Intracellular signaling involved in estrogen regulation of serotonin reuptake

The impact of estradiol on serotonin, glutamate, and dopamine systems

Estradiol, the most potent and prevalent member of the estrogen class of steroid hormones and is expressed in both sexes. Functioning as a neuroactive steroid, it plays a crucial role in modulating neurotransmitter systems affecting neuronal circuits and brain functions including learning and memory, reward and sexual behaviors. These neurotransmitter systems encompass the serotonergic, dopaminergic, and glutamatergic signaling pathways. Consequently, this review examines the pivotal role of estradiol and its receptors in the regulation of these neurotransmitter systems in the brain. Through a comprehensive analysis of current literature, we investigate the multifaceted effects of estradiol on key neurotransmitter signaling systems, namely serotonin, dopamine, and glutamate. Findings from rodent models illuminate the impact of hormone manipulations, such as gonadectomy, on the regulation of neuronal brain circuits, providing valuable insights into the connection between hormonal fluctuations and neurotransmitter regulation. Estradiol exerts its effects by binding to three estrogen receptors: estrogen receptor alpha (ERα), estrogen receptor beta (ERβ), and G protein-coupled receptor (GPER). Thus, this review explores the promising outcomes observed with estradiol and estrogen receptor agonists administration in both gonadectomized and/or genetically knockout rodents, suggesting potential therapeutic avenues. Despite limited human studies on this topic, the findings underscore the significance of translational research in bridging the gap between preclinical findings and clinical applications. This approach offers valuable insights into the complex relationship between estradiol and neurotransmitter systems. The integration of evidence from neurotransmitter systems and receptor-specific effects not only enhances our understanding of the neurobiological basis of physiological brain functioning but also provides a comprehensive framework for the understanding of possible pathophysiological mechanisms resulting to disease states. By unraveling the complexities of estradiol's impact on neurotransmitter regulation, this review contributes to advancing the field and lays the groundwork for future research aimed at refining understanding of the relationship between estradiol and neuronal circuits as well as their involvement in brain disorders.

Exploring sex differences: insights into gene expression, neuroanatomy, neurochemistry, cognition, and pathology

Increased knowledge about sex differences is important for development of individualized treatments against many diseases as well as understanding behavioral and pathological differences. This review summarizes sex chromosome effects on gene expression, epigenetics, and hormones in relation to the brain. We explore neuroanatomy, neurochemistry, cognition, and brain pathology aiming to explain the current state of the art. While some domains exhibit strong differences, others reveal subtle differences whose overall significance warrants clarification. We hope that the current review increases awareness and serves as a basis for the planning of future studies that consider both sexes equally regarding similarities and differences.

Effects of jaw clenching and mental stress on persistent inward currents estimated by two different methods

Spinal motoneuron firing depends greatly on persistent inward currents (PICs), which in turn are facilitated by the neuromodulators serotonin and noradrenaline. The aim of this study was to determine whether jaw clenching (JC) and mental stress (MS), which may increase neuromodulator release, facilitate PICs in human motoneurons. The paired motor unit (MU) technique was used to estimate PIC contribution to motoneuron firing. Surface electromyograms were collected using a 32-channel matrix on gastrocnemius medialis (GM) during voluntary, ramp, plantar flexor contractions. MU discharges were identified, and delta frequency (ΔF), a measure of recruitment-derecruitment hysteresis, was calculated. Additionally, another technique was used (VibStim) that evokes involuntary contractions that persist after cessation of combined Achilles tendon vibration and triceps surae neuromuscular electrical stimulation. VibStim measures of plantar flexor torque and soleus activity may reflect PIC activation. ΔF was not significantly altered by JC (p = .679, n = 18, 9 females) or MS (p = .147, n = 14, 5 females). However, all VibStim variables quantifying involuntary torque and muscle activity during and after vibration cessation were significantly increased in JC (p < .011, n = 20, 10 females) and some, but not all, increased in MS (p = .017-.05, n = 19, 10 females). JC and MS significantly increased the magnitude of involuntary contractions (VibStim) but had no effect on GM ΔF during voluntary contractions. Effects of increased neuromodulator release on PIC contribution to motoneuron firing might differ between synergists or be context dependent. Based on these data, the background level of voluntary contraction and, hence, both neuromodulation and ionotropic inputs could influence neuromodulatory PIC enhancement.

Progesterone and Its Metabolites Play a Beneficial Role in Affect Regulation in the Female Brain

Premenstrual dysphoric disorder is a female affective disorder that is defined by mood symptoms. The condition is linked to unstable progesterone concentrations. Progestin supplementation is given in cases of threatened or recurrent miscarriage and for luteal phase support. Progesterone is essential for implantation, immune tolerance, and modulation of uterine contractility. For a long time, the administration of progestins was associated with an unfavorable impact on mood, leading to negative affect, and, therefore, was contraindicated in existing mood disorders. Establishing the role of the natural progesterone derivative allopregnanolone in advances in the treatment of postpartum depression has shed new light on the general pathophysiology of mood disorders. Allopregnanolone directly interacts with gamma-aminobutyric acid type A (GABA-A) receptors even at nanomolar concentrations and induces significant anti-depressant, anti-stress, sedative, and anxiolytic effects. Postpartum depression is caused by a rapid drop in hormones and can be instantly reversed by the administration of allopregnanolone. Premenstrual dysphoric disorder can also be considered to result from insufficient neuroactive steroid action due to low progesterone derivative concentration, unstable hormone levels, or decreased receptor sensitivity. The decrease in progesterone levels in perimenopause is also associated with affective symptoms and an exacerbation of some psychosomatic syndromes. Bioidentical progesterone supplementation encounters several obstacles, including limited absorption, first-pass effect, and rapid metabolism. Hence, non-bioidentical progestins with better bioavailability were widely applied. The paradoxical, unfavorable effect of progestins on mood can be explained by the fact that progestins suppress ovulation and disturb the endocrine function of the ovary in the luteal phase. Moreover, their distinct chemical structure prevents their metabolism to neuroactive, mood-improving derivatives. A new understanding of progesterone-related mood disorders can translate the study results from case series and observational studies to cohort studies, clinical trials, and novel, effective treatment protocols being developed.

