Strain differences in paroxetine-induced reduction of immobility time in the forced swimming test in mice: role of serotonin

Terahertz photoneuromodulation of lateral orbitofrontal cortex neurons ameliorates stress-induced depression and cognitive impairment

Depression imposes a staggering global socioeconomic burden. Current pharmacotherapies face major limitations, including slow efficacy, adverse effects, and non-response rates of up to 55%, necessitating novel therapeutic modalities. This study introduces terahertz (THz) photoneuromodulation as an innovative physical intervention for depression, offering several advantages over conventional pharmacological or optogenetic approaches. Mild THz photoneuromodulation circumvents the need for exogenous agents or genetic modifications, mitigating potential risks while precisely modulating neurotransmitter levels and neuronal excitability to alleviate depression-like behaviors. In a chronic restraint stress (CRS) mouse model, THz photostimulation rapidly attenuated hyperactivity and increased serotonin levels by 107.5% ± 45.3% in lateral orbitofrontal cortex glutamatergic neurons (OFCGlu) compared to those treated with antidepressants. This led to marked improvements in depressive-like behaviors and cognitive function. Furthermore, THz modulation of OFC activity recapitulated the effects of chemogenetic inhibition, underscoring the OFC's pivotal role in regulating depressive states. This research unveils THz photoneuromodulation as a promising, safe, rapid-acting, and durable neurotherapeutic strategy addressing persistent unmet needs in depression treatment.

Boosting Serotonin Synthesis Is Not Sufficient to Improve Motor Coordination of Mecp2 Heterozygous Mouse Model of Rett Syndrome

Motor deficit is a core symptom of Rett syndrome, a rare neurological disease caused in most cases by mutations of the methyl-CpG-binding protein2 (MECP2) gene. Serotonin reuptake inhibitors improve motor coordination in Mecp2 heterozygous (Het) mice and serotonin depletion prevented this effect. Here, we assess alterations in indole levels in various brain regions and whether boosting brain serotonin synthesis with the serotonin precursors tryptophan, 5-hydroxytryptophan and α-lactalbumin rescued motor coordination deficit of Mecp2 Het mice. Motor coordination was assessed in the accelerated rotarod during and after systemic administration of serotonin precursors for 2-3 weeks. Since no data are available, the effect of α-lactalbumin on tryptophan, serotonin and 5-hydroxyindoleacetic acid levels was evaluated in various brain regions in order to identify the dose of ALAC to evaluate on motor coordination. As compared to WT, Mecp2 Het mice show reduced levels of serotonin in the whole brain, hippocampus, brainstem and cerebral cortex, but not the striatum. Reduced levels of 5-hydroxyindoleacetic acid were observed in the hippocampus and brainstem. Doses of serotonin precursors increasing brain tryptophan and/or serotonin production and metabolism had no effect on motor coordination. The results indicate that boosting serotonin synthesis is not sufficient to improve motor coordination of Mecp2 Het mice.

Therapeutic potential of cannabidiol in depression

Major depressive disorder (MDD) is a widespread and debilitating condition affecting a significant portion of the global population. Traditional treatment for MDD has primarily involved drugs that increase brain monoamines by inhibiting their uptake or metabolism, which is the basis for the monoaminergic hypothesis of depression. However, these treatments are only partially effective, with many patients experiencing delayed responses, residual symptoms, or complete non-response, rendering the current view of the hypothesis as reductionist. Cannabidiol (CBD) has shown promising results in preclinical models and human studies. Its mechanism is not well-understood, but may involve monoamine and endocannabinoid signaling, control of neuroinflammation and enhanced neuroplasticity. This chapter will explore CBD's effects in preclinical and clinical studies, its molecular mechanisms, and its potential as a treatment for MDD.

Lipopolysaccharide-induced depression-like model in mice: meta-analysis and systematic evaluation

Depression is a complex and biologically heterogeneous disorder. Recent studies have shown that central nervous system (CNS) inflammation plays a key role in the development of depression. Lipopolysaccharide (LPS)-induced depression-like model in mice is commonly used to studying the mechanisms of inflammation-associated depression and the therapeutic effects of drugs. Numerous LPS-induced depression-like models in mice exist and differ widely in animal characteristics and methodological parameters. Here, we systematically reviewed studies on PubMed from January 2017 to July 2022 and performed cardinal of 170 studies and meta-analyses of 61 studies to support finding suitable animal models for future experimental studies on inflammation-associated depression. Mouse strains, LPS administration, and behavioral outcomes of these models have been assessed. In the meta-analysis, forced swimming test (FST) was used to evaluate the effect size of different mouse strains and LPS doses. The results revealed large effect sizes in ICR and Swiss mice, but less heterogeneity in C57BL/6 mice. For LPS intraperitoneal dose, the difference did not affect behavioral outcomes in C57BL/6 mice. However, in ICR mice, the most significant effect on behavioral outcomes was observed after the injection of 0.5 mg/kg LPS. Our results suggests that mice strains and LPS administration play a key role in the evaluation of behavioral outcomes in such models.

Mirtazapine attenuates the cocaine-induced locomotor sensitization in male and female C57BL/6J and BALBA/cJ mouse

BACKGROUND

Clinical studies have described the efficacy of various therapeutic approaches. Results are inconsistent and clinical application is limited. Clinical trials have suggested that individual variability in the response to pharmacological therapies and sex affects the efficacy of some antidepressant drugs. Mouse strain-dependent variability influenced the response to antidepressant drugs. Some mouse strains respond faster and better to antidepressants than other mouse strains. We recently reported a series of preclinical studies that showed that dosing of mirtazapine, a noradrenergic and serotonergic antidepressant, in male and female Wistar rats decreased cocaine-induced locomotor activity and attenuated the induction and expression of cocaine-induced locomotor sensitization. Therefore, the aim of this study was to evaluate the mirtazapine effects, on cocaine-induced locomotor activity and cocaine-induced locomotor sensitization in male and female mice of the C57BL/6J and BALB/cJ strains, which differ in sensitivity to the cocaine motor effects and response to antidepressant drugs.

METHODS

Male and female BALB/cJ and C57BL/6J inbred mice (20-25 g) were daily dosed with 10 mg/kg of cocaine during the induction and expression of locomotor sensitization. During drug withdrawal, cocaine was withdrawn, and the groups received daily mirtazapine (30 mg/kg, i.p.) or saline. Mirtazapine was administered 30 min before cocaine. After each administration, locomotor activity for each animal was recorded for 30 min in transparent Plexiglass activity chambers.

RESULTS

Cocaine-induced locomotor activity were greater in C57BL/6J strain mice than BALB/cJ strain mice during the induction and expression phase of locomotor sensitization. The female mice of both strains showed a higher cocaine locomotor response than males and mirtazapine significantly decreased cocaine-induced locomotor activity, as well as the induction and expression of locomotor sensitization, regardless of mouse strain or sex.

CONCLUSION

The results suggest mirtazapine may be considered an effective therapeutic option to treat cocaine use disorder in men and women with very diverse genetic backgrounds.

