A functional Tph2 C1473G polymorphism causes an anxiety phenotype via compensatory changes in the serotonergic system

Bile duct ligation elevates 5-HT levels in cerebral cortex of rats partly due to impairment of brain UGT1A6 expression and activity via ammonia accumulation

Hepatic encephalopathy (HE) is often associated with endogenous serotonin (5-HT) disorders. However, the reason for elevated brain 5-HT levels due to liver failure remains unclear. This study aimed to investigate the mechanism by which liver failure increases brain 5-HT levels and the role in behavioral abnormalities in HE. Using bile duct ligation (BDL) rats as a HE model, we verified the elevated 5-HT levels in the cortex but not in the hippocampus and striatum, and found that this cortical 5-HT overload may be caused by BDL-mediated inhibition of UDP-glucuronosyltransferase 1A6 (UGT1A6) expression and activity in the cortex. The intraventricular injection of the UGT1A6 inhibitor diclofenac into rats demonstrated that the inhibition of brain UGT1A6 activity significantly increased cerebral 5-HT levels and induced HE-like behaviors. Co-immunofluorescence experiments demonstrated that UGT1A6 is primarily expressed in astrocytes. In vitro studies confirmed that NH4Cl activates the ROS-ERK pathway to downregulate UGT1A6 activity and expression in U251 cells, which can be reversed by the oxidative stress antagonist N-acetyl-l-cysteine and the ERK inhibitor U0126. Silencing Hepatocyte Nuclear Factor 4α (HNF4α) suppressed UGT1A6 expression whilst overexpressing HNF4α increased Ugt1a6 promotor activity. Meanwhile, both NH4Cl and the ERK activator TBHQ downregulated HNF4α and UGT1A6 expression. In the cortex of hyperammonemic rats, we also found activation of the ROS-ERK pathway, decreases in HNF4α and UGT1A6 expression, and increases in brain 5-HT content. These results prove that the ammonia-mediated ROS-ERK pathway activation inhibits HNF4α expression to downregulate UGT1A6 expression and activity, thereby increasing cerebral 5-HT content and inducing manic-like HE symptoms. This is the first study to reveal the mechanism of elevated cortical 5-HT concentration in a state of liver failure and elucidate its association with manic-like behaviors in HE.

The Role of C1473G Polymorphism in Mouse Triptophan Hydroxylase 2 Gene in the Acute Effects of Ethanol on the c-fos Gene Expression and Metabolism of Biogenic Amines in the Brain

Tryptophan hydroxylase 2 is a key enzyme in the synthesis of the neurotransmitter serotonin, which plays an important role in the regulation of behavior and various physiological functions. We studied the effect of acute ethanol administration on the expression of the early response c-fos gene and metabolism of serotonin and catecholamines in the brain structures of B6-1473C and B6-1473G congenic mouse strains differing in the single-nucleotide substitution C1473G in the Tph2 gene and activity of the encoded enzyme. Acute alcoholization led to a significant upregulation of the c-fos gene expression in the frontal cortex and striatum of B6-1473G mice and in the hippocampus of B6-1473C mice and caused a decrease in the index of serotonin metabolism in the nucleus accumbens in B6-1473C mice and in the hippocampus and striatum of B6-1473G mice, as well as to the decrease in the norepinephrine level in the hypothalamus of B6-1473C mice. Therefore, the C1473G polymorphism in the Tph2 gene has a significant effect of acute ethanol administration on the c-fos expression pattern and metabolism of biogenic amines in the mouse brain.

Novel aspects of enteric serotonergic signaling in health and brain-gut disease

Gastrointestinal (GI) comorbidities are common in individuals with mood and behavioral dysfunction. Similarly, patients with GI problems more commonly suffer from co-morbid psychiatric diagnoses. Although the central and enteric nervous systems (CNS and ENS, respectively) have largely been studied separately, there is emerging interest in factors that may contribute to disease states involving both systems. There is strong evidence to suggest that serotonin may be an important contributor to these brain-gut conditions. Serotonin has long been recognized for its critical functions in CNS development and function. The majority of the body's serotonin, however, is produced in the GI tract, where it plays key roles in ENS development and function. Further understanding of the specific impact that enteric serotonin has on brain-gut disease may lay the foundation for the creation of novel therapeutic targets. This review summarizes the current data focusing on the important roles that serotonin plays in ENS development and motility, with a focus on novel aspects of serotonergic signaling in medical conditions in which CNS and ENS co-morbidities are common, including autism spectrum disorders and depression.

