Prenatal selective serotonin reuptake inhibitor (SSRI) exposure induces working memory and social recognition deficits by disrupting inhibitory synaptic networks in male mice
Selective serotonin reuptake inhibitors (SSRIs) are commonly prescribed antidepressant drugs in pregnant women. Infants born following prenatal exposure to SSRIs have a higher risk for behavioral abnormalities, however, the underlying mechanisms remains unknown. Therefore, we examined the effects of prenatal fluoxetine, the most commonly prescribed SSRI, in mice. Intriguingly, chronic in utero fluoxetine treatment impaired working memory and social novelty recognition in adult males. In the medial prefrontal cortex (mPFC), a key region regulating these behaviors, we found augmented spontaneous inhibitory synaptic transmission onto the layer 5 pyramidal neurons. Fast-spiking interneurons in mPFC exhibited enhanced intrinsic excitability and serotonin-induced excitability due to upregulated serotonin (5-HT) 2A receptor (5-HT2AR) signaling. More importantly, the behavioral deficits in prenatal fluoxetine treated mice were reversed by the application of a 5-HT2AR antagonist. Taken together, our findings suggest that alterations in inhibitory neuronal modulation are responsible for the behavioral alterations following prenatal exposure to SSRIs.
KeywordsPrenatal Serotonin (5-HT) Selective serotonin reuptake inhibitor (SSRI) Fluoxetine Working memory Social recognition Serotonin 2A receptor (5-HT2AR)
- 5-HT (serotonin)
5-HT 2A receptor
Miniature excitatory postsynaptic currents
Miniature inhibitory postsynaptic currents
Medial prefrontal cortex
Spontaneous excitatory postsynaptic currents
Spontaneous inhibitory postsynaptic currents
Selective serotonin reuptake inhibitor
Anti-depressants are commonly prescribed to treat major depression and post-traumatic stress disorder. Currently, 17% of pregnant women experience major depression, and approximately 10% of these women use anti-depressants [1, 2, 3]. The most commonly prescribed anti-depressants, selective serotonin reuptake inhibitors (SSRIs), are believed to increase the ambient level of 5-hydrotryptamine (5-HT, serotonin) in synaptic clefts by preventing its reabsorption [4, 5, 6]. However, the exact mechanism by which SSRIs mitigates depression remains unknown. A recent systematic review showed that the potential adverse effects of SSRIs might outweigh their beneficial effects on depression [7, 8, 9]. In addition, fluoxetine (FLX), one of the most widely used SSRIs with a moderately long half-life (t ½ = 48 h), can cross the placental and blood-brain barriers and is also detected in breast milk, suggesting potential accumulation of FLX as well as 5-HT in the fetal brain . However, little is known about the safety of FLX use during pregnancy. Moreover, the long-term consequences of prenatal FLX exposure for adverse behavioral outcomes in offspring are uncertain and sometimes conflicting; these conflicting findings are likely due to the independent association between maternal depression and negative pregnancy outcomes in human [11, 12].
Ansorge et al. firstly observed that postnatal FLX exposure produced anxiety behaviors and disrupted learning in rodent offspring [13, 14]. In subsequent studies, manipulations of brain 5-HT levels during early development produced abnormal neuronal circuit formation in the cortex and promoted aggressive or anxiety-related behaviors [15, 16, 17, 18, 19]. However, a thorough assessment of animal behaviors induced by prenatal SSRI exposure has not been performed. Furthermore, the underlying molecular and circuit mechanisms of these behavioral changes have not been investigated and, for this reason, no rescue experiment has been performed in offspring exposed to SSRIs during prenatal period.
Using a combination of behavioral analyses and electrophysiological investigations of affected neuronal circuits, we examined how chronic prenatal exposure to exogenous FLX influences animal behaviors and neuronal circuitry and function. We observed impaired social recognition and working memory in male mice chronically exposed to FLX. Furthermore, we observed reduced frequencies in spontaneous excitatory postsynaptic currents recorded from layer (L) 5 pyramidal neurons in the prelimbic cortex of affected mice. Intriguingly, these reduced excitatory neuronal activities were caused by enhanced serotonergic modulation of fast-spiking (FS) interneurons in L5 due to enhanced 5-HT2A receptors (5-HT2ARs). Moreover, the acute treatment with the 5-HT2AR antagonist MDL100907 (MDL) normalized the impaired social recognition and working memory impairment in these animals .
