Prenatal selective serotonin reuptake inhibitor (SSRI) exposure induces working memory and social recognition deficits by disrupting inhibitory synaptic networks in male mice

Conditioning and pseudoconditioning differently change intrinsic excitability of inhibitory interneurons in the neocortex

Many studies indicate a broad role of various classes of GABAergic interneurons in the processes related to learning. However, little is known about how the learning process affects intrinsic excitability of specific classes of interneurons in the neocortex. To determine this, we employed a simple model of conditional learning in mice where vibrissae stimulation was used as a conditioned stimulus and a tail shock as an unconditioned one. In vitro whole-cell patch-clamp recordings showed an increase in intrinsic excitability of low-threshold spiking somatostatin-expressing interneurons (SST-INs) in layer 4 (L4) of the somatosensory (barrel) cortex after the conditioning paradigm. In contrast, pseudoconditioning reduced intrinsic excitability of SST-LTS, parvalbumin-expressing interneurons (PV-INs), and vasoactive intestinal polypeptide-expressing interneurons (VIP-INs) with accommodating pattern in L4 of the barrel cortex. In general, increased intrinsic excitability was accompanied by narrowing of action potentials (APs), whereas decreased intrinsic excitability coincided with AP broadening. Altogether, these results show that both conditioning and pseudoconditioning lead to plastic changes in intrinsic excitability of GABAergic interneurons in a cell-specific manner. In this way, changes in intrinsic excitability can be perceived as a common mechanism of learning-induced plasticity in the GABAergic system.

Prenatal exposure to fluoxetine modulates emotionality and aversive memory in male and female rat offspring

During pregnancy, women are prone to depression, for which selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine, are usually the first-line treatment. However, fluoxetine can cross the placental barrier and affect fetuses, causing changes in serotonin levels early in life. Long-term effects in the brain circuits that control cognitive and emotional behavior are related to early fluoxetine exposure during development. In this study, we aimed to investigate whether fluoxetine exposure (10 mg/kg/day) from the 13th gestational day (GD13) to GD21 may lead to behavioral emotional-cognitive changes in male and female rat offspring approximately 90 days postnatally (~PN90). We have analyzed the performance of individuals in the open field and in the plus-maze discriminative avoidance task, which assesses anxiety and learning/memory processing behaviors. We have found that prenatal (GD13-GD21) exposure to fluoxetine strengthened aversive memory and induced higher anxiety levels in males, and quick extinction of aversive memory in females. Taken together, these results suggest that early exposure to fluoxetine impairs the basal state of anxiety and the cognitive functions of rats during adulthood, which may be in a sex-specific manner because males appear more susceptible than females.

Encoding of social novelty by sparse GABAergic neural ensembles in the prelimbic cortex

Although the prelimbic (PrL) area is associated with social behaviors, the neural ensembles that regulate social preference toward novelty or familiarity remain unknown. Using miniature two-photon microscopy (mTPM) to visualize social behavior-associated neuronal activity within the PrL in freely behaving mice, we found that the Ca2+ transients of GABAergic neurons were more highly correlated with social behaviors than those of glutamatergic neurons. Chemogenetic suppression of social behavior-activated GABAergic neurons in the PrL disrupts social novelty behaviors. Restoring the MeCP2 level in PrL GABAergic neurons in MECP2 transgenic (MECP2-TG) mice rescues the social novelty deficits. Moreover, we identified and characterized sparsely distributed NewPNs and OldPNs of GABAergic interneurons in the PrL preferentially responsible for new and old mouse exploration, respectively. Together, we propose that social novelty information may be encoded by the responses of NewPNs and OldPNs in the PrL area, possibly via synergistic actions on both sides of the seesaw.

