Transmembrane signaling in the brain by serotonin, a key regulator of physiology and emotion

Contribution of the serotonergic system to developmental brain abnormalities in autism spectrum disorder

This review highlights a key role of the serotonergic system in brain development and in distortions of normal brain development in early stages of fetal life resulting in cascades of abnormalities, including defects of neurogenesis, neuronal migration, neuronal growth, differentiation, and arborization, as well as defective neuronal circuit formation in the cortex, subcortical structures, brainstem, and cerebellum of autistic subjects. In autism, defects in regulation of neuronal growth are the most frequent and ubiquitous developmental changes associated with impaired neuron differentiation, smaller size, distorted shape, loss of spatial orientation, and distortion of cortex organization. Common developmental defects of the brain in autism include multiregional focal dysplastic changes contributing to local neuronal circuit distortion, epileptogenic activity, and epilepsy. There is a discrepancy between more than 500 reports demonstrating the contribution of the serotonergic system to autism's behavioral anomalies, highlighted by lack of studies of autistic subjects' brainstem raphe nuclei, the center of brain serotonergic innervation, and of the contribution of the serotonergic system to the diagnostic features of autism spectrum disorder (ASD). Discovery of severe fetal brainstem auditory system neuronal deficits and other anomalies leading to a spectrum of hearing deficits contributing to a cascade of behavioral alterations, including deficits of social and verbal communication in individuals with autism, is another argument to intensify postmortem studies of the type and topography of, and the severity of developmental defects in raphe nuclei and their contribution to abnormal brain development and to the broad spectrum of functional deficits and comorbid conditions in ASD.

5-HTP inhibits eosinophilia via intracellular endothelial 5-HTRs; SNPs in 5-HTRs associate with asthmatic lung function

Background

Previous research showed that 5-hydroxytryptophan (5HTP), a metabolic precursor of serotonin, reduces allergic lung inflammation by inhibiting eosinophil migration across endothelial monolayers.

Objective

It is unknown if serotonin receptors are involved in mediating this 5HTP function or if serotonin receptor (HTR) single nucleotide polymorphisms (SNPs) associate with lung function in humans.

Methods

Serotonin receptor subtypes were assessed by qPCR, western blot, confocal microscopy, pharmacological inhibitors and siRNA knockdown. HTR SNPs were assessed in two cohorts.

Results

Pharmacological inhibition or siRNA knockdown of the serotonin receptors HTR1A or HTR1B in endothelial cells abrogated the inhibitory effects of 5HTP on eosinophil transendothelial migration. In contrast, eosinophil transendothelial migration was not inhibited by siRNA knockdown of HTR1A or HTR1B in eosinophils. Surprisingly, these HTRs were intracellular in endothelial cells and an extracellular supplementation with serotonin did not inhibit eosinophil transendothelial migration. This is consistent with the inability of serotonin to cross membranes, the lack of selective serotonin reuptake receptors on endothelial cells, and the studies showing minimal impact of selective serotonin reuptake inhibitors on asthma. To extend our HTR studies to humans with asthma, we examined the CHIRAH and GALA cohorts for HTR SNPs that affect HTR function or are associated with behavior disorders. A polygenic index of SNPs in HTRs was associated with lower lung function in asthmatics.

Conclusions

Serotonin receptors mediate 5HTP inhibition of transendothelial migration and HTR SNPs associate with lower lung function. These results may serve to aid in design of novel interventions for allergic inflammation.

Possible Role of 5-Hydroxytryptamine (5-HT) Receptor on Human Sperm Motility Regulation

The purpose of this study is to examine whether 5-hydroxytryptamine (5-HT, also known as serotonin) regulates human sperm motility, focusing on 5-HT receptors. Immunofluorescent staining revealed the existence of seven types of 5-HT receptors with a heterogeneous pattern of reactive sites. In detail, 5-HT1B, 5-HT6, and 5-HT7 were detected in the post-acrosomal and mid-piece regions. The 5-HT2A and 5-HT5A receptors were mainly localized in the equatorial segment. 5-HT3A and 5-HT4 receptors were present in the neck and post-acrosomal regions. When examining the effects of 5-HT receptor antagonists on sperm motility, only the 5-HT2A receptor antagonist significantly reduced sperm motility. This suggests that the 5-HT2A receptor may have a regulatory function in sperm motility. Eventually, progressive motility should be attenuated to penetrate the oocyte for fertilization. The current study indicated heterogenous expression patterns and plausible functions of 5-HT receptors in human sperm.

Activation of 5-HT1A Receptors Normalizes the Overexpression of Presynaptic 5-HT1A Receptors and Alleviates Diabetic Neuropathic Pain

Neuropathic pain is a well-documented phenomenon in experimental and clinical diabetes; however, current treatment is unsatisfactory. Serotoninergic-containing neurons are key components of the descending autoinhibitory pathway, and a decrease in their activity may contribute at least in part to diabetic neuropathic pain (DNP). A streptozotocin (STZ)-treated rat was used as a model for type 1 diabetes mellitus (T1DM). Pain transmission was evaluated using well-established nociceptive-based techniques, including the Hargreaves apparatus, cold plate and dynamic plantar aesthesiometer. Using qRT-PCR, Western blotting, immunohistochemistry, and HPLC-based techniques, we also measured in the central nervous system and peripheral nervous system of diabetic animals the expression and localization of 5-HT1A receptors (5-HT1AR), levels of key enzymes involved in the synthesis and degradation of tryptophan and 5-HT, including tryptophan hydroxylase-2 (Tph-2), tryptophan 2,3-dioxygenase (Tdo), indoleamine 2,3-dioxygenase 1 (Ido1) and Ido2. Moreover, spinal concentrations of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA, a metabolite of 5-HT) and quinolinic acid (QA, a metabolite of tryptophan) were also quantified. Diabetic rats developed thermal hyperalgesia and cold/mechanical allodynia, and these behavioral abnormalities appear to be associated with the upregulation in the levels of expression of critical molecules related to the serotoninergic nervous system, including presynaptic 5-HT1AR and the enzymes Tph-2, Tdo, Ido1 and Ido2. Interestingly, the level of postsynaptic 5-HT1AR remains unaltered in STZ-induced T1DM. Chronic treatment of diabetic animals with 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT), a selective 5-HT1AR agonist, downregulated the upregulation of neuronal presynaptic 5-HT1AR, increased spinal release of 5-HT (↑ 5-HIAA/5-HT) and reduced the concentration of QA, decreased mRNA expression of Tdo, Ido1 and Ido2, arrested neuronal degeneration and ameliorated pain-related behavior as exemplified by thermal hyperalgesia and cold/mechanical allodynia. These data show that 8-OH-DPAT alleviates DNP and other components of the serotoninergic system, including the ratio of 5-HIAA/5-HT and 5-HT1AR, and could be a useful therapeutic agent for managing DNP.

