Response of 5 hpf zebrafish embryos to low-dose microbeam protons

The microbeam irradiation system (single-particle irradiation system to cell, acronym as SPICE) at the National Institute of Radiological Sciences (NIRS), Japan, was employed to irradiate dechorionated embryos of the zebrafish, Danio rerio, at 5 h post-fertilization (hpf) by protons each having an energy of ∼3.4 MeV. Either 1 or 10 positions on the cells were irradiated with different number of protons. The levels of apoptosis in zebrafish embryos at 25 hpf were quantified through terminal dUTP transferase-mediated nick end-labeling assay. Triphasic dose–esponses were obtained (Fig. 1), including (i) a subhormetic zone with an increase in apoptotic signals for a small number of irradiated protons per position, (ii) a hormetic zone with a reduction in the apoptotic signals below the spontaneous level for a larger number of irradiated protons per position and (iii) a toxic zone with an increase in apoptotic signals again if the number of irradiated protons per position was further increased.