Modulation of serotonin transporter function by kappa-opioid receptor ligands

Resolution of tonic concentrations of highly similar neurotransmitters using voltammetry and deep learning

With advances in our understanding regarding the neurochemical underpinnings of neurological and psychiatric diseases, there is an increased demand for advanced computational methods for neurochemical analysis. Despite having a variety of techniques for measuring tonic extracellular concentrations of neurotransmitters, including voltammetry, enzyme-based sensors, amperometry, and in vivo microdialysis, there is currently no means to resolve concentrations of structurally similar neurotransmitters from mixtures in the in vivo environment with high spatiotemporal resolution and limited tissue damage. Since a variety of research and clinical investigations involve brain regions containing electrochemically similar monoamines, such as dopamine and norepinephrine, developing a model to resolve the respective contributions of these neurotransmitters is of vital importance. Here we have developed a deep learning network, DiscrimNet, a convolutional autoencoder capable of accurately predicting individual tonic concentrations of dopamine, norepinephrine, and serotonin from both in vitro mixtures and the in vivo environment in anesthetized rats, measured using voltammetry. The architecture of DiscrimNet is described, and its ability to accurately predict in vitro and unseen in vivo concentrations is shown to vastly outperform a variety of shallow learning algorithms previously used for neurotransmitter discrimination. DiscrimNet is shown to generalize well to data captured from electrodes unseen during model training, eliminating the need to retrain the model for each new electrode. DiscrimNet is also shown to accurately predict the expected changes in dopamine and serotonin after cocaine and oxycodone administration in anesthetized rats in vivo. DiscrimNet therefore offers an exciting new method for real-time resolution of in vivo voltammetric signals into component neurotransmitters.

Kappa Opioid Receptors Reduce Serotonin Uptake and Escitalopram Efficacy in the Mouse Substantia Nigra Pars Reticulata

The serotonin and kappa opioid receptor (KOR) systems are strongly implicated in disorders of negative affect, such as anxiety and depression. KORs expressed on axon terminals inhibit the release of neurotransmitters, including serotonin. The substantia nigra pars reticulata (SNr) is involved in regulating affective behaviors. It receives the densest serotonergic innervation in the brain and has high KOR expression; however, the influence of KORs on serotonin transmission in this region is yet to be explored. Here, we used ex vivo fast-scan cyclic voltammetry (FSCV) to investigate the effects of a KOR agonist, U50, 488 (U50), and a selective serotonin reuptake inhibitor, escitalopram, on serotonin release and reuptake in the SNr. U50 alone reduced serotonin release and uptake, and escitalopram alone augmented serotonin release and slowed reuptake, while pretreatment with U50 blunted both the release and uptake effects of escitalopram. Here, we show that the KOR influences serotonin signaling in the SNr in multiple ways and short-term activation of the KOR alters serotonin responses to escitalopram. These interactions between KORs and serotonin may contribute to the complexity in the responses to treatments for disorders of negative affect. Ultimately, the KOR system may prove to be a promising pharmacological target, alongside traditional antidepressant treatments.

Traumatic Stress-Induced Vulnerability to Addiction: Critical Role of the Dynorphin/Kappa Opioid Receptor System

Substance use disorders (SUD) may emerge from an individual’s attempt to limit negative affective states and symptoms linked to stress. Indeed, SUD is highly comorbid with chronic stress, traumatic stress, or post-traumatic stress disorder (PTSD), and treatments approved for each pathology individually often failed to have a therapeutic efficiency in such comorbid patients. The kappa-opioid receptor (KOR) and its endogenous ligand dynorphin (DYN), seem to play a key role in the occurrence of this comorbidity. The DYN/KOR function is increased either in traumatic stress or during drug use, dependence acquisition and DYN is released during stress. The behavioural effects of stress related to the DYN/KOR system include anxiety, dissociative and depressive symptoms, as well as increased conditioned fear response. Furthermore, the DYN/KOR system is implicated in negative reinforcement after the euphoric effects of a drug of abuse ends. During chronic drug consumption DYN/KOR functions increase and facilitate tolerance and dependence. The drug-seeking behaviour induced by KOR activation can be retrieved either during the development of an addictive behaviour, or during relapse after withdrawal. DYN is known to be one of the most powerful negative modulators of dopamine signalling, notably in brain structures implicated in both reward and fear circuitries. KOR are also acting as inhibitory heteroreceptors on serotonin neurons. Moreover, the DYN/KOR system cross-regulate with corticotropin-releasing factor in the brain. The sexual dimorphism of the DYN/KOR system could be the cause of the gender differences observed in patients with SUD or/and traumatic stress-related pathologies. This review underlies experimental and clinical results emphasizing the DYN/KOR system as common mechanisms shared by SUD or/and traumatic stress-related pathologies, and suggests KOR antagonist as a new pharmacological strategy to treat this comorbidity.

