Chewing ameliorates stress-induced suppression of spatial memory by increasing glucocorticoid receptor expression in the hippocampus.

Chewing alters hypothalamic-pituitary-adrenal axis function and improves the ability to cope with stress in rodents. Given that stress negatively influences hippocampus-dependent learning and memory, we aimed to elucidate whether masticatory movements, namely chewing, improve the stress-induced impairment of spatial memory in conjunction with increased hippocampal glucocorticoid receptor expression. Male Sprague-Dawley rats were subjected to restraint stress by immobilization for 2h: the stress with chewing (SC) group were allowed to chew on a wooden stick during the latter half of the immobilization period, whereas the stress without chewing (ST) group were not allowed to do so. Performance in the Morris water maze test was significantly impaired in the ST group compared with the SC group. Further, the numbers of glucocorticoid receptor immunopositive neurons in the hippocampal cornu ammonis 1 region were significantly lower in the ST group than in the control and SC groups. The control and SC rats showed no significant differences in both the water maze performance and the numbers of glucocorticoid receptor-immunopositive neurons. The immunohistochemical finding correlated with the performance in the water maze test. These results suggest that chewing is a behavioral mechanism to cope with stress by increasing hippocampal glucocorticoid receptor expression.