Molecular biology of serotonin (5-HT) receptors

Depression and Antidepressants During Pregnancy: Craniofacial Defects Due to Stem/Progenitor Cell Deregulation Mediated by Serotonin

Depression is a common and debilitating mood disorder that increases in prevalence during pregnancy. Worldwide, 7 to 12% of pregnant women experience depression, in which the associated risk factors include socio-demographic, psychological, and socioeconomic variables. Maternal depression could have psychological, anatomical, and physiological consequences in the newborn. Depression has been related to a downregulation in serotonin levels in the brain. Accordingly, the most commonly prescribed pharmacotherapy is based on selective serotonin reuptake inhibitors (SSRIs), which increase local serotonin concentration. Even though the use of SSRIs has few adverse effects compared with other antidepressants, altering serotonin levels has been associated with the advent of anatomical and physiological changes in utero, leading to defects in craniofacial development, including craniosynostosis, cleft palate, and dental defects. Migration and proliferation of neural crest cells, which contribute to the formation of bone, cartilage, palate, teeth, and salivary glands in the craniofacial region, are regulated by serotonin. Specifically, craniofacial progenitor cells are affected by serotonin levels, producing a misbalance between their proliferation and differentiation. Thus, it is possible to hypothesize that craniofacial development will be affected by the changes in serotonin levels, happening during maternal depression or after the use of SSRIs, which cross the placental barrier, increasing the risk of craniofacial defects. In this review, we provide a synthesis of the current research on depression and the use of SSRI during pregnancy, and how this could be related to craniofacial defects using an interdisciplinary perspective integrating psychological, clinical, and developmental biology perspectives. We discuss the mechanisms by which serotonin could influence craniofacial development and stem/progenitor cells, proposing some transcription factors as mediators of serotonin signaling, and craniofacial stem/progenitor cell biology. We finally highlight the importance of non-pharmacological therapies for depression on fertile and pregnant women, and provide an individual analysis of the risk-benefit balance for the use of antidepressants during pregnancy.

Targeting Serotonin1A Receptors for Treating Chronic Pain and Depression

The association of chronic pain with depression is becoming increasingly recognized. Treating both the conditions together is essential for an effective treatment outcome. In this regard, it is important to identify a shared mechanism involved in the association of chronic pain with depression. Central serotonin (5-hydroxytryptamine; 5-HT) neurotransmission has long been known to participate in the processing of signals related to pain. It also plays a key role in the pathogenesis and treatment of depression. Although functional responses to serotonin are mediated via the activation of multiple receptor types and subtypes, the 5-HT1A subtype is involved in the processing of nociception as well as the pathogenesis and treatment of depression. This receptor is located presynaptically, as an autoreceptor, on the perikaryon and dendritic spines of serotonin-containing neurons. It is also expressed as a heteroreceptor on neurons receiving input from serotonergic neurons. This arti-cle targets the 5-HT1A receptors to show that indiscriminate activation of pre and postsynaptic 5-HT1A receptors is likely to produce no therapeutic benefits; biased activation of the 5-HT heteroreceptors may be a useful strategy for treating chronic pain and depression individually as well as in a comorbid condition.

A systematic review of clinical guidelines on choice, dose, and duration of antipsychotics treatment in first- and multi-episode schizophrenia

Clinical guidelines provide evidence-based recommendations to regulate pharmacological treatment of psychotic disorders. However, the quality of evidence, country of origin, and publication dates of such guidelines vary, which leads to discrepancies between recommendations. This systematic review aimed to examine consensus and disparities between clinical recommendations on the choice, dose, and duration of antipsychotic treatment for first- and multi-episode schizophrenia patients. A literature search through The Cochrane Library, Embase, Medline, PsycINFO, PubMed, Scopus, Web of Sciences, and relevant bibliographies revealed 24 guidelines that met the inclusion criteria. The guidelines indicated mostly consistent recommendations regarding the optimal dose range of antipsychotics, while guidance with regards to the choice and duration of treatment remains somewhat controversial. Current trends in guidelines emphasize that there is simply no 'one-size-fits-all' method to manage schizophrenia patients. Further research is needed not only to address discrepancies between guidelines, but also to justify the gap between theory and practice.

The Impact of Different Antimigraine Compounds on Platelet and Erythrocyte Aggregation

Clinical and experimental data suggest that ergotamine compounds and triptans may contribute to vascular events such as myocardial infarction and stroke. The role of blood cell aggregation in this context is, however, not clarified. We aimed to evaluate the impact of different acute antimigraine compounds on platelet and erythrocyte aggregation in a human ex vivo experimental design. In 20 healthy subjects without migraine and in 20 healthy subjects with migraine without aura, platelet and erythrocyte aggregation were measured before and after intake of placebo, acetylsalicylic acid, ergotamine tartrate, zolmitriptan and sumatriptan. Platelet aggregation was measured by the so-called platelet reactivity index. Erythrocyte aggregation was measured by photometric assessment in an aggregometer. Ergotamine tartrate induced a significant increase of platelet aggregation, whereas acetylsalicylic acid induced a significant decrease in both subject groups. After placebo, after sumatriptan and after zolmitriptan, no significant changes of platelet aggregation were noted. Erythrocyte aggregation was affected by neither compound. We can conclude that platelet aggregation, but not erythrocyte aggregation, is increased after intake of ergotamine tartrate. This may in part contribute to vascular side-effects of this compound. Acetylsalicylic acid and the triptans appeared to be safe with respect to platelet and erythrocyte aggregation.

5-HT2 receptors facilitate JC polyomavirus entry

The human JC polyomavirus (JCPyV) causes the rapidly progressing demyelinating disease progressive multifocal leukoencephalopathy (PML). The disease occurs most often in individuals with AIDS but also occurs in individuals receiving immunomodulatory therapies for immune-related diseases such as multiple sclerosis. JCPyV infection of host cells requires the pentasaccharide lactoseries tetrasaccharide c (LSTc) and the serotonin receptor 5-hydroxytryptamine (5-HT) receptor 5-HT2AR. While LSTc is involved in the initial attachment of virus to cells via interactions with VP1, the mechanism by which 5-HT2AR contributes to infection is not clear. To further define the roles of serotonin receptors in infection, HEK293A cells, which are poorly permissive to JCPyV, were transfected with 14 different isoforms of serotonin receptor. Only 5-HT2 receptors were found to support infection by JCPyV. None of the other 11 isoforms of serotonin receptor supported JCPyV infection. Expression of 5-HT2 receptors did not increase binding of JCPyV to cells, but this was not unexpected, given that the cells uniformly expressed the major attachment receptor, LSTc. Infection of these cells remained sensitive to inhibition with soluble LSTc, confirming that LSTc recognition is required for JCPyV infection. Virus internalization into HEK293A cells was significantly and specifically enhanced when 5HT2 receptors were expressed. Taken together, these data confirm that the carbohydrate LSTc is the attachment receptor for JCPyV and that the type 2 serotonin receptors contribute to JCPyV infection by facilitating entry.

Context-dependent modulation of auditory processing by serotonin

Context-dependent plasticity in auditory processing is achieved in part by physiological mechanisms that link behavioral state to neural responses to sound. The neuromodulator serotonin has many characteristics suitable for such a role. Serotonergic neurons are extrinsic to the auditory system but send projections to most auditory regions. These projections release serotonin during particular behavioral contexts. Heightened levels of behavioral arousal and specific extrinsic events, including stressful or social events, increase serotonin availability in the auditory system. Although the release of serotonin is likely to be relatively diffuse, highly specific effects of serotonin on auditory neural circuitry are achieved through the localization of serotonergic projections, and through a large array of receptor types that are expressed by specific subsets of auditory neurons. Through this array, serotonin enacts plasticity in auditory processing in multiple ways. Serotonin changes the responses of auditory neurons to input through the alteration of intrinsic and synaptic properties, and alters both short- and long-term forms of plasticity. The infrastructure of the serotonergic system itself is also plastic, responding to age and cochlear trauma. These diverse findings support a view of serotonin as a widespread mechanism for behaviorally relevant plasticity in the regulation of auditory processing. This view also accommodates models of how the same regulatory mechanism can have pathological consequences for auditory processing.