Prenatal fluoxetine treatment induced deficits in working memory and social recognition
Effects of prenatal SAL and FLX treatment on number, gender composition and weight of offspring
Total number of litters
Total number of offspring
Total number of male mice
Average number of pups per litter
4.73 ± 0.44
4.17 ± 0.37
Average number of male pups per litter
2.64 ± 0.33
2.13 ± 0.21
Percentage of male pups in each litter
55.88 ± 4.98%
54.26 ± 4.19%
Weight of male mice at P28
16.44 ± 0.41 g, n = 20
15.89 ± 0.33 g, n = 23
To further examine the role of prenatal FLX in working memory and cognitive function, we subjected FLX-treated mice to the Y-maze spontaneous alternation task . Briefly, both control and FLX treated mice were allowed to freely access three arms of the Y-maze for 10 min. Mice prefer to explore a previously uncharted arm of the maze rather than returning to a previously visited arm . Intriguingly, the alternation rate of prenatally FLX-treated mice was lower than that of SAL-treated mice (t  =3.05, p < 0.01; Fig. 1b) without changes in general activity, as measured by the total arm entries (p > 0.05; Fig. 1c). To test whether this reduced alternation rate was due to either behavioral perseverance or recognition of a new environment, we performed the novel object recognition test. Interestingly, both prenatally SAL- and FLX-treated mice were able to distinguish the novel and familiar objects, as assessed by the time spent exploring the novel object (Additional file 1: Figure S1E). Additionally, there was no significant difference in grooming behaviors between the two groups, indicating that the behavioral perseverance of FLX-treated mice was not associated with repetitive or obsessive-compulsive behaviors (Additional file 1: Figure S1F).
Next, we subjected the two groups of mice to the social interaction test by using a 3-chamber apparatus [25, 26]. During the 10-min habituation phase, neither groups showed any side preference in the 3-chamber apparatus as reflected by the lack of differences in the time spent sniffing two empty wire pencil holders in the left and right chambers (p > 0.05; Additional file 1: Figures S1G, H). Over the next 10 min, both groups spent significantly more time sniffing the juvenile male mouse (social stimulus) than the dummy object (t  =7.72, p < 0.001 for SAL; t  =10.39, p < 0.001 for FLX; Fig. 1d-f). During the last 10 min, prenatally SAL-treated mice spent significantly more time sniffing the novel juvenile male mouse than the familiar juvenile mouse (t  =3.28, p < 0.01), confirming a preference for social novelty in SAL mice. In contrast, prenatally FLX-treated mice failed to show this preference, as indicated by the similar time spent in investigating the novel and familiar juvenile mice (t  = 0.25, p = 0.81; Fig. 1g-i). Taken together, these data indicate that prenatally FLX-treated mice exhibited deficits in working memory and social novelty recognition.
Increased frequency of spontaneous and miniature inhibitory synaptic currents in layer 5 pyramidal neurons in the prefrontal cortex of FLX-treated mice
To test whether inhibitory synaptic transmission in the PFC was affected in FLX-treated mice, we recorded both spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs, respectively) in L5 pyramidal neurons within the PrL . Surprisingly, we observed a significant increase in the frequency of sIPSCs (SAL: 22.46 ± 0.72 Hz; FLX: 29.99 ± 0.96 Hz, p < 0.01), but not the amplitude (SAL: 36.88 ± 4.57 pA; FLX: 39.29 ± 3.36 pA) (Fig. 2g-i). In contrast, neither the frequency nor the amplitude of mIPSCs were affected in L5 neurons from FLX-treated mice (Fig. 2j-l). These data indicate that prenatal FLX exposure increased spontaneous inhibitory network activity in L5 pyramidal neurons within the PrL.