Improving clinical outcomes through attention to sex and hormones in research

Biological sex, fluctuations in sex steroid hormones throughout life and gender as a social construct all influence every aspect of health and disease. Yet, for decades, most basic and clinical studies have included only male individuals. As modern health care moves towards personalized medicine, it is clear that considering sex and hormonal status in basic and clinical studies will bring precision to the development of novel therapeutics and treatment paradigms. To this end, funding, regulatory and policy agencies now require inclusion of female animals and women in basic and clinical studies. However, inclusion of female animals and women often does not mean that information regarding potential hormonal interactions with pharmacological treatments or clinical outcomes is available. All sex steroid hormones can interact with receptors for drug targets, metabolism and transport. Genetic variation in receptors or in enzymatic function might contribute to sex differences in therapeutic efficacy and adverse drug reactions. Outcomes from clinical trials are often not reported by sex, and, if the data are available, they are not translated into clinical practice guidelines. This Review will provide a historical perspective for the current state of research related to hormone trials and provide concrete strategies that, if implemented, will improve the health of all people.

Voluntary binge-patterned alcohol drinking and sex-specific influences on monoamine-related neurochemical signatures in the mouse gut and brain

BACKGROUND

Altered monoamine (i.e., serotonin, dopamine, and norepinephrine) activity following episodes of alcohol abuse plays key roles not only in the motivation to ingest ethanol, but also physiological dysfunction related to its misuse. Although monoamine activity is essential for physiological processes that require coordinated communication across the gut-brain axis (GBA), relatively little is known about how alcohol misuse may affect monoamine levels across the GBA. Therefore, we evaluated monoamine activity across the mouse gut and brain following episodes of binge-patterned ethanol drinking.

METHODS

Monoamine and select metabolite neurochemical concentrations were analyzed by ultra-high-performance liquid chromatography in gut and brain regions of female and male C57BL/6J mice following "Drinking in the Dark" (DID), a binge-patterned ethanol ingestion paradigm.

RESULTS

First, we found that alcohol access had an overall small effect on gut monoamine-related neurochemical concentrations, primarily influencing dopamine activity. Second, neurochemical patterns between the small intestine and the striatum were correlated, adding to recent evidence of modulatory activity between these areas. Third, although alcohol access robustly influenced activity in brain areas in the mesolimbic dopamine system, binge exposure also influenced monoaminergic activity in the hypothalamic region. Finally, sex differences were observed in the concentrations of neurochemicals within the gut, which was particularly pronounced in the small intestine.

CONCLUSION

Together, these data provide insights into the influence of alcohol abuse and biological sex on monoamine-related neurochemical changes across the GBA, which could have important implications for GBA function and dysfunction.

Role of Estradiol in the Expression of Genes Involved in Serotonin Neurotransmission: Implications for Female Depression

BACKGROUND

In women, changes in estrogen levels may increase the incidence and/or symptomatology of depression and affect the response to antidepressant treatments. Estrogen therapy in females may provide some mood benefits as a single treatment or might augment clinical response to antidepressants that inhibit serotonin reuptake.

OBJECTIVE

We analyzed the mechanisms of estradiol action involved in the regulation of gene expression that modulates serotonin neurotransmission implicated in depression.

METHOD

Publications were identified by a literature search on PubMed.

RESULTS

The participation of estradiol in depression may include regulation of the expression of tryptophan hydroxylase-2, monoamine oxidase A and B, serotonin transporter and serotonin-1A receptor. This effect is mediated by estradiol binding to intracellular estrogen receptor that interacts with estrogen response elements in the promoter sequences of tryptophan hydroxylase-2, serotonin transporter and monoamine oxidase-B. In addition to directly binding deoxyribonucleic acid, estrogen receptor can tether to other transcription factors, including activator protein 1, specificity protein 1, CCAAT/enhancer binding protein β and nuclear factor kappa B to regulate gene promoters that lack estrogen response elements, such as monoamine oxidase-A and serotonin 1A receptor.

CONCLUSION

Estradiol increases tryptophan hydroxylase-2 and serotonin transporter expression and decreases the expression of serotonin 1A receptor and monoamine oxidase A and B through the interaction with its intracellular receptors. The understanding of molecular mechanisms of estradiol regulation on the protein expression that modulates serotonin neurotransmission will be helpful for the development of new and more effective treatment for women with depression.

Effects of separate or combined exposure of nonylphenol and octylphenol on central 5-HT system and related learning and memory in the rats

This study evaluated toxic effects of nonylphenol (NP) and octylphenol (OP) on central 5-hydroxytryptamine (5-HT) system and related learning and memory in the rats. Male Sprague-Dawley rats were exposed to NP (30, 90, or 270 mg/kg), OP (40, 120, or 360 mg/kg), or a mixture of NP and OP [(mixed with the corresponding NP, OP alone exposed low, medium and high dose according to the natural environment exists NP:OP = 4:1; NOL (24 mg/kg NP+8 mg/kg OP), NOM (72 mg/kg NP+24 mg/kg OP), NOH (216 mg/kg NP+72 mg/kg OP)] by gavage every other day for 30 d. Learning and memory were assessed using a passive-avoidance test. Levels of estrogen receptor β (ERβ), 5-HT, tryptophan hydroxylase 2 (TPH2), monoamine oxidase (MAOA) enzyme, serotonin transporter (SERT), the vesicular monoamine transporter 2 (VMAT2), 5-hydroxytryptamine 1 A (5-HT_1A), 5-hydroxytryptamine 3 A (5-HT_3A), 5-hydroxytryptamine 3B (5-HT_3B), 5-hydroxytryptamine 4 A (5-HT_4A) and 5-hydroxytryptamine 6 A (5-HT_6A) were measured using ELISA kits. Levels of ERβ, MAOA, SERT, VMAT2, 5-HT_1A, 5-HT_3A, 5-HT_3B, 5-HT_4A and 5-HT_6A in rat hippocampal reduced by a high dose of NP and/or OP. Levels of TPH2 in rat midbrain and 5-HT in rat hippocampal increased by a high dose of NP and/or OP. In addition, latency was significantly shorter and errors were significantly greater in the high dose NP and NP+OP (NO) groups. Taken together, these results suggest that NP and/or OP may affect learning and memory in rats by inhibiting levels of ERβ, which could then lead to decreases in levels of 5-HT_1A, 5-HT_3A, 5-HT_3B, 5-HT_4A, and 5-HT_6A in the rat hippocampus. These findings suggested that separate and combined exposure to NP and OP could produce toxic effects on central 5-HT system and related learning and memory in the rats.