Pharmacological alterations of anxious behaviour in mice depending on both strain and the behavioural situation

A previous study comparing non-emotive mice from the strain C57BL/6/ByJ with ABP/Le mice showed ABP/Le to be more anxious in an open-field situation. In the present study, several compounds affecting anxiety were assayed on ABP/Le and C57BL/6/ByJ mice using three behavioural models of anxiety: the elevated plus-maze, the light-dark discrimination test and the free exploratory paradigm. The compounds used were the full benzodiazepine receptor agonist, chlordiazepoxide, and the antagonist, flumazenil, the GABA(A) antagonist, bicuculline, the full 5-HT(1A) agonist 8-OH-DPAT, and the mixed 5-HT(1A)/5-HT(1B) agonist, RU 24969. Results showed the effect of the compounds to be dependent on both the strain and the behavioural task. Several compounds found to be anxiolytic in ABP/Le mice had an anxiogenic effect on C57BL/6/ByJ mice. More behavioural changes were observed for ABP/Le in the elevated plus-maze, but the clearest findings for C57BL/6/ByJ mice were observed in the light-dark discrimination apparatus. These data demonstrate that anxious behaviour is a complex phenomenon which cannot be described by a single behavioural task nor by the action of a single compound.

Imaging of cholinergic and monoaminergic neurochemical changes in neurodegenerative disorders

Positron emission tomography (PET) or single photon emission computer tomography (SPECT) imaging provides the means to study neurochemical processes in vivo. These methods have been applied to examine monoaminergic and cholinergic changes in neurodegenerative disorders. These investigations have provided important insights into disorders, such as Alzheimer's disease (AD) and Parkinson's disease (PD). The most intensely studied monoaminergic transmitter is dopamine. The extent of presynaptic nigrostriatal dopaminergic denervation can be quantified in PD and may serve as a diagnostic biomarker. Dopaminergic receptor imaging may help to distinguish idiopathic PD from atypical parkinsonian disorders. Cholinergic denervation has been identified not only in AD but also in PD and more severely in parkinsonian dementia. PET or SPECT can also provide biomarkers to follow progression of disease or evaluate the effects of therapeutic interventions. Cholinergic receptor imaging is expected to play a major role in new drug development for dementing disorders.

Mechanism of action of atypical antipsychotic drugs and the neurobiology of schizophrenia

Atypical antipsychotics have greatly enhanced the treatment of schizophrenia. The mechanisms underlying the effectiveness and adverse effects of these drugs are, to date, not sufficiently explained. This article summarises the hypothetical mechanisms of action of atypical antipsychotics with respect to the neurobiology of schizophrenia.When considering treatment models for schizophrenia, the role of dopamine receptor blockade and modulation remains dominant. The optimal occupancy of dopamine D(2) receptors seems to be crucial to balancing efficacy and adverse effects - transient D(2) receptor antagonism (such as that attained with, for example, quetiapine and clozapine) is sufficient to obtain an antipsychotic effect, while permanent D(2) receptor antagonism (as is caused by conventional antipsychotics) increases the risk of adverse effects such as extrapyramidal symptoms. Partial D(2) receptor agonism (induced by aripiprazole) offers the possibility of maintaining optimal blockade and function of D(2) receptors. Balancing presynaptic and postsynaptic D(2) receptor antagonism (e.g. induced by amisulpride) is another mechanism that can, through increased release of endogenous dopamine in the striatum, protect against excessive blockade of D(2) receptors. Serotonergic modulation is associated with a beneficial increase in striatal dopamine release. Effects on the negative and cognitive symptoms of schizophrenia relate to dopamine release in the prefrontal cortex; this can be modulated by combined D(2) and serotonin 5-HT(2A) receptor antagonism (e.g. by olanzapine and risperidone), partial D(2) receptor antagonism or the preferential blockade of inhibitory dopamine autoreceptors. In the context of the neurodevelopmental disconnection hypothesis of schizophrenia, atypical antipsychotics (in contrast to conventional antipsychotics) induce neuronal plasticity and synaptic remodelling, not only in the striatum but also in other brain areas such as the prefrontal cortex and hippocampus. This mechanism may normalise glutamatergic dysfunction and structural abnormalities and affect the core pathophysiological substrates for schizophrenia.

Expression of mRNA for multiple serotonin (5-HT) receptor types/subtypes by the peripheral blood mononuclear cells of rhesus macaques

To find out whether rhesus macaque peripheral blood mononuclear cells (PBMCs) express mRNA for 5-HT receptors, blood samples from normal healthy rhesus monkeys were used to isolate PBMCs by Ficoll-paque density gradient centrifugation. Total RNA was extracted from MT-2 cells, Hut-78 cells, naive or phytohemagglutinin (PHA) stimulated human and monkey PBMCs. One tube RT-PCR was performed using primers specific for human 5-HT1A, 5-HT1B, 5-HT1E, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT3, 5-HT4, 5-HT6, and 5-HT7 receptors. Amplicons of expected sizes were obtained from human cell lines as well as both human and monkey PBMCs. Both PHA stimulated human and monkey PBMCs express mRNAs for 5-HT1A, 5-HT1B, 5-HT1E, 5-HT2A, 5-HT3, 5-HT4, 5-HT6 receptor types/subtypes. However, mRNAs for 5-HT1B, 5-HT1E and 5-HT2A cannot be confidently detected in some of the PBMC samples without PHA stimulation. 5-HT2B and 5-HT7 receptor mRNA was not detected in most of the samples and 5-HT2C receptor mNRA was not detected at all. FACS analysis revealed that CD3+ lymphocyte increased more than 20% among lymphocytes in the PHA stimulated PBMCs. These data indicate that similar to human PBMC, rhesus macaque PBMC may express multiple types of 5-HT receptors and the expression profile could change after PHA stimulation due to either the changes in cell composition or changes in gene transcription level. This provided a basis for further studies on the neuroimmunomodulatory interactions of 5-HT in rhesus macaques.

Reduced density of functional 5-HT1A receptors in the brain, medulla and spinal cord of monoamine oxidase-A knockout mouse neonates

Abnormally high brain 5-HT levels in monoamine oxidase-A knockout (MAO-A KO) mouse neonates raise the question of whether the distribution and density of the 5-HT1A receptors (5-HT1AR) expressed in the brain by postnatal day P7 are affected and, if so, whether the 5-HT1A autoreceptors in the dorsal raphe are modified in the same way as the postsynaptic 5-HT1AR present in raphe target structures. [3H]8-OH-DPAT binding and quantitative autoradiography were performed to answer these questions. Binding specificity was first confirmed in adult wild-type mice and rat brain sections. 5-HT1AR binding was then analyzed in four MAO-A mutant vs. five wild-type neonatal brains, from olfactory bulb to cervical cord. Among 12 structures expressing postsynaptic 5-HT1AR in wild-type neonates, the highest densities involved the retrosplenial cortex, entorhinal cortex, and septum (52-46 fmol/mg tissue); low densities occurred in the hippocampus and spinal cord (24 fmol/mg tissue); in addition, the raphe autoreceptor density was only 20 fmol/mg tissue. In mutants, the distribution of postsynaptic 5-HT1AR was unchanged, but an overall decrease in density occurred (-32% to -63%); the raphe autoreceptors decreased in mutants by at least -79%. Data are discussed with reference to the ectopic 5-HT uptake and accumulation reported to occur during the first 10 postnatal days in wild-type and MAO-A KO mice. As previously suggested to explain the raphe autoreceptor loss in 2-month-old MAO-A KO mice, the overall 5-HT1AR down-regulation in mutant pups probably results from extracellular 5-HT excess in both raphe and target structures. The greater the 5-HT excess, the more the functional receptor density decreases.

