Fluoxetine disrupts food intake and estrous cyclicity in Fischer female rats

The cycle of stress: A systematic review of the impact of chronic psychological stress models on the rodent estrous cycle

Stress is known to impair reproduction through interactions between the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes. However, while it is well accepted that stress can alter estrous cycle regularity, a key indicator of female's HPG axis function, effects of different types of psychological stress have been inconsistent. This systematic review evaluated the impact of rodent models of psychological stress on estrous cyclicity, while reporting biological parameters pertaining to HPA or HPG axis function assessed within these studies. We performed a systematic database search and included articles that implemented a psychological stress model in rodents and reported estrous cyclicity for at least two cycles after initiation of stress. Of the 32 studies included, 62.5% reported post-stress alterations to estrous cyclicity, with Chronic Mild Stress (CMS) models showing the most conclusive effects. Twenty-five studies measured HPG or HPA axis markers, with cycle disruptions being commonly observed in parallel with altered estradiol and increased corticosterone levels. Our review highlights gaps in reporting estrous cyclicity assessments and makes recommendations to improve comparability between studies.

Role of estrogen in treatment of female depression

Depression is a neurological disorder that profoundly affects human physical and mental health, resulting in various changes in the central nervous system. Despite several prominent hypotheses, such as the monoaminergic theory, hypothalamic-pituitary-adrenal (HPA) axis theory, neuroinflammation, and neuroplasticity, the current understanding of depression's pathogenesis remains incomplete. Importantly, depression is a gender-dimorphic disorder, with women exhibiting higher incidence rates than men. Given estrogen's pivotal role in the menstrual cycle, it is reasonable to postulate that its fluctuating levels could contribute to the pathogenesis of depression. Estrogen acts by binding to a diversity of receptors, which are widely distributed in the central nervous system. An abundance of research has established that estrogen and its receptors play a crucial role in depression, spanning pathogenesis and treatment. In this comprehensive review, we provide an in-depth analysis of the fundamental role of estrogen and its receptors in depression, with a focus on neuroinflammation, neuroendocrinology, and neuroplasticity. Furthermore, we discuss potential mechanisms underlying the therapeutic effects of estrogen in the treatment of depression, which may pave the way for new antidepressant drug development and alternative treatment options.

Serotonin neurons in mating female mice are activated by male ejaculation

Sexual stimulation triggers changes in female physiology and behavior, including sexual satiety and preparing the uterus for pregnancy. Serotonin (5-HT) is an important regulator of reproductive physiology and sexual receptivity, but the relationship between sexual stimulation and 5-HT neural activity in females is poorly understood. Here, we investigated dorsal raphe 5-HT neural activity in female mice during sexual behavior. We found that 5-HT neural activity in mating females peaked specifically upon male ejaculation and remained elevated above baseline until disengagement. Artificial intravaginal mechanical stimulation was sufficient to elicit increased 5-HT neural activity but the delivery of ejaculatory fluids was not. Distal penis expansion ("penile cupping") at ejaculation and forceful expulsion of ejaculatory fluid each provided sufficient mechanical stimulation to elicit 5-HT neuron activation. Our study identifies a female ejaculation-specific signal in a major neuromodulatory system and shows that intravaginal mechanosensory stimulation is necessary and sufficient to drive this signal.

Serotonin neurons in mating female mice are activated by male ejaculation

Sexual stimulation triggers changes in female physiology and behavior, including sexual satiety and preparing the uterus for pregnancy. Serotonin is an important regulator of reproductive physiology and sexual receptivity, but the relationship between sexual stimulation and serotonin neural activity in females is poorly understood. Here, we investigated dorsal raphe serotonin neural activity in females during sexual behavior. We found that serotonin neural activity in mating females peaked specifically upon male ejaculation, and remained elevated above baseline until disengagement. Artificial intravaginal mechanical stimulation was sufficient to elicit increased 5-HT neural activity but the delivery of ejaculatory fluids was not. Distal penis erectile enlargement ("penile cupping") at ejaculation and forceful expulsion of ejaculatory fluid each provided sufficient mechanical stimulation to elicit serotonin neuron activation. Our study identifies a female ejaculation-specific signal in a major neuromodulatory system and shows that intravaginal mechanosensory stimulation is necessary and sufficient to drive this signal.

Therapeutic treatment with fluoxetine using the chronic unpredictable stress model induces changes in neurotransmitters and circulating miRNAs in extracellular vesicles

The most widely prescribed antidepressant, fluoxetine (FLX), is known for its antioxidant and anti-inflammatory effects when administered post-stress. Few studies have evaluated the effects of FLX treatment when chronic stress has induced deleterious effects in patients. Our objective was to evaluate FLX treatment (20 mg/kg/day, i.v.) once these effects are manifested, and the drug's relation to extracellular circulating microRNAs associated with inflammation, a hedonic response (sucrose intake), the forced swim test (FST), and corticosterone levels (CORT) and monoamine concentrations in limbic areas. A group of Wistar rats was divided into groups: Control; FLX; CUMS (for six weeks of exposure to chronic, unpredictable mild stress); and CUMS + FLX, a mixed group. After CUMS, the rats performed the FST, and serum levels of CORT and six microRNAs (miR-16, -21, -144, -155, -146a, -223) were analyzed, as were levels of dopamine, noradrenaline, and serotonin in the prefrontal cortex, hippocampus, and hypothalamus. CUMS reduced body weight, sucrose intake, and hippocampal noradrenaline levels, but increased CORT, immobility behavior on the FST, dopamine concentrations in the prefrontal cortex, and all miRNAs except miR-146a expression. Administering FLX during CUMS reduced CORT levels and immobility behavior on the FST and increased the expression of miR-16, -21, -146a, -223, and dopamine. FLX protects against the deleterious effects of stress by reducing CORT and has an antidepressant effect on the FST, with minimally-modified neurotransmitter levels. FLX increased the expression of miRNAs as part of the antidepressant effect. It also regulates both neuroinflammation and serotoninergic neurotransmission through miRNAs, such as the miR-16.

