Effect of fluoxetine treatment on neurotoxicity induced by lysolecithin in male rats

Demyelination disorder is an unusual pathologic event, which occurs in the central nervous system (CNS). Multiple sclerosis (MS) is an inflammatory demyelinating disease that affects the CNS, and it is the leading cause of disability in young adults. Lysolecithin (LPC) is one of the best toxin-induced demyelination models. In this study, a suitable model is created, and the effect of fluoxetine treatment is examined on this model. In this case, it was assumed that daily fluoxetine treatment had increased the endogenous remyelination in the LPC model. This study was focused on investigating the influence of the fluoxetine dose of 5 or 10 mg/kg per day for 1 and 4 weeks on LPC-induced neurotoxicity in the corpus callosum region. It was performed as a demyelinating model in male Wistar rats. After 3 days, fluoxetine was injected intraperitoneally (5 or 10 mg/kg per day) for 1 and 4 weeks in each group. After completing the treatment course, the corpus callosum was removed to examine the gene expression and histological analysis was performed. The results of the histopathological study of hematoxylin and eosin staining of the corpus callosum showed that in 1 and 4-week treatment groups, fluoxetine has reduced the level of inflammation at the LPC injection site (5 and 10 mg/kg per day). Fluoxetine treatment in the luxol fast blue (LFB) staining of the corpus callosum has been led to an increase in myelination capacity in all doses and times. The results of the genetic study showed that the fluoxetine has significantly reduced the expression level of tumor necrosis factor-α, nuclear factor κβ, and induced nitric oxide synthase in comparison with the untreated LPC group. Also, the fluoxetine treatment has enhanced the expression level of the forkhead box P3 (FOXP3) gene in comparison with the untreated group. Fluoxetine has increased the expression level of myelination and neurotrophic genes such as myelin basic protein (MBP), oligodendrocyte transcription factor 2 (OLIG2), and brain-derived neurotrophic factor (BDNF). The outcomes demonstrated that fluoxetine reduces inflammation and strengthens the endogenous myelination in the LPC-induced demyelination model; however, supplementary studies are required for specifying the details of its mechanisms.

The Developmental Role of Serotonin: A Comparison of Short- and Long-term Blockade of the Serotonin Transporter on Behavioural Outcomes in Adulthood

<p>Serotonin is an important neurotransmitter that regulates a range of processes within the brain and is implicated in several psychiatric disorders. In addition, serotonin acts as a developmental signal during critical periods of prenatal development, influencing processes such as neuronal proliferation, migration, and synaptogenesis (Gaspar et al., 2003). The serotonin transporter (5- HTT) plays a key role in regulating extracellular serotonin levels and is the main target of selective-serotonin reuptake inhibitors (SSRIs), a class of drugs that have anti-anxiety and anti- depressive activity. SSRIs cause an acute increase in extracellular serotonin and are commonly prescribed as a treatment for depression and anxiety during pregnancy (Tran & Robb, 2015). Given that these drugs alter serotonin transmission and can pass to the developing fetus via the placenta, it is vital that the outcomes of prenatal SSRI exposure are investigated. In humans, a genetic variant of the gene that codes for the 5-HTT (SLC6A4) has been linked to increased risk for developing depression and anxiety (Caspi et al., 2003). The functional consequences of this genetic polymorphism are life-long alterations in 5-HTT activity, resulting in increased extracellular levels of serotonin (Nakamura et al., 2000). Given prenatal SSRI exposure results in a time-locked blockade of 5-HTT during critical periods of development, it follows that alterations in serotonin during development might similarly result in enhanced risk for depression and anxiety later in life. Outcomes in children prenatally exposed to SSRIs are difficult to study due to confounds of pre- existing maternal depression. Therefore, the current thesis presents two experiments that aimed to further investigate the role of altered extracellular serotonin levels during development in an animal model. Experiment one aimed to develop a method of voluntary oral administration of the SSRI fluoxetine to pregnant rat dams. This method was then applied in experiment two to create a time-locked blockade of 5-HTT during critical periods of development in an animal model of life-long 5-HTT blockade. The aim of experiment two was to directly assess the contribution of short- and long-term 5-HTT blockade on anxiety and depression phenotypes in adult male offspring. In addition, maternal behaviour was assessed to determine whether fluoxetine treatment had an influence on mother-pup interactions that could confound results. To test for anxiety and depression phenotypes, the novel affective disorder test (ADT) was used to assess anxiety behaviour and the deficits in anticipatory pleasure indicative of anhedonia. In the current study, fluoxetine treatment did not have an effect on litter outcomes or mother-pup interactions. Crucially, no significant group differences were found indicating that neither short- nor long- term blockade of 5-HTT resulted in increased anxiety- or depressive-like behaviours in the current experiment. However, limitations with methodological design limit the translatability of these results to the broader literature, and validation of the ADT is required before these results can be generalised beyond this thesis.</p>