Serotonin-estrogen interactions: What can we learn from pregnancy?

We have reviewed the scientific literature related to four diseases in which to serotonin (5-HT) is involved in the etiology, herein named 5-HT-linked diseases, and whose prevalence is influenced by estrogenic status: depression, migraine, irritable bowel syndrome and eating disorders. These diseases all have in common a sex-dimorphic prevalence, with women more frequently affected than men. The co-occurrence between these 5-HT-linked diseases suggests that they have common physiopathological mechanisms. In most 5-HT-linked diseases (except for anorexia nervosa and irritable bowel syndrome), a decrease in the serotonergic tone is observed and estrogens are thought to contribute to the improvement of symptoms by stimulating the serotonergic system. Human pregnancy is characterized by a unique 5-HT and estrogen synthesis by the placenta. Pregnancy-specific disorders, such as hyperemesis gravidarum, gestational diabetes mellitus and pre-eclampsia, are associated with a hyperserotonergic state and decreased estrogen levels. Fetal programming of 5-HT-linked diseases is a complex phenomenon that involves notably fetal-sex differences, which suggest the implication of sex steroids. From a mechanistic point of view, we hypothesize that estrogens regulate the serotonergic system, resulting in a protective effect against 5-HT-linked diseases, but that, in turn, 5-HT affects estrogen synthesis in an attempt to retrieve homeostasis. These two processes (5-HT and estrogen biosynthesis) are crucial for successful pregnancy outcomes, and thus, a disruption of this 5-HT-estrogen relationship may explain pregnancy-specific pathologies or pregnancy complications associated with 5-HT-linked diseases.

Fluoxetine and its active metabolite norfluoxetine disrupt estrogen synthesis in a co-culture model of the feto-placental unit

The effects of fluoxetine, one of the most prescribed selective serotonin-reuptake inhibitors (SSRIs) during pregnancy, and its active metabolite norfluoxetine were studied on placental aromatase (CYP19) and feto-placental steroidogenesis. Fluoxetine did not alter estrogen secretion in co-culture of fetal-like adrenocortical (H295R) and trophoblast-like (BeWo) cells used as a model of the feto-placental unit, although it induced CYP19 activity, apparently mediated by the serotonin (5-HT)_2A receptor/PKC signaling pathway. Norfluoxetine decreased estrogen secretion in the feto-placental co-culture and competitively inhibited catalytic CYP19 activity in BeWo cells. Decreased serotonin transporter (SERT) activity in the co-culture was comparable to 17β-estradiol treatment of BeWo cells. This work shows that the complex interaction of fluoxetine and norfluoxetine with placental estrogen production, involves 5-HT-dependent and -independent mechanisms. Considering the crucial role of estrogens during pregnancy, our results raise concern about the impact of SSRI treatment on placental function and fetal health.

High-Dose Testosterone Treatment Increases Serotonin Transporter Binding in Transgender People

BACKGROUND

Women are two times more likely to be diagnosed with depression than men. Sex hormones modulating serotonergic transmission are proposed to partly underlie these epidemiologic findings. Here, we used the cross-sex steroid hormone treatment of transsexuals seeking sex reassignment as a model to investigate acute and chronic effects of testosterone and estradiol on serotonin reuptake transporter (SERT) binding in female-to-male and male-to-female transsexuals.

METHODS

Thirty-three transsexuals underwent [(11)C]DASB positron emission tomography before start of treatment, a subset of which underwent a second scan 4 weeks and a third scan 4 months after treatment start. SERT nondisplaceable binding potential was quantified in 12 regions of interest. Treatment effects were analyzed using linear mixed models. Changes of hormone plasma levels were correlated with changes in regional SERT nondisplaceable binding potential.

RESULTS

One and 4 months of androgen treatment in female-to-male transsexuals increased SERT binding in amygdala, caudate, putamen, and median raphe nucleus. SERT binding increases correlated with treatment-induced increases in testosterone levels, suggesting that testosterone increases SERT expression on the cell surface. Conversely, 4 months of antiandrogen and estrogen treatment in male-to-female transsexuals led to decreases in SERT binding in insula, anterior, and mid-cingulate cortex. Increases in estradiol levels correlated negatively with decreases in regional SERT binding, indicating a protective effect of estradiol against SERT loss.

CONCLUSIONS

Given the central role of the SERT in the treatment of depression and anxiety disorders, these findings may lead to new treatment modalities and expand our understanding of the mechanism of action of antidepressant treatment properties.

Tamoxifen antagonizes the effects of ovarian hormones to induce anxiety and depression-like behavior in rats

The effects of tamoxifen (TAM) on anxiety and depression-like behavior in ovariectomized (OVX) and naïve female rats were investigated. The animals were divided into Sham-TAM, OVX-TAM, Sham and OVX groups. Tamoxifen (1 mg/kg) was administered for 4 weeks. In the forced swimming test, the immobility times in the OVX and Sham-TAM groups were higher than in the Sham group. In the open field, the numbers of central crossings in the OVX and Sham-TAM groups were lower than the number in the Sham group, and the number of peripheral crossings in the OVX group was lower than the number in the Sham group. In the elevated plus maze, the numbers of entries to the open arm among the animals in the Sham-TAM and OVX groups were lower than the number in the Sham group, while the number of entries to the open arm in the OVX-TAM group was higher than the number in the OVX group. It was shown that deletion of ovarian hormones induced anxiety and depression-like behavior. Administration of tamoxifen in naïve rats led to anxiety and depression-like behavior that was comparable with the effects of ovarian hormone deletion. It can be suggested that tamoxifen antagonizes the effects of ovarian hormones. It also seems that tamoxifen has anxiolytic effects on ovariectomized rats.