The Effect of Early Life Stress on Emotional Behaviors in GPR37KO Mice

GPR37 is an orphan G-protein-coupled receptor, a substrate of parkin which is linked to Parkinson's disease (PD) and affective disorders. In this study, we sought to address the effects of early life stress (ELS) by employing the paradigm of limited nesting material on emotional behaviors in adult GPR37 knockout (KO) mice. Our results showed that, while there was an adverse effect of ELS on various domains of emotional behaviors in wild type (WT) mice in a sex specific manner (anxiety in females, depression and context-dependent fear memory in males), GPR37KO mice subjected to ELS exhibited less deteriorated emotional behaviors. GPR37KO female mice under ELS conditions displayed reduced anxiety compared to WT mice. This was paralleled by lower plasma corticosterone in GPR37KO females and a lower increase in P-T286-CaMKII by ELS in the amygdala. GPR37KO male mice, under ELS conditions, showed better retention of hippocampal-dependent emotional processing in the passive avoidance behavioral task. GPR37KO male mice showed increased immobility in the forced swim task and increased P-T286-CaMKII in the ventral hippocampus under baseline conditions. Taken together, our data showed overall long-term effects of ELS-deleterious or beneficial depending on the genotype, sex of the mice and the emotional context.

Tryptophan Hydroxylase 2 Deficiency Modifies the Effects of Fluoxetine and Pargyline on the Behavior, 5-HT- and BDNF-Systems in the Brain of Zebrafish (Danio rerio)

The mechanisms of resistance to antidepressant drugs is a key and still unresolved problem of psychopharmacology. Serotonin (5-HT) and brain-derived neurotrophic factor (BDNF) play a key role in the therapeutic effect of many antidepressants. Tryptophan hydroxylase 2 (TPH2) is the rate-limiting enzyme in 5-HT synthesis in the brain. We used zebrafish (Danio rerio) as a promising model organism in order to elucidate the effect of TPH2 deficiency caused by p-chlorophenylalanine (pCPA) on the alterations in behavior and expression of 5-HT-related (Tph2, Slc6a4b, Mao, Htr1aa, Htr2aa) and BDNF-related (Creb, Bdnf, Ntrk2a, Ngfra) genes in the brain after prolonged treatment with two antidepressants, inhibitors of 5-HT reuptake (fluoxetine) and oxidation (pargyline). In one experiment, zebrafish were treated for 72 h with 0.2 mg/L fluoxetine, 2 mg/L pCPA, or the drugs combination. In another experiment, zebrafish were treated for 72 h with 0.5 mg/L pargyline, 2 mg/L pCPA, or the drugs combination. Behavior was studied in the novel tank diving test, mRNA levels were assayed by qPCR, 5-HT and its metabolite concentrations were measured by HPLC. The effects of interaction between pCPA and the drugs on zebrafish behavior were observed: pCPA attenuated “surface dwelling” induced by the drugs. Fluoxetine decreased mRNA levels of Tph2 and Htr2aa genes, while pargyline decreased mRNA levels of Slc6a4b and Htr1aa genes. Pargyline reduced Creb, Bdnf and Ntrk2a genes mRNA concentration only in the zebrafish treated with pCPA. The results show that the disruption of the TPH2 function can cause a refractory to antidepressant treatment.

Sex-based changes in rat brain serotonin and behavior in a model of altitude-related vulnerability to treatment-resistant depression

RATIONALE

Rates of depression and suicide increase with altitude. In our animal model, rats housed at moderate altitude vs. at sea level exhibit increased depressive symptoms in the forced swim test (FST) and lack of response to selective serotonin reuptake inhibitors (SSRIs). Depression and SSRI resistance are linked to disrupted serotonergic function, and hypobaric hypoxia may reduce the oxygen-dependent synthesis of serotonin. We therefore tested brain serotonin in rats housed at altitude.

METHODS

Sprague-Dawley rats were housed at altitude (4,500 ft, 10,000 ft) vs. sea level for 7-36 days. Brain serotonin was measured by ELISA, or behavior evaluated in the FST, sucrose preference (SPT), or open-field tests (OFT).

RESULTS

After 2 weeks at 4,500 ft or 10,000ft vs. sea level, serotonin levels decreased significantly at altitude in the female prefrontal cortex, striatum, hippocampus, and brainstem, but increased with altitude in the male hippocampus and brainstem. Female brain serotonin decreased from 7 to 36 days at 4,500 ft, but males did not vary. At 2 weeks and 24 days, females at altitude exhibit lower brain serotonin and increased depressive symptoms in the FST and SPT, with motor behavior unaltered. In males, serotonin, passive coping in the FST and OFT immobility increased with altitude at 2 weeks, but not at 24 days. Male SPT behavior did not change with altitude.

CONCLUSIONS

Females may be more vulnerable to depressive symptoms at altitude, while males may be resilient. Chronic hypoxic stress at altitudes as low as 4,500 ft may cause a brain serotonin imbalance to worsen vulnerability to depression and SSRI resistance, and potentially worsen suicide risk.

Opposite genotype-specific effects of serotoninergic treatments on Pavlovian Conditioned Approach in mice of two inbred strains C57 BL/6J and DBA/2J

Individual variability in the response to pharmacological therapies is a major problem in the treatment of psychiatric disorders. Comparative studies of phenotypes expressed by mice of the C57BL/6J (C57) and DBA/2J (DBA) inbred strains can help identify neurobiological determinants of this variability at preclinical levels. We have recently demonstrated that whereas young adult mice of both strains develop sign-tracking in a Pavlovian Conditioned Approach (PCA), a trait associated with dysfunctional behavior in rat models, in full adult C57 mice acquisition of this phenotype is inhibited by pre-frontal cortical (PFC) serotonin (5HT) transmission. These findings suggest a different role of 5HT transmission on sign-tracking development in mice of the two genotypes. In the present experiments, we tested the effects of the 5-HT synthesis booster 5-hydroxytryptophan (5-HTP) and of the selective 5HT reuptake inhibitor (SSRI) fluoxetine on the development and expression of sign-tracking in young adult mice from both inbred strains. In mice of the C57 strain, administration of 5-HTP before each training session blocked the training-induced shift to positive PCA scores which indicates the development of sign-tracking, whereas the same treatment was ineffective in mice of DBA strain. On the other hand, a single administration of fluoxetine was ineffective in unhandled saline- and 5-HTP-treated C57 mice, whereas it enhanced the expression of positive PCA scores by mice of DBA strain treated with 5-HTP during training. These findings confirm the strain-specific inhibitory role of PFC 5-HT transmission on sign-tracking development by mice of the C57 strain and support the hypothesis that different genotype-specific neurobiological substrates of dysfunctional phenotypes contribute to variable effects of pharmacotherapies.

