Serotonin receptor knockouts: a moody subject

A label-free strategy for immobilization of GPCRs using site-specific encoded non-natural amino acids to develop a selectively chromatographic approach for pursuing potential ligands binding to 5-hydroxytryptamine 1A receptor

G protein-coupled receptors (GPCRs) are one of the most prominent targets for drug discovery. Immobilizing GPCRs has proven to be an effective strategy for expanding the utility of GPCRs into nonbiological contexts. However, traditional strategies of immobilizing GPCRs have been severely challenged due to the loss of receptor function. Here, we reported a novel and general approach to realize the label-free and site-selective immobilization of 5-hydroxytryptamine 1A receptor (5-HT1AR) and the application in developing a chromatographic method with improved specificity for pursuing 5-HT1AR ligands from natural products. This method involved the use of a clickable non-natural amino acid, O-allyl-L-tyrosine (O-ALTyr) to immobilize the receptor onto thiol-functionalized silica gels through a 'thiol-ene' click chemistry, which allowed us to avoid the purification step and directly immobilize 5-HT1AR on silica gels. The immobilized receptor was characterized using immunofluorescence assay, and receptor-ligand interaction analysis was conducted through frontal analysis. To test the feasibility of the immobilized 5-HT1ARO-ALTyr in complex matrices, bioactive compounds in Ziziphi Spinosae Semen (ZSS) were screened and their interaction with the receptor was assessed using zonal elution. Our findings indicated that immobilizing the receptor through nnAAs effectively minimizes the chromatographic peak tailing and broadening of specific ligands, leading to a significant improvement in chromatographic performance. The association constants of the three ligands to 5-HT1AR were approximately one order of magnitude greater than those of Halo-tag attachment. These results demonstrated that the immobilized 5-HT1AR exhibits high specificity and the ability to recognize receptor ligands from complex matrices. This allowed us to identify magnoflorine (Mag) as a potential ligand of 5-HT1AR from ZSS extract. In vivo assay also proved that Mag presented a promising anxiolytic effect by promoting the expression of 5-HT1AR in mice brain. The above findings pointed to the fact that the immobilized 5-HT1AR affinity chromatographic strategy relying on the site-specific encoded non-natural amino acid is a powerful alternative for precisely determining the drug-protein interaction and discovering the specific ligand of GPCRs from complex matrixes.

A novel optical biosensor for direct and selective determination of serotonin in serum by Solid Surface-Room Temperature Phosphorescence

This paper describes a novel biosensor which combines the use of nanotechnology (non-woven nanofibre mat) with Solid Surface-Room Temperature Phosphorescence (SS-RTP) measurement for the determination of serotonin in human serum. The developed biosensor is simple and can be directly applied in serum; only requires a simple clean-up protocol. Therefore it is the first time that serotonin is analysed directly in serum with a non-enzymatic technique. This new approach is based on the covalent immobilization of serotonin directly from serum on a functional nanofibre material (Tiss®-Link) with a preactivated surface for direct covalent immobilization of primary and secondary amines, and the subsequent measurement of serotonin phosphorescent emission from the solid surface. The phosphorescent detection allows avoiding the interference from any fluorescence emission or scattering light from any molecule present in the serum sample which can be also immobilised on the nanofibre material. The determination of serotonin with this SS-RTP sensor overcomes some limitations, such as large interference from the matrix and high cost and complexity of many of the methods widely used for serotonin analysis. The potential applicability of the sensor in the clinical diagnosis was demonstrated by analysing serum samples from seven healthy volunteers. The method was validated with an external reference laboratory, obtaining a correlation coefficient of 0.997 which indicates excellent correlation between the two methods.

Behavioral studies on anxiety and depression in a drug discovery environment: keys to a successful future

The review describes a personal journey through 25 years of animal research with a focus on the contribution of rodent models for anxiety and depression to the development of new medicines in a drug discovery environment. Several classic acute models for mood disorders are briefly described as well as chronic stress and disease-induction models. The paper highlights a variety of factors that influence the quality and consistency of behavioral data in a laboratory setting. The importance of meta-analysis techniques for study validation (tolerance interval) and assay sensitivity (Monte Carlo modeling) are demonstrated by examples that use historic data. It is essential for successful discovery of new potential drugs to maintain a high level of control in animal research and to bridge knowledge across in silico modeling, and in vitro and in vivo assays. Today, drug discovery is a highly dynamic environment in search of new types of treatments and new animal models which should be guided by enhanced two-way translation between bench and bed. Although productivity has been disappointing in the search of new and better medicines in psychiatry over the past decades, there has been and will always be an important role for in vivo models in-between preclinical discovery and clinical development. The right balance between good science and proper judgment versus a decent level of innovation, assay development and two-way translation will open the doors to a very bright future.