The Developmental Role of Serotonin: A Comparison of Short- and Long-term Blockade of the Serotonin Transporter on Behavioural Outcomes in Adulthood

<p>Serotonin is an important neurotransmitter that regulates a range of processes within the brain and is implicated in several psychiatric disorders. In addition, serotonin acts as a developmental signal during critical periods of prenatal development, influencing processes such as neuronal proliferation, migration, and synaptogenesis (Gaspar et al., 2003). The serotonin transporter (5- HTT) plays a key role in regulating extracellular serotonin levels and is the main target of selective-serotonin reuptake inhibitors (SSRIs), a class of drugs that have anti-anxiety and anti- depressive activity. SSRIs cause an acute increase in extracellular serotonin and are commonly prescribed as a treatment for depression and anxiety during pregnancy (Tran & Robb, 2015). Given that these drugs alter serotonin transmission and can pass to the developing fetus via the placenta, it is vital that the outcomes of prenatal SSRI exposure are investigated. In humans, a genetic variant of the gene that codes for the 5-HTT (SLC6A4) has been linked to increased risk for developing depression and anxiety (Caspi et al., 2003). The functional consequences of this genetic polymorphism are life-long alterations in 5-HTT activity, resulting in increased extracellular levels of serotonin (Nakamura et al., 2000). Given prenatal SSRI exposure results in a time-locked blockade of 5-HTT during critical periods of development, it follows that alterations in serotonin during development might similarly result in enhanced risk for depression and anxiety later in life. Outcomes in children prenatally exposed to SSRIs are difficult to study due to confounds of pre- existing maternal depression. Therefore, the current thesis presents two experiments that aimed to further investigate the role of altered extracellular serotonin levels during development in an animal model. Experiment one aimed to develop a method of voluntary oral administration of the SSRI fluoxetine to pregnant rat dams. This method was then applied in experiment two to create a time-locked blockade of 5-HTT during critical periods of development in an animal model of life-long 5-HTT blockade. The aim of experiment two was to directly assess the contribution of short- and long-term 5-HTT blockade on anxiety and depression phenotypes in adult male offspring. In addition, maternal behaviour was assessed to determine whether fluoxetine treatment had an influence on mother-pup interactions that could confound results. To test for anxiety and depression phenotypes, the novel affective disorder test (ADT) was used to assess anxiety behaviour and the deficits in anticipatory pleasure indicative of anhedonia. In the current study, fluoxetine treatment did not have an effect on litter outcomes or mother-pup interactions. Crucially, no significant group differences were found indicating that neither short- nor long- term blockade of 5-HTT resulted in increased anxiety- or depressive-like behaviours in the current experiment. However, limitations with methodological design limit the translatability of these results to the broader literature, and validation of the ADT is required before these results can be generalised beyond this thesis.</p>

Exercise rather than fluoxetine promotes oligodendrocyte differentiation and myelination in the hippocampus in a male mouse model of depression

Although selective serotonin reuptake inhibitor (SSRI) systems have been meaningfully linked to the clinical phenomena of mood disorders, 15–35% of patients do not respond to multiple SSRI interventions or even experience an exacerbation of their condition. As we previously showed, both running exercise and fluoxetine reversed depression-like behavior. However, whether exercise reverses depression-like behavior more quickly than fluoxetine treatment and whether this rapid effect is achieved via the promotion of oligodendrocyte differentiation and/or myelination in the hippocampus was previously unknown. Sixty male C57BL/6 J mice were used in the present study. We subjected mice with unpredictable chronic stress (UCS) to a 4-week running exercise trial (UCS + RN) or intraperitoneally injected them with fluoxetine (UCS + FLX) to address these uncertainties. At the behavioral level, mice in the UCS + RN group consumed significantly more sugar water in the sucrose preference test (SPT) at the end of the 7th week than those in the UCS group, while those in the UCS + FLX group consumed significantly more sugar water than mice in the UCS group at the end of the 8th week. The unbiased stereological results and immunofluorescence analyses revealed that running exercise, and not fluoxetine treatment, increased the numbers of CC1+ and CC1+/Olig2+/BrdU+ oligodendrocytes in the CA1 subfield in depressed mice exposed to UCS. Moreover, running exercise rather than fluoxetine increased the level of myelin basic protein (MBP) and the G-ratio of myelinated nerve fibers in the CA1 subfield in the UCS mouse model. Unlike fluoxetine, exercise promoted hippocampal myelination and oligodendrocyte differentiation and thus has potential as a therapeutic strategy to reduce depression-like behaviors induced by UCS.