Pianp deficiency links GABAB receptor signaling and hippocampal and cerebellar neuronal cell composition to autism-like behavior

Pianp (also known as Leda-1) is a type I transmembrane protein with preferential expression in the mammalian CNS. Its processing is characterized by proteolytic cleavage by a range of proteases including Adam10, Adam17, MMPs, and the γ-secretase complex. Pianp can interact with Pilrα and the GB1a subunit of the GABA_B receptor (GBR) complex. A recent case description of a boy with global developmental delay and homozygous nonsense variant in PIANP supports the hypothesis that PIANP is involved in the control of behavioral traits in mammals. To investigate the physiological functions of Pianp, constitutive, global knockout mice were generated and comprehensively analyzed. Broad assessment did not indicate malformation or malfunction of internal organs. In the brain, however, decreased sizes and altered cellular compositions of the dentate gyrus as well as the cerebellum, including a lower number of cerebellar Purkinje cells, were identified. Functionally, loss of Pianp led to impaired presynaptic GBR-mediated inhibition of glutamate release and altered gene expression in the cortex, hippocampus, amygdala, and hypothalamus including downregulation of Erdr1, a gene linked to autism-like behavior. Behavioral phenotyping revealed that Pianp deficiency leads to context-dependent enhanced anxiety and spatial learning deficits, an altered stress response, severely impaired social interaction, and enhanced repetitive behavior, which all represent characteristic features of an autism spectrum disorder-like phenotype. Altogether, Pianp represents a novel candidate gene involved in autism-like behavior, cerebellar and hippocampal pathology, and GBR signaling.

Targeted Manipulation of Brain Serotonin: RNAi-Mediated Knockdown of Tryptophan Hydroxylase 2 in Rats

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of the biogenic monoamine serotonin (5-hydroxytryptamine, 5-HT). Two existing TPH isoforms are responsible for the generation of two distinct serotonergic systems in vertebrates. TPH1, predominantly expressed in the gastrointestinal tract and pineal gland, mediates 5-HT biosynthesis in non-neuronal tissues, while TPH2, mainly found in the raphe nuclei of the brain stem, is accountable for the production of 5-HT in the brain. Neuronal 5-HT is a key regulator of mood and behavior and its deficiency has been implicated in a variety of neuropsychiatric disorders, e.g., depression and anxiety. To gain further insights into the complexity of central 5-HT modulations of physiological and pathophysiological processes, a new transgenic rat model, allowing an inducible gene knockdown of Tph2, was established based on doxycycline-inducible shRNA-expression. Biochemical phenotyping revealed a functional knockdown of Tph2 mRNA expression following oral doxycycline administration, with subsequent reductions in the corresponding levels of TPH2 enzyme expression and activity. Transgenic rats showed also significantly decreased tissue levels of 5-HT and its degradation product 5-Hydroxyindoleacetic acid (5-HIAA) in the raphe nuclei, hippocampus, hypothalamus, and cortex, while peripheral 5-HT concentrations in the blood remained unchanged. In summary, this novel transgenic rat model allows inducible manipulation of 5-HT biosynthesis specifically in the brain and may help to elucidate the role of 5-HT in the pathophysiology of affective disorders.

