New mouse 5-HT2-like receptor. Expression in brain, heart and intestine

Normal Pregnancy-Induced Islet Beta Cell Proliferation in Mouse Models That Are Deficient in Serotonin-Signaling

During mouse pregnancy placental lactogens stimulate prolactin receptors on pancreatic islet beta cells to induce expression of the tryptophan hydroxylase Tph1, resulting in the synthesis and secretion of serotonin. Presently, the functional relevance of this phenomenon is unclear. One hypothesis is that serotonin-induced activation of 5-HT_2B receptors on beta cells stimulates beta cell proliferation during pregnancy. We tested this hypothesis via three different mouse models: (i) total Tph1KO mice, (ii) 129P2/OlaHsd mice, which are incompetent to upregulate islet Tph1 during pregnancy, whereas Tph1 is normally expressed in the intestine, mammary glands, and placenta, and (iii) Htr2b-deficient mice. We observed normal pregnancy-induced levels of beta cell proliferation in total Tph1KO mice, 129P2/OlaHsd mice, and in Htr2b^-/- mice. The three studied mouse models indicate that islet serotonin production and its signaling via 5-HT_2B receptors are not required for the wave of beta cell proliferation that occurs during normal mouse pregnancy.

First quantitative dosages: Strong correlations between non-5-HT2Rs serotonin receptors on normal human heart valves

Objectives

Although critical in animal and human development and pathology, a measurement of the quantitative expression of 5-HTR serotonin receptors on animal or human valvular tissues has never been performed.

Methods

Quantification of the most frequent 5-HTRs reported as being present in human peripheral tissue was performed using radiolabeled agonists/antagonists. A membrane protein extract from normal human valves (aortic/mitral/tricuspid and some pulmonary) and associated diseased left myocardium, all unusable in clinics, were obtained from the Homograft bank.

Results

We analyzed 5-HT1AR/5-HT1B/DR/5-HT2AR/5-HT2BR/5-HT 2CR/5-HT4R/5-HT7R from 28 hearts. We confirmed the presence of tissue and measured the quantitative content for respective proteins in femtomol/mg of protein extracts: for 5-HT2AR (35.9+/−0.7), 5-HT2BR (28.8+/−1.3) but also a newly observed and robust expression for 5-HT4R (38+/−4.2). We identified one, 5-HT1ARs (4.9+/−0.3), and the possible expression, but at a very low level, of previously reported 5-HT1B/DRs (1.3+/−0.5) as well as the new 5-HT7Rs (3.5+/0.1) and 5-HT2CRs (1.2+/−0.1). Interestingly, by using univariate analysis, we were able to observe many correlations between the different 5-HTR levels of expression especially between 5-HT1AR/5-HT1B/DR and also between 5-HT4R/5-HT7R, but none were observed between 5-HT2AR and 5-HT2BR. Using multivariate analyses for a specific 5-HTR level of expression, after adjustment for implantation sites and other 5-HTRs, we found that 5-HT1AR was correlated with 5-HT1B/DR;5-HT4R with 5-HT7R and 5-HT1AR;5-HT2BR with 5-HT2AR only. For 5-HT2C, no correlation was observed.

Conclusion

5-HT2AR/5-HT2BR and 5-HT4R were all observed to have a high and equal level of expression on human valves, but that of 5-HT1AR was more limited. Since these non-5-HT2Rs are coupled with different G-proteins, with specific signaling, theoretically they may control the main 5-HT2R signaling (i.e., PLC/DAG-PKC-ERK/Ras/Src signaling) involved in valvular fibrosis and degeneration.

"Selective" serotonin 5-HT2A receptor antagonists

Blockade of the serotonin 5-HT_2A G protein-coupled receptor (5-HT_2AR) is a fundamental pharmacological characteristic of numerous antipsychotic medications, which are FDA-approved to treat schizophrenia, bipolar disorder, and as adjunctive therapies in major depressive disorder. Meanwhile, activation of the 5-HT_2AR by serotonergic psychedelics may be useful in treating neuropsychiatric indications, including major depressive and substance use disorders. Serotonergic psychedelics and other 5-HT_2AR agonists, however, often bind other receptors, and standard 5-HT_2AR antagonists lack sufficient selectivity to make well-founded mechanistic conclusions about the 5-HT_2AR-dependent effects of these compounds and the general neurobiological function of 5-HT_2ARs. This review discusses the limitations and strengths of currently available "selective" 5-HT_2AR antagonists, the molecular determinants of antagonist selectivity at 5-HT_2ARs, and the utility of molecular pharmacological and computational methods in guiding the discovery of novel unambiguously selective 5-HT_2AR antagonists.

Multiple facets of serotonergic modulation

The serotonergic system of the central nervous system (CNS) has been implicated in a broad range of physiological functions and behaviors, such as cognition, mood, social interaction, sexual behavior, feeding behavior, sleep-wake cycle and thermoregulation. Serotonin (5-hydroxytryptamine, 5-HT) establishes a plethora of interactions with neurochemical systems in the CNS via its numerous 5-HT receptors and autoreceptors. The facets of this control are multiple if we consider the molecular actors playing a role in the autoregulation of 5-HT neuron activity including the 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_2B, 5-HT₇ receptors as well as the serotonin transporter. Moreover, extrinsic loops involving other neurotransmitters giving the other 5-HT receptors the possibility to impact 5-HT neuron activity. Grasping the complexity of these interactions is essential for the development of a variety of therapeutic strategies for cognitive defects and mood disorders. Presently we can illustrate the plurality of the mechanisms and only conceive that these 5-HT controls are likely not uniform in terms of regional and neuronal distribution. Our understanding of the specific expression patterns of these receptors on specific circuits and neuronal populations are progressing and will expand our comprehension of the function and interaction of these receptors with other chemical systems. Thus, the development of new approaches profiling the expression of 5-HT receptors and autoreceptors should reveal additional facets of the 5-HT controls of neurochemical systems in the CNS.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Serotonergic modulation across sensory modalities

The serotonergic system has been widely studied across animal taxa and different functional networks. This modulatory system is therefore well positioned to compare the consequences of neuromodulation for sensory processing across species and modalities at multiple levels of sensory organization. Serotonergic neurons that innervate sensory networks often bidirectionally exchange information with these networks but also receive input representative of motor events or motivational state. This convergence of information supports serotonin's capacity for contextualizing sensory information according to the animal's physiological state and external events. At the level of sensory circuitry, serotonin can have variable effects due to differential projections across specific sensory subregions, as well as differential serotonin receptor type expression within those subregions. Functionally, this infrastructure may gate or filter sensory inputs to emphasize specific stimulus features or select among different streams of information. The near-ubiquitous presence of serotonin and other neuromodulators within sensory regions, coupled with their strong effects on stimulus representation, suggests that these signaling pathways should be considered integral components of sensory systems.

Serotonin receptors in depression and anxiety: Insights from animal studies

Serotonin regulates many physiological processes including sleep, appetite, and mood. Thus, serotonergic system is an important target in the treatment of psychiatric disorders, such as major depression and anxiety. This natural neurotransmitter interacts with 7 families of its receptors (5-HT1-7), which cause a variety of pharmacological effects. Using genetically modified animals and selective or preferential agonists and antagonist, numerous studies demonstrated the involvement of almost all serotonin receptor subtypes in antidepressant- or anxiolytic-like effects. In this review, based on animal studies, we discuss the possible involvement of serotonin receptor subtypes in depression and anxiety.

