Early intervention with fluoxetine reverses abnormalities in the serotonergic system and behavior of rats exposed prenatally to dexamethasone
Highlights
► Prenatal dexamethasone (DEX) treatment induced anxiety-related behaviors. ► Prenatal DEX treatment decreased 5-HT1A-R mRNA and 5-HT in the brain. ► Prenatal DEX treatment decreased BDNF protein in the mPFC and hippocampus. ► 3 weeks of postnatal fluoxetine treatment prevented the changes induced by DEX. ► 3 weeks of postnatal 8-OH-DPAT treatment prevented DEX-induced behavioral changes.
Introduction
The development of therapeutic strategies against anxiety disorders is a critical issue in psychiatry. Anxiety disorders may affect up to 20% of the population at some point in their lifetime (Leonardo and Hen, 2008). It is therefore important to understand the neurobiological mechanisms that underlie the development of anxiety disorders. The use of animal models is very helpful for this purpose because changes that occur in the brain during the development of diseases can be examined.
Increasing evidence shows that stress loading (Weinstock, 2008, Kinsella and Monk, 2009) or exposure to corticosteroids (Velíšek, 2005) during the prenatal period induces anxiety traits in human offspring. Similar findings have been reported in animal models of prenatal exposure to maternal stress (Owen et al., 2005, Weinstock, 2008) or corticosteroids (Welberg et al., 2001, Owen et al., 2005). These results indicate that excessive prenatal glucocorticoid exposure induces long-lasting changes in the brain and in later life results in emotional abnormalities, such as anxiety. In our laboratory, we showed that prenatal dexamethasone (DEX) exposure induced not only anxiety-related behaviors in adult male offspring but also earlier presenting abnormalities in the hypothalamic–pituitary–adrenal (HPA) axis during postnatal development (Nagano et al., 2008). Our results indicate that prenatal DEX exposure induces long-lasting changes in the brain before the emergence of an adult anxiety-like phenotype. On the other hand, adverse early postnatal environments—such as when the subject is neglected, undergoes maternal separations (Pryce et al., 2005), or undergoes neonatal handling—are also critical for the development of emotionality in mammals (Wakshlak and Weinstock, 1990, Pryce et al., 2005). Thus, both the prenatal and early postnatal environments are critical for emotional development.
Pharmacological and gene targeting studies have implicated the serotonergic system in the brain as a key system in anxiety disorders. Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are effective in the treatment of anxiety (Vaswani et al., 2003), and the 5-HT 1A receptor (5-HT1A-R) is one of the targets of non-benzodiazepine anti-anxiety drugs (Akimova et al., 2009). Currently, some 5-HT1A-R agonists, such as buspirone and tandospirone, are also in clinical use for anxiety (Ohno, 2011). In fact, some studies with 5-HT1A-R mutant mice have shown elevated anxiety levels in these animals (Heisler et al., 1998, Parks et al., 1998, Ramboz et al., 1998). Using conditional 5-HT1A-R knockout mice, it was shown that the expression of 5-HT1A-R in the brain during the early postnatal period is necessary for establishing normal anxiety-like behavior in adulthood (Gross et al., 2002). Moreover, early postnatal overexpression of 5-HT1A-R resulted in reduced anxiety-related behaviors in adulthood (Kusserow et al., 2004). However, the 5-HT2A-R seems to play an opposite role in the pathophysiology of anxiety. A 5-HT2A-R antagonist showed anxiolytic activity (Kehne et al., 1996), and 5-HT2A-R deletion reduced anxiety in mice (Weisstaub et al., 2006). These lines of evidence suggest that 5-HT signaling, especially via the 5-HT1A-R and the 5-HT2A-R, is important in anxiety.
Although studies on mutant mice have shown the importance of 5-HT signaling in the brain (Heisler et al., 1998, Parks et al., 1998, Ramboz et al., 1998, Gross et al., 2002, Kusserow et al., 2004, Weisstaub et al., 2006), these have not clarified the brain regions responsible for anxiety. The medial prefrontal cortex (mPFC) is a brain region that expresses 5-HT1A-Rs and controls the firing of 5-HT neurons in the raphe nucleus (RN) (Sharp et al., 2007). It is also an important structure in executive function and the regulation of emotional behaviors including anxiety (Evans et al., 2006, Moghaddam and Homayoun, 2008). Another report showed that the mPFC volume is decreased by corticosteroid treatment and chronic stress (Cerqueira et al., 2005, Cerqueira et al., 2007). The hippocampus is a region to which some neurons of the RN send their projections (McKenna and Vertes, 2001), and a role of the hippocampus in anxiety has also been described in several studies. For example, intrahippocampal microinfusions of a 5-HT1A-R agonist have been shown to be anxiolytic (Engin and Treit, 2007). Moreover, suggesting a mechanism for the effects of early adverse experiences, corticosteroid exposure was shown to induce neuronal loss in the hippocampus (Duksal et al., 2009), a region in which many glucocorticoid receptors are expressed (Han et al., 2005).
In the present study, we focused on the serotonergic system and examined the effects of prenatal DEX exposure on the serotonergic system in the mPFC and hippocampus, as these are regions that seem to be involved in anxiety. To examine whether early modulation of the serotonergic system reverses molecular and behavioral abnormalities induced by DEX exposure, we investigated the effects of early postnatal treatment with fluoxetine (FLX), an SSRI commonly used for anxiety.
Section snippets
Animals
Sprague–Dawley female rats at gestational day (GD) 14 were obtained from Japan SLC (Japan) and housed individually. All rats were kept at a controlled temperature (22 °C ± 1 °C) and under a regular light/dark cycle (lights on from 06:00 to 20:00) with free access to food and water. Only male offspring underwent the examinations. All experiments were conducted in accordance with the National Institute of Health guidelines for the care and use of laboratory animals and were approved by the
Prenatal DEX treatment disturbed 5-HT signaling in the brain at PW4
Prenatal DEX treatment suppressed 5-HT1A-R mRNA expression in the mPFC at PW4 (79.5 ± 3.3% of the control, p < 0.01; Fig. 1a). Although the 5-HT1A-R mRNA expression concentration of the control offspring was much higher in the hippocampus (dorsal: 5.1 ± 0.6 × 106 molecules/μg total RNA, ventral: 7.9 ± 0.5 × 106 molecules/μg total RNA) than in the mPFC (1.3 ± 0.1 × 106 molecules/μg total RNA), the expression concentration in the hippocampus was unaffected by the prenatal DEX treatment (dorsal:
Discussion
The present study clearly shows that early postnatal FLX treatment ameliorates anxiety-related behaviors in adulthood induced by prenatal DEX treatment, and this effect is associated with the partial restoration of changes in the central serotonergic system. Furthermore, the 5-HT1A-R full agonist 8-OH-DPAT also suppressed anxiety-like behaviors to a similar degree as FLX in DEX-exposed offspring at adulthood.
In the present study, all of the pregnant rats were purchased at GD14 and the treatment
Acknowledgments
This study was supported by Japan Science and Technology Agency, Core Research for Evolutional Science and Technology (to M.N. and H.S.) and the Grants-in-Aid for Science Research (C) (project nos. 21591335 (to M.N.) and 22590249 (to H.S.)) and a grant (S0801035 to H.S.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors have no conflict of interest in this study.
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