Sex hormones affect neurotransmitters and shape the adult female brain during hormonal transition periods

Sex hormones have been implicated in neurite outgrowth, synaptogenesis, dendritic branching, myelination and other important mechanisms of neural plasticity. Here we review the evidence from animal experiments and human studies reporting interactions between sex hormones and the dominant neurotransmitters, such as serotonin, dopamine, GABA and glutamate. We provide an overview of accumulating data during physiological and pathological conditions and discuss currently conceptualized theories on how sex hormones potentially trigger neuroplasticity changes through these four neurochemical systems. Many brain regions have been demonstrated to express high densities for estrogen- and progesterone receptors, such as the amygdala, the hypothalamus, and the hippocampus. As the hippocampus is of particular relevance in the context of mediating structural plasticity in the adult brain, we put particular emphasis on what evidence could be gathered thus far that links differences in behavior, neurochemical patterns and hippocampal structure to a changing hormonal environment. Finally, we discuss how physiologically occurring hormonal transition periods in humans can be used to model how changes in sex hormones influence functional connectivity, neurotransmission and brain structure in vivo.

Mechanisms of pain modulation by sex hormones in migraine

A number of pain conditions, acute as well as chronic, are much more prevalent in women, such as temporomandibular disorder (TMD), irritable bowel syndrome, fibromyalgia, and migraine. The association of female sex steroids with these nociceptive conditions is well known, but the mechanisms of their effects on pain signaling are yet to be deciphered. We reviewed the mechanisms through which female sex steroids might influence the trigeminal nociceptive pathways with a focus on migraine. Sex steroid receptors are located in trigeminal circuits, providing the molecular substrate for direct effects. In addition to classical genomic effects, sex steroids exert rapid nongenomic actions to modulate nociceptive signaling. Although there are only a handful of studies that have directly addressed the effect of sex hormones in animal models of migraine, the putative mechanisms can be extrapolated from observations in animal models of other trigeminal pain disorders, like TMD. Sex hormones may regulate sensitization of trigeminal neurons by modulating expression of nociceptive mediator such as calcitonin gene-related peptide. Its expression is mostly positively regulated by estrogen, although a few studies also report an inverse relationship. Serotonin (5-Hydroxytryptamine [5-HT]) is a neurotransmitter implicated in migraine; its synthesis is enhanced in most parts of brain by estrogen, which increases expression of the rate-limiting enzyme tryptophan hydroxylase and decreases expression of the serotonin re-uptake transporter. Downstream signaling, including extracellular signal-regulated kinase activation, calcium-dependent mechanisms, and cAMP response element-binding activation, are thought to be the major signaling events affected by sex hormones. These findings need to be confirmed in migraine-specific animal models that may also provide clues to additional ion channels, neuropeptides, and intracellular signaling cascades that contribute to the increased prevalence of migraine in women.

Quantitative changes in intracellular calcium and extracellular-regulated kinase activation measured in parallel in CHO cells stably expressing serotonin (5-HT) 5-HT2A or 5-HT2C receptors

Background

The serotonin (5-HT) 2A and 2C receptors (5-HT_2AR and 5-HT_2CR) are involved in a wide range of physiological and behavioral processes in the mammalian central and peripheral nervous systems. These receptors share a high degree of homology, have overlapping pharmacological profiles, and utilize many of the same and richly diverse second messenger signaling systems. We have developed quantitative assays for cells stably expressing these two receptors involving minimal cell sample manipulations that dramatically improve parallel assessments of two signaling responses: intracellular calcium (Ca_i⁺⁺) changes and activation (phosphorylation) of downstream kinases. Such profiles are needed to begin to understand the simultaneous contributions from the multiplicity of signaling cascades likely to be initiated by serotonergic ligands.

Results

We optimized the Ca_i⁺⁺ assay for stable cell lines expressing either 5-HT_2AR or 5-HT_2CR (including dye use and measurement parameters; cell density and serum requirements). We adapted a quantitative 96-well plate immunoassay for pERK in the same cell lines. Similar cell density optima and time courses were observed for 5-HT_2AR- and 5-HT_2CR-expressing cells in generating both types of signaling. Both cell lines also require serum-free preincubation for maximal agonist responses in the pERK assay. However, 5-HT_2AR-expressing cells showed significant release of Ca_i⁺⁺ in response to 5-HT stimulation even when preincubated in serum-replete medium, while the response was completely eliminated by serum in 5-HT_2CR-expressing cells. Response to another serotonergic ligand (DOI) was eliminated by serum-replete preincubation in both cells lines.

Conclusions

These data expand our knowledge of differences in ligand-stimulated signaling cascades between 5-HT_2AR and 5-HT_2CR. Our parallel assays can be applied to other cell and receptor systems for monitoring and dissecting concurrent signaling responses.

Estradiol and progesterone modify the effects of the serotonin reuptake transporter polymorphism on serotonergic responsivity to citalopram

Individual vulnerability to psychopathologies is linked to a number of genetic polymorphisms including the serotonin transporter (5HTT) promoter polymorphic region (5HTTLPR). A single copy of the short variant (s-variant) allele of 5HTTLPR confers increased susceptibility to anxiety disorders and depression and decreased efficacy of serotonin-releasing agents in pharmacotherapy compared to the homozygous long 5HTTLPR variant (l/L). The data suggesting that the 5HTTLPR polymorphism modulates the efficacy of serotonin-releasing agents in pharmacotherapy is inconsistent. Other factors such as age, gender, and hormonal status could interact with 5HTTLPR genotype to affect individual physiological and behavioral responses to serotonin reuptake inhibitors such as citalopram. Indeed, estradiol and progesterone, the primary female steroid hormones, exert an array of effects on the serotonergic system, including 5HTT expression. The present study used ovariectomized female rhesus monkeys to determine the interaction between the 5HTTLPR polymorphism and the effects of midfollicular levels of estradiol and luteal levels of progesterone on serotonergic responsivity to acute citalopram administration. The increase in serum prolactin, a surrogate measure of serotonin activity, following citalopram administration was significantly larger in l/L females than in s-variant females over the course of two hours during concurrent estradiol and progesterone hormone replacement only. These data suggest that ovarian function and the 5HTTLPR polymorphism interact to gate serotonergic reactivity in females, suggesting that clinicians should be aware of the ovarian status and 5HTTLPR genotype of women when considering serotonergic pharmacotherapy in women.

Antidepressive and anxiolytic activity of selective estrogen receptor modulators in ovariectomized mice submitted to chronic unpredictable stress

Estradiol has antidepressive and anxiolytic actions. However, its therapeutic use is limited by its peripheral effects. Selective estrogen receptor modulators may represent an alternative to estradiol for the treatment of depressive symptoms. Here we report that tamoxifen and raloxifene decrease immobility time in the forced swim test and increases the time spent in open arms in the elevated plus maze in ovariectomized mice submitted to chronic unpredictable stress.