Duloxetine attenuates pain in association with downregulation of platelet serotonin transporter in patients with burning mouth syndrome and atypical odontalgia

OBJECTIVE

The aim of this study was evaluation of the association between severity of pain and expression of total or ubiquitinated serotonin transporter (SERT) protein in patients with burning mouth syndrome and atypical odontalgia (BMS/AO), who were treated by duloxetine.

METHODS

Patients with BMS/AO were assessed for severity of pain using the visual analog scale (VAS), and expression of total and ubiquitinated SERT protein in platelets before (baseline) and 12 weeks after duloxetine-treatment.

RESULTS

The expression of total and ubiquitinated SERT protein at baseline in all patients (n = 33) were higher and lower, respectively, compared to those in healthy controls. 12 weeks after duloxetine-treatment, there was no difference in the total SERT protein levels between patients (n = 21) and healthy controls. In the 16 patients who could be measured, mean VAS scores and total SERT protein levels were significantly decreased after the treatment, compared to those at baseline. There was tendency for a positive correlation between total SERT protein levels and VAS scores in these patients.

CONCLUSIONS

Our findings indicate that duloxetine relieves pain in association with downregulation of platelet SERT expression in patients with BMS/AO.

Amygdala dynorphin/κ opioid receptor system modulates depressive-like behavior in mice following chronic social defeat stress

Major depression disorder is a severe and recurrent neuropsychological disorder characterized by lowered mood and social activity and cognitive impairment. Owing to unclear molecular mechanisms of depression, limited interventions are available in clinic. In this study we investigated the role of dynorphin/κ opioid receptor system in the development of depression. Mice were subjected to chronic social defeat stress for 14 days. Chronic social defeat stress induced significant social avoidance in mice characterized by decreased time duration in the interaction zone and increased time duration in the corner zone. Pre-administration of a κ opioid receptor antagonist norBNI (10 mg/kg, i.p.) could prevent the development of social avoidance induced by chronic social defeat stress. Social avoidance was not observed in κ opioid receptor knockout mice subjected to chronic social defeat stress. We further revealed that social defeat stress activated c-fos and ERK signaling in the amygdala without affecting the NAc, hippocampus and hypothalamus, and ERK activation was blocked by systemic injection of norBNI. Finally, the expression of dynorphin A, the endogenous ligand of κ opioid receptor, was significantly increased in the amygdala following social defeat stress; microinjection of norBNI into the amygdala prevented the development of depressive-like behaviors caused by social defeat stress. The present study demonstrates that upregulated dynorphin/κ opioid receptor system in the amygdala leads to the emergence of depression following chronic social defeat stress, and sheds light on κ opioid receptor antagonists as potential therapeutic agents for the prevention and treatment of depression following chronic stress.

Kappa Opioid Receptor Mediated Differential Regulation of Serotonin and Dopamine Transporters in Mood and Substance Use Disorder

Dynorphin (DYN) is an endogenous neurosecretory peptide which exerts its activity by binding to the family of G protein-coupled receptors, namely the kappa opioid receptor (KOR). Opioids are associated with pain, analgesia, and drug abuse, which play a central role in mood disorders with monoamine neurotransmitter interactions. Growing evidence demonstrates the cellular signaling cascades linked to KOR-mediated monoamine transporters regulation in cell models and native brain tissues. This chapter will review DYN/KOR role in mood and addiction in relevance to dopaminergic and serotonergic neurotransmissions. Also, we discuss the recent findings on KOR-mediated differential regulation of serotonin and dopamine transporters (SERT and DAT). These findings led to a better understanding of the role of DYN/KOR system in aminergic neurotransmission via its modulatory effect on both amine release and clearance. Detailed knowledge of these processes at the molecular level enables designing novel pharmacological reagents to target transporter motifs to treat mood and addiction and reduce unwanted side effects such as aversion, dysphoria, sedation, and psychomimesis.

