Alterations in the brain serotonin system and serotonin-regulated behavior during aging in zebrafish males and females

The brain serotonin (5-HT) system performs a neurotrophic function and supports the plasticity of the nervous system, while its age-related changes can increase the risk of senile neurodegeneration. Zebrafish brain is highly resistant to damage and neurodegeneration due to its high regeneration potential and it is a promising model object in searching for molecular factors preventing age-related neurodegeneration. In the present study alterations in 5-HT-related behavior in the home tank and the novel tank diving test, as well as 5-HT, 5-HIAA levels, tryptophan hydroxylase (TPH), monoamine oxidase (MAO) activity and the expression of genes encoding TPH, MAO, 5-HT transporter and 5-HT receptors in the brain of 6, 12, 24 and 36 month old zebrafish males and females are investigated. Marked sexual dimorphism in the locomotor activity in the novel tank test is revealed: females of all ages move slower than males. No sexual dimorphism in 5-HT-related traits is observed. No changes in 5-HT and 5-HIAA levels in zebrafish brain during aging is observed. At the same time, the aging is accompanied by a decrease in the locomotor activity, TPH activity, tph2 and htr1aa genes expression as well as an increase in the MAO activity and slc6a4a gene expression in their brain. These results indicate that the brain 5-HT system in zebrafish is resistant to age-related alterations.

Distinct Effects of Escitalopram and Vortioxetine on Astroglial L-Glutamate Release Associated with Connexin43

It has been established that enhancement of serotonergic transmission contributes to improvement of major depression; however, several post-mortem studies and experimental depression rodent models suggest that functional abnormalities of astrocytes play important roles in the pathomechanisms/pathophysiology of mood disorders. Direct effects of serotonin (5-HT) transporter inhibiting antidepressants on astroglial transmission systems has never been assessed in this context. Therefore, to explore the effects of antidepressants on transmission associated with astrocytes, the present study determined the effects of the selective 5-HT transporter inhibitor, escitalopram, and the 5-HT partial agonist reuptake inhibitor, vortioxetine, on astroglial L-glutamate release through activated hemichannels, and the expression of connexin43 (Cx43), type 1A (5-HT1AR) and type 7 (5-HT7R) 5-HT receptor subtypes, and extracellular signal-regulated kinase (ERK) in astrocytes using primary cultured rat cortical astrocytes in a 5-HT-free environment. Both escitalopram and 5-HT1AR antagonist (WAY100635) did not affect basal astroglial L-glutamate release or L-glutamate release through activated hemichannels. Subchronic (for seven days) administrations of vortioxetine and the 5-HT7R inverse agonist (SB269970) suppressed both basal L-glutamate release and L-glutamate release through activated hemichannels, whereas 5-HT1AR agonist (BP554) inhibited L-glutamate release through activated hemichannels, but did not affect basal L-glutamate release. In particular, WAY100635 did not affect the inhibitory effects of vortioxetine on L-glutamate release. Subchronic administration of vortioxetine, BP554 and SB269970 downregulated 5-HT1AR, 5-HT7R and phosphorylated ERK in the plasma membrane fraction, but escitalopram and WAY100635 did not affect them. Subchronic administration of SB269970 decreased Cx43 expression in the plasma membrane but did not affect the cytosol; however, subchronic administration of BP554 increased Cx43 expression in the cytosol but did not affect the plasma membrane. Subchronic vortioxetine administration increased Cx43 expression in the cytosol and decreased it in the plasma membrane. WAY100635 prevented an increased Cx43 expression in the cytosol induced by vortioxetine without affecting the reduced Cx43 expression in the plasma membrane. These results suggest that 5-HT1AR downregulation probably increases Cx43 synthesis, but 5-HT7R downregulation suppresses Cx43 trafficking to the plasma membrane. These results also suggest that the subchronic administration of therapeutic-relevant concentrations of vortioxetine inhibits both astroglial L-glutamate and Cx43 expression in the plasma membrane via 5-HT7R downregulation but enhances Cx43 synthesis in the cytosol via 5-HT1AR downregulation. This combination of the downregulation of 5-HT1AR, 5-HT7R and Cx43 in the astroglial plasma membrane induced by subchronic vortioxetine administration suggest that astrocytes is possibly involved in the pathophysiology of depression.

