Exposure to early life stress regulates Bdnf expression in SERT mutant rats in an anatomically selective fashion.

Although the causes of psychiatric disorders are not fully understood, it is well established that mental illness originates from the interaction between genetic and environmental factors. In this regard, compelling evidence demonstrates that depression can be the consequence of altered, and often maladaptive, response to adversities during pre- and early post-natal life. In this study, we investigated the impact of chronic maternal separation (MS) on the expression of the neurotrophin brain-derived neurotrophic factor (BDNF) in serotonin transporter (SERT) knockout rats in the ventral and dorsal hippocampus as well as the ventromedial and dorsomedial prefrontal cortex (PFC). We found that both SERT deletion and the MS led to an overall reduction in Bdnf expression in the ventral hippocampus and the ventromedial PFC, whereas in the dorsal hippocampus and in the dorsomedial PFC, we observed a significant increase in the neurotrophin gene expression after MS exposure, specifically in the heterozygous SERT rats. In summary, we show that the modulation of Bdnf expression in SERT mutant rats exposed to MS reflects the complex functional consequences of this gene-environment interaction with a clear distinction between the ventral and the dorsal subfields of the hippocampus and of the PFC. Early life stress differently affects the expression of Bdnf in an anatomically distinct manner as a function of SERT genotype. Specifically, both SERT deletion and the maternal separation (MS) led to an overall reduction in Bdnf expression in the ventral hippocampus and in the ventromedial prefrontal cortex, whereas in the dorsal hippocampus and in the dorsomedial prefrontal cortex, we observed a significant increase in the neurotrophin gene expression after MS exposure specifically in the heterozygous SERT rats. We think that these findings may provide novel cues for modulating neurotrophin function, which is dys-regulated in several psychiatric conditions.

Constitutive and LPS-regulated expression of interleukin-18 receptor beta variants in the mouse brain.

Interleukin (IL)-18 is a pro-inflammatory cytokine that is proposed to be involved in physiological as well as pathological conditions in the adult brain. IL-18 acts through a heterodimer receptor comprised of a subunit alpha (IL-18Rα) required for binding, and a subunit beta (IL-18Rß) necessary for activation of signal transduction. We recently demonstrated that the canonical alpha binding chain, and its putative decoy isoform, are expressed in the mouse central nervous system (CNS) suggesting that IL-18 may act on the brain by directly binding its receptor. Considering that the co-expression of the beta chain seems to be required to generate a functional receptor and, a short variant of this chain has been described in rat and human brain, in this study we have extended our investigation to IL-18Rß in mouse. Using a multi-methodological approach we found that: (1) a short splice variant of IL-18Rß was expressed in the CNS even if at lower levels compared to the full-length IL-18Rß variants, (2) the canonical IL-18Rß is expressed in the CNS particularly in areas and nuclei belonging to the limbic system as previously observed for IL-18Rα and finally (3) we have also demonstrated that both IL-18Rß isoforms are up-regulated in different brain areas three hours after a single lipopolysaccharide (LPS) injection suggesting that IL-18Rß in the CNS might be involved in mediating the endocrine and behavioral effects of LPS. Our data highlight the considerable complexity of the IL-18 regulation activity in the mouse brain and further support an important central role for IL-18.

Early life stress and serotonin transporter gene variation interact to affect the transcription of the glucocorticoid and mineralocorticoid receptors, and the co-chaperone FKBP5, in the adult rat brain.

The short allelic variant of the serotonin transporter (5-HTT) promoter-linked polymorphic region (5-HTTLPR) has been associated with the etiology of major depression by interaction with early life stress (ELS). A frequently observed endophenotype in depression is the abnormal regulation of levels of stress hormones such as glucocorticoids. It is hypothesized that altered central glucocorticoid influence on stress-related behavior and memory processes could underlie the depressogenic interaction of 5-HTTLPR and ELS. One possible mechanism could be the altered expression of the genes encoding the glucocorticoid and mineralocorticoid receptors (GR, MR) and their inhibitory regulator FK506-binding protein 51 (FKBP5) in stress-related forebrain areas. To test this notion, we exposed heterozygous (5-HTT(+/-)) and homozygous (5-HTT(-/-)) serotonin transporter knockout rats and their wildtype littermates (5-HTT(+/+)) to daily 3 h maternal separations from postnatal day 2 to 14. In the medial prefrontal cortex (mPFC) and hippocampus of the adult male offspring, we found that GR, MR, and FKBP5 mRNA levels were affected by ELS × 5-HTT genotype interaction. Specifically, 5-HTT(+/+) rats exposed to ELS showed decreased GR and FKBP5 mRNA in the dorsal and ventral mPFC, respectively. In contrast, 5-HTT(+/-) rats showed increased MR mRNA levels in the hippocampus and 5-HTT(-/-) rats showed increased FKBP5 mRNA in the ventral mPFC after ELS exposure. These findings indicate that 5-HTT genotype determines the specific adaptation of GR, MR, and FKBP5 expression in response to early life adversity. Therefore, altered extra-hypothalamic glucocorticoid signaling should be considered to play a role in the depressogenic interaction of ELS and 5-HTTLPR.