Modulation of human serotonin transporter expression by 5-HTTLPR in colon cells

Serotonin (5-HT) is a monoamine neurotransmitter and plays important roles in several of the human body's systems. Known as a primary target for psychoactive drug development, the 5-HT transporter (5-HTT, SERT) plays a critical role in the regulation of serotonergic function by reuptaking 5-HT. The allelic variation of 5-HTT expression is caused by functional gene promoter polymorphism with two principal variant alleles, 5-HTT gene-linked polymorphic region (5-HTTLPR). It has been demonstrated that 5-HTTLPR is associated with numerous neuropsychiatric disorders. The functional roles of 5-HTTLPR have been reported in human choriocarcinoma (JAR), lymphoblast and raphe cells. To date, the significance of 5-HTTLPR in gastrointestinal tract-derived cells has never been elucidated. Thus, the impact of 5-HTTLPR on 5-HTT transcription was studied in SW480 human colon carcinoma cells, which were shown to express 5-HTT. We found 42-bp fragment in long (L) allele as compared to short (S) allele, and this allelic difference resulted in 2-fold higher transcriptional efficiency of L allele (P < 0.05) as demonstrated using a functional reporter gene assay. Nevertheless, the transcriptional effect of estrogen and glucocorticoid on 5-HTT expression via 5-HTTLPR was not found in this cell line. Our study was the first to demonstrate the molecular role of this allelic variation in gastrointestinal tract cells.

Estradiol-dependent modulation of serotonergic markers in auditory areas of a seasonally breeding songbird

Because no organism lives in an unchanging environment, sensory processes must remain plastic so that in any context, they emphasize the most relevant signals. As the behavioral relevance of sociosexual signals changes along with reproductive state, the perception of those signals is altered by reproductive hormones such as estradiol (E2). We showed previously that in white-throated sparrows, immediate early gene responses in the auditory pathway of females are selective for conspecific male song only when plasma E2 is elevated to breeding-typical levels. In this study, we looked for evidence that E2-dependent modulation of auditory responses is mediated by serotonergic systems. In female nonbreeding white-throated sparrows treated with E2, the density of fibers immunoreactive for serotonin transporter innervating the auditory midbrain and rostral auditory forebrain increased compared with controls. E2 treatment also increased the concentration of the serotonin metabolite 5-HIAA in the caudomedial mesopallium of the auditory forebrain. In a second experiment, females exposed to 30 min of conspecific male song had higher levels of 5-HIAA in the caudomedial nidopallium of the auditory forebrain than birds not exposed to song. Overall, we show that in this seasonal breeder, (a) serotonergic fibers innervate auditory areas; (b) the density of those fibers is higher in females with breeding-typical levels of E2 than in nonbreeding, untreated females; and (c) serotonin is released in the auditory forebrain within minutes in response to conspecific vocalizations. Our results are consistent with the hypothesis that E2 acts via serotonin systems to alter auditory processing.

Ontogeny and regulation of the serotonin transporter: providing insights into human disorders

Serotonin (5-hydroxytryptamine, 5-HT) was one of the first neurotransmitters for which a role in development was identified. Pharmacological and gene knockout studies have revealed a critical role for 5-HT in numerous processes, including cell division, neuronal migration, differentiation and synaptogenesis. An excess in brain 5-HT appears to be mechanistically linked to abnormal brain development, which in turn is associated with neurological disorders. Ambient levels of 5-HT are controlled by a vast orchestra of proteins, including a multiplicity of pre- and post-synaptic 5-HT receptors, heteroreceptors, enzymes and transporters. The 5-HT transporter (SERT, 5-HTT) is arguably the most powerful regulator of ambient extracellular 5-HT. SERT is the high-affinity uptake mechanism for 5-HT and exerts tight control over the strength and duration of serotonergic neurotransmission. Perturbation of its expression level or function has been implicated in many diseases, prominent among them are psychiatric disorders. This review synthesizes existing information on the ontogeny of SERT during embryonic and early postnatal development though adolescence, along with factors that influence its expression and function during these critical developmental windows. We integrate this knowledge to emphasize how inappropriate SERT expression or its dysregulation may be linked to the pathophysiology of psychiatric, cardiovascular and gastrointestinal diseases.

Anxiety-like behaviors and expression of SERT and TPH in the dorsal raphé of estrogen- and fluoxetine-treated ovariectomized rats

The anxiolytic effect of fluoxetine (Flx) was often ineffective in postmenopausal and estrogen-deficient patients, but such effect had not been experimentally demonstrated, particularly in the female rat model of estrogen deficiency. Here we determined the anxiety-like behaviors in ovariectomized (Ovx) rats treated for 4weeks with 10μg/kg 17β-estradiol s.c. (Ovx+E2), 10mg/kg Flx p.o. (Ovx+Flx) or a combination of both (Ovx+E2+Flx). Since Flx is known to induce anxiolysis in males, we first evaluated the Flx regimen in male rats. The results showed that anxiety-like behaviors were reduced in Flx-treated male rats. In contrast, Ovx+Flx rats still exhibited the same anxiety-like behaviors as in Ovx rats. Both Ovx+E2 and Ovx+E2+Flx rats, however, showed comparable reductions in anxiety-like behaviors, suggesting that Flx had no anxiolytic-like effect. Furthermore, E2 and E2+Flx similarly upregulated the mRNA expression of serotonin reuptake transporter (SERT) and tryptophan hydroxylase-2 in the dorsal raphé of Ovx rats, while having no effect on SERT expression in the frontal cortex, hippocampus, septum, amygdala and periaqueductal gray. In conclusion, Flx induced anxiolytic-like action in male rats. In Ovx rats, it was E2 and not Flx that exerted the anxiolytic-like action, which was mediated, in part, by altering serotonin metabolism in the dorsal raphé.