The antidepressant fluoxetine (Prozac®) modulates estrogen signaling in the uterus and alters estrous cycles in mice

Selective serotonin reuptake inhibitors (SSRI) are the most used antidepressants. However, up to 80% of women taking SSRI suffer from sexual dysfunction. We investigated the effects of fluoxetine (Prozac®) (low and high dose, n = 6-7/group) on reproductive function and the regulation of the estrous cycle. All mice treated with high dose of fluoxetine had interruption of estrous cycles within a few days after onset of treatment. When treated for 14 days, mice in the high dose group had fewer CL, often lack of any CL, and antral follicles. Uterine expression of estrogen receptor alpha, G-protein coupled estrogen receptor, and steroidogenesis enzymes were upregulated in the high dose group. Nevertheless, decreased expression of connexin 43 and alkaline phosphatase and increased expression of insulin-like growth factor-binding protein 3 and monoamine oxidase A are consistent with decreased estrogen signaling and the decreased uterine weight. Taken together, fluoxetine modulates estrogen synthesis/signaling and dysregulates estrous cycles.

Effect of Low and High Doses of Two Selective Serotonin Reuptake Inhibitors on Pregnancy Outcomes and Neonatal Mortality

Selective serotonin reuptake inhibitors (SSRI) are the most common antidepressant used by pregnant women; however, they have been associated with adverse pregnancy outcomes and perinatal morbidity in pregnant women and animal models. We investigated the effects of two SSRI, fluoxetine and sertraline, on pregnancy and neonatal outcomes in mice. Wild-type mice were treated daily with low and high doses of fluoxetine (2 and 20 mg/kg) and sertraline (10 and 20 mg/kg) from the day of detection of a vaginal plug until the end of lactation (21 days postpartum). Pregnancy rate was decreased only in the high dose of fluoxetine group. Maternal weight gain was reduced in the groups receiving the high dose of each drug. Number of pups born was decreased in the high dose of fluoxetine and low and high doses of sertraline while the number of pups weaned was decreased in all SSRI-treated groups corresponding to increased neonatal mortality in all SSRI-treated groups. In conclusion, there was a dose-dependent effect of SSRI on pregnancy and neonatal outcomes in a non-depressed mouse model. However, the distinct placental transfer of each drug suggests that the effects of SSRI on pup mortality may be mediated by SSRI-induced placental insufficiency rather than a direct toxic effect on neonatal development and mortality.

Antidepressants' effects on testosterone and estrogens: What do we know?

Various antidepressants are commonly used to treat depression and anxiety disorders, and sex differences have been identified in their efficacy and side effects. Steroids, such as estrogens and testosterone, both in the periphery and locally in the brain, are regarded as important modulators of these sex differences. This review presents published data from preclinical and clinical studies that measure testosterone and estrogen level changes during and/or after acute or chronic administration of different antidepressants. The majority of studies show an interaction between sex hormones and antidepressants on sexual function and behavior, or in depressive symptom alleviation. However, most of the studies omit to investigate antidepressants' effects on circulating levels of gonadal hormones. From data reviewed herein, it is evident that most antidepressants can influence testosterone and estrogen levels. Still, the evidence is conflicting with some studies showing an increase, others decrease or no effect. Most studies are conducted in male animals or humans, underscoring the importance of considering sex as an important variable in such investigations, especially as depression and anxiety disorders are more common in women than men. Therefore, research is needed to elucidate the extent to which antidepressants can influence both peripheral and brain levels of testosterone and estrogens, in males and females, and whether this impacts the effectiveness or side effects of antidepressants.

Female rat sexual behavior is unaffected by perinatal fluoxetine exposure

Serotonin plays an important role in adult female sexual behavior, however little is known about the influence of serotonin during early development on sexual functioning in adulthood. During early development, serotonin acts as neurotrophic factor, while it functions as a modulatory neurotransmitter in adulthood. The occurrence of serotonin release, could thus have different effects on behavioral outcomes, depending on the developmental period in which serotonin is released. Because serotonin is involved in the development of the HPG axis which is required for puberty establishment, serotonin could also alter expression patterns of for instance the estrogen receptor ɑ (ERɑ). The aim of our study was to investigate the effects of increased serotonin levels during early development on adult female rat sexual behavior during the full behavioral estrus in a seminatural environment. To do so, rats were perinatally exposed with the selective serotonin reuptake inhibitor (SSRI) fluoxetine (10 mg/kg FLX) and sexual performance was tested during adulthood. All facets of female sexual behavior between the first and last lordosis (behavioral estrus), and within each copulation bout of the behavioral estrus were analyzed. Besides the length and onset of the behavioral estrus and the sexual behaviors patterns, other social and conflict behavior were also investigated. In addition, we studied the effects of perinatal FLX exposure on ERɑ expression patterns in the medial preoptic nucleus, ventromedial nucleus of the hypothalamus, medial amygdala, bed nucleus of the stria terminalis, and the dorsal raphé nucleus. The results showed that perinatal fluoxetine exposure has no effect on adult female sexual behavior. The behavioral estrus of FLX-females had the same length and pattern as CTR-females. In addition, FLX- and CTR-females showed the same amount of paracopulatory behavior and lordosis, both during the full behavioral estrus and the "most active bout". Furthermore, no differences were found in the display of social and conflict behaviors, nor in ERɑ expression patterns in the brain. We conclude that increases in serotonin levels during early development do not have long-term consequences for female sexual behavior in adulthood.

