Stress-related activities induced by predator odor may become indistinguishable by hinokitiol odor

Predator odors, such as 2,5-dihydro-2,4,5-trimethylthiazoline (TMT), induce a stress-like behavior in some rodents, and there is activation of a complex mix of brain regions including the anterior piriform cortex (APC) and the bed nucleus of stria terminalis (BST). In contrast, rose odor can counteract TMT-induced activation of the ventrorostral part of APC and the medial part of BST. In the present study, two novel odors, woody (hinokitiol) and caraway [S(+)-carvone] odors, were evaluated to determine whether they have an antistress effect. Plasma adrenocorticotropic hormone levels, a marker of stress, and the number of c-Fos-immunopositive cells were determined in APC and BST. Plasma adrenocorticotropic hormone levels were increased by TMT alone and in combination with S(+)-carvone; however, hinokitiol with or without TMT did not have an effect. The number of activated cells in the medial part of BST was increased by TMT alone and in combination with S(+)-carvone or hinokitiol. Although TMT alone activated the medial part of BST, a mixture of TMT and hinokitiol activated both the medial and the lateral part of BST. These data suggest that the selective responses to TMT in the medial part of BST were obscured by activation of more odor-related regions by hinokitiol with TMT. In addition, the ratio of medial to lateral BST activation may be critical in stress-related behavior. In conclusion, hinokitiol can alleviate TMT-induced stress; however, the underlying mechanism appears to be different from that of the rose odor, as found in our previous study.