Assessment of retention and attenuation of motor-learning memory by repeated rotor-rod analyses

Abstract

Retention of acquired learning memory is essential for reasonable behavior and crisis avoidance of individuals. Therefore, establishment of a system suitable for analysis of the retention/attenuation of acquired memory is desired. In the present study, mice were conducted on the repeated rotor-rod test, consisting of two series of experiments (Series 1 and 2) of 10 trials each. When rotating speed was 9 rpm, retention time on the rod was gradually increased and reached the maximum value within 10 trials in Series 1. When Series 2 was performed 1 or 3 days after Series 1, retention time of trials 1-3 in Series 2 was not significantly different from that of trials 8-10 in Series 1. On the other hand, retention time of trials 1-3 in Series 2 was significantly declined from that of trials 8-10 in Series 1 when Series 2 was conducted day 7, and returned to the initial level, the same level with trials 1-3 in Series 1, when Series 2 was conducted on days 14 or 30. These results indicate that acquired motor-learning memory is retained for 3 days at least, began to decline by day 7 and returned to the initial level by day 14. In older mice of 10-11 months old, there was a delay in the acquisition of motor learning in Day 0, whereas the retention was not impaired in Day 7. The repeated rotor-rod analyses may useful for research on factors affecting retention/attenuation and acquisition of motor-learning memory and proceed our understanding of motor-learning memory.

Fig. 1

Effect of speed of rotating rod on retention time on the rod. (A–E) Retention time of the mice placed on the rod rotating at 7 (A), 8 (B), 9 (C), 10 (D) and 11 (E) rpm. Mean ± S.E.M. (n = 10 in each group). Bonferroni’s multiple comparison test was used for statistical analysis, after one-way ANOVA.

Fig. 2

Retention time of mice conducted on 2 series of rotor-rod tests. Series of rotor-rod test (9 rpm) consisting of 10 trials were repeated two times with interval of 1 (A; n = 10), 3 (B; n = 12), 7 (C; n = 12), 14 (D; n = 10) or 30 (E; n = 8) days. Retention time of each trial in the first series (Day 0; left column) and second series (Day 1, 3, 7, 14 or 30; right column) was expressed as Mean ± S.E.M.

Fig. 3

Retention/attenuation of motor-learning memory on days 1–30. (A–E) Averaged retention times of trials 1–3, 4–7 and 8–10 in series 1 (left column; Day 0) and series 2 (right column; Day 1 (A; n = 10), 3 (B; n = 12), 7 (C; n = 12), 14 (D; n = 10) or 30 (E; n = 8)). Mean ± S.E.M. (n = 7 in each group). ⁺⁺p < 0.01, ⁺⁺⁺p < 0.001, compared with the value of trials 1–3 in Series 1, and *p < 0.05, **p < 0.01,***p < 0.001, compared with the value of trials 8–10 in Series 1 by Bonferroni’s multiple comparison test after one-way repeated measures ANOVA.

Fig. 4

Decline in motor-learning memory from Day 1 to Day 30. Retention times of trials 1–3 (A), 4–7 (B), 8–10 (C) in Series 2, normalized by value of trials 8–10 in Series 1. Rotor-rod test in Series 2 were conducted 1 (n = 10), 3 (n = 8), 7 (n = 9), 14 (n = 8) or 30 (n = 8) days after the Series 1. Mean ± S.E.M. *p < 0.05, **p < 0.01, ***p < 0.001, compared with the value of Day 1 by Bonferroni’s multiple comparison test after one-way ANOVA.

Fig. 5

Effects of aging on motor learning analyzed by the repeated rotor-rod test. Young (Y; n = 12) and aged (A; n = 8) mice were conducted on 2 series of rotor-rod tests (9 rpm) with interval of 7 days. (A, B) Retention time of each trial in the first series (left column; Day 0; young (Y) and aged (A) mice) and second series (right column; Day 7) was expressed as Mean ± S.E.M. Data of young mice were the same with those in Fig. 2C. (C, D) Averaged retention times of trials 1–3, 4–7 and 8–10 in series 1 (left column; Day 0) and series 2 (right column; Day 7) of young (Y) and aged (A) mice. Mean ± S.E.M. Data of young mice were the same with those in Fig. 3C. ⁺⁺p < 0.01, ⁺⁺⁺p < 0.001, compared with the value of trials 1–3 in Series 1, and *p < 0.05, compared with the value of trials 8–10 in Series 1 by Bonferroni’s multiple comparison test after one-way repeated measures ANOVA. (E) Comparison of retention time normalized by that of trials 8–10 in Day 0 between young and aged mice. Mean ± S.E.M.

Fig. 6

Effects of exercise in the first series on the second series of exercise. In the first series (Day 0; left column), mice were placed on the rod rotating at 7 (A; n = 7), 9 (B; n = 9) or 11 (C; n = 6) rpm. In the second series (Day 7; right column), mice were placed on the rod rotating at 9 rpm. Mean ± S.E.M.

Fig. 7

Effects of lower or higher exercise load on the second series of exercise. (A–C) The data in Fig. 5 were grouped into Trials 1–3, 4–7 and 8–10 in Series 1 (Day 0; left column) and Series 2 (Day 7; right column). Mice were placed on the rod rotating at 7 (A; n = 7), 9 (B; n = 9) or 11 (C; n = 6) in series 1, and 9 rpm in series 2. Mean ± S.E.M. Bonferroni’s multiple comparison test was used for statistical analysis, after one-way repeated measures ANOVA. (D) First-order correlation between total retention time (sum of the retention time of trials 1–10) in Series 1 and the retention time of Trials 1–3 in Series 2 (p = 0.0083, n = 22).