Positive regulation of raphe serotonin neurons by serotonin 2B receptors

Serotonin is a neurotransmitter involved in many psychiatric diseases. In humans, a lack of 5-HT_2B receptors is associated with serotonin-dependent phenotypes, including impulsivity and suicidality. A lack of 5-HT_2B receptors in mice eliminates the effects of molecules that directly target serotonergic neurons including amphetamine derivative serotonin releasers, and selective serotonin reuptake inhibitor antidepressants. In this work, we tested the hypothesis that 5-HT_2B receptors directly and positively regulate raphe serotonin neuron activity. By ex vivo electrophysiological recordings, we report that stimulation by the 5-HT_2B receptor agonist, BW723C86, increased the firing frequency of serotonin Pet1-positive neurons. Viral overexpression of 5-HT_2B receptors in these neurons increased their excitability. Furthermore, in vivo 5-HT_2B-receptor stimulation by BW723C86 counteracted 5-HT_1A autoreceptor-dependent reduction in firing rate and hypothermic response in wild-type mice. By a conditional genetic ablation that eliminates 5-HT_2B receptor expression specifically and exclusively from Pet1-positive serotonin neurons (Htr2b ^5-HTKO mice), we demonstrated that behavioral and sensitizing effects of MDMA (3,4-methylenedioxy-methamphetamine), as well as acute behavioral and chronic neurogenic effects of the antidepressant fluoxetine, require 5-HT_2B receptor expression in serotonergic neurons. In Htr2b ^5-HTKO mice, dorsal raphe serotonin neurons displayed a lower firing frequency compared to control Htr2b ^lox/lox mice as assessed by in vivo extracellular recordings and a stronger hypothermic effect of 5-HT_1A-autoreceptor stimulation was observed. The increase in head-twitch response to DOI (2,5-dimethoxy-4-iodoamphetamine) further confirmed the lower serotonergic tone resulting from the absence of 5-HT_2B receptors in serotonin neurons. Together, these observations indicate that the 5-HT_2B receptor acts as a direct positive modulator of serotonin Pet1-positive neurons in an opposite way as the known 5-HT_1A-negative autoreceptor.

The serotonin 5-HT2C receptor and the non-addictive nature of classic hallucinogens

Classic hallucinogens share pharmacology as serotonin 5-HT_2A, 5-HT_2B, and 5-HT_2C receptor agonists. Unique among most other Schedule 1 drugs, they are generally non-addictive and can be effective tools in the treatment of addiction. Mechanisms underlying these attributes are largely unknown. However, many preclinical studies show that 5-HT_2C agonists counteract the addictive effects of drugs from several classes, suggesting this pharmacological property of classic hallucinogens may be significant. Drawing from a comprehensive analysis of preclinical behavior, neuroanatomy, and neurochemistry studies, this review builds rationale for this hypothesis, and also proposes a testable, neurobiological framework. 5-HT_2C agonists work, in part, by modulating dopamine neuron activity in the ventral tegmental area-nucleus accumbens (NAc) reward pathway. We argue that activation of 5-HT_2C receptors on NAc shell, GABAergic, medium spiny neurons inhibits potassium Kv1.x channels, thereby enhancing inhibitory activity via intrinsic mechanisms. Together with experiments that show that addictive drugs, such as cocaine, potentiate Kv1.x channels, thereby suppressing NAc shell GABAergic activity, this hypothesis provides a mechanism by which classic hallucinogen-mediated stimulation of 5-HT_2C receptors could thwart addiction. It also provides a potential reason for the non-addictive nature of classic hallucinogens.

Toll-like receptors 2 and 4 modulate the contractile response induced by serotonin in mouse ileum: analysis of the serotonin receptors involved

BACKGROUND

Microbiota through toll-like receptors (TLR) may regulate gastrointestinal motility by activating neuroendocrine mechanisms. We evaluated the influence of TLR2 and TLR4 in the spontaneous contractions and serotonin (5-HT)-induced motor response in mouse ileum, and the 5-HT receptors involved.

METHODS

Muscle contractility studies to evaluate the spontaneous intestinal motility and the response to 5-HT were performed in the ileum from wild type (WT), TLR2(-/-), TLR4(-/-), and TLR2/4 double knockout (DKO) mice. 5-HT receptor expression was determined by real-time PCR.

KEY RESULTS

The amplitude of spontaneous contractions in ileum was higher in TLR2(-/-), TLR4(-/-), and TLR2/4 DKO mice with respect to WT. 5-HT evoked concentration-dependent contractile responses in the ileum from TLR2(-/-) and TLR4(-/-) mice similar to WT. However, in ileum from TLR2/4 DKO, 5-HT did not induce any contractile response. Expression of 5-HT2A, 5-HT2B, 5-HT2C, and 5-HT3 receptors resulted increased in ileum from TLR4(-/-) and TLR2/4 DKO. Expression of the 5-HT4 receptor was diminished in TLR2(-/-) and TLR2/4 DKO. High levels of 5-HT7 receptor expression were found in TLR2/4 DKO but not in TLR2(-/-) or TLR4(-/-). In WT and TLR4(-/-), 5-HT2, 5-HT3, 5-HT4, and 5-HT7 receptor antagonists reduced the contractile response evoked by 5-HT. In TLR2(-/-) mice, 5-HT4 antagonist did not reduce the 5-HT response. In TLR2/4 DKO mice, only 5-HT4 and 5-HT7 receptor antagonists reduced the relaxing response induced by 5-HT.

CONCLUSIONS & INFERENCES

TLR2 and TLR4 signaling may modulate the spontaneous contractions and the serotonin contractile response by acting on 5-HT2, 5-HT3, 5-HT4, and 5-HT7 receptors.

Structure and function of serotonin G protein-coupled receptors

Serotonin receptors are prevalent throughout the nervous system and the periphery, and remain one of the most lucrative and promising drug discovery targets for disorders ranging from migraine headaches to neuropsychiatric disorders such as schizophrenia and depression. There are 14 distinct serotonin receptors, of which 13 are G protein-coupled receptors (GPCRs), which are targets for approximately 40% of the approved medicines. Recent crystallographic and biochemical evidence has provided a converging understanding of the basic structure and functional mechanics of GPCR activation. Currently, two GPCR crystal structures exist for the serotonin family, the 5-HT1B and 5-HT2B receptor, with the antimigraine and valvulopathic drug ergotamine bound. The first serotonin crystal structures not only provide the first evidence of serotonin receptor topography but also provide mechanistic explanations into functional selectivity or biased agonism. This review will detail the findings of these crystal structures from a molecular and mutagenesis perspective for driving rational drug design for novel therapeutics incorporating biased signaling.

Controversies on the role of 5-HT(2C) receptors in the mechanisms of action of antidepressant drugs

Evidence from the various sources indicates alterations in 5-HT2C receptor functions in anxiety, depression and suicide, and other stress-related disorders treated with antidepressant drugs. Although the notion of a 5-HT2C receptor desensitization following antidepressant treatments is rather well anchored in the literature, this concept is mainly based on in vitro assays and/or behavioral assays (hypolocomotion, hyperthermia) that have poor relevance to anxio-depressive disorders. Our objective herein is to provide a comprehensive overview of the studies that have assessed the effects of antidepressant drugs on 5-HT2C receptors. Relevant molecular (second messengers, editing), neurochemical (receptor binding and mRNA levels), physiological (5-HT2C receptor-induced hyperthermia and hormone release), behavioral (5-HT2C receptor-induced changes in feeding, anxiety, defense and motor activity) data are summarized and discussed. Setting the record straight about drug-induced changes in 5-HT2C receptor function in specific brain regions should help to determine which pharmacotherapeutic strategy is best for affective and anxiety disorders.

