Recent studies show that neuronal and glial plasticity are important for therapeutic action of antidepressants. We previously reported that antidepressants increase glial cell line-derived neurotrophic factor (GDNF) production in rat C6 glioma cells (C6 cells). Here, we found that amitriptyline, a tricyclic antidepressant, increased both GDNF mRNA expression and release, which were selectively and completely inhibited by mitogen-activated protein kinase kinase inhibitors. Indeed, treatment of amitriptyline rapidly increased extracellular signal-regulated kinase (ERK) activity, as well as p38 mitogen-activated protein kinase and c-Jun NH 2 -terminal kinase activities. Furthermore, different classes of antidepressants also rapidly increased ERK activity. The extent of acute ERK activation and GDNF release were significantly correlated to each other in individual antidepressants, suggesting an important role of acute ERK activation in GDNF production. Furthermore, antidepressants increased the acute ERK activation and GDNF mRNA expression in normal human astrocytes as well as C6 cells. Although 5-hydroxytryptamine (serotonin) (5-HT), but not noradrenaline or dopamine, increased ERK activation and GDNF release via 5-HT 2A receptors, ketanserin, a 5-HT 2A receptor antagonist, did not have any effect on the amitriptyline-induced ERK activation. Thus, GDNF production by amitriptyline was independent of monoamine. Both of the amitriptyline-induced ERK activation and GDNF mRNA expression were blocked by genistein, a general protein tyrosine kinase (PTK) inhibitor. Actually, we found that amitriptyline acutely increased phosphorylation levels of several phosphotyrosine-containing proteins. Taken together, these findings indicate that ERK activation through PTK regulates antidepressant-induced GDNF production and that the GDNF production in glial cells may be a novel action of the antidepressant, which is independent of monoamine.Major depression is a common and severe illness affecting a large number of individuals during their lifetime, and it is primarily treated with antidepressants. Most of the antidepressants are known to inhibit 5-hydroxytryptamine (serotonin, 5-HT) and/or noradrenaline (NA) reuptake; however, the efficacy of these antidepressants cannot be solely explained by their actions on the monoaminergic system. The molecular and cellular adaptations that underlie the therapeutic action of antidepressants have remained obscure. ; JNK, c-Jun NH 2 -terminal protein kinase; LDH, lactate dehydrogenase; ANOVA, analysis of variance; PLSD, protected least significant difference; HSD, honestly significant difference; MEK, mitogen-activated protein kinase kinase; PKA, protein kinase A; PKC, protein kinase C; DA, dopamine; PI3K, phosphoinositide-3 kinase; SP600125, anthra(1,9-cd)pyrazol-6(2H)-one.

Brain imaging studies have revealed that both the hippocampus and prefrontal cortex undergo selective volume reduction in major depressive disorder (MDD).2 One of the most consistent findings, associated with the volume reduction, in postmortem studies of MDD is a decrease in the density and number of glia in several cortical areas (1). The decreases in glial density are accompanied by a reduction of astrocytic markers, such as glial fibrillary acidic protein and glutamine synthetase (1, 2), thus suggesting that glia, especially astrocytes might be involved in the pathophysiology of MDD.One of the major role of astrocytes is the production of neurotrophic factors, such as nerve growth factor (NGF), brainderived neurotrophic factor, fibroblast growth factor (FGF), and glial cell line-derived neurotrophic factor (GDNF), which support neurogenesis, gliogenesis, development, plasticity, and survival (3).GDNF, a member of the transforming growth factor-␤ superfamily, was originally purified from a rat glial cell line supernatant as a trophic factor for midbrain dopamine neurons and was later found to have pronounced effects on other neuronal populations and glia (4). GDNF improves cognitive function (5, 6) while also inhibiting drug-induced dependence (7). These results suggest that GDNF plays a crucial role in not only neuronal development but also neuronal and glial plasticity in higher-ordered brain function.A growing body of evidence suggests that GDNF as well as brain-derived neurotrophic factor is involved in the pathophysiology of . GDNF has been shown to decrease in the peripheral blood of patients with MDD (8). In addition, the decreased blood level of GDNF in MDD has been reported to increase after antidepressant treatment (13). We have also previously shown that antidepressants increase the GDNF production in C6 glioma cells (C6 cells), rat astrocytes, and normal human astrocytes (NHA) (14,15). Treatment with antidepressants alters the GDNF levels in rodents in vivo and glial cell culture in vitro (16 -18). These findings suggest that an increase of GDNF production may be involved in the therapeutic effect *

We previously reported that serotonin (5‐HT) increased glial cell line‐derived neurotrophic factor (GDNF) release in a 5‐HT2 receptor (5‐HT2R) and mitogen‐activated protein kinase kinase/extracellular signal‐related kinase (MEK/ERK)‐dependent manner in rat C6 glioma cells (C6 cells), a model of astrocytes. We herein found that 5‐HT‐induced rapid ERK phosphorylation was blocked by 5‐HT2R antagonists in C6 cells. We therefore examined 5‐HT‐induced ERK phosphorylation to reveal the mechanism of 5‐HT‐induced GDNF mRNA expression. As 5‐HT‐induced ERK phosphorylation was blocked by inhibitors for Gαq/11 and fibroblast growth factor receptor (FGFR), but not for second messengers downstream of Gαq/11, 5‐HT2R‐mediated FGFR transactivation was suggested to be involved in the ERK phosphorylation. Although FGFR1 and 2 were functionally expressed in C6 cells, 5‐HT selectively phosphorylated FGFR2. Indeed, small interfering RNA for FGFR2, but not for FGFR1, blocked 5‐HT‐induced ERK phosphorylation. As Src family tyrosine kinase inhibitors and microtubule depolymerizing agents blocked 5‐HT‐induced FGFR2 phosphorylation, Src family tyrosine kinase and stabilized microtubules were suggested to act upstream of FGFR2. Finally, 5‐HT‐induced GDNF mRNA expression was also inhibited by the blockade of 5‐HT2R, FGFR, and Src family tyrosine kinase. In conclusion, our findings suggest that 5‐HT induces GDNF mRNA expression via 5‐HT2R‐mediated FGFR2 transactivation in C6 cells.

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