Effect of photoperiodic alterations on depression-like behavior and the brain serotonin system in mice genetically different in tryptophan hydroxylase 2 activity

Reduction of natural illumination in fall/winter months causes seasonal affective disorders (SAD) in vulnerable individuals. Neurotransmitter serotonin (5-HT) is involved in the mechanism of SAD. Tryptophan hydroxylase-2 (TPH2) is the key enzyme of 5-HT synthesis in the brain. C1473 G polymorphism in the Tph2 gene is a key factor defining the enzyme activity in the mouse brain. The main aims of the study were to investigate the effects of C1473 G polymorphism on behavior and brain 5-HT system responses to photoperiod alterations. The experiment was carried out on adult mouse males of B6-1473C and B6-1473 G congenic lines with normal and low TPH2 activities, respectively. B6-1473C and B6-1473 G mice were divided into four groups of 8 each and exposed for 28 days to standard-day (14 h light and 10 h darkness) or short-day (4 h light and 20 h darkness) conditions. No effect of photoperiod on locomotor, exploratory activities and anxiety in the open field test was observed. At the same time, photoperiod alterations affected depressive-like immobility in the forced swim test, the 5-HT, 5-hydroxyindoleacetic acid (5-HIAA) levels, 5-HIAA/5-HT ratio and the Htr2a mRNA level in hippocampus and midbrain. The effect of the interaction between C1473 G polymorphism and photoperiod on 5-HT level and 5-HIAA/5-HT ratio in hippocampus was revealed. Short-day conditions reduced the level and increased 5-HIAA/5-HT ratio in this structure only in B6-1473 G mice. At the same time, C1473 G polymorphism does not alter effects of short-day conditions on immobility time in the forced swim test and the Htr2a mRNA level in the brain.

Single nucleotide polymorphisms, variable number tandem repeats and allele influence on serotonergic enzyme modulators for aggressive and suicidal behaviors: A review

The serotonergic system plays key regulatory roles in cognition and emotion. Several lines of evidence suggest that genetic variation is associated with aggressive and suicidal behaviors. Genetic studies have largely focused on three types of variations: single nucleotide polymorphisms (SNPs), variable number tandem repeats (VNTRs), and alleles. 95 published papers (49 papers for aggression and 46 for suicide) were reviewed to summarize the impact of SNPs, VNTRs, and alleles of tryptophan hydroxylase (TPH, the rate-limiting enzyme in serotonin [5-HT] synthesis), 5-HT transporter (5-HTT), serotonergic receptors, monoamine oxidase (an enzyme that catalyzes 5-HT degradation) on aggression and suicidal behaviors. These study samples include healthy controls, psychiatric disease patients, and animal models. This article mainly reviews studies on the relationship between 5-HT transmissions and genetic variations involved in aggression (particularly impulsive aggression) or suicide in people with different ethnicities and psychiatric disorders. We found that most SNPs, VNTRs, and alleles exerted influences on aggression or suicide. Only A128C in TPH1, A138G in 5-HT2A, and L type in the VNTR of monoamine oxidase A (MAOA) affected both aggression and suicide. The associations between some genetic variations and aggression/suicide may be influenced by gender, age, ethnicity, psychiatric disease, and even parenting or prenatal stress. These findings may help clarify how genetic and environmental factors influence the development of aggressive and suicidal behaviors.

Loss of Adult 5-HT1A Autoreceptors Results in a Paradoxical Anxiogenic Response to Antidepressant Treatment