Estrogens of multiple classes and their role in mental health disease mechanisms

Gender and sex hormones can influence a variety of mental health states, including mood, cognitive development and function, and vulnerability to neurodegenerative diseases and brain damage. Functions of neuronal cells may be altered by estrogens depending upon the availability of different physiological estrogenic ligands; these ligands and their effects vary with life stages, the genetic or postgenetic regulation of receptor levels in specific tissues, or the intercession of competing nonphysiological ligands (either intentional or unintentional, beneficial to health or not). Here we review evidence for how different estrogens (physiological and environmental/dietary), acting via different estrogen receptor subtypes residing in alternative subcellular locations, influence brain functions and behavior. We also discuss the families of receptors and transporters for monoamine neurotransmitters and how they may interact with the estrogenic signaling pathways.

Activation of transgenic estrogen receptor-beta by selected phytoestrogens in a stably transduced rat serotonergic cell line

Many flavonoids, a major group of phenolic plant-derived secondary metabolites, are known to possess estrogen-like bioactivities. However, little is known about their estrogenic properties in the central nervous system due to the lack of suitable cellular models expressing sufficient amounts of functional estrogen receptor beta (ERbeta). To overcome this deficit, we have created a cellular model, which is serotonergic in origin, to study properties of estrogenic substances by stably transducing RN46A-B14 cells derived from raphe nuclei region of the rat brain with a lentiviral vector encoding a human ERbeta. We clearly showed that the transgenic human ERbeta is a spontaneously expressed and a functional receptor. We have further assessed the estrogenicity of three different isoflavones and four different naringenin-type flavanones in this cell line utilizing a luciferase reporter gene assay. Genistein (GEN), Daidzein (DAI), Equol (EQ), Naringenin (NAR) and 8-prenylnaringenin (8-PN) showed strong estrogenic activity in a concentration-dependent manner as compared to 7-(O-prenyl)naringenin-4'-acetate (7-O-PN) which was only slightly estrogenic and 6-(1,1-dimethylallyl)naringenin (6-DMAN) that neither showed estrogenic nor anti-estrogenic activity in our model. All observed effects could be antagonized by the anti-estrogen fulvestrant. Moreover, co-treatment of cells with 17beta-estradiol (E2) and either GEN or DAI showed a slight additive effect as compared to EQ. On the other hand, 8-PN in addition to 7-O-PN, but not NAR and 6-DMAN, were able to slightly antagonize the responses triggered by E2. Our newly established cellular model may prove to be a useful tool in explicating basic physiological properties of ERbeta in the brain and may help unravel molecular and cellular mechanisms involved in serotonergic mood regulation by estrogen or potential plant-derived secondary metabolites.

The unliganded long isoform of estrogen receptor beta stimulates brain ryanodine receptor single channel activity alongside with cytosolic Ca2+

Ca(2+) release from intracellular stores mediated by endoplasmic reticulum membrane ryanodine receptors (RyR) plays a key role in activating and synchronizing downstream Ca(2+)-dependent mechanisms, in different cells varying from apoptosis to nuclear transcription and development of defensive responses. Recently discovered, atypical "nongenomic" effects mediated by estrogen receptors (ER) include rapid Ca(2+) release upon estrogen exposure in conditions implicitly suggesting involvement of RyRs. In the present study, we report various levels of colocalization between RyR type 2 (RyR2) and ER type beta (ER beta) in the neuronal cell line HT-22, indicating a possible functional interaction. Electrophysiological analyses revealed a significant increase in single-channel ionic currents generated by mouse brain RyRs after application of the soluble monomer of the long form ER beta (ER beta 1). The effect was due to a strong increase in open probability of RyR higher open channel sublevels at cytosolic [Ca(2+)] concentrations of 100 nM, suggesting a synergistic action of ER beta 1 and Ca(2+) in RyR activation, and a potential contribution to Ca(2+)-induced Ca(2+) release rather than to basal intracellular Ca(2+) concentration level at rest. This RyR/ER beta interaction has potential effects on cellular physiology, including roles of shorter ER beta isoforms and modulation of the RyR/ER beta complexes by exogenous estrogens.

Effects of estrogen on AF64A-induced apoptosis in NG108-15 cells

In this study, we show that pretreatment with physiological concentrations (1-100 nM) of 17beta-estradiol decreased apoptosis induced by ethylcholine aziridinium (AF64A), a choline toxin, in the cholinergic neuronal cell line NG108-15. These protective effects were observed after short-term (30 min) pretreatment, and were blocked by treatment with an estrogen receptor antagonist and inhibitors of phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase kinase (MEK). The protective effects were, however, not reversed by a protein synthesis inhibitor. Furthermore, we examined the effects of 17beta-estradiol on choline uptake in NG108-15 cells. Although choline uptake was inhibited by a selective inhibitor of choline uptake, hemicholinium-3, it was not altered by treatment with 17beta-estradiol. These results indicated that the protective effect of 17beta-estradiol on AF64A-induced apoptosis could be nongenomic, and that this effect may be due to the activation of PI3K/Akt and/or MEK/extracellular signal-regulated kinase (ERK) pathways.

Differential regulation of dopamine transporter function and location by low concentrations of environmental estrogens and 17beta-estradiol

BACKGROUND

The effects of 17beta-estradiol (E2) and xenoestrogens (XEs) on dopamine transport may have important implications for the increased incidence of neurologic disorders, especially in women during life stages characterized by frequent hormonal fluctuations.

OBJECTIVE

We examined low concentrations of XEs [dieldrin, endosulfan, o', p'-dichlorodiphenyl-ethylene (DDE), nonylphenol (NP), and bisphenol A (BPA)] for nongenomic actions via action of membrane estrogen receptors (ERs).

METHODS

We measured activity of the dopamine transporter (DAT) by the efflux of 3H-dopamine in nontransfected nerve growth factor-differentiated PC12 rat pheochromocytoma cells expressing membrane DAT, ER-alpha, ER-beta, and G-protein-coupled receptor 30. We used a plate immunoassay to monitor trafficking of these proteins.

RESULTS

All compounds at 1 nM either caused efflux or inhibited efflux, or both; each compound evoked a distinct oscillatory pattern. At optimal times for each effect, we examined different concentrations of XEs. All XEs were active at some concentration < 10 nM, and dose responses were all nonmonotonic. For example, 10(-14) to 10(-11) M DDE caused significant efflux inhibition, whereas NP and BPA enhanced or inhibited efflux at several concentrations. We also measured the effects of E2/XE combinations; DDE potentiated E(2)-mediated dopamine efflux, whereas BPA inhibited it. In E2-induced efflux, 15% more ER-alpha trafficked to the membrane, whereas ER-beta waned; during BPA-induced efflux, 20% more DAT was trafficked to the plasma membrane.