Examination of associations between personality traits, and polymorphisms of MAO-A and 5-HTT with the severity of menopausal symptoms and depression levels

The aim of the present study was to investigate the association between different personality traits with monoamine oxidase A (MAO-A) and serotonin transporter (5-HTT), and to examine their effects on the severity of menopause symptoms as well as the levels of depression in menopausal women. The study was designed as an analytical cross-sectional study, performed on 132 healthy post-menopausal women in Iran. Personality was assessed using the Revised Edition of the NEO Personality Index (Neuroticism-Extroversion-Openness-Five Factor Inventory). The symptoms score sheet, used for measuring menopausal symptoms, was used to classify the participants according to their menopausal symptoms. Furthermore, Beck Depression Inventory short version was also used to assess depressive symptoms. Examination of blood samples taken from all participants was to identify DNA polymorphisms of 5-HTT using PCR. The results of the present study showed that a high level of neuroticism (P<0.0001), low levels of extroversion (P<0.002), an openness to experience (P=0.039) and conscientiousness (P=0.001) were all positively associated with the severity of menopausal symptoms. In addition, a high level of neuroticism (P<0.0001), low levels of extroversion (P<0.0001), and a low level of agreeableness (P<0.024) and conscientiousness (P<0.0001) were all positively associated with depressive symptoms. There was no statistically significant association between MAO-A and 5-HTT polymorphisms with menopause and depression scores. Based on these results, there appears to be a significant association between personality traits with both depression and menopausal symptoms. Identification of homogeneous groups of women who are predisposed to depression and severe menopausal symptoms may allow for the implementation of early prevention programs.

Smoking moderates association of 5-HTTLPR and in vivo availability of serotonin transporters

Although preclinical studies clearly indicate an effect of 5-HTTLPR genotype on 5-HT transporter (5-HTT) expression, studies in humans provided inconclusive results, hypothetically due to environmental factors and differences in individual behavior. For example, nicotine and other constituents of tobacco smoke elevate serotonin (5-HT) levels in the brain and may thereby cause homeostatic adaptations in 5-HTT availability that moderate effects of 5-HTTLPR genotype. To test whether 5-HTT availability in the midbrain is affected by smoking status and 5-HTTLPR genotype, we pooled data from prior studies on in vivo 5-HTT availability (BP_ND) measured with positron emission tomography (PET) and [¹¹C]DASB. In total, we reanalyzed 5-HTT availability in 116 subjects using ANCOVA statistics. ROI analysis revealed that current smokers and non-smokers do not differ in midbrain BP_ND. Interestingly, smoking status significantly interacted with 5-HTTLPR genotype: active smoking was associated with reduced 5-HTT availability only in LL subjects but not in carriers of the S-allele. From the perspective of genotype effects, non-smokers showed the expected association with 5-HTTLPR, i.e. higher 5-HTT availability in LL subjects compared to carriers of the S-allele, whereas this pattern was actually reversed for active smokers. Our study indicates that smoking status moderates the association of 5-HTTLPR genotype and 5-HTT expression, which may help to explain inconsistent findings in previous studies. Regarding the mechanism, we suggest that smoking may induce epigenetic processes such as methylation of SLC6A4, which can differ depending on its genetic constitution.

Protein Kinases Alter the Allosteric Modulation of the Serotonin Transporter In Vivo and In Vitro

AIM

Studies using S- and R-enantiomers of the SSRI citalopram have shown that R-citalopram exerts an antagonistic effect on the efficacy of the antidepressant S-citalopram (escitalopram) through an interaction at an allosteric modulator site on the serotonin transporter (SERT). Here, we show that protein kinase signaling systems are involved in the allosteric modulation of the SERT in vivo and in vitro.

METHODS

We assessed the effects of nonspecific protein kinase inhibitor staurosporine in the action of escitalopram and/or R-citalopram using electrophysiological and behavioral assays in rats and cell surface SERT expression measures in serotoninergic cells.

RESULTS

Acute administration of R-citalopram counteracted the escitalopram-induced suppression of the serotonin (5-HT) neuronal firing activity and increase of the head twitches number following L-5-hydroxytryptophan injection. Importantly, these counteracting effects of R-citalopram were abolished by prior systemic administration of staurosporine. Interestingly, the preventing effect of staurosporine on 5-HT neuronal firing activity was abolished by direct activation of protein kinase C with phorbol 12-myristate 13-acetate. Finally, in vitro, quantification of the amount of cell surface-expressed SERT molecules revealed that R-citalopram prevented escitalopram-induced SERT internalization that was completely altered by staurosporine.

CONCLUSION

Taken together, these results highlight for the first time an involvement of protein kinases in the allosteric modulation of SERT function.