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are first-line antidepressants but require several weeks to elicit their actions. Chronic SSRI treatment induces desensitization of 5-HT1A autoreceptors to enhance 5-HT neurotransmission. Mice (both sexes) with gene deletion of 5-HT1A autoreceptors in adult 5-HT neurons (1AcKO) were tested for response to SSRIs. Tamoxifen-induced recombination in adult 1AcKO mice specifically reduced 5-HT1A autoreceptor levels. The 1AcKO mice showed a loss of 5-HT1A autoreceptor-mediated hypothermia and electrophysiological responses, but no changes in anxiety- or depression-like behavior. Subchronic fluoxetine (FLX) treatment induced an unexpected anxiogenic effect in 1AcKO mice in the novelty suppressed feeding and elevated plus maze tests, as did escitalopram in the novelty suppressed feeding test. No effect was seen in wild-type (WT) mice. Subchronic FLX increased 5-HT metabolism in prefrontal cortex, hippocampus, and raphe of 1AcKO but not WT mice, suggesting hyperactivation of 5-HT release. To detect chronic cellular activation, FosB⁺ cells were quantified. FosB⁺ cells were reduced in entorhinal cortex and hippocampus (CA2/3) and increased in dorsal raphe 5-HT cells of 1AcKO mice, suggesting increased raphe activation. In WT but not 1AcKO mice, FLX reduced FosB⁺ cells in the median raphe, hippocampus, entorhinal cortex, and median septum, which receive rich 5-HT projections. Thus, in the absence of 5-HT1A autoreceptors, SSRIs induce a paradoxical anxiogenic response. This may involve imbalance in activation of dorsal and median raphe to regulate septohippocampal or fimbria-fornix pathways. These results suggest that markedly reduced 5-HT1A autoreceptors may provide a marker for aberrant response to SSRI treatment.SIGNIFICANCE STATEMENT Serotonin-selective reuptake inhibitors (SSRIs) are effective in treating anxiety and depression in humans and mouse models. However, in some cases, SSRIs can increase anxiety, but the mechanisms involved are unclear. Here we show that, rather than enhancing SSRI benefits, adulthood knockout (KO) of the 5-HT1A autoreceptor, a critical negative regulator of 5-HT activity, results in an SSRI-induced anxiety effect that appears to involve a hyperactivation of the 5-HT system in certain brain areas. Thus, subjects with very low levels of 5-HT1A autoreceptors, such as during childhood or adolescence, may be at risk for an SSRI-induced anxiety response.

Comprehensive behavioral analysis and quantification of brain free amino acids of C57BL/6J congenic mice carrying the 1473G allele in tryptophan hydroxylase-2

Aim

Tryptophan hydroxylase 2 (Tph2) is a rate‐limiting enzyme for the biosynthesis of 5‐hydroxytryptamine (5‐HT, serotonin). Previous studies have reported that C1473G polymorphism of the murine Tph2 gene leads to decreased 5‐HT levels in the brain and abnormal behavioral phenotypes, such as impaired anxiety‐ and depression‐like behaviors. In this study, to confirm the effect of the C1473G polymorphism on mouse phenotypes, we conducted a comprehensive battery of behavioral tests and measured the amounts of brain free amino acids involved in the production of 5‐HT.

Methods

We obtained C57BL/6J congenic mice that were homozygous for the 1473G allele of Tph2 (1473G) and subjected them and their wild‐type littermates (1473C) to a battery of behavioral tests. Using reverse‐phase high‐performance liquid chromatography (HPLC), we measured the amounts of free amino acids in the 5‐HT and epinephrine synthetic/metabolic pathways in the frontal cortex, hippocampus, striatum, and midbrain.

Results

We failed to detect significant differences between genotypes in depression‐like behaviors, anxiety‐like behaviors, social behaviors, sensorimotor gaiting, or learning and memory, while 1473G mice exhibited a nominally significant impairment in gait analysis, which failed to reach study‐wide significance. In the HPLC analysis, there were no significant differences in the amounts of 5‐HT, dopamine, norepinephrine, and epinephrine in the frontal cortex, hippocampus, striatum, and midbrain.

Conclusion

Our findings do not support the idea that congenic C57BL/6J mice carrying the 1473G allele may represent an animal model of mood disorder under normal conditions without stress.

We assessed the behavioral and biochemical phenotypes of congenic C57BL/6J mice carrying the 1473G allele and failed to identify significant differences between the 1473G allele‐carrying mice and their wild‐type littermates. Thus, our findings do not support the use of 1473G allele‐carrying C57BL/6J mice as an animal model of mood disorder under normal conditions without stress.

Social approach, anxiety, and altered tryptophan hydroxylase 2 activity in juvenile BALB/c and C57BL/6J mice