Of minds and embryos: left-right asymmetry and the serotonergic controls of pre-neural morphogenesis

Serotonin is a clinically important neurotransmitter regulating diverse aspects of cognitive function, sleep, mood, and appetite. Increasingly, it is becoming appreciated that serotonin signaling among non-neuronal cells is a novel patterning mechanism existing throughout diverse phyla. Here, we review the evidence implicating serotonergic signaling in embryonic morphogenesis, including gastrulation, craniofacial and bone patterning, and the generation of left-right asymmetry. We propose two models suggesting movement of neurotransmitter molecules as a novel mechanism for how bioelectrical events may couple to downstream signaling cascades and gene activation networks. The discovery of serotonin-dependent patterning events occurring long before the development of the nervous system opens exciting new avenues for future research in evolutionary, developmental, and clinical biology.

Serotonin-related enhancement of recovery of hind limb motor functions in spinal rats after grafting of embryonic raphe nuclei

Recently, we demonstrated improvements in hind limb locomotor-like movements following grafting of embryonic raphe nuclei cells into the spinal cord below the level of total transection in adult rats. The purpose of the present study was to clarify whether this improvement was due to newly established serotonergic innervation between the graft and the host. Two months after intraspinal grafting of the embryonic raphe nuclei, the spinalized rats, when put on a treadmill, could be induced to walk with regular alternating hind limb movements with the plantar contact with the ground during the stance phase, and ankle dorsiflexion during the swing phase of each step cycle. In the same situation the spinal rats, that did not receive the graft, were not able to initiate the dorsiflexion of the ankle joint during the swing phase and very often the dorsal surface of the foot was dragged along the ground. Intraperitoneal application of directly acting 5-HT2 antagonist Cyproheptadine (1 mg/kg) impaired reversibly the hind limb locomotor-like movements in grafted rats. This impairment lasted for 2-3 h. The same procedure in control rats did not markedly alter the hind limb locomotor-like movements. The effect of Cyproheptadine in grafted rats was reversed by i.p. injections of the 5-HT2 agonist Quipazine (0.5 mg/kg). These results show that the graft-induced restitution of hind limb locomotor abilities in adult spinal rats is brought about by the new serotonergic innervation of the host spinal cord circuitry from the grafted neurons and is mediated by 5-HT2 receptors.

Distribution of mRNA that codes for 5-hydroxytryptamine receptor subtypes in the gastrointestinal tract of dairy cows

OBJECTIVE

To describe the distribution of mRNA that codes for 8 subtypes of 5-hydroxytryptamine receptors (5-HTRs) in the digestive tract of dairy cows.

SAMPLE POPULATION

Fresh full-thickness wall specimens from the abomasum (fundus, corpus, and antrum), ileum, cecum, proximal loop of ascending colon, and 4 locations of the spiral colon collected from 10 healthy cows at slaughter.

PROCEDURE

Concentrations of mRNA that code for 5-HTR subtypes (5-HTR1A. 5-HTR1B, 5-HTR1D, 5-HTR1F, 5-HTR2A, 5-HTR2B, 5-HTR2C, and 5-HTR4) in the bovine digestive tract were measured by use of a quantitative real-time reverse transcription-polymerase chain reaction assay. Results were reported in relation to mRNA expression of the housekeeping gene glyceraldehyde phosphate dehydrogenase (GAPDH).

RESULTS

Mean relative mRNA concentrations for 5-HTR were low (range, 0% to 1.32% of GAPDH), and mRNA that codes for 5-HTR1A was not detected. In the abomasum, mRNA expression was highest for 5-HTR1B and 5-HTR2B, followed by subtypes 1F 2A, 1D, and 4, whereas 5-HTR2C was not detected. In intestinal samples, concentrations of subtypes 1B, 2B, and 4 were highest, followed by 1D, 1F, 2A, and 2C. Relative concentrations of mRNA that code for 5-HTR2A were significantly higher in the abomasum than the intestines, but lower for 5-HTR2B, 5-HTR2C, and 5-HTR4.

CONCLUSIONS AND CLINICAL RELEVANCE

Relative concentrations of mRNA that code for 5-HTRs differ among locations in the gastrointestinal tract of cattle. Understanding differences in the distribution of 5-HTRs in healthy cattle and cattle with gastrointestinal tract disease may lead to improved therapeutic approaches for abomasal and cecal motility disorders.

Quantitative mRNA analysis of eight bovine 5-HT receptor subtypes in brain, abomasum, and intestine by real-time RT-PCR

Serotoninergic pathways are involved in economically important bovine gastrointestinal (GI) motility disorders such as displaced abomasum and cecal dilatation/dislocation. The existing research tools to investigate the role of serotoninergic pathways in such disorders in ruminants comprise functional pharmacological methods, e.g., in vitro contractility studies in tissue baths, and electromyographical recordings in vivo. However, no tools for quantification of bovine serotonin receptor [5-hydroxytryptamine receptor (5-HTR)] expression were available so far. This study aimed to develop real-time RT-PCR assays for quantitative mRNA analysis of bovine 5-HTR subtypes. Because the bovine 5-HTR coding sequences (CDSs) were completely unknown, multiple species (human, mouse, and rat) alignment of complete CDS was used for primer design in highly homologous regions. LightCycler real-time RT-PCR assays (partial CDS) for the following bovine 5-HTR subtypes were developed and validated: 5-HTR1A, 5-HTR1B, 5-HTR1D, 5-HTR1F, 5-HTR2A, 5-HTR2B, 5-HTR2C, and 5-HTR4. Intra- and inter-assay coefficients of variation (CV) for the eight established assays were small, ranging from 0.49% to 2.46%. As a first physiological application, 5-HTR mRNA expression levels were measured in brain, abomasum, and intestine of 10 healthy, lactating dairy cows. The 5-HTR expression was quantified by normalization to the housekeeping gene glyceraldehyde-phosphate-dehydrogenase (GAPDH). The 5-HTR subtype expression levels ranged from 0.001% (5-HTR2C in intestine) to 1% 5-HTR/GAPDH (5-HTR1B and 5-HTR4 in intestine). There were high variations of 5-HTR subtype mRNA expression within tissues across receptor subtypes and within receptor subtypes across tissues. In conclusion, accurate real-time RT-PCR assays for quantitative analysis of bovine 5-HTR subtype gene expression were developed and validated.

Re-screening serotonin receptors for genetic variants identifies population and molecular genetic complexity

We have re-screened the genes for the 5-HT1A, 5-HT2A, 5-HT2C, and 5-HT7 serotonin receptors for genetic variants in a large African-American and Caucasian-American population sample. We have identified eight novel variants in these genes including four that are predicted to cause amino acid substitutions. These variants are additional candidates for association studies of behavioral disorders such as depression and schizophrenia as well as quantitative personality traits. We have also detected some, but not all, previously identified variants in these genes suggesting that many previously identified variants are unique to specific populations. The results of this study, and previous screens of serotonin receptors, demonstrate that the genes for serotonin receptors display marked population and molecular genetic complexity. These levels of complexity may have a substantial effect on genetic association studies of human behavioral variability related to these genes. We discuss the implications of these findings and propose methods to address complexity in genetic association studies.