II. Antidepressants and sexual behavior: Acute fluoxetine, but not ketamine, disrupts paced mating behavior in sexually experienced female rats

Female sexual dysfunction is both a symptom of depression and exacerbated by treatments for depression. Ketamine, a novel treatment for depression, has been shown to enhance, whereas fluoxetine has been shown to impair sexual motivation. Sexual experience leads to more robust partner preference and paced mating behavior in female rats. Whether acute ketamine and fluoxetine similarly affect sexual motivation and mating behavior in sexually experienced female rats is unknown. Sexually experienced female rats received 10 mg/kg i.p. of ketamine or saline vehicle (Experiment 1) or 10 mg/kg i.p. of fluoxetine or water vehicle (Experiment 2) 30 min before a 10-min no-contact partner preference test followed immediately by a 15-intromission paced mating test. Partner preference and paced mating behavior did not differ between ketamine- and saline-treated rats. In contrast, rats treated with fluoxetine spent significantly less time with either stimulus animal and were less active during the partner preference test than water-treated rats. Additionally, contact-return latency to ejaculation was significantly longer in fluoxetine-treated rats and they spent less time with the male during paced mating in comparison to water-treated rats. Thus, even with sexual experience, fluoxetine disrupts sexual function whereas ketamine has no detrimental effects on sexual behavior in female rats. A growing body of evidence suggests that ketamine is an encouraging new approach to treat depression particularly because it is not associated with sexual dysfunction.

Environmentally relevant exposure to an antidepressant alters courtship behaviours in a songbird

Pharmaceuticals in the environment are a recently identified global threat to wildlife, including birds. Like other human pharmaceuticals, the antidepressant fluoxetine (Prozac) enters the environment via sewage and has been detected at wastewater treatment plants. Birds foraging on invertebrates at these sites can be exposed to pharmaceuticals, although the implications of exposure are poorly understood. We conducted experiments to test whether chronic exposure to a maximally environmentally relevant concentration of fluoxetine (2.7 μg day^-1) altered courtship behaviour and female reproductive physiology in wild-caught starlings (Sturnus vulgaris), a species commonly found foraging on invertebrates at wastewater treatment plants. When paired with a female over two days, males sang less and were more aggressive towards fluoxetine-treated females than controls. Fluoxetine-treated females were initially aggressive towards males, becoming significantly less aggressive by the second day. In contrast, control females expressed intermediate levels of aggression throughout. We found no effect of female treatment on female courtship behaviour. Female body condition, circulating testosterone and circulating oestradiol were unaffected by treatment and did not account for male preference. Our findings suggest that exposure to an antidepressant reduced female attractiveness, adding to growing evidence that environmental concentrations of pharmaceuticals can alter important traits related to individual fitness and population dynamics.

Effects of selective serotonin reuptake inhibitors on three sex steroids in two versions of the aromatase enzyme inhibition assay and in the H295R cell assay

Selective serotonin reuptake inhibitors are known to have a range of disorders that are often linked to the endocrine system e.g. hormonal imbalances, breast enlargement, sexual dysfunction, and menstrual cycle disorders. The mechanisms behind most of these disorders are not known in details. In this study we investigated whether the endocrine effect due to SSRI exposure could be detected in well adopted in vitro steroidogenesis assays, two versions of the aromatase enzyme inhibition assay and the H295R cell assay. The five drugs citalopram, fluoxetine, fluvoxamine, paroxetine and sertraline, were shown to inhibit the aromatase enzyme in both types of aromatase assays. The IC50 values ranged from 3 to 600 μM. All five SSRIs, were further investigated in the H295R cell line. All compounds altered the steroid secretion from the cells, the lowest observed effect levels were 0.9 μM and 3.1 μM for sertraline and fluvoxamine, respectively. In general the H295R cell assay was more sensitive to SSRI exposure than the two aromatase assays, up to 20 times more sensitive. This indicates that the H295R cell line is a better tool for screening endocrine disrupting effects. Our findings show that the endocrine effects of SSRIs may, at least in part, be due to interference with the steroidogenesis.

Estrogenic/antiestrogenic activity of selected selective serotonin reuptake inhibitors

BACKGROUND AND AIMS

Selective serotonin reuptake inhibitors (SSRIs) are one of the most prescribed classes of psychotropics. Even though the SSRI class consists of 6 molecules (citalopram, escitalopram, fluoxetine, fluvoxamine, paroxetine and sertraline), only fluoxetine was intensively studied for endocrine disruptive effects, while the other SSRIs received less attention. This study was designed to evaluate the estrogenic/antiestrogenic effect of fluoxetine, sertraline and paroxetine.

METHODS

The in vitro (anti)estrogenic activity was assessed using a firefly luciferase reporter construct in the T47D-KBluc breast cancer cell line. These cells express nuclear estrogen receptors that can activate the transcription of the luciferase reporter gene upon binding of estrogen receptor agonists.

RESULTS

All three compounds were found to interact with the estrogen receptor. Fluoxetine had dual properties, weak estrogenic at lower concentrations and antiestrogenic effect at higher concentrations. Sertraline shared the same properties with fluoxetine, but also increased the estradiol-mediated transcriptional activity. Paroxetine presented only one type of effect, the ability to increase the estradiol-mediated transcriptional activity.

CONCLUSIONS

Overall, our results indicate a possible interaction of SSRIs with the estrogen receptor. As SSRIs are being used by all categories of population, including pregnant women or children, establishing whether they can affect the endocrine mediated mechanisms should be a priority.

Dose-dependent effects of the antiprogestin, RU486, on sexual behavior of naturally cycling Fischer rats

Regularly cycling Fischer female rats were treated with either a low (5mg/kg) or high (5mg/RAT; approximately 30mg/kg) dose of the antiprogestin, RU486, before the morning of proestrus or on the morning of proestrus. The emergence of sexual behavior after treatment with RU486 was examined in a mating test with a sexually active male rat. Lordosis behavior was remarkably resistant to the effects of RU486. Only the high dose of RU486 given the evening before proestrus, approximately 22h before mating, reduced lordosis behavior. Independent of dose or time of treatment, proceptivity was reduced and resistance to the male's attempts to mount was increased by RU486 treatment. In addition, the effect of a 5min restraint stress on sexual behavior was examined. In contrast to the relative resistance of lordosis behavior of unrestrained rats to RU486 treatment, RU486 treated rats showed a significant decline in lordosis behavior after restraint. These findings allow the suggestion that the emergence of lordosis behavior is relatively resistant to the antiprogestin while the maintenance of lordosis behavior after restraint may require participation of intracellular progesterone receptors.

