Abstract: The generic term mitogen-activated protein kinases (MAPKs) is used to denote a family of signal transduction mediators that regulate a diverse array of cellular functions via activation of a sequential phosphorylation cascade involving a three-protein cassette. Among several MAPKs cascades that have been characterized, the extracellular signal-regulated kinases 1 and 2 (ERK1/2) are the most widely investigated. ERK1/2 are extensively distributed throughout the central nervous system and prominently found in the hippocampus and prefrontal cortex. At present, ERK1/2 are being extensively studied in the field of learning and memory. Recent data have demonstrated that ERK has a vital role in the hippocampus-dependent learning and memory and prefrontal cortex-dependent conditioning. It can facilitate learning and memory consolidation and regulate neuronal plasticity. However, there is rather little evidence concerning the role of ERK1/2 in stress response and emotional regulation. The purpose of the present study was to detect the effects of chronic forced swimming stress on emotion and ERK1/2 in the hippocampus and the prefrontal cortex in rats and to determine the relation between emotion and ERK1/2 of brain.
Thirty rats were randomly divided into three groups. They were the swimming stress group, apparatus control group, and the control group. The stressed animals received swimming stress for 5 minutes once a day for 14 days. The apparatus controls were placed in a novel environment for 5 minutes once a day for 14 consecutive days. The controls were free of stress. After stress, rats were tested with an open-field, elevated-plus maze and saccharin preference test. They were then decapitated and dissected to detect the ERK1/2 of the hippocampus and prefrontal cortex with blotting. It was found that both the swimming stress group and the apparatus control group showed significant emotional disorders. The body weight gain of the stressed group was significantly lower than that of the control group and the apparatus controls. The defecation of the apparatus controls was more than that of the control group and the stressed group. The locomotion of the stressed group was significantly lower than that of the other two groups. Compared to the controls, the stressed animals spent a shorter time in open arms and longer time in closed arms. The saccharine solution consumed and the ratio of the saccharine solution consumed to the total liquid consumed of both the stressed group and the apparatus controls were significantly lower than that of the control group. There were enhanced levels of ERK1/2 in the prefrontal cortex of the stressed group and the apparatus controls, with no change in the hippocampus. The ERK2 level of the prefrontal cortex was negatively correlated with the amount of the saccharin solution consumed. The results suggested chronic swimming stress could induce emotional disorders and increase the level of ERK1/2 in the prefrontal cortex. ERK1/2 was closely linked to emotional response and could provide the mechanism underlying protective reaction under stressful situations

Key words: swimming stress, depression, hippocampus, prefrontal cortex, extracellular signal-regulated kinase