Regulation of dendrite formation of Purkinje cells by serotonin through serotonin1A and serotonin2A receptors in culture

Serotonergic fibers and receptors appear in the rat cerebellum during early postnatal development. In the present study, we investigated the actions of serotonin (5-HT) and its receptors in the dendrite formation of Purkinje cells in organotypic cultures of anterior and posterior lobes of the cerebellum at postnatal day 7. In anterior lobes after 4 days in vitro (4DIV), the dendritic areas and branchings of Purkinje cells were increased by the treatment of 2 microM 5-HT, but decreased by 20 microM 5-HT. In posterior lobes after 4DIV, the dendritic areas of Purkinje cells were increased by 5-HT (2, 20 and 200 microM). In contrast, 5-HT treatment decreased dendritic areas of Purkinje cells in both anterior and posterior lobes after 7DIV. Next, we determined the actions of specific 5-HT receptors in mediating the effects of 5-HT by treatment with selective 5-HT receptor agonists. In anterior lobes after 4DIV, dendritic areas of Purkinje cells were increased by a 5-HT1A receptor agonist (8-OH-DPAT), whereas decreased by a 5-HT2A receptor agonist (DOI). The present study suggested that the dendrite formation of Purkinje cells is promoted by 5-HT through 5-HT1A receptors, but inhibited by 5-HT through 5-HT2A receptors.

Modifiers of gastrointestinal motility of cattle

Little clinically relevant, evidence-based data about the effect of motility modifiers on the GI tract of cattle are currently available. Additionally, some of the published results seem to be contradictory. Three main facts explain this apparent discrepancy: (1) Results may not be transferred from one species to another, because presence, concentration, location, distribution, and function of specific receptors may differ significantly among species. (2) The lack of a significant effect on smooth muscle preparations in vitro does not necessarily exclude a certain drug's motility-modifying property on affected animals in vivo. Certain drugs bind to receptors outside the myenteric plexus. Others, such as lidocaine and adrenergic-, dopaminergic-, and opioid-antagonists increase only GI motility, if inhibitory reflexes or a hyperactive state of the inhibitory sympathetic nervous system preexists. (3) Effects of motility modifiers as found in healthy experimental animals in vivo may not be similar to those found in spontaneously diseased animals. Accurate and reliable data on the effect of modifiers of GI motility of cattle will be obtained only from double-blinded, evidence-based, in vivo studies on spontaneously affected animals. Because well-documented results from such studies are extremely rare, intensive research in this field is warranted in the future.

Modification of spontaneous contractility of smooth muscle preparations from the bovine abomasal antrum by serotonin receptor agonists

The effects of serotonin (5-hydroxytryptamine, 5-HT), a 5-HT2-receptor agonist, alpha-methyl-5-hydroxytryptamine (alpha-M-5-HT) and RS-67506, a 5-HT4 receptor partial agonist, on spontaneous contractility of bovine abomasal smooth muscle preparations were investigated in vitro. Preparations from the abomasal antrum of freshly slaughtered healthy dairy cows were cut parallel to the longitudinal fibres, suspended in isolated organ baths, and concentration-response curves were performed by cumulative application of the 5-HT receptor agonists. Blockade of 5-HT2-induced response was tested with atropine and hexamethonium. Serotonin evoked a significant increase in the area under curve (AUC), whilst the 5-HT2 receptor agonist alpha-M-5-HT significantly increased the AUC and resting tone (RT). RS-67506 induced a significant increase in AUC and RT and a significant decrease in the maximum force. The effect of alpha-M-5-HT was mediated by a muscarinic cholinergic pathway, as the effect of alpha-M-5-HT was inhibited in the presence of atropine but not hexamethonium. It is concluded that 5-HT2 and 5-HT4 receptors are present in the bovine abomasal antrum. Muscarinic receptors are involved in the increase in RT seen after 5-HT2 receptor stimulation.

Characterization of a novel G-protein coupled receptor from the parasitic nematode H. contortus with high affinity for serotonin

The neurotransmitter serotonin (5HT) has been shown to modulate mobility, feeding, egg-laying, and defecation behaviors in the saprophytic nematode Caenorhabditis elegans. Although the effects of serotonin on these behaviors in parasitic nematodes is under study, little is known about the diversity, ontogeny, signaling, and pharmacology of serotonin receptors in these organisms. In an effort to increase our understanding of this system, we cloned and characterized a novel cDNA (5HT1Hc) from the parasitic nematode Haemonchus contortus that has high amino acid sequence homology with known G-protein coupled 5HT1-receptors from invertebrates and vertebrates. Transcript expression studies in four development stages (egg, L1/L2, L3, and adult) revealed the presence of the mRNA in the L1/L2, L3, and adult stages. Membranes from insect cells (Sf9) expressing the 5HT1Hc-receptor cDNA displayed nanomolar binding affinity to serotonin and a unique pharmacological profile distinct from known invertebrate and mammalian 5HT-receptors. Receptor signaling studies with mammalian AV12 cells expressing the 5HT1Hc-receptor and the promiscuous G-protein, Galpha15, demonstrated dose-dependent intracellular signals with serotonin acting as an agonist. Together, these studies describe a novel invertebrate 5HT-receptor with high affinity for the indolealkylamine, serotonin, and pharmacological properties that do not conform to any known members of this superfamily of metabotropic receptors.

Central serotonin and melanocortin pathways regulating energy homeostasis

It is now established that the hypothalamus is essential in coordinating endocrine, autonomic, and behavioral responses to changes in energy availability. However, the interaction of key peptides, neuropeptides, and neurotransmitters systems within the hypothalamus has yet to be delineated. Recently, we investigated the mechanisms through which central serotonergic (5-hydroxytryptamine, 5-HT) systems recruit leptin-responsive hypothalamic pathways, such as the melanocortin systems, to affect energy balance. Through a combination of functional neuroanatomy, feeding, and electrophysiology studies in rodents, we found that 5-HT drugs require functional melanocortin pathways to exert their effects on food intake. Specifically, we observed that anorectic 5-HT drugs activate pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (Arc). We provide evidence that the serotonin 2C receptor (5-HT(2C)R) is expressed on POMC neurons and contributes to this effect. Finally, we found that 5-HT drug-induced hypophagia is attenuated by pharmacological or genetic blockade of downstream melanocortin 3 and 4 receptors. We review candidate brain regions expressing melanocortin 3 and 4 receptors that play a role in energy balance. A model is presented in which activation of the melanocortin system is downstream of 5-HT and is necessary to produce the complete anorectic effect of 5-HT drugs. The data reviewed in this paper incorporate the central 5-HT system to the growing list of metabolic signals that converge on melanocortin neurons in the hypothalamus.

5-HT(1A) receptor subtype mRNA expression in cochlear nucleus

Previous studies indicate expression of various serotonin receptor subtypes, including the 5-HT(1A) receptor subtype, in rodent cochlear nucleus. Our long-term goal is to identify the types of cochlear nucleus neurons, which are well described in cat, that express 5-HT receptors. In the current study, the reverse transcriptase/polymerase chain reaction and the in situ hybridization method were used to detect the mRNA encoding a portion of the 5-HT(1A) receptor subtype in the cochlear nucleus of the cat.

5-HT(6) receptor antagonism potentiates the behavioral and neurochemical effects of amphetamine but not cocaine

The localization of serotonin 5-HT(6) receptors in limbic and motor brain regions, and the high affinity of these receptors for several antipsychotic agents, suggest that they may be involved in motor activity, reward-related behaviors, and psychotic disorders. The present study characterized the effects of a novel 5-HT(6) receptor antagonist, SB 258510A, on psychostimulant-induced motor activity, self-administration, and increases in extracellular dopamine in the nucleus accumbens and frontal cortex of male Wistar rats. The locomotor-activating effects of amphetamine (1mg/kg) were dose-dependently enhanced by pretreatment with SB 258510A (3, 10mg/kg). Similarly, amphetamine self-administration was dose-dependently altered by SB 258510A in a manner indicative of enhanced reinforcing effects of amphetamine on both fixed and progressive ratio schedules of reinforcement. SB 258510A treatment had no effect on either cocaine-induced locomotor activity or cocaine self-administration. Dual-probe in vivo microdialysis revealed that pretreatment with 3mg/kg SB 258510A potentiated an amphetamine-induced increase in extracellular dopamine more robustly in the frontal cortex than in the nucleus accumbens. These data indicate that activation of 5-HT(6) receptors may regulate behaviors related to amphetamine but not cocaine, and point to the frontal cortex as a possible site of action for these effects.