Postnatal maintenance of the 5-Ht1a-Pet1 autoregulatory loop by serotonin in the raphe nuclei of the brainstem

Background

Despite the importance of 5-HT1A as a major target for the action of several anxiolytics/antidepressant drugs, little is known about its regulation in central serotonin (5-hydroxytryptamine, 5-HT) neurons.

Results

We report that expression of 5-HT1A and the transcription factor Pet1 was impaired in the rostral raphe nuclei of mice lacking tryptophan hydroxylase 2 (Tph2) after birth. The downregulation of Pet1 was recapitulated in 5-Ht1a^-/- mice. Using an explant culture system, we show that reduction of Pet1 and 5-HT1A was rescued in Tph2^-/- brainstem by exogenous 5-HT. In contrast, 5-HT failed to rescue reduced expression of Pet1 in 5-Ht1a^-/- brainstem explant culture.

Conclusions

These results suggest a causal relationship between 5-HT1A and Pet1, and reveal a potential mechanism by which 5-HT1A-Pet1 autoregulatory loop is maintained by 5-HT in a spatiotemporal-specific manner during postnatal development. Our results are relevant to understanding the pathophysiology of certain psychiatric and developmental disorders.

Social defeat: impact on fear extinction and amygdala-prefrontal cortical theta synchrony in 5-HTT deficient mice

Emotions, such as fear and anxiety, can be modulated by both environmental and genetic factors. One genetic factor is for example the genetically encoded variation of the serotonin transporter (5-HTT) expression. In this context, the 5-HTT plays a key role in the regulation of central 5-HT neurotransmission, which is critically involved in the physiological regulation of emotions including fear and anxiety. However, a systematic study which examines the combined influence of environmental and genetic factors on fear-related behavior and the underlying neurophysiological basis is missing. Therefore, in this study we used the 5-HTT-deficient mouse model for studying emotional dysregulation to evaluate consequences of genotype specific disruption of 5-HTT function and repeated social defeat for fear-related behaviors and corresponding neurophysiological activities in the lateral amygdala (LA) and infralimbic region of the medial prefrontal cortex (mPFC) in male 5-HTT wild-type (+/+), homo- (−/−) and heterozygous (+/−) mice. Naive males and experienced losers (generated in a resident-intruder paradigm) of all three genotypes, unilaterally equipped with recording electrodes in LA and mPFC, underwent a Pavlovian fear conditioning. Fear memory and extinction of conditioned fear was examined while recording neuronal activity simultaneously with fear-related behavior. Compared to naive 5-HTT+/+ and +/− mice, 5-HTT−/− mice showed impaired recall of extinction. In addition, 5-HTT−/− and +/− experienced losers showed delayed extinction learning and impaired recall of extinction. Impaired behavioral responses were accompanied by increased theta synchronization between the LA and mPFC during extinction learning in 5-HTT-/− and +/− losers. Furthermore, impaired extinction recall was accompanied with increased theta synchronization in 5-HTT−/− naive and in 5-HTT−/− and +/− loser mice. In conclusion, extinction learning and memory of conditioned fear can be modulated by both the 5-HTT gene activity and social experiences in adulthood, accompanied by corresponding alterations of the theta activity in the amygdala-prefrontal cortex network.