Novel Targets to Treat Depression: Opioid-Based Therapeutics

After participating in this activity, learners should be better able to:• Identify the effects of dysregulated opioid signalling in depression• Evaluate the use of opioid compounds and ketamine in patients with depression ABSTRACT: Major depressive disorder (MDD) remains one of the leading causes of disability and functional impairment worldwide. Current antidepressant therapeutics require weeks to months of treatment prior to the onset of clinical efficacy on depressed mood but remain ineffective in treating suicidal ideation and cognitive impairment. Moreover, 30%-40% of individuals fail to respond to currently available antidepressant medications. MDD is a heterogeneous disorder with an unknown etiology; novel strategies must be developed to treat MDD more effectively. Emerging evidence suggests that targeting one or more of the four opioid receptors-mu (MOR), kappa (KOR), delta (DOR), and the nociceptin/orphanin FQ receptor (NOP)-may yield effective therapeutics for stress-related psychiatric disorders. Furthermore, the effects of the rapidly acting antidepressant ketamine may involve opioid receptors. This review highlights dysregulated opioid signaling in depression, evaluates clinical trials with opioid compounds, and considers the role of opioid mechanisms in rapidly acting antidepressants.

Genetic deletion of Rgs12 in mice affects serotonin transporter expression and function in vivo and ex vivo

BACKGROUND

Regulator of G protein Signaling (RGS) proteins inhibit G protein-coupled receptor (GPCR) signaling, including the signals that arise from neurotransmitter release. We have shown that RGS12 loss diminishes locomotor responses of C57BL/6J mice to dopamine transporter (DAT)-targeting psychostimulants. This diminution resulted from a brain region-specific upregulation of DAT expression and function in RGS12-null mice. This effect on DAT prompted us to investigate whether the serotonin transporter (SERT) exhibits similar alterations upon RGS12 loss in C57BL/6J mice.

AIMS

Does RGS12 loss affect (a) hyperlocomotion to the preferentially SERT-targeting psychostimulant 3,4-methylenedioxymethamphetamine (MDMA), (b) SERT expression and function in relevant brain regions, and/or (c) serotonergically modulated behaviors?

METHODS

Open-field and spontaneous home-cage locomotor activities were quantified. 5-HT, 5-HIAA, and SERT levels in brain-region homogenates, as well as SERT expression and function in brain-region tissue preparations, were measured using appropriate biochemical assays. Serotonergically modulated behaviors were assessed using forced swim and tail suspension paradigms, elevated plus and elevated zero maze tests, and social interaction assays.

RESULTS

RGS12-null mice displayed no hyperlocomotion to 10 mg/kg MDMA. There were brain region-specific alterations in SERT expression and function associated with RGS12 loss. Drug-naïve RGS12-null mice displayed increases in both anxiety-like and anti-depressive-like behaviors.

CONCLUSION

RGS12 is a critical modulator of serotonergic neurotransmission and serotonergically modulated behavior in mice; lack of hyperlocomotion to low dose MDMA in RGS12-null mice is related to an alteration of steady-state SERT expression and 5-HT uptake.

Glycogen synthase kinase-3ß supports serotonin transporter function and trafficking in a phosphorylation-dependent manner