Urocortin1-induced anorexia is regulated by activation of the serotonin 2C receptor in the brain

This study was conducted to determine the mechanisms by which serotonin (5-hydroxytryptamine, 5-HT) receptors are involved in the suppression of food intake in a rat stress model and to observe the degree of activation in the areas of the brain involved in feeding. In the stress model, male Sprague-Dawley rats (8 weeks old) were given intracerebroventricular injections of urocortin (UCN) 1. To determine the role of the 5-HT2c receptor (5-HT2cR) in the decreased food intake in UCN1-treated rats, specific 5-HT2cR or 5-HT2b receptor (5-HT2bR) antagonists were administered. Food intake was markedly reduced in UCN1-injected rats compared with phosphate buffered saline treated control rats. Intraperitoneal administration of a 5-HT2cR antagonist, but not a 5-HT2bR antagonist, significantly inhibited the decreased food intake. To assess the involvement of neural activation, we tracked the expression of c-fos mRNA as a neuronal activation marker. Expression of the c-fos mRNA in the arcuate nucleus, ventromedial hypothalamic nucleus (VMH) and rostral ventrolateral medulla (RVLM) in UNC1-injected rats showed significantly higher expression than in the PBS-injected rats. Increased c-fos mRNA was also observed in the paraventricular nucleus (PVN), the nucleus of the solitary tract (NTS), and the amygdala (AMG) after injection of UCN1. Increased 5-HT2cR protein expression was also observed in several areas. However, increased coexpression of 5-HT2cR and c-fos was observed in the PVN, VMH, NTS, RVLM and AMG. Whereas, pro-opiomelanocortin mRNA expression was not changed. In an UNC1-induced stress model, 5-HT2cR expression and activation was found in brain areas involved in feeding control.

Multiple conformations of 5-HT2A and 5-HT 2C receptors in rat brain: an autoradiographic study with [125I](±)DOI

Earlier autoradiographic studies with the 5-HT2 receptor agonist [(125)I](±)DOI in human brain showed unexpected biphasic competition curves for various 5-HT2A antagonists. We have performed similar studies in rat brain regions with selective 5-HT2A (M100907) and 5-HT2C (SB242084) antagonists together with ketanserin and mesulergine. The effect of GTP analogues on antagonist competition was also studied. Increasing concentrations of Gpp(NH)p or GTPγS resulted in a maximal inhibition of [(125)I](±)DOI-specific binding of approximately 50 %. M100907 competed biphasically in all regions. In the presence of 100 μM Gpp(NH)p, M100907 still displaced biphasically the remaining [(125)I](±)DOI binding. Ketanserin showed biphasic curves in some regions and monophasic curves in others. In the latter, Gpp(NH)p evidenced an additional high-affinity site. SB242084 competed biphasically in brainstem nuclei and monophasically in the other regions. In most areas, SB242084 affinities were not notably altered by Gpp(NH)p. Mesulergine competed monophasically in all regions without alteration by Gpp(NH)p. These results conform with the extended ternary complex model of receptor action: receptor exists as an equilibrium of multiple conformations, i.e. ground (R), partly activated (R*) and activated G-protein-coupled (R*G) conformation/s. Thus, [(125)I](±)DOI would label multiple conformations of both 5-HT2A and 5-HT2C receptors in rat brain, and M100907 and ketanserin would recognise these conformations with different affinities.

Presynaptic control of serotonin on striatal dopamine function

RATIONALE

The influences of the serotonergic system on dopamine (DA) neuron activity have received considerable attention during the last three decades due to the real opportunity to improve disorders related to central DA neuron dysfunctions such as Parkinson's disease, schizophrenia, or drug abuse with serotonergic drugs. Numerous biochemical and behavioral data indicate that serotonin (5-HT) affects dopaminergic terminal function in the striatum.

OBJECTIVE

The authors propose a thorough examination of data showing controversial effects induced by striatal 5-HT on dopaminergic activity.

RESULTS

Inhibitory and excitatory effects of exogenous 5-HT have been reported on DA release and synthesis, involving various striatal 5-HT receptors. 5-HT also promotes an efflux of DA through reversal of the direction of DA transport. By analogy with the mechanism of action described for amphetamine, the consequences of 5-HT entering DA terminals might explain both the excitatory and inhibitory effects of 5-HT on presynaptic DA terminal activity, but the physiological relevance of this mechanism is far from clear. The recent data suggest that the endogenous 5-HT system affects striatal DA release in a state-dependent manner associated with the conditional involvement of various 5-HT receptors such as 5-HT(2A), 5-HT(2C), 5-HT(3), and 5-HT(4) receptors.

CONCLUSION

Methodological and pharmacological issues have prevented a comprehensive overview of the influence of 5-HT on striatal DA activity. The distribution of striatal 5-HT receptors and their restricted influence on DA neuron activity suggest that the endogenous 5-HT system exerts multiple and subtle influences on DA-mediated behaviors.

Immunohistochemical location of serotonin and serotonin 2B receptor in the small intestine of pigs

The distribution of serotonin and serotonin 2B receptor in the small intestines of pigs newborn, 5, 15 and 100 days of age were examined qualitatively and quantitatively by immunohistochemical labeling, microscopic observation and image analysis. The results showed serotonin immunopositive cells distributed diffusely among the epithelial cells of the middle and more basal parts of villi and intestinal glands in all segments of all pigs examined. Serotonin 2B receptor was first localized in the duodenum of 15-day-old pigs, whereas in 100-day-old pigs, serotonin 2B receptor was immunolabeled abundantly in all segments. Serotonin 2B receptor was distributed in the connective tissue of the small intestinal mucosa, lamina propria and in some myenteric neurons. The density of serotonin 2B receptor immunopositive cells in the duodenum of 100-day-old pigs was higher than that of 15-day-old pigs. The density of serotonin 2B receptor immunopositive cells in the duodenum was the highest among the three segments of the 100-day-old pigs. The study indicates that the distribution of serotonin 2B receptor is species different in the pig small intestine and the intensity of serotonin 2B receptor becomes stronger as the small intestine matures.

Molecular biology of 5-HT receptors

Serotonin (5-hydroxytryptamine; 5-HT) is a monoamine neurotransmitter whose effects are mediated by at least 13 distinct G protein-coupled receptors (GPCRs) of the type A family which includes the monoamine receptors and a combination of ligand-gated ion channels (5-HT3) of the Cys loop family which constitutes heteropentamers. 5-HT receptors are currently divided into seven classes (5-HT1 to 5-HT7), based on structural, transductional and operational features. While this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity is supported by the existence of a great number of splice and editing variants for several 5-HT receptors, their possible modulation by accessory proteins and chaperones, as well as their potential to form homo or heteromers both at the GPCR and at the ligand-gated channel level.

