Combination of chewing and stress up-regulates hippocampal glucocorticoid receptor in contrast to the increase of mineralocorticoid receptor under stress only.

In general, acute immobilization stress increases plasma corticosterone levels that signal the hypothalamic-pituitary-adrenal axis. Mineralocorticoid receptors and glucocorticoid receptors in the hippocampus perform crucial roles in this feedback mechanism. In the present study, we investigated the effects of chewing under stress on the rat hippocampal mineralocorticoid and glucocorticoid receptors by immunohistochemistry. We separated rats into a control group, a 2-h immobilization stress group (stress only group), and a 2-h immobilization stress group that was allowed to chew on a wooden stick for the latter 1h (stress with chewing group). Mineralocorticoid receptor immunoreactive cells with nucleus staining in the hippocampal CA1 area were scattered in the pyramidal cell layer. The stress only group showed the densest distribution of immunoreactive cells; however, the density of the immunoreactive cells in the stress with chewing group was similar to that of the control group. Changes in immunoreactive cell density were not visible in other areas of the hippocampus, namely, the CA3 area and dentate gyrus. Image analysis indicated that the increase in the mineralocorticoid receptor immunoreactive area within a fixed area in the stress only group was statistically significant compared with those in the control group and the stress with chewing group. On the other hand, glucocorticoid receptor immunoreactive cells in the CA1 area seemed to be increased in the stress with chewing group, but not in the stress only group. Image analysis indicated that this increase was statistically significant. These results suggest that immobilization and immobilization with chewing differentially affect these two types of glucocorticoid receptors in the rat hippocampus. Considering that chewing has alleviative effects against stress, glucocorticoid receptor elevation in the hippocampal CA1 area is one of the neuronal mechanisms of coping with stress.