Quantitation of 5HT3 receptors in forebrain of serotonin transporter deficient mice

5-HT Receptors and the Development of New Antidepressants

Serotonin modulates several physiological and cognitive pathways throughout the human body that affect emotions, memory, sleep, and thermal regulation. The complex nature of the serotonergic system and interactions with other neurochemical systems indicate that the development of depression may be mediated by various pathomechanisms, the common denominator of which is undoubtedly the disturbed transmission in central 5-HT synapses. Therefore, the deliberate pharmacological modulation of serotonergic transmission in the brain seems to be one of the most appropriate strategies for the search for new antidepressants. As discussed in this review, the serotonergic system offers great potential for the development of new antidepressant therapies based on the combination of SERT inhibition with different pharmacological activity towards the 5-HT system. The aim of this article is to summarize the search for new antidepressants in recent years, focusing primarily on the possibility of benefiting from interactions with various 5-HT receptors in the pharmacotherapy of depression.

Perturbed Developmental Serotonin Signaling Affects Prefrontal Catecholaminergic Innervation and Cortical Integrity

Proper development of the medial prefrontal cortex (mPFC), crucial for correct cognitive functioning, requires projections from, among others, the serotonergic (5-HT) and catecholaminergic systems, but it is unclear how these systems influence each other during development. Here, we describe the parallel development of the 5-HT and catecholaminergic prefrontal projection systems in rat and demonstrate a close engagement of both systems in the proximity of Cajal-Retzius cells. We further show that in the absence of the 5-HT transporter (5-HTT), not only the developing 5-HT but also the catecholaminergic system, including their projections towards the mPFC, are affected. In addition, the layer identity of the mPFC neurons and reelin-positive interneuron number and integration are altered in the absence of the 5-HTT. Together, our data demonstrate a functional interplay between the developing mPFC 5-HT and catecholaminergic systems, and call for a holistic approach in studying neurotransmitter systems-specific developmental consequences for adult behavior, to eventually allow the design of better treatment strategies for neuropsychiatric disorders.

Meta-analysis of the interaction between serotonin transporter promoter variant, stress, and posttraumatic stress disorder

Exposure to stress predicts the occurrence of posttraumatic stress disorder (PTSD) in individuals harboring the serotonin transporter promoter variant 5-HTTLPR. We carried out a meta-analysis of studies investigating the interaction between 5-HTTLPR, stress, and PTSD to clarify the interrelatedness of these factors. We reviewed all relevant studies published in English before May 2016. The Lipták-Stouffer z-score method for meta-analysis was applied to combined data. The z score was separately calculated for the stressful life events, childhood adversity, bi- and triallelic loci, and cross-sectional and longitudinal studies subgroups. A total of 14 studies with 15,883 subjects met our inclusion criteria. We found strong evidence that the presence of 5-HTTLPR influenced the relationship between stress and PTSD (P = 0.00003), with the strongest effects observed in the cross-sectional and longitudinal groups (P = 0.01 and 2.0 × 10-6, respectively). Stressful life events and childhood adversity separately interacted with 5-HTTLPR in PTSD (P = 2.0 × 10-8 and 0.003, respectively). When the studies were stratified by locus classification, the evidence was stronger for the triallelic (P = 4.0 × 10-8) than for the biallelic (P = 0.054) locus subgroup. There was strong evidence that 5-HTTLPR influences the relationship between stress and PTSD.