Increased excitability and serotonergic modulation of L5 fast-spiking interneurons in PrL of FLX-treated mic
The mPFC receives dense serotonergic innervation from the raphe nuclei, and both pyramidal and interneurons within the mPFC express several 5-HT receptor subtypes, with a particularly high density of 5-HT1A and 5-HT2ARs [31, 32, 33]. Chronic increases in ambient synaptic 5-HT due to SSRI-mediated blockade of the serotonin transporter (5-HTT) could potentially result in desensitization, internalization, or compensatory level changes via transcription and translations of certain 5-HT receptors [24, 34]. To test this hypothesis, qRT-PCR was performed on the PrL tissues from either SAL-treated or FLX-treated dams. The qRT-PCR results showed a significant increase in the levels of the 5-HT2AR mRNAs (SAL vs FLX fold change in 5-HT2AR: 1.49 ± 0.14; p = 0.044, unpaired t-test; SAL, n = 5; FLX, n = 5). Although not statistically significant, there was an upward trend in the level of the 5-HT1AR mRNAs (SAL vs FLX fold change in 5-HT1AR: 1.48 ± 0.17; p = 0.054, unpaired t-test; SAL, n = 5; FLX, n = 5)(Additional file 1: Figures S4A, B). In contrast, we did not observe any significant changes in the mRNA expression levels of other 5-HT receptors and transporters in both qRT-PCR and microarray analyses (Additional file 1: Figures S4C, D).
We wondered whether the upregulation of 5-HT 2ARs resulted in increased spontaneous inhibitory synaptic transmission in L5 excitatory pyramidal neurons. To address this question, we first investigated functional changes in sIPSCs recorded from L5 pyramidal neurons upon acute exogenous 5-HT treatment. Consistent with results from previous studies [35, 36], treatment with exogenous 5-HT (30 μM) significantly enhanced both the frequency and amplitude of sIPSCs in L5 pyramidal neurons in SAL-treated mice by 32.74 ± 6.65% and 29.22 ± 11.09%, respectively (n = 5, p < 0.05, Additional file 1: Figures S4E, F). Intriguingly, L5 pyramidal neurons in FLX-treated mice exhibited substantial increases in the frequency and amplitude of sIPSCs (53.25% ± 7.85 and 36.93 ± 6.22%, n = 5, p < 0.05, Additional file 1, Figures S4E, F).
We further tested the effect of 5-HT treatment on L5 pyramidal neurons. Compared with FS interneurons, L5 pyramidal neurons showed a significantly reduced spike frequency in response to 5-HT application (Additional file 1: Figures S6A-C). Furthermore, no significant difference in the 5-HT-mediated reduction in the sEPSC frequency in L5 pyramidal neurons was observed between SAL- and FLX-treated mice (Additional file 1: Figures S6D, G).
Behavioral deficits of FLX-treated mice were ameliorated by acute treatment with 5-HT2AR antagonists
We showed that prenatally FLX-treated mice exhibited deficits in a working memory task and social novelty recognition paradigm via enhanced inhibitory synaptic activities in the L5 neurons of the mPFC resulting from enhanced 5-HT2AR signaling in FS PV neurons. More importantly, the acute inhibition of 5-HT2AR signaling in FLX-treated mice successfully reversed the observed behavioral deficits. Although 5-HT generally plays a critical role in mammalian neuronal development and behavior, the causal relationship between alterations in 5-HT homeostasis during pregnancy and adverse behavioral consequences in adulthood is poorly understood. Previously, several studies have attempted to address this question using both genetic deletion of SERT and SSRI administration in rodents. However, these studies suffered from inconsistent behavioral phenotypes, which were partially due to the use of different rodent strains and the type, dosage, and timing of administered SSRIs (see Additional file 1: Table S1). In the present study, we adopted a treatment scheme similar to that of Noorlander et al. This treatment mimicked SSRI exposure before the 3rd trimester in humans, in which doctors recommend that pregnant women abstain from (or reduce the dose of) SSRIs during late pregnancy . In this paradigm, we consistently observed behavioral deficits in Y-maze spontaneous alternation tasks in prenatally FLX-treated mice without anxiety-related behaviors. More importantly, SSRI-treated mice exhibited normal sociability but impaired preference for social novelty in the three-chamber test (Fig. 1g-i), which is strikingly similar to the behaviors of mice lacking integrin β3, whose activities are linked to 5-HT transport and the pathophysiology of hyperserotonemia and autism [40, 41], as well as other mice lacking genes associated with autism [42, 43, 44].
