Tricyclic antidepressant amitriptyline activates fibroblast growth factor receptor (FGFR) signaling in glial cells: Involvement in glial cell line-derived neurotrophic factor (GDNF) 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 (PTK), extracellular signal-regulated kinase (ERK) and cAMP responsive element binding protein (CREB) activation in glial cells (Hisaoka et al., 2007, 2008). 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 non-antidepressants, acutely increased the phosphorylation of FGFRs and FGFR substrate 2 (FRS2. Amitriptyline-induced CREB phosphorylation and GDNF production were blocked by FGFR tyrosine kinase inhibitors. Therefore, antidepressants activate the FGFR/FRS2/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 et al., 2008); 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.