Depression-like behaviors in tree shrews and comparison of the effects of treatment with fluoxetine and carbetocin

• Social withdrawal, anhedonia and learned helplessness were developed in tree shrews.
• Both fluoxetine and carbetocin treatment could reverse learned helplessness.
• Both fluoxetine and carbetocin treatment could elevate sucrose preference and improved social interaction behaviors.
• Carbetocin might be a candidate antidepressant with potential applications in humans.

Tree shrews, a species phylogenetically close to primates, are regarded as a suitable and naturalistic animal model for depression studies. However, psychological symptoms that are essential for depression diagnosis and treatment, such as helplessness and social withdrawal, have not been studied in this model. Therefore, in this study, we first investigated learned helplessness, social interaction and sucrose preference induced by two chronic stress paradigms: uncontrollable foot shocks (1-week foot shocks) and multiple unpredictable stimuli (1-week foot shocks and 3-week unpredictable stressors) in tree shrews. Our results showed that uncontrollable foot shocks could only induce learned helplessness in animals; whereas animals treated with multiple unpredictable stimuli exhibited more depression-like behaviors including social withdrawal, anhedonia and learned helplessness. These findings suggested that multiple unpredictable stimuli could effectively induce various depression-like behaviors in tree shrews. More importantly, we compared the antidepressant effects of fluoxetine and carbetocin, a long-acting oxytocin analog, on specific depression-like behaviors. Our present data displayed that, compared with fluoxetine, carbetocin was also effective in reversing learned helplessness, elevating sucrose preference and improving social interaction behaviors in depression-like animals. Therefore, carbetocin might be a potential antidepressant with applications in humans.