Differential effects of cholesterol and desmosterol on the ligand binding function of the hippocampal serotonin(1A) receptor: implications in desmosterolosis

Cholesterol is a unique molecule in terms of high level of in-built stringency, fine tuned by natural evolution for its ability to optimize physical properties of higher eukaryotic cell membranes in relation to biological functions. We previously demonstrated the requirement of membrane cholesterol in maintaining the ligand binding activity of the hippocampal serotonin(1A) receptor. In order to test the molecular stringency of the requirement of cholesterol, we depleted cholesterol from native hippocampal membranes followed by replenishment with desmosterol. Desmosterol is an immediate biosynthetic precursor of cholesterol in the Bloch pathway differing only in a double bond at the 24th position in the alkyl side chain. Our results show that replenishment with desmosterol does not restore ligand binding activity of the serotonin(1A) receptor although replenishment with cholesterol led to significant recovery of ligand binding. This is in spite of similar membrane organization (order) in these membranes, as monitored by fluorescence anisotropy measurements. The requirement for restoration of ligand binding activity therefore appears to be more stringent than the requirement for the recovery of overall membrane order. These novel results have potential implications in understanding the interaction of membrane lipids with this important neuronal receptor in diseases such as desmosterolosis.

Interest of using genetically manipulated mice as models of depression to evaluate antidepressant drugs activity: a review

Among the multiple possibilities to study human depressive disorders, animal models remain important preclinical tools. They allow the understanding of the mechanisms of action of antidepressant drugs. Primarily developed in rat, animal models of depression have been adapted to the mouse, an easy-to-use mammal with better genetic possibilities than rats. As an example, genetic manipulation of the serotoninergic 5-hydroxytryptamine-HT; (5-HT) system provided important opportunities to investigate the role of this monoamine in mood disorders. The contribution of either constitutive knockout (KO), tissue specific, or inducible KO mice and animal models in the current knowledge of the pathophysiology and treatment of depression is unanimously recognized. The phenotype of genetically manipulated animals is strongly influenced by both the genetic background of the animal as well as environmental factors. For these reasons, it is necessary to underline that KO mice have been generated on various genetic backgrounds, which strongly influence the behavioral and neurochemical responses to the tests. The present review will thus focus on KO mice lacking G protein-coupled monoaminergic receptors (e.g; 5-HT1B, 5-HT1A, and 5-HT4 receptors) and the 5-HT serotonin transporter, which is the main target of antidepressant drugs (or strategies). The importance of KO mice for neurotrophic factors, particularly for brain-derived neurotrophic factor and its main receptor displaying a tyrosine kinase activity, will also be addressed to illustrate the fact that in preclinical studies, combination of genetic manipulations with pharmacological ones should allow further progress in the field of neuropsychopharmacology.

Mutant mouse models and antidepressant drug research: focus on serotonin and brain-derived neurotrophic factor

Several lines of knockout (KO) mice have been evaluated as models of depression-related behavioral and neurobiological changes, and used to investigate molecular and cellular mechanisms underlying the activity of antidepressant drugs. Adult neurogenesis and brain 5-hydroxytryptamine (5-HT)/neurotrophic factor interactions have recently attracted great interest in relation to the mechanism of action of antidepressant drugs. The present review focuses primarily on genetic manipulation of the serotoninergic (5-HT) system. Basal neurochemical and behavioral changes occurring in mice lacking the 5-HT transporter (SERT), which is the main target of antidepressant drugs, as well as in those lacking G protein-coupled serotonin receptors (e.g. 5-HT1B, 5-HT1A, and 5-HT4 receptors) are described and evaluated. The importance of KO mice for neurotrophic factors, particularly for brain-derived neurotrophic factor and its high-affinity receptor (R-TrkB), is also addressed. Constitutive KO, tissue specific, or inducible KO mice targeting both 5-HT and brain-derived neurotrophic factor systems may potentially make an important contribution to knowledge of the pathophysiology and treatment of depression.

Genetic disruption of both tryptophan hydroxylase genes dramatically reduces serotonin and affects behavior in models sensitive to antidepressants

The neurotransmitter serotonin (5-HT) plays an important role in both the peripheral and central nervous systems. The biosynthesis of serotonin is regulated by two rate-limiting enzymes, tryptophan hydroxylase-1 and -2 (TPH1 and TPH2). We used a gene-targeting approach to generate mice with selective and complete elimination of the two known TPH isoforms. This resulted in dramatically reduced central 5-HT levels in Tph2 knockout (TPH2KO) and Tph1/Tph2 double knockout (DKO) mice; and substantially reduced peripheral 5-HT levels in DKO, but not TPH2KO mice. Therefore, differential expression of the two isoforms of TPH was reflected in corresponding depletion of 5-HT content in the brain and periphery. Surprisingly, despite the prominent and evolutionarily ancient role that 5-HT plays in both vertebrate and invertebrate physiology, none of these mutations resulted in an overt phenotype. TPH2KO and DKO mice were viable and normal in appearance. Behavioral alterations in assays with predictive validity for antidepressants were among the very few phenotypes uncovered. These behavioral changes were subtle in the TPH2KO mice; they were enhanced in the DKO mice. Herein, we confirm findings from prior descriptions of TPH1 knockout mice and present the first reported phenotypic evaluations of Tph2 and Tph1/Tph2 knockout mice. The behavioral effects observed in the TPH2 KO and DKO mice strongly confirm the role of 5-HT and its synthetic enzymes in the etiology and treatment of affective disorders.