Diverse actions of ovarian steroids in the serotonin neural system

All of the serotonin-producing neurons of the mammalian brain are located in 10 nuclei in the mid- and hindbrain regions. The cells of the rostal nuclei project to almost every area of the forebrain and regulate diverse neural processes from higher order functions in the prefrontal cortex such as integrative cognition and memory, to limbic system control of arousal and mood, to diencephalic functions such as pituitary hormone secretion, satiety, and sexual behavior. The more caudal serotonin neurons project to the spinal cord and interact with numerous autonomic and sensory systems. All of these neural functions are sensitive to the presence or absence of the ovarian hormones, estrogen and progesterone. We have shown that serotonin neurons in nonhuman primates contain estrogen receptor beta and progestin receptors. Thus, they are targets for ovarian steroids which in turn modify gene expression. Any change in serotoninergic neural function could be manifested by a change in any of the projection target systems and in this manner, serotonin neurons integrate steroid hormone information and partially transduce their action in the CNS. This article reviews the work conducted in this laboratory on the actions of estrogens and progestins in the serotonin neural system of nonhuman primates. Comparisons to results obtained in other laboratory animal models are made when available and limited clinical data are referenced. The ability of estrogens and progestins to alter the function of the serotonin neural system at various levels provides a cellular mechanism whereby ovarian hormones can impact cognition, mood or arousal, hormone secretion, pain, and other neural circuits.

Neurotransmitter action in osteoblasts: expression of a functional system for serotonin receptor activation and reuptake

Neurotransmitter regulation of bone metabolism has been the subject of increasing interest and investigation. Because serotonin (5-HT) plays a role as a regulator of craniofacial morphogenesis, we investigated the expression and function of 5-HT receptors and the 5-HT transporter (5-HTT) in bone. Primary cultures of rat osteoblasts (rOB) and a variety of clonal osteoblastic cell lines, including ROS 17/2.8, UMR 106-H5, and Py1a, showed mRNA expression for 5-HTT as well as the 5-HT(1A), 5-HT(1D), 5-HT(2A), and 5-HT(2B) receptors by reverse transcription-polymerase chain reaction (RT-PCR) analysis. Protein expression of the 5-HT(1A), 5-HT(2A), and 5-HT(2B) receptors was confirmed by immunoblot. 5-HTT binding sites were assessed in ROS 17/2.8 and UMR 106-H5 cells by binding of the stable cocaine analog [125I]RTI-55, which showed a relatively high density of nanomolar affinity binding sites. Imipramine and fluoxetine, antagonists with specificity for 5-HTT, showed the highest potency to antagonize [125I]RTI-55 binding in ROS and UMR cells. GBR-12935, a relatively selective dopamine transporter antagonist, had a much lower potency, as did desipramine, a selective norepinephrine transporter antagonist. The maximal [3H]5-HT uptake rate in ROS cells was 110 pmol/10 min per well, with a K(m) value of 1.13 micromol/L. Imipramine and fluoxetine inhibited specific [3H]5-HT uptake with IC(50) values in the nanomolar range. In normal differentiating rOB cultures, 5-HTT functional activity was observed initially at day 25, and activity increased almost eightfold by day 31. In mature rOB cultures, the estimated density of [125I]RTI-55 binding sites was 600 fmol/mg protein. Functional downregulation of transporter activity was assessed after PMA treatment, which caused a significant 40% reduction in the maximal uptake rate of [3H]5-HT, an effect that was prevented by pretreatment with staurosporine. The affinity of 5-HT for the transporter was significantly increased following PMA treatment. We assessed the functional significance of expression of the 5-HT receptors by investigating the interaction between 5-HT and parathyroid hormone (PTH) signaling. 5-HT potentiates the PTH-induced increase in AP-1 activity in UMR cells. These results demonstrate that osteoblastic cells express a functional serotonin system, with mechanisms for responding to and regulating uptake of 5-HT.

Serotonin effects on frequency tuning of inferior colliculus neurons

We investigated the modulatory effects of serotonin on the tuning of 114 neurons in the central nucleus of the inferior colliculus (ICc) of Mexican free-tailed bats and how serotonin-induced changes in tuning influenced responses to complex signals. We obtained a "response area" for each neuron, defined as the frequency range that evoked discharges and the spike counts evoked by those frequencies at a constant intensity. We then iontophoretically applied serotonin and compared response areas obtained before and during the application of serotonin. In 58 cells, we also assessed how serotonin-induced changes in response areas correlated with changes in the responses to brief frequency-modulated (FM) sweeps whose structure simulated natural echolocation calls. Serotonin profoundly changed tone-evoked spike counts in 60% of the neurons (68/114). In most neurons, serotonin exerted a gain control, facilitating or depressing the responses to all frequencies in their response areas. In many cells, serotonergic effects on tones were reflected in the responses to FM signals. The most interesting effects were in those cells in which serotonin selectively changed the responsiveness to only some frequencies in the neuron's response area and had little or no effect on other frequencies. This caused predictable changes in responses to the more complex FM sweeps whose spectral components passed through the neurons' response areas. Our results suggest that serotonin, whose release varies with behavioral state, functionally reconfigures the circuitry of the IC and may modulate the perception of acoustic signals under different behavioral states.

Analyses of [18F]altanserin bolus injection PET data. II: Consideration of radiolabeled metabolites in humans

Imaging serotonin-2A (5-HT(2A)) neuroreceptors with positron emission tomography (PET) and [(18)F]altanserin has been the focus of a series of PET studies, as [(18)F]altanserin is one of the most selective 5-HT(2A) antagonist radiotracers. Previous animal studies showed that radiolabeled metabolites (radiometabolites) of [(18)F]altanserin crossed the blood-brain barrier (BBB) to localize nonspecifically in brain, consistent with a constant radioactivity "background." In this work, we evaluated human bolus injection [(18)F]altanserin PET data with detailed consideration of the impact of BBB-permeable metabolites on the specific binding parameters. Data were quantified using either single (parent radiotracer), dual (parent radiotracer and radiometabolites), or no arterial input function(s) (cerebellum as reference tissue input function). A step-gradient high-performance liquid chromatography (HPLC) analysis provided distinct separation of [(18)F]altanserin and four radiolabeled components in plasma. After [(18)F]altanserin injection, the step-gradient data showed that the major BBB-permeable radiometabolites approached constant levels in plasma (>50 min), consistent with a constant metabolite "background." The single-input Logan graphical results were highly correlated with the dual-input results and its bias was fairly constant across regions and subjects, as similarly observed for a nongraphical reference tissue method. The most comprehensive and quantitatively valid analysis for bolus [(18)F]altanserin PET data was the dual-input method that specifically accounted for BBB-permeable metabolites, although the Logan analysis was preferred because it provided a good compromise between validity, sensitivity, and reliability of implementation. Further study is needed to better understand how the cerebellar kinetics of [(18)F]altanserin and its radiometabolites impact the reference tissue measures.