The 5-HT and PLC Signaling Pathways Regulate the Secretion of IL-1β, TNF-α and BDNF from NG2 Cells

The present study was clarified the relationship between NG2 glial cells and 5-hydroxytryptamine (5-HT) to further revealed a role in the regulation of cortical excitability. The co-localization of NG2 cells and 5-HT in rat prefrontal cortex was determined using immunofluorescence. Different concentrations of 5-HT were applied to cultured NG2 cells. Real-time PCR measured the expression of interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α) and brain-derived neurotrophic factor (BDNF). Changes in the expression of IL-1β, TNF-α, and BDNF in NG2 cells were detected after the addition of 5-HT receptor specific blockers and phospholipase C (PLC) specific activators and inhibitors. The results confirmed that the NG2 protein and 5-HT co-localized in the prefrontal cortex. 5-HT treatment of NG2 cells significantly reduced the expression of IL-1β and BDNF mRNA and increased the expression of TNF-α. The 5-HT receptor specific inhibitors alverine citrate, ketanserin, ondansetron and SB-399885 blocked the regulatory effects of 5-HT on NG2 cells. The PLC signal was linked to the secretion of IL-1β, TNF-α and BDNF in NG2 cells. These results indicated that 5-HT affected IL-1β, TNF-α, and BDNF secretion from NG2 cells via the 5-HT1A, 5-HT2A, 5-HT3, 5-HT6 receptors and the PLC signaling pathway.

Influence of Prenatal Drug Exposure, Maternal Inflammation, and Parental Aging on the Development of Autism Spectrum Disorder

Autism spectrum disorder (ASD) affects reciprocal social interaction and produces abnormal repetitive, restrictive behaviors and interests. The diverse causes of ASD are divided into genetic alterations and environmental risks. The prevalence of ASD has been rising for several decades, which might be related to environmental risks as it is difficult to consider that the prevalence of genetic disorders related to ASD would increase suddenly. The latter includes (1) exposure to medications, such as valproic acid (VPA) and selective serotonin reuptake inhibitors (SSRIs) (2), maternal complications during pregnancy, including infection and hypertensive disorders of pregnancy, and (3) high parental age. Epidemiological studies have indicated a pathogenetic role of prenatal exposure to VPA and maternal inflammation in the development of ASD. VPA is considered to exert its deleterious effects on the fetal brain through several distinct mechanisms, such as alterations of γ-aminobutyric acid signaling, the inhibition of histone deacetylase, the disruption of folic acid metabolism, and the activation of mammalian target of rapamycin. Maternal inflammation that is caused by different stimuli converges on a higher load of proinflammatory cytokines in the fetal brain. Rodent models of maternal exposure to SSRIs generate ASD-like behavior in offspring, but clinical correlations with these preclinical findings are inconclusive. Hypertensive disorders of pregnancy and advanced parental age increase the risk of ASD in humans, but the mechanisms have been poorly investigated in animal models. Evidence of the mechanisms by which environmental factors are related to ASD is discussed, which may contribute to the development of preventive and therapeutic interventions for ASD.

Modulating role of serotonergic signaling in sleep and memory

Serotonin is an important neurotransmitter with various receptors and wide-range effects on physiological processes and cognitive functions including sleep, learning, and memory. In this review study, we aimed to discuss the role of serotonergic receptors in modulating sleep-wake cycle, and learning and memory function. Furthermore, we mentioned to sleep deprivation, its effects on memory function, and the potential interaction with serotonin. Although there are thousands of research articles focusing on the relationship between sleep and serotonin; however, the pattern of serotonergic function in sleep deprivation is inconsistent and it seems that serotonin has not a certain role in the effects of sleep deprivation on memory function. Also, we found that the injection type of serotonergic agents (systemic or local), the doses of these drugs (dose-dependent effects), and up- or down-regulation of serotonergic receptors during training with various memory tasks are important issues that can be involved in the effects of serotonergic signaling on sleep-wake cycle, memory function, and sleep deprivation-induced memory impairments. This comprehensive review was conducted in the PubMed, Scopus, and ScienceDirect databases in June and July 2021, by searching keywords sleep, sleep deprivation, memory, and serotonin.

Maternal Psychiatric Conditions, Treatment With Selective Serotonin Reuptake Inhibitors, and Neurodevelopmental Disorders

BACKGROUND

This study aims to clarify relationships of maternal psychiatric conditions and selective serotonin reuptake inhibitor (SSRI) use during preconception and pregnancy with risk of neurodevelopmental disorders in offspring.

