Loss of neuronal protein expression in mouse hippocampus after irradiation

The effects of radiation on neurons are incompletely characterized. We evaluated changes in the expression of neuronal nuclear and other proteins in the mouse hippocampus after 17-Gy whole-brain irradiation. Expression of neuronal nuclei (NeuN), neuron-specific enolas, prospero-related homeobox 1 (Prox1), calbindin D28k, and synaptophysin 1 in the CA1, CA3, and dentate gyrus of the hippocampus was determined by immunohistochemistry; neuronal numbers were estimated by design-based stereology. At 7 days after irradiation, there was a marked reduction of NeuN neurons in CA3. Stereologic estimates confirmed a significant reduction in NeuN neurons in CA3 at 7 days, in the dentate gyrus at 7 days, 3 weeks and 2 months, and in CA1 at 2 months compared with controls; neuron-specific enolase and prospero-related homeobox 1-positive neurons in the CA3 subregion were also decreased at 7 days. The numbers of granule and pyramidal cells identified by 4'6-diamidino-2-phenylindole nuclear staining, however, remained unchanged, and there were no changes in calbindin D28k or synaptophysin 1 immunoreactivity after irradiation. We conclude that irradiation may result in a temporary loss of neuronal protein expression in mouse hippocampus. These changes do not necessarily indicate loss of neurons and indicate the need for caution regarding the use of phenotypic markers such as NeuN to estimate changes in neuronal numbers after irradiation.