Reduced neuroplasticity in aged rats: a role for the neurotrophin brain-derived neurotrophic factor.

Aging is a physiological process characterized by a significant reduction of neuronal plasticity that might contribute to the functional defects observed in old subjects. Even if the neurobiological mechanisms that contribute to such impairment remain largely unknown, a role for neurotrophic molecules, such as the neurotrophin brain-derived neurotrophic factor (BDNF), has been postulated. On this basis, the purpose of this study was to provide a detailed investigation of the BDNF system, at transcriptional and translational levels, in the ventral and dorsal hippocampus and in the prefrontal cortex of middle-aged and old rats, compared with in adult animals. The expression of major players in BDNF regulation and response, including the transcription factors, calcium-responsive transcription factor, cyclic adenosine monophosphate (cAMP) responsive element-binding protein (CREB), and neuronal Per Arnt Sim (PAS) domain protein 4, and the high-affinity receptor tropomyosin receptor kinase B (TrkB), was also analyzed. Our results demonstrate that the BDNF system is affected at different levels in aged rats with global impairment including reduced transcription, impaired protein synthesis and processing, and decreased activation of the TrkB receptors. These modifications might contribute to the cognitive deficits associated with aging and suggest that pharmacological strategies aimed at restoring reduced neurotrophism might be useful to counteract age-related cognitive decline.

Glucocorticoid receptor and FKBP5 expression is altered following exposure to chronic stress: modulation by antidepressant treatment.

Major depression is thought to originate from the interaction between susceptibility genes and adverse environmental events, in particular stress. The hypothalamus-pituitary-adrenal (HPA) axis is the major system involved in stress response and its dysregulation is an important element in the pathogenesis of depression. The stress response is therefore finely tuned through a series of mechanisms that control the trafficking of glucocorticoid receptors (GRs) to the nucleus, including binding to the chaperone protein FKBP5 and receptor phosphorylation, suggesting that these elements may also be affected under pathologic conditions. On these bases, we investigated FKBP5 and GR expression and phosphorylation in the hippocampus (ventral and dorsal) and in the prefrontal cortex of rats exposed to chronic mild stress (CMS) and we analyzed the effect of a concomitant antidepressant treatment. We found that animals exposed to CMS show increased expression of FKBP5 as well as enhanced cytoplasmic levels of GR, primarily in ventral hippocampus and prefrontal cortex. Chronic treatment with the antidepressant duloxetine is able to normalize such alterations, mainly in the prefrontal cortex. Moreover, we demonstrate that CMS-induced alterations of GR trafficking and transcription may be sustained by changes in receptor phosphorylation, which are also modulated by pharmacological intervention. In summary, while GR-related changes after CMS might be relevant for the depressive phenotype, the ability of antidepressant treatment to correct some of these alterations may contribute to the normalization of HPA axis dysfunctions associated with stress-related disorders.

Lack of serotonin transporter alters BDNF expression in the rat brain during early postnatal development.

It is well established that alterations of the serotoninergic system may contribute to the pathophysiology of mood disorders. Accordingly, it has been demonstrated that genetic deletion of the serotonin transporter (SERT) in rodents leads to an anxious and depressive phenotype, which is also associated with reduced neuronal plasticity. Indeed, we have demonstrated that adult SERT(-/-) animals show decreased brain-derived neurotrophic factor (BDNF) expression, as well as reduced levels of transcription factor regulating the neurotrophin transcription. While these changes may represent long-term consequences of impaired function of the transporter during development, no information exists with respect to the developmental profile of such changes. Using SERT(-/-) rats at different ages, we found that the impairment in neuroplasticity originates early in development and worsens during the first 3 weeks of life. Indeed, we observed that BDNF expression was reduced at birth and that the magnitude of these changes became more pronounced starting from PND21, being sustained by epigenetic mechanisms as well as alterations in the expression of specific transcription factors, including Npas4 and CaRF. These results suggest that an impairment of SERT may affect BDNF expression throughout postnatal development. These early changes may increase stress susceptibility during critical windows of brain maturation, which may eventually lead to the heightened predisposition to mood disorders found in individual carrying genetic variants of the serotonin transporter.

Modulation of the inflammatory response in rats chronically treated with the antidepressant agomelatine.