5HT3 receptors: Target for new antidepressant drugs

5HT3 receptors (5HT3Rs) have long been identified as a potential target for antidepressants. Several studies have reported that antagonism of 5HT3Rs produces antidepressant-like effects. However, the exact role of 5HT3Rs and the mode of antidepressant action of 5HT3R antagonists still remain a mystery. Here, we provide a comprehensive overview of 5HT3Rs: (a) regional and subcellular distribution of 5HT3Rs in discrete brain regions, (b) preclinical and clinical evidence supporting the antidepressant effect of 5HT3R antagonists, and (c) neurochemical, biological and neurocellular signaling pathways associated with the antidepressant action of 5HT3R antagonists. 5HT3Rs located on the serotonergic and other neurotransmitter interneuronal projections control their release and affect mood and emotional behavior; however, new evidence suggests that apart from modulating the neurotransmitter functions, 5HT3R antagonists have protective effects in the pathogenic events including hypothalamic-pituitary-adrenal-axis hyperactivity, brain oxidative stress and impaired neuronal plasticity, pointing to hereby unknown and novel mechanisms of their antidepressant action. Nonetheless, further investigations are warranted to establish the exact role of 5HT3Rs in depression and antidepressant action of 5HT3R antagonists.

Abnormal cell-intrinsic and network excitability in the neocortex of serotonin-deficient Pet-1 knockout mice

Neurons originating from the raphe nuclei of the brain stem are the exclusive source of serotonin (5-HT) to the cortex. Their serotonergic phenotype is specified by the transcriptional regulator Pet-1, which is also necessary for maintaining their neurotransmitter identity across development. Transgenic mice in which Pet-1 is genetically ablated (Pet-1(-/-)) show a dramatic reduction (∼80%) in forebrain 5-HT levels, yet no investigations have been carried out to assess the impact of such severe 5-HT depletion on the function of target cortical neurons. Using whole cell patch-clamp methods, two-dimensional (2D) multielectrode arrays (MEAs), 3D morphological neuronal reconstructions, and animal behavior, we investigated the impact of 5-HT depletion on cortical cell-intrinsic and network excitability. We found significant changes in several parameters of cell-intrinsic excitability in cortical pyramidal cells (PCs) as well as an increase in spontaneous synaptic excitation through 5-HT3 receptors. These changes are associated with increased local network excitability and oscillatory activity in a 5-HT2 receptor-dependent manner, consistent with previously reported hypersensitivity of cortical 5-HT2 receptors. PC morphology was also altered, with a significant reduction in dendritic complexity that may possibly act as a compensatory mechanism for increased excitability. Consistent with this interpretation, when we carried out experiments with convulsant-induced seizures to asses cortical excitability in vivo, we observed no significant differences in seizure parameters between wild-type and Pet-1(-/-) mice. Moreover, MEA recordings of propagating field potentials showed diminished propagation of activity across the cortical sheath. Together these findings reveal novel functional changes in neuronal and cortical excitability in mice lacking Pet-1.

Association and interaction analyses of 5-HT3 receptor and serotonin transporter genes with alcohol, cocaine, and nicotine dependence using the SAGE data

Previous studies have implicated genes encoding the 5-HT3AB receptors (HTR3A and HTR3B) and the serotonin transporter (SLC6A4), both independently and interactively, in alcohol (AD), cocaine (CD), and nicotine dependence (ND). However, whether these genetic effects also exist in subjects with comorbidities remains largely unknown. We used 1,136 African-American (AA) and 2,428 European-American (EA) subjects from the Study of Addiction: Genetics and Environment (SAGE) to determine associations between 88 genotyped or imputed variants within HTR3A, HTR3B, and SLC6A4 and three types of addictions, which were measured by DSM-IV diagnoses of AD, CD, and ND and the Fagerström Test for Nicotine Dependence (FTND), an independent measure of ND commonly used in tobacco research. Individual SNP-based association analysis revealed a significant association of rs2066713 in SLC6A4 with FTND in AA (β = -1.39; P = 1.6E - 04). Haplotype-based association analysis found one major haplotype formed by SNPs rs3891484 and rs3758987 in HTR3B that was significantly associated with AD in the AA sample, and another major haplotype T-T-G, formed by SNPs rs7118530, rs12221649, and rs2085421 in HTR3A, which showed significant association with FTND in the EA sample. Considering the biologic roles of the three genes and their functional relations, we used the GPU-based Generalized Multifactor Dimensionality Reduction (GMDR-GPU) program to test SNP-by-SNP interactions within the three genes and discovered two- to five-variant models that have significant impacts on AD, CD, ND, or FTND. Interestingly, most of the SNPs included in the genetic interaction model(s) for each addictive phenotype are either overlapped or in high linkage disequilibrium for both AA and EA samples, suggesting these detected variants in HTR3A, HTR3B, and SLC6A4 are interactively contributing to etiology of the three addictive phenotypes examined in this study.