Expanding the concept of serotoninomics: perspectives for serotonin studies in the 20's of the 21st century

Surely, Vittorio Erspamer, discoverer of Enteramine in 1935, and Irvine Page, Maurice M. Rapport and Arda Green, discoverers of Serotonin in 1948, never imagined the biological importance that this fundamental molecule has in the living beings of our planet; from its physiological, passing through endocrine, neural, developmental and reproductive functions and even its role in evolution. For this reason, our workgroup is commemorating these researchers and celebrating their great discovery, which deeply influenced science and medicine, in the present perspective article. As a consequence of their seminal work, and the work of many other researchers in the field of serotonin over the following years, now we stand in front of the practical concept of "Serotoninomics," which we think will contribute to find out precise answers regarding basic, clinical, and translational research related to serotonin, just as the emerging medical and "omics" sciences have done before.

The G Protein-Coupled Serotonin 1A Receptor Augments Protein Kinase Cε-Mediated Neurogenesis in Neonatal Mouse Hippocampus-PKCε-Mediated Signaling in the Early Hippocampus

The neurotransmitter serotonin (5-HT) plays an important role in mood disorders. It has been demonstrated that 5-HT signaling through 5-HT1A receptors (5-HT1A-R) is crucial for early postnatal hippocampal development and later-life behavior. Although this suggests that 5-HT1A-R signaling regulates early brain development, the mechanistic underpinnings of this process have remained unclear. Here we show that stimulation of the 5-HT1A-R at postnatal day 6 (P6) by intrahippocampal infusion of the agonist 8-OH-DPAT (D) causes signaling through protein kinase Cε (PKCε) and extracellular receptor activated kinase ½ (ERK1/2) to boost neuroblast proliferation in the dentate gyrus (DG), as displayed by an increase in bromodeoxy-uridine (BrdU), doublecortin (DCX) double-positive cells. This boost in neuroproliferation was eliminated in mice treated with D in the presence of a 5-HT1A-R antagonist (WAY100635), a selective PKCε inhibitor, or an ERK1/2-kinase (MEK) inhibitor (U0126). It is believed that hippocampal neuro-progenitors undergoing neonatal proliferation subsequently become postmitotic and enter the synaptogenesis phase. Double-staining with antibodies against bromodeoxyuridine (BrdU) and neuronal nuclear protein (NeuN) confirmed that 5-HT1A-R → PKCε → ERK1/2-mediated boosted neuroproliferation at P6 also leads to an increase in BrdU-labeled granular neurons at P36. This 5-HT1A-R-mediated increase in mature neurons was unlikely due to suppressed apoptosis, because terminal deoxynucleotidyl transferase dUTP nick-end labeling analysis showed no difference in DNA terminal labeling between vehicle and 8-OH-DPAT-infused mice. Therefore, 5-HT1A-R signaling through PKCε may play an important role in micro-neurogenesis in the DG at P6, following which many of these new-born neuroprogenitors develop into mature neurons.

The Roles of Serotonin in Neuropsychiatric Disorders

The serotonergic system extends throughout the central nervous system (CNS) and the gastrointestinal (GI) tract. In the CNS, serotonin (5-HT, 5-hydroxytryptamine) modulates a broad spectrum of functions, including mood, cognition, anxiety, learning, memory, reward processing, and sleep. These processes are mediated through 5-HT binding to 5-HT receptors (5-HTRs), are classified into seven distinct groups. Deficits in the serotonergic system can result in various pathological conditions, particularly depression, schizophrenia, mood disorders, and autism. In this review, we outlined the complexity of serotonergic modulation of physiologic and pathologic processes. Moreover, we provided experimental and clinical evidence of 5-HT's involvement in neuropsychiatric disorders and discussed the molecular mechanisms that underlie these illnesses and contribute to the new therapies.

Indolaminergic System in Adult Rat Testes: Evidence for a Local Serotonin System

Serotonin (5-HT) is member of a family of indolamine molecules that participate in a wide variety of biological processes. Despite its important role in the regulation of local blood systems, little is known about the physiological function of 5-HT in reproductive organs, its functional implications, and its role in the reproduction of mammals. In the present work, we evaluated the localization and distribution of 5-HT (using histochemical analysis of indolamines) and different components of the serotoninergic system in rat testes. We detected local synthesis and degradation through immunofluorescence and western blot analyses against the TPH1, MAO_A, 5-HT_T, and VMAT1 serotonin transporters. We also identified the localization and distribution of the 5-HT_1B, 5-HT_2A, and 5-HT_3A receptors. RT-PCR results showed the presence of the Tph1, Maoa, Slc6a4, and Htr3a genes in testes and in the brain stem (Tph1 was used as a negative control). High-performance liquid chromatography was used to determine the presence of 5-HT and the activity of tryptophan hydroxylase in testes homogenates in vitro. Our observations suggest that TPH1 activity and local 5-HT synthesis befall in rat testes. We propose that 5-HT could participate in the regulation of testosterone synthesis and in the spermatogenesis process via local serotoninergic system. However, more studies are needed before concluding that rat testes, or those of other mammals, contain an active form of tryptophan hydroxylase and produce 5-HT.

