Social defeat and isolation induce clear signs of a depression-like state, but modest cardiac alterations in wild-type rats

Adverse social environments play a relevant role in the onset and progression of mood disorders. On the other hand, depression is an independent risk factor for cardiovascular morbidity. This study was aimed at (i) corroborating the validity of a rat model of depression based on a negative social episode followed by social isolation and (ii) verifying its impact on cardiac function and structure. Pair housed, wild-type Groningen rats (Rattus norvegicus) were implanted with radiotransmitters for ECG, temperature and activity recordings. They were either exposed to a social defeat episode followed by 4-week isolation or left undisturbed with their female partners. The social challenge induced a series of biological changes that are commonly taken as markers of depression in rats, including decreased body weight gain and reduced preference for sucrose consumption, functional and structural changes of the hypothalamic-pituitary-adrenocortical axis, increased anxiety in the elevated plus maze test. The cardiovascular alterations consisted in (i) transitory heart rate circadian rhythm alterations, (ii) lack of habituation of cardiac autonomic responsivity (tachycardia and vagal withdrawal) to an acute stressor, and (iii) moderate hypertrophy affecting the right ventricle of the heart. These results indicate that a depression-like state induced via this model of social challenge was associated with a few modest cardiovascular changes. Further studies are required to confirm the validity of this rat model of depression as a valid preclinical approach to the comprehension of the biological substrates underlying depression-cardiovascular comorbidity.