Fluoxetine increases brain MeCP2 immuno-positive cells in a female Mecp2 heterozygous mouse model of Rett syndrome through endogenous serotonin

Motor skill deficit is a common and invalidating symptom of Rett syndrome (RTT), a rare disease almost exclusively affecting girls during the first/second year of life. Loss-of-function mutations of the methyl-CpG-binding protein2 (MECP2; Mecp2 in rodents) gene is the cause in most patients. We recently found that fluoxetine, a selective serotonin (5-HT) reuptake inhibitor and antidepressant drug, fully rescued motor coordination deficits in Mecp2 heterozygous (Mecp2 HET) mice acting through brain 5-HT. Here, we asked whether fluoxetine could increase MeCP2 expression in the brain of Mecp2 HET mice, under the same schedule of treatment improving motor coordination. Fluoxetine increased the number of MeCP2 immuno-positive (MeCP2+) cells in the prefrontal cortex, M1 and M2 motor cortices, and in dorsal, ventral and lateral striatum. Fluoxetine had no effect in the CA3 region of the hippocampus or in any of the brain regions of WT mice. Inhibition of 5-HT synthesis abolished the fluoxetine-induced rise of MeCP2+ cells. These findings suggest that boosting 5-HT transmission is sufficient to enhance the expression of MeCP2 in several brain regions of Mecp2 HET mice. Fluoxetine-induced rise of MeCP2 could potentially rescue motor coordination and other deficits of RTT.

Mapping the living mouse brain neural architecture: strain-specific patterns of brain structural and functional connectivity

Mapping brain structural and functional connectivity (FC) became an essential approach in neuroscience as network properties can underlie behavioral phenotypes. In mouse models, revealing strain-related patterns of brain wiring is crucial, since these animals are used to answer questions related to neurological or neuropsychiatric disorders. C57BL/6 and BALB/cJ strains are two of the primary "genetic backgrounds" for modeling brain disease and testing therapeutic approaches. However, extensive literature describes basal differences in the behavioral, neuroanatomical and neurochemical profiles of the two strains, which raises questions on whether the observed effects are pathology specific or depend on the genetic background of each strain. Here, we performed a systematic comparative exploration of brain structure and function of C57BL/6 and BALB/cJ mice using Magnetic Resonance Imaging (MRI). We combined deformation-based morphometry (DBM), diffusion MRI and high-resolution fiber mapping (hrFM) along with resting-state functional MRI (rs-fMRI) and demonstrated brain-wide differences in the morphology and "connectome" features of the two strains. Essential inter-strain differences were depicted regarding the size and the fiber density (FD) within frontal cortices, along cortico-striatal, thalamic and midbrain pathways as well as genu and splenium of corpus callosum. Structural dissimilarities were accompanied by specific FC patterns, emphasizing strain differences in frontal and basal forebrain functional networks as well as hubness characteristics. Rs-fMRI data further indicated differences of reward-aversion circuitry and default mode network (DMN) patterns. The inter-hemispherical FC showed flexibility and strain-specific adjustment of their patterns in agreement with the structural characteristics.

Genetic Background Underlying 5-HT1A Receptor Functioning Affects the Response to Fluoxetine

The influence of genetic background on sensitivity to drugs represents a topical problem of personalized medicine. Here, we investigated the effect of chronic (20 mg/kg, 14 days, i.p.) antidepressant fluoxetine treatment on recombinant B6-M76C mice, differed from control B6-M76B mice by CBA-derived 102.73–110.56 Mbp fragment of chromosome 13 and characterized by altered sensitivity of 5-HT_1A receptors to chronic 8-OH-DPAT administration and higher 5-HT_1A receptor mRNA levels in the frontal cortex and hippocampus. Significant changes in the effects of fluoxetine treatment on behavior and brain 5-HT system in recombinant B6-M76C mice were revealed. In contrast to B6-M76B mice, in B6-M76C mice, fluoxetine produced pro-depressive effects, assessed in a forced swim test. Fluoxetine decreased 5-HT_1A receptor mRNA levels in the cortex and hippocampus, reduced 5-HT_1A receptor protein levels and increased receptor silencer Freud-1 protein levels in the hippocampus of B6-M76C mice. Fluoxetine increased mRNA levels of the gene encoding key enzyme for 5-HT synthesis in the brain, tryptophan hydroxylase-2, but decreased tryptophan hydroxylase-2 protein levels in the midbrain of B6-M76B mice. These changes were accompanied by increased expression of the 5-HT transporter gene. Fluoxetine reduced 5-HT and 5-HIAA levels in cortex, hippocampus and midbrain of B6-M76B and in cortex and midbrain of B6-M76C; mice. These data demonstrate that changes in genetic background may have a dramatic effect on sensitivity to classic antidepressants from the Selective Serotonin Reuptake Inhibitors family. Additionally, the results provide new evidence confirming our idea on the disrupted functioning of 5-HT_1A autoreceptors in the brains of B6-M76C mice, suggesting these mice as a model of antidepressant resistance.

Key role of the 5-HT1A receptor addressing protein Yif1B in serotonin neurotransmission and SSRI treatment

BACKGROUND

Altered function of serotonin receptor 1A (5-HT1AR) has been consistently implicated in anxiety, major depressive disorder and resistance to antidepressants. Mechanisms by which the function of 5-HT1AR (expressed as an autoreceptor in serotonergic raphe neurons and as a heteroreceptor in serotonin [5-HT] projection areas) is altered include regulation of its expression, but 5-HT1AR trafficking may also be involved.

METHODS

We investigated the consequences of the lack of Yif1B (the 5-HT1AR trafficking protein) on 5-HT neurotransmission in mice, and whether Yif1B expression might be affected under conditions known to alter 5-HT neurotransmission, such as anxious or depressive states or following treatment with fluoxetine (a selective serotonin reuptake inhibitor) in humans, monkeys and mice.

RESULTS

Compared with wild-type mice, Yif1B-knockout mice showed a significant decrease in the forebrain density of 5-HT projection fibres and a hypofunctionality of 5-HT1A autoreceptors expressed on raphe 5-HT neurons. In addition, social interaction was less in Yif1B-knockout mice, which did not respond to the antidepressant-like effect of acute fluoxetine injection. In wild-type mice, social defeat was associated with downregulated Yif1B mRNA in the prefrontal cortex, and chronic fluoxetine treatment increased Yif1B expression. The expression of Yif1B was also downregulated in the postmortem prefrontal cortex of people with major depressive disorder and upregulated after chronic treatment with a selective serotonin reuptake inhibitor in monkeys.

LIMITATIONS

We found sex differences in Yif1B expression in humans and monkeys, but not in mice under the tested conditions.

CONCLUSION

These data support the concept that Yif1B plays a critical role in 5-HT1AR functioning and brain 5-HT homeostasis. The opposite changes in its expression observed in anxious or depressive states and after therapeutic fluoxetine treatment suggest that Yif1B might be involved in vulnerability to anxiety and depression, and fluoxetine efficacy.