The T102C polymorphism of 5HT2A receptor in oral epithelial dysplasia: A pilot case-control study

OBJECTIVE

investigate the T102C polymorphism of 5HT2A receptor in dysplasia in oral potentially malignant lesions and its association with smoking and alcohol habits.

METHODS

case-control study that included patients with oral potentially malignant lesions (OPML) histopathologically diagnosed with dysplasia and healthy controls, and within these group patients with and without smoking and alcohol consumption habits. Cell samples from the oral lesions were collected with the patients previously anesthetized using disposable cytological brushes. Deoxyribonucleic acid (DNA) extraction was performed and the T102C polymorphism (rs6313) was genotyped in a real-time polymerase chain reaction (PCR) allelic discrimination assays.

RESULTS

110 individuals were included in this study (38 with dysplasia and 72 controls). The genotype (p = 0.016), allele (p = 0.020) and smoking habits (<0.001) distribution differed significantly between dysplasia and control group, where the CT and TT (C - cytosine/ T - thymine) genotype and the T allele showed a higher frequency in dysplasia (65.6, 18.8 and 84.4 %, respectively) than in controls (55.7, 4.9 and 60.7). Concerning smoking habits, the higher frequency was in the dysplasia group. The multivariate logistic regression analysis, associating variables of interest and the presence of dysplasia, showed that individuals with smoking habits present 7.58 increase risk to develop dysplasia than non-smokers; and individuals carrying the T allele for the T102C polymorphism have a 4.6 increased risk to develop oral dysplasia in OPML.

CONCLUSIONS

the T102C polymorphism is associated with oral dysplasia in OPML, however, failed to show association with smoking and alcohol habits in OPML dysplasia.

Individual Differences in the Speed of Facial Emotion Recognition Show Little Specificity but Are Strongly Related with General Mental Speed: Psychometric, Neural and Genetic Evidence

Facial identity and facial expression processing are crucial socio-emotional abilities but seem to show only limited psychometric uniqueness when the processing speed is considered in easy tasks. We applied a comprehensive measurement of processing speed and contrasted performance specificity in socio-emotional, social and non-social stimuli from an individual differences perspective. Performance in a multivariate task battery could be best modeled by a general speed factor and a first-order factor capturing some specific variance due to processing emotional facial expressions. We further tested equivalence of the relationships between speed factors and polymorphisms of dopamine and serotonin transporter genes. Results show that the speed factors are not only psychometrically equivalent but invariant in their relation with the Catechol-O-Methyl-Transferase (COMT) Val158Met polymorphism. However, the 5-HTTLPR/rs25531 serotonin polymorphism was related with the first-order factor of emotion perception speed, suggesting a specific genetic correlate of processing emotions. We further investigated the relationship between several components of event-related brain potentials with psychometric abilities, and tested emotion specific individual differences at the neurophysiological level. Results revealed swifter emotion perception abilities to go along with larger amplitudes of the P100 and the Early Posterior Negativity (EPN), when emotion processing was modeled on its own. However, after partialling out the shared variance of emotion perception speed with general processing speed-related abilities, brain-behavior relationships did not remain specific for emotion. Together, the present results suggest that speed abilities are strongly interrelated but show some specificity for emotion processing speed at the psychometric level. At both genetic and neurophysiological levels, emotion specificity depended on whether general cognition is taken into account or not. These findings keenly suggest that general speed abilities should be taken into account when the study of emotion recognition abilities is targeted in its specificity.

Pharmacogenetics of chemotherapy-induced nausea and vomiting

Chemotherapy-induced nausea and vomiting (CINV) is associated with distressing adverse effects observed in patients during cytotoxic chemotherapy. One of the potential factors explaining suboptimal response to currently used antiemetics is variability in genes encoding enzymes and proteins that play a role in the action of antiemetic drugs. Pharmacogenomics studies of CINV are sparse and focus mainly on polymorphisms associated with serotonin receptor, drug metabolism and drug transport. Currently, the role of pharmacogenetics in mechanisms of CINV has not been fully unraveled, and it is premature to implement results of pharmacogenetic association studies of antiemetic drugs in clinical practice. More uniform studies, with genetic profiles and biomarkers relevant for the proposed target and transporter mechanisms, are needed.

Cyclophosphamide causes activation of protein kinase A (PKA) in the brainstem of vomiting least shrews (Cryptotis parva)

Complete control of emesis caused by cyclophosphamide (CPA) is of immense interest to both patients and physicians. Serotonin 5-HT3- and tachykinin NK1-receptor antagonists are widely used antiemetics in clinic, but they fail to completely control CPA-induced emesis. New antiemetic targets for the full control of CPA-induced vomiting are lacking. We therefore examined the effects of CPA on emetic targets downstream of 5-HT3- and NK1- receptors in an attempt to better understand the molecular bases of CPA-induced emesis. Acute CPA (200 mg/kg, i.p.) administration in the least shrew caused a biphasic pattern of emesis over a 40 h observation period, with maximal peak vomit frequency during the 1st hour of treatment (acute phase), followed by a delayed-phase which peaks at 27th hour. The NK1 receptor mRNA levels increased significantly at 8 h post-CPA treatment in the brainstem, and at 28 h in the whole intestine. Substance P mRNA levels tended to increase both in the brainstem and intestine at most time-points post-CPA injection, however due to large variability, they failed to attain significance. Likewise, protein expression profiles of both NK1- and 5-HT3 -receptors in the brainstem were unchanged at any time-point. However, phosphorylation levels of protein kinase A (PKA), but not of extracellular signal-regulated protein kinase 1/2 (ERK1/2), were increased at 2, 8, 22, 28, and 33 h time-points after the treatment with CPA. Moreover, brainstem but not frontal cortex cAMP tissue levels tended to be elevated at most time-points, but significant increases occurred only at 1 and 2 h post-CPA treatment. The phosphodiesterase inhibitor, rolipram, caused significant increases in shrew brainstem cAMP levels which were associated with its capacity to produce vomiting, while pretreatment with SQ22536, an inhibitor of adenylyl cyclase, prevented rolipram-induced emesis. The results demonstrate that accumulation of cAMP and subsequent activation of PKA in the brainstem may help to initiate and sustain emesis induced by CPA in the least shrew. Our findings suggest that suppression of the cAMP/PKA cascade may have antiemetic potential in the management of CPA-induced emesis.