Solubilization of human serotonin1A receptors expressed in neuronal cells

The serotonin 1A receptor is an important member of the G-protein coupled receptor family, and is involved in a variety of cognitive, behavioral, and developmental functions. None of the subtypes of G-protein coupled serotonin receptors have yet been purified to homogeneity from natural sources. We report here, for the first time, the solubilization of human serotonin(1A) receptors stably expressed in neuronal (HN2) cells. Importantly, ligand binding assay shows that the serotonin 1A receptor solubilized this way is functionally active. The effective solubilization of the serotonin 1A receptor from neuronal cells represents an important step toward the purification of the receptor in native-like membrane environment.

Membrane Organization and Function of the Serotonin1A Receptor

(1) The serotonin(1A) receptor is a G-protein coupled receptor involved in several cognitive, behavioral, and developmental functions. It binds the neurotransmitter serotonin and signals across the membrane through its interactions with heterotrimeric G-proteins. (2) Lipid-protein interactions in membranes play an important role in the assembly, stability, and function of membrane proteins. The role of membrane environment in serotonin(1A) receptor function is beginning to be addressed by exploring the consequences of lipid manipulations on the ligand binding and G-protein coupling of serotonin(1A) receptors, the ability to functionally solubilize the serotonin(1A) receptor, and the factors influencing the membrane organization of the serotonin(1A) receptor. (3) Recent developments involving the application of detergent-based and detergent-free approaches to understand the membrane organization of the serotonin(1A) receptor under conditions of ligand activation and modulation of membrane lipid content, with an emphasis on membrane cholesterol, are described.

Differential effects of cholesterol and 7-dehydrocholesterol on the ligand binding activity of the hippocampal serotonin(1A) receptor: implications in SLOS

The requirement of membrane cholesterol in maintaining ligand binding activity of the hippocampal serotonin(1A) receptor has previously been demonstrated. In order to test the stringency of the requirement of cholesterol, we depleted cholesterol from native hippocampal membranes followed by replenishment with 7-dehydrocholesterol. The latter sterol is an immediate biosynthetic precursor of cholesterol differing only in a double bond at the 7th position in the sterol ring. Our results show, for the first time, that replenishment with 7-dehydrocholesterol does not restore ligand binding activity of the serotonin(1A) receptor, in spite of recovery of the overall membrane order. The requirement for restoration of ligand binding activity therefore is more stringent than the requirement for the recovery of overall membrane order. These novel results have potential implications in understanding the interaction of membrane lipids with this important neuronal receptor under pathogenic conditions such as the Smith-Lemli-Opitz syndrome.

Membrane Organization of the Serotonin 1A Receptor Monitored by a Detergent-Free Approach

: 1. Insolubility of membrane constituents in nonionic detergents such as Triton X-100 has been a widely used biochemical criterion to indicate their localization in membrane domains. However, concerns on the possibility of membrane perturbation in the presence of detergents have led to the development of detergent-free approaches.2. We have explored the organization of the serotonin(1A) receptor, an important G-protein coupled receptor, from bovine hippocampus and CHO cells using a detergent-free approach in order to address the points of agreement with our previous results using Triton X-100.3. A significant fraction of the serotonin(1A) receptor has been found to be localized in a heavy density fraction obtained using a detergent-free approach to isolate membrane domains. In addition, we have characterized the membrane fractions isolated in terms of their lipid composition and membrane physical properties.4. The results obtained on the membrane localization of the serotonin(1A) receptor from the present experiments using a detergent-free approach correlate well with our earlier findings obtained using a detergent-based method (Kalipatnapu, S., and Chattopadhyay, A., FEBS Lett. 576:455-460, 2004). These results provide important information on the membrane organization of the hippocampal serotonin(1A) receptor and are relevant in view of the concerns on the use of detergent in determination of membrane organization of constituent proteins and lipids.