Fluoxetine rescues rotarod motor deficits in Mecp2 heterozygous mouse model of Rett syndrome via brain serotonin

Motor skill is a specific area of disability of Rett syndrome (RTT), a rare disorder occurring almost exclusively in girls, caused by loss-of-function mutations of the X-linked methyl-CpG-binding protein2 (MECP2) gene, encoding the MECP2 protein, a member of the methyl-CpG-binding domain nuclear proteins family. Brain 5-HT, which is defective in RTT patients and Mecp2 mutant mice, regulates motor circuits and SSRIs enhance motor skill learning and plasticity. In the present study, we used heterozygous (Het) Mecp2 female and Mecp2-null male mice to investigate whether fluoxetine, a SSRI with pleiotropic effects on neuronal circuits, rescues motor coordination deficits. Repeated administration of 10 mg/kg fluoxetine fully rescued rotarod deficit in Mecp2 Het mice regardless of age, route of administration or pre-training to rotarod. The motor improvement was confirmed in the beam walking test while no effect was observed in the hanging-wire test, suggesting a preferential action of fluoxetine on motor coordination. Citalopram mimicked the effects of fluoxetine, while the inhibition of 5-HT synthesis abolished the fluoxetine-induced improvement of motor coordination. Mecp2 null mice, which responded poorly to fluoxetine in the rotarod, showed reduced 5-HT synthesis in the prefrontal cortex, hippocampus and striatum, and reduced efficacy of fluoxetine in raising extracellular 5-HT as compared to female mutants. No sex differences were observed in the ability of fluoxetine to desensitize 5-HT_1A autoreceptors upon repeated administration. These findings indicate that fluoxetine rescues motor coordination in Mecp2 Het mice through its ability to enhance brain 5-HT and suggest that drugs enhancing 5-HT neurotransmission may have beneficial effects on motor symptoms of RTT.

Increased depression-related behavior during the postpartum period in inbred BALB/c and C57BL/6 strains

Pregnancy and lactation are characterized by dramatic changes in the endocrine system and brain in mammalian females. These changes, with stress before pregnancy, are potential risk factors for the development of postpartum depression (PPD). A valid animal model of PPD is needed to understand the neurobiological basis of the depressive state of females. To explore a mouse model of PPD, we first assessed anxiety-like and depression-related behaviors in nulliparous (virgin), nonlactating primiparous, and lactating primiparous females in four inbred strains of mice (C57BL/6J, C57BL/6JJcl, BALB/cAnNCrlCrlj, and BALB/cAJcl). Pups from the nonlactating female group were removed one day after parturition to examine the effects of physical interaction with pups on the postpartum behaviors. Second, we investigated the additional effects of prepregnancy stress (restraint stress for 6 h/day for 21 days) on postpartum behaviors in the BALB/cAJcl strain. We found that females of the two BALB/c substrains showed decreased locomotor activity and increased anxiety-like and depression-related behaviors compared with females of the two C57BL/6 substrains. Behavioral differences were also observed between the two substrains of each strain. Additionally, pregnancy- and lactation-dependent behavioral differences were found in some strains: lactating BALB/cAJcl females traveled shorter distance than the females of the other reproductive state groups, while nonlactating and lactating BALB/cAJcl and C57BL/6J females showed increased depression-related behavior compared with nulliparous females. Lactating BALB/cAJcl and C57BL/6JJcl females exhibited decreased sucrose preference or anhedonia-like behavior compared with nulliparous and nonlactating females, although these results did not reach statistical significance after correction for multiple testing. An additional independent experiment replicated the marked behavioral changes in lactating BALB/cAJcl females. Moreover, increased anxiety-like behavior was observed in lactating BALB/cAJcl females that experienced prepregnancy stress. These results suggest genetic contributions to the regulation of anxiety-like and depression-related behaviors in female mice. Furthermore, this study suggests that pregnancy and lactation cause decreased locomotor activity and increased depression-related behaviors, which was consistently found in our results, and that prepregnancy stress enhances anxiety-like behavior in the BALB/cAJcl strain. The inbred strain of female mice may be used as a potential model of PPD to further study the genetic and neurobiological mechanisms underlying the development of this disorder.

Tryptophan hydroxylase 2 as a therapeutic target for psychiatric disorders: focus on animal models

Introduction: Tryptophan hydroxylase 2 (TPH2) is the key, rate-limiting enzyme of serotonin (5-HT) synthesis in the brain. Some polymorphic variants of the human Tph2 gene are associated with psychiatric disorders. Area covered: This review focuses on the mechanisms underlying the association between the TPH2 activity and behavioral disturbances in models of psychiatric disorders. Specifically, it discusses: 1) genetic and posttranslational mechanisms defining the TPH2 activity, 2) behavioral effects of knockout and loss-of-function mutations in the mouse Tph2 gene, 3) pharmacological inhibition and the activation of the TPH2 activity and 4) alterations in the brain TPH2 activity in animal models of psychiatric disorders. We show the dual role of the TPH2 activity: both deficit and excess of the TPH2 activity cause significant behavioral disturbances in animal models of depression, anxiety, aggression, obsessive-compulsive disorders, schizophrenia, and catalepsy. Expert opinion: Pharmacological chaperones correcting the structure of the TPH2 molecule are promising tools for treatment of some hereditary psychiatric disorders caused by loss-of-function mutations in the human Tph2 gene; while some stress-induced affective disorders, associated with the elevated TPH2 activity, may be effectively treated by TPH2 inhibitors. This dual role of TPH2 should be taken into consideration during therapy of psychiatric disorders.

Hypobaric hypoxia exposure in rats differentially alters antidepressant efficacy of the selective serotonin reuptake inhibitors fluoxetine, paroxetine, escitalopram and sertraline

Treatment-resistant depression, a chronic condition that affects 30% of depressed patients on antidepressants, is highly linked to suicidal behavior. Chronic hypoxia exposure via living at altitude (hypobaric hypoxia) or with chronic hypoxic diseases is demographically linked to increased risk for depression and suicide. We previously demonstrated that housing rats at altitude for a week incrementally increases depression-like behavior in the forced swim test (FST) in females, but not males. In animal models, high altitude exposure reduces brain serotonin, and selective serotonin reuptake inhibitors (SSRIs) can lose efficacy when brain serotonin levels are low. To address whether residence at moderate altitude is detrimental to SSRI function, we examined SSRI efficacy in the FST after a week of housing rats at altitudes of 4500 ft. or 10,000 ft. as compared to at sea level. In females, the tricyclic antidepressant desipramine (positive control) functioned well in all groups, increasing latency to immobility and decreasing immobility, by increasing climbing. However, the SSRIs fluoxetine, paroxetine and escitalopram were ineffective in females in all groups: only paroxetine improved swimming in the FST as expected of a SSRI, while all three unexpectedly reduced climbing. Fluoxetine was also ineffective in male rats. Sertraline was the only SSRI with antidepressant efficacy at altitude in both females and males, increasing swimming, climbing and latency to immobility, and reducing immobility. Hypobaric hypoxia thus appears to be detrimental to efficacy of the SSRIs fluoxetine, paroxetine and escitalopram, but not of sertraline. Unlike the other SSRIs, sertraline can improve both serotonergic and dopaminergic transmission, and may be less impacted by a hypoxia-induced serotonin deficit. A targeted approach may thus be necessary for successful antidepressant treatment in patients with depression who live at altitude or with chronic hypoxic diseases, and that sertraline may be the SSRI of choice for prescription for this population.