METHODS

We used data from the Study to Explore Early Development, a multisite case-control study conducted in the United States among children born between 2003 and 2011. Final study group classifications of autism spectrum disorder (ASD) (n = 1367), developmental delays or disorders (DDs) (n = 1750), and general population controls (n = 1671) were determined by an in-person standardized developmental assessment. Maternal psychiatric conditions and SSRI use during pregnancy were ascertained from both self-report and medical records. We used logistic regression to evaluate associations of ASD and DDs (vs. population controls) with maternal psychiatric conditions and SSRI treatment in pregnancy. To reduce confounding by indication, we also examined SSRI associations in analyses restricted to mothers with psychiatric conditions during pregnancy.

RESULTS

Psychiatric conditions and SSRI use during pregnancy were significantly more common among mothers of children with either ASD or DDs than among population controls. Odds of ASD were similarly elevated among mothers with psychiatric conditions who did not use SSRIs during pregnancy (adjusted odds ratio 1.81, 95% confidence interval 1.44-2.27) as in mothers who did use SSRIs (adjusted odds ratio 2.05, 95% confidence interval 1.50-2.80). Among mothers with psychiatric conditions, SSRI use was not significantly associated with ASD in offspring (adjusted odds ratio 1.14, 95% confidence interval 0.80-1.62). Primary findings for DDs exhibited similar relationships to those observed with ASD.

CONCLUSIONS

Maternal psychiatric conditions but not use of SSRIs during pregnancy were associated with increased risk of neurodevelopmental disorders in offspring.

Prenatal fluoxetine impairs non-hippocampal but not hippocampal memory in adult male rat offspring

Fluoxetine is often prescribed to treat depression during pregnancy. Rodent studies have shown that fluoxetine exposure during early development can induce persistent changes in the emotional behavior of the offspring. However, the effects of prenatal fluoxetine on memory have not been elucidated. This study evaluates the memory of adult male offspring from rat dams orally administered with a clinically relevant dose of 0.7 mg/kg fluoxetine from 9 weeks before pregnancy to 1 week before delivery. Hippocampal-dependent (Morris Water Maze, MWM) and non-hippocampal-dependent (Novel Object Recognition, NOR) memory paradigms were assessed. Anxiety- and depressive-like symptoms were also evaluated using the Open Field Test, Tail Suspension Test and Sucrose Preference Test. Male rats exposed to fluoxetine during gestation displayed NOR memory impairments during adulthood, as well as increased anxiety- and depressive-like symptoms. In the MWM, the offspring of fluoxetine-treated dams did not show learning deficits. However, a retention impairment was found on remote memory, 15 days after the end of training. Molecular analyses showed increased expression of NMDA subunit NR_2B, and a decrease in NR_2A-to- NR_2B ratio in the temporal cortex, but not in the hippocampus, suggesting changes in NMDA receptor composition. These results suggest that in utero exposure to fluoxetine induces detrimental effects on non-hippocampal memory and in remote retention of hippocampal-dependent memory, which is believed to be stored in the temporal cortex, possibly due to changes in cortical NMDA receptor subunit stoichiometry. The present results warrant the need for studies on potential remote memory deficits in human offspring exposed to fluoxetine in utero.

Postnatal exposure to fluoxetine led to cognitive-emotional alterations and decreased parvalbumin positive neurons in the hippocampus of juvenile Wistar rats

The exposure to selective serotonin reuptake inhibitors (SSRIs) during development results in behavioural impairment in adulthood in humans and animal models. Indeed, serotonergic overexpression in early life leads to structural and functional changes in brain circuits that control cognition and emotion. However, the effects of developmental exposure to these substances on the behaviour of adolescent rats are conflicting and remain poorly characterised. We performed a behavioural screening to investigate the effects of postnatal exposure to fluoxetine on memory and behaviours related to anxiety, anhedonia, and depression, as well we evaluate the parvalbumin expression in hippocampus of juvenile (~PND45) female and male rats. Fluoxetine (daily 20 mg/kg s.c. injections from PND7-PND21)- or vehicle-treated adolescent rats went through several behavioural tasks (from PND 38 to PND52) and were subject to transcardial perfusion and brain removal for immunohistochemical analysis (PND53). We found that postnatal exposure to fluoxetine increased anxiety- and depression-like behaviours in the open field and sucrose preference and forced swimming tests, respectively. In addition, this treatment induced working memory and short-term (but not long-term) recognition memory impairments, and reduced parvalbumin-positive interneurons in the hippocampus. In addition, the results revealed developmental sex-dependent effects of fluoxetine postnatal treatment on adolescent rats' behaviour. These outcomes indicate that affective disorders and mnemonic alterations caused by SSRIs perinatal exposure can be present at adolescence.

