Differential stress responsivity in diet-induced obese and resistant rats

The relationship between stress and obesity was assessed in male rats selectively bred to develop either diet-induced obesity (DIO) or diet resistance (DR) when fed a high-energy, 31% fat diet for 3 wk followed by 2 wk on a hyperphagic liquid diet (Ensure). One-half of the rats of each phenotype were subjected to moderate daily, unpredictable stress (cage changing, exposure to conspecific, swim, and immobilization stress, intraperitoneal saline injection) during the 5 wk. Both stressed and unstressed DIO rats were 26% heavier and ate 27% more than comparable DR rats at onset and had 48% lower basal morning plasma corticosterone levels. Stressed DR rats gained less weight and had significant elevations of basal morning corticosterone but reduced basal sympathetic activity (24-h urine norepinephrine) over 5 wk compared with their unstressed DR controls. Terminally, there was a 35% increase in the paraventricular nucleus corticotropin-releasing hormone mRNA expression. On the other hand, stressed DIO rats showed only a transient early increase in open-field activity and a terminal increase in basal corticosterone levels as the only effects of stress. Thus DIO rats are hyporesponsive to chronic stress compared with DR rats. This is in keeping with several other known differences in hypothalamopituitary and autonomic function in this model.