The stress effects of a single injection of isotonic saline solution: systemic (blood) and central (frontal cortex and dorsal and ventral hippocampus)

In experiments with animals, a group that is injected with the vehicle in which a drug of interest is dissolved is often used as a control. However, even a single injection of a vehicle is a stressor, i.e., “treatment stress,” which may significantly affect some stress-sensitive indices. In the present study, we report some data on the effects of a single intraperitoneal injection of isotonic saline solution on the contents of corticosterone, nitric oxide metabolites, and oxidative capacity, as well as on the expression of proteins and mRNAs of proinflammatory cytokines in the blood and brain regions of rats within one day after the injection as compared to intact animals. At the early time points after the injection, corticosterone contents were substantially elevated in the blood and ventral hippocampus. The content of nitric oxide metabolites decreased in the blood and remained stably low within 2–24 h after the injection. The injection did not affect the contents of proinflammatory cytokines in the blood; however, early after the injection the expression of IL-1s mRNA decreased in the ventral hippocampus and frontal cortex, whereas 24 h after this treatment, the expression of TNF-a mRNA increased by a factor of 4 in the frontal cortex. Thus, a single injection of isotonic saline solution had a clear stress-producing effect, which was observed at the systemic level and in stress-sensitive brain regions. The strength of this stressful event was sufficient to activate the hypothalamus–pituitary–adrenal axis but not sufficient to induce a significant inflammatory response. The frontal cortex was most sensitive to this treatment; the alterations in the ventral hippocampus were less expressed, whereas the dorsal hippocampus was most stress resistant. Our data show that it is important to consider and thoroughly analyze the effects of “treatment stress” in experiments using injections of biologically active substances.