Increased Anxiety and Anhedonia in Female Rats Following Exposure to Altitude

Sheth, Chandni, Hendrik Ombach, Paul Olson, Perry F. Renshaw, and Shami Kanekar. Increased anxiety and anhedonia in female rats following exposure to altitude. High Alt Med Biol. 19:81-90, 2018.-Anxiety disorders are chronic, highly prevalent conditions, often comorbid with depression. Both anxiety and depression form major risk factors for suicide. Living at altitude is associated with higher rates of depression and suicide, leading us to address whether anxiety disorders may also be amplified at altitude. Using a novel translational animal model, we previously showed that depression-like behavior increases with altitude of housing in female, but not male rats. We now use this model to examine the effects of altitude on both anxiety-like behavior and anhedonia, a core symptom of depression. After housing for a week at sea level, 4500 or 10,000 ft, rats were evaluated for anxiety in the open-field test or the elevated plus maze, and anhedonia in the sucrose preference test. Another group was tested at baseline. Anxiety-like behavior increased in females housed at altitude. In females, lower sucrose preference was seen in those housed at 10,000 ft versus those at sea level. Males showed no change in anxiety or anhedonia across groups. These data suggest that living at moderate-high altitude may pose a risk factor for those vulnerable to anxiety disorders, with the potential to be particularly detrimental to females at altitude.

Purine and pyrimidine metabolism: Convergent evidence on chronic antidepressant treatment response in mice and humans

Selective Serotonin Reuptake Inhibitors (SSRIs) are commonly used drugs for the treatment of psychiatric diseases including major depressive disorder (MDD). For unknown reasons a substantial number of patients do not show any improvement during or after SSRI treatment. We treated DBA/2J mice for 28 days with paroxetine and assessed their behavioral response with the forced swim test (FST). Paroxetine-treated long-time floating (PLF) and paroxetine-treated short-time floating (PSF) groups were stratified as proxies for drug non-responder and responder mice, respectively. Proteomics and metabolomics profiles of PLF and PSF groups were acquired for the hippocampus and plasma to identify molecular pathways and biosignatures that stratify paroxetine-treated mouse sub-groups. The critical role of purine and pyrimidine metabolisms for chronic paroxetine treatment response in the mouse was further corroborated by pathway protein expression differences in both mice and patients that underwent chronic antidepressant treatment. The integrated -omics data indicate purine and pyrimidine metabolism pathway activity differences between PLF and PSF mice. Furthermore, the pathway protein levels in peripheral specimens strongly correlated with the antidepressant treatment response in patients. Our results suggest that chronic SSRI treatment differentially affects purine and pyrimidine metabolisms, which may explain the heterogeneous antidepressant treatment response and represents a potential biosignature.

5-HT6 receptor agonism facilitates emotional learning

Serotonin (5-HT) and its receptors play crucial roles in various aspects of mood and cognitive functions. However, the role of specific 5-HT receptors in these processes remains to be better understood. Here, we examined the effects of the selective and potent 5-HT6 agonist (WAY208466) on mood, anxiety and emotional learning in mice. Male C57Bl/6J mice were therefore tested in the forced swim test (FST), elevated plus-maze (EPM), and passive avoidance tests (PA), respectively. In a dose-response experiment, mice were treated intraperitoneally with WAY208466 at 3, 9, or 27 mg/kg and examined in an open field arena open field test (OFT) followed by the FST. 9 mg/kg of WAY208466 reduced immobility in the FST, without impairing the locomotion. Thus, the dose of 9 mg/kg was subsequently used for tests of anxiety and emotional learning. There was no significant effect of WAY208466 in the EPM. In the PA, mice were trained 30 min before the treatment with saline or WAY208466. Two separate sets of animals were used for short term memory (tested 1 h post-training) or long term memory (tested 24 h post-training). WAY208466 improved both short and long term memories, evaluated by the latency to enter the dark compartment, in the PA. The WAY208466-treated animals also showed more grooming and rearing in the light compartment. To better understand the molecular mechanisms and brain regions involved in the facilitation of emotional learning by WAY208466, we studied its effects on signal transduction and immediate early gene expression. WAY208466 increased the levels of phospho-Ser⁸⁴⁵-GluA1 and phospho-Ser^217/221-MEK in the caudate-putamen. Levels of phospho-Thr^202/204-Erk1/2 and the ratio mature BDNF/proBDNF were increased in the hippocampus. Moreover, WAY208466 increased c-fos in the hippocampus and Arc expression in both hippocampus and prefrontal cortex (PFC). The results indicate antidepressant efficacy and facilitation of emotional learning by 5-HT6 receptor agonism via mechanisms that promote neuronal plasticity in caudate putamen, hippocampus, and PFC.

Viral vector mediated expression of mutant huntingtin in the dorsal raphe produces disease-related neuropathology but not depressive-like behaviors in wildtype mice

Huntington׳s disease (HD) is a neurodegenerative disorder caused by a mutation in the HTT gene (mHTT) encoding the protein huntingtin. An expansion in the gene׳s CAG repeat length renders a misfolded, dysfunctional protein with an abnormally long glutamine (Q) stretch at the N terminus that often incorporates into inclusion bodies and leads to neurodegeneration in many regions of the brain. HD is characterized by motor and cognitive decline as well as mood disorders, with depression being particularly common. Approximately 40% of the HD population suffers from depressive symptoms. Because these symptoms often manifest a decade or more prior to the knowledge that the person is at risk for the disease, a portion of the early depression in HD appears to be a consequence of the pathology arising from expression of the mutant gene. While the depression in HD patients is often treated with serotonin agonists, there is scant experimental evidence that the depression in HD responds well to these serotonin treatments or in a similar manner to how non-HD depression tends to respond. Additionally, at very early sub-threshold depression levels, abnormal changes in several neuronal populations are already detectable in HD patients, suggesting that a variety of brain structures may be involved. Taken together, the serotonin system is a viable candidate. However, at present there is limited evidence of the precise nuclei or circuits that play a role in HD depression. With this in mind, the current study was designed to control for the widespread brain neuropathology that occurs in HD and in transgenic mouse models of HD and focuses specifically on the influence of the midbrain dorsal raphe nucleus (DRN). The DRN provides the majority of the serotonin to the forebrain and exhibits cell loss in non-HD depression. Therefore, we employed a viral vector delivery system to investigate whether the over-expression of mHTT in the DRN׳s ventral sub-nuclei alone is sufficient to produce depressive-like behaviors. Wildtype mice were injected with an adeno-associated virus (AAV2/1) encoding HTT containing either a pathogenic (N171-82Q) or control (N171-16Q) CAG repeat length into the ventral DRN and depressive-like behaviors and motor behaviors were assessed for 12 weeks post-surgery. Quantitative PCR and immunohistochemistry (IHC) verified positive transduction in the ventral aspects of the DRN, including the ventral sub-nucleus (DRv) and interfascicular sub-nucleus (DRif). IHC demonstrated microgliosis in and around the injection site and mHTT-positive inclusions in serotonin-producing neurons and a small percentage of astrocytes in animals injected with N171-82Q compared to controls. Moreover, N171-82Q injected mice showed a 75% reduction in cells that stained positive for the serotonin synthesis enzyme, tryptophan hydroxylase-2 (TPH2) compared to controls (p<0.05). Despite mHTT-mediated pathology in the DRv and DRif, no significant changes in depressive-like behavior were detected. Consequently, we conclude that 12 weeks of N171-82Q expression in the ventral sub-nuclei of the DRN of wildtype mice causes characteristic disease-related cellular neuropathology but is not sufficient to elicit depressive-like behaviors. Ongoing studies are investigating whether a larger injection volume that transfects a larger percentage of the DRN and/or a longer time course of mHTT expression might elicit depressive-like behaviors. Moreover, mHTT expression in other regions of the brain, such as the hippocampal dentate gyrus and/or the frontal cortex might be necessary to elicit HD depression. Together, these results may prove helpful in addressing which therapeutic and/or pharmacological strategies might be most efficacious when treating depressive symptomology in patients suffering from HD.