Effects of intracerebroventricular administered fluoxetine on cardio-ventilatory functions in rainbow trout (Oncorhynchus mykiss)

Fluoxetine (FLX) is a selective serotonin (5-HT) reuptake inhibitor present in the aquatic environment which is known to bioconcentrate in the brains of exposed fish. FLX acts as a disruptor of various neuroendocrine functions in the brain, but nothing is known about the possible consequence of FLX exposure on the cardio-ventilatory system in fish. Here we undertook to investigate the central actions of FLX on ventilatory and cardiovascular function in unanesthetized rainbow trout (Oncorhynchus mykiss). Intracerebroventricular (ICV) injection of FLX (dosed between 5 and 25 μg) resulted in a significantly elevated total ventilation (VTOT), with a maximum hyperventilation of +176% (at a dose of 25μg) compared with vehicle injected controls. This increase was due to an increase in ventilatory amplitude (VAMP: +126%) with minor effects on ventilatory frequency. The highest dose of FLX (25 μg) produced a significant increase in mean dorsal aortic blood pressure (PDA: +20%) without effects on heart rate (ƒH). In comparison, intra-arterial injections of FLX (500-2,500 μg) had no effect on ventilation but the highest doses increased both PDA and ƒH. The ICV and IA cardio-ventilatory effects of FLX were very similar to those previously observed following injections of 5-HT, indicating that FLX probably acts via stimulating endogenous 5-HT activity through inhibition of 5-HT transporter(s). Our results demonstrate for the first time in fish that FLX administered within the brain exerts potent stimulatory effects on ventilation and blood pressure increase. The doses of FLX given to fish in our study are higher than the brain concentrations of FLX in fish that result from acute exposure to FLX through the water. Nonetheless, our results indicate possible disrupting action of long term exposure to FLX discharged into the environment on central target sites sensitive to 5-HT involved in cardio-ventilatory control.

Environmental concentrations of the selective serotonin reuptake inhibitor fluoxetine impact specific behaviors involved in reproduction, feeding and predator avoidance in the fish Pimephales promelas (fathead minnow)

Pharmaceuticals and personal care products (PPCPs) have been found in surface waters worldwide, but little is understood of their effects on the wildlife that inhabit these waters. Fluoxetine (Prozac; Eli Lilly), a highly prescribed selective serotonin reuptake inhibitor (SSRI), is a commonly found PPCP in surface water. The purpose of this project was to determine if environmentally relevant concentrations of fluoxetine impact behavior that is important for population survival in native fish species, including reproduction, feeding and predator avoidance. Chronic 4-week exposures were conducted with doses ranging from 100 ng/L to 100 μg/L to cover a range of environmentally relevant concentrations up to higher concentrations comparable to other published studies with the same drug that have documented various physiological impacts. Pimephales promelas (fathead minnow), a species native to North America, was used as it conducts a range of specific mating behaviors and therefore serves as an excellent model of specific impacts on brain function. Fluoxetine concentrations as low as 1 μg/L, a concentration that has been found in many freshwater environments, were found to significantly impact mating behavior, specifically nest building and defending in male fish. Males were also found to display aggression, isolation, and repetitive behaviors at higher concentrations. Female mating behavior was largely unaffected. In addition, predator avoidance behaviors in males and females were also impacted at 1 μg/L. Feeding was impacted at 10 μg/L and in the highest exposure (100 μg/L), egg production was limited by deaths of females due to significant male aggressive behaviors in the first two weeks of exposure. Specific behavioral changes occurred at each concentration (most noticeably 1 μg/L and 100 μg/L) indicating a dose dependent effect that triggered different responses at lower exposures versus higher exposures or differential impacts of dose depending on brain region. Length of exposure also had an impact on aggressive behavior. Changes in hormone levels, indicating significant neuroendocrine changes, suggested as a mechanism of response in higher dose and acute studies, were not linked to changes in behaviors at the doses used in this study. This research provides detailed data on how exposures to fluoxetine impact specific fish behaviors and reproduction and that the effects are dose dependent.

Developmental fluoxetine exposure facilitates sexual behavior in female offspring

RATIONALE

A growing number of infants are being exposed to selective serotonin reuptake inhibitor (SSRI) medications during the perinatal period. SSRIs target the serotoninergic system and are a popular treatment for maternal mood disorders. Serotonin itself plays a key role in the sexual differentiation through its role in the development of the hypothalamic-pituitary-gonadal axis, and previous research has shown that developmental SSRI exposure has an effect on sexual behavior in male offspring.

OBJECTIVES

Our aim was to determine the role of developmental exposure to a popular SSRI medication, fluoxetine, on sexual differentiation of the brain and behavior in female offspring using a rodent model of maternal adversity.

METHODS

Stressed and non-stressed Sprague-Dawley rat dams were chronically treated with either fluoxetine (5 mg/kg/day) or vehicle beginning on postnatal day 1. Four groups of female offspring were used: (1) control + vehicle, (2) control + fluoxetine, (3) prenatal stress + vehicle, and (4) prenatal stress + fluoxetine.

RESULTS

Primary results show that in adult female offspring, developmental fluoxetine exposure facilitates proceptive and receptive behaviors with a significant increase in the number of proceptive behaviors, a significant increase in the lordosis quotient, and a significant decrease in the rejection quotient.

CONCLUSIONS

This research contributes in the understanding of the long-term impact developmental fluoxetine exposure on the hypothalamus-pituitary-gonadal (HPG) system in adult female offspring.

Pharmacology of serotonin and female sexual behavior

In this review, first a historical perspective of serotonin's (5-HT) involvement in female sexual behavior is presented. Then an overview of studies implicating 5-HT is presented. The effect of drugs that increase or decrease CNS levels of 5-HT is reviewed. Evidence is presented that drugs which increase 5-HT have negative effects on female sexual behavior while a decrease in 5-HT is associated with facilitation of sexual behavior. Studies with compounds that act on 5-HT₁, 5-HT₂ or 5-HT₃ receptors are discussed. Most evidence indicates that 5-HT₁A receptor agonists inhibit sexual behavior while 5-HT₂ or 5-HT₃ receptors may exert a positive influence. There is substantial evidence to support a role for 5-HT in the modulation of female consummatory sexual behavior, but studies on the role of 5-HT in other elements of female sexual behavior (e.g. desire, motivation, sexual appetite) are few. Future studies should be directed at determining if these additional components of female sexual behavior are also modulated by 5-HT.