Effect of Regular Physical Exercise on Gut Microbiota and Depressive Behaviors in Rats

Objective. The gut microbiota, as the critical mediator of the gut-brain axis, can produce and transport neuroactive substances, thus playing a significant role in the pathogenesis of depression. Although regular physical exercise is an important nondrug antidepressant, its specific effector mechanism is still unclear. Methods. Rats were randomly divided into four different groups (n = 10 for each group) as follows: normal group (G1), depression group (G2), fluoxetine treatment group (G3), and regular exercise treatment group (G4). All rats underwent forced swimming tests, tail suspension tests, open field tests, and elevated plus-maze tests to detect behavioristics. Then, corticosterone levels were detected by ELISA. Additionally, taxonomic analysis of the gut microbiota in all rats was performed after they were exercised regularly for 60 days. Results. Compared with the G1 group, the rats in the G2 group showed significant depression-like behaviors, with increased serum corticosterone levels. The proportions of Bacteroides, Actinomycetes, Proteobacteria, Saccharomyces, and Cyanobacteria in rats of the G2 group were lower than those in the G1 group, while the proportions of Firmicutes, Tenicotte, Deferrobacteria, and Fusobacteria were increased. Furthermore, after regular exercise treatment, the gut microbiota of rats was effectively improved, almost returning to the level of the G1 group, and depressive behavior and corticosterone levels were also restored, which was almost the same as the effect of fluoxetine treatment. Conclusion. Regular physical exercise could alleviate depressive-like behaviors by modulating the species and function of the gut microbiota.

Constitutive Serotonin Tone Modulates Molecular and Behavioral Response to Chronic Fluoxetine Treatment: A Study on Genetic Rat Model

Selective serotonin reuptake inhibitors (SSRIs) are the most commonly prescribed medications for the treatment of mood disorders. Yet, individual response to SSRIs is highly variable, with only a portion of patients showing the desired therapeutic effect. To better understand the molecular basis underlying individual variability in response to SSRIs, here we comparatively studied behavioral and molecular consequences of chronic treatment with fluoxetine, a widely used SSRI, in two sublines of rats with constitutionally different serotonin (5HT) homeostasis: the high-5HT and low-5HT sublines. Platelet 5HT levels, a recognized indicator of SSRI efficacy, were decreased by fluoxetine treatment in both 5HT-sublines. On the other hand, biologically active plasma 5HT levels were reduced only in high-5HT rats. The anxiolytic effect of fluoxetine was also evident only in high-5HT rats, as supported by spatio-temporal and ethological behavioral measures in the elevated plus maze (EPM) test and exploratory behavior measures in the open field (OF) test. None of the behavioral EPM or OF measures were significantly altered by fluoxetine treatment in low-5HT rats. Unexpectedly, 5HT levels in cerebral cortices tended to be reduced only in low-5HT rats. Moreover, the effects of fluoxetine on cortical expression levels of 5HT-related proteins were also present only in low-5HT rats, with serotonin transporter (5HTT) and serotonin receptor type 1a (Htr1a) being down-regulated, while serotonin receptor type 4 (Htr4) was up-regulated by fluoxetine treatment. The obtained results support a role of individual 5HT tone as an important influencing factor on the biological actions of SSRI antidepressants.

Environmental Enrichment Induces Meningeal Niche Remodeling through TrkB-Mediated Signaling

Neural precursors (NPs) present in the hippocampus can be modulated by several neurogenic stimuli, including environmental enrichment (EE) acting through BDNF-TrkB signaling. We have recently identified NPs in meninges; however, the meningeal niche response to pro-neurogenic stimuli has never been investigated. To this aim, we analyzed the effects of EE exposure on NP distribution in mouse brain meninges. Following neurogenic stimuli, although we did not detect modification of the meningeal cell number and proliferation, we observed an increased number of neural precursors in the meninges. A lineage tracing experiment suggested that EE-induced β3-Tubulin+ immature neuronal cells present in the meninges originated, at least in part, from GLAST+ radial glia cells. To investigate the molecular mechanism responsible for meningeal reaction to EE exposure, we studied the BDNF-TrkB interaction. Treatment with ANA-12, a TrkB non-competitive inhibitor, abolished the EE-induced meningeal niche changes. Overall, these data showed, for the first time, that EE exposure induced meningeal niche remodeling through TrkB-mediated signaling. Fluoxetine treatment further confirmed the meningeal niche response, suggesting it may also respond to other pharmacological neurogenic stimuli. A better understanding of the neurogenic stimuli modulation for meninges may be useful to improve the effectiveness of neurodegenerative and neuropsychiatric treatments.