INTRODUCTIONRadiation therapy is often used for the treatment of brain tumors; however, radiationinduced brain injury, including both anatomical and functional changes, is sometimes observed after the irradiation (1). Radiation-induced brain injury is mostly observed and symptomatically found chronically after radiation therapy, and the main underlying cause is thought to be cell death. The acute effects of irradiation are well known, but very few studies have investigated the underlying mechanisms.Radiation-induced brain injury is classified into acute, early delayed and late delayed injury. Early delayed injuries start to occur more than 1 month after irradiation. For example, progressive depletion of adult neurogenesis in the hippocampus by X-irradiation induces impairment of cognitive function 1-2 months after the exposure (2-4). In contrast, acute injury has been reported to manifest within days of exposure. For example, impairment of long term potentiation has been observed 48 h after -irradiation (5). Because mature neurons are relatively radiation-resistant, the acute effects of injury might be associated with synaptic dysfunction.It has been reported recently that the mislocalization and dysregulation of postsynaptic structural proteins result in synaptopathy (6). We focused on drebrin, a protein whose loss from dendritic spines is a surrogate marker of synaptopathy. In dementias, such as Alzheimer's disease, mild cognitive impairment and Down syndrome, drebrin loss from the dendritic spines 2 of hippocampal neurons is observed (7).Cognitive function strongly correlates with synaptic plasticity (8, 9), and dynamic morphological changes in dendritic spines play an important role in synaptic plasticity.Dendritic spines are actin-rich structures that protrude from dendritic shafts. F-actin in the core of the dendritic spine is stabilized by drebrin, which is an actin-binding protein (10, 11), and regulation of F-actin stability is necessary for synaptic plasticity (12).We recently reported that X-irradiation induces an acute decrease in drebrin in the dendritic spines of cultured hippocampal neurons (13). This suggests that the X-irradiation causes acute alterations in synaptic function. Therefore in the present study, we examined the acute effect of X-irradiation using behavioral and immunohistochemical analyses within 24 h of exposure.
MATERIALS AND METHODS
AnimalsTen-to 11-week-old male mice were used (C57BL/6N; CLEA Japan, Inc., Japan; SLC Co., Ltd., Shizuoka, Japan). All animal experiments were performed according to guidelines set by the Animal Care and Experimentation Committee (Gunma University, Showa Campus, Maebashi, Japan). We used 10-13 mice per group for behavioral analysis, 3-4 mice (4-7 slices from one animal) per group for immunohistochemical analysis and three mice per group for western blot analysis. The mice were maintained under standard animal facility conditions with 3 food and water available ad libitum. All efforts were made to minimize animal suffering and to reduce the number of ...