Tryptophan hydroxylase 1 (Tph-1)-targeted bone anabolic agents for osteoporosis

Tryptophan hydroxylase 1 (Tph-1), the principal enzyme for peripheral serotonin biosynthesis, provides a novel target to design anabolic agents for osteoporosis. Here, we present a design, synthesis of a novel series of ursolic acid derivatives under the guidance of docking technique, and bioevaluation of the derivatives using RBL2H3 cells and ovariectomized (OVX) rats. Of the compounds, 9a showed a potent inhibitory activity on serotonin biosynthesis. Further investigations revealed that 9a, as an efficient Tph-1 binder identified by SPR (estimated KD: 6.82 μM), suppressed the protein and mRNA expressions of Tph-1 and lowered serotonin contents in serum and gut without influence on brain serotonin. Moreover, oral administration of 9a elevated serum level of N-terminal propeptide of procollagen type 1 (P1NP), a bone formation marker, and improved bone microarchitecture without estrogenic side effects in ovariectomized rats. Collectively, 9a may serve as a new candidate for bone anabolic drug discovery.

Serotonin modulates outward potassium currents in mouse olfactory receptor neurons

Monoaminergic neurotransmitter 5-hydroxytryptamine (5-HT), also known as serotonin, plays important roles in modulating the function of the olfactory system. However, thus far, the knowledge about 5-HT and its receptors in olfactory receptor neurons (ORNs) and their physiological role have not been fully characterized. In the present study, reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed the presence of 5-HT(1A) and 5-HT(1B) receptor subtypes in mouse olfactory epithelium at the mRNA level. With subtype selective antibodies and standard immunohistochemical techniques, both receptor subtypes were found to be positively labeled. To further elucidate the molecular mechanisms of 5-HT act on the peripheral olfactory transduction, the whole-cell patch clamp techniques were used on freshly isolated ORNs. We found that 5-HT decreased the magnitude of outward K(+) current in a dose-dependent manner and these inhibitory effects were markedly attenuated by the 5-HT(1A) receptor blocker WAY-100635 and the 5-HT(1B) receptor antagonist GR55562. These data suggested that 5-HT may play a role in the modulation of peripheral olfactory signals by regulating outward potassium currents, both 5-HT(1A) and 5-HT(1B) receptors were involved in this regulation.

Serotonin in testes of bat Myotis velifer during annual reproductive cycle: expression, localization, and content variations

The mechanism of reproduction in mammals is very complex and in some cases is quite particular. For example in some bat species, the male presents a reproductive mechanism characterized by an annual testicular cycle that goes from recrudescence to regression (spermatogenesis to inactivity period, respectively). After recrudescence, the spermatozoa arrive at epididymis and wait to be expelled at the time of ejaculation during the mating period, which occurs some months later. Because serotonin (5-HT) has gained reproductive importance in the last years, the aim of the present study was to analyze the expression of this indolamine and both tryptophan hydroxylase and monoamine oxidase isoform A-enzymes involved in its metabolism-in Myotis velifer testes, a seasonal reproductive bat species that shows temporal asynchrony in its sexual cycle, across the principal periods of their reproductive cycle. By using both Falck-Hillarp histochemistry and immunofluorescence techniques, we found serotonin in vesicles of Leydig cells and probably Sertoli cells too; interestingly, both intracellular localization and concentration was variable across the different stages of the reproductive cycle, being lower during spermatogenesis phase and increasing during the mating phase. These results suggest that 5-HT is present in bat testes and it could play an important role in testicular function during their reproductive cycle.

Evidence of 5-HT components in human sperm: implications for protein tyrosine phosphorylation and the physiology of motility

Serotonin (5-hydroxytryptamine; C(10)H(12)N(2)O (5-HT)) is produced in the CNS and in some cells of peripheral tissues. In the mammalian male reproductive system, both 5-HT and tryptophan hydroxylase (TPH) have been described in Leydig cells of the testis and in principal cells of the caput epididymis. In capacitated hamster sperm, it has been shown that 5-HT promotes the acrosomal reaction. The aim of this work was to explore the existence of components of the serotoninergic system and their relevance in human sperm physiology. We used both immunocytochemistry and western blot to detect serotoninergic markers such as 5-HT, TPH1, MAO(A), 5-HT(1B), 5-HT(3), and 5HT(T); HPLC for TPH enzymatic activity; Computer Assisted Semen Analysis assays to measure sperm motility parameters and pharmacological approaches to show the effect of 5-HT in sperm motility and tyrosine phosphorylation was assessed by western blot. We found the presence of serotoninergic markers (5-HT, TPH1, MAO(A), 5-HT(1B), 5-HT(2A), 5-HT(3), 5-HT(T), and TPH enzymatic activity) in human sperm. In addition, we observed a significant increase in tyrosine phosphorylation and changes in sperm motility after 5-HT treatment. In conclusion, our data demonstrate the existence of components of a serotoninergic system in human sperm and support the notion for a functional role of 5-HT in mammalian sperm physiology, which can be modulated pharmacologically.