Molecular actions and clinical pharmacogenetics of lithium therapy

Mood disorders, including bipolar disorder and depression, are relatively common human diseases for which pharmacological treatment options are often not optimal. Among existing pharmacological agents and mood stabilizers used for the treatment of mood disorders, lithium has a unique clinical profile. Lithium has efficacy in the treatment of bipolar disorder generally, and in particular mania, while also being useful in the adjunct treatment of refractory depression. In addition to antimanic and adjunct antidepressant efficacy, lithium is also proven effective in the reduction of suicide and suicidal behaviors. However, only a subset of patients manifests beneficial responses to lithium therapy and the underlying genetic factors of response are not exactly known. Here we discuss preclinical research suggesting mechanisms likely to underlie lithium's therapeutic actions including direct targets inositol monophosphatase and glycogen synthase kinase-3 (GSK-3) among others, as well as indirect actions including modulation of neurotrophic and neurotransmitter systems and circadian function. We follow with a discussion of current knowledge related to the pharmacogenetic underpinnings of effective lithium therapy in patients within this context. Progress in elucidation of genetic factors that may be involved in human response to lithium pharmacology has been slow, and there is still limited conclusive evidence for the role of a particular genetic factor. However, the development of new approaches such as genome-wide association studies (GWAS), and increased use of genetic testing and improved identification of mood disorder patients sub-groups will lead to improved elucidation of relevant genetic factors in the future.

Individual differences and the characterization of animal models of psychopathology: a strong challenge and a good opportunity

Despite the development of valuable new techniques (i.e., genetics, neuroimage) for the study of the neurobiological substrate of psychiatric diseases, there are strong limitations in the information that can be gathered from human studies. It is thus critical to develop appropriate animal models of psychiatric diseases to characterize their putative biological bases and the development of new therapeutic strategies. The present review tries to offer a general perspective and several examples of how individual differences in animals can contribute to explain differential susceptibility to develop behavioral alterations, but also emphasizes methodological problems that can lead to inappropriate or over-simplistic interpretations. A critical analysis of the approaches currently used could contribute to obtain more reliable data and allow taking full advantage of new and sophisticated technologies. The discussion is mainly focused on anxiety-like and to a lower extent on depression-like behavior in rodents.

Sildenafil, a phosphodiesterase type 5 inhibitor, reduces antidepressant-like activity of paroxetine in the forced swim test in mice

BACKGROUND

Sildenafil, a selective phosphodiesterase 5 (PDE5) inhibitor, has recently been reported to influence the antidepressant activity of some antidepressant drugs. The present study was undertaken to investigate the involvement of the nitric oxide/cyclic guanosine 3',5'-monophosphate/PDE5 (NO/cGMP/PDE5) signaling pathway in the antidepressant activity of paroxetine and to assess the interaction between paroxetine and sildenafil, in the forced swim test in mice.

METHODS

Swim trials were conducted by placing mice in glass cylinders filled with water for 6 min. Total behavioral immobility was measured during the last 4 min of the test. Changes in locomotor activity were measured with photoresistor actimeters. Serum and brain paroxetine concentrations were assayed by the HPLC method.

RESULTS

Paroxetine at a dose of 1 mg/kg significantly decreased immobility time in the forced swim test, while sildenafil (5, 10 and 20 mg/kg) in a dose-dependent manner reduced the antidepressant activity of paroxetine. Pharmacokinetic studies did not show any significant changes in paroxetine concentration in serum and brain tissue as compared to paroxetine treatment alone.

CONCLUSIONS

The results suggest that paroxetine may exert its antidepressant action by decreasing cGMP levels and sildenafil, as a drug which has the opposite effect on the processes mediated via the NO/cGMP/PDE5 signaling pathway, may decrease the efficacy of paroxetine. However, the co-administration of paroxetine with sildenafil resulted in a potent reduction (80%) of locomotor activity, which suggests that the reversal of antidepressant action of paroxetine may have been a result of locomotor deficits. Further studies are required to explain the mechanism underlying this phenomenon.

Differential responses to distinct psychotropic agents of selectively bred dominant and submissive animals

Dominance and submissiveness are two opposite poles of behavior representing important functional elements in the development of social interactions. We previously demonstrated the inheritability of these traits by selective breeding based upon the dominant-submissive relationships (DSR) food competition paradigm. Continued multigenerational behavioral selection of Sabra mice yielded animal populations with strong and stable features of dominance and submissiveness. We found that these animals react differentially to stressogenic triggers, antidepressants and mood stabilizing agents. The anxiolytic compound diazepam (1.5mg/kg, i.p.) reduced anxiety-like behavior of submissive animals, but showed anxiogenic effects among dominant animals. In the Forced Swim test, the antidepressant paroxetine (1, 3 and 10mg/kg, i.p.) markedly reduced immobility of submissive animals, demonstrating antidepressant-like effect. In contrast, when administered to dominant animals, paroxetine caused extreme (frenetic) activity. The mood stabilizer lithium (0.4%, p.o.) selectively influenced dominant mice, without affecting the behavior of submissive animals. In summary, we describe here two distinct animal populations possessing strong dominant and submissive phenotypes. We suggest that these populations hold potential as tools for studying the molecular basis and pharmacogenetics of dominant and submissive behavior.