Serotonin (5-HT) transporter (SERT) plays a crucial role in serotonergic transmission in the central nervous system, and any aberration causes serious mental illnesses. Nevertheless, the cellular mechanisms that regulate SERT function and trafficking are not entirely understood. Growing evidence suggests that several protein kinases act as modulators. Here, we delineate the molecular mechanisms by which glycogen synthase kinase-3ß (GSK3ß) regulates SERT. When mouse striatal synaptosomes were treated with the GSK3α/ß inhibitor CHIR99021, we observed a significant increase in SERT function, V_max , surface expression with a reduction in 5-HT K_m and SERT phosphorylation. To further study how the SERT molecule is affected by GSK3α/ß, we used HEK-293 cells as a heterologous expression system. As in striatal synaptosomes, CHIR99021 treatment of cells expressing wild-type hSERT (hSERT-WT) resulted in a time and dose-dependent elevation of hSERT function with a concomitant increase in the V_max and surface transporters because of reduced internalization and enhanced membrane insertion; silencing GSK3α/ß in these cells with siRNA also similarly affected hSERT. Converting putative GSK3α/ß phosphorylation site serine at position 48 to alanine in hSERT (hSERT-S48A) completely abrogated the effects of both the inhibitor CHIR99021 and GSK3α/ß siRNA. Substantiating these findings, over-expression of constitutively active GSK3ß with hSERT-WT, but not with hSERT-S48A, reduced SERT function, V_max , surface density, and enhanced transporter phosphorylation. Both hSERT-WT and hSERT-S48A were inhibited similarly by PKC activation or by inhibition of Akt, CaMKII, p38 MAPK, or Src kinase. These findings provide new evidence that GSK3ß supports basal SERT function and trafficking via serine-48 phosphorylation.

Ex vivo Quantitative Proteomic Analysis of Serotonin Transporter Interactome: Network Impact of the SERT Ala56 Coding Variant

Altered serotonin (5-HT) signaling is associated with multiple brain disorders, including major depressive disorder (MDD), obsessive-compulsive disorder (OCD), and autism spectrum disorder (ASD). The presynaptic, high-affinity 5-HT transporter (SERT) tightly regulates 5-HT clearance after release from serotonergic neurons in the brain and enteric nervous systems, among other sites. Accumulating evidence suggests that SERT is dynamically regulated in distinct activity states as a result of environmental and intracellular stimuli, with regulation perturbed by disease-associated coding variants. Our lab identified a rare, hypermorphic SERT coding substitution, Gly56Ala, in subjects with ASD, finding that the Ala56 variant stabilizes a high-affinity outward-facing conformation (SERT^∗) that leads to elevated 5-HT uptake in vitro and in vivo. Hyperactive SERT Ala56 appears to preclude further activity enhancements by p38α mitogen-activated protein kinase (MAPK) and can be normalized by pharmacological p38α MAPK inhibition, consistent with SERT Ala56 mimicking, constitutively, a high-activity conformation entered into transiently by p38α MAPK activation. We hypothesize that changes in SERT-interacting proteins (SIPs) support the shift of SERT into the SERT^∗ state which may be captured by comparing the composition of SERT Ala56 protein complexes with those of wildtype (WT) SERT, defining specific interactions through comparisons of protein complexes recovered using preparations from SERT^–/– (knockout; KO) mice. Using quantitative proteomic-based approaches, we identify a total of 459 SIPs, that demonstrate both SERT specificity and sensitivity to the Gly56Ala substitution, with a striking bias being a loss of SIP interactions with SERT Ala56 compared to WT SERT. Among this group are previously validated SIPs, such as flotillin-1 (FLOT1) and protein phosphatase 2A (PP2A), whose functions are believed to contribute to SERT microdomain localization and regulation. Interestingly, our studies nominate a number of novel SIPs implicated in ASD, including fragile X mental retardation 1 protein (FMR1) and SH3 and multiple ankyrin repeat domains protein 3 (SHANK3), of potential relevance to long-standing evidence of serotonergic contributions to ASD. Further investigation of these SIPs, and the broader networks they engage, may afford a greater understanding of ASD as well as other brain and peripheral disorders associated with perturbed 5-HT signaling.

Histamine Receptors Regulate the Activity, Surface Expression, and Phosphorylation of Serotonin Transporters