In contrast to previous studies by Ansorge et al.  and Noorlander et al.  we did not find an anxiety-like effect in adult mice in our experiments. This could be due to two reasons: First, Ansorge et al.  employed postnatal treatment from P4-P21 and the difference in timeline of exposure to SSRI between the two protocols could have resulted in different developmental effects on different brain circuits. While the timing of our protocol was identical to that of Noorlander et al., , the difference in dosage of FLX that was used (ours: 0.6 mg/kg/day; Noorlander et al.: 0.8 mg/kg/day) could have different effect on the maturation and/or modification of emotional brain circuits. Although the dose difference between two protocols seemed to be small, Noorlander et al.  reported an increased mortality rate of offspring, which we did not observe in ours.
We recorded spontaneous synaptic activities induced by local inhibitory and excitatory networks in single neurons within L5, a major output neuronal layer within the mPFC. A significant increase in the frequency of sIPSCs was observed in the mPFC of FLX-treated mice compared with that in SAL-treated mice (Fig. 2g-i), but the frequency of sEPSCs recorded in the same neurons was decreased (Fig. 2a-c). Because both sIPSCs and sEPSCs were recorded in the same L5 pyramidal neurons, these data indicate that the ratio of excitatory to inhibitory drive onto those neurons is abnormally low in FLX-treated mice. As increase in sIPSC frequency is caused by activity-dependent changes such as excitability, we next blocked APs in a slice with TTX and recorded mIPSCs in L5 pyramidal neurons. Intriguingly, significant differences in both frequency and amplitude of mIPSCs were not observed between FLX- and SAL-treated mice, indicating that the increased frequency of inhibitory currents observed in L5 mPFC neurons was due to activity-dependent GABAergic release. Because these excitatory neurons received inhibitory synaptic inputs from PV-positive FS interneurons, we measured the intrinsic excitability of L5 FS interneurons and observed a significant increase in intrinsic excitability without marked changes in channel properties or input resistance (Additional file 1: Figure S3). We reasoned that the compensatory augmentation of specific 5-HT receptors could arise from prolonged exposure to 5-HT due to SSRI treatment and observed a concurrent increase in two 5-HT receptors, 5-HT1AR and 2AR, using qPCR analysis. Because of the lack of suitable antibodies against 5-HT receptors for immunohistochemical analyses, we performed electrophysiological recordings and pharmacology to test the contribution of the increased abundance of specific 5-HT receptors in the PFC of FLX-treated mice. Surprisingly, increases in activity- and 5-HT-dependent changes in the excitability of FS interneurons were mediated by 5-HT2ARs, but not 5-HT1ARs (Fig. 3d-g and Additional file 1: Figure S5). Although we do not clearly understand why 5-HT2AR signaling or expression was enhanced specifically in FS interneurons, Athilingam et al. recently showed that 5-HT treatment in FS interneurons resulted in the suppression of an inward-rectifying potassium conductance, which eventually lead to increased excitability of these interneurons via 5-HT2ARs , which might explain the excitability change in FS interneurons upon chronic SSRI treatment.
Changes in sIPSC frequency or altered inhibitory drive relative to excitatory drive have been observed in other animal models of neurodevelopmental disorders in which working memory deficits have also been reported [29, 46, 47, 48]. Our findings support the hypothesis that environmental changes induced by a single drug during pregnancy elicits an imbalance in the inhibitory/excitatory drive onto major output neurons in the L5 microcircuits within the PFC and subsequently alters animal behavior through non-genetic, compensatory upregulation of unique classes of 5-HT receptors in specific neuronal types. However, the potential mechanism of this compensatory 5-HT-receptor upregulation in FS interneurons needs to be addressed. Furthermore, studies to determine whether L2/3 neurons in the mPFC or hippocampal neurons exhibit comparable changes in the intrinsic excitability of neurons and excitatory/inhibitory (E/I) imbalance in the network will be interesting. Our data also support the findings from recent optogenetic studies showing that acute modulation of the excitability of FS PV-positive interneurons within the mPFC elicits changes in network oscillation and the performance of cognitive flexibility behavior in mice [49, 50]. Therefore, measurements of network oscillations in FLX-treated mice during working memory or social novelty task using in vivo multi-electrode recordings will be exciting.