Serotonin 2B receptor: Upregulated with age and hearing loss in mouse auditory system

Serotonin (5-HT) is a monoamine neurotransmitter. Serotonin may modulate afferent fiber discharges in the cochlea, inferior colliculus (IC) and auditory cortex. Specific functions of serotonin are exerted upon its interaction with specific receptors; one of those receptors is the serotonin 2B receptor. The aim of this study was to investigate the differences in gene expression of serotonin 2B receptors with age in cochlea and IC, and the possible correlation between gene expression and functional hearing measurements in CBA/CaJ mice. Immunohistochemical examinations of protein expression of IC in mice of different age groups were also performed. Gene expression results showed that serotonin 2B receptor gene was upregulated with age in both cochlea and IC. A significant correlation between gene expression and functional hearing results was established. Immunohistochemical protein expression studies of IC showed more serotonin 2B receptor cells in old mice relative to young adult mice, particularly in the external nucleus. We conclude that serotonin 2B receptors may play a role in the pathogenesis of age-related hearing loss.

Endogenous 5-HT2B receptor activation regulates neonatal respiratory activityin vitro

An implication of 5-HT(2B) receptors in central nervous system has not yet been clearly elucidated. We studied the role of different 5-HT(2) receptor subtypes in the medullary breathing center, the pre-Bötzinger complex, and on hypoglossal motoneurons in rhythmically active transversal slice preparations of neonatal rats and mice. Local microinjection of 5-HT(2) receptor agonists revealed tonic excitation of hypoglossal motoneurons. Excitatory effects of the 5-HT(2B) receptor agonist BW723C86 could be blocked by bath application of LY272015, a highly selective 5-HT(2B) receptor antagonist. Excitatory effects of the 5-HT(2A/B/C) receptor agonist alpha-methyl 5-HT could be blocked by the preferential 5-HT(2A) receptor antagonist ketanserin. Therefore, 5-HT-induced excitation of hypoglossal motoneurons is mediated by convergent activation of 5-HT(2A) and 5-HT(2B) receptors. Local microinjection of BW723C86 in the pre-Bötzinger complex increased respiratory frequency. Bath application of LY272015 blocked respiratory activity, whereas ketanserin had no effect. Therefore, endogenous 5-HT appears to support tonic action on respiratory rhythm generation via 5-HT(2B) receptors. In preparations of 5-HT(2B) receptor-deficient mice, respiratory activity appeared unaltered. Whereas BW723C86 and LY272015 had no effects, bath application of ketanserin disturbed and blocked rhythmic activity. This demonstrates a stimulatory role of endogenous 5-HT(2B) receptor activation at the pre-Bötzinger complex and hypoglossal motoneurons that can be taken up by 5-HT(2A) receptors in the absence of 5-HT(2B) receptors. The presence of functional 5-HT(2B) receptors in the neonatal medullary breathing center indicates a potential convergent regulatory role of 5-HT(2B) and -(2A) receptors on the central respiratory network.

Effects of excess and deprivation of serotonin on in vitro neuronal differentiation

The neurotransmitter serotonin (5HT) possesses developmental functions in vertebrates and invertebrates. Rodent embryos express 5HT receptors even before neural development, but the role of this neurochemical seems to be particularly important during axonal morphogenesis and differentiation and in neural crest cell migration. Moreover, 5HT inhibitors are teratogenic in mammals, inducing brain and heart abnormalities. The aim of this study was to investigate the effects of nonphysiological concentrations of 5HT (5HT excess as well as deprivation) on developing rat neural cells using the micromass method. This simple and rapid micromass method allows the culture of mesencephalic cells capable of achieving and maintaining a significant degree of differentiation. Mesencephalic cells from 13 d post coitum (pc) rat were cultured and exposed to exogenous 5HT (1, 10, 50, or 100 microM) or to the specific 5HT2 receptor inhibitor mianserin (0.5, 5, 25, or 50 microM) during the whole culture period (5 d). The micromass morphology, the cytoskeletal organization, the pathological apoptosis, and the differentiative capability of cultured mesencephalic cells have been analyzed. The results show that 10-100 microM 5HT and 0.5-50 microM mianserin are able to disrupt the normal micromass morphology; 5HT and mianserin are unable to interfere with the cytoskeletal structures; mianserin (but not 5HT) induces pathological apoptosis on micromass cells at concentration levels of 0.5-50 microM; 5HT (but not mianserin) alters the neural differentiation at concentration levels of 10-100 microM. In conclusion, our results demonstrate that an excess of 5HT inhibits the capability of mesencephalic neurons to differentiate as shown by the alterations of the expression of the neuronal differentiative proteins glial-derived neurotrophic factor and Neu-N; on the other hand, the blocking of 5HT2 receptors induces apoptosis in differentiating neurons.

Differential effects of ethanol on gamma-aminobutyric acid-A receptor-mediated synaptic currents in congenic strains of inbred long and short-sleep mice

BACKGROUND

Ethanol enhances gamma-aminobutyric acid (GABA)A receptor-mediated responses in the brain, and this enhancement is greater in a mouse line behaviorally more sensitive to ethanol (long sleep) than in a line (short sleep) behaviorally less ethanol sensitive (assayed by loss of righting; sleep time). Quantitative trait locus (QTL) analysis of inbred long sleep (ILS) and inbred short sleep (ISS) phenotypes revealed four chromosomal regions (Lore1, Lore2, Lore4, and Lore5) that together account for approximately 50% of ethanol-induced sleep-time variance. Congenic strains were generated, each of which is homozygous for one of four ISS Lore QTLs on the ILS background. These congenic mouse strains are ideally suited for asking which QTL regions might correlate with other phenotypes that differ between ILS and ISS mice. Here we used the congenics to investigate altered GABAA responses to ethanol.

METHODS

Evoked GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) were measured by whole-cell voltage-clamp recording procedures in CA1 pyramidal neurons in hippocampal brain slices.

RESULTS

GABAA IPSC responses in hippocampal brain slices from ILS mice were significantly enhanced by 80 mM ethanol, whereas those from ISS mice were not affected. ILS.Lore2 and ILS.Lore5 congenic strains were significantly enhanced by 80 mM ethanol, similar to the background (control) ILS mice. However, ethanol had no significant effect on GABAA responses in ILS.Lore1 and ILS.Lore4 congenic mice, similar to the ISS mice, thus reflecting the influence of ISS alleles on the ILS phenotype.

CONCLUSIONS

Our results suggest that alleles located in the Lore1 and Lore4 QTL regions confer ethanol sensitivity of GABAA receptor-mediated IPSCs. Thus, for these QTLs, GABAA IPSCs may represent an endophenotype of sedative/hypnotic sensitivity to ethanol. Although the Lore2 and Lore5 QTL regions have a significant effect on sleep time, they do not play a significant role in the differential ethanol enhancement of GABAA IPSCs between ILS and ISS mice.