Abnormal phrenic motoneuron activity and morphology in neonatal monoamine oxidase A-deficient transgenic mice: possible role of a serotonin excess

In rodent neonates, the neurotransmitter serotonin (5-HT) modulates the activity of both the medullary respiratory rhythm generator and the cervical phrenic motoneurons. To determine whether 5-HT also contributes to the maturation of the respiratory network, experiments were conducted in vitro on the brainstem-spinal cord preparation of neonatal mice originating from the control strain (C3H) and the monoamine oxidase A-deficient strain, which has a brain perinatal 5-HT excess (Tg8). At birth, the Tg8 respiratory network is unable to generate a respiratory pattern as stable as that produced by the C3H network, and the modulation by 5-HT of the network activity present in C3H neonates is lacking in Tg8 neonates. In addition, the morphology of the phrenic motoneurons is altered in Tg8 neonates; the motoneuron dendritic tree loses the C3H bipolar aspect but exhibits an increased number of spines and varicosities. These abnormalities were prevented in Tg8 neonates by treating pregnant Tg8 dams with the 5-HT synthesis inhibitor p-chlorophenylalanine or a 5-HT(2A) receptor antagonist but were induced in wild-type neonates by treating C3H dams with a 5-HT(2A) receptor agonist. We conclude that 5-HT contributes, probably via 5-HT(2A) receptors, to the normal maturation of the respiratory network but alters it when present in excess. Disorders affecting 5-HT metabolism during gestation may therefore have deleterious effects on newborns.

Serotonergic innervation of mesencephalic trigeminal nucleus neurons: a light and electron microscopic study in the rat

Neurons of the mesencephalic trigeminal nucleus (MTN) are considered to be homologous to mechanosensitive neurons in the sensory ganglia. The sites of origin of serotonin (5HT)-immunoreactive axons on neuronal cell bodies in the MTN were studied in the rat by combining immunofluorescence histochemical techniques with retrograde tracing of Fluoro-Gold (FG) and anterograde tracing of biotin-conjugated dextran amine (BDA). The tracing studies, which were combined with multiple-labeling immunohistochemistry and confocal microscopy, indicated that 5HT-immunoreactive axon terminals on the cell bodies of MTN neurons originated from the medullary raphe nuclei, such as the nucleus raphes magmus (RMg), alpha part of the nucleus reticularis gigantocellularis (GiA) and nucleus raphes obscurus (ROb), as well as from the mesopontine raphe nuclei, such as the nucleus raphes dorsalis (DR), nucleus raphes pontis (PnR) and nucleus raphes medianus (MnR); mainly from the RMg, GiA and DR, and additionally from the ROb, PnR and MnR. The cell bodies in close apposition to 5HT-immunoreactive axon terminals were found through the whole rostrocaudal extent of the MTN. Electron microscopically a number of axon terminals that were labeled with BDA injected into the raphe nuclei were confirmed to be in asymmetric synaptic contact with the cell bodies of MTN neurons. It was also indicated that substance P existed in some of the 5HT-containing axosomatic terminals arising from the ROb, RMg and GiA. The present results indicated that proprioceptive sensory signals from the muscle spindles or periodontal ligament might be modulated at the level of the primary afferent cell bodies in the MTN by 5HT-containing axons from the mesopontine and medullary raphe nuclei including the GiA.

Derivatives of (R)-1,11-methyleneaporphine: synthesis, structure, and interactions with G-protein coupled receptors

The design and synthesis of a well-characterized novel ring system, (R)-lambda1,11-methyleneaporphine [(R)-4], and 15 derivatives thereof are presented. The addition of various nucleophiles to (R)-lambda1,11-carbonylaporphine [(R)-11] or to the 1,11-hydroxymethyleneaporphine epimers gave separable mixtures of epimers. The epimeric ratios obtained in most reactions seem to be a result of steric factors directing the nucleophilic attack. The structure of the epimers was determined by a combination of X-ray crystallography (5 derivatives), NMR spectroscopy, and chemical correlation. Interesting and diverse pharmacological profiles of the derivatives were revealed through binding studies at serotonin 5-HT(7) and 5-HT(1A) receptors as well as at dopamine D(2A) receptors. Two derivatives appeared to be selective 5-HT(7) receptor antagonists. It is evident from our results that the novel ring system [(R)-4] provides a useful complement to other scaffolds available to medicinal chemists involved in studies of GPC receptors.

The role of serotonin(2) receptors in mediating cocaine-induced convulsions

Previous research in our laboratory suggests that serotonin (5-HT) neurotransmission mediates the expression of cocaine-induced convulsions. The role of 5-HT in mediating this toxic effect of cocaine appears to be due to activation of 5-HT(2) receptors, because cocaine-induced convulsions are blocked by the 5-HT(2) antagonists cinanserin, ketanserin, and pirenperone. The present study utilized a number of compounds that display a high affinity for 5-HT(2) receptors to further examine the role of these sites in mediating this toxic effect of cocaine. Cocaine-induced convulsions were observed following pretreatment with various doses of the following 5-HT(2) antagonists: mianserin, metergoline, MDL 11939, and methiothepin. In addition, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine (NAN 190) was tested to examine the influence of 5-HT(1) sites and the agonist compound 1-(3-triflurormethylphenyl)piperazine (TFMPP) was examined to further explore the role of 5-HT(1) and 5-HT(2) sites. Each 5-HT(2) antagonist attenuated cocaine-induced convulsions. Conversely, NAN 190 did not alter this toxic effect of cocaine. In addition, TFMPP significantly potentiated cocaine-induced convulsions. The results from this study support the hypothesis that 5-HT neurotransmission, acting primarily at 5-HT(2) receptors, plays an important role in mediating cocaine-induced convulsions.

Segregation of serotonin 5-HT2A and 5-HT3 receptors in inhibitory circuits of the primate cerebral cortex

An emerging concept of cortical network organization is that distinct segments of the pyramidal neuron tree are controlled by functionally diverse inhibitory microcircuits. We compared the expression of two serotonin receptor subtypes, the G-protein-coupled 5-hydroxytryptamine2A receptors and the ion-channel gating 5-HT3 receptors, in cortical neuron types, which control these microcircuits. Here we show, using light and electron microscopic immunocytochemical techniques, that 5-HT2A receptors are segregated from 5-HT3 receptors in the macaque cerebral cortex. 5-HT2A receptor immunolabel was found in pyramidal cells and also in GABAergic interneurons known to specialize in the perisomatic inhibition of pyramidal cells: large and medium-size parvalbumin- and calbindin-containing interneurons. In contrast, 5-HT3 label was only present in small GABA-, substance P receptor-, and calbindin-containing neurons and in medium-size calretinin-containing neurons: interneurons known to preferentially target the dendrites of pyramidal cells. This cellular segregation indicates a serotonin-receptor-specific segmentation of the GABAergic inhibitory actions along the pyramidal neuron tree.

Biological phenotypes associated with individuals at high risk for developing alcohol-related disorders: Part 1

This article reviews the results of studies concerning particular classes of biological phenotypes that may have relevance for alcohol dependence. Broadly defined, these classes include brain neurotransmitter systems and neuroelectric potentials. Evidence is presented concerning genotypic variation in alcoholics and high-risk relatives suggesting that the etiology of alcoholism and other addictive diseases is mediated in part through suboptimal neurotransmitter functioning. Research opportunities are offered with respect to specific candidate genes that have been cloned from these neurotransmitter systems that could be most fully utilized in family-based genetic analyses. Additional evidence is offered, suggesting that characteristics of particular neuroelectric potentials (e.g. the amplitude of the P300 component of the event-related potential) may provide another dimension of potential markers that could be used to identify children at risk. Finally, methodological considerations specific to high risk studies are discussed. Among these are the need to include a plan for studying more severe cases of alcohol dependence that are relatively uncomplicated by other major psychiatric disorders. Plans for long-term follow-up of children at highest risk for developing the disorder should also be included. Multiple domains of inquiry should not be viewed as "unfocused" but rather as an economical means for utilizing highly characterized samples of individuals meeting rigorous research criteria.