The increased 5-HT2AR-mediated changes in the excitability of FS interneurons in FLX-treated mice prompted us to test whether the behavioral deficits of these mice were modulated by the application of a 5-HT2AR antagonist, such as MDL. The impairments of working memory and social novelty recognition were rescued by an in vivo treatment with MDL (Fig. 4 b-i). It is intriguingly, because a MDL treatment rescues attentional performance deficits in phencyclidine-treated  and NMDAR antagonist-treated  rats. Because patients with schizophrenia, who usually exhibit working memory deficits, have very high 5-HT2AR occupancy in the frontal cortex , the enhanced 5-HT2AR activity observed in our study may represent a general pathogenic mechanism of behavioral deficits in some mental disorders, and systemic administration of MDL might potentially restore synaptic and behavioral deficits in patients with disorders with similar etiologies.
Taken together, our data revealed that altered prenatal 5-HT homeostasis results in mPFC-dependent behavioral deficits, and systemic modulation of the augmented 5-HT2AR sufficiently rescues these behavioral deficits. Furthermore, our findings may potentially provide new opportunities for the pharmacological treatment of patients who have been prenatally exposed to psychotropic medications during the prenatal period.
Materials and methods
Animals were housed in a specific pathogen-free facility maintained below 22 °C and 55% humidity under a 12-h light-dark cycle (lights on at 0700 h) with food and water provided ad libitum . Timed pregnancy was achieved by breeding wild-type C57BL6J (The Jackson Laboratory) male mice with female mice.
Mice were perfused with PBS, followed by 4% (w/v) paraformaldehyde in PBS (pH 7.4). Brains were harvested and fixed with 4% paraformaldehyde overnight, transferred to 30% (w/v) sucrose in PBS, and then cryosectioned into 40 μm thick slices. For immunohistochemistry, brain slices were permeabilized with 0.2% Triton X-100 in PBS for 10 min and transferred to blocking buffer (5% donkey serum, 2% BSA and 0.2% Triton X-100 in PBS) for 1 h at room temperature. Next, the sections were incubated with primary antibody for parvalbumin (1:1000 diluted in blocking buffer, PV-235, Swant) overnight at 4 °C. The sections were subsequently incubated with appropriate secondary antibodies (1:500 diluted in blocking buffer, anti-streptavidin Alexa Fluor 488, anti-mouse Alexa Fluor 555; Invitrogen) for 2 h at room temperature. All sections were then stained with DAPI (1:5000 diluted with 0.2% Triton X-100 in PBS, Sigma-Aldrich) and mounted with Fluorsafe (Merck Millipore). Images were captured using an LSM 710 confocal microscope (Zeiss).
L5 neurons were studied in acute coronal slices of the medial prefrontal cortex (mPFC) from male postnatal day 90–150 mice. After the mice were anesthetized by inhalation of 5% isoflurane, they were decapitated, and their brains were quickly removed and chilled in ice-cold, high-magnesium cutting solution containing the following components (in mM): 110 ChCl, 26 NaHCO3, 3.2 KCl, 0.5 CaCl2, 7 MgCl2, 1.25 NaH2PO4, 10 glucose, 2 sodium pyruvate, and 3 ascorbate . The pH was adjusted to 7.4 by saturation with carbogen (95% O2 and 5% CO2), and the osmolality was approximately 300 mOsmol/L. The isolated brain was glued onto the stage of a vibrating blade Compresstome (VF-200, Precisionary), and 300 μm-thick slices were cut. The slices were incubated at 34 °C for 30 min in the same solution and thereafter maintained at room temperature. For experiments, we transferred a slice that recovered for at least one hour to a recording chamber superfused with artificial cerebrospinal fluid (aCSF) containing the following components (in mM): 124 NaCl, 26 NaHCO3, 3.2 KCl, 2.5 CaCl2, 1.3 MgCl2, 1.25 NaH2PO4, and 10 glucose. The aCSF was bubbled with 95% O2 and 5% CO2. Whole-cell voltage- or current-clamp recordings (one cell per slice) were performed at 32 ± 1 °C, and the rate of aCSF perfusion was maintained at 1–1.5 ml min− 1. Recordings were performed in somata with a Multiclamp 700B amplifier. Patch pipettes for current-clamp mode were filled with internal solutions containing the following components (in mM): 115 K-gluconate, 20 KCl, 10 