Increased wakefulness, motor activity and decreased theta activity after blockade of the 5-HT2B receptor by the subtype-selective antagonist SB-215505

Serotonin-2 receptor antagonists, like ritanserin, greatly enhance deep slow wave sleep (SWS-2) and low-frequency EEG power in humans and rodents. 5-HT(2A) and 5-HT(2C) receptors may be involved in these effects, but the role of the 5-HT(2B) receptor is still unclear. To investigate the role of the 5-HT(2B) receptor in regulation of the sleep-wake cycle, the subtype-selective antagonist SB-215505 (0.1, 0.3 and 1.0 mg kg(-1) i.p.) was administered to Sprague-Dawley rats at light onset (beginning of passive phase). EEG, EMG and motor activity were recorded during the subsequent 8 h. SB-215505 dose-dependently increased wakefulness (W) at the expense of the intermediate stage of sleep, paradoxical sleep (PS) and SWS-2 in the first hour. Parallel to increased W, significantly increased motor activity was found. Spectral analysis of the EEG in W showed a dose-dependent decrease in power density in the 3-8 Hz frequency range (maximum effect at 6 Hz). In light slow wave sleep and SWS-2, the drug reduced low-frequency (<8 Hz) EEG power, suggesting decreased sleep intensity after SB-215505 treatment. In PS, the drug dose-dependently decreased EEG power solely in the theta (6-9 Hz) band, primarily affecting the peak power value (7 Hz). The well-known SWS-2 enhancing effect of 5-HT(2) receptor antagonists is mediated by 5-HT(2A) and/or 5-HT(2C) receptors. In contrast, blockade of 5-HT(2B) receptors increases motor activity and W along with decreased theta activity during W and PS. Activation of 5-HT(2B) receptors may contribute to initiation of sleep and to theta generation during W and PS under physiological conditions.

Effects of sarpogrelate, a novel 5-HT2 antagonist, on 5-HT-induced endothelium-dependent relaxations in porcine coronary artery

The aim of the present study was to examine the effects of sarpogrelate, a 5-HT2 antagonist, on 5-HT-induced endothelium-dependent relaxation in isolated porcine coronary artery preincubated with ketanserin (3 x 10(-6) M) and precontracted by U 46619 (5 x 10(-9) M) and compare its effects with other 5-HT2 antagonists such as ritanserin and cyproheptadine. The investigation showed that sarpogrelate (10(-7)-10(-5) M) had a weak antagonistic effect on 5-HT-induced relaxation and its effect was weaker than that of ritanserin (10(-9)-10(-7) M) and cyproheptadine (10(-8)-10(-6) M). The rank order of the antagonistic effects was: ritanserin > cyproheptadine > sarpogrelate. The study also showed that both sarpogrelate and ritanserin had no inhibitory effect on bradykinin-induced relaxation. In our previous study, we investigated the binding affinity of sarpogrelate, ritanserin and cyproheptadine to the 5-HT2A-receptor in rabbit cerebral cortex membranes and the pKi values found were 7.22, 8.98 and 7.54, respectively (M. Rashid et al., Jpn J Pharmacol 87, 189-194, 2001). Rank order of the calculated ratio of concentration of pA2 or pD'2 vs Ki was: sarpogrelate > ritanserin > cyproheptadine. Thus, these findings suggest that sarpogrelate has the lowest antagonistic effect on 5-HT-induced endothelium-dependent relaxation and the highest selectivity towards 5-HT2A receptor and might also be the safest drug with respect to its clinical implications in comparison with ritanserin and cyproheptadine.

The serotonin binding site of human and murine 5-HT2B receptors: molecular modeling and site-directed mutagenesis

Bacteriorhodopsin and rhodopsin crystal structures were used as templates to build structural models of the mouse and human serotonin (5-HT)-2B receptors (5-HT(2B)Rs). Serotonin was docked to the receptors, and the amino acids predicted to participate to its binding were subjected to mutagenesis. 5-HT binding affinity and 5-HT-induced inositol triphosphate production were measured in LMTK(-) cells transfected with either wild-type or mutated receptor genes. According to these measurements, the bacteriorhodopsin-based models of the 5-HT(2B)Rs appear more confident than the rhodopsin-based ones. Residues belonging to the transmembrane domains 3 and 6, i.e. Asp(3.32), Ser(3.36), Phe(6.52), and Asn(6.55), make direct contacts with 5-HT. In addition, Trp(3.28), Phe(3.35), Phe(6.52), and Phe(7.38) form an aromatic box surrounding 5-HT. The specificity of human and mouse 5-HT(2B)Rs may be reflected by different rearrangements of the aromatic network upon 5-HT binding. Two amino acids close to Pro(5.50) in the human transmembrane domain 5 sequence were permuted to introduce a "mouse-like" sequence. This change was enough to confer the human 5-HT(2B)R properties similar to those of the mouse. Taken together, the computed models and the site-directed mutagenesis experiments give a structural explanation to (i) the different 5-HT pK(D) values measured with the human and mouse 5-HT(2B)Rs (7.9 and 5.8, respectively) and (ii) the specificity of 5-HT binding to 5-HT(2B)Rs as compared with other serotonergic G-protein coupled receptors.

5-HT(2A) receptors: location and functional analysis in intestines of wild-type and 5-HT(2A) knockout mice

The distribution and function of the 5-hydroxytryptamine (5-HT(2A)) receptor were investigated in the intestines of wild-type (5-HT(2A) +/+) and knockout (5-HT(2A) -/-) mice. In 5-HT(2A) +/+ mice, rats, and guinea pigs, 5-HT(2A) receptor immunoreactivity was found on circular and longitudinal smooth muscle cells, neurons, enterocytes, and Paneth cells. Muscular 5-HT(2A) receptors were concentrated in caveolae; neuronal 5-HT(2A) receptors were found intracellularly and on the plasma membranes of nerve cell bodies and axons. Neuronal 5-HT(2A) immunoreactivity was detected as early as E14 in ganglia, intravillus nerves, and the deep muscle plexus. The 5-HT(2A) -/- colon did not express 5-HT(2A) receptors and did not contract in response to exogenous 5-HT. 5-HT(2A) -/- enterocytes were smaller, Paneth cells fewer, and muscle layers thinner (and showed degeneration) compared with those of 5-HT(2A) +/+ littermates. The 5-HT(2A) receptor may thus be required for the maintenance and/or development of enteric neuroeffectors and other enteric functions, although gastrointestinal and colonic transit times in 5-HT(2A) -/- and +/+ mice did not differ significantly.

Serotonin 5- HT (2C) receptor knockout mice: autoradiographic analysis of multiple serotonin receptors

Quantitative receptor autoradiography was used to study possible alterations of the densities of multiple serotonin (5-HT) receptor subtypes and of serotonin transporter in the brain of 5-HT(2C) receptor knockout mice. The radioligands employed were [(3)H]citalopram, [(3)H]WAY100,635, [(3)H]8-OH-DPAT, [(3)H]GR125743, [(3)H]sumatriptan, [(3)H]MDL100,907, [(125)I](+/-)DOI, [(3)H]mesulergine, [(3)H]5-HT, [(3)H]GR113808, and [(3)H]5-CT. As expected, radioligands that label 5-HT(2C) receptors showed a complete absence of labeling in mutant mice choroid plexus and significantly reduced densities in other brain regions expressing 5-HT(2C) receptors. With the rest of the radioligands, no significant alterations in the densities of labeled sites were found in any brain region. In situ hybridization showed no changes in 5-HT(2A) receptor and serotonin transporter mRNA levels, whereas 5-HT(2C) receptor mRNA levels were reduced in certain brain regions. The present results indicate that the mouse serotonergic system does not exhibit compensatory up- or down-regulation of the majority of its components (serotonin transporter and most 5-HT receptor subtypes) in response to the absence of 5-HT(2C) receptors.