Membrane Organization and Dynamics of the G-Protein-Coupled Serotonin1A Receptor Monitored Using Fluorescence-Based Approaches

The G-protein-coupled receptor (GPCR) superfamily represents one of the largest classes of molecules involved in signal transduction across the plasma membrane. Fluorescence-based approaches have provided valuable insights into GPCR functions such as receptor-receptor and receptor-ligand interactions, real-time assessment of signal transduction, receptor dynamics on the plasma membrane, and intracellular trafficking of receptors. This has largely been possible with the use of fluorescent probes such as the green fluorescent protein (GFP) from the jellyfish Aequoria victoria and its variants. We discuss the potential of fluorescence-based approaches in providing novel information on the membrane organization and dynamics of the G-protein-coupled serotonin1A receptor tagged to the enhanced yellow fluorescent protein (EYFP).

Role of cholesterol in ligand binding and G-protein coupling of serotonin1A receptors solubilized from bovine hippocampus

The serotonin(1A) (5-HT(1A)) receptor is an important member of the superfamily of seven transmembrane domain G-protein-coupled receptors. We report here that solubilization of the hippocampal 5-HT(1A) receptor by the zwitterionic detergent CHAPS is accompanied by loss of membrane cholesterol which results in a reduction in specific agonist binding activity and extent of G-protein coupling. Importantly, replenishment of cholesterol to solubilized membranes using MbetaCD-cholesterol complex restores the cholesterol content of the membrane and significantly enhances the specific agonist binding activity and G-protein coupling. These novel results provide useful information on the role of cholesterol in solubilization of G-protein-coupled receptors, an important step for molecular characterization of these receptors.

Effect of Mg2+ on guanine nucleotide sensitivity of ligand binding to serotonin1A receptors from bovine hippocampus

The serotonin1A (5-HT1A) receptor is an important member of the superfamily of seven transmembrane domain G-protein coupled receptors (GPCRs). We report here that guanine nucleotide sensitivity of agonist binding to hippocampal 5-HT1A receptors is dependent on the concentration of Mg2+. Our results show that agonist binding to 5-HT1A receptors is relatively insensitive to guanine nucleotides in the absence of Mg2+. In contrast to this, the specific antagonist binding is insensitive to guanine nucleotides, even in the presence of Mg2+. These results point out the requirement of an optimal concentration of Mg2+ which could be used in assays toward determining guanine nucleotide sensitivity of ligand binding to GPCRs such as the 5-HT1A receptor. Our results provide novel insight into the requirement and concentration dependence of Mg2+ in relation to guanine nucleotide sensitivity for the 5-HT1A receptor in particular, and GPCRs in general.

Uptake of serotonin by adult rat corpus callosum is partially reduced by common antidepressants

The corpus callosum (CC) is the main white matter tract involved in interhemispheric brain communication. We establish that uptake of [3H]5-hydroxytryptamine (5-HT) in CC is partially inhibited by some antidepressants. Slices of the adult rat CC had a high-affinity uptake of 5-HT. About 80% of this uptake was Na+ dependent, with a Michaelis-Menten constant, Km, of 420 +/- 80 nM and a rate of 5-HT uptake, Vmax, of 9.5 +/- 0.8 pmol/mg protein/min. The 5-HT uptake was reduced approximately 60% at pH 5 compared with that at pH 7. Fluoxetine (Prozac) inhibited only 43% of 5-HT uptake in a concentration-dependent manner, with an affinity constant, Ki, of 44.7 +/- 10.0 nM. We also studied the effects of other monoamine uptake inhibitors, all at 10 microM, and found that zimelidine, imipramine, and clomipramine inhibited 5-HT uptake in the CC by approximately 30-40%. The fluoxetine-insensitive 5-HT uptake was not altered by high concentrations of dopamine plus norepinephrine. The present data show that Na(+)-dependent 5-HT uptake occurs in the CC and optic nerve and that this uptake is partially sensitive to antidepressants and probably mediated by the serotonin transporter, which may be relevant during depression.

