Major depressive disorder is a debilitating disease. Unfortunately, treatment with antidepressants (ADs) has limited therapeutic efficacy since resistance to AD is common. Research in this field is hampered by the lack of a reliable natural animal model of AD resistance. Depression resistance is related to various factors, including the attendance of cardiovascular risk factors and past depressive episodes. We aimed to design a rodent model of depression resistance to ADs, associating cardiovascular risk factors with repeated unpredicted chronic mild stress (UCMS). Male BALB/c mice were given either a regular (4% fat) or a high fat diet (45% fat) and subjected to two 7-week periods of UCMS separated by 6 weeks. From the second week of each UCMS procedure, vehicle or fluoxetine (10 mg/kg, i.p.) was administrated daily. The effects of the UCMS and fluoxetine in both diet conditions were assessed using physical (coat state and body weight) and behavioural tests (the reward maze test and the splash test). The results demonstrate that during the second procedure, UCMS induced behavioural changes, including coat state degradation, disturbances in self-care behaviour (splash test) and anhedonia (reward maze test) and these were reversed by fluoxetine in the regular diet condition. In contrast, the high-fat diet regimen prevented the AD fluoxetine from abolishing the UCMS-induced changes. In conclusion, by associating UCMS—an already validated animal model of depression—with high-fat diet regimen, we designed a naturalistic animal model of AD resistance related to a sub-nosographic clinical entity of depression.

As chronic stress and depression have become recognized as significant risk factors for peripheral vascular disease in patients with no prior history of vasculopathy, we interrogated this relationship utilizing an established mouse model of chronic stress/depressive symptoms from behavioral research. Male mice were exposed to 8 wk of unpredictable chronic mild stress (UCMS; e.g., wet bedding, predator sound/smell, random disruption of light/dark cycle), with indexes of depressive behavior (coat status, grooming, and mobility) becoming exacerbated vs. controls. In vascular rings, constrictor (phenylephrine) and endothelium-independent dilator (sodium nitroprusside) responses were not different between groups, although endothelium-dependent dilation (methacholine) was attenuated with UCMS. Nitric oxide synthase (NOS) inhibition was without effect in UCMS but nearly abolished reactivity in controls, while cyclooxygenase inhibition blunted dilation in both. Combined blockade abolished reactivity in controls, although a significant dilation remained in UCMS that was abolished by catalase. Arterial NO production was attenuated by UCMS, although H₂O₂ production was increased. UCMS mice demonstrated an increased, although variable, insulin resistance and inflammation. However, while UCMS-induced vascular impairments were consistent, the predictive power of aggregate plasma levels of insulin, TNF-α, IL-1β, and C-reactive peptide were limited. However, when separated into tertiles with regard to vascular outcomes, insulin resistance and hypertension were predictive of the most severe vascular impairments. Taken together, these data suggest that aggregate insulin resistance, inflammation, and hypertension in UCMS mice are not robust predictors of vascular dysfunction, suggesting that unidentified mechanisms may be superior predictors of poor vascular outcomes in this model.