Autism spectrum disorder (ASD) is a heterogeneous and highly heritable condition with multiple aetiologies. Although the biological mechanisms underlying ASD are not fully understood, evidence suggests that dysregulation of serotonergic systems play an important role in ASD psychopathology. Preclinical models using mice with altered serotonergic neurotransmission may provide insight into the role of serotonin in behaviours relevant to clinical features of ASD. For example, BALB/c mice carry a loss-of-function single nucleotide polymorphism (SNP; C1473 G) in tryptophan hydroxylase 2 (Tph2), which encodes the brain-specific isoform of the rate-limiting enzyme for serotonin synthesis, and these mice frequently have been used to model symptoms of ASD. In this study, juvenile male BALB/c (G/G; loss-of-function variant) and C57BL/6 J (C/C; wild type variant) mice, were exposed to the three-chamber sociability test, and one week later to the elevated plus-maze (EPM). Tryptophan hydroxylase 2 (TPH2) activity was measured following injection of the aromatic amino acid decarboxylase (AADC)-inhibitor, NSD-1015, and subsequent HPLC detection of 5-hydroxytryptophan (5-HTP) within subregions of the dorsal raphe nucleus (DR) and median raphe nucleus (MnR). The BALB/c mice showed reduced social behaviour and increased anxious behaviour, as well as decreased 5-HTP accumulation in the rostral and mid-rostrocaudal DR. In the full cohort of mice, TPH2 activity in the mid-rostrocaudal DR was correlated with anxious behaviour in the EPM, however these correlations were not statistically significant within each strain, suggesting that TPH2 activity was not directly associated with either anxiety or sociability. Further research is therefore required to more fully understand how serotonergic systems are involved in mouse behaviours that resemble some of the clinical features of ASD.

Genetically driven brain serotonin deficiency facilitates panic-like escape behavior in mice

Multiple lines of evidence implicate brain serotonin (5-hydroxytryptamine; 5-HT) system dysfunction in the pathophysiology of stressor-related and anxiety disorders. Here we investigate the influence of constitutively deficient 5-HT synthesis on stressor-related anxiety-like behaviors using Tryptophan hydroxylase 2 (Tph2) mutant mice. Functional assessment of c-Fos after associated foot shock, electrophysiological recordings of GABAergic synaptic transmission, differential expression of the Slc6a4 gene in serotonergic neurons were combined with locomotor and anxiety-like measurements in different contextual settings. Our findings indicate that constitutive Tph2 inactivation and consequential lack of 5-HT synthesis in Tph2 null mutant mice (Tph2^-/-) results in increased freezing to associated foot shock and a differential c-Fos activity pattern in the basolateral complex of the amygdala. This is accompanied by altered GABAergic transmission as observed by recordings of inhibitory postsynaptic currents on principal neurons in the basolateral nucleus, which may explain increased fear associated with hyperlocomotion and escape-like responses in aversive inescapable contexts. In contrast, lifelong 5-HT deficiency as observed in Tph2 heterozygous mice (Tph^+/-) is able to be compensated through reduced GABAergic transmission in the basolateral nucleus of the amygdala based on Slc6a4 mRNA upregulation in subdivisions of dorsal raphe neurons. This results in increased activity of the basolateral nucleus of the amygdala due to associated foot shock. In conclusion, our results reflect characteristic syndromal dimensions of panic disorder and agoraphobia. Thus, constitutive lack of 5-HT synthesis influence the risk for anxiety- and stressor-related disorders including panic disorder and comorbid agoraphobia through the absence of GABAergic-dependent compensatory mechanisms in the basolateral nucleus of the amygdala.

Genetic influences on social attention in free-ranging rhesus macaques

An ethological approach to attention predicts that organisms orient preferentially to valuable sources of information in the environment. For many gregarious species, orienting to other individuals provides valuable social information but competes with food acquisition, water consumption and predator avoidance. Individual variation in vigilance behaviour in humans spans a continuum from inattentive to pathological levels of interest in others. To assess the comparative biology of this behavioural variation, we probed vigilance rates in free-ranging macaques during water drinking, a behaviour incompatible with the gaze and postural demands of vigilance. Males were significantly more vigilant than females. Moreover, vigilance showed a clear genetic component, with an estimated heritability of 12%. Monkeys carrying a relatively infrequent 'long' allele of TPH2, a regulatory gene that influences serotonin production in the brain, were significantly less vigilant compared to monkeys that did not carry the allele. These findings resonate with the hypothesis that the serotonin pathway regulates vigilance in primates and by extension provoke the idea that individual variation in vigilance and its underlying biology may be adaptive rather than pathological.