8-[3H]-hydroxy-2-(di-n-propylamino)tetralin binding sites in blood lymphocytes of rats and the modulation by mitogens and immobilization

Serotonin 5-HT(1A) receptors were characterized in rat resting lymphocytes obtained by cardiac puncture with the use of the ligand [3H]8-hydroxy-2-(di-n-propylamino)tetralin. Selectivity of the specific binding was demonstrated by inhibition experiments with various serotonergic and nonserotonergic drugs. The rank order of potency for inhibition was WAY-100478>pindobind>NAN-190>buspirone>imipramine>serotonin. While pimozide, desipramine, nomifensine, haloperidol and sulpiride did not inhibit the binding. Kinetic parameters calculated from saturation experiments indicated one site of interaction, with an equilibrium dissociation constant of 2.50 nM and maximum binding capacity of 487.21 nmol/10(6) cells. Complete dissociation was obtained with serotonin as the displacement agent, and equilibrium dissociation constant calculated by association and dissociation experiments was 2.03 nM. Thus, serotonin 5-HT(1A) receptors are present in resting lymphocytes. The in vivo administration of the mitogens lipopolysacharide (0.1 mg/kg, 18 h) or concanavalin A (0.2 mg/kg, 18 h) increased the number of sites. The elevation produced by the latter was of higher magnitude than that of lipopolysacharide, and two sites of the binding were determined by isotopic dilution. Immobilization stress (1 h daily for 7 days) also resulted in a significant increase of binding capacity, but was smaller than that produced by the mitogens. The affinity of binding was not affect by the treatments. The results indicate that serotonin 5-HT(1A) receptors are modulated by unspecific and specific immune system activation, as well as by a potent stress condition, which might result in relevant functional modifications in the response of rat lymphocytes.

Serotonin transporter genetic variation and the response of the human amygdala

A functional polymorphism in the promoter region of the human serotonin transporter gene (SLC6A4) has been associated with several dimensions of neuroticism and psychopathology, especially anxiety traits, but the predictive value of this genotype against these complex behaviors has been inconsistent. Serotonin [5- hydroxytryptamine, (5-HT)] function influences normal fear as well as pathological anxiety, behaviors critically dependent on the amygdala in animal models and in clinical studies. We now report that individuals with one or two copies of the short allele of the serotonin transporter (5-HTT) promoter polymorphism, which has been associated with reduced 5-HTT expression and function and increased fear and anxiety-related behaviors, exhibit greater amygdala neuronal activity, as assessed by BOLD functional magnetic resonance imaging, in response to fearful stimuli compared with individuals homozygous for the long allele. These results demonstrate genetically driven variation in the response of brain regions underlying human emotional behavior and suggest that differential excitability of the amygdala to emotional stimuli may contribute to the increased fear and anxiety typically associated with the short SLC6A4 allele.

Genotype/age interactions on aggressive behavior in gonadally intact estrogen receptor beta knockout (betaERKO) male mice

Estrogen, as an aromatized metabolite of testosterone, has a facilitatory effect on male aggressive behavior in mice. Two subtypes of estrogen receptors, alpha (ER-alpha) and beta (ER-beta), in the brain are known to bind estrogen. Previous studies revealed that the lack of ER-alpha gene severely reduced the induction of male aggressive behavior. In contrast, mice that lacked the ER-beta gene tended to be more aggressive than wild type (WT) control mice, although the behavioral effects of ER-beta gene disruption were dependent on their social experience. These findings lead us to hypothesize that estrogen may facilitate aggression via ER-alpha whereas it may inhibit aggression via ER-beta. In the present study, we further investigated the role of ER-beta in the regulation of aggressive behavior by examining developmental changes starting at the time of first onset, around the age of puberty. Aggressive behaviors of ER-beta gene knockout (betaERKO) mice were examined in three different age groups, puberty, young-adult, and adult. Each mouse was tested every other day for three times in a resident-intruder paradigm against olfactory bulbectomized intruder mice and their trunk blood was collected for measurements of serum testosterone after the completion of the study. Overall, betaERKO mice were significantly more aggressive than WT. These genotype differences were more pronounced in puberty and young adult age groups, but not apparent in the adult age group, in which betaERKO mice were less aggressive than those in two younger age groups. Serum testosterone levels of betaERKO mice were significantly higher than those of WT mice only in the pubertal age group, but not in young adult (when betaERKO mice were still significantly more aggressive than WT mice) and adult (when no genotype differences in aggression were found) age groups. These results suggest that ER-beta mediated actions of gonadal steroids may more profoundly be involved in the inhibitory regulation of aggressive behavior in pubertal and young adult mice.

