Naoto Kajitani

Antidepressant acts on astrocytes leading to an increase in the expression of neurotrophic/growth factors: differential regulation of FGF-2 by noradrenaline.

Recently, multiple neurotrophic/growth factors have been proposed to play an important role in the therapeutic action of antidepressants. In this study, we prepared astrocyte- and neuron-enriched cultures from the neonatal rat cortex, and examined the changes in neurotrophic/growth factor expression by antidepressant treatment using real-time PCR. Treatment with amitriptyline (a tricyclic antidepressant) significantly increased the expression of fibroblast growth factor-2 (FGF-2), brain-derived neurotrophic factor, vascular endothelial growth factor and glial cell line-derived neurotrophic factor mRNA with a different time course in astrocyte cultures, but not in neuron-enriched cultures. Only the expression of FGF-2 was higher in astrocyte cultures than in neuron-enriched cultures. We focused on the FGF-2 production in astrocytes. Several different classes of antidepressants, but not non-antidepressants, also induced FGF-2 mRNA expression. Noradrenaline (NA) is known to induce FGF-2 expression in astrocyte cultures, as with antidepressants. Therefore, we also assessed the mechanism of NA-induced FGF-2 expression, in comparison to amitriptyline. NA increased the FGF-2 mRNA expression via α1 and β-adrenergic receptors; however, the amitriptyline-induced FGF-2 mRNA expression was not mediated via these adrenergic receptors. Furthermore, the amitriptyline-induced FGF-2 mRNA expression was completely blocked by cycloheximide (an inhibitor of protein synthesis), while the NA-induced FGF-2 mRNA was not. These data suggest that the regulation of FGF-2 mRNA expression by amitriptyline was distinct from that by NA. Taken together, antidepressant-stimulated astrocytes may therefore be important mediators that produce several neurotrophic/growth factors, especially FGF-2, through a monoamine-independent and a de novo protein synthesis-dependent mechanism.

Tricyclic Antidepressant Amitriptyline Activates Fibroblast Growth Factor Receptor Signaling in Glial Cells INVOLVEMENT IN GLIAL CELL LINE-DERIVED NEUROTROPHIC FACTOR PRODUCTION

Recently, both clinical and animal studies demonstrated neuronal and glial plasticity to be important for the therapeutic action of antidepressants. Antidepressants increase glial cell line-derived neurotrophic factor (GDNF) production through monoamine-independent protein-tyrosine kinase, extracellular signal-regulated kinase (ERK), and cAMP responsive element-binding protein (CREB) activation in glial cells (Hisaoka, K., Takebayashi, M., Tsuchioka, M., Maeda, N., Nakata, Y., and Yamawaki, S. (2007) J. Pharmacol. Exp. Ther. 321, 148–157; Hisaoka, K., Maeda, N., Tsuchioka, M., and Takebayashi, M. (2008) Brain Res. 1196, 53–58). This study clarifies the type of tyrosine kinase and mechanism of antidepressant-induced GDNF production in C6 glioma cells and normal human astrocytes. The amitriptyline (a tricyclic antidepressant)-induced ERK activation was specifically and completely inhibited by fibroblast growth factor receptor (FGFR) tyrosine kinase inhibitors and siRNA for FGFR1 and -2. Treatment with amitriptyline or several different classes of antidepressants, but not non-antidepressants, acutely increased the phosphorylation of FGFRs and FGFR substrate 2α (FRS2α). Amitriptyline-induced CREB phosphorylation and GDNF production were blocked by FGFR-tyrosine kinase inhibitors. Therefore, antidepressants activate the FGFR/FRS2α/ERK/CREB signaling cascade, thus resulting in GDNF production. Furthermore, we attempted to elucidate how antidepressants activate FGFR signaling. The effect of amitriptyline was inhibited by heparin, non-permeant FGF-2 neutralizing antibodies, and matrix metalloproteinase (MMP) inhibitors. Serotonin (5-HT) also increased GDNF production through FGFR2 (Tsuchioka, M., Takebayashi, M., Hisaoka, K., Maeda, N., and Nakata, Y. (2008) J. Neurochem. 106, 244–257); however, the effect of 5-HT was not inhibited by heparin and MMP inhibitors. These results suggest that amitriptyline-induced FGFR activation might occur through an extracellular pathway, in contrast to that of 5-HT. The current data show that amitriptyline-induced FGFR activation might occur by the MMP-dependent shedding of FGFR ligands, such as FGF-2, thus resulting in GDNF production.