The complex role of the serotonin transporter in adult neurogenesis and neuroplasticity. A critical review

OBJECTIVES

Studies on the serotonin transporter (SERT) with regard to neurogenesis and neuroplastic effects on the adult brain are scarce. This is intriguing since neurogenesis is believed to play a decisive role in modulating the effect of selective serotonin reuptake inhibitors (SSRI), which are targeting SERT.

METHODS

Therefore, we reviewed the current scientific literature about the influence of serotonin on neurogenesis with particular emphasis on SERT in various settings, both in vivo and in vitro.

RESULTS

Experiments using SERT KO (knock-out) animal models showed that SERT does not directly or indirectly influence neurogenesis in vitro, whereas compensatory mechanism seem to participate in vivo.

CONCLUSION

At least with regard to adult neural stem cells, the impact of serotonin (5-HT) on neuroplasticity and neurogenesis is not due to SERT-mediated effcts. Instead, serotonergic fine-tuning may be exerted by a number of other different mechanisms including endogenous production of 5-HT in adult neural stem cells, uptake of 5-HT into adult neural stem cells by other monoamine transporters, and actions of the 5-HT1A receptors present on these cells.

Involvement of nitric oxide in granisetron improving effect on scopolamine-induced memory impairment in mice

Granisetron, a serotonin 5-HT(3) receptor antagonist, widely used as an antiemetic drug following chemotherapy, has been found to improve learning and memory. In this study, effects of granisetron on spatial recognition memory and fear memory and the involvement of nitric oxide (NO) have been determined in a Y-maze and passive avoidance test. Granisetron (3, 10mg/kg, intraperitoneally) was administered to scopolamine-induced memory-impaired mice prior to acquisition, consolidation and retrieval phases, either in the presence or in the absence of a non-specific NO synthase inhibitor, l-NAME (3, 10mg/kg, intraperitoneally); a specific inducible NO synthase (iNOS) inhibitor, aminoguanidine (100mg/kg); and a NO precursor, l-arginine (750 mg/kg). It is demonstrated that granisetron improved memory acquisition in a dose-dependent manner, but it was ineffective on consolidation and retrieval phases of memory. The beneficial effect of granisetron (10mg/kg) on memory acquisition was significantly reversed by l-NAME (10mg/kg) and aminoguanidine (100mg/kg); however, l-arginine (750 mg/kg) did not potentiate the effect of sub-effective dose of granisetron (3mg/kg) in memory acquisition phase. It is concluded that nitric oxide is probably involved in improvement of memory acquisition by granisetron in both spatial recognition memory and fear memory. This article is part of a Special Issue entitled The Cognitive Neuroscience.

Psychotropic and nonpsychotropic cannabis derivatives inhibit human 5-HT(3A) receptors through a receptor desensitization-dependent mechanism