Genetic dysfunction of serotonin 2A receptor hampers response to antidepressant drugs: A translational approach

Pharmacological studies have yielded valuable insights into the role of the serotonin 2A (5-HT2A) receptor in major depressive disorder (MDD) and antidepressant drugs (ADs) response. However, it is still unknown whether genetic variants in the HTR2A gene affect the therapeutic outcome of ADs and the mechanism underlying the regulation of such response remains poorly described. In this context, a translational human-mouse study offers a unique opportunity to address the possibility that variations in the HTR2A gene may represent a relevant marker to predict the efficacy of ADs. In a first part of this study, we investigated in depressed patients the effect of three HTR2A single nucleotide polymorphisms (SNPs), selected for their potential functional consequences on 5-HT2A receptor (rs6313, rs6314 and rs7333412), on response and remission rates after 3 months of antidepressant treatments. We also explored the consequences of the constitutive genetic inactivation of the 5-HT2A receptor (i.e. in 5-HT2A(-/-) mice) on the activity of acute and prolonged administration of SSRIs. Our clinical data indicate that GG patients for the rs7333412 SNP were less prone to respond to ADs than AA/AG patients. In the preclinical study, we demonstrated that the 5-HT2A receptor exerts an inhibitory influence on the neuronal activity of the serotonergic system after acute administration of SSRIs. However, while the chronic administration of the SSRIs escitalopram or fluoxetine elicited a progressive increased in the firing rate of 5-HT neurons in 5-HT2A(+/+) mice, it failed to do so in 5-HT2A(-/-) mutants. These electrophysiological impairments were associated with a decreased ability of the chronic administration of fluoxetine to stimulate hippocampal plasticity and to produce antidepressant-like activities. Genetic loss of the 5-HT2A receptor compromised the activity of chronic treatment with SSRIs, making this receptor a putative marker to predict ADs response.

Central 5-HT neurotransmission modulates weight loss following gastric bypass surgery in obese individuals

The cerebral serotonin (5-HT) system shows distinct differences in obesity compared with the lean state. Here, it was investigated whether serotonergic neurotransmission in obesity is a stable trait or changes in association with weight loss induced by Roux-in-Y gastric bypass (RYGB) surgery. In vivo cerebral 5-HT2A receptor and 5-HT transporter binding was determined by positron emission tomography in 21 obese [four men; body mass index (BMI), 40.1 ± 4.1 kg/m(2)] and 10 lean (three men; BMI, 24.6 ± 1.5 kg/m(2)) individuals. Fourteen obese individuals were re-examined after RYGB surgery. First, it was confirmed that obese individuals have higher cerebral 5-HT2A receptor binding than lean individuals. Importantly, we found that higher presurgical 5-HT2A receptor binding predicted greater weight loss after RYGB and that the change in 5-HT2A receptor and 5-HT transporter binding correlated with weight loss after RYGB. The changes in the 5-HT neurotransmission before and after RYGB are in accordance with a model wherein the cerebral extracellular 5-HT level modulates the regulation of body weight. Our findings support that the cerebral 5-HT system contributes both to establish the obese condition and to regulate the body weight in response to RYGB.

Mice with compromised 5-HTT function lack phosphotyrosine-mediated inhibitory control over prefrontal 5-HT responses

The activity of the prefrontal cortex is essential for normal emotional processing and is strongly modulated by serotonin (5-HT). Yet, little is known about the regulatory mechanisms that control the activity of the prefrontal 5-HT receptors. Here, we found and characterized a deregulation of prefrontal 5-HT receptor electrophysiological signaling in mouse models of disrupted serotonin transporter (5-HTT) function, a risk factor for emotional and cognitive disturbances. We identified a novel tyrosine kinase-dependent mechanism that regulates 5-HT-mediated inhibition of prefrontal pyramidal neurons. We report that mice with compromised 5-HTT, resulting from either genetic deletion or brief treatment with selective serotonin reuptake inhibitors during development, have amplified 5-HT1A receptor-mediated currents in adulthood. These greater inhibitory effects of 5-HT are accompanied by enhanced downstream coupling to Kir3 channels. Notably, in normal wild-type mice, we found that these larger 5-HT1A responses can be mimicked through inhibition of Src family tyrosine kinases. By comparison, in our 5-HTT mouse models, the larger 5-HT1A responses were rapidly reduced through inhibition of tyrosine phosphatases. Our findings implicate tyrosine phosphorylation in regulating the electrophysiological effects of prefrontal 5-HT1A receptors with implications for neuropsychiatric diseases associated with emotional dysfunction, such as anxiety and depressive disorders.

Effects of olanzapine and betahistine co-treatment on serotonin transporter, 5-HT2A and dopamine D2 receptor binding density