Amitriptyline up-regulates connexin43-gap junction in rat cultured cortical astrocytes via activation of the p38 and c-Fos/AP-1 signalling pathway

Intercellular communication via gap junctions, comprised of connexin (Cx) proteins, allow for communication between astrocytes, which in turn is crucial for maintaining CNS homeostasis. The expression of Cx43 is decreased in post‐mortem brains from patients with major depression. A potentially novel mechanism of tricyclic antidepressants is to increase the expression and functioning of gap junctions in astrocytes.

Tricyclic Antidepressant Amitriptyline-induced Glial Cell Line-derived Neurotrophic Factor Production Involves Pertussis Toxin-sensitive Gαi/o Activation in Astroglial Cells

Background: A significant non-neural, monoamine-independent mechanism underlies the antidepressant effect of amitriptyline. Results: Amitriptyline-evoked GDNF production is mediated by pertussis toxin (PTX)-sensitive Gαi/o. Conclusion: PTX-sensitive Gαi/o activation is critical for the cascade that underpins the biological effect of amitriptyline. Significance: Further elaboration of the intracellular mechanism of amitriptyline could lead to greater understanding of depression and novel antidepressant treatments. Further elaborating the mechanism of antidepressants, beyond modulation of monoaminergic neurotransmission, this study sought to elucidate the mechanism of amitriptyline-induced production of glial cell line-derived neurotrophic factor (GDNF) in astroglial cells. Previous studies demonstrated that an amitriptyline-evoked matrix metalloproteinase (MMP)/FGF receptor (FGFR)/FGFR substrate 2α (FRS2α)/ERK cascade is crucial for GDNF production, but how amitriptyline triggers this cascade remains unknown. MMP is activated by intracellular mediators such as G proteins, and this study sought to clarify the involvement of G protein signaling in amitriptyline-evoked GDNF production in rat C6 astroglial cells (C6 cells), primary cultured rat astrocytes, and normal human astrocytes. Amitriptyline-evoked GDNF mRNA expression and release were inhibited by pertussis toxin (PTX), a Gαi/o inhibitor, but not by NF449, a Gαs inhibitor, or YM-254890, a Gαq inhibitor. The activation of the GDNF production cascade (FGFR/FRS2α/ERK) was also inhibited by PTX. Deletion of Gαο1 and Gαi3 by RNAi demonstrated that these G proteins play important roles in amitriptyline signaling. G protein activation was directly analyzed by electrical impedance-based biosensors (CellKeyTM assay), using a label-free (without use of fluorescent proteins/probes or radioisotopes) and real time approach. Amitriptyline increased impedance, indicating Gαi/o activation that was suppressed by PTX treatment. The impedance evoked by amitriptyline was not affected by inhibitors of the GDNF production cascade. Furthermore, FGF2 treatment did not elicit any effect on impedance, indicating that amitriptyline targets PTX-sensitive Gαi/o upstream of the MMP/FGFR/FRS2α/ERK cascade. These results suggest novel targeting for the development of antidepressants.

Amitriptyline induces brain-derived neurotrophic factor (BDNF) mRNA expression through ERK-dependent modulation of multiple BDNF mRNA variants in primary cultured rat cortical astrocytes and microglia.

A significant role of brain-derived neurotrophic factor (BDNF) has been previously implicated in the therapeutic effect of antidepressants. To ascertain the contribution of specific cell types in the brain that produce BDNF following antidepressant treatment, the effects of the tricyclic antidepressant amitriptyline on rat primary neuronal, astrocytic and microglial cortical cultures were examined. Amitriptyline increased the expression of BDNF mRNA in astrocytic and microglial cultures but not neuronal cultures. Antidepressants with distinct mechanisms of action, such as clomipramine, duloxetine and fluvoxamine, also increased BDNF mRNA expression in astrocytic and microglial cultures. There are multiple BDNF mRNA variants (exon I, IIA, IV and VI) expressed in astrocytes and microglia and the variant induced by antidepressants has yet to be elaborated. Treatment with antidepressants increased the expression of exon I, IV and VI in astrocyte and microglia. Clomipramine alone significantly upregulated expression of exon IIA. The amitriptyline-induced expression of both total and individual BDNF mRNA variants (exon I, IV and VI) were blocked by MEK inhibitor U0126, indicating MEK/ERK signaling is required in the expression of BDNF. These findings indicate that non-neural cells are a significant target of antidepressants and further support the contention that glial production of BDNF is crucial role in the therapeutic effect of antidepressants. The current data suggest that targeting of glial function could lead to the development of antidepressants with a truly novel mechanism of action.