Δ⁹ tetrahydrocannabinol (THC) and cannabidiol (CBD) are the principal psychoactive and nonpsychoactive components of cannabis. While most THC-induced behavioral effects are thought to depend on endogenous cannabinoid 1 (CB1) receptors, the molecular targets for CBD remain unclear. Here, we report that CBD and THC inhibited the function of human 5-HT(3A) receptors (h5-HT(3A)Rs) expressed in HEK 293 cells. The magnitude of THC and CBD inhibition was maximal 5 min after a continuous incubation with cannabinoids. The EC₅₀ values for CBD and THC-induced inhibition were 110 nM and 322 nM, respectively in HEK 293 cells expressing h5-HT(3A)Rs. In these cells, CBD and THC did not stimulate specific [³⁵S]-GTP-γs binding in membranes, suggesting that the inhibition by cannabinoids is unlikely mediated by a G-protein dependent mechanism. On the other hand, both CBD and THC accelerated receptor desensitization kinetics without significantly changing activation time. The extent of cannabinoid inhibition appeared to depend on receptor desensitization. Reducing receptor desensitization by nocodazole, 5-hydroxyindole and a point-mutation in the large cytoplasmic domain of the receptor significantly decreased CBD-induced inhibition. Similarly, the magnitude of THC and CBD-induced inhibition varied with the apparent desensitization rate of h5-HT(3A)Rs expressed in Xenopus oocytes. For instance, with increasing amount of h5-HT(3A)R cRNA injected into the oocytes, the receptor desensitization rate at steady state decreased. THC and CBD-induced inhibition was correlated with the change in the receptor desensitization rate. Thus, CBD and THC inhibit h5-HT(3A) receptors through a mechanism that is dependent on receptor desensitization.

Genetic variation in cortico-amygdala serotonin function and risk for stress-related disease

The serotonin system is strongly implicated in the pathophysiology and therapeutic alleviation of stress-related disorders such as anxiety and depression. Serotonergic modulation of the acute response to stress and the adaptation to chronic stress is mediated by a myriad of molecules controlling serotonin neuron development (Pet-1), synthesis (tryptophan hydroxylase 1 and 2 isozymes), packaging (vesicular monoamine transporter 2), actions at presynaptic and postsynaptic receptors (5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C, 5-HT3A, 5-HT4, 5-HT5A, 5-HT6, 5-HT7), reuptake (serotonin transporter), and degradation (monoamine oxidase A). A growing body of evidence from preclinical rodents models, and especially genetically modified mice and inbred mouse strains, has provided significant insight into how genetic variation in these molecules can affect the development and function of a key neural circuit between the dorsal raphe nucleus, medial prefrontal cortex and amygdala. By extension, such variation is hypothesized to have a major influence on individual differences in the stress response and risk for stress-related disease in humans. The current article provides an update on this rapidly evolving field of research.

A pharmacological analysis of mice with a targeted disruption of the serotonin transporter

RATIONALE

Partial or complete ablation of serotonin transporter (SERT) expression in mice leads to altered responses to serotonin receptor agonists and other classes of drugs.

OBJECTIVES

In the current report, we review and integrate many of the major behavioral, physiological, and neurochemical findings in the current literature regarding pharmacological assessments made in SERT mutant mice.

RESULTS

The absence of normal responses to serotonin reuptake inhibiting (SRI) antidepressants in SERT knockout (-/-) mice demonstrates that actions on SERT are a critical principle mechanism of action of members of this class of antidepressants. Drugs transported by SERT, (+)-3,4-methylenedioxymethamphetamine (MDMA) and 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH(2)-MPTP), are also inactive in SERT -/- mice. Temperature, locomotor, and electrophysiological responses to various serotonin receptor agonists, including 8-hydroxy-2-(di-n-propylamino)-tetraline (8-OH-DPAT), ipsapirone, and RU24969, are reduced in SERT -/- mice, despite comparatively lesser reductions in Htr1a and Htr1b binding sites, G-proteins, and other signaling molecules. SERT -/- mice exhibit an approximately 90% reduction in head twitches in response to the Htr2a/2c agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), associated with a profound reduction in arachidonic acid signaling, yet only modest changes in Htr2a and Htr2c binding sites. SERT -/- mice also exhibit altered behavioral responses to cocaine and ethanol, related to abnormal serotonin, and possibly dopamine and norepinephrine, homeostasis.

