Depression‐related behavioural and neuroendocrine changes in the Spontaneously Diabetic Torii (SDT) fatty rat, an animal model of type 2 diabetes mellitus

: Depression is one of the most common psychiatric diseases and is commonly comorbid with type 1 or 2 diabetes mellitus (DM). However, the pathophysiology underlying the depressive state in DM remains poorly understood. Animal models are useful tools to investigate the association between depression and DM. In the present study we investigated whether the Spontaneously Diabetic Torii (SDT) fatty rat, a novel animal model of type 2 DM, shows depression-related features. We assessed depression-like behaviour, hyperactivation of the hypothalamic-pituitary-adrenal (HPA) axis, and neurotransmitter levels in the brain. Behaviour was evaluated using a forced swimming test, and the HPA axis was evaluated with changes in plasma corticosterone levels after a swimming stress exposure or dexamethasone challenge. In addition, serotonin (5-hydroxytryptamine; 5-HT), noradrenaline, glutamate and γ-aminobutyric acid (GABA) concentrations in the frontal cortex, hippocampus and brain stem were measured. In the forced swimming test, SDT fatty rats exhibited increased duration of immobility compared with control Sprague-Dawley (SD) rats. Moreover, basal corticosterone levels were significantly elevated in SDT fatty compared with control SD rats. However, there were no stress-induced increases or changes in dexamethasone-induced suppression of corticosterone in SDT fatty compared with control SD rats. Furthermore, there were significant changes in 5-HT concentrations in the prefrontal cortex, and in GABA and glutamate concentrations in the hippocampus in SDT fatty compared with controls. The results of the present study suggest that the SDT fatty rat may be an appropriate model for diabetes with comorbid depression associated with neurotransmitter impairments and aberrant basal HPA hyperactivity.