CONCLUSIONS

Low levels of environmental estrogen contaminants acting as endocrine disruptors via membrane ERs can alter dopamine efflux temporal patterning and the trafficking of DAT and membrane ERs, providing a cellular mechanism that could explain the disruption of physiologic neurotransmitter function.

Nongenomic actions of low concentration estrogens and xenoestrogens on multiple tissues

Nongenomic estrogenic mechanisms offer an opportunity to explain the conundrum of environmental estrogen and plant estrogen effects on cells and animals at the very low concentrations which are prevalent in our environments and diets. Heretofore the actions of these compounds have not been adequately accounted for by laboratory tests utilizing assays for actions only via the genomic pathway of steroid action and the nuclear forms of estrogen receptor alpha and beta. Membrane versions of these receptors, and the newly described GPR30 (7TMER) receptor protein provide explanations for the more potent actions of xenoestrogens. The effects of estrogens on many tissues demand a comprehensive assessment of the receptors, receptor levels, and mechanisms that might be involved, to determine which of these estrogen mimetic compounds are harmful and which might even be used therapeutically, depending upon the life stage at which we are exposed to them.

Plasma serotonin levels and the platelet serotonin transporter

Serotonin (5HT) is a platelet-stored vasoconstrictor. Altered concentrations of circulating 5HT are implicated in several pathologic conditions, including hypertension. The actions of 5HT are mediated by different types of receptors and terminated by a single 5HT transporter (SERT). Therefore, SERT is a major mechanism that regulates plasma 5HT levels to prevent vasoconstriction and thereby secure a stable blood flow. In this study, the response of platelet SERT to the plasma 5HT levels was examined within two models: (i) in subjects with chronic hypertension or normotension; (ii) on platelets isolated from normotensive subjects and pretreated with 5HT at various concentrations. The platelet 5HT uptake rates were lower during hypertension due to a decrease in Vmax with a similar Km; also, the decrease in Vmax was primarily due to a decrease in the density of SERT on the platelet membrane, with no change in whole cell expression. Additionally, while the platelet 5HT content decreased 33%, the plasma 5HT content increased 33%. Furthermore, exogenous 5HT altered the 5HT uptake rates by changing the density of SERT molecules on the plasma membrane in a biphasic manner. Therefore, we hypothesize that in a hypertensive state, the elevated plasma 5HT levels induces a loss in 5HT uptake function in platelets via a decrease in the density of SERT molecules on the plasma membrane. Through the feedback effect of this proposed mechanism, plasma 5HT controls its own concentration levels by modulating the uptake properties of platelet SERT.

Serotonin transporter function, but not expression, is dependent on brain-derived neurotrophic factor (BDNF): in vivo studies in BDNF-deficient mice

In the present study, we used high-speed chronoamperometry to examine serotonin (5-HT) transporter (5-HTT) function in vivo in 2-, 5-, and 10-month-old brain-derived neurotrophic factor (BDNF)+/- mice. The rate of clearance of exogenously applied 5-HT was measured in CA3 region of hippocampus. In 2-month-old mice, the rate of 5-HT clearance did not differ between BDNF+/+ and BDNF+/- mice. In BDNF+/+ mice, 5-HT clearance rate (Tc) increased markedly with age. In contrast, Tc remained relatively static in BDNF+/- mice across 2-, 5-, and 10-month age groups. At 5 months of age, female BDNF+/+ mice had a lower maximal velocity (Vmax) for 5-HT clearance than male BDNF+/+ mice. There was a similar trend in 5-month-old BDNF+/- mice, but this did not reach statistical significance. There was an age-dependent increase in KT value for 5-HT clearance (i.e., decreased in vivo affinity of 5-HTT), but no significant effect of genotype or gender. 5-HTT density, as measured by [3H]cyanoimipramine binding, was not different between BDNF+/+ and BDNF+/- mice, although there was a significant increase in 5-HTT binding with age. The selective 5-HT reuptake inhibitor fluvoxamine (50 and 100 pmol) significantly decreased 5-HT clearance in BDNF+/+ mice, but not in BDNF+/- mice. Our data suggest that the profoundly reduced ability of 5- and 10-month-old BDNF+/- mice to clear 5-HT is not because of a decrease in the total number of 5-HTTs, but may be due to functional deficits in the 5-HTT, e.g., in the machinery/signaling required for insertion of 5-HTTs into the plasma membrane and/or activation of the 5-HTT once it is positioned to take up 5-HT from extracellular fluid.

Puberty, hormones, and sex differences in alcohol abuse and dependence

Sex differences in patterns of drinking and rates of alcohol abuse and dependence begin to emerge during the transition from late puberty to young adulthood. Increases in pubertal hormones, including gonadal and stress hormones, are a prominent developmental feature of adolescence and could contribute to the progression of sex differences in alcohol drinking patterns during puberty. This paper reviews experimental and correlational studies of gonadal and stress-related hormone changes and their effects on alcohol drinking and other associated actions of alcohol. Mechanisms are suggested by which reproductive hormones and stress-related hormones may modulate neural circuits within the brain reward system to produce sex differences in alcohol drinking patterns and vulnerability to alcohol abuse and dependence which become apparent during the late pubertal period.

Estradiol effects on the dopamine transporter - protein levels, subcellular location, and function

Background

The effects of estrogens on dopamine (DA) transport may have important implications for the increased incidence of neurological disorders in women during life stages when hormonal fluctuations are prevalent, e.g. during menarche, reproductive cycling, pregnancy, and peri-menopause.

