Apoptotic death of beta cells after optic nerve transection in adult cats

We have revealed previously that the survival rate of beta cells of cat retinal ganglion cells (RGCs) rapidly decreased to 29% on day 7 after optic nerve transection, whereas that of alpha cells slowly decreased to 64% on day 14 (Watanabe et al., 2001). The reason that beta cells die more rapidly than alpha cells was not clear. In the present study, we tested the possibility that the rapid death of beta cells is attributable to apoptosis, as shown for some axotomized RGCs in rats. The following results were obtained. First, the proportion of pyknotic cells in Nissl-stained cat retinas started to increase sharply starting on day 4 and reached a peak on day 6 after optic nerve transection. The time course of occurrence of pyknotic cells corresponded well with that of the rapid death of axotomized beta cells. Secondly, the proportion of pyknotic cells was the highest in the area centralis (AC), in which beta cells are densely distributed. The preferential death of axotomized RGCs in the AC was also confirmed by terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling staining in cross sections. Thirdly, after the intravitreal injection of caspase 3 inhibitor (z-DEVD-cmk) the survival of axotomized beta cells on day 7 was significantly enhanced, whereas no such survival-promoting effect was obtained in axotomized alpha cells. Taken together, these results suggest that the rapid death of axotomized beta cells is attributable mainly to apoptosis, which is mediated by caspase 3.