Early Developmental Exposure to Fluoxetine and Citalopram Results in Different Neurodevelopmental Outcomes

Although selective serotonin reuptake inhibitors are commonly prescribed for prenatal depression, there exists controversy over adverse effects of SSRI use on fetal development. Few studies have adequately isolated outcomes due to SSRI exposure and those due to maternal psychiatric conditions. Here, we directly investigated outcomes of exposure to widely-used SSRIs Fluoxetine and Citalopram on the developing nervous system of Xenopus laevis tadpoles, using an integrative experimental approach. We exposed tadpoles to low doses of Citalopram and Fluoxetine during a critical developmental period and found that different experimental groups displayed opposing behavioral effects. While both groups showed reduced schooling behavior, the Fluoxetine group showed increased seizure susceptibility and reduced startle habituation. In contrast, Citalopram treated tadpoles had decreased seizure susceptibility and increased habituation. Both groups had abnormal dendritic morphology in the optic tectum, a brain area important for behaviors tested. Whole-cell electrophysiological recordings of tectal neurons showed no differences in synaptic function; however, tectal cells from Fluoxetine-treated tadpoles had decreased voltage gated K+ currents while cells in the Citalopram group had increased K+ currents. Both behavioral and electrophysiological findings indicate that cells and circuits in the Fluoxetine treated optic tecta are hyperexcitable, while the Citalopram group exhibits decreased excitability. Taken together, these results show that early developmental exposure to SSRIs is sufficient to induce neurodevelopmental effects, however these effects can be complex and vary depending on the SSRI. This may explain some discrepancies across human studies, and further underscores the importance of serotonergic signaling for the developing nervous system.

Sex-Specific Disruption of Distinct mPFC Inhibitory Neurons in Spared-Nerve Injury Model of Neuropathic Pain

The medial prefrontal cortex (mPFC) modulates a range of behaviors, including responses to noxious stimuli. While various pain modalities alter mPFC function, our understanding of changes to specific cell types underlying pain-induced mPFC dysfunction remains incomplete. Proper activity of cortical GABAergic interneurons is essential for normal circuit function. We find that nerve injury increases excitability of layer 5 parvalbumin-expressing neurons in the prelimbic (PL) region of the mPFC from male, but not female, mice. Conversely, nerve injury dampens excitability in somatostatin-expressing neurons in layer 2/3 of the PL region; however, effects are differential between males and females. Nerve injury slightly increases the frequency of spontaneous excitatory post-synaptic currents (sEPSCs) in layer 5 parvalbumin-expressing neurons in males but reduces frequency of sEPSCs in layer 2/3 somatostatin-expressing neurons in females. Our findings provide key insight into how nerve injury drives maladaptive and sex-specific alterations to GABAergic circuits in cortical regions implicated in chronic pain.

The effects of maternal SSRI exposure on the serotonin system, prefrontal protein expression and behavioral development in male and female offspring rats

Fluoxetine (FLX), a commonly used selective serotonin reuptake inhibitor, is often used to treat depression during pregnancy. However, prenatal exposure to FLX has been associated with a series of neuropsychiatric illnesses. The use of a rodent model can provide a clear indication as to whether prenatal exposure to SSRIs, independent of maternal psychiatric disorders or genetic syndromes, can cause long-term behavioral abnormalities in offspring. Thus, the present study aimed to explore whether prenatal FLX exposure causes long-term neurobehavioral effects, and identify the underlying mechanism between FLX and abnormal behaviors. In our study, 12/mg/kg/day of FLX or equal normal saline (NS) was administered to pregnant Sprague-Dawley (SD) rats (FLX = 30, NS = 27) on gestation day 11 till birth. We assessed the physical development and behavior of offspring, and in vivo magnetic resonance spectroscopy (MRS) was conducted to quantify biochemical alterations in the prefrontal cortex (PFC). Ex vivo measurements of brain serotonin level and a proteomic analysis were also undertaken. Our results showed that the offspring (male offspring in particular) of fluoxetine exposed mothers showed delayed physical development, increased anxiety-like behavior, and impaired social interaction. Moreover, down-regulation of 5-HT and SERT expression were identified in the PFC. We also found that prenatal FLX exposure significantly decreased NAA/tCr with 1H-MRS in the PFC of offspring. Finally, a proteomic study revealed sex-dependent differential protein expression. These findings may have translational importance suggesting that using SSRI medication alone in pregnant mothers may result in developmental delay in their offspring. Our results also help guide the choice of outcome measures in identifying of molecular and developmental mechanisms.

