We compared changes in the liver structure in stress-resistant and stress-sensitive rats under conditions of chronic stress exposure. The number of degenerative cells, specific area of degenerative part of hepatocyte cytoplasm, and specific area of intralobular sinusoidal capillaries increased in animals of both groups. These parameters were significantly higher in stresss-ensitive rats. Reparative processes that manifested in increased in number of binucleated hepatocytes and number of nucleoli in nuclei were observed in the liver parenchyma. These changes were more pronounced in animals with high resistance to stress. Nucleus hypertrophy was also found in these rats.

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Experiments on rats showed that the selective opioid κ receptor agonist dynorphin A (1-13) had marked antioxidant actions in immobilization stress of different durations, apparent as reductions in lipid peroxidation products in liver tissues. The effects of this peptide on the activities of the antioxidant defense enzymes superoxide dismutase and catalase depended on the duration of stress. While the peptide had a notable stimulatory effect on catalase activity in animals subjected to immobilization for 3 h, this effect was signifi cantly smaller in rats exposed to stress for 6 or 12 h. These studies demonstrated that dynorphin A (1-13) has a stimulatory action on superoxide dismutase activity in animals exposed to immobilization for 6 or 12 h, this being seen at all study time points. Peptide had no effect on the activity of this enzyme in rats exposed to immobilization stress for 3 h.