Models for depression in drug screening and preclinical studies: Future directions

The basic consideration in the field of antidepressants is that tests to model depression do not exist, as depression etiopathology is unknown. So far, any kind of proposed model for depression needs to satisfy construct, face and predictive validities. In the present editorial, this idea is challenged, based on the fact that “old” methods can only reveal therapeutical “me-too” drugs and that there is no longer a need of therapeutical “me-too” drugs in the field of antidepressants. Since reduction in the number of antidepressant non-responders is a real medical need, the predictive validity of animal models will be challenged in the future, as the new methods should be based on antidepressant-insensitive animals. Moreover, antidepressants exert similar effects in depressed and non-depressed subjects, but mood normalization is only induced in depressed patients. This implies that the use of normal cells and animals only involves pharmacological rather than therapeutical actions of drugs. Therefore, the use of environmental-induced changes, in the hope that these can evidence antidepressant-insensitive animals, will predominantly be used in the future. In the choice of experimental settings, other factors need to be taken into consideration: (1) gender of animals, as depression affects females more than males, (2) natural rhythmicity in drug effects; (3) pharmacokinetics; and (4) possible biomarker(s) to be measured. There are no golden recipes to discover new antidepressants but the experimental long-term strategy should very clearly be declared before starting the experiments.

The mouse forced swim test

The forced swim test is a rodent behavioral test used for evaluation of antidepressant drugs, antidepressant efficacy of new compounds, and experimental manipulations that are aimed at rendering or preventing depressive-like states. Mice are placed in an inescapable transparent tank that is filled with water and their escape related mobility behavior is measured. The forced swim test is straightforward to conduct reliably and it requires minimal specialized equipment. Successful implementation of the forced swim test requires adherence to certain procedural details and minimization of unwarranted stress to the mice. In the protocol description and the accompanying video, we explain how to conduct the mouse version of this test with emphasis on potential pitfalls that may be detrimental to interpretation of results and how to avoid them. Additionally, we explain how the behaviors manifested in the test are assessed.

Differential long term effects of early diisopropylfluorophosphate exposure in Balb/C and C57Bl/J6 mice

The long-term effect of postnatal administration of a sub-toxic dose of the irreversible acetylcholinesterase inhibitor diisopropylfluorophosphate (DFP) on depression and anxiety behavior was compared in two strains of inbred mice. C57BL/6J and Balb/C mice were injected for 7 consecutive days with either 1 mg/kg DFP or saline on postnatal days 14-20. Mice were tested at age 3-4 months for initial and learned anxiety using double-exposure elevated plus maze and to a novel enclosed environment. Depression was assayed using the sweet preference model of anhedonia and the forced swim test for despair. Postnatal DFP pretreatment led to less activity and more immobility in the elevated plus maze in both mouse strains in the first session. The effect was attenuated in the second session in the C57BL/6J strain but not the Balb/C strain. DFP did not affect the sweet preference or forced swim tests, suggesting a dissociation between the long-term effects of DFP on immobility in the context of approach-avoidance conflict (elevated plus maze) versus despair (forced swim).

Association between tryptophan hydroxylase-2 genotype and the antidepressant effect of citalopram and paroxetine on immobility time in the forced swim test in mice

Tryptophan hydroxylase-2 (TPH2) is the rate limiting enzyme of serotonin synthesis in the brain. The 1473G allele of the C1473G polymorphism in mTPH2 gene is associated with reduced enzyme activity and serotonin synthesis rate in the mouse brain. Here, the influence of the 1473G allele on the antidepressant effect of selective serotonin reuptake inhibitors (SSRIs), citalopram (2.5 or 5.0mg/kg) and paroxetine (5.0 or 10.0mg/kg), in the forced swim test was studied using B6-1473G and B6-1473C congenic mouse lines with the 1473G (decreased TPH2 activity) or 1473C (normal TPH2 activity) alleles, respectively, transferred to the genome of C57BL/6 mouse strain. Paroxetine (5.0 or 10.0mg/kg) and citalopram (2.5 or 5.0mg/kg) decreased immobility time in B6-1473C mice, while both doses of paroxetine and 2.5mg/kg of citaloprame did not alter immobility time in B6-1473G mice. However, 5.0mg/kg of citalopram reduced immobility in B6-1473G mice. The results provided genetic evidence of moderate association between 1473G allele and reduced sensitivity to SSRIs in mice.

The role of serotonin receptor subtypes in treating depression: a review of animal studies

RATIONALE

Serotonin reuptake inhibitors (SSRIs) are effective in treating depression. Given the existence of different families and subtypes of 5-HT receptors, multiple 5-HT receptors may be involved in the antidepressant-like behavioral effects of SSRIs.

OBJECTIVE

Behavioral pharmacology studies investigating the role of 5-HT receptor subtypes in producing or blocking the effects of SSRIs were reviewed.

RESULTS

Few animal behavior tests were available to support the original development of SSRIs. Since their development, a number of behavioral tests and models of depression have been developed that are sensitive to the effects of SSRIs, as well as to other types of antidepressant treatments. The rationale for the development and use of these tests is reviewed. Behavioral effects similar to those of SSRIs (antidepressant-like) have been produced by agonists at 5-HT(1A), 5-HT(1B), 5-HT(2C), 5-HT(4), and 5-HT(6) receptors. Also, antagonists at 5-HT(2A), 5-HT(2C), 5-HT(3), 5-HT(6), and 5-HT(7) receptors have been reported to produce antidepressant-like responses. Although it seems paradoxical that both agonists and antagonists at particular 5-HT receptors can produce antidepressant-like effects, they probably involve diverse neurochemical mechanisms. The behavioral effects of SSRIs and other antidepressants may also be augmented when 5-HT receptor agonists or antagonists are given in combination.

CONCLUSIONS

The involvement of 5-HT receptors in the antidepressant-like effects of SSRIs is complex and involves the orchestration of stimulation and blockade at different 5-HT receptor subtypes. Individual 5-HT receptors provide opportunities for the development of a newer generation of antidepressants that may be more beneficial and effective than SSRIs.

Antidepressant response to chronic citalopram treatment in eight inbred mouse strains

RATIONALE

The antidepressant response exhibits a characteristic delay. BALB/cJ mice respond to chronic, but not subchronic, treatment with selective serotonin reuptake inhibitors (SSRIs), providing a model of antidepressant onset. Identification of other mouse strains exhibiting this phenotype will provide additional tools for studying mechanisms of the antidepressant response.

OBJECTIVES

We aimed to identify inbred mouse strains that respond to chronic, but not subchronic, SSRI treatment in the forced swim test (FST). We also assessed whether response correlated with genotype at the functional C1473G polymorphism in tryptophan hydroxylase-2 (Tph2).

METHODS

BALB/cJ, three closely related strains (BALB/cByJ, SEA/GnJ, A/J), and four distantly related strains (C57BL/6J, C57BL/10J, CAST/EiJ, SM/J) received the SSRI citalopram (0-30 mg/kg/day in drinking water) for ~4 weeks and were assessed for locomotion and FST behavior. Citalopram-responsive strains were assessed identically following ~1 week of treatment. C1473G genotypes were determined.

