Relationship between light and dark period activity cycles and oral condition in senescence-accelerated mice

Abstract

IntroductionThe aging of the Japanese society is associated with an increase in the number of patients with senile dementia, and provisions to deal with this problem are becoming increasingly important. There is a clear relationship between the oral condition and general health 1,2) , but the relationship between oral conditions with aging and dementia r emains unclear 3) . In the present study, we used s enescenceaccelerated mice (SAMP1) to evaluate the effects of tooth absence and masticatory abnormalities induced during the juvenile period on life cycle and activity at later ages during which peripheral symptoms of dementia appear. Rodents, however, are nocturnal animals with higher activity levels in the dark period than in the light period, showing an activity cycle that follows the biological clock, and diurnal activity levels in senile rats aged 90 to 108 weeks is significantly shorter than that in juvenile rats, with no differences in the activity levels between the light and dark periods. Further, spontaneous activity is decreased, indicating that the nocturnal behavior of senescent animals is disturbed 4) . In the present study, this characteristic was used as an index of dementia. Materials and MethodsFifty male SAMP1 mice were used. The strain was kindly donated by the Institute of Frontier Medical Sciences, Kyoto University. We bred brother-sister in the animal facility of the Central Research Division at the School of Dentistry, Asahi University. The SAMP8 mouse has an average life span of 12 months (about half life span of normal mice) and begins to show deficits in learning and memory and aging at 5 months. The characteristics of this strain were described in detail by Takeda et al. 5,6) . The a nimals were bred and maintained under conventional conditions; housed in groups of 5 in plastic cages (175 W × 245 D × 125 H mm) under temperature (23 ± 1°C), humidity (55 ± 2%), and light (12 h; light period, 6:00 -18:00; dark period, 18:00 -6:00) controlled conditions; and water was available ad libitum. This experiment was conducted Okajimas Folia Anat. Jpn., 88(1): 29-36, May, 2011 Relationship between light and dark period activity cycles and oral condition in senescence-accelerated mice Summary: We investigated the effect of tooth absence and masticatory abnormalities due to powdered food feeding starting during the juvenile period on light and dark period activity cycles in senescence-accelerated mice (SAMP1) . SAMP1 were divided into 5 groups: Group 1, maxillo-mandibular molar tooth extraction; Group 2, maxillary molar tooth extraction; Group 3, mandibular molar tooth extraction; Group 4, powdered food; and Group 5, sham-operated control. Senescence was observed earliest in the powdered food group. Total 24-hour activity was higher in the control group than in the four other groups. In the powdered food group, the dark period activity decreased to less than 60% of the total activity in the 36th week. In the tooth extraction groups (Groups 1-3), dark period activity decreased to less than 60% of...