Evidence for endorphin-mediated cross-tolerance between chronic stress and the behavioral effects of ionizing radiation.

C57BL/6J mice exhibit a naloxone-reversible locomotor hyperactivity after exposure to ionizing radiation. These data implicate endogenous opiates in this radiogenic behavioral change. Similarly, endorphins mediate analgesia produced by chronic stress (e.g., foot shock or restraint) and levels of plasma Beta-endorphin are elevated following exposure to acute stress. Therefore, the present study sought to determine if behavioral cross-tolerance could be obtained between endorphin-producing stressors and radiation exposure. Repeated pretreatment with foot shock or restraint subsequently inhibited the locomotor-activating effects of radiation. These data are consistent with the hypothesis that cross-tolerance developed between the effects of stress-induced endogenous opiate release and the radiation-induced release of endorphins.