Effect of long-term social isolation on behavior and brain plasticity in mice with tumor necrosis factor gene knockout

Abstract

Prolonged social isolation can disrupt the functional activity of the serotonin (5-HT) neurotransmitter system and neurotrophic support of the brain, activate neuroinflammatory processes, and cause various behavioral disorders [1]. On the contrary, the pro-inflammatory cytokine, specifically tumor necrosis factor (TNF), affects the synthesis of serotonin and the expression of neurotrophins in the brain [2]. Additionally, TNF gene knockout alters the severity of depression-like behavior and cognitive functions in rodents [3]. The objective of this study was to examine the impact of prolonged social isolation on the 5-HT system in the brain as well as the expression of the neurotrophic factors BDNF and NGF in Tnf gene knockout (TNF KO) mice and wild-type C57BL/6 mice. Mice from each strain were divided into two groups: “control”, which were kept in groups, and “experimental”, which were isolated in cages for six weeks. The mice were subjected to a battery of tests including the “open field”, “three-chamber”, and “forced swimming” tests. The expression of genes was gauged in the brain structure of mice through real-time RT-PCR and protein content was analyzed through Western blotting. Serotonin and its metabolite 5-HIAA were quantified using HPLC. The results were analyzed using a two-way analysis of variance followed by Fisher’s multiple comparisons. The animals’ locomotor activity did not differ between groups. Social isolation in TNF KO mice led to reduced exploratory activity and increased anxiety (p 0.05) in the open field test. In isolated wild-type mice, social object preference in the three-chamber test decreased (p 0.01). Isolation had no effect on depression-like freezing in the “forced swimming” test and cognitive functions in the “new object” test in animals of both strains. Social isolation resulted in decreased expression of the tryptophan hydroxylase 2 gene (synthesizes 5-HT) in the midbrain of wild-type mice (p 0.05) and increased expression of the 5-HT1A receptor gene in this structure in knockout animals (p 0.05). Only the knockout mice exhibited a reduction in 5-HT levels in the hippocampus due to isolation (p 0.05). However, there were no differences observed in the levels of the neurotransmitter and its metabolite 5-HIAA in the frontal cortex and midbrain between groups of both strains of mice. In TNF KO mice exposed to isolation, the mRNA level of the nerve growth factor gene NGF increased in the frontal cortex (p 0.01); additionally, the content of proBDNF protein (a precursor of the BDNF factor) increased in both the hippocampus and frontal cortex (p 0.05). Our findings indicate that Tnf gene knockout modifies the influence of long-term social isolation on behavior, the 5-HT system, and the expression of neurotrophic factors in the brain.