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.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Developmentally regulated serotonin 5-HT2B receptors

Serotonin (5-hydroxytryptamine, 5-HT) binds to numerous cognate receptors to initiate its biological effects. In this review, we have focused on the 5-HT2B receptor to address how signaling and expression of this receptor is specifically implicated in embryonic development and adult health and disease. Transduction of the 5-HT2B signaling is complex, including phospholipase C and A2 stimulation, cGMP production and a mitogenic signal that integrates the tyrosine kinase-signaling pathway. Furthermore, 5-HT, through the 5-HT2B receptors, has the ability to control serotonergic differentiation of committed neuron-like cells. In addition, 5-HT2B receptors are actively involved in the transient action of 5-HT during embryonic morphogenesis. Our recent data presented the first genetic evidence that 5-HT via 5-HT2B receptors regulates cardiac embryonic development and adult functions and suggested that this receptor subtype may be involved in other physiopathological situations. In particular, 5-HT-dependent molecular mechanisms may be involved in embryonic development and postnatal maturation of the enteric nervous system. Also, the involvement of the 5-HT2B receptor in the vascular growth often observed in hypertension is likely. These probably result from reactivation of developmentally regulated receptors in pathological situations. Finally, embryonic functions of 5-HT2 receptors observed in Drosophila gastrulation suggest evolutionary conserved mechanisms.

Dissociation of systemic and hippocampal modulation of rat locomotor activity by 5-HT(2C) receptors

In the present study, the ability of 5-hydroxytryptamine(2C) (5-HT(2C)) receptors in the hippocampus to enhance locomotor activity in rats was investigated by local infusion. Intraperitoneal injection of the selective 5-HT(2C) antagonist SB 242084 (1 mg/kg) significantly increased rats motor activity, while the effects of non-selective 5-HT(2C) agonist m-chlorophenylpiperazine (m-CPP, 10 mg/kg; i.p.) on motor activity were lower than those of the control group. In the day hours, the local infusion of non-selective 5-HT(2C) agonist, m-CPP (1.0 mM) into the bilateral hippocampus via microdialysis probes increased locomotor activity in contrast with intraperitoneal injection. This increase was completely reversed by the combined infusion of the selective 5-HT(2C) antagonist SB 242084 (100 mM). In the night hours, the local infusion of SB 242084 (100 mM) into the bilateral hippocampus significantly inhibited the nocturnal hyperactivity, which was reversed by the combined infusion of m-CPP. The present study demonstrates that the 5-HT(2C) receptors in the hippocampus act to increase rat locomotor activity.

Sonic hedgehog promotes G(1) cyclin expression and sustained cell cycle progression in mammalian neuronal precursors

Sonic hedgehog (Shh) signal transduction via the G-protein-coupled receptor, Smoothened, is required for proliferation of cerebellar granule neuron precursors (CGNPs) during development. Activating mutations in the Hedgehog pathway are also implicated in basal cell carcinoma and medulloblastoma, a tumor of the cerebellum in humans. However, Shh signaling interactions with cell cycle regulatory components in neural precursors are poorly understood, in part because appropriate immortalized cell lines are not available. We have utilized primary cultures from neonatal mouse cerebella in order to determine (i) whether Shh initiates or maintains cell cycle progression in CGNPs, (ii) if G(1) regulation by Shh resembles that of classical mitogens, and (iii) whether individual D-type cyclins are essential components of Shh proliferative signaling in CGNPs. Our results indicate that Shh can drive continued cycling in immature, proliferating CGNPs. Shh treatment resulted in sustained activity of the G(1) cyclin-Rb axis by regulating levels of cyclinD1, cyclinD2, and cyclinE mRNA transcripts and proteins. Analysis of CGNPs from cyclinD1(-/-) or cyclinD2(-/-) mice demonstrates that the Shh proliferative pathway does not require unique functions of cyclinD1 or cyclinD2 and that D-type cyclins overlap functionally in this regard. In contrast to many known mitogenic pathways, we show that Shh proliferative signaling is mitogen-activated protein kinase independent. Furthermore, protein synthesis is required for early effects on cyclin gene expression. Together, our results suggest that Shh proliferative signaling promotes synthesis of regulatory factor intermediates that upregulate or maintain cyclin gene expression and activity of the G(1) cyclin-Rb axis in proliferating granule neuron precursors.

Behavioral effects of AMI-193, a 5-HT(2A)- and dopamine D(2)-receptor antagonist, in the squirrel monkey

8-[3-(4-Fluorophenoxy) propyl]-1-phenyl-1,3,8-triazaspiro[4, 5]decan-4-one (AMI-193) was developed as a 5-HT(2A)-selective antagonist with in vivo activity suitable for behavioral studies. However, AMI-193 is a potent dopamine D(2)-receptor antagonist with low nanomolar affinity. Accordingly, D(2)-actions may contribute to its behavioral pharmacology. In the present study, the effects of AMI-193 on operant behavior were characterized in squirrel monkeys. In subjects trained under a fixed-interval (FI) schedule of stimulus termination, AMI-193 (0.003-0.01 mg/kg) dose-dependently decreased response rate. When administered in combination with cocaine (0.03-3. 0 mg/kg) or the selective dopamine uptake inhibitor, GBR 12909 (0. 03-3.0 mg/kg), the rate-decreasing effects of AMI-193 were reversed by both dopamine indirect agonists. In drug-discrimination experiments, AMI-193 (0.003 and 0.01 mg/kg) attenuated the discriminative-stimulus effects of cocaine. AMI-193 (0.003 and 0.01 mg/kg) also reduced response rate under a second-order schedule of i. v. self-administration of cocaine (0.1 mg/infusion). The profile of behavioral effects and drug interactions observed in the present study, in conjunction with the relatively high affinity of AMI-193 for dopamine D(2) receptors, suggests that its D(2)-antagonist effects play a prominent role in the behavioral pharmacology of AMI-193.

The multi-PDZ domain protein MUPP1 is a cytoplasmic ligand for the membrane-spanning proteoglycan NG2

A yeast two-hybrid screen was employed to identify ligands for the cytoplasmic domain of the NG2 chondroitin sulfate proteoglycan. Two overlapping cDNA clones selected in the screen are identical in sequence to a DNA segment coding for the most amino-terminal of the 13 PDZ domains found in the multi-PDZ-protein MUPP1. Antibodies made against recombinant polypeptides representing these two clones (NIP-2 and NIP-7) are reactive with the same 250-kDa molecule recognized by anti-MUPP1 antibodies, confirming the presence of the NIP-2 and NIP-7 sequences in the MUPP1 protein. NIP-2 and NIP-7 GST fusion proteins effectively recognize NG2 in pull-down assays, demonstrating the ability of these polypeptide segments to interact with the intact proteoglycan. The fusion proteins fail to bind NG2 missing the C-terminal half of the cytoplasmic domain, emphasizing the role of the NG2 C-terminus in the interaction with MUPP1. The existence of an NG2/MUPP1 interaction in situ is demonstrated by the ability of NG2 antibodies to co-immunoprecipitate both NG2 and MUPP1 from detergent extracts of cells expressing the two molecules. MUPP1 may serve as a multivalent scaffold that provides a means of linking NG2 with key structural and/or signaling components in the cytoplasm.

