Knockdown of Connexin43 in Astrocytes Is Involved in the Therapeutic Effect of Tricyclic Antidepressant Amitriptyline

Background The number of depressive patients is increasing, and it is one of the most common social problems in the world. In general, antidepressants exert therapeutic effect by the increase of monoamine concentration at synaptic cleft in central nervous system. It is known that antidepressants generally increase the amount of monoamine in the brain immediately after administration, but it takes several months for the therapeutic effect to be observed in patients with depression. Therefore, other mechanisms, which are independent of monoamine, are involved in antidepressant-induced response. Moreover, the therapeutic satisfaction is very low because antidepressant drugs are not effective for one-third of patients and the most effective drugs are different for each individual patient. Therefore, it is necessary to develop useful biomarkers to assess the therapeutic efficacy of antidepressants. Recently, it was reported that antidepressants exert therapeutic effects mediated by astrocytes, which are most abundant glia cells in the brain, without involvement of monoamines. Thus, it is suggested that astrocytes are central of the monoamine-independent mechanism by antidepressants, but the detailed mechanism is unknown. Connexin43 (Cx43) is highly expressed in astrocytes and contributes to not only intercellular molecular transportation but also intracellular signaling. It is demonstrated that Cx43 expression is reduced in the prefrontal cortex and hippocampus of depressive patients, suggesting that Cx43 may be involved in the pathology of depression. However, the significance of Cx43 reduction for the antidepressant effect is still uncovered. Aims & Objectives The current study examined the significance of downregulation of Cx43 in depression by using the animal model with depression and primary cultured astrocytes. Method Animal models with depression were prepared using ddY mice with chronic pain stress. Among three branches of sciatic nerve, common peroneal and tibial nerves were ligated with 6-0 silk suture and cleaved to prepare depression model mice induced by chronic pain. Forced swim test was used to assess depressed behavior. Primary cultured astrocytes were prepared from the cerebral cortex of neonatal Wistar rats. The expression of Cx43 was downregulated by RNA interference. The expression of mRNA and protein were measured by real-time PCR and Western blotting, respectively. Results In the animal model, Cx43 expression in hippocampus was negatively correlated with the degree of antidepressant effect of amitriptyline, tricyclic antidepressant, but not that depression-like symptom. In cultured astrocytes, downregulation of Cx43 potentiated the amitriptyline-induced brain-derived neurotrophic factor (BDNF) expression. This potentiation was significantly suppressed by the blockade of the downstream signal of lysophosphatidic acid (LPA)1/3 receptors and Src tyrosine kinase. Discussion & Conclusions The current study revealed that Cx43 expression is downregulated in depression model mice, which is correlated with the effect of amitriptyline. In addition, the knockdown of Cx43 potentiates amitriptyline-induced BDNF expression through the LPA1/3-mediated Src-extracellular signal-regulated kinase (ERK) signaling in primary cultured astrocytes. These results indicate the downregulation of Cx43 observed in depressive patients might contribute to therapeutic effect of antidepressants.