Abstract
Purpose: Recently, it has become clear that loss of teeth is one of the causes of learning and memory ability impairment. On the other hand, learning and memory ability impairment occurs spontaneously with aging. However, little consideration has so far been given to the impairment of learning and memory ability due to the effect of loss of teeth, aging, or both of them in human beings. In this study, we consider the effect of age-related changes in learning and memory ability, the effect of molar teeth loss and longitudinal molarless condition on learning and memory ability using SAMP 8//Ymi (Senescence-Accelerated Mouse P 8//Ymi).
Methods: SAMP 8//Ymi shows that the age-accelerated characteristic in a short life span and median survival time is 12 months. In addition, age-related deterioration of learning and memory has been demonstrated. 60 Male SAMP 8//Ymi aged 2 months were divided into 6 groups. Each of the three control groups of 10 mice was kept until 3 (young adult), 5 (middle age), 8 (old age) months of age (normal condition). The remaining three groups of 10 mice each had their maxillary molar teeth extracted and were kept until 3, 5, 8 months of age (molarless condition). For the behavioral study, a passive avoidance test of the step-through type was adopted. The two-compartment stepthrough passive avoidance apparatus consisted of an illuminated chamber and a dark chamber. The two chambers were separated by a guillotine door and the dark chamber had a grid floor. In an acquisition trial, the mouse was placed in the illuminated chamber and the time before it entered the dark chamber was recorded. As soon as the mouse entered the dark chamber, the door was closed and an AC 0.3mA scrambled footshock was applied to the floor grid for 3 seconds. A Retention test was carried out by replacing the mouse in the illuminated chamber 24 hours after the acquisition trial, and the latency to enter the dark chamber was measured. The learning and memory ability were evaluated on the basis of the latency time.
The degree of senescence was evaluated using the grading score system for SAM. The Grading score was recorded at intervals of one month.
Statistical analyses of data were performed by the Kruskal-Wallis test and the Mann-Whitney U-test.
Result: In SAMP 8//Ymi with molar teeth, 8-month-old mice showed significantly shorter latency time than 3 and 5-month-old mice (p<0.05). In SAMP 8//Ymi without maxillary molar teeth, 5 and 8- month-old mice showed significantly shorter latency time than 3-month-old mice (p<0.05). As a result, in 5-month-old mice, the ones without molar teeth showed significant shorter latency time than the mice with molar teeth (p<0.05). In the degree of senescence, the grading score increased significantly in mice with and without molar teeth associated with aging (p<0.05). The mice without molar teeth showed a significantly higher grading score than the mice with molar teeth at 5-months-old (p<0.05). However, there were no significant differences between 3-month-old mice and 8-month-old mice.
Conclusions: SAMP 8 with molar teeth showed age-related changes in impairment of learning and memory ability. SAMP 8 without molar teeth showed earlier impairment of learning and memory abil-ity than SAMP 8 with molar teeth. In addition to this, accelerated senescence was observed in SAMP 8 without molar teeth. Accordingly, we suggest that the earlier aging phenomenon and the impairment of learning and memory ability are due to the loss of molar teeth occur at the same time.