Cisplatin causes over-expression of tachykinin NK(1) receptors and increases ERK1/2- and PKA- phosphorylation during peak immediate- and delayed-phase emesis in the least shrew (Cryptotis parva) brainstem

Scant information is available regarding the effects of cisplatin on the expression profile of tachykinin NK(1) receptors and downstream signaling during cisplatin-induced emesis. Cisplatin causes peak early- and delayed-phase emesis in the least shrew at 1-2 and 33 h post-injection. To investigate the expression profile of NK(1) receptor during both emetic phases, we cloned the cDNA corresponding to a ~700 base pairs of mRNA flanked by two stretches of nucleotides conserved among different species and demonstrated that the shrew NK(1) receptor nucleotide sequence shares ~90% sequence identity with the human NK(1) receptor. Of the 12 time-points tested, significant increases in expression levels of NK(1) receptor mRNA in the shrew brainstem occurred at 2 and 28 h post-cisplatin injection, whereas intestinal NK(1) receptor mRNA was increased at 28 h. Shrew brainstem and intestinal substance P mRNA levels also tended to increase during the two phases. Furthermore, expression levels of NK(1) receptor protein were significantly increased in the brainstem at 2, 8, and 33 h post-cisplatin. No change in brainstem 5-HT(3) receptor protein expression was observed. The temporal enhancements in NK(1) receptor protein expression were mirrored by significant increases in the phosphorylation status of the brainstem ERK1/2 at 2, 8, and 33 h post-cisplatin. Phosphorylation of PKA significantly increased at 33rd and 40th hour. Our results indicate associations between cisplatin's peak immediate- and delayed-phase vomiting frequency with increased: (1) expression levels of NK(1) receptor mRNA and its protein level, and (2) downstream NK(1) receptor-mediated phosphorylation of ERK1/2 and PKA signaling.

Serotonin 1A receptor-mediated signaling through ERK and PKCα is essential for normal synaptogenesis in neonatal mouse hippocampus

Aberrant expression of the presynaptic serotonin 1A receptor (5-HT(1A)-R) because of a polymorphism in the 5-HT(1A)-R gene is associated with severe depression in human, whereas its absence up to postnatal day 21 (P21) in the forebrain of mice results in heightened anxiety in adulthood. These observations collectively indicate that the 5-HT(1A)-R has a crucial role in brain development. To understand the mechanistic underpinnings of this phenomenon, we used organotypic slice cultures of hippocampi from C57BL6 mice (C57) at P15, which coincides with the peak of neonatal synaptogenesis. Stimulation of the hippocampal 5-HT(1A)-R caused a dramatic increase in PSD95 expression and dendritic spine and synapse formation through sequential activation of the mitogen-activated protein kinase isozymes Erk1/2 and protein kinase C (PKC). Intrahippocampal infusion of 5-HT(1A)-R agonists and signaling inhibitors at P15 revealed that the same pathway through PKCα augments PSD95 expression and synaptogenesis in vivo in 24 h in both C57 as well as Swiss Webster mice. Furthermore, intrahippocampal infusion of the antidepressant fluoxetine, a serotonin reuptake inhibitor, also augmented PSD95 expression and synaptogenesis through the same pathway. This increased synaptogenesis was observed even 5 days after treatment. Finally, compared with the wild type, the 5-HT(1A)-R(-/-) mice harbor significantly less synapses in the hippocampus, but infusion of the PKC-stimulator and Alzheimer drug bryostatin into the 5-HT(1A)-R(-/-) mice to bypass the non-existent 5-HT(1A)-R boosted PSD95 expression and synaptogenesis. The elucidated signaling cascade explains how 5-HT(1A)-R regulates hippocampal sculpting and function, which may determine the affective phenotype of an adult.

Functional crosstalk and heteromerization of serotonin 5-HT2A and dopamine D2 receptors

The serotonin 5-HT(2A) receptor (5-HT(2A)R) and dopamine D(2) receptor (D(2)R) are high-affinity G protein-coupled receptor targets for two different classes of antipsychotic drugs used to treat schizophrenia. Interestingly, the antipsychotic effects are not based on the regulation of same signaling mediators since activation of the 5-HT(2A)R and of the D(2)R regulate G(q/11) protein and G(i/o) protein, respectively. Here we use radioligand binding and second messenger production assays to provide evidence for a functional crosstalk between 5-HT(2A)R and D(2)R in brain and in HEK293 cells. D(2)R activation increases the hallucinogenic agonist affinity for 5-HT(2A)R and decreases the 5-HT(2A)R induced inositol phosphate production. In vivo, 5-HT(2A)R expression is necessary for the full effects of D(2)R antagonist on MK-801-induced locomotor activity. Co-immunoprecipitation studies show that the two receptors can physically interact in HEK293 cells and raise the possibility that a receptor heterocomplex mediates the crosstalk observed. The existence of this 5-HT(2A)R-D(2)R heteromer and crosstalk may have implications for diseases involving alterations of serotonin and dopamine systems and for the development of new classes of therapeutic drugs.

Chronic exposure to fluoxetine (Prozac) causes developmental delays in Rana pipiens larvae

Selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine, are among the many pharmaceuticals detected in aquatic ecosystems. Although the acute effects of SSRIs on select organisms have been reported, little is understood about the chronic effects of these drugs on amphibians, which are particularly sensitive to environmental pollutants. Serotonin plays important roles in many physiological functions, including a wide array of developmental processes. Exposure to SSRIs during development may cause developmental complications in a variety of organisms, but little is known about the degree of exposure necessary to cause deleterious effects. Here, we sought to gain a better understanding of the effects of SSRIs on amphibian development by use of a combined laboratory and outdoor mesocosm study. Tadpoles in a laboratory setting were exposed to a low (0.029 µg/L) and a high (0.29 µg/L) concentration of the common SSRI fluoxetine from stages 21 and 22 through completion of metamorphosis. Tadpoles in outdoor mesocosms were exposed to fluoxetine concentrations ranging from 0.1 to 0.3 µg/L. Exposed tadpoles in the laboratory showed delayed development compared with controls when stage was assessed throughout the experiment. Control tadpoles also gained weight faster than treatment tadpoles, which may be explained by reduced food intake. Mesocosm tadpoles exhibited similar trends, but no significant differences were detected. These results indicate that ecologically relevant levels of fluoxetine may cause developmental delays in amphibians.