Olanzapine is widely used in treating multiple domains of schizophrenia symptoms but induces serious metabolic side-effects. Recent evidence has showed that co-treatment of betahistine (a histaminergic H1 receptor agonist and H3 receptor antagonist) is effective for preventing olanzapine-induced weight gain/obesity, however it is not clear whether this co-treatment affects on the primary therapeutic receptor binding sites of olanzapine such as serotonergic 5-HT2A receptors (5-HT2AR) and dopaminergic D2 receptors (D2R). Therefore, this study investigated the effects of this co-treatment on 5-HT2AR, 5-HT transporter (5-HTT) and D2R bindings in various brain regions involved in antipsychotic efficacy. Female Sprague Dawley rats were administered orally (t.i.d.) with either olanzapine (1mg/kg), betahistine (2.7 mg/kg), olanzapine plus betahistine (O+B), or vehicle (control) for 2 weeks. Quantitative autoradiography was used to detect the density of [(3)H]ketanserin, [(3)H]paroxetine and [(3)H]raclopride binding site to 5-HT2AR, 5-HTT and D2R. Compared to the controls, olanzapine significantly decreased [(3)H]ketanserin bindings to 5-HT2AR in the prefrontal cortex, cingulate cortex, and nucleus accumbens. Similar changes in 5-HT2AR bindings in these nuclei were also observed in the O+B co-treatment group. Olanzapine also significantly decreased [(3)H]paroxetine binding to 5-HTT in the ventral tegmental area and substantia nigra, however, both olanzapine only and O+B co-treatment did not affect [(3)H]raclopride binding to D2R. The results confirmed the important role of 5-HT2AR in the efficacy of olanzapine, which is not influenced by the O+B co-treatment. Therefore, betahistine co-treatment would be an effective combination therapy to reduce olanzapine-induced weight gain side-effects without affecting olanzapine's actions on 5-HT2AR transmissions.

Effect of glial cell line-derived neurotrophic factor on behavior and key members of the brain serotonin system in mouse strains genetically predisposed to behavioral disorders

The effect of glial cell line-derived neurotrophic factor (GDNF) on behavior and on the serotonin (5-HT) system of a mouse strain predisposed to depressive-like behavior, ASC/Icg (Antidepressant Sensitive Cataleptics), in comparison with the parental "nondepressive" CBA/Lac mice was studied. Within 7 days after acute administration, GDNF (800 ng, i.c.v.) decreased cataleptic immobility but increased depressive-like behavioral traits in both investigated mouse strains and produced anxiolytic effects in ASC mice. The expression of the gene encoding the key enzyme for 5-HT biosynthesis in the brain, tryptophan hydroxylase-2 (Tph-2), and 5-HT1A receptor gene in the midbrain as well as 5-HT2A receptor gene in the frontal cortex were increased in GDNF-treated ASC mice. At the same time, GDNF decreased 5-HT1A and 5-HT2A receptor gene expression in the hippocampus of ASC mice. GDNF failed to change Tph2, 5-HT1A , or 5-HT2A receptor mRNA levels in CBA mice as well as 5-HT transporter gene expression and 5-HT1A and 5-HT2A receptor functional activity in both investigated mouse strains. The results show 1) a GDNF-induced increase in the expression of key genes of the brain 5-HT system, Tph2, 5-HT1A , and 5-HT2A receptors, and 2) significant genotype-dependent differences in the 5-HT system response to GDNF treatment. The data suggest that genetically defined cross-talk between neurotrophic factors and the brain 5-HT system underlies the variability in behavioral response to GDNF.

Animal models of the serotonin syndrome: a systematic review

The serotonin syndrome (SS) is a potentially life-threatening disorder in humans which is induced by ingestion of an overdose or by combination of two or more serotonin (5-HT)-enhancing drugs. In animals, acute administration of direct and indirect 5-HT agonists also leads to a set of behavioral and autonomic responses. In the current review, we provide an overview of the existing versions of the animal model of the SS. With a focus on studies in rats and mice, we analyze the frequency of behavioral and autonomic responses following administration of 5-HT-enhancing drugs and direct 5-HT agonists administered alone or in combination, and we briefly discuss the receptor mediation of these responses. Considering species differences, we identify a distinct set of behavioral and autonomic responses that are consistently observed following administration of direct and indirect 5-HT agonists. Finally, we discuss the importance of a standardized assessment of SS responses in rodents and the utility of animal models of the SS in translational studies, and provide suggestions for future research.

Ascending monoaminergic systems alterations in Alzheimer's disease. translating basic science into clinical care

Extensive neuropathological studies have established a compelling link between abnormalities in structure and function of subcortical monoaminergic (MA-ergic) systems and the pathophysiology of Alzheimer's disease (AD). The main cell populations of these systems including the locus coeruleus, the raphe nuclei, and the tuberomamillary nucleus undergo significant degeneration in AD, thereby depriving the hippocampal and cortical neurons from their critical modulatory influence. These studies have been complemented by genome wide association studies linking polymorphisms in key genes involved in the MA-ergic systems and particular behavioral abnormalities in AD. Importantly, several recent studies have shown that improvement of the MA-ergic systems can both restore cognitive function and reduce AD-related pathology in animal models of neurodegeneration. This review aims to explore the link between abnormalities in the MA-ergic systems and AD symptomatology as well as the therapeutic strategies targeting these systems. Furthermore, we will examine possible mechanisms behind basic vulnerability of MA-ergic neurons in AD.