Perinatal SSRI exposure affects brain functional activity associated with whisker stimulation in adolescent and adult rats

Selective serotonin reuptake inhibitors (SSRI), such as fluoxetine, are used as first-line antidepressant medication during pregnancy. Since SSRIs cross the placenta the unborn child is exposed to the maternal SSRI medication, resulting in, amongst others, increased risk for autism in offspring. This likely results from developmental changes in brain function. Studies employing rats lacking the serotonin transporter have shown that elevations in serotonin levels particularly affect the development of the whisker related part of the primary somatosensory (barrel) cortex. Therefore, we hypothesized that serotonin level disturbances during development alter brain activity related to whisker stimulation. We treated female dams with fluoxetine or vehicle from gestational day 11 onwards for 21 days. We investigated offspring's brain activity during whisker stimulation using functional magnetic resonance imaging (fMRI) at adolescence and adulthood. Our results indicate that adolescent offspring displayed increased activity in hippocampal subareas and the mammillary body in the thalamus. Adult offspring exhibited increased functional activation of areas associated with (higher) sensory processing and memory such as the hippocampus, perirhinal and entorhinal cortex, retrospinal granular cortex, piriform cortex and secondary visual cortex. Our data imply that perinatal SSRI exposure leads to complex alterations in brain networks involved in sensory perception and processing.

Gestational Factors throughout Fetal Neurodevelopment: The Serotonin Link

Serotonin (5-HT) is a critical player in brain development and neuropsychiatric disorders. Fetal 5-HT levels can be influenced by several gestational factors, such as maternal genotype, diet, stress, medication, and immune activation. In this review, addressing both human and animal studies, we discuss how these gestational factors affect placental and fetal brain 5-HT levels, leading to changes in brain structure and function and behavior. We conclude that gestational factors are able to interact and thereby amplify or counteract each other's impact on the fetal 5-HT-ergic system. We, therefore, argue that beyond the understanding of how single gestational factors affect 5-HT-ergic brain development and behavior in offspring, it is critical to elucidate the consequences of interacting factors. Moreover, we describe how each gestational factor is able to alter the 5-HT-ergic influence on the thalamocortical- and prefrontal-limbic circuitry and the hypothalamo-pituitary-adrenocortical-axis. These alterations have been associated with risks to develop attention deficit hyperactivity disorder, autism spectrum disorders, depression, and/or anxiety. Consequently, the manipulation of gestational factors may be used to combat pregnancy-related risks for neuropsychiatric disorders.

Pro-neurogenic effect of fluoxetine in the olfactory bulb is concomitant to improvements in social memory and depressive-like behavior of socially isolated mice

Although loneliness is a human experience, it can be estimated in laboratory animals deprived from physical contact with conspecifics. Rodents under social isolation (SI) tend to develop emotional distress and cognitive impairment. However, it is still to be determined whether those conditions present a common neural mechanism. Here, we conducted a series of behavioral, morphological, and neurochemical analyses in adult mice that underwent to 1 week of SI. We observed that SI mice display a depressive-like state that can be prevented by enriched environment, and the antidepressants fluoxetine (FLX) and desipramine (DES). Interestingly, chronic administration of FLX, but not DES, was able to counteract the deleterious effect of SI on social memory. We also analyzed cell proliferation, neurogenesis, and astrogenesis after the treatment with antidepressants. Our results showed that the olfactory bulb (OB) was the neurogenic niche with the highest increase in neurogenesis after the treatment with FLX. Considering that after FLX treatment social memory was rescued and depressive-like behavior decreased, we propose neurogenesis in the OB as a possible mechanism to unify the FLX ability to counteract the deleterious effect of SI.