Comparison of female Fischer and Sprague-Dawley rats in the response to ketanserin

The effects of the 5-HT2A/2C receptor antagonist, ketanserin, on lordosis behavior were examined in hormonally primed, ovariectomized Fischer and Sprague-Dawley females. Rats were primed with 0.067μg/g body weight estradiol benzoate and 3.33μg/g body weight progesterone. After a pretest for sexual behavior, rats were injected with 0.416 to 10mg/kg ketanserin. In both strains, lordosis behavior, lordosis quality, and proceptivity were significantly reduced by ketanserin. There was modest evidence of a strain difference with Sprague-Dawley females slightly more sensitive to ketanserin. In a second experiment, the effects of 10mg/kg fluoxetine, 1mg/kg ketanserin, and their combination were examined to determine if the two drugs would have additive effects on sexual behavior. There was no evidence that the drugs were additive in their effect and the strains did not differ in their response to the combined treatment. These findings are discussed in relation to prior evidence for strain differences in the sexual behavioral response to fluoxetine and to a receptor agonist acting preferentially at 5-HT1A receptors.

Sprague-Dawley and Fischer female rats differ in acute effects of fluoxetine on sexual behavior

INTRODUCTION

The selective serotonin reuptake inhibitor (SSRI), fluoxetine, leads to sexual dysfunction in a substantial proportion of women. In studies with the Fischer inbred rat, the 5-HT(1A) receptor has been implicated in this sexual dysfunction. Whether this association with 5-HT(1A) receptors holds for other rat strains is not known.

AIM

The effects of acute fluoxetine on sexual behavior in two strains of rats that differ in their response to a 5-HT(1A) receptor agonist were examined. Whether the strain difference is comparable in naturally cycling and hormonally primed, ovariectomized rats was determined.

METHODS

Proestrous rats and ovariectomized rats, hormonally primed with estradiol benzoate and progesterone, were treated with varying doses of fluoxetine. Sexual behavior was examined before and after treatment with the SSRI.

MAIN OUTCOME MEASURES

Lordosis to mount ratios, lordosis quality, and proceptive behaviors were quantified. Sprague-Dawley and Fischer females were compared on each of these measures. The IC(50) for inhibition of lordosis behavior was determined.

RESULTS

In both the intact and the hormonally primed, ovariectomized model, Sprague-Dawley females were less sensitive to the effects of fluoxetine on sexual behavior. In both groups, fluoxetine showed dose dependency in behavioral inhibition, but a higher dose was required for Sprague-Dawley than for Fischer females. Naturally cycling, proestrous rats required a higher dose of fluoxetine than hormonally primed ovariectomized rats to produce significant inhibition of sexual behavior. Thus, the strain difference in the response to fluoxetine does not parallel strain differences in the response to a 5-HT(1A) receptor agonist.

CONCLUSIONS

Acute treatment with fluoxetine inhibits lordosis behavior in both Fischer and Sprague-Dawley females and the strain difference cannot be explained by reported strain differences in the response to a 5-HT(1A) receptor agonist. Fluoxetine's inhibition of female rat sexual behavior may involve effects of the SSRI in addition to activation of the 5-HT(1A) receptor.

In vivo and in vitro estrogenic activity of the antidepressant fluoxetine

Recent years have seen an increase in the use of antidepressant drugs, especially fluoxetine (FLX), in sensitive populations, such as pregnant and lactating women. Although some evidence suggests a possible endocrine action of FLX, no specific studies have been performed to investigate this hypothesis. In the present study, we investigated the possible (anti)androgenic and (anti)estrogenic actions of FLX using Hershberger, uterotrophic (0.4, 1.7, and 17mg/kg), and reporter gene (7.6-129μM) assays. In the Hershberger assay, no differences were observed in androgen-dependent organ weights. However, the uterotrophic and gene reporter assays indicated a possible estrogenic action of FLX. Uterine weight increased in the 1.7 and 17mg/kg/day groups in the 3-day uterotrophic assay in immature rats. Additionally, noncytotoxic concentrations of FLX induced estrogenic responses and increased the estrogenic response of estradiol in MCF-7 breast cancer cells transfected with luciferase.

Paroxetine-induced reduction of sexual incentive motivation in female rats is not modified by 5-HT1B or 5-HT2C antagonists

RATIONALE

Clinical data show that paroxetine causes sexual dysfunction in a substantial proportion of women taking this compound.

OBJECTIVES

This work was conducted to determine whether chronic paroxetine reduces sexual incentive motivation in female rats and whether this compound can modify any aspect of paced mating. The role of the 5-HT(1B) and 5-HT(2C) receptors in any potential effects was also evaluated.

METHODS

Ovariectomized female rats were implanted with osmotic minipumps releasing 10 mg/kg per day of paroxetine or vehicle for 28 days. Tests for sexual incentive motivation and paced mating were performed just before implantation and at regular intervals thereafter. The females were primed with estradiol benzoate (25 μg/rat) and progesterone (1 mg/rat) before each of these tests. On days 25-27 of treatment, the females were injected with the 5-HT(1B) antagonist GR125,743 (5 mg/kg), the 5-HT(2C) antagonist SB206,553 (5 mg/kg) and vehicle in counterbalanced order. Preinjection time was 30 min.

RESULTS

Paroxetine reduced sexual incentive motivation on day 20 of treatment without affecting any aspect of paced mating. None of the antagonists modified the inhibitory effect of paroxetine on sexual incentive motivation. In the group chronically treated with vehicle, SB206,553 reduced proceptive behaviors in the paced mating test. No other effect was obtained.

