Changes in behaviors of male C57BL/6J mice across adult life span and effects of dietary restriction

Abstract

Behavioral analysis is a high-end read-out of aging impact on an organism, and here, we have analyzed behaviors in 4-, 22-, and 28-month-old male C57BL/6J with a broad range of tests. For comparison, a group of 28-month-old males maintained on dietary restriction (DR) was included. The most conspicuous alteration was the decline in exploration activity with advancing age. Aging also affected other behaviors such as motor skill acquisition and grip strength, in contrast to latency to thermal stimuli and visual placement which were unchanged. Object recognition tests revealed intact working memory at 28 months while memory recollection was impaired already at 22 months. Comparison with female C57BL/6J (Fahlström et al., Neurobiol Aging 32:1868-1880, 2011) revealed that alterations in aged males and females are similar and that several of the behavioral indices correlate with age in both sexes. Moreover, we examined if behavioral indices in 22-month-old males could predict remaining life span as suggested in the study by Ingram and Reynolds (Exp Aging Res 12(3):155-162, 1986) and found that exploratory activity and motor skills accounted for up to 65% of the variance. Consistent with that a high level of exploratory activity and preserved motor capacity indicated a long post-test survival, 28-month-old males maintained on DR were more successful in such tests than ad libitum fed age-matched males. In summary, aged C57BL/6J males are marked by a reduced exploratory activity, an alteration that DR impedes. In light of recently published data, we discuss if a diminishing drive to explore may associate with aging-related impairment of central aminergic pathways.

Fig. 1

Elevated plus maze (EPM). Stacked columns showing a time spent in the different locations of the maze and b number of exploratory entries into the arms. In both early and advanced aging, there is a dramatic reduction in time spent in the open arm (OA). Also the number of entries into the OA declined in early and advanced aged mice (p < 0.01). While the overall exploratory activity assessed by total number of entries were significantly reduced in 28-month-old mice (p < 0.01), age-matched mice maintained on DR were not significantly different from adult controls. Black = OA, white = CA, and gray = central platform (CP). Values indicated are mean ± SEM. Statistical significance: *p < 0.05; **p < 0.01; ***p < 0.001

Fig. 2

Exploration of dark and light arena. a Latency to entry of the dark arena and b time spent exploring the dark and light arena, respectively, during 300 s. While latency to enter dark area did not change significantly across adult life span, latency was increased in aged dietary restricted mice. Both ad libitum fed and DR fed aged mice spent more time exploring the light arena than adults and mice in early aging. Values indicated are mean ± SEM. Statistical significance: *p < 0.05; **p < 0.01; ***p < 0.001

Fig. 3

Individual exploration trajectories and rears in dark and light arena (300 s). Representative individual trajectories showing the change in exploratory activity across adult life span. Brown lines indicate locomotion (horizontal movements) while blue indicates rears (vertical movements). Exploratory activity declines with age and more time is spent in the light arena. Aged mice maintained on DR explore more than ad libitum fed age-matched controls. In accord with younger mice, aged DR mice explore the full arena including all corners

Fig. 4

Changes in exploration activities in the open field arena (180 s). a Stacked columns showing total locomotor distance and fraction of this occurring in the peripheral and central field, respectively. b Number of rears and c stacked columns showing fractions of the 180 s used to rest and move, respectively. Across adult life span locomotion and rearing decline while periods of rest increase. Values indicated are mean ± SEM. Statistical significance: *p < 0.05; **p < 0.01; ***p < 0.001

Fig. 5

Changes in exploration activities in the open field arena (60 min). a Total locomotor distance arranged in consecutive 10 min BINs, b number of rears, c time moving in seconds, and d fraction of total locomotor distance occurring in the central field of the arena. Key to animal groups in a. Under the first 10 min, all groups of aged mice had a lower level of activity than adults (a–d). After this initial period, 28-month-old DR mice showed an exploratory activity similar to young adult mice. At later time points, they showed less habituation than young adults and continued to explore all regions of the arena (a–d; see also Fig. S1). The level of activity among 22-month-old mice was in-between adults and 28-month-old mice, while the latter group consistently showed less activity throughout the test period. Values indicated are mean ± SEM. Statistical significance: *p < 0.05; **p < 0.01; ***p < 0.001

Fig. 6

Changes across adult life span in sensorimotor behavior. a Thermal nociceptive threshold tested by “hot plate,” b forelimb grip strength assessed by the grip strength test, and c, d locomotor speed and quality score assessment of the behavior, respectively, on the beam. Values indicated are mean ± SEM. Statistical significance: *p < 0.05; **p < 0.01; ***p < 0.001

Fig. 7

Rotarod performance across adult life span. Key to age groups in graph. Data points were interconnected to ease tracing of each series. Young adult mice, 22 and 28 months old on DR all showed some improvement across trial. In the 28-month-old group of ad libitum fed mice, the improvement was minimal and this group performed significantly worse than young adults in trials 3 and 4. As indicated in Table 2, two 4-month-old mice jumped off the RR and were discarded from this test. Values indicated are mean ± SEM. Statistical significance: *p < 0.05; **p < 0.01; ***p < 0.001

Fig. 8

Working memory and memory consolidation. a Pretrial on working memory with two identical objects (left and right). Fraction of the time spent at each object on the ordinate, while object position is displayed on the abscissa. Key to age group in a. b Probe trial with familiar and novel object and all age groups show a preference for the novel object (p < 0.05 and p < 0.01). c The pretrial of the object memory consolidation test and d probe trial after 24 h disclosing that the aged mice were unsuccessful to retain the memory of the familiar object while 4-month-old male mice managed this task