Adult AMPA GLUA1 receptor subunit loss in 5-HT neurons results in a specific anxiety-phenotype with evidence for dysregulation of 5-HT neuronal activity

Both the glutamatergic and serotonergic (5-HT) systems are implicated in the modulation of mood and anxiety. Descending cortical glutamatergic neurons regulate 5-HT neuronal activity in the midbrain raphe nuclei through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors. To analyze the functional role of GLUA1-containing AMPA receptors in serotonergic neurons, we used the Cre-ERT2/loxP-system for the conditional inactivation of the GLUA1-encoding Gria1 gene selectively in 5-HT neurons of adult mice. These Gria1(5-HT-/-) mice exhibited a distinct anxiety phenotype but showed no alterations in locomotion, depression-like behavior, or learning and memory. Increased anxiety-related behavior was associated with significant decreases in tryptophan hydroxylase 2 (TPH2) expression and activity, and subsequent reductions in tissue levels of 5-HT, its metabolite 5-hydroxyindoleacetic acid (5-HIAA), and norepinephrine in the raphe nuclei. However, TPH2 expression and activity as well as monoamine levels were unchanged in the projection areas of 5-HT neurons. Extracellular electrophysiological recordings of 5-HT neurons revealed that, while α1-adrenoceptor-mediated excitation was unchanged, excitatory responses to AMPA were enhanced and the 5-HT1A autoreceptor-mediated inhibitory response to 5-HT was attenuated in Gria1(5-HT-/-) mice. Our data show that a loss of GLUA1 protein in 5-HT neurons enhances AMPA receptor function and leads to multiple local molecular and neurochemical changes in the raphe nuclei that dysregulate 5-HT neuronal activity and induce anxiety-like behavior.

Strain-dependent variations in stress coping behavior are mediated by a 5-HT/GABA interaction within the prefrontal corticolimbic system

BACKGROUND

Serotonin and γ-aminobutyric acid (GABA) transmission is crucial in coping strategies.

METHODS

Here, using mice from 2 inbred strains widely exploited in behavioral neurochemistry, we investigated whether serotonin transmission in medial prefrontal cortex and GABA in basolateral amygdala determine strain-dependent liability to stress response and differences in coping.

RESULTS

C57BL/6J mice displayed greater immobility in the forced swimming test, higher serotonin outflow in medial prefrontal cortex, higher GABA outflow in basolateral amygdala induced by stress, and higher serotonin 1A receptor levels in medial prefrontal cortex accompanied by lower GABAb receptor levels in basolateral amygdala than DBA/2J mice. In assessing whether serotonin in medial prefrontal cortex determines GABA functioning in response to stress and passive coping behavior in C57BL/6J and DBA/2J mice, we observed that selective prefrontal serotonin depletion in C57BL/6J and DBA/2J reduced stress-induced GABA outflow in basolateral amygdala and immobility in the forced swimming test.

CONCLUSIONS

These results show that strain-dependent prefrontal corticolimbic serotonin/GABA regulation determines the strain differences in stress-coping behavior in the forced swimming test and point to a role of a specific neuronal system in genetic susceptibility to stress that opens up new prospects for innovative therapies for stress disorders.

Mice genetically depleted of brain serotonin do not display a depression-like behavioral phenotype

Reductions in function within the serotonin (5HT) neuronal system have long been proposed as etiological factors in depression. Selective serotonin reuptake inhibitors (SSRIs) are the most common treatment for depression, and their therapeutic effect is generally attributed to their ability to increase the synaptic levels of 5HT. Tryptophan hydroxylase 2 (TPH2) is the initial and rate-limiting enzyme in the biosynthetic pathway of 5HT in the CNS, and losses in its catalytic activity lead to reductions in 5HT production and release. The time differential between the onset of 5HT reuptake inhibition by SSRIs (minutes) and onset of their antidepressant efficacy (weeks to months), when considered with their overall poor therapeutic effectiveness, has cast some doubt on the role of 5HT in depression. Mice lacking the gene for TPH2 are genetically depleted of brain 5HT and were tested for a depression-like behavioral phenotype using a battery of valid tests for affective-like disorders in animals. The behavior of TPH2(-/-) mice on the sucrose preference test, tail suspension test, and forced swim test and their responses in the unpredictable chronic mild stress and learned helplessness paradigms was the same as wild-type controls. While TPH2(-/-) mice as a group were not responsive to SSRIs, a subset responded to treatment with SSRIs in the same manner as wild-type controls with significant reductions in immobility time on the tail suspension test, indicative of antidepressant drug effects. The behavioral phenotype of the TPH2(-/-) mouse questions the role of 5HT in depression. Furthermore, the TPH2(-/-) mouse may serve as a useful model in the search for new medications that have therapeutic targets for depression that are outside of the 5HT neuronal system.