Erk1/2-dependent phosphorylation of PKCalpha at threonine 638 in hippocampal 5-HT(1A) receptor-mediated signaling

Stimulation of the serotonin 1A receptor (5-HT(1A)-R) causes activation of extracellular signal-regulated protein kinase (Erk) and protein kinase C alpha (PKCalpha) in both hippocampal HN2-5 cells and cultured hippocampal slices from postnatal day-15 (P15) mice. Our earlier studies demonstrated that PKCalpha is co-immunoprecipitated with Erk and the phosphorylation of PKCalpha in this Erk-PKCalpha complex is dependent on the Erk pathway. Furthermore, the T(638) residue, which must be phosphorylated for the complete activation of PKCalpha, is within an authentic Erk consensus domain (S/TP), and the PKCalpha protein also contains two docking sites for Erk such as KRGRIYL and KRGIIYRDLKL. Using Föster Resonance Energy Transfer (FRET) we have confirmed an association between Erk and PKCalpha. Employing PKCalpha and Erk mutants we next demonstrated that Erk causes direct phosphorylation and activation of PKCalpha. By mutating the phosphoinositide-dependent kinase-1 (PDK-1)-promoted phosphorylation site (S(497)) and the kinase site (K(368)) in PKCalpha, we observed that both of these autophosphorylation-deficient mutants are phosphorylated at T(638) in an Erk-dependent manner. To confirm that Erk indeed catalyzes phosphorylation of PKCalpha at T(638), we used a mutant Erk construct in which a relatively large amino acid residue in the ATP binding site (Q(103)) had been replaced with glycine, enabling this mutant to utilize a bulky analog of ATP, cyclopentyl ATP. An in vitro kinase assay using this mutant Erk protein, radiolabeled cyclopentyl ATP, and a synthetic oligopeptide containing the S/TP site of PKCalpha demonstrated phosphorylation of the peptide by Erk1/2. These results confirm the novel possibility that PKCalpha is a direct substrate of Erk1/2 in neuronal cells and help link two important signaling molecules that regulate maturation and protection of hippocampal neurons as well as many other cell types.

Benzimidazole derivatives as new serotonin 5-HT6 receptor antagonists. Molecular mechanisms of receptor inactivation

On the basis of our previously described pharmacophore model for serotonin 5-HT(6) receptor (5-HT(6)R) antagonists, we have designed, synthesized, and pharmacologically characterized a series of benzimidazole derivatives 1-20 that represent a new family of potent antagonists at the human 5-HT(6)R. Site-directed mutagenesis and a beta(2)-adrenoceptor-based homology model of the 5-HT(6)R were used to predict the mode of binding of antagonist SB-258585 and the new synthesized ligands. Substitution of W6.48, F6.52, or N6.55 by Ala fully impedes compound 4 to block 5-HT-induced activation. Thus, we propose that D3.32 in TM 3 anchors the protonated piperazine ring, the benzimidazole ring expands parallel to EL 2 to hydrogen bond N6.55 in TM 6, and the aromatic ring is placed between TMs 3 and 5 in CH(2)-containing compounds and between TMs 3 and 6 in CO-containing compounds. This combined experimental and computational study has permitted to propose the molecular mechanisms by which the new benzimidazole derivatives act as 5-HT(6)R antagonists.

Secretion of S100B, an astrocyte-derived neurotrophic protein, is stimulated by fluoxetine via a mechanism independent of serotonin

S100B is a calcium-binding protein, produced and secreted by astrocytes, which has a putative paracrine neurotrophic activity. Clinical studies have suggested that peripheral elevation of this protein is positively correlated with a therapeutic antidepressant response, particularly to selective serotonin reuptake inhibitors (SSRIs); however, the mechanism underlying this response remains unclear. Here, we measured S100B secretion directly in hippocampal astrocyte cultures and hippocampal slices exposed to fluoxetine and observed a significant increment of S100B release in the presence of this SSRI, apparently dependent on protein kinase A (PKA). Moreover, we found that serotonin (possibly via the 5HT1A receptor) reduces S100B secretion and antagonizes the effect of fluoxetine on S100B secretion. These data reinforce the effect of fluoxetine, independently of serotonin and serotonin receptors, suggesting a putative role for S100B in depressive disorders and suggesting that other molecular targets may be relevant for antidepressant activity.

Distribution of serotonin immunoreactivity in the spiral ganglion neurons of mouse cochlea

OBJECTIVE

Serotonin (5-hydroxytryptamine, 5-HT) is a neuromodulator/neurotransmitter with multiple biological functions. Spiral ganglion cells in the cochlea are the primary neurons of the afferent system in the auditory transmission. In this study, we used the immunohistochemical technique to investigate the distribution of serotonin in the spiral ganglion of mouse cochlea.

MATERIALS AND METHODS

The cochlea tissue of four adult mice was dissected and fixed. The immunohistochemical staining was applied by using goat anti-serotonin polyclonal antibody as primary antibody. Tissue sections were treated with biotin-labeled rabbit anti-goat immunoglobulin G, followed by adding streptavidin-biotin-peroxidase complex. Finally, the sections were stained with 3,3-diaminobenzidine (DAB) solution.

RESULTS

The spiral ganglion exhibited pronounced immunoreactivity for serotonin. Specifically, serotonin immunoreactivity was detected in the cytoplasma of spiral ganglion neurons located in Rosenthal's canal of the bony modiolus of mouse cochlea.

CONCLUSIONS

Since spiral ganglion neurons are the afferent neurons to the auditory sense organ, our result strongly suggests that serotonin molecule may function as a neuromodulator/neurotransmitter in the peripheral auditory processing.