CONCLUSIONS

Together, these studies demonstrate a complex and varied array of modified drug responses after constitutive deletion of SERT and provide insight into the role of serotonin, and in particular, its transporter, in the modulation of complex behavior and in the pharmacological actions of therapeutic agents and drugs of abuse.

Transmission disequilibrium analysis of the functional 5-HT3A receptor variant C178T in early-onset obsessive compulsive-disorder

The 5-HT(3) receptor is unique among the serotonin receptors in that it is a ligand-gated ion channel. Dysfunction of the serotonin system is thought to contribute to the pathogenesis of obsessive-compulsive disorder (OCD). Apart from the standard treatment with serotonin reuptake inhibitors and behavioural therapy, a 5-HT(3) receptor antagonist has recently been shown to benefit some OCD patients, suggesting the 5-HT(3) receptor as a serotonergic candidate gene in the polygenic aetiology of OCD. A functional regulatory variant of the 5-HT(3A) receptor influences 5-HT(3) receptor expression, serotonin metabolites in cerebrospinal fluid, and amygdala reactivity. We therefore assessed whether this C178T variant influences the risk of developing OCD. In a family-based approach employing the transmission disequilibrium test, we analysed a unique sample of 75 children and adolescents with OCD, as well as their biological parents. We found no evidence for a preferential transmission of either allele to the patients--the estimated transmission rate for the C allele was 0.51 (95% CI 0.36-0.65). This argues against an involvement of the 5-HT(3A) receptor in the polygenic aetiology of early-onset OCD.

Adult neurogenesis in serotonin transporter deficient mice

Serotonin (5-HT) is a regulator of morphogenetic activities during early brain development and neurogenesis, including cell proliferation, migration, differentiation, and synaptogenesis. The 5-HT transporter (5-HTT, SLC6A4) mediates high-affinity reuptake of 5-HT into presynaptic terminals and thereby fine-tunes serotonergic neurotransmission. Inactivation of the 5-HTT gene in mice reduces 5-HT clearance resulting in persistently increased concentrations of synaptic 5-HT. In the present study, we investigated the effects of elevated 5-HT levels on adult neurogenesis in the hippocampus of 5-HTT deficient mice, including stem cell proliferation, survival, and differentiation. Using an in vivo approach, we showed an increase in proliferative capacity of hippocampal adult neural stem cells in aged 5-HTT knockout mice (approximately 14.5 months) compared to wildtype controls. In contrast, in vivo and additional in vitro analyses of younger adult 5-HTT knockout mice (approximately 7 weeks and approximately 3.0 months) did not reveal significant changes in proliferation of neural stem cells or survival of newborn cells. We showed that the cellular fate of newly generated cells in 5-HTT knockout mice is not different with respect to the total number and percentage of neurons or glial cells from wildtype controls. Our findings indicate that elevated synaptic 5-HT concentration throughout early development and later life of 5-HTT deficient mice does not induce adult neurogenesis in adult mice, but that elevated 5-HT levels in aged mice influence stem cell proliferation.

Mechanical, thermal and formalin-induced nociception is differentially altered in 5-HT1A-/-, 5-HT1B-/-, 5-HT2A-/-, 5-HT3A-/- and 5-HTT-/- knock-out male mice

