Blood immunological parameters (cytokine profile and interferon status) and the level of monoamines and their metabolites in various brain structures (amygdala, hippocampus, septum, and hypothalamus) were studied in rats kept under standard conditions or in overpopulated cages. Long-term overcrowding was associated with reduced expression of IL-4 gene, increased transcription of IL-17, and decreased production of IFN-γ, which attested to impaired humoral and cell-mediated immunity and disturbances in IFN-γ synthesis at the post-transcriptional level. Under these conditions, the levels of norepinephrine and dopamine decreased in the septum, but increased in the hypothalamus. The amount of dopamine metabolite dihydroxyphenylacetic acid decreased in both these structures, and the index of dopamine metabolism (dihydroxyphenylacetic acid/dopamine ratio, DOPAC/dopamine) decreased only in the hypothalamus. Overcrowding was not followed by changes in the parameters of noradrenergic and dopaminergic systems in the amygdala and hippocampus and serotoninergic system in all study structures.

Behavioral indicators of anxiety and body weight dynamics were compared in rats kept under standard conditions (4-5 animals per cage) and in overpopulated cages (17 rats per cage). The behavior of rats exposed to overcrowding for a short and long period was evaluated in the open field, light-darkness, and elevated plus-maze tests. The anxiety parameters of animals increased only in the light-darkness test during the acute period of overcrowding and in all tests during chronic overcrowding. Behavioral tests had a negative impact on body weight gain in controls and rats living in overpopulated cages. If the behavioral tests were carried out at the beginning of the experiment, body weight ceased to increase in animals exposed to overcrowding and continued to increase in the controls. Overcrowding without behavioral tests had no effects on the body weight dynamics. The model of chronic (but not acute) overcrowding may be used for evaluation of anxiolytic effects of drugs.

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Effects of chronic overcrowding on the development of depressive-like behavior and changes in monoamine contents in the brain structures involved in the development of depression were studied in rats. It was shown that overcrowding increased grooming duration and did not change the number of crossed squares and postures in the open-field test. In the forced swimming test, overcrowding increased immobility time and deceased the duration of the first episode of active swimming, which attested to depressive-like behavior. Overcrowding reduced the content of dopamine in the striatum, its metabolites (homovanilic and dioxyphenylacetic acids) and dopamine metabolism rate in the hypothalamus, and increased the concentrations of norepinephrine, homovanilic acid, and dioxyphenylacetic acid in the olfactory bulb and homovanilic acid in the prefrontal cortex. No changes were found in the nucleus accumbens. Serotonin content did not change in all analyzed structures. It was hypothesized that the observed changes in the content of monoamines and their metabolites in certain brain structures could underlie the depressive-like behavior induced by overcrowding in rats.

No abstract