Combining genetic and genomic approaches to study mood disorders

Recent technological advances in genetic manipulations and DNA microarrays are profoundly altering the landscape of biological research, offering opportunities to investigate biological questions that were only dreamed of a few years ago. With this revolution comes the hope of being able to tackle some of the more arduous challenges that the central nervous system has presented to the research community. Specifically, a major goal in the study of neuropsychiatric disorders has been to identify underlying mechanisms of brain dysfunction with the expectation that these insights may allow a better diagnosis, prevention and effective treatments for these disorders. For the most part, treatments of these disorders have relied on serendipitous discovery of pharmacological entities with therapeutic efficacy, while the causes of the disorders have remained unknown. The serotonin system, and the serotonin(1A) (5-HT(1A)) receptor in particular, have been under intense investigation, mostly due to the fact that serotonergic drugs that directly or indirectly affect the 5-HT(1A) receptor, are effective therapeutic agents in treating patients with various neuropsychiatric disorders, including anxiety and depression. Genetic deletion of the receptor in mouse results in increased anxiety, thus supporting an active role for this receptor in mood regulation. However, the analysis of genetic deletion experiments can be confounded by hidden developmental roles of the missing receptor, by adaptive compensatory mechanisms, as well by the fact that the genes or gene products that are responsible for the cellular and molecular aspects of the phenotype may be several steps removed from the genetic intervention. Here, we present a combined methodological approach of tissue specific and conditional genetic manipulations, with large-scale search for altered gene expression, as an experimental framework to investigate the role of genes with complex functions and/or complex expression patterns. The 5-HT(1A) receptor is used as a model of gene product with complex functions and distributions, and as a prototypical system to which these new genetic approaches are currently being applied.

Altered fear circuits in 5-HT(1A) receptor KO mice

The study of genetically altered mice has been used successfully to determine the influence of different neurotransmitter receptors on fear and anxiety. Mice with a genetic deletion of the serotonin 1A receptor (5-HT(1A)R knockout [KO]) have been shown to be more fearful in a number of behavioral conflict tests, confirming the important role of this receptor in modulating anxiety. Factor analysis of the behavior of WT and 5-HT(1A)R KO mice in the open field test shows that locomotion and anxiety measures segregate independently, supporting the idea that the anxious behavior of the KO mice is not the result of altered locomotion. KO mice also show increased anxiety in the novelty-suppressed feeding task, which differs from the other conflict tests in the motivational drive of the animals. In response to a discrete aversive stimulus, foot shock, the KO mice show increased freezing and increased tachycardia. However, activation of the hypothalamic-pituitary-adrenal axis in response to stress appears to be slightly blunted in the KO animals. Together, these data support the idea that the 5-HT(1A)R modulates an important fear circuit in the brain. The dual function of the 5-HT(1A)R as both a presynaptic autoreceptor, negatively regulating serotonin activity, and a postsynaptic heteroreceptor, inhibiting the activity of nonserotonergic neurons in forebrain structures, has complicated interpretation of the anxious phenotype of these KO mice. A more complete understanding of the function of the 5-HT(1A)R awaits further study of its role in behaving animals using tissue-specific antagonists and novel transgenic mice with tissue-specific expression of the receptor.

Mouse models of serotonin receptor function: toward a genetic dissection of serotonin systems

Central serotonin (5-hydroxytryptamine, 5-HT) systems regulate a wide variety of complex behaviors, and are targeted by drugs used in the treatment of diverse neuropsychiatric disorders. The actions of 5-HT are mediated by a large and heterogeneous family of 5-HT receptor subtypes. Studies of the functional significance of individual subtypes have been complicated by the limited availability of selective receptor agonist and antagonist drugs. Molecular genetic techniques offer complementary approaches for studying the behavioral roles of individual 5-HT receptor subtypes through the generation of gene-targeted and transgenic lines of mice with altered expression of 5-HT receptor genes. This review will examine insights into the serotonergic regulation of behavior that have been produced by the study of these lines, as well as discuss important caveats to the interpretation of these studies.