Results

The activity of the DA transporter (DAT) was measured by the specific uptake of ³H-DA. We found that low concentrations (10^-14 to 10^-8 M) of 17β-estradiol (E₂) inhibit uptake via the DAT in PC12 cells over 30 minutes, with significant inhibition taking place due to E₂ exposure during only the last five minutes of the uptake period. Such rapid action suggests a non-genomic, membrane-initiated estrogenic response mechanism. DAT and estrogen receptor-α (ERα) were elevated in cell extracts by a 20 ng/ml 2 day NGFβ treatment, while ERβ was not. DAT, ERα and ERβ were also detectable on the plasma membrane of unpermeabilized cells by immunocytochemical staining and by a fixed cell, quantitative antibody (Ab)-based plate assay. In addition, PC12 cells contained RNA coding for the alternative membrane ER GPR30; therefore, all 3 ER subtypes are candidates for mediating the rapid nongenomic actions of E₂. At cell densities above 15,000 cells per well, the E₂-induced inhibition of transport was reversed. Uptake activity oscillated with time after a 10 nM E₂ treatment; in a slower room temperature assay, inhibition peaked at 9 min, while uptake activity increased at 3 and 20–30 min. Using an Ab recognizing the second extracellular loop of DAT (accessible only on the outside of unpermeabilized cells), our immunoassay measured membrane vs. intracellular/nonvesicular DAT; both were found to decline over a 5–60 min E₂ treatment, though immunoblot analyses demonstrated no total cellular loss of protein.

Conclusion

Our results suggest that physiological levels of E₂ may act to sequester DAT in intracellular compartments where the transporter's second extramembrane loop is inaccessible (inside vesicles) and that rapid estrogenic actions on this differentiated neuronal cell type may be regulated via membrane ERs of several types.

Serotonin regulation of serotonin uptake in RN46A cells

AIM

The role of the serotonin transporter (SERT) is to remove serotonin (5-HT) from the synaptic space. In vitro studies have shown that 5-HT uptake via SERT is influenced by the availability of its substrate, 5-HT. We used RN46A cells, a line that expresses SERT, to investigate 5-HT regulation of 5-HT uptake and the intracellular signaling pathways involved. RN46A cells also express mRNAs for 5-HT receptors (5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(2C)) and as cAMP and intracellular Ca(2+) are modulated by different 5-HT receptors, we studied both pathways.

METHODS

5-HT uptake was determined as imipramine-inhibitable uptake of [(3)H]5-HT, intracellular cAMP was measured by RIA and intracellular Ca(2+) changes were determined using the ratiometric method of intracellular Ca(2+) imaging.

RESULTS

For uptake experiments, cells were kept for 30 min either with or without 1 microM 5-HT in the medium before measuring uptake. Removal of 5-HT for 30 min significantly decreased [(3)H]5-HT uptake. The absence of 5-HT for 15 min failed to induce any changes in intracellular cAMP levels. Removal of 5-HT from the medium did not change intracellular Ca(2+) levels either; however, adding 1 microM 5-HT after 5 min in 5-HT-free conditions rapidly increased intracellular Ca(2+) levels in 50% of the cells. The remaining cells showed no changes in the intracellular Ca(2+) levels.

CONCLUSIONS

We have shown that in RN46A cells, that endogenously express SERT and mRNAs for several 5-HT receptors, changes in 5-HT levels influence 5-HT uptake rate as well as induce changes in intracellular Ca(2+) levels. This suggests that 5-HT may utilize intracellular Ca(2+) to regulate 5-HT uptake.

The rationale, efficacy and safety evidence for tegaserod in the treatment of irritable bowel syndrome

A growing body of evidence implicates abnormal serotonergic regulation of gastrointestinal function in the pathogenesis of the irritable bowel syndrome (IBS). Drugs targeting this system are therefore attractive concepts. The partial 5-HT4 receptor agonist tegaserod might be predicted to have positive therapeutic effects on a constipated and uncomfortable gut. However, IBS runs a chronic, benign course and carries no associated mortality, so it is imperative that the safety profile of new pharmacological agents made available to physicians is exemplary. The authors review the evidence for 5-HT in the aetiology of IBS and its symptoms, and the data available concerning the partial 5-HT4 receptor agonist tegaserod, in terms of rationale, efficacy and safety.

Neuroactive steroid effects on cognitive functions with a focus on the serotonin and GABA systems

This article will review neuroactive steroid effects on serotonin and GABA systems, along with the subsequent effects on cognitive functions. Neurosteroids (such as estrogen, progesterone, and allopregnanolone) are synthesized in the central and peripheral nervous system, in addition to other tissues. They are involved in the regulation of mood and memory, in premenstrual syndrome, and mood changes related to hormone replacement therapy, as well as postnatal and major depression, anxiety disorders, and Alzheimer's disease. Estrogen and progesterone have their respective hormone receptors, whereas allopregnanolone acts via the GABA(A) receptor. The action of estrogen and progesterone can be direct genomic, indirect genomic, or non-genomic, also influencing several neurotransmitter systems, such as the serotonin and GABA systems. Estrogen alone, or in combination with antidepressant drugs affecting the serotonin system, has been related to improved mood and well being. In contrast, progesterone can have negative effects on mood and memory. Estrogen alone, or in combination with progesterone, affects the brain serotonin system differently in different parts of the brain, which can at least partly explain the opposite effects on mood of those hormones. Many of the progesterone effects in the brain are mediated by its metabolite allopregnanolone. Allopregnanolone, by changing GABA(A) receptor expression or sensitivity, is involved in premenstrual mood changes; and it also induces cognitive deficits, such as spatial-learning impairment. We have shown that the 3beta-hydroxypregnane steroid UC1011 can inhibit allopregnanolone-induced learning impairment and chloride uptake potentiation in vitro and in vivo. It would be important to find a substance that antagonizes allopregnanolone-induced adverse effects.

Estrous cycle influence on individual differences in the response to novelty and cocaine in female rats

In rats, individual differences in vulnerability to self-administration of drugs of abuse can be predicted by individual locomotor responses to a novel environment. This phenomenon has been well described for male rats, however very little information is available with regard to female rats and the added complication of estrous cycle hormone changes influencing activity levels. This study was designed to explore the relationship between individual responsiveness to exposure to a novel environment, the estrous cycle, and the response to cocaine in intact, cycling female rats. Locomotor activity of naive female rats was measured upon exposure to a novel environment followed by determination of estrous cycle stage and level of circulating estradiol. Rats were identified as high-responder (HR; 15% most active) or low-responder (LR; 15% least active) rats based on the locomotor response. Hyperactivity in response to cocaine was greater in HR than in LR rats. These data in combination with evaluation of the stage of estrous suggest that the estrous cycle interacts with individual phenotypic characteristics to modify the sensitivity to cocaine in female animals.