Long-term effects of irradiation with iron-56 particles on the nigrostriatal dopamine system

Exposure to heavy ions during a Mars mission might damage the brain, thus compromising mission success and the quality of life of returning astronauts. Several workers have suggested that the dopamine system is particularly sensitive to heavy ion radiation, but direct evidence for this notion is lacking. We examined measures of brain dopamine viability at times up to 15 months after acute exposure of rats to Fe-56 (1.2-2.4 Gy). No effects were seen in brain sections stained for tyrosine hydroxylase, the classical marker for dopamine cells and nerve terminals. Locomotion stimulated by cocaine, which directly activates the dopamine system, was reduced at 6 months but not at 12 months. Furthermore, in a visually cued lever-pressing test, reaction times, which are prolonged by dopamine system damage, were identical in irradiated and control animals. However, learning times were increased by irradiation. Our data suggest that the midbrain dopamine system is not especially sensitive to damage by Fe-56 particles at doses much higher than would be associated with travel to and from Mars.