Reuptake and clearance of released serotonin (5-HT) are critical in serotonergic neurotransmission. Serotonin transporter (SERT) is mainly responsible for clearing the extracellular 5-HT. Controlled trafficking, phosphorylation, and protein stability have been attributed to robust SERT activity. H₃ histamine receptors (H₃Rs) act in conjunction and regulate 5-HT release. H₃Rs are expressed in the nervous system and located at the serotonergic terminals, where they act as heteroreceptors. Although histaminergic and serotonergic neurotransmissions are thought to be two separate events, whether H₃Rs influence SERT in the CNS to control 5-HT reuptake has never been addressed. With a priori knowledge gained from our studies, we explored the possibility of using rat hippocampal synaptosomal preparations. We found that treatment with H₃R/H₄R-agonists immepip and (R)-(-)-α-methyl-histamine indeed resulted in a time- and concentration-dependent decrease in 5-HT transport. On the other hand, treatment with H₃R/H₄R-inverse agonist thioperamide caused a moderate increase in 5-HT uptake while blocking the inhibitory effect of H₃R/H₄R agonists. When investigated further, immepip treatment reduced the level of SERT on the plasma membrane and its phosphorylation. Likewise, CaMKII inhibitor KN93 or calcineurin inhibitor cyclosporine A also inhibited SERT function; however, an additive effect with immepip was not seen. High-speed in vivo chronoamperometry demonstrated that immepip delayed 5-HT clearance while thioperamide accelerated 5-HT clearance from the extracellular space. Immepip selectively inhibited SERT activity in the hippocampus and cortex but not in the striatum, midbrain, and brain stem. Thus, we report here a novel mechanism of regulating SERT activity by H₃R-mediated CaMKII/calcineurin pathway in a brain-region-specific manner and perhaps synaptic 5-HT in the CNS that controls 5-HT clearance.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Targeting opioid dysregulation in depression for the development of novel therapeutics

Since the serendipitous discovery of the first class of modern antidepressants in the 1950's, all pharmacotherapies approved by the Food and Drug Administration for major depressive disorder (MDD) have shared a common mechanism of action, increased monoaminergic neurotransmission. Despite the widespread availability of antidepressants, as many as 50% of depressed patients are resistant to these conventional therapies. The significant length of time required to produce meaningful symptom relief with these medications, 4-6 weeks, indicates that other mechanisms are likely involved in the pathophysiology of depression which may yield more viable targets for drug development. For decades, no viable candidate target with a different mechanism of action to that of conventional therapies proved successful in clinical studies. Now several exciting avenues for drug development are under intense investigation. One of these emerging targets is modulation of endogenous opioid tone. This review will evaluate preclinical and clinical evidence pertaining to opioid dysregulation in depression, focusing on the role of the endogenous ligands endorphin, enkephalin, dynorphin, and nociceptin/orphanin FQ (N/OFQ) and their respective receptors, mu (MOR), delta (DOR), kappa (KOR), and the N/OFQ receptor (NOP) in mediating behaviors relevant to depression and anxiety. Finally, putative opioid based antidepressants that are under investigation in clinical trials, ALKS5461, JNJ-67953964 (formerly LY2456302 and CERC-501) and BTRX-246040 (formerly LY-2940094) will be discussed. This review will illustrate the potential therapeutic value of targeting opioid dysregulation in developing novel therapies for MDD.

Dysbiosis contributes to chronic constipation development via regulation of serotonin transporter in the intestine

Chronic constipation is a prevalent functional gastrointestinal disorder accompanied with intestinal dysbiosis. However, causal relationship between dysbiosis and constipation remains poorly understood. Serotonin transporter (SERT) is a transmembrane transport protein which re-uptakes excessive 5-hydroxytryptamine (5-HT) from effective location to terminate its physiological effects and involves in regulating gastrointestinal motility. In this study, fecal microbiota from patients with constipation and healthy controls were transplanted into the antibiotic depletion mice model. The mice which received fecal microbiota from patients with constipation presented a reducing in intestinal peristalsis and abnormal defecation parameters including the frequency of pellet expulsion, fecal weight and fecal water content. After fecal microbiota transplantation, the SERT expression in the colonic tissue was significantly upregulated, and the content of 5-HT was decreased which negatively correlated with the gastrointestinal transit time. Moverover, fecal microbiota from the mice which received fecal microbiota from patients with constipation also upregulated SERT in Caco-2 cells. Besides, this process accompanied with the decreased abundance of Clostridium, Lactobacillus, Desulfovibrio, and Methylobacterium and an increased tend of Bacteroides and Akkermansia, which also involved in the impairment of intestinal barrier after FMT. Taken together, intestinal dysbiosis may upregulate the SERT expression and contribute to the development of chronic constipation.