Selective Noradrenergic Activation of BDNF Translation by Mirtazapine

Antidepressants are known for their neurotrophic effects, particularly through the regulation of brain-derived neurotrophic factor (BDNF) expression. Mirtazapine, a tetracyclic noradrenergic and specific serotonergic antidepressant (NaSSA) has been observed to upregulate BDNF, though its underlying mechanism remains unclear. In this study, we used the human neuroblastoma SH-SY5Y cell line to investigate whether mirtazapine could enhance BDNF translation by modulating serotonin and/or norepinephrine and their receptors. A 1-h stimulation with 1 or 10 µM mirtazapine led to downregulation of serotonergic receptors 5HT1A, while increasing ADRA2A and ADRB2 receptors. Mirtazapine at 10 µM upregulated endogenous BDNF after 3h, but not 1h stimulation. To investigate the translation of major BDNF transcripts, we used chimeric BDNF-luciferase constructs with the untranslated 5'UTR exons I, IIc, IV, or VI, and the long version of the 3'UTR. Luciferase assays and Western blotting revealed that mirtazapine selectively enhanced exon-IIc-BDNF-long3'UTR-Luciferase translation. This increase was associated with norepinephrine release and was inhibited by blocking ADRA2A or ADRB2 adrenoceptors for the exon-IIc-BDNF-long3'UTR-Luciferase, and ADR2B for endogenous BDNF. These findings provide a new perspective on the critical role of the noradrenergic system in mediating mirtazapine's effects on BDNF translation. We propose a novel mechanism of action in which mirtazapine promotes norepinephrine release and noradrenergic responses by upregulating ADRA2A and ADRB2 while downregulating serotonergic receptors.