CONCLUSION

The effects of paroxetine seen in female rats are similar to those observed in women, suggesting that disturbances of sexual incentive motivation in rats are predictive of sexual dysfunction in women. The 5-HT(1B) and 5-HT(2C) receptors do not seem to be of any importance for paroxetine's inhibitory effect.

The age-dependent effects of selective serotonin reuptake inhibitors in humans and rodents: A review

The selective serotonin reuptake inhibitor (SSRI) Prozac® (fluoxetine) is widely prescribed for the treatment of depression and anxiety-related disorders. While extensive research has established that fluoxetine is safe for adults, safety is not guaranteed for (unborn) children and adolescents. Some clinical studies have reported adverse outcomes, such as premature birth, neonatal cardiovascular abnormalities, and pulmonary hypertension in children whose mothers used SSRIs during pregnancy. In addition, several reports show that adolescent fluoxetine treatment increases risk for suicidal behavior. Despite these studies, fluoxetine is not contraindicated in the treatment of depressed pregnant women and adolescents. Longitudinal research in humans is limited because of ethical reasons and time constraints, and to overcome these limitations, rodents are used to increase insight in the age-dependent effects of fluoxetine exposure. It has been established that neonatal and adolescent fluoxetine exposure leads to paradoxical anxiety- and depression-like features in later life of rats and mice, although in some studies adolescent fluoxetine exposure was without effects. These age-dependent outcomes of fluoxetine may be explained by serotonin's neurotrophic effects, which may vary according to the developmental stage of the brain due to epigenetic modifications. Here we review the existing evidence for the age-dependent effects of fluoxetine in humans and rodents, address the gaps in our current knowledge and propose directions for future research. Given the overlap between human and rodent findings, rodents provide heuristic value in further research on the age-dependent effects of SSRIs.

Fluoxetine prevents 8-OH-DPAT-induced hyperphagia in Fischer inbred rats

Ovariectomized, Fischer rats were hormonally primed with 10 μg estradiol benzoate and 50 μg progesterone or were treated with the sesame seed oil vehicle. Food intake was measured 2 h and 24 h after treatment with 0.25 mg/kg of the 5-HT(1A) receptor agonist, (±)-8-hydroxy 2-(di-n-propylamino) tetralin (8-OH-DPAT), 5 mg/kg of the selective serotonin reuptake inhibitor, fluoxetine, or their combination. Consistent with prior studies, two hour food intake of rats given fluoxetine and 8-OH-DPAT did not differ from vehicle controls. 8-OH-DPAT-induced hyperphagia, evident at 2 h, was blocked by co-treatment with fluoxetine. However, in contrast to prior studies, 5 mg/kg fluoxetine, alone, had only modest effects on food intake. Differences in our experimental protocols and/or the strain of rat may account for the lower anorectic response to fluoxetine. Nevertheless, the absence of a significant response to fluoxetine, alone, coupled with the drug's attenuation of the hyperphagic effect of 8-OH-DPAT, leads to the suggestion that the behavioral response to the combined treatment is more complex than that of simple additivity. Consistent with this suggestion, 24 h food intake of rats given 8-OH-DPAT and fluoxetine was lower than that of vehicle or 8-OH-DPAT-treated rats.

Pharmaceuticals as neuroendocrine disruptors: lessons learned from fish on Prozac

Pharmaceuticals are increasingly detected in a variety of aquatic systems. One of the most prevalent environmental pharmaceuticals in North America and Europe is the antidepressant fluoxetine, a selective serotonin reuptake inhibitor (SSRI) and the active ingredient of Prozac. Usually detected in the range below 1 μg/L, fluoxetine and its active metabolite norfluoxetine are found to bioaccumulate in wild-caught fish, particularly in the brain. This has raised concerns over potential disruptive effects of neuroendocrine function in teleost fish, because of the known role of serotonin (5-HT) in the modulation of diverse physiological processes such as reproduction, food intake and growth, stress and multiple behaviors. This review describes the evolutionary conservation of the 5-HT transporter (the therapeutic target of SSRIs) and reviews the disruptive effects of fluoxetine on several physiological endpoints, including involvement of neuroendocrine mechanisms. Studies on the goldfish, Carassius auratus, whose neuroendocrine regulation of reproduction and food intake are well characterized, are described and represent a reliable model to study neuroendocrine disruption. In addition, fish studies investigating the effects of fluoxetine, not only on reproduction and food intake, but also on stress and behavior, are discussed to complement the emerging picture of neuroendocrine disruption of physiological systems in fish exposed to fluoxetine. Environmental relevance and key lessons learned from the effects of the antidepressant fluoxetine on fish are highlighted and may be helpful in designing targeted approaches for future risk assessments of pharmaceuticals disrupting the neuroendocrine system in general.

Waterborne fluoxetine disrupts the reproductive axis in sexually mature male goldfish, Carassius auratus