Acceleration of gastrointestinal transit by momordin Ic in mice: possible involvement of 5-hydroxytryptamine, 5-HT(2) receptors and prostaglandins

Possible involvement of 5-hydroxytryptamine (5-HT), 5-HT receptors and prostaglandins in the acceleration of gastrointestinal transit by momordin Ic was investigated in mice. Accelerative effect of momordin Ic (25 mg/kg, p.o.) on gastrointestinal transit was attenuated by pretreatment with a bolus of DL-p-chlorophenylalanine methyl ester (an inhibitor of 5-HT synthesizing enzyme), but not repeated pretreatment with DL-p-chlorophenylalanine methyl ester. Furthermore, cyproheptadine (a nonselective 5-HT(2) receptor antagonist), ritanserin (a 5-HT(2A/2B/2C) receptor antagonist) and clozapine (a 5-HT(2A/2C) receptor antagonist) also attenuated the effect of momordin Ic, but methiothepin (a 5-HT(1) receptor antagonist), MDL 72222 (3-tropanyl-3,5-dichlorobenzoate) and metoclopramide (5-HT(3) receptor antagonists), tropisetron (a 5-HT(3/4) receptor antagonist), ketanserin and haloperidol (5-HT(2A) receptor antagonists) did not. These results suggested a possible involvement of endogenous 5-HT and 5-HT(2B/2C) over 5-HT(2A) receptors. Attenuation by pretreatment with indomethacin (an inhibitor of prostaglandins synthesis) suggested involvement of prostaglandins. It is postulated that momordin Ic accelerates gastrointestinal transit partially by stimulating synthesis of 5-HT to act through 5-HT(2), possibly 5-HT(2C) and/or 5-HT(2B) receptors, which, in turn, increases synthesis of prostaglandins.

Serotonin and the 5-HT(2B) receptor in the development of enteric neurons

We tested the hypothesis that 5-HT promotes the differentiation of enteric neurons by stimulating a developmentally regulated receptor expressed by crest-derived neuronal progenitors. 5-HT and the 5-HT(2) agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine(.)HCl (DOI) enhanced in vitro differentiation of enteric neurons, both in dissociated cultures of mixed cells and in cultures of crest-derived cells isolated from the gut by immunoselection with antibodies to p75(NTR). The promotion of in vitro neuronal differentiation by 5-HT and DOI was blocked by the 5-HT(1/2) antagonist methysergide, the pan-5-HT(2) antagonist ritanserin, and the 5-HT(2B/2C)-selective antagonist SB206553. The 5-HT(2A)-selective antagonist ketanserin did not completely block the developmental effects of 5-HT. 5-HT induced the nuclear translocation of mitogen-activated protein kinase. This effect was blocked by ritanserin. mRNA encoding 5-HT(2A) and 5-HT(2B) receptors was detected in the fetal bowel (stomach and small and large intestine), but that encoding the 5-HT(2C) receptor was not. mRNA encoding the 5-HT(2B) receptor and 5-HT(2B) immunoreactivity were found to be abundant in primordial [embryonic day 15 (E15)-E16] but not in mature myenteric ganglia. 5-HT(2B)-immunoreactive cells were found to be a subset of cells that expressed the neuronal marker PGP9.5. These data demonstrate for the first time that the 5-HT(2B) receptor is expressed in the small intestine as well as the stomach and that it is expressed by enteric neurons as well as by muscle. It is possible that by stimulating 5-HT(2B) receptors, 5-HT affects the fate of the large subset of enteric neurons that arises after the development of endogenous sources of 5-HT.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Anxiolytic-like actions of BW 723C86 in the rat Vogel conflict test are 5-HT2B receptor mediated

The 5-HT2B receptor agonist, BW 723C86 (10, 30(mg/kg i.p. 30 min pre-test), increased the number of punishments accepted in a rat Vogel drinking conflict paradigm over 3 min, as did the benzodiazepine anxiolytics, chlordiazepoxide (2.5-10 mg/kg p.o. 1 h pre-test) and alprazolam (0.2-5 mg/kg p.o. 1 h pre-test), but not the 5-HT2C/2B receptor agonist, m-chlorophenylpiperazine (mCPP, 0.3-3 mg/kg i.p) or the 5-HT1A receptor agonist, buspirone (5-20 mg/kg p.o. 1 h pre-test). The effect of BW 723C86 was unlikely to be secondary to enhanced thirst, as BW 723C86 did not increase the time that rats with free access to water spent drinking, nor did it reduce sensitivity to shock in the apparatus. The anti-punishment effect of BW 723C86 was opposed by prior treatment with the 5-HT2/2B receptor antagonist, SB-206553 (10 and 20 mg/kg p.o. 1 h pre-test), and the selective 5-HT2B receptor antagonist, SB-215505 (1 and 3 mg/kg p.o. 1 h pre-test), but not by the selective 5-HT2C receptor antagonist, SB-242084 (5 mg/kg p.o.), or the 5-HT1A receptor antagonist, WAY 100635 (0.1 or 0.3 mg/kg s.c. 30 min pre-test). Thus, the anti-punishment action of BW 723C86 is likely to be 5-HT2B receptor mediated. This is consistent with previous reports that BW 723C86 exhibited anxiolytic-like properties in both the social interaction and Geller-Seifter conflict tests.

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.

Paradoxical effects of lithium on serotonergic receptor function: an immunocytochemical, behavioural and autoradiographic study

Lithium is the preferred treatment for bipolar affective disorder, yet its mechanism of action is poorly understood. Our study was designed to investigate the effect of lithium on the 5-HT2A or 5-HT2C (5-HT2A/2C) receptor subtypes, by comparing the consequences of chronic pre-treatment of rats with lithium on 5-HT2A/2C receptor-mediated behavioural responses, Fos expression, and the density of these receptors in the brain. In addition, the time-course and persistence of the effect of chronic lithium on 5-HT2A/2C receptor-mediated Fos expression was examined. Furthermore, the acute action of lithium on Fos expression was also examined. In an investigation of the dose response of Fos to the 5-HT2A/2C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), rats received saline or 1, 2, 4, 8, 12, 16, 24 or 32 mg/kg DOI, then were sacrificed 3 h later for immunocytochemical localisation of Fos. In a chronic lithium study, rats received either control or lithium-containing (0.1% LiCO3) chow for 3 weeks prior to challenge with 8 mg/kg DOI. DOI-induced locomotor activity was measured for 30 min immediately following the drug challenge, then 150 min later, the animals were sacrificed for Fos immunocytochemistry. The brains of another group of rats, also receiving either control or lithium-containing diet for 3 weeks, were analysed for the distribution and density of 5-HT2A receptor binding sites by quantitative [3H]ketanserin autoradiography. One group of chronic lithium treated rats received ritanserin (0.4 mg/kg), a 5-HT2A/2C receptor antagonist, 40 min before DOI challenge and were sacrificed 3 h later for Fos localisation. In the time-course experiment, rats received lithium-containing diet for 3 weeks followed by normal, control diet for 48 h, 1, 2 or 4 weeks prior to DOI or saline challenge. A further group of animals received an injection of LiCl (3 mM/kg) before being challenged with DOI or saline 12, 24, 36 or 48 h later. The dose-response experiment revealed that little Fos-like immunoreactivity was evident above basal levels following administration of 1 mg/kg DOI. However, at all other doses examined, Fos-like immunoreactivity was elevated in a number of brain areas, particularly in cerebral cortex, olfactory tubercle and amygdala. Following 24 mg/kg DOI, the number of Fos-positive nuclei appeared to have reached a plateau level. Treatment of rats with chronic lithium significantly enhanced DOI-induced locomotor activity and Fos-like immunoreactivity throughout the cerebral cortex. This elevation in Fos-like immunoreactivity was completely abolished by prior treatment with ritanserin. In contrast, chronic lithium treatment had no effect on the density of [3H]ketanserin binding to 5-HT2A receptors in any brain region examined. The results of the time-course experiment demonstrated that the enhancing effect of lithium on 5-HT2A/2C receptor-mediated Fos expression was short-lived such that Fos-like immunoreactivity returned to untreated levels within 48 h. In the acute lithium experiment, administration of lithium to rats 12 or 24 h before DOI resulted in a similar elevation of Fos-like immunoreactivity to that seen in chronically treated animals. Administration of acute lithium 36 or 48 h before DOI had no effect. The effects of lithium on 5-HT2A/2C receptor function thus appear to be complex. In particular, the results of this study indicate that the enhancing effects of lithium on DOI-induced locomotor activity and Fos-like immunoreactivity are not accompanied by any alteration in the density of 5-HT2A receptor binding sites. If changes in receptor numbers therefore do not account for the physiological effect of chronic lithium, other explanations must be sought. The study also suggests that the inositol depletion hypothesis of lithium's therapeutic action does not adequately explain the mechanism of action of lithium in man.