Fluoxetine-induced proliferation and differentiation of neural progenitor cells isolated from rat postnatal cerebellum

Previous studies have shown that the serotonin-reuptake inhibitor (SSRI) fluoxetine affects neural progenitors derived from postnatal cerebellum or adult hippocampus and stimulates their proliferation. In the human cerebellum, the proliferation of cerebellar granule cells (CGC) continues until the 11th postnatal month and could be influenced in infants by breastfeeding-delivered SSRIs. Current information about fluoxetine effects on postnatal cerebellar neural progenitors is limited. Here we report the characterization of fluoxetine actions on rat postnatal cerebellar neural progenitors. RT-PCR and immunostaining revealed the expression of serotonin transporter (SERT), 5HT(1A) receptors, tryptophan hydroxylase (TPH), and serotonin (5HT). Protracted in vitro fluoxetine treatment increased cell proliferation and differentiation. The proliferative effects of fluoxetine, 5HT, and the selective agonist of 5HT(1A) receptors trans-8-hydroxy-2-(N-n-propyl-N-3'-iodo-2'-propenyl)aminotetralin (8-OH-PIPAT) were abolished by the selective antagonist of 5HT(1A) receptors, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY-100635). Furthermore, fluoxetine-induced activation of both the cAMP-response element-binding (CREB) protein and extracellular signal-regulated protein kinases (ERK1/2), which was abolished by the selective inhibitor of MAP kinase kinase (MEK) 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene (U0126), and increased cyclin D1 expression. All these effects were prevented by WAY-100635. Collectively, our results demonstrate that rat postnatal cerebellum contains neural progenitors capable of proliferating and differentiating in response to fluoxetine exposure, possibly through the activation of 5HT(1A) receptors. The relevance of these findings for possible SSRI effects on the developing postnatal/infant human cerebellum should be explored.

Physiological endpoints for potential SSRI interactions in fish

Selective serotonin reuptake inhibitors (SSRIs) are among the pharmaceutical compounds frequently detected in sewage treatment plant effluents and surface waters, albeit at very low concentrations, and have therefore become a focus of interest as environmental pollutants. These neuroactive drugs are primarily used in the treatment of depression but have also found broader use as medication for other neurological dysfunctions, consequently resulting in a steady increase of prescriptions worldwide. SSRIs, via inhibition of the serotonin (5-hydroxytryptamine, 5-HT) reuptake mechanism, induce an increase in extracellular 5-HT concentration within the central nervous system of mammals. The phylogenetically ancient and highly conserved neurotransmitter and neurohormone 5-HT has been found in invertebrates and vertebrates, although its specific physiological role and mode of action is unknown for many species. Consequently, it is difficult to assess the impact of chronic SSRI exposure in the environment, especially in the aquatic ecosystem. In view of this, the current knowledge of the functions of 5-HT in fish physiology is reviewed and, via comparison to the physiological role and function of 5-HT in mammals, a characterization of the potential impact of chronic SSRI exposure on fish is provided. Moreover, the insight on the physiological function of 5-HT strongly suggests that the experimental approaches currently used are inadequate if not entirely improper for routine environmental risk assessment of pharmaceuticals (e.g., SSRIs), as relevant endpoints are not assessed or impossible to determine.

Attenuating effects of prior oestradiol benzoate priming on 5-HT-mediated lordosis behavior in rats are dose-dependent

The present study was designed to investigate the role of the 5-HT7 receptors in lordosis and compare the lordotic responses with 5-HT1A agent under the influence of different steroid-priming regimens in ovariectomized, non-receptive and receptive rats. 8-OH DPAT, a 5-HT1A agonist and 5-CT, a 5-HT7 agonist inhibited the lordosis differently in non-receptive and receptive rats, however, the response was attenuated in a dose-dependent manner following 5-CT treatment in the first two tests. Treatment with 5-HT1A antagonist, WAY 100 135 caused a protective effect which was evident in the second test only. Priming with 25 microg OB attenuated in the first test in non-receptive rats whereas the same dose repeated a similar pattern in receptive rats. The attenuation of LQ was evident in rats treated with 5-HT7 antagonist, SB 269970-A. This finding shows that WAY 100 135, a 5-HT1A antagonist has potency to attenuate inhibitory influence of 8-OH DPAT by enhancing lordosis behavior acutely in female rats with a low estrous state. Treatment with 5-CT and SB 269970-A as 5-HT7, agonist and antagonist, respectively, have mimicked 5-HT-mediated lordotic response as moderate affinity towards 5-HT1A receptors has been reported. This offers a comparable effect on lordosis as a result of the two 5-HT agents used.

Infrared and Ultraviolet Spectral Signatures and Conformational Preferences of Jet-Cooled Serotonin

The ultraviolet and infrared spectroscopy of single conformations of neutral serotonin (5-hydroxytryptamine) have been studied in the gas phase using a combination of methods including laser-induced fluorescence, resonance-enhanced two-photon ionization, UV-UV hole-burning spectroscopy, and resonant ion-dip infrared spectroscopy. By comparison to its close analogue tryptamine, for which firm assignments to seven low-energy conformations have been made, UV and IR transitions due to eight conformations of serotonin are observed and assigned. The ultraviolet spectrum divides into two subsets of transitions separated from one another by approximately 230 cm-1 ascribable to syn and anti conformations of the 5-OH group. These two subsets are also distinguishable via their 5-OH stretch fundamentals, with the anti-OH subset shifted by approximately 4-5 cm-1 to lower frequency than those due to syn-OH conformers. The existing firm assignments for tryptamine play a decisive role in assignments in serotonin, where the alkyl CH stretch infrared spectrum is diagnostic of the conformation of the ethylamine side chain. Conformer A of serotonin (SERO(A)), with S1 <-- S0 origin transition at 32584 cm-1, is assigned to Gpy(out)/anti-OH, SERO(B) at 32548 cm-1 to Gpy(up)/anti, SERO(C) at 32545 cm-1 to Gph(out)/anti, SERO(D) at 32560 cm-1 to Anti(py)/anti, SERO(E) at 32537 cm-1 to Anti(up)/anti, SERO(F) at 32353 cm-1 to Gpy(out)/syn, SERO(G) at 32313 cm-1 to Gpy(up)/syn, and SERO(H) at 32282 cm-1 to Gph(out)/syn. The conformational preferences of serotonin differ from those of tryptamine most notably in the selective stabilization observed for the Gph(out)/anti-OH conformer SERO(C), which makes it the second-most intense transition in the ultraviolet spectrum, surpassed only by the Gpy(out)/anti-OH conformer SERO(A).