Fluoxetine (FLX) is a pharmaceutical acting as a selective serotonin reuptake inhibitor and is used to treat depression in humans. Fluoxetine and the major active metabolite norfluoxetine (NFLX) are released to aquatic systems via sewage-treatment effluents. They have been found to bioconcentrate in wild fish, raising concerns over potential endocrine disrupting effects. The objective of this study was to determine effects of waterborne FLX, including environmental concentrations, on the reproductive axis in sexually mature male goldfish. We initially cloned the goldfish serotonin transporter to investigate tissue and temporal expression of the serotonin transporter, the FLX target, in order to determine target tissues and sensitive exposure windows. Sexually mature male goldfish, which showed the highest levels of serotonin transporter expression in the neuroendocrine brain, were exposed to FLX at 0.54μg/L and 54μg/L in a 14-d exposure before receiving vehicle or sex pheromone stimulus consisting of either 4.3nM 17,20β-dihydroxy-4-pregnene-3-one (17,20P) or 3nM prostaglandin F₂(α) (PGF₂(α)). Reproductive endpoints assessed included gonadosomatic index, milt volume, and blood levels of the sex steroids testosterone and estradiol. Neuroendocrine function was investigated by measuring blood levels of luteinizing hormone, growth hormone, pituitary gene expression of luteinizing hormone, growth hormone and follicle-stimulating hormone and neuroendocrine brain expression of isotocin and vasotocin. To investigate changes at the gonadal level of the reproductive axis, testicular gene expression of the gonadotropin receptors, both the luteinizing hormone receptor and the follicle-stimulating hormone receptor, were measured as well as expression of the growth hormone receptor. To investigate potential impacts on spermatogenesis, testicular gene expression of the spermatogenesis marker vasa was measured and histological samples of testis were analyzed qualitatively. Estrogen indices were measured by expression and activity analysis of gonadal aromatase, as well as liver expression analysis of the estrogenic marker, esr1. After 14d, basal milt volume significantly decreased at 54μg/L FLX while pheromone-stimulated milt volume decreased at 0.54μg/L and 54μg/L FLX. Fluoxetine (54μg/L) inhibited both basal and pheromone-stimulated testosterone levels. Significant concentration-dependent reductions in follicle-stimulating hormone and isotocin expression were observed with FLX in the 17,20P- and PGF₂(α)-stimulated groups, respectively. Estradiol levels and expression of esr1 concentration-dependently increased with FLX. This study demonstrates that FLX disrupts reproductive physiology of male fish at environmentally relevant concentrations, and potential mechanisms are discussed.

Role of 5-HT(1A) receptors in fluoxetine-induced lordosis inhibition

The selective serotonin reuptake inhibitor (SSRI), fluoxetine (Prozac(R)), is an effective antidepressant that is also prescribed for other disorders (e.g. anorexia, bulimia, and premenstrual dysphoria) that are prevalent in females. However, fluoxetine also produces sexual side effects that may lead patients to discontinue treatment. The current studies were designed to evaluate several predictions arising from the hypothesis that serotonin 1A (5-HT(1A)) receptors contribute to fluoxetine-induced sexual dysfunction. In rodent models, 5-HT(1A) receptors are potent negative modulators of female rat sexual behavior. Three distinct experiments were designed to evaluate the contribution of 5-HT(1A) receptors to the effects of fluoxetine. In the first experiment, the ability of the 5-HT(1A) receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635), to prevent fluoxetine-induced lordosis inhibition was examined. In the second experiment, the effects of prior treatment with fluoxetine on the lordosis inhibitory effect of the 5-HT(1A) receptor agonist, (+/-)-8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT), were studied. In the third experiment, the ability of progesterone to reduce the acute response to fluoxetine was evaluated. WAY100635 attenuated the effect of fluoxetine; prior treatment with fluoxetine decreased 8-OH-DPAT's potency in reducing lordosis behavior; and progesterone shifted fluoxetine's dose-response curve to the right. These findings are consistent with the hypothesis that 5-HT(1A) receptors contribute to fluoxetine-induced sexual side effects.

Fluoxetine-induced decrements in sexual responses of female rats and hamsters are reversed by 3α,5α-THP

INTRODUCTION

Sexual dysfunction, as a result of selective-serotonin reuptake inhibitor (SSRI) treatment among women, is relatively common and is a factor in medication compliance. The mechanisms that underlie these side-effects of SSRIs are not well-understood. SSRIs can alter activity of catabolic enzymes that are involved in progesterone's conversion to 5 α-pregnan-3 α-ol-20-one (3 α,5 α-THP). 3 α,5 α-THP plays a key role in female reproductive physiology and behavior.

AIMS

This study aimed to determine whether 3 α,5 α-THP, in the midbrain ventral tegmental area (VTA) may be a potential mechanism for fluoxetine's reduction in sexual responding of female rodents. We hypothesized that if fluoxetine induces decrements in sexual responding in part through actions of 3 α,5 α-THP, then fluoxetine will inhibit sexual receptivity concomitant with reducing 3 α,5 α-THP levels, effects which can be reversed by 3 α,5 α-THP administration.

METHODS

  Experiment 1 investigated effects of acute systemic fluoxetine [20 mg/kg intraperitoneal (IP)] and/or 3 α,5 α-THP [500 µg, subcutaneous (SC)] administration on sexual responding of ovariectomized, hormone-primed rats. Experiment 2 examined effects of 3 α,5 α-THP administration to the midbrain VTA (100 ng) on fluoxetine-induced decrements in lordosis of ovariectomized, hormone-primed rats and hamsters.

MAIN OUTCOME MEASURES

Sexual responding was determined in rats and hamsters. For rats, the percentage of times that the lordosis response occurred following mounting by a sexually-vigorous male (lordosis quotients) was utilized. For hamsters, lateral displacement, the pelvic movement that females will make to facilitate intromissions by a male hamster, was utilized.

RESULTS

Fluoxetine significantly reduced lordosis, and this was reversed SC 3 α,5 α-THP. Intra-VTA 3 α,5 α-THP attenuated fluoxetine's detrimental effects on lordosis quotients and lateral displacement of rats and hamsters, respectively.

CONCLUSIONS

Thus, fluoxetine's effects to disrupt female sexual responses may involve its effects on progestogens in the midbrain VTA.

Fluoxetine does not prevent interspecific mating between two hamster species

In a recent study we showed that female Syrian hamsters (Mesocricetus auratus) from a laboratory stock readily mated with male Turkish hamsters (M. brandti). We hypothesized that captivity and/or unconscious selection of the most receptive females by researchers or animal caretakers results in heightened female sexual receptivity and reduces the tendency to reject heterospecific males. To test this hypothesis, we decided to decrease female receptivity by injection of fluoxetine, which increases the levels of serotonin in a number of brain loci, including areas involved in mediating sexual behavior, and determine whether such a decrease in receptivity would result in better discrimination of heterospecific males and rejection of such males as mates. We treated estrous female Syrian hamsters with 20mg/kg fluoxetine or vehicle and paired them with both a conspecific and a heterospecific male (Turkish hamster) in two sequential tests. All females showed similar behavior - they exhibited lordosis toward both conspecific and heterospecific males. We did not observe any sign of aggression. Latency to display lordosis and the duration of lordosis were not affected by fluoxetine. Fluoxetine thus did not have an effect on the behavior of females toward conspecific or heterospecific males. Independent of treatment (fluoxetine or vehicle injection), females did display lordosis significantly faster and for a longer duration in the presence of a conspecific male, even though these differences were small.