Adaptive changes in serotonin metabolism preserve normal behavior in mice with reduced TPH2 activity

Polymorphisms in the TPH2 gene coding for the serotonin synthesizing enzyme in the brain are considered as risk factors associated with depression and anxiety in humans. However, whether a certain variation in the TPH2 gene leads to decreased brain serotonin production and development of psychological abnormalities remains unresolved. We generated a new mouse model, carrying one Tph2-null allele and one Tph21473G-allele, coding for a hypoactive form of the enzyme. We tested these mice along with C57BL/6 mice (Tph2C/C), congenic C57BL/6 mice homozygous for the Tph21473G-allele (Tph2G/G), and heterozygous Tph2-deficient mice (Tph2C/-) for anxiety- and depression-like behavior, and evaluated brain serotonin metabolism and 5-HT1AR signaling by high-performance liquid chromatography and quantitative autoradiography, respectively. Progressive reduction in TPH2 activity had no effect on emotional behavior, and only slightly affected brain serotonin levels. However, serotonin degradation rate was drastically decreased in mice with reduced TPH2 activity, thereby compensating for the lowered rate of serotonin production in these mice. In addition, the hypothermic response to the 5-HT1AR agonist, 8-OH-DPAT, was attenuated in mice with reduced serotonin production. In contrast, 5-HT1A autoreceptor density and G-protein coupling were not changed in mice with gradual decrease in central serotonin. Taken together, these data suggest that in conditions of reduced serotonin production lowered serotonin degradation rate contributes to the maintenance of brain serotonin at levels sufficient for adequate behavior responses. These findings reveal that decreased TPH2 activity cannot be considered a reliable predisposition factor for impaired emotional behavior.

Differential postpartum sensitivity to the anxiety-modulating effects of offspring contact is associated with innate anxiety and brainstem levels of dopamine beta-hydroxylase in female laboratory rats

In female mammals, the postpartum period involves dramatic shifts in many socioemotional behaviors. This includes a suppression of anxiety-related behaviors that requires recent physical contact with offspring. Factors contributing to differences among females in their susceptibility to the anxiety-modulating effect of offspring contact are unknown, but could include their innate anxiety and brain monoaminergic activity. Anxiety behavior was assessed in a large group of nulliparous female rats and the least-anxious and most-anxious tertiles were mated. Anxiety was assessed again postpartum after females were permitted or prevented from contacting their offspring 4 h before testing. Levels of dopamine β-hydroxylase (DBH, norepinephrine synthesizing enzyme) and tryptophan hydroxylase-2 (TPH2, serotonin synthesizing enzyme) were measured in the brainstem and dorsal raphe, respectively. It was found that anxiety-related behavior in the two groups did not differ when dams were permitted contact with offspring before testing. Removal of the offspring before testing, however, differentially affected anxiety based on dams' innate anxiety. Specifically, dams reverted back to their pre-mating levels of anxiety such that offspring removal slightly increased anxiety in the most-anxious females but greatly lowered anxiety in the least-anxious females. This reduction in anxiety in the least-anxious females after litter removal was associated with lower brainstem DBH. There was no relationship between females' anxiety and dorsal raphe TPH2. Thus, a primary effect of recent contact with offspring on anxiety-related behavior in postpartum rats is to shift females away from their innate anxiety to a more moderate level of responding. This effect is particularly true for females with the lowest anxiety, may be mediated by central noradrenergic systems, and has implications for their ability to attend to their offspring.