BBB in endothelial transport of various substances into the brain. More specifically, permeability functions of BBB in transendothelial transport of various substances such as metabolic fuels, ethanol, amino acids, proteins, peptides, lipids, vitamins, neurotransmitters, monocarbxylic acids, gases, water, and minerals in the peripheral circulation and into the brain have been widely explained. In addition, roles of various receptors, ATP powered pumps, channels, and transporters in transport of vital molecules in maintenance of homeostasis and normal body functions have been described in detail. Major role of integral membrane proteins, carriers, or transporters in drug transport is highlighted. Both diffusion and carrier mediated transport mechanisms which facilitate molecular trafficking through transcellular route to maintain influx and outflux of important nutrients and metabolic substances are elucidated. Present review paper aims to emphasize role of important transport systems with their recent advancements in CNS protection mainly for providing a rapid clinical aid to patients. This review also suggests requirement of new well-designed therapeutic strategies mainly potential techniques, appropriate drug formulations, and new transport systems for quick, easy, and safe delivery of drugs across blood brain barrier to save the life of tumor and virus infected patients.