Growing evidence suggests that the activation of the inflammatory/immune system contributes to depression pathogenesis, a hypothesis that might hold strong clinical implication. Indeed more than 30% of depressed patients fail to achieve remission, which poses the necessity to identify systems that may represent novel targets for medications. Accordingly, goal of this study was to evaluate the ability of the antidepressant agomelatine to modulate specific components of the immune response in the rat brain following an inflammatory challenge with lipopolysaccharide (LPS). To this aim, adult male rats were chronically treated with agomelatine before being acutely challenged with LPS 16 h after the last drug administration. Rats were sacrificed 2, 6, or 24h after the challenge and several components of the inflammatory response have been investigated by using real-time PCR or ELISA. We found that agomelatine significantly reduced the LPS-induced up-regulation of the pro-inflammatory cytokines interleukin-1ß and interleukin-6 in the rat brain as well as at peripheral level. At central level, these effects are associated to the inhibition of NF-κB translocation as well as to alterations of mechanisms responsible for microglia activation. In addition, we found that agomelatine was also able to alter the expression of enzymes related to the kynurenine pathway that are thought to represent important mediators to inflammation-related depression. These data disclose novel properties that may contribute to the therapeutic effect of agomelatine providing evidence for a crucial role of specific components of the immune/inflammatory system in the antidepressant response and thereby in depression etiopathology.

Stress-induced changes of hippocampal NMDA receptors: modulation by duloxetine treatment.

It is now well established that the glutamatergic system contributes to the pathophysiology of depression. Exposure to stress, a major precipitating factor for depression, enhances glutamate release that can contribute to structural abnormalities observed in the brain of depressed subjects. On the other hand, it has been demonstrated that NMDA antagonists, like ketamine, exert an antidepressant effect at preclinical and clinical levels. On these bases, the purpose of our study was to investigate whether chronic mild stress is associated with specific alterations of the NMDA receptor complex, in adult rats, and to establish whether concomitant antidepressant treatment could normalize such deficits. We found that chronic stress increases the expression of the obligatory GluN1 subunit, as well as of the accessory subunits GluN2A and GluN2B at transcriptional and translational levels, particularly in the ventral hippocampus. Concomitant treatment with the antidepressant duloxetine was able to normalize the increase of glutamatergic receptor subunit expression, and correct the changes in receptor phosphorylation produced by stress exposure. Our data suggest that prolonged stress, a condition that has etiologic relevance for depression, may enhance glutamate activity through post-synaptic mechanisms, by regulating NMDA receptors, and that antidepressants may in part normalize such changes. Our results provide support to the notion that antidepressants may exert their activity in the long-term also via modulation of the glutamatergic synapse.

Developmental influence of the serotonin transporter on the expression of npas4 and GABAergic markers: modulation by antidepressant treatment.

Alterations of the serotonergic system are involved in the pathophysiology of mood disorders and represent an important target for its pharmacological treatment. Genetic deletion of the serotonin transporter (SERT) in rodents leads to an anxious and depressive phenotype, and is associated with reduced neuronal plasticity as indicated by decreased brain-derived neurotrophic factor (Bdnf) expression levels. One of the transcription factors regulating Bdnf is the neuronal PAS domain protein 4 (Npas4), which regulates activity-dependent genes and neuroprotection, and has a critical role in the development of GABA synapses. On the basis of these premises, we investigated the expression of Npas4 and GABAergic markers in the hippocampus and prefrontal cortex of homozygous (SERT(-/-)) and heterozygous (SERT(+/-)) knockout rats, and analyzed the effect of long-term duloxetine treatment on the expression of these targets. We found that Npas4 expression was reduced in both the brain structures of adult SERT(+/-) and SERT(-/-) animals. This effect was already present in adolescent SERT(-/-), and could be mimicked by prenatal exposure to the antidepressant fluoxetine. Moreover, SERT(-/-) rats showed a strong impairment of the GABAergic system, as indicated by the reduction of several markers, including the vesicular transporter (Vgat), glutamic acid decarboxylase-67 (Gad67), the receptor subunit GABA A receptor, gamma 2 (GABA(A)-γ2), and calcium-binding proteins that label subgroups of the GABAergic neurons. Interestingly, chronic treatment with the antidepressant duloxetine was able to restore the physiological levels of Npas4 and GABAergic markers in SERT(-/-) rats, although some differences in the modulation of GABAergic genes exist between hippocampus and prefrontal cortex. Our results demonstrate that SERT knockout rats, an animal model of mood disorders, have reduced Npas4 expression that correlates with decreased expression of Bdnf exon I and IV. These changes lead to an impairment of the GABAergic system that may contribute to the anxious and depressive phenotype associated with inherited SERT downregulation.