Social threat and novel cage stress-induced sustained extracellular-regulated kinase1/2 (ERK1/2) phosphorylation but differential modulation of brain-derived neurotrophic factor (BDNF) expression in the hippocampus of NMRI mice.

The extracellular signal-regulated kinase1/2 (ERK1/2) pathway has a key role in cell survival and brain plasticity, processes that are impaired following exposure to stressful situations. We have recently validated two repeated intermittent stress procedures in male NMRI mice, social threat and repeated exposure to a novel cage, which result in clear behavioral effects following 4 weeks of application. The present results demonstrate that both repeated intermittent stress procedures alter the activity of the ERK1/2 pathway in the brain, as shown by changes in phosphorylated ERK1/2 (phospho-ERK1/2) protein expression and in the expression of downstream proteins: phosphorylated cAMP response element binding protein (CREB) and brain-derived neurotrophic factor (BDNF), in the hippocampus, the frontal cortex and the hypothalamus. The hippocampus showed greater responsiveness to stress as the two stressors increased phospho-ERK1/2 and BDNF expression under acute condition. Following repeated stress, hyperphosphorylation of ERK1/2 was associated with up-regulation of hippocampal BDNF expression in the social threat group but not in mice exposed to novel cage. This lack of a pro-survival effect of ERK1/2 with repeated novel cage exposure may constitute an early event in stress-mediated brain pathology. The sustained BDNF up-regulation in the hippocampi of mice subjected to repeated social threat could be related to rewarding aspects of aggressive interactions, suggested by our previous studies.