Na2-phosphocreatine, 10 HEPES, 2 Mg-ATP, 0.3 NaGTP, and 0.1% biocytin. For voltage-clamp recordings, we used internal solutions containing the following components (in mM): 120 Cs-methane sulfonate, 10 CsCl, 10 TEA-Cl, 1 MgCl2, 10 HEPES, 0.1 EGTA, 0.4 Tris-GTP, 3 Mg-ATP, and 5 Na2-phosphocreatine. We recorded series resistance throughout the experiments and excluded neurons with series resistance > 20 MΩ from data analysis. Membrane potential values were presented as recorded without correcting for liquid junction potentials. MDL10090, CNQX, APV, picrotoxin, and TTX were purchased from Tocris Bioscience. All other drugs were purchased from Sigma-Aldrich. Stock solutions of drugs were made by dissolving in deionized water or DMSO according to manufacturer’s specifications and were stored at − 20 °C. On the day of the experiment, one aliquot was thawed and used. The concentration of DMSO in solutions was maintained at 0.1%.
Behavioral assays and analyses
Animals and fluoxetine treatment
Male and female mice breeders were co-housed until pregnancy. The date when a plug was first noted was classified as ED0. From ED4 to ED19, pregnant females were housed individually and received daily i.p. injections of either FLX (0.6 mg/kg/day in a volume of 10 ml/kg, Sigma) or equal volumes of SAL . Females used for injection were used only once to minimize the potential cross-generation effects of FLX administration. At postnatal day 21–23, offspring were weaned and housed with their same-sex littermates.
For behavioral testing, we used adult males treated prenatally with either FLX or SAL. All animals were 12 weeks old at the time of testing. All tests except for the open-field test were conducted during the light phase. All behavioral apparatuses were cleaned with 70% ethanol and dried with tissue between each animal.
The open-field test was conducted to measure general exploratory behavior. Mice were placed into the center of a Plexiglas cage (40.5 cm × 40.5 cm × 16 cm) for a 60-min test. Horizontal locomotion (i.e., distance traveled) was automatically recorded and analyzed by using Versamax software (AccuScan Instruments Inc).
Elevated zero maze
Anxiety-related behavior was measured using the elevated zero maze (CSI-MZ-ZR, Cleversys), which consists of a circular runway subdivided into two closed and two open sections elevated 60 cm above the floor. Mice were initially introduced into one of the closed runways and were allowed to freely explore the apparatus for 10 min. During the 10-min test, the percentage of time spent on the open arms and the number of transitions between two closed arms were scored using Topscan software (Cleversys).
To allow the mice to discriminate between the three arms of the Y-maze apparatus (San Diego Instruments), the sides of one arm of was lined with a pattern of black vertical bars on a white background with a black square at the end. The sides of another arm were lined with a pattern of solid circles with black triangles on a white background and a triangle marking the end. The remaining arm was not marked. The Y-maze test was conducted for 10 min. The first minute was not coded and treated as the habituation period. Subsequently, the entries into each arm were recoded. The spontaneous alternation index was calculated as the number of non-repeating triplets (for example, if each arm was labeled “A”, “B” or “C”, “ABCAC” = 2) divided by the total number of possible non-repeating triplets (total number of entries into each arm - 2).
Novel object recognition test
The novel object recognition test was conducted in the open-field apparatus, which was subdivided equally into two arenas with a red plastic divider. For training, two identical objects constructed out of Lego blocks were placed at either end of the arena. Mice were allowed to explore and familiarize themselves with the two objects for 10 mins. They were returned to their cages for 20 min while the arena and the objects were cleaned to remove any odor cues. Afterwards, a test of short-term memory was conducted by placing the familiar object and a novel object that differed in shape, color, and size at either end of the arena. The duration of sniffing and bouts of sniffing were manually recorded using Topscan software (CleverSys).
