Masticatory dysfunction and neuroactive opioids

Objectives: We investigated the effects of masticatory dysfunction on endogenous opioids in the cerebral limbic system and hypothalamus, and examined the possible association with stress using a corticosterone synthesis inhibitor, metyrapone.

Methods: Masticatory dysfunction was made by raising the height of mouse molar with a dental resin. Immunohistochemistry was performed according to the ABC method. A group of dysfunctional mice was pretreated with metyrapone.

Results: Among tested opioids, dynorphin-A immunoreactive intensity and immunopositive cell number were increased in both the amygdala (especially in the central amygdala) and hypothalamus (especially in the paraventricular hypothalamic nucleus), in comparison with sham-operated mice. Pretreatment of metyrapone seemed to prohibit these changes in the central amygdala, but not in the hypothalamus.

Conclusion: The present data suggest the involvement of masticatory dysfunction in the emotional integration processes that may be coupled with an endogenous opioid, dynorphin-A in the amygdala and hypothalamus. Since corticosterone mediates stress response, the present results also suggest the involvement of stress with the increased dynorphin-A-like immunoreactivity in the amygdala.

This work was supported by an Open Research Center subsidy (H18) from MEXT.