Serotonin differentially modulates responses to tones and frequency-modulated sweeps in the inferior colliculus

Although almost all auditory brainstem nuclei receive serotonergic innervation, little is known about its effects on auditory neurons. We address this question by evaluating the effects of serotonin on sound-evoked activity of neurons in the inferior colliculus (IC) of Mexican free-tailed bats. Two types of auditory stimuli were used: tone bursts at the neuron's best frequency and frequency-modulated (FM) sweeps with a variety of spectral and temporal structures. There were two main findings. First, serotonin changed tone-evoked responses in 66% of the IC neurons sampled. Second, the influence of serotonin often depended on the type of signal presented. Although serotonin depressed tone-evoked responses in most neurons, its effects on responses to FM sweeps were evenly mixed between depression and facilitation. Thus in most cells serotonin had a different effect on tone-evoked responses than it did on FM-evoked responses. In some neurons serotonin depressed responses evoked by tone bursts but left the responses to FM sweeps unchanged, whereas in others serotonin had little or no effect on responses to tone bursts but substantially facilitated responses to FM sweeps. In addition, serotonin could differentially affect responses to various FM sweeps that differed in temporal or spectral structure. Previous studies have revealed that the efficacy of the serotonergic innervation is partially modulated by sensory stimuli and by behavioral states. Thus our results suggest that the population activity evoked by a particular sound is not simply a consequence of the hard wiring that connects the IC to lower and higher regions but rather is highly dynamic because of the functional reconfigurations induced by serotonin and almost certainly other neuromodulators as well.

Effects of clozapine and haloperidol on 5-HT6 receptor mRNA levels in rat brain

The high affinity of 5-HT6 receptors for atypical antipsychotic drugs, and their localization in limbic and cortical regions of the brain, suggest that they might play a role in the pathophysiology of schizophrenia. To determine if this receptor is regulated by antipsychotics, rats were injected with clozapine (20 mg/kg/day), haloperidol (2 mg/kg/day), or vehicle daily for 2 weeks, and 5-HT6 receptor mRNA levels were measured by in situ hybridization. Clozapine but not haloperidol significantly decreased 5-HT6 expression in all subfields of the hippocampus. No drug effects were observed in cortical or forebrain structures. These results suggest that downregulation of this receptor in the hippocampus might be a characteristic of atypical antipsychotic drugs, although this hypothesis will require testing with other atypical antipsychotics.

Test-retest variability of serotonin 5-HT2A receptor binding measured with positron emission tomography and [18F]altanserin in the human brain

The role of serotonin in CNS function and in many neuropsychiatric diseases (e.g., schizophrenia, affective disorders, degenerative dementias) support the development of a reliable measure of serotonin receptor binding in vivo in human subjects. To this end, the regional distribution and intrasubject test-retest variability of the binding of [18F]altanserin were measured as important steps in the further development of [18F]altanserin as a radiotracer for positron emission tomography (PET) studies of the serotonin 5-HT2A receptor. Two high specific activity [18F]altanserin PET studies were performed in normal control subjects (n = 8) on two separate days (2-16 days apart). Regional specific binding was assessed by distribution volume (DV), estimates that were derived using a conventional four compartment (4C) model, and the Logan graphical analysis method. For both analysis methods, levels of [18F]altanserin binding were highest in cortical areas, lower in the striatum and thalamus, and lowest in the cerebellum. Similar average differences of 13% or less were observed for the 4C model DV determined in regions with high receptor concentrations with greater variability in regions with low concentrations (16-20%). For all regions, the absolute value of the test-retest differences in the Logan DV values averaged 12% or less. The test-retest differences in the DV ratios (regional DV values normalized to the cerebellar DV) determined by both data analysis methods averaged less than 10%. The regional [18F]altanserin DV values using both of these methods were significantly correlated with literature-based values of the regional concentrations of 5-HT2A receptors determined by postmortem autoradiographic studies (r2 = 0.95, P < 0.001 for the 4C model and r2 = 0.96, P < 0.001 for the Logan method). Brain uptake studies in rats demonstrated that two different radiolabeled metabolites of [18F]altanserin (present at levels of 3-25% of the total radioactivity in human plasma 10-120 min postinjection) were able to penetrate the blood-brain barrier. However, neither of these radiolabeled metabolites bound specifically to the 5-HT2A receptor and did not interfere with the interpretation of regional [18F]altanserin-specific binding parameters obtained using either a conventional 4C model or the Logan graphical analysis method. In summary, these results demonstrate that the test-retest variability of [18F]altanserin-specific binding is comparable to that of other PET radiotracers and that the regional specific binding of [18F]altanserin in human brain was correlated with the known regional distribution of 5-HT2A receptors. These findings support the usefulness of [18F]altanserin as a radioligand for PET studies of 5-HT2A receptors.

The molecular biology of serotonin receptors: therapeutic implications for the interface of mood and psychosis

This review summarizes the molecular biology of serotonin (5-HT; 5-hydroxytryptamine) receptors and indicates the potential relevance of this information for the treatment of mood and psychotic disorders. At least 15 separate subtypes of 5-HT receptors have been identified by molecular cloning techniques to be distinct genetic entities. Subtle differences in the primary amino acid sequences of these receptors can yield large differences in ligand selectivity. Additionally, it has recently been discovered that drugs such as atypical antipsychotic drugs and serotonin-selective reuptake inhibitors may interact with a large number of heretofore unknown 5-HT receptors. Thus clozapine, for instance, has high affinity for at least four separate 5-HT receptors, and it is unknown which of these receptors is essential for its unique therapeutic efficacy. One way to approach these questions is to test subtype-selective agents, although there are few of these currently available. Approaches to the design of subtype-selective ligands are described, including structure-based drug design and combinatorial approaches. Modes of regulation of 5-HT receptors are also summarized, and it is emphasized that antipsychotic drugs and antidepressants likely exert their effects via nontranscriptional and posttranslational means. Understanding the cellular mechanisms by which 5-HT receptors are regulated by psychopharmacologic agents is likely to yield novel insights into drug action.

Ovarian steroids and serotonin neural function

The serotonin neural system originates from ten nuclei in the mid- and hindbrain regions. The cells of the rostral nuclei project to almost every area of the forebrain, including the hypothalamus, limbic regions, basal ganglia, thalamic nuclei, and cortex. The caudal nuclei project to the spinal cord and interact with numerous autonomic and sensory systems. This article reviews much of the available literature from basic research and relevant clinical research that indicates that ovarian steroid hormones, estrogens and progestins, affect the function of the serotonin neural system. Experimental results in nonhuman primates from this laboratory are contrasted with studies in rodents and humans. The sites of action of ovarian hormones on the serotonin neural system include effects within serotonin neurons as well as effects on serotonin afferent neurons and serotonin target neurons. Therefore, information on estrogen and progestin receptor-containing neurons was synthesized with information on serotonin afferent and efferent circuits. The ability of estrogens and progestins to alter the function of the serotonin neural system at various levels provides a cellular mechanism whereby ovarian hormones can impact mood, cognition, pain, and numerous other autonomic functions.