RESULTS

BALB/cJ and related strains carried the 1473G allele and responded to chronic citalopram treatment in the FST. BALB/cJ, BALB/cByJ, and SEA/GnJ mice showed either no response or an attenuated response to subchronic treatment. Distantly related strains carried the 1473C allele and showed no response to citalopram. No relationship was found between the antidepressant response and baseline immobility or locomotion.

CONCLUSIONS

BALB/cJ and related strains exhibit an antidepressant response to chronic SSRI treatment that emerges over time and is likely a heritable trait. This antidepressant response is associated with carrying the 1473G allele in Tph2. In conclusion, BALB/cJ and related strains provide valuable models for studying the therapeutic mechanisms of SSRIs.

Strain-dependent serotonin neuron feedback control: role of serotonin 2C receptors

We investigated the role of serotonin(2C) receptor-mediated feedback mechanisms in the response to citalopram in C57BL/6 and DBA/2 mice, which are respectively responders and non-responders to selective serotonin reuptake inhibitors in the forced swimming test. The microdialysis technique was used to assess changes in extracellular serotonin and GABA in the mouse dorsal raphé (DR). Citalopram (1.25-20 mg/kg) raised extracellular serotonin and GABA in the DR of both mouse strains. These effects were abolished by depleting brain serotonin with p-chlorophenylalanine (300 mg/kg × 3). Systemic and/or intra-DR infusion of the serotonin(2C) receptor antagonist 6-chloro-5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-indoline (1 mg/kg and 0.1 μM, respectively) enhanced citalopram's effect on extracellular serotonin in the DR and medial prefrontal cortex and abolished the rise of GABA in the DR of DBA/2 mice but had no effect in C57BL/6 mice. The serotonin(2C) receptor agonist Ro60-0175 (0.03-3.0 mg/kg) reduced extracellular serotonin and raised GABA in the DR of DBA/2 mice but had much less effect in C57BL/6 mice. These findings show that the sensitivity of serotonin(2C) receptors determines the efficacy of augmentation strategies aimed at enhancing the effect of serotonin reuptake inhibitors on extracellular serotonin through the suppression of serotonin(2C) receptor-mediated feedback control of serotonin neurons.

Tryptophan hydroxylase 2 genotype determines brain serotonin synthesis but not tissue content in C57Bl/6 and BALB/c congenic mice

Tryptophan hydroxylase 2 (TPH2) catalyzes the rate-limiting step in the synthesis of brain serotonin (5-HT). In a previous report, a single nucleotide polymorphism in mTph2 (C1473G) reduced 5-HT synthesis by 55%. Mouse strains expressing the 1473C allele, such as C57Bl/6, have higher 5-HT synthesis rates than strains expressing the 1473G allele, such as BALB/c. Many studies have attributed strain differences to Tph2 genotype without ruling out the potential role of alterations in other genes. To test the role of the C1473G polymorphism in strain differences, we generated C57Bl/6 and BALB/c mice congenic for the Tph2 locus. We found that the 1473G allele reduced 5-HT synthesis in C57Bl/6 mice but had no effect on 5-HT tissue content except for a slight reduction (15%) in the frontal cortex. In BALB/c mice, the 1473C allele increased 5-HT synthesis but again did not affect 5-HT tissue content. At the same time, 5-hydroxyindoleacetic acid (5-HIAA) was significantly elevated in BALB/c congenic mice. In C57Bl/6 mice, there was no effect of genotype on 5-HIAA levels. BALB/c mice had lower expression of monoamine oxidase A and B than C57Bl/6 mice, but there was no effect of Tph2 genotype. On the tail suspension test, escitalopram treatment reduced immobility regardless of genotype. These data demonstrate that the C1473G polymorphism determines differences in 5-HT synthesis rates among strains but only minimally affects 5-HT tissue levels.

Strain and regional dependence of alternate splicing of acetylcholinesterase in the murine brain following stress or treatment with diisopropylfluorophosphate

Induction of the rare readthrough variant of acetylcholinesterase (AChE-R) by an acetylcholinesterase (AChE) inhibitor or by stress was tested in four mouse strains that differ in their behavioural profiles on tests of anxiety and depression. BALB/C, C57Bl/6, C3H/He and CD-1 mouse strains were tested in the elevated plus maze in two sessions, separated by 48h. All strains, except CD-1, showed the expected reduction in open arm exploration on the second session. BALB/C and C3H mice spent a greater proportion of the time in the open arms on the first exposure, but spent more time immobile in the maze compared to the CD1 and C57 strains. Immobility was attenuated upon the second exposure in all strains, except the BALB/C mice. Real-time PCR was used to investigate regional and strain differences in induction of AChE-R mRNA following four daily injections of diisopropylfluorophosphate (DFP) (.1mg/kg). AChE-R induction was found in the frontal cortex, but not in amygdala, hippocampus or striatum of CD-1 mice. Nor was there AChE-R induction in the brains of the inbred strains. Four daily sessions of swim stress were used to investigate stress-induced induction of AChE-R. BALB/C mice showed significantly more immobility in the forced swim test (FST) compared to the other strains. FST did not induce AChE-R mRNA in any brain region tested; however, AChE-R mRNA expression in the frontal cortex was negatively correlated with immobility in the FST.

Enhancement of cortical extracellular 5-HT by 5-HT1A and 5-HT2C receptor blockade restores the antidepressant-like effect of citalopram in non-responder mice

We recently found that the response of DBA/2 mice to SSRIs in the forced swim test (FST) was impaired and they also had a smaller basal and citalopram-stimulated increase in brain extracellular serotonin (5-HT) than 'responder' strains. We employed intracerebral microdialysis, FST and selective antagonists of 5-HT1A and 5-HT2C receptors to investigate whether enhancing the increase in extracellular 5-HT reinstated the anti-immobility effect of citalopram in the FST. WAY 100635 (0.3 mg/kg s.c.) or SB 242084 (1 mg/kg s.c.), respectively a selective 5-HT1A and 5-HT2C receptor antagonist, raised the effect of citalopram (5 mg/kg) on extracellular 5-HT in the medial prefrontal cortex of DBA/2N mice (citalopram alone 5.2+/-0.3 fmol/20 microl, WAY 100635+citalopram 9.9+/-2.1 fmol/20 microl, SB 242084+ citalopram 7.6+/-1.0 fmol/20 microl) to the level reached in 'responder' mice given citalopram alone. The 5-HT receptor antagonists had no effect on the citalopram-induced increase in extracellular 5-HT in the dorsal hippocampus. The combination of citalopram with WAY 100635 or SB 242084 significantly reduced immobility time in DBA/2N mice that otherwise did not respond to either drug singly. Brain levels of citalopram in mice given citalopram alone or with 5-HT antagonists did not significantly differ. The results confirm that impaired 5-HT transmission accounts for the lack of effect of citalopram in the FST and suggest that enhancing the effect of SSRIs on extracellular 5-HT, through selective blockade of 5-HT1A and 5-HT2C receptors, could be a useful strategy to restore the response in treatment-resistant depression.