Fibroblast growth factor 2 mRNA expression evoked by amitriptyline involves extracellular signal-regulated kinase-dependent early growth response 1 production in rat primary cultured astrocytes

Recently, we demonstrated that several antidepressants including amitriptyline increased fibroblast growth factor 2 (FGF2) mRNA expression slowly over 24 h through de novo protein synthesis in rat primary cultured astrocytes. This study defined the signaling cascade that mediates amitriptyline‐induced FGF2 production in rat primary cultured astrocytes. Amitriptyline treatment significantly increased early growth response 1 (EGR1), a transcription factor known to regulate FGF2 expression. Knockdown of EGR1 using siRNA blocked amitriptyline‐evoked FGF2 mRNA expression. Treatment with several different classes of antidepressants leads to expression of EGR1 mRNA as well as FGF2 mRNA. These results confirm that EGR1 production is likely to be indispensable for the amitriptyline‐evoked FGF2 mRNA expression. Signal transduction inhibitors were used to elaborate the cellular signaling cascade that leads to EGR1‐mediated FGF2 expression following amitriptyline treatment. Amitriptyline‐evoked EGR1‐mediated FGF2 mRNA expression was blocked by mitogen‐activated protein kinase/extracellular signal‐regulated kinase kinase (MEK)1/2 inhibitor. Furthermore, extracellular signal‐regulated kinase/EGR1‐mediated FGF2 mRNA expression evoked by amitriptyline was blocked by inhibitors of the FGF receptor, epidermal growth factor receptor (EGFR), and matrix metalloproteinase. Taken together, these results demonstrate that amitriptyline increases FGF2 mRNA expression through a matrix metalloproteinase/receptor tyrosine kinases (RTK) (FGF receptor and EGFR)/extracellular signal‐regulated kinase/EGR1 signaling pathway in rat primary cultured astrocytes.

Tricyclic antidepressant amitriptyline indirectly increases the proliferation of adult dentate gyrus-derived neural precursors: an involvement of astrocytes.

Antidepressants increase the proliferation of neural precursors in adult dentate gyrus (DG), which is considered to be involved in the therapeutic action of antidepressants. However, the mechanism underlying it remains unclear. By using cultured adult rat DG-derived neural precursors (ADP), we have already shown that antidepressants have no direct effects on ADP. Therefore, antidepressants may increase the proliferation of neural precursors in adult DG via unknown indirect mechanism. We have also shown that amitriptyline (AMI), a tricyclic antidepressant, induces the expressions of GDNF, BDNF, FGF2 and VEGF, common neurogenic factors, in primary cultured astrocytes (PCA). These suggest that AMI-induced factors in astrocytes may increase the proliferation of neural precursors in adult DG. To test this hypothesis, we examined the effects of AMI-induced factors and conditioned medium (CM) from PCA treated with AMI on ADP proliferation. The effects of CM and factors on ADP proliferation were examined with BrdU immunocytochemistry. AMI had no effect on ADP proliferation, but AMI-treated CM increased it. The receptors of GDNF, BDNF and FGF2, but not VEGF, were expressed in ADP. FGF2 significantly increased ADP proliferation, but not BDNF and GDNF. In addition, both of a specific inhibitor of FGF receptors and anti-FGF2 antibody significantly counteracted the increasing effect of CM on ADP proliferation. In addition, FGF2 in brain is mainly derived from astrocytes that are key components of the neurogenic niches in adult DG. These suggest that AMI may increase ADP proliferation indirectly via PCA and that FGF2 may a potential candidate to mediate such an indirect effect of AMI on ADP proliferation via astrocytes.