Characteristics of agonist displacement of [3H]ketanserin binding to platelet 5-HT2A receptors: implications for psychiatric research

Brain 5-HT(2A) receptors exist in two agonist affinity states as a function of their coupling to Gq protein. This has not yet been shown in platelets. We examined [3H]ketanserin's saturable binding to platelet 5-HT2A receptors and characteristics of agonist displacement curves of [3H]ketanserin binding in healthy control subjects. [3H]ketanserin saturation curves showed a trend for a two-site model, reflecting two independent binding sites. At low [3H]ketanserin concentrations, agonist displacement curves were flat and best fit a two-site model, indicating the existence of two agonist affinity states. Guanylyl 5'-imidotriphosphate [Gpp(NH)p] induced a significant rightward shift in agonist displacement curves to fit a one-site model. Platelet membrane 5-HT2A receptors exist in two agonist affinity states that are regulated by Gq protein. Platelet 5-HT2A receptors provide an accessible model for examining possible dysregulation in the agonist affinity or coupling efficiency to the phosphoinositide system in psychiatric disorders and their modulation by psychotropic medications.

Why tryptophan hydroxylase is difficult to purify: a reactive oxygen-derived species-mediated phenomenon that may be implicated in human pathology

1. Attempts and apparently successful procedures to obtain reasonable quantities of electrophoretically homogenous mammalian brain-derived tryptophan hydroxylase, (TPH), have been described, starting in the early 1970s. This work has been carried out with the primary objective to obtain specific antisera to this enzyme to map out serotonergic pathways in the nervous system. 2. By using a multitude of techniques, antisera have indeed been fabricated and employed. However, it is doubtful if pure, native TPH has ever been produced. Indeed, there is strong evidence that more than one isoform of TPH exists in the rat brain. Thus, these antisera are probably directed against TPH-derived polypeptides and not the holoenzyme(s). 3. The difficulty in the purification of TPH lies not only in its subjectivity to proteolysis, but more importantly in its probable capacity to produce superoxide leading to hydrogen perioxide formation. This, in turn, may undergo Fenton chemistry with iron at the active site of the protein to produce hydroxyl radicals that directly attack and destroy the enzyme molecule. Evidence for such a mechanism is presented together with possible protocols that might be used to produce pure stable holo TPH(s). 4. It is hypothesized that similar oxidative events may take place in vivo under certain conditions leading to pathological results. Strategies to block these events are suggested.

Intradermal 5-HT induces Fos expression in rat dorsal horn neurons not via 5-HT3 but via 5-HT2A receptors

We investigated the effects of peripherally administered 5-HT on the secondary neurons in the spinal cord of rats using Fos-like immunoreactivity (FLI) as a marker of neuronal activation. The intradermal administration of 5-HT (30, 60 microg) induced a large number of FLI neurons in the ipsilateral dorsal horn. In animals given 5-HT2A receptor agonists (DOI: 0.28 to 2.8 micromol/kg, alpha-methyl 5-HT: 0.28 to 2.8 micromol/kg) intradermally, immunoreactive neurons were evoked in the same manner as those given 5-HT. Other agonists, including 5-HT3 receptor agonists (m-CPG: 16 to 32 micromol/kg, 2-methyl 5-HT: 0.0028 to 2.8 micromol/kg), did not induce FLI neurons at any dose examined. Furthermore, 5-HT2A receptor antagonist (ketanserin: 1 mg/kg, i.p.) suppressed the expression of FLI in the dorsal horn caused by peripheral 5-HT, but 5-HT3 receptor antagonist (tropisetron: 1 mg/kg, i.p.) did not. These findings suggest that the 5-HT-induced nociceptive response is mediated by 5-HT2A receptors in the periphery.

Mobilization of arachidonate and docosahexaenoate by stimulation of the 5-HT2A receptor in rat C6 glioma cells

In this study, we demonstrate that astroglial 5-HT2A receptors are linked to the mobilization of polyunsaturated fatty acids (PUFA). Stimulation of C6 glioma cells, prelabeled with [3H]arachidonate (AA, 20:4n6) and [14C]docosahexaenoate (DHA, 22:6n3), with serotonin and the 5-HT(2A/2C) receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) resulted in the mobilization of both [3H] and [14C] into the supernatant of the cell monolayers. The increased radioactivity in the supernatant was mainly associated with free fatty acids. Experiments using inhibitors of phosphoinositide-specific phospholipase C and PLA2, inhibited the DOI-stimulated mobilization of AA and DHA, suggesting the involvement of both phospholipases. Ketanserin (1 microM), a 5-HT(2A/2C) receptor antagonist, and MDL 100,907 (R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-pi peridine-methanol) (1 microM), a highly selective antagonist for 5-HT2A receptors, significantly decreased the DOI-stimulated release of AA and DHA. These results indicate that the 5-HT2A receptor is coupled to the mobilization of PUFA. The release of AA and DHA in response to serotonin may represent a mechanism through which astroglia provide these polyunsaturated fatty acids to neurons.

Serotonergic inhibition of phrenic motoneuron activity: an in vitro study in neonatal rat

In vitro experiments were conducted on neonatal rat brainstem-spinal cord preparations to test the hypothesis of an inhibitory modulation of phrenic activity by serotonin (5-HT) via non-5-HT2A receptors [Lindsay, A.D. and Feldman, J.L., Modulation of respiratory activity of neonatal rat phrenic motoneurones by serotonin, J. Physiol., 461 (1993) 213-233]. The changes induced by 5-HT and related agents on phrenic root discharges and membrane currents in identified phrenic motoneurons were analysed after blockade of spinal 5-HT2A receptors. Spinal application of 5-HT1B (but not 5-HT1A) receptor agonists depressed the phrenic activity and the effect was prevented by pretreatment with 5-HT1B (but not 5-HT1A, 5-HT2A and 5-HT3) receptor antagonists. Results from phrenic motoneuron whole cell recordings do not reject a presynaptic location of the 5-HT receptors responsible for this depression.

Intravenous cocaine self-administration in mice lacking 5-HT1B receptors

The present experiment tested the hypothesis that 5-HT1B receptors are involved in the reinforcing effects of cocaine. Transgenic mice lacking 5-HT1B receptors were used as subjects and compared with wild-type mice for the acquisition and maintenance of intravenous (IV) cocaine self-administration. Male 129/Sv-ter and 5-HT1B-minus 129/Sv-ter inbred mice (Columbia University, New York) were initially trained to press a lever under a fixed-ratio schedule 2, first for sweetened condensed milk as reinforcer and subsequently for cocaine (2.0 mg/kg/infusion). When a stable baseline of responding was obtained, each subject was tested under different doses of cocaine (1.0, 2.0, and 4.0 mg/kg), with the number of reinforcers per hour used as the dependent variable. Both strains successfully acquired food-shaping and cocaine self-administration, but the mutant mice presented a significantly shorter latency to meet IV cocaine self-administration acquisition criteria (p < 0.05). However, both wild-type and mutant mice had similar dose-response to cocaine. These results suggest that the 5-HT1B receptors may be implicated in the propensity to self-administer cocaine, but other mechanisms might be involved in the maintenance of cocaine self-administration.

Pretreatment with 5-HT1A receptor agonist flesinoxan attenuates Fos protein in rat hypothalamus

The 5-HT1A receptor agonist flesinoxan has anxiolytic activity and concurrently enhances plasma corticosterone levels in rats. After a second injection of flesinoxan 24 h later, the corticosterone response disappears, but not the anxiolytic effects. Male rats received two injections with either flesinoxan or vehicle within 24 h. Flesinoxan challenge enhanced Fos immunoreactivity in the paraventricular nucleus of the hypothalamus, the central amygdala, and the dorsolateral part of the bed nucleus of the stria terminalis and plasma corticosterone levels in the vehicle-pretreated rats. Flesinoxan pretreatment resulted in an attenuated response of plasma corticosterone levels and Fos-positive neurons in the paraventricular nucleus of the hypothalamus, but not in the central amygdala and the bed nucleus after a flesinoxan challenge. The differential desensitization levels for both behaviour and neuroendocrine responses after flesinoxan treatment seem to correspond to different organization levels in the brain, like limbic system and hypothalamus.