Influences of Housing Environment on Functional State of the Body after Action of Ulcerogenic Stressor

Abstract

Here we studied influences of housing environment on functional state of the body after action of ulcerogenic stressor. The effects of ulcerogenic stressor on the gastric mucosa, the hypothalamic‐pituitary‐adrenocortical (HPA) axis reactivity and glucocorticoid receptors in hippocampus, behavior and somatic pain sensitivity were compared in rats kept in enriched environment, social isolation, and standard laboratory conditions (control group). The rats aged 30 days were divided into groups and placed in standard cages 6 rats per cage (control) or 1 rats per cage (isolation) or in enriched cages 6 rats per cage (enriched environment). One month later, all rats (after 24 h fasting) were stressed by cold restraint (10˚C) for 3 h. In 3 h one half of rats in each group were decapitated to examine the gastric mucosa. Another half of rats were subjected to behavior tests (open field and elevated plus maze) and somatic pain sensitivity test (hot plate) as well as the HPA axis reactivity testing by plasma corticosterone levels induced by mild procedural stressor. Glucocorticoid receptor expression was estimated in hippocampus by immunohistochemistry. Body weight gradually increased in all groups during 1 month before exposure of stressor, however, in the isolated rats, the weight gain was greater than that in enriched and control groups. In all groups cold restraint caused the gastric erosion, the mean areas of which were not significantly differed. In behavior tests, the animals of isolated group were approximately equally divided into vulnerable (showing depressive‐like reactions in the open field) and resistant (not differed from the non‐stressed control) to ulcerogenic stress (active subgroup). All isolated rats showed a reduced level of exploratory activity in the elevated plus maze. Rats from enriched environment showed greater motor and exploratory activity, as well as lower anxiety level compared to those in isolated group. Isolated rats were most sensitive to action of pain, whereas rats kept in enriched environment were less sensitive to pain exposure. In the isolated group, the weakest corticosterone release among all groups was observed in response to mild procedural stress. Low HPA stress‐reactivity of isolated rats was combined with enhanced expression of glucocorticoid receptors in the hippocampus indicating a sensitization of the mechanisms of negative glucocorticoid feedback following prolonged social isolation. Thus, social isolation is a stressor leading to weight gain, a decrease in stress reactivity accompanied by an increase of glucocorticoid receptor expression in the hippocampus, a decrease of exploratory activity and increased of somatic pain sensitivity whereas enriched environment exerts beneficial action on functional state of the body: increases motor and exploratory activity, decreases anxiety and somatic pain sensitivity.