Corticosteroids regulate 5-HT(1A) but not 5-HT(1B) receptor mRNA in rat hippocampus

The role of mineralocorticoid and glucocorticoid receptors (MR and GR, respectively) in the regulation of serotonin receptors has not been clearly delineated. There is no consensus regarding the regulation of 5-HT(1A) receptors, and corticosteroid regulation of 5-HT(1B) mRNA has not been previously studied. We compared the effects of long-term (two week) adrenalectomy (no MR or GR activation) and several hormone replacement protocols designed to stimulate MR selectively (ALDO), MR and GR (HCT), and continuous MR with cyclical GR activation (SHAM adrenalectomy). 5-HT(1A) and 5-HT(1B) mRNAs were measured by in situ hybridization in hippocampus and raphe nuclei. None of the experimental manipulations altered 5-HT(1B) mRNA levels in the hippocampus or dorsal raphe, and also had no effect on 5-HT(1A) mRNA in dorsal or median raphe. However, 5-HT(1A) mRNA levels were regulated in a complex manner in the different subfields of hippocampus. We conclude that both MR and GR play an integrated role in regulating 5-HT(1A) mRNA levels in hippocampus while having no effect on 5-HT(1B) mRNA levels under these conditions.

The ETS domain factor Pet-1 is an early and precise marker of central serotonin neurons and interacts with a conserved element in serotonergic genes

Serotonin (5-HT) plays a crucial neuromodulatory role in numerous physiological and behavioral functions, and dysfunction of the serotonergic system has been implicated in several psychiatric disorders. Despite the widespread importance of the central serotonergic neurotransmitter system, little is known about the molecular mechanisms controlling the development of 5-HT neurons. We previously identified an ETS domain transcription factor, Pet-1, that is expressed in a small number of tissues, including the brain. Here, we show that expression of Pet-1 RNA in the brain is restricted to, and marks, the entire rostrocaudal extent of rat serotonergic hindbrain raphe nuclei. Remarkably, Pet-1 RNA colocalizes with tryptophan hydroxylase-positive neurons in raphe nuclei but not with their nonserotonergic neuron or non-neuronal neighbors. Pet-1 RNA is limited to two domains in the developing hindbrain, which precedes the appearance of 5-HT in each domain by approximately a half day. Conserved Pet-1 binding sites are present in or near the promoter regions of the human and mouse 5-HT1a receptor, serotonin transporter, tryptophan hydroxylase, and aromatic L-amino acid decarboxylase genes whose expression is characteristic of the serotonergic neuron phenotype. These sites are capable of supporting transcriptional activation through interactions with the Pet-1 ETS domain and can function as enhancers. Together, our findings establish Pet-1 as an early and precise marker of 5-HT neurons and suggest that it functions specifically in the differentiation and maintenance of these neurons.

The ETS domain factor Pet-1 is an early and precise marker of central serotonin neurons and interacts with a conserved element in serotonergic genes

Serotonin (5-HT) plays a crucial neuromodulatory role in numerous physiological and behavioral functions, and dysfunction of the serotonergic system has been implicated in several psychiatric disorders. Despite the widespread importance of the central serotonergic neurotransmitter system, little is known about the molecular mechanisms controlling the development of 5-HT neurons. We previously identified an ETS domain transcription factor, Pet-1, that is expressed in a small number of tissues, including the brain. Here, we show that expression of Pet-1 RNA in the brain is restricted to, and marks, the entire rostrocaudal extent of rat serotonergic hindbrain raphe nuclei. Remarkably, Pet-1 RNA colocalizes with tryptophan hydroxylase-positive neurons in raphe nuclei but not with their nonserotonergic neuron or non-neuronal neighbors. Pet-1 RNA is limited to two domains in the developing hindbrain, which precedes the appearance of 5-HT in each domain by approximately a half day. Conserved Pet-1 binding sites are present in or near the promoter regions of the human and mouse 5-HT1a receptor, serotonin transporter, tryptophan hydroxylase, and aromatic L-amino acid decarboxylase genes whose expression is characteristic of the serotonergic neuron phenotype. These sites are capable of supporting transcriptional activation through interactions with the Pet-1 ETS domain and can function as enhancers. Together, our findings establish Pet-1 as an early and precise marker of 5-HT neurons and suggest that it functions specifically in the differentiation and maintenance of these neurons.