Effects of fluoxetine on the reproduction of two prosobranch mollusks: Potamopyrgus antipodarum and Valvata piscinalis

Fluoxetine is a widely used antidepressant, frequently found in aquatic ecosystems. We investigated its effects on two freshwater prosobranch gastropods: Valvata piscinalis (European valve snail) and Potamopyrgus antipodarum (New Zealand mudsnail), which have different reproductive modes. The fecundity of V. piscinalis (cumulate number of eggs at day 42) was not affected with an NOEC of 100 mvg/L nominal concentration (69 microg/L measured concentration). The mudsnail P. antipodarum responded in a biphasic dose-effect curve at low concentrations. The cumulate number of neonates at day 42 had an LOEC of 100 microg/L (69 microg/L) and an NOEC of 33.3 microg/L (13 microg/L), whereas the embryos in the brood pouch at day 42 only showed an LOEC of 3.7 microg/L (1 microg/L). We also observed histological effects in P. antipodarum (gonadal thickness). Among the sexual steroids we measured only testosterone which varied, independent of reproduction. Moreover the use of two closely related species highlights the interspecific variability.

Modest effects of repeated fluoxetine on estrous cyclicity and sexual behavior in Sprague Dawley female rats

In an earlier study, we reported that daily fluoxetine treatment (10 mg/kg/day) rapidly disrupted estrous cyclicity and sexual receptivity in adult, regularly cycling Fischer rats. The current study was designed to investigate if comparable fluoxetine treatment would similarly affect intact, regularly cycling Sprague Dawley rats. In the first experiment, fluoxetine was injected for 24 days. After 11-14 days of daily fluoxetine treatment, 40% of the rats showed a transient disturbance of the estrous cycle with elimination of sexual receptivity. In these affected rats, reduced sexual receptivity generally preceded disruption of vaginal cyclicity. In a second experiment, a shorter exposure was used to attempt to dissociate effects of fluoxetine on behavior and estrous cyclicity. Nine days of fluoxetine treatment eliminated sexual receptivity and proceptivity (hops/darts) in 40% and 46%, respectively, of rats without altering the estrous cycle. Female rats then received a 10th fluoxetine injection 30 min prior to assessment of sexual motivation (measured with the male preference paradigm). There was no effect of fluoxetine on male preference, but fluoxetine significantly reduced the number of crossings and seconds of grooming during preference testing. Therefore, effects of fluoxetine on estrous cyclicity and behavior of Sprague Dawley female rats were smaller and required longer to develop than previously reported in Fischer female rats. These findings reinforce a probable relationship between fluoxetine's effect on sexual activity and neuroendocrine disturbances and illustrate the importance of strain selection in attempting to model human disease.

Daily male exposure attenuates estrous cycle disruption by fluoxetine

Fluoxetine (Prozac) produces sexual dysfunction in a substantial number of patients. In the few animal studies designed to address this sexual dysfunction in females, data have been inconsistent. Some investigators report that the drug disrupts sexual behavior without affecting the estrous cycle while we have reported robust effects of fluoxetine on the estrous cycle. The current studies were designed to initiate examination of procedural differences that may account for these contradictory outcomes. In the first experiment, intact, regularly cycling female rats were injected daily for 10 days with 10 mg/kg fluoxetine (intraperitoneally) or vehicle. Male-exposed, fluoxetine- or vehicle-treated rats were housed in a room with males and placed for 5 min/day into a male's cage. Other fluoxetine-treated females were housed in a room separate from males. In the second experiment, this protocol was repeated for 20 days and an additional group of females were exposed to male bedding for 5 min/day. Without male exposure, fluoxetine rapidly disrupted vaginal estrus and sexual receptivity so that approximately 50% of the rats failed to show vaginal estrus during the first 5 days; and the majority of the rats had a blocked cycle by 10 days of treatment. With male exposure, these reproductive effects were attenuated. The majority of rats cycled normally during the first 5 days of treatment and more than half cycled throughout the experiment. Loss of behavioral receptivity occurred even when normal estrous cyclicity was present. Although exposure to the male's bedding may have delayed the onset of estrous cycle disruption, five min daily exposure to a male's bedding did not prevent the disruptive effects of fluoxetine. These findings are consistent with evidence that fluoxetine's effect on female sexual dysfunction may result, in part, from the drugs' disruption of the hypothalamic-pituitary-gonadal axis. However, the data also evidence dissociation between the effects of fluoxetine on vaginal and behavioral estrus. These findings may also explain why different laboratories have reported the presence or absence of estrous cycle disturbances following daily treatment with fluoxetine.

Effects of fluoxetine on hippocampal-dependent and hippocampal-independent learning tasks

It is generally assumed that fluoxetine does not produce cognitive impairments, based on observations that fluoxetine-treated animals do not show impairment in learning the spatial water-maze task. As fluoxetine has different effects on different brain regions and as learning is not a unitary phenomenon, it may be the case that fluoxetine has different effects on different types of learning and memory paradigms. In this study, 15 male Sprague-Dawley rats were given chronic injections of either fluoxetine or saline and received training in two hippocampal-independent tasks in addition to a spatial water-maze task. The two hippocampal-independent tasks were a short-delay appetitive Pavlovian-conditioning task and an object-recognition task. The results showed that the fluoxetine-injected rats did not show any impairment relative to the saline controls in either the acquisition or the retention phases of the water-maze task, but were significantly impaired in both of the hippocampal-independent tasks. Fluoxetine-injected rats spent significantly less time exploring the novel object in the object-recognition task and took longer to learn the association between the conditional stimulus and the appetitive unconditional stimulus in the appetitive Pavlovian-conditioning task.