Social cognition tests
Social preference and social recognition were tested using a three-chamber apparatus (CSI-SO-PP, CleverSys). The entire test consists of three consecutive 10 min blocks. For the first block, mice were habituated to the entire apparatus and two empty wire pencil holders placed at opposite corners. For the second block, one juvenile male mouse (social stimulus) was placed in one holder, and a dummy object (non-social stimulus) was placed in the other holder to test for social preference. For the last block, the juvenile mouse remained in one holder (familiar), but the dummy object was replaced by a novel juvenile male mouse (novel) to test for social discrimination.
Golgi staining and tracing
Mice were perfused with PBS and placed in impregnation solution (FD Rapid Golgi-Stain Kit, FD Neurotech) for one week. 100 μm sections were cut on a cryostat, processed using the kit’s standard staining procedure and images were taken with the confocal microscope. Z-stack images were traced and analyzed with the Simple Neurite Tracer plugin for ImageJ.
Gene expression study
Total RNA was extracted from punched samples of the mPFC using an RNeasy kit (Qiagen) followed by cDNA synthesis using a Quantitect Reverse Transcription kit (Qiagen). Real-time polymerase chain reaction (PCR) was performed on three independent sets of templates using iQ SYBR Green Supermix (Bio-Rad). The amplification mixture consisted of 1 μM primers, 10 μl of iQ SYBR Green Supermix (Bio-Rad), and 100 ng of template DNA in a total volume of 20 μl. Cycling parameters were 95 °C for 15 s, 57 °C for 1 min and 72 °C for 40 cycles using a CFX96 real-time PCR detection system (Bio-Rad). For each assay, PCR was performed after melting curve analysis. To reduce variability, we ran each sample in duplicate and included control qPCR reactions without the template in each run.
For microarray analyses, we normalized all microarray data using RMA method provided by the affy package  and analyzed differentially expressed genes using the limma package . For gene-probe mapping information and GO term annotation, we used the EnsEMBL database (version 90).
Data were analyzed and plotted using GraphPad Prism (GraphPad Software) and are presented as the means ± SEM with symbols representing individual subjects. Data from the open-field test data were analyzed using two-way repeated-measures ANOVA, followed by Tukey’s post hoc test. The rest of the data were analyzed using the Mann-Whitney test, Wilcoxon Signed rank test, and unpaired t-test. P < 0.05 was considered statistically significant. In addition, extreme values were subjected to Grubb’s test for outliers (http://graphpad.com/quickcalcs/Grubbs1.cfm) and excluded from the analysis.
We would like to thank Dr. Ann-Marie Chacko and her team for the Duke-NUS Laboratory for Translational and Molecular Imaging (LTMI) for their helpful discussions and generously sharing reagents. We thank Drs. Justin Lee, Chul-Hoon Kim, and Eddy Leman for their critical comments on the manuscript.
This work was supported by Singapore Ministry of Education (MOE) Academic Research Fund (MOE2012-T2–1-021, MOE2014-T2–2-071), National Medical Research Council Individual Research Grant (NMRC/CBRG/0075/2014 and NMRC/OFIRG/0050/2017) and Translational Clinical Research Programme in Parkinson’s Disease (NMRC/TCR/013-NNI/2014), A*Star Translational Collaborative Research Partnership Grant (TCRP, 13/1/96/688), National Research Foundation (NRF-CRP17–2017-04), Duke-NUS Signature Research Program Block Grant (all to H.S.J.), and a Khoo Postdoctoral Fellowship Award (Duke-NUS-KPFA/2017/0016, to S. T.). T.K. was supported by Basic Science Research Program via the National Research Foundation of Korea (NRF-2016R1C1B2009302) and the UNIST block grant (1.170009.01).
Availability of data and materials
Please contact author for data requests.
WY designed and performed most of electrophysiological recordings; YW, YHL, and ST performed behavioral assays and immunohistochemistry; YX performed electrophysiological recordings and immunohistochemistry; AT and HG performed behavioral assays and analyzed data; HL and TK performed qPCR and genechip analyses; HSJ supervised the project. HSJ contributed to ideas, designed, and wrote the manuscript.
All animal procedures were approved by the Institutional Animal Care and Use Committee of Duke-NUS Medical School and SingHealth (IACUC #2010/SHS/590 and #2014/SHS/999).
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