Extensive studies in rodents suggest that serotonin (5-HT) modulates nociceptive responses through the stimulation of several receptor types. However, it remains to demonstrate that these receptors participate in the control of nociception under physiological conditions. Pain behaviors of mutants which do not express 5-HT1A, 5-HT1B, 5-HT2A or 5-HT3A receptors, or lacking the 5-HT transporter, compared to paired wild-type mice of the same genetic background, were examined using validated tests based on different sensory modalities. Mechanical (von Frey filaments, tail pressure, tail clip tests), thermal (radiant heat, 46 degrees C water bath, hot-plate test) and formalin-induced nociception were determined in 2- to 3-month-old males. 5-HT1A knock-out mice differed from wild-types by higher thermal sensitivity (hot-plate test only), and 5-HT1B knock-out mice by higher thermal and formalin sensitivity. Both 5-HT2A and 5-HT3A knock-out mice differed from wild-types by a dramatic decrease in the formalin-induced nociceptive responses for phase II (16-45 min after injection/inflammatory phase). In contrast, neither mechanical, thermal nor formalin-induced nociception differed between mutants lacking the 5-HT transporter and paired wild-type mice. Although differences in spontaneous locomotor activity in 5-HT1B-/- (increase) and 5-HT3A-/- (decrease) knock-out mice versus paired wild-types might have confounded differences in nociception, acute 5-HT receptor blockade by selective antagonists was found to replicate in wild-type mice the effects on pain behavior, but not on locomotor activity, of the respective gene knock-out in mutants. These results support the conclusion that the complex control of pain mechanisms by 5-HT, acting at multiple receptors, is physiologically relevant in mice.

Cellular and molecular alterations in mice with deficient and reduced serotonin transporters

The function of serotonin transporters (SERTs) is related to mood regulation. Mice with deficient or reduced SERT function (SERT knockout mice) show several behavioral changes, including increased anxiety-like behavior, increased sensitivity to stress, and decreases in aggressive behavior. Some of these behavioral alterations are similar to phenotypes found in humans with short alleles of polymorphism in the 5-hydroxytryptamine (5-HT) transporter-linked promoter region (5-HTTLPR). Therefore, SERT knockout mice can be used as a tool to study 5-HTTLPR-related variations in personality and may be the etiology of affective disorders. This article focuses on the cellular and molecular alterations in SERT knockout mice, including changes in 5-HT concentrations and its metabolism, alterations in 5-HT receptors, impaired hypothalamic-pituitary-adrenal gland axis, developmental changes in the neurons and brain, and influence on other neurotransmitter transporters and receptors. It also discusses the possible relationships between these alterations and the behavioral changes in these mice. The knowledge provides the foundation for understanding the cellular and molecular mechanisms that mediate the SERT-related mood regulation, which may have significant impact on understanding the etiology of affective disorders and developing better therapeutic approaches for affective disorders.

Inactivation of 5HT transport in mice: modeling altered 5HT homeostasis implicated in emotional dysfunction, affective disorders, and somatic syndromes

Animal models have not only become an essential tool for investigating the neurobiological function of genes that are involved in the etiopathogenesis of human behavioral and psychiatric disorders but are also fundamental in the development novel therapeutic strategies. As an example, inactivation of the serotonin (5HT) transporter (5Htt, Slc6a4) gene in mice expanded our view of adaptive 5HT uptake regulation and maintenance of 5HT homeostasis in the developing human brain and molecular processes underlying anxiety-related traits, as well as affective spectrum disorders including depression. 5Htt-deficient mice have been employed as a model complementary to direct studies of genetically complex traits and disorders, with important findings in biochemical, morphological, behavioral, and pharmacological areas. Based on growing evidence for a critical role of the 5HTT in the integration of synaptic connections in the rodent, nonhuman primate, and human brain during critical periods of development and adult life, more in-depth knowledge of the molecular mechanisms implicated in these fine-tuning processes is currently evolving. Moreover, demonstration of a joint influence of the 5HTT variation and environmental sources during early brain development advanced our understanding of the mechanism of genexgene and genexenvironment interactions in the developmental neurobiology of anxiety and depression. Lastly, imaging techniques, which become increasingly elaborate in displaying the genomic influence on brain system activation in response to environmental cues, have provided the means to bridge the gap between small effects of 5HTT variation and complex behavior, as well as psychopathological dimensions. The combination of elaborate genetic, epigenetic, imaging, and behavioral analyses will continue to generate new insight into 5HTT's role as a master control gene of emotion regulation.