Functional characteristics of heterologously expressed 5-HT receptors

Over the past 10 years, molecular cloning has revealed the presence of 15 serotonin (5-hydroxytryptamine; 5-HT) receptor subtypes, which can be subdivided in seven subfamilies. Except for the 5-HT3 receptors, which are ligand-gated ion channels, all 5-HT receptors belong to the superfamily of G-protein-coupled receptors. The large multiplicity of 5-HT receptor subtypes has been suggested to be a direct result of the evolutionary age of the 5-HT system. Molecular information on G-protein-coupled 5-HT receptors is currently available for several mammalian species as well as for a limited number of invertebrate species (insects, molluscs). The aim of this review is to give an overview of all cloned 5-HT receptor subtypes belonging to the superfamily of G-protein-coupled receptors with specific emphasis on the pharmacological and signaling properties of the receptors upon expression in several heterologous expression systems.

Distribution of the 5-hydroxytryptamine2C receptor protein in adult rat brain and spinal cord determined using a receptor-directed antibody: effect of 5,7-dihydroxytryptamine

A synthetic peptide, corresponding to the N-terminal decapeptide (+Y11C12) of the rat 5-hydroxytryptamine2C (5-HT2C) receptor protein was used to produce a sheep polyclonal antiserum. Western blot analysis showed that the resultant antibody G241 recognised two membrane proteins, one (55 kDa) approximating the molecular mass of the 5-HT2C receptor (52 kDa) and a second (63 kDa), which may be a glycosylated form of the receptor protein. HEK 293 cells transfected with human 5-HT2C cDNA displayed intense cell surface immunoreactivity with the 5-HT2C antiserum, which was completely prevented by incubating the antibody with the synthetic 5-HT2C peptide (10 microM), whilst neither non-immune serum nor untransfected cells displayed any immunoreactivity. A radioimmunoassay was developed to quantify the regional distribution of 5-HT2C-like immunoreactivity (LI) in the adult rat brain. The choroid plexus contained five-fold higher levels of 5-HT2C-LI than any brain region but high levels were found in the frontal cortex, septum, hypothalamus, and striatum, intermediate levels in the thalamus and midbrain, and lower levels in brainstem, cerebellum, and spinal cord. In rat cortical membranes, the B(max) value from [3H]-mesulergine binding was ten-fold lower than 5-HT2C-LI levels determined by radioimmunoassay, which may reflect measurement of internalised receptor protein by radioimmunoassay which is not detected with conventional 5-HT2C ligands. Ten days after depletion of 5-HT with the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), there was a significant increase in 5-HT2C-LI in the choroid plexus and the ventral cervical spinal cord, suggesting that receptors therein are located post-synaptic to destroyed serotonergic nerve terminals. In contrast, the significant reduction in 5-HT2C-LI observed in the midbrain, brainstem, and dorsal thoracic spinal cord following 5,7-DHT implies that 5-HT2C receptors may be located on 5-HT nerve terminals in these regions.

Identification and localization of a skeletal muscle secrotonin 5-HT2A receptor coupled to the Jak/STAT pathway

The neurotransmitter serotonin mediates a wide variety of peripheral and central physiological effects through the binding to multiple receptor subtypes (Wilkinson, L. O., and Dourish, C. T. (1991) in Serotonin Receptor Subtypes: Basic and Clinical Aspects (Peroutka, S. J., ed) Vol. 15, pp.147-210, Wiley-Liss, New York). Among them, serotonin 5-HT2A receptors are known to activate the phospholipase C-beta second messenger pathway (Peroutka, S. J. (1995) Trends Neurosci. 18, 68-69). We identified and localized in rat skeletal muscle myoblasts a functional serotonin 5-HT2A receptor. This receptor was detected on the plasma membrane, in myoblasts, and at the level of T-tubules in contracting myotubes. Binding of serotonin to its receptor increases the expression of genes involved in myogenic differentiation. Unexpectedly, the 5-HT2A receptor is able to activate another signaling pathway; it triggers a rapid and transient tyrosine phosphorylation of Jak2 kinase in response to serotonin. Jak2 auto-phosphorylation is followed by the tyrosine phosphorylation of STAT3 (signal transducers and activators of transcription) and its translocation into the nucleus. We also find that the 5-HT2A receptor and STAT3 co-precipitate with Jak2, indicating that they are physically associated. We conclude that the serotonin 5-HT2A receptor identified in skeletal muscle myoblasts is able to activate the intracellular phosphorylation pathway used by cytokines. The presence of serotonin receptors in T-tubules suggests a role for serotonin in excitation-contraction coupling and (or) an effect in skeletal muscle fiber repairing.

A comparative study of the effects of selective and non-selective 5-HT2 receptor subtype antagonists in rat and mouse models of anxiety

Although there is some evidence that compounds acting at 5-HT2 receptors show anxiolytic activity, little is known about the specific involvement of the different 5-HT2 receptor subtypes in the modulation of anxiety-related responses. In the present study, the behavioural effects of mianserin, a non-selective 5-HT2 receptor antagonist, MDL 100,907, a selective 5-HT2A receptor antagonist, and SB 206553, a selective 5-HT2B/2C receptor antagonist, were investigated in two rat (the Vogel drinking conflict and the elevated plus-maze tests) and two mouse (i.e. the mouse defense test battery (MDTB) and the light/dark choice test) models of anxiety. Diazepam was used as a positive control. In the Vogel drinking test, mianserin (10 mg/kg) and SB 206553 (3-30 mg/kg), but not MDL 100,907, increased punished responding. Similarly, mianserin (1 mg/kg) and SB 206553 (3-10 mg/kg), but not MDL 100,907, increased entries into the open arms of the elevated plus-maze. These effects are consistent with anxiolytic-like actions of mianserin and SB 206553, although the magnitude of the effects of these two compounds was less than those of diazepam. In addition, in the MDTB, the 5-HT2 antagonists did not clearly affect the defensive reactions of mice exposed to a rat stimulus and they failed to reverse the avoidance of the illuminated box in the light/dark choice test. These results indicate a lack of anxiolytic-like action of the compounds in mice. These behavioural profiles suggest that blockade of the 5-HT2A receptor may not reduce anxiety and demonstrate that 5-HT2B and/or 5-HT2C receptor subtypes may be primarily involved in the anxiolytic-like effects of mianserin and SB 206553 in rats.