Plasticity of 5‐HT1Areceptor‐mediated signaling during early postnatal brain development

The presence of serotonin 1A receptor (5-HT(1A)-R) in the hippocampus, amygdala, and most regions of the frontal cortex is essential between postnatal day-5-21 (P5-21) for the expression of normal anxiety levels in adult mice. Thus, the 5-HT(1A)-R plays a crucial role in this time window of brain development. We show that the 5-HT(1A)-R-mediated stimulation of extracellular signal-regulated kinases 1 and 2 (Erk1/2) in the hippocampus undergoes a transition between P6 and P15. At P6, a protein kinase C (PKC) isozyme is required for the 5-HT(1A)-R -->Erk1/2 cascade, which causes increased cell division in the dentate gyrus. By contrast, at P15, PKC alpha participates downstream of Erk1/2 to augment synaptic transmission through the Schaffer Collateral pathway but does not cause increased cell division. Our data demonstrate that the 5-HT(1A)-R -->Erk1/2 cascade uses PKC isozymes differentially, first boosting the cell division to form new hippocampal neurons at P6 and then undergoing a plastic change in mechanism to strengthen synaptic connections in the hippocampus at P15.

Intracellular signaling pathways pave roads to recovery for mood disorders

Mood disorders, including major depression and bipolar disorder, remain a major unmet medical need as current antidepressant and mood stabilizing therapies require chronic treatment for efficacy and are not effective in all patients. Multiple deficits, including cell atrophy and loss, have been observed in limbic and cortical brain regions of patients with mood disorders and in stressed animals. It is thought that antidepressant and mood stabilizing medications restore these deficits by reestablishing proper patterns of gene expression and function. In support of this hypothesis, numerous changes in gene expression and activity have been observed in limbic and cortical brain regions of mood disorder patients, and thymoleptic therapies have been shown to reciprocally regulate many of these changes. These findings have implicated four main signaling pathways in the pathophysiology and/or treatment of mood disorders, namely the cyclic-AMP, phosphoinositol, mitogen-activated protein kinase, and glycogen synthase kinase signaling cascades. Below we review this literature, and discuss potential targets for novel antidepressant and mood stabilizing drug design that are highlighted by these findings.

A genomic view of the sea urchin nervous system

The sequencing of the Strongylocentrotus purpuratus genome provides a unique opportunity to investigate the function and evolution of neural genes. The neurobiology of sea urchins is of particular interest because they have a close phylogenetic relationship with chordates, yet a distinctive pentaradiate body plan and unusual neural organization. Orthologues of transcription factors that regulate neurogenesis in other animals have been identified and several are expressed in neurogenic domains before gastrulation indicating that they may operate near the top of a conserved neural gene regulatory network. A family of genes encoding voltage-gated ion channels is present but, surprisingly, genes encoding gap junction proteins (connexins and pannexins) appear to be absent. Genes required for synapse formation and function have been identified and genes for synthesis and transport of neurotransmitters are present. There is a large family of G-protein-coupled receptors, including 874 rhodopsin-type receptors, 28 metabotropic glutamate-like receptors and a remarkably expanded group of 161 secretin receptor-like proteins. Absence of cannabinoid, lysophospholipid and melanocortin receptors indicates that this group may be unique to chordates. There are at least 37 putative G-protein-coupled peptide receptors and precursors for several neuropeptides and peptide hormones have been identified, including SALMFamides, NGFFFamide, a vasotocin-like peptide, glycoprotein hormones and insulin/insulin-like growth factors. Identification of a neurotrophin-like gene and Trk receptor in sea urchin indicates that this neural signaling system is not unique to chordates. Several hundred chemoreceptor genes have been predicted using several approaches, a number similar to that for other animals. Intriguingly, genes encoding homologues of rhodopsin, Pax6 and several other key mammalian retinal transcription factors are expressed in tube feet, suggesting tube feet function as photosensory organs. Analysis of the sea urchin genome presents a unique perspective on the evolutionary history of deuterostome nervous systems and reveals new approaches to investigate the development and neurobiology of sea urchins.

Nicotine and serotonin in immune regulation and inflammatory processes: a perspective

Nicotine and serotonin modulate the innate and adaptive immune responses and the inflammatory states. Several nicotinic cholinergic and serotonergic receptor subtypes have been characterized in B and T lymphocytes, monocytes, macrophages, and dendritic cells. The use of knockout mice has allowed a better characterization of nicotinic receptors and their role in anti-inflammatory processes in these cells. Cytokines play a crucial role in controlling inflammatory reactions. Nicotine and serotonin have been reported to regulate cytokine release. Cholinergic mechanisms also play an important role in inflammation through endogenous acetylcholine. Nicotine mimics this effect by activating the cholinergic anti-inflammatory pathways. New concepts of reciprocal interactions between nicotine and serotonin are emerging. The role of nicotine as an anti-inflammatory agent has been established, whereas that of serotonin remains more controversial.

Prolonged treatment with ligands affects ligand binding to the human serotonin(1A) receptor in Chinese hamster ovary cells

1. The serotonin(1A) receptors are members of a superfamily of seven transmembrane domain receptors that couple to G-proteins, and appear to be involved in several behavioral and cognitive functions. 2. We monitored the effect of prolonged treatment of the human serotonin(1A) receptor expressed in Chinese hamster ovary (CHO) cells with pharmacologically well-characterized ligands on its binding to the agonist 8-hydroxy-2(di-N-propylamino)tetralin (8-OH-DPAT) and antagonist 4-(2'-methoxy)-phenyl-1-[2'-(N-2''-pyridinyl)-p-fluorodobenzamido]ethyl-piperazine (p-MPPF). 3. Our results indicate that prolonged treatment with the specific agonist (8-OH-DPAT) differentially affects subsequent binding of the agonist and antagonist to the receptor in a manner independent of receptor-G-protein coupling. Importantly, our results show that prolonged treatment with the commonly used antagonist p-MPPF, and its iodinated analogue 4-(2'-methoxy)-phenyl-1-[2'-(N-2''-pyridinyl)-p-iodobenzamido]ethyl-piperazine (p-MPPI), which have earlier been reported to display similar binding properties to serotonin(1A) receptors, induces significantly different effects on the ligand binding function of serotonin(1A) receptors.