The effects of sildenafil on neuroplasticity in human neuroblastoma cells

Abstract

The antidepressant treatment of depression, currently the most debilitating psychiatric disorder, is plagued by delayed onset of action, troublesome side-effects and treatment resistance. However, a comprehensive understanding of the biological basis and treatment of depression remains elusive, prompting extensive ongoing research. The neuroplasticity hypothesis of depression has gained support from various lines of experimental and clinical evidence, whereas chronic antidepressant treatments reverse impaired neuroplasticity. The NO/cGMP pathway is believed to play an important role in the dysregulated neuroplasticity and has been a target for novel antidepressant strategies. Our laboratory recently demonstrated antidepressant-like effects of the phosphodiesterase type 5 (POE5) inhibitor sildenafil when combined with the antimuscarinic drug atropine in rats. Unpublished data suggested that sildenafil may up-regulate genes encoding for the expression of anti-apoptotic proteins in vitro. Therefore, the primary study objective was to investigate the effects of POE5 inhibitors and other modulators of the NO/cGMP pathway on neuroplasticity. The human neuroblastoma (SH-SY5Y) and non-neuronal Chinese hamster ovary (CHO-K1) cell lines were subjected to various biological stressors associated with the neuropathology of major depression, including glutamate-induced excitotoxicity, H2O2-induced oxidative stress and serum deprivation. The latter was selected as optimal stressor in SH-SY5Y cells where after they were incubated for 24 hours with the antidepressat drugs imipramine, f1uoxetine and tianeptine, the mood stabilizer lithium, the POE5 inhibitors sildenafil and tadalafil, the POE4 inhibitor, rolipram and the cGMP analogue db-cGMP under conditions of serum deprivation. Also, the incubation with sildenafil and db-cGMP was performed with and without the soluble guanylate cyclase inhibitor (ODQ) or the protein kinase G (PKG) inhibitor (RP-PETcGMPS). Thereafter cell viability was measured with the MTT-and Trypan blue assays and ONA repair capacity was measured with the comet assay. The results indicated that sildenafil exerts a protective effect against oxidative stress, as measured in the MTT assay. This property was shared by lithium, but not by db-cGMP or the antidepressants. In addition, ODQ and RP-PET-cGMPS reversed the protective effect of sildenafil, but with ODQ having a significantly greater effect. Results from the comet assay indicate that all antidepressant drugs, PDE inhibitors and db-cGMP significantly increased DNA repair capacity of SH-SY5Y cells. ODQ and RP-PET-cGMPS reversed the enhancing effects of sildenafil and db-cGMP on DNA repair capacity. Main conclusions are that 24 hour serum deprivation of human neuroblastoma (SHSY5Y) cells provides a suitable in vitro biological stressor. Serum deprivation in combination with DNA repair capacity as biological marker of oxidative stress, exhibit appropriate predictive validity in evaluating the effects of antidepressant drugs on neuroplasticity.. Sildenafil, but not tadalafil, may possess a unique protective property against oxidative stress to increase mitochondrial function, not shared by classical antidepressants. PDE5 inhibitors share the protective property of classical antidepressants to enhance DNA repair capacity under conditions of oxidative stress. The mechanism whereby the PDE5 inhibitors exert their protective property to enhance DNA repair capacity, involve increasing levels of cGMP and is PKG mediated. Finally, this study suggests that neuroprotective effects may contribute to the antidepressant-like activity of PDE5 inhibitors as observed in animal studies.