Electroconvulsive seizure induces thrombospondin-1 in the adult rat hippocampus

Synaptic dysfunction has recently gained attention for its involvement in mood disorders. Electroconvulsive therapy (ECT) possibly plays a role in synaptic repair. However, the underlying mechanisms remain uncertain. Thrombospondin-1 (TSP-1), a member of the TSP family, is reported to be secreted by astrocytes and to regulate synaptogenesis. We investigated the effects of electroconvulsive seizure (ECS) on the expression of TSPs in the adult rat hippocampus. Single and repeated ECS significantly increased TSP-1 mRNA expression after 2h and returned to sham levels at 24h. Conversely, the TSP-2 and -4 mRNA levels did not change. Only repeated ECS induced TSP-1 proteins. ECS also induced glial fibrillary acidic protein (GFAP) expression. The GFAP expression occurred later than the TSP-1 mRNA expression following single ECS; however, it occurred earlier and was more persistent following repeated ECS. ECS had no effect on the α2δ-1 or neuroligin-1 expressions, both of which are TSP-1 receptors. Furthermore, chronic treatment with antidepressants did not induce the expression of TSP-1 or GFAP. These findings suggest that repeated ECS, but not chronic treatment with antidepressants, induces TSP-1 expression partially via the activation of astrocytes. Therefore, TSP-1 is possibly involved in the synaptogenic effects of ECS.

Identification of Lysophosphatidic Acid Receptor 1 in Astroglial Cells as a Target for Glial Cell Line-derived Neurotrophic Factor Expression Induced by Antidepressants

Preclinical and clinical evidence suggests that glial cell line-derived neurotrophic factor (GDNF) is important in the therapeutic effect of antidepressants. A previous study demonstrated that the tricyclic antidepressant amitriptyline induces Gαi/o activation, which leads to GDNF expression in astrocytes. However, the specific target expressed in astrocytes that mediates antidepressant-evoked Gαi/o activation has yet to be identified. Thus, the current study explored the possibility that antidepressant-induced Gαi/o activation depends on lysophosphatidic acid receptor 1 (LPAR1), a Gαi/o-coupled receptor. GDNF mRNA expression was examined using real-time PCR and Gαi/o activation was examined using the cell-based receptor assay system CellKeyTM in rat C6 astroglial cells and rat primary cultured astrocytes. LPAR1 antagonists blocked GDNF mRNA expression and Gαi/o activation evoked by various classes of antidepressants (amitriptyline, nortriptyline, mianserin, and fluoxetine). In addition, deletion of LPAR1 by RNAi suppressed amitriptyline-evoked GDNF mRNA expression. Treatment of astroglial cells with the endogenous LPAR agonist LPA increased GDNF mRNA expression through LPAR1, whereas treatment of primary cultured neurons with LPA failed to affect GDNF mRNA expression. Astrocytic GDNF expression evoked by either amitriptyline or LPA utilized, in part, transactivation of fibroblast growth factor receptor and a subsequent ERK cascade. The current results suggest that LPAR1 is a novel, specific target of antidepressants that leads to GDNF expression in astrocytes.

Altered Serum Levels of Matrix Metalloproteinase-2, -9 in Response to Electroconvulsive Therapy for Mood Disorders

Background: Inflammatory processes could underlie mood disorders. Matrix metalloproteinases (MMPs) and tissue inhibitors of MMPs (TIMP) are inflammation-related molecules. The current study sought an association between mood disorders and systemic levels of MMPs and TIMPs. Methods: Serum was obtained from patients with mood disorders (n=21) and patients with schizophrenia (n=13) scheduled to undergo electroconvulsive therapy. Serum was also obtained from healthy controls (n=40). Clinical symptoms were assessed by the Hamilton Rating Score for Depression and the Brief Psychiatric Rating Scale. Serum levels of MMPs and TIMPs were quantified by ELISA. Results: The serum levels of MMP-2 in mood disorder patients, but not in schizophrenia patients, prior to the first electroconvulsive therapy session (baseline) was significantly lower than that of healthy controls. At baseline, levels of MMP-9 and TIMP-2, -1 were not different between patients with mood disorder and schizophrenia and healthy controls. After a course of electroconvulsive therapy, MMP-2 levels were significantly increased in mood disorder patients, but MMP-9 levels were significantly decreased in both mood disorder and schizophrenia patients. In mood disorder patients, there was a significant negative correlation between depressive symptoms and serum levels of MMP-2 and a positive correlation between depressive symptoms and MMP-9. In addition, alterations of serum levels of MMP-2 and MMP-9 were significantly correlated each other and were associated with certain depressive symptoms. Conclusion: A change in inflammatory homeostasis, as indicated by MMP-2 and MMP-9, could be